Abstract 102 - SGTSG_Cause_&_Effects_of_Deformation_in_the_Lithosphere_Waratah Bay_2012

Geological Society of Australia

ABSTRACTS Number 102

Specialist Group in Tectonics and Structural Geology Cause and Effects of Deformation in the Lithosphere" s o T s <5

o

Waratah Bay, Victoria, Australia January

February

2012

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Geological Society of Australia Specialist Group in Tectonics and Structural Geology conference SGTSG Specialist Group in Tectonics and Structural Geology "Cause and Effects of deformation in the Lithosphere''

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January 29th - February 3rd Waratah Bay, VICTORIA Australia

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Specialist Group in Tectonics and Structural Geology 2012 Conference Cause and Effects of Deformation in the Lithosphere Geological Society of Australia Abstracts Number 102 SGTSG biennial conference, Waratah Bay, VIC, Australia. Jan 29th -Feb 3rd Edited by: Peter Betts, Chris Wilson, Tim Rawling, Caroline Venn.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

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Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Evidence for rapid tectonic switches and long-lived structural reactivations in the northern Mount Painter Province, South Australia RJ. Armit - School of Geosciences, Monash University, VIC 3800, Australia.

P.G. Betts - School of Geosciences, Monash University, VIC 3800, Australia.

B.F. Schaefer - GEMOC, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia.

L Ailleres - School of Geosciences, Monash University, VIC 3800, Australia.

Corresponding author email: Robin.Armit@monash.edu Detailed structural mapping from the northern Mount Painter Province indicates that the metasediments of the Radium Creek Group underwent both Mesoproterozoic and Palaeozoic polydeformation. Early north directed DlD2 deformation is accompanied by upper amphibolite fades metamorphism.

This deformation is characterised by recumbent folds that are preserved in the hinges of NE-SW trending doubly-plunging upright F3 folds (Armit et al., 2012).

U-Pb-Hf dating of zircons from the Radium Creek Group identifies a consistent maximum depositional age for the Radium Creek Metamorphics of ca. 1596-1592 Ma across the Paralana Fault with a cHf range between -5.5 and +4. The timing of D1-D2 is constrained between this ca.

1592 Ma maximum depositional age and the minimum depositional age indicated by the truncation of the S1-S2 fabrics during the shallow-level emplacement of the ca. 1585-1569 Ma (Elburg et al., 2001; Eraser and Neumann, 2010; Neumann, 2001; Neumann et al., 2009) porphyritic Mount Neill Granite. This indicates that the Radium Creek Group were deposited, buried to mid-crustal levels and exhumed within 7 Myrs between 1592 and 1585 Ma before switching once again during the onset of NW-SE directed shortening during D3 (Armit et al., 2012). This time-frame is similar to that of modern orogens such as the Late Miocene-Present Kaikoura Orogeny of southern New Zealand (Landis and

Coombs, 1964; Kamp, 1984). An S3 fabric overprints the earlier S1-S2 fabrics in the metasediments and extends into the Mount Neill Granite as a steeply dipping NE-SW trending gneissosity. The D3 event is constrained by the presence of this S3 foliation in the ca. 1585-1569 Ma Mount Neill Granite, the ca. 1552 Ma emplacement age (Eraser and Neumann, 2010) of the undeformed Hodgkinson Granodiorite and dating of ca. 1555 Ma metamorphic overgrowths on zircons from the Radium Creek Group (Armit et al., 2012). Repeated reactivation of the Proterozoic basement architecture constitutes significant deformational phases in the Mount Painter Province.

During the development of the Neoproterozoic Adelaidean basins a series of shallow north-plunging quartz defined stretch lineations along the NESW trending Paralana Fault Zone record a phase of transtensional movement.

NW-SE directed D5 deformation produced reverse movement along the Paralana Fault and tightened the preexisting upright F3 folds producing a shallow NE-SW trending crenulation lineation on the steeply dipping bedding surfaces of the Radium Creek Group.

NE-SW directed D6 shortening warped the existing structures producing type 2 interference patterns with the rotation of F3 and F5 folds axes. These Palaeozoic D5-D6 deformational events most likely reflect the ca. 500 Ma (Dutch et al., 2005; Foden et al., 2006; Harrison and McDougall, 1981; Sandiford et al..

Geological Society ofAustralia, Abstract No. 102 SGTSG2012: Cause and Effects of Deformation in the Lithosphere

1998) Delamerian Orogeny but might relate to deformation during the Ordovician AHce Springs Orogeny (McLaren et al., 2002). Further brittle deformation that overprints the D6 structures along the Paralana Fault such as a hematite bearing fault breccia at the Mount Neill Granite and metasediment interface highlight the protracted geodynamic evolution of the Paralana Fault Zone.

The Mesoproterozoic geodynamic evolution component of the Mount Painter Province differs to that of the southern Gawler Craton and Curnamona Province, but correlates better with the Mount Isa Inlier and the northern Gawler Craton. This provides important Early Mesoproterozoic constraints for future tectonic models of eastern Proterozoic Australia.

References

Armit, RJ., Betts, P.G., Schaefer, B.F., Ailleres, L., 2012. Mesoproterozoic and Palaeozoic constraints on long-lived poly-deformation in the northern Mount Painter Inlier. Gondwana Research.

Dutch, R.A., Hand, M., Clark, C., 2005. Cambrian reworking of the southern Australian Proterozoic Curnamona Province: constraints from regional shear-zone systems. Journal of the Geological Society of London 162, 763-775.

Elburg, M.A., Bons, P.O., Dougherty-Page, J., Janka, C.E., Neumann, N., Schaefer, B., 2001. Age and metasomatic alteration of the Mt Neil Granite at Nooldoonooldoona waterhole, Mt Painter Inlier, South Australia. Australian Journal of Earth Sciences 48,721-730.

Foden, J., Elburg, M.A., Dougherty-Page, J., Burtt, A., 2006. The Timing and Duration of the Delamerian Orogeny: Correlation with the Ross

Orogen and Implications for Gondwana Assembly. The Journal of Geology 114,189-210.

Eraser, G.L., Neumann, N.L., 2010. New SHRIMP U-Pb zircon ages from the Gawler Craton and Curnamona Province, South Australia, 2008 2010. Geoscience Australia.

Harrison, T.M., McDougall, I., 1981. Excess 40Ar in metamorphic rocks from Broken Hill, New South Wales: implications for 40Ar/39Ar age spectra and the thermal history of the region.

Earth and Planetary Science Letters 55,123-149.

Kamp, P.J.J., 1984. Neocene and Quaternary extent and geometry of the subducted Pacific Plate beneath North Island, New Zealand: implications for Kaikoura tectonics.

Tectonophysics 108, 241-266.

Landis, C.A., Coombs, D.S., 1967. Metamorphic belts and orogenesis in southern New Zealand.

Tectonophysics 4, 501-518.

McLaren, S., Dunlap, W.J., Sandiford, M., McDougall, I., 2002. Thermochronology of high heat-producing crust at Mount Painter, South Australia: Implications for tectonic reactivation of continental interiors. Tectonics 21.

Neumann, N., 2001. Geochemical and isotopic characteristics of South Australian Proterozoic granites: implications for the origin and evolution of high heat-producing terrains..

Department of Geology and Geophysics.

University of Adelaide., Adelaide Neumann, N., Hore, S., Eraser, G., 2009. New SHRIMP geochronology from the Mount Painter Province, South Australia, In: Korsch, R.J. (Ed.), Broken Hill Exploration Initiative: Abstracts for the 2009 Conference. Geoscience Australia.

Sandiford, M., Hand, M., McLaren, S., 1998. High geothermal gradient metamorphism during thermal subsidence. Earth and Planetary Science Letters 163,149-165.

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Nature and significance of a synkinematic vein network associated with the Atacama Fault Zone, Northern Chile Gloria Arancibia - Pontificia Universidad Catolica de Chile, Santiago, CHILE Jose Cembrano - Pontificia Universidad Catolica de Chile, Santiago, CHILE Rodrigo Gomila - Universidad Catolica del Norte, Antofagasta,

CHILE

Veronica Herrena - Universidad Catolica del Norte, Antofagasta,

CHILE

Viviana Olivares - Universidad Catolica del Norte, Antofagasta,

CHILE

Claudia Hidalgo - Pontificia Universidad Catolica de Chile, Santiago, CHILE Corresponding author email: garancibia@ing.puc.cl Fault zones have long been recognized as sites of focused deformation and hydrothermal fluid flow. Commonly, the interaction between fluid flow migration and host rock deformation is reflected as mineral precipitation in a vein network. However, the precise mechanism by which syntectonic veins form remains controversial. Arguments are centered over the need of void formation prior to vein growth, and whether veins record advective and/or diffusive mass transfer in the crust.

Most workers propose different mechanisms to explain vein textures: fibrous textures are related to diffusional mass transfer, whereas other textures (e.g. blocky) are related to advective flow regimes (e.g. Cox et al., 2001; Oliver et al., 2001; Hilgers et al., 2001; Baker et al., 2006). We here examine an example of hydrothermal vein structural mesh, where syntectonic fault-vein and extensional veins are interconnected.

Our case study lies within a regionalscale strike-slip duplex belonging to the Mesozoic Atacama fault system in the Central Andes. The duplex is defined by two kilometer-scale, NNW-striking sinistral strike-slip fault (Bolfin and Jorgillo faults) joined by a series of NW striking, second-order faults with sinistral-normal kinematics (Cembrano et al., 2005). Previous work on these

hydrothermal synkinematic veins (Herrera et al., 2005; Olivares et al., 2010), suggest that the internal texture of veins reflect their formation mechanisms and that they are spatially and temporally related to seismiccoseismic cycle (e.g. Sibson, 1996). We concentrate on the microscopic analysis of excellently preserved and exposed fault-fracture networks developed on metadioritic rock at the southern termination of the Bolfin fault (Fig. la), where a kilometer-scale horsetail structure has also been recognized at mesoscopic and microscopic scales (101 to 10-3 m long) (Jensen etal., 2011).

Mesoscopic and structural analysis

microscopic

Samples were collected from the damage zone of Bolfin fault (e.g.

Mitchell and Faulkner, 2009; Jensen et al., 2011) characterized by millimeterto-centimeter wide fault-veins and extensional veins where the main mineralogical precipitation sequence is chlorite-quartz-epidote-calcite (Herrera et al., 2005; Olivares et al., 2010). The fault-vein analyzed here is 0.2 mm wide and show an epidote striated surface with calcite mineral growth behind the steps indicating normal-sinistral slip. An adjacent extensional vein is 0.1-0.3 mm wide and lies oblique to the fault-vein.

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere References

Both veins host the same mineralogical composition (epidote-calcite-quartz) and are spatially interconnected. The vein microstructure, based on crystal morphology and/or mineral growth direction (e.g. Ramsay and Huber, 1987; Passchier and Trouw, 1996; Bons, 2000), corresponds to:

Barker, S.L.L.; Cox, S.F.; Eggins, S.M.; Gagan, M.K. (2006). Microchemical evidence for episodic growth of antitaxial veins during fracture controlled fluid flow. Earth and Planetary Science Letters, 250,331-344.

Bons, P. 2000. The formation of veins and their microstructures. In Stress, Strain and Structure (Means, W.D.; Jessell, M.W.; Urai, J.L; editors).

Journal of the Virtual Explorer 2: 42 p.

1) Extensional veins are characterized, from the edge to the center, by a fine-grained blocky texture of epidote - calcite ± quartz without preferred orientation; followed by syntaxial long/wide ratio 3 crystals of epidote with their long axes oriented perpendicular to the vein walls. The latter show no evidence of competitive growth. Calcite crystals occur in the central part of the vein, filling open spaces without evidence of either competitive growth or later deformation (Fig. l b ) .

2) Fault-veins show a cataclastic texture consisting of banded finegrained to medium grained matrix of epidote-calcite-quartz, with angular broken, non-preferentially oriented epidote and elongated irregular clasts of both calcite and quartz (Fig Ic).

Cembrano, J., Gonzalez, G., Arancibia, G., Ahumada, I., Olivares, V., Herrera, V. (2005).

Fault zone development and strain partitioning in an extensional strike-slip duplex: a case study from the Mesozoic Atacama fault system, Northern Chile. Tectonophysics 400 (1-4), 105125.

Cox, S.; Etheridge, M. (2001). Crack-seal fibre growth mechanisms and their significance in the development of oriented layer silicate microstructures. Tectonophysics, 92,147-170.

Herrera, V.; Cembrano, J.; Olivares, V.; Kojima, S. (2005). Precipitacion por despresurizacion y ebullicion en vetas hospedadas en un duplex de rumbo extensional: Evidencias microestructurales y microtermometricas.

Revista Geologica de Chile 32 (2): 207-227 Hilgers, C.; Koehn, D.; Bons, P.; Urai, J. (2001).

Development of crystal morphology during unitaxial growth in a progressively widening vein; II. Numerical simulations of the evolution of antitaxial fibrous veins. Journal of Structural Geology, 23, 873-885.

The extensional vein show evidence of opening rate > precipitation rate (ep-ccqtz) suggesting that fluid flow occurred by advective transport, where material is carried by the migrating fluid either throughout a permeable rock mass or within discrete fractures. The cataclastic vein, in turn, probably first formed as an open or hybrid fracture filled by hydrothermal minerals (epidote-calcitequartz), then followed by crushing and newly fluid-filled voids (calcite).

Precipitation in open spaces (cc in steps) within the fault-vein probably occurred during single seismic events with high precipitation rates, whereas in the extensional vein, precipitation may have taken place during the interseismic period of the earthquake cycle where growth rates are expected to be lower.

Jensen, E., Cembrano, J., Faulkner, D., Veloso, E., Arancibia, G. (2011) Development of a selfsimilar strike-slip duplex system in the Atacama Fault system, Chile. Journal of Structural Geology, doi:10.1016/j.jsg.2011.09.002 Mitchell, T.M., Faulkner, D.R. (2009) The nature and origin of off-fault damage surrounding strike-slip fault zones with a wide range of displacements: a field study from the Atacama fault system, northern Chile. Journal of Structural Geology 31 (8), 802-816.

Olivares, V., Cembrano, J., Arancibia, G., Reyes, N., Herrera, V. 2010. Significado tectonico y migracion de fluidos de fallas y vetas en la porcion sur del duplex Caleta Coloso, Norte de Chile. Andean Geology 37 (2): 473-497.

Oliver, N.; Bons, P. (2001). Mechanisms of fluid flow and fluid-rock interaction in fossil metamorphic hydrothermal systems inferred from vein-wallrock patterns, geometry and microstructure, Geofluids 1,137-162.

Acknowledgments Fondecyt Project 1100464 is funding this research.

Passchier, C.; Trouw, R. 1996. Dilatation Sites:

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Fibrous Veins, Strain Shadows, Strain Fringes and Boudins. In Micro-Tectonics (Passchier, C.;Trouw, R.; editors). Springer-Verlag, Berlin, Heidelberg: 131-15 I.Germany.

Ramsay, J.G.; Huber, M.l. (1987). Mechanical Analysis of fractures. In: The techniques of

modern Structural Geology. Vol. 2: folds and fractures. Academic Press, London, Ramsay, J. and Huber, M. (eds), 561-594.

Sibson, R.H. 1996. Structural permeability of fluid-driven fault-fracture meshes. Journal of Structural Geology, Vol. 18, No. 8, p. 1031-1042.

^vein

yvein

Figure 1. (A) Regional geological map showing two NNW-striking, sinistral strike-slip faults (Bolfin & Jorgillo faults) which are joined by a series of NW-strlking sinistral-normal second-order faults. A key structural element of the Bolfin fault Is a horsetail structure located at its southernmost end (black square). (B) MIcrophotograph of extensional epidote-calcite vein shows a fine-grained blocky texture on the vein edges and syntaxial growth towards the center (C) Microphotograph of epidote-calcite fault-vein with cataclastic texture (parallel-polarised light).

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Geothermal constraints on an ultra-high temperature orogen, Eastern Ghats Belt, India Andrew Barker - The University of Adelaide Corresponding author email: andrew.barker@adelaide.edu.au This observation suggests that the Thbearing phase (monazite) was stable during UHT metamorphism, or saturated in any fluid phase. Thermal conductivity data suggest that thermal anisotropy and foliation may play a significant role in the thermal evolution of orogens.

Stochastic numerical models constructed using the observed thermal properties and the crustal structure constrained by seismic profiles can match high heat flow values observed in the Godavari Graben, a Gondwana Basin that cross-cuts the Eastern Ghats Province. These models suggest high heat flow in the Eastern Ghats province due to the high heat producing granulite facies rocks.

UHT metamorphic conditions (c. 1000 Ma) could have been attained by burial of the present day upper crust to 20-30 km depth without the requirement of addition heat input from magmatism, lithospheric thinning, or asthenospheric upwelling.

The Eastern Ghats province of India is a polycyclic metamorphic belt that records ultra-high temperature (>900°C) metamorphism (-1000-900 Ma). The tectonic setting of this UHT metamorphism is unclear, so to constrain possible mechanisms of generating UHT conditions w e collected radiogenic heat production and thermal conductivity data for rocks from the Eastern Ghats province to constrain the thermal properties of the crust. The granulite-facies khondalites, K-feldspar megacrystic granites and quartzofeldspathic gneisses have high average heat production values (3.76±0.53 Wm3, 2.79±0.53 Wm-3 and 5.49±0.69 Wm3 respectively), whereas the UHT mafic granulites have a low heat production of 0.69±0.23 Wm-3. Most samples have anomalously high concentrations of thorium with respect to uranium ([Th]/[U] -- 14.3) such that the component of heat production due to Th is four times higher than that from U.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

The influence of mantle plumes on subduction zone dynamics Peter Betts - School ofGeosciences, Monash University, Clayton VIC 3800.

Wendy Mason - School of Geosciences, Monash University, Clayton VIC 3800.

Louis Moresi - School ofGeosciences and School of Mathematical Sciences, Monash University, Clayton VIC 3800.

Corresponding author email: Peter.Betts@monash.edu

The geological record and seismic tomography show plumes and subducting oceanic slabs collisions are common, however the nature of these collisions and the mechanism for the transfer of the plume from the subducting to the overriding plate are uncertain. Slab-plume interactions occur when a subducting slab retreats over an oceanic hotspot. Murphy et al.

(1998) defined the term 'plumemodified' orogenesis to describe the interaction of a mantle plume and a convergent margin. Ancient examples include: the Tertiary subduction of the ancestral Yellowstone plume (Murphy et al., 1998); retreat of the Gondwanan margin above the Jurassic KarrooFerrar plume (Dalziel et al, 2000); and Mesoproterozoic orogenesis and voluminous felsic magmatism throughout Precambrian Australia (Betts et al., 2009).

We've investigated the behaviour of the subducting slab as it retreats towards and interacts with a buoyant mantle plume head using 3D finiteelement numerical simulations (Moresi et al, 2007) to quantify how a plume modifies trench and slab geometry, and predicts the fate of plumes once they are entrained in a retreating slab. Our models include cases with a slab viscosity 200 times that of the upper mantle and a 'weak' or 'strong' yield stress (equating to 52 MPa or 104 MPa at 100 km depth respectively). A 100 km thick plume head is imposed immediately beneath the centre of the 100 km thick slab with a viscosity 100

times lower than the upper mantle. The plume head density (3200 and 3250 kg/m^) and dimension (between 400x400 km and 800x800 km) were modified.

When a "strong" slab interacts with plumes of different sizes and densities there is relatively minor modification of the trench geometry. Plume material is incorporated into the strong subducting slabs, suggesting that subduction of "strong" oceanic lithosphere prevents interaction of the plume with the overriding plate.

However, in the models with a weak slab and a 3200 kg/m^ density plume there was significant modification of the subducting slab and trench geometry. For plumes of all sizes the leading edge of the plume subducted and became entrained by the subducting slab and a slab window developed. The size of the slab window increased with size of the plume. When the plume breached the surface before interacting with the trench it significantly impacted slab geometry. For the largest and most buoyant plumes the slab window was enormous, creating a potential conduit for the plume to transfer from the subducting plate to the overriding plate without requiring thermal or chemical assimilation of the subducting slab as proposes in many previous models.

The presence of a plume in all of our models produced local trench advance.

The weaker the slab, and the larger and more buoyant the plume, the greater 11

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

the rate of trench advance. Away from the plume the trench continued to roll back. The degree of trench curvature was significantly greater for "weak" slab models with a 3200 kg/m^ density plume because of a combination of toroidal mantle flow through the slab windows promoting slab rollback at the peripherals of the slab-plume interaction, and trench advance where the plume was entrained into the trench. For the large 700x700 and 800x800 km sized 3200 kg/m^ density plumes trench advance rates at the centre of the plume reached 1.82 and 3.14 cm/yr respectively by the time the plumes reached the surface. This is considered sufficient to contribute to development of an orogenic belt in the overriding plate (Schellart, 2008; Schellart et al., 2007).

Smaller 400x400 and 600x600 km sized plumes exhibited modest and protracted trench advance rates up to 0.85 and 1.02 cm/yr respectively.

This research was supported under Australian Research Council's Discovery Projects funding scheme (DP0878501 and DP1095166).

The software Underworld, StGermain and gLucifer have been developed as part of AuScope Ltd, funded under the National Collaborative Research Infrastructure Strategy (NCRIS), an Australian Commonwealth Government Programme.

References Betts, P. G., Giles, D., Foden, ]., Schaefer, B. R, Mark, G., Pankhurst, M. J., Forbes, C.)., Williams, H. A.,

Chalmers,

N.

C., and

Hills,

Q.,

2009,

Mesoproterozoic plume-modified orogenesis in eastern Precambrian Australia: Tectonics, v. 28, no. 3.

Dalziel, I. W . D., Lawver, L. A., and Murphy, J. B., 2000, Plumes, orogenesis, and supercontinental fragmentation:

Earth

and

Planetary

Science

Letters, v. 178, no. 1-2, p. 1-11.

Moresi, L., Quenette, S., Lemiale, V , Meriaux, C., Appelbe,

B.,

and

Computational

Muhlhaus,

approaches

to

H.

B.,

2007,

studying

non-

linear dynamics of the crust and mantle: Physics of the Earth and Planetary Interiors, v. 163, no. 1 4, p. 69-82.

The results of the models are consistent with spatial and temporal observations of ancient interpreted examples plume modified orogenesis.

Murphy, J. B., Oppliger, G. L., Brimhall Jr, G. H., and Hynes, A., 1998, Plume-modified orogeny: An example

from

the

western

United

States:

Overriding

plate

Geology, v. 26, no. 8, p. 731-734.

Acknowledgments We acknowledge the support of the Monash e-Research Centre, in particular Philip Chan. Numerical simulations presented in this paper were run on the Monash Sun Grid (MSG), submitted via the Australian Research Collaboration Service (now eResearch Collaborative Services) (ARCS) Compute Grid using the ARCS grid submission client Grisu.

Schellart,

W.

P.,

shortening

and

extension

2008,

above

subduction

zones: A parametric study to explain formation of the Andes Mountains: Geological Society of America Bulletin, v. 120, no. 11-12, p. 1441-1454.

Schellart, W .

P., Freeman, J., Stegman,

D. R.,

Moresi, L , and May, D., 2007, Evolution

and

diversity of subduction zones controlled by slab width: Nature, v. 446, no. 7133, p. 308-311.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Syn-deformational porosity development in Alpine Fault mylonites: Interplay between dislocation migration, tangling and selective dissolution by metamorphic fluids Marco A. Billia - Department of Geology, University ofOtago, PO Box 56, Dunedin, New Zealand Virginia G. Toy - Department of Geology, University ofOtago, PO Box 56, Dunedin, New Zealand Nick E. Timms - Western Australia Geothermal Centre of Excellence, Department of Applied Geology, Curtin University, Perth, GPO Box U1987, Western Australia, 6845, Australia David J. Prior - Department of Geology, University ofOtago, PO Box 56, Dunedin, New Zealand Rob Hart - Centre for Materials Research, Curtin University, GPO Box U1987, Perth, WA 6845, Australia Corresponding author email: marco@geology.co.nz Despite strong evidence that ductile mid-crustal shear zones are preferential pathways for migrating fluids, the ambient porosity of high-strain, exhumed ductile fault rocks is extremely low. The micromechanisms by which fluids migrate through mylonites without brittle fractures are still poorly understood.

beam milling of transmission electron microscopy (TEM) lamellae. Those TEM investigations show that micropores are mainly associated with GB and that LAB also interact with GB to form pores.

Dislocation tangles, threads, dipoles, loops and strain fields are present within subgrains, as well as in host grains.

Micropores are therefore interpreted to be the morphological expression of the interaction of high dislocation densities (< 5-109 cm-2) in individual quartz grains with grain and subgrain boundary surfaces. These represent the pinning of dislocations on GB and subsequent dissolution on these high-energy sites by aqueous fluids during deformation.

We characterized high-strain quartz microstructures employing electron backscatter diffraction and secondary electron imaging of broken surfaces on several ultramylonite specimens from a 120 km long section of New Zealand's Alpine Fault zone. Quartz occurs both within mixed quartz-feldspar-mica aggregate domains and within monomineralic domains both containing sigmoidal feldspar porphyroclasts.

Quartz grains are 2-1000 |im in diameter and exhibit a strong crystallographic preferred orientation imposed during dislocation creep. Grain boundaries (GB) and some subgrain boundaries (LAB) are decorated with micropores that commonly have unimodal size distributions and consistent, pseudo-tetrahedral morphology on a given crystal face. We specifically targeted low- and high-angle boundaries with different misorientation angles by focussed ion

Subgrain formation by progressive stacking of dislocations to form crystallographically controlled GB at high temperatures enhanced porosity by producing smaller, recrystallized grains that still contain dislocations on their boundaries. These boundaries are still preferable sites for nucleation of new micropores. However, partial late recrystallisation by grain boundary bulging at higher crustal levels produced subgrains without these highstress sites that prevented dynamic porosity from developing.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Our observations reveal a transient feedback mechanism between dislocation and diffusion creep resulting in a dynamic porosity structure in a

variety of natural polymineralic and polycrystalline aggregates during ductile deformation at lower- to midcrustal levels.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Topography of Continental Hinterlands: Plumes or Plate Tectonics?

Thomas Blenkinsop - School of Earth and Environmental Science James Cook University, Australia Andy Moore - African Queen Mines Ltd., Box 66, Maun, Botsv\^ana Fenton (Woody) Cotterill - AEON - Africa Earth Observatory Network, Geoecodynamics Research Hub, University ofStellenbosch, Private Bag XI, Matieland 7602, South Africa Corresponding author email: Thomas.Blenkinsop@jcu.edu.au Continental hinterlands, inland of passive margins, may preserve a detailed record of epeirogenic movements from sedimentation, drainage evolution, and uplift histories constrained by low temperature thermochronology.

These vertical movements may theoretically be due to thermal, isostatic, or dynamic (from either upper or lower mantle anomalies) effects, or buckling due to plate boundary stresses (Fig. 1).

the predicted area of uplift (e.g. the Khomas highlands in Namibia and the Bie Plateau in Angola). Present day drainage can be described by three concentric uplift axes, sub-parallel to the coast. These axes evolved .at Ma, --83 Ma and 44-33 Ma, and correspond with timing of unconformities in the Congo basin, and episodes of alkaline volcanism.

An important clue to the origin of the uplift axes is given by their temporal correlation with major episodes of plate tectonic reorganization in the Atlantic and Indian Oceans, which are recognised from the offshore sedimentary and paleomagnetic records. The uplift axes are likely to relate to plate boundary stresses transmitted into the continental interior. Long distance stress transfer through the continental crust has been demonstrated for Europe in the Cretaceous and Cenozoic. While this evidence does not preclude a component of dynamic uplift, it suggests that horizontal stresses can play at the least a modifying role in continental hinterland topography.

The elevation of southern Africa, typically over 2000 km from the nearest plate margin, has been explained as a consequence of dynamic topography over a mantle plume. One of the largest mantle velocity anomalies in the world indeed underlies the crust in the Atlantic ocean to the west of the elevated region.

However, neither present day topography, nor drainage evolution, are compatible with the dynamic uplift model. This model predicts a broad domal pattern of uplift, yet within southern Africa there are large low lying areas (e.g. the Mozambique plane) as well as high areas on the periphery of Flexure: Elastic stresses

Passive Margin

Erosion: Underplating: Isostacy Heating and Isostasy

Dynamic Topography

Plate Boundary

Continental Hinterland

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Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

New insights from numerical modelling to high strains Paul Bans - Department ofGeosciences, Eberhard Karls University Tubingen, Germany BdlintMorvai - Department of Geosciences, Eberhard Karls University Tubingen, Germany Gema Llorens - Department of Geosciences, Eberhard Karls University Tubingen, Germany Mark W. Jessell - IRD LMTG UMR 5563,14 avenue Edouard Belin, 31400 Toulouse, France Enrique Gomez-Rivas - Department of Geosciences, Eberhard Karls University Tubingen, Germany Corresponding author email: paul.bons@uni-tuebingen.de to 1 5 - 3 0 % less than the initial shear stress. This initial hardening and subsequent softening is solely caused by the microstructural change, as viscosities of the phase regions are set to remain constant.

The initial hardening is probably due to the development of a foliation at an angle to the shear plane. Only when the foliation has further strengthened and has rotated towards the shear plane does it facilitate shearing and cause softening of the bulk rock. The initial hardening and subsequent softening may play a role in creating the instabilities that lead to localisation of deformation and the development of shear zones.

Finite-element modelling of high-strain deformation structures is possible in the numerical mod-elling platform Elle (Bons et al., 2008), thanks to wrapping boundaries, continuous remeshing and additional strategies to avoid or reduce negative effects of boundary conditions and collapse of the model. Here we give two examples of structures that develop in simple shear at high strains: shear bands and folds.

We modelled the deformation of twophase rocks in simple shear up to shear strains of 7.5. The material is described by a contiguous sets of polygons, each having either a low or high viscosity.

Simulations were run with 30, 50 and 7 0 % hard phase and viscosity contrasts of 1, 5, and 10. Sim-pie shear deformation was applied, using the module "Basil", in small shear strain increments of 0.025. Full stress and strain rate distribution for each strain increment were recorded.

Theory, experimental and numerical models of folding are almost exclusively restricted to pure shear deformation.

We modelled folding of stiff layers in a viscous matrix in simple shear up to a shear strain of four. Simple shear folds only show subtle differences with pure shear folds at the same strain and competency contrast. In general, simple shear folds tend to be more variable in wavelength, amplitude and axial plane orientation.

Asymmetric cleavage refraction appears to be the main indicator of non-coaxial deformation.

Microstructural development is, as expected, a function of the composition, i.e. the ratio of soft to hard phase, and the viscosity ratio (Jessell et al., 2009).

Strain localisation (formation of shear bands) is strongest in case of an initially hard-phase load-bearing framework.

Effective viscosity contrast is enhanced in case of power-law creep, which thus also enhances shear band formation.

References Bons, P.D., Koehn, D. and Jessell M.W. (Eds.), 2008. Microdynamics Simulation. Lecture Notes in Earth Sciences 106. Springer-Verlag.

The shear stress needed to maintain a constant strain rate initially increases by 0 - 1 0 % up to a shear strain of about one and then decreases asymptotically

Jessell, M.W., Bons, P.D., Griera, A., Evans, L.A., Wilson, C.J.L., 2009. A tale of two viscosities.

Journal of Structural Geology 31, 7 1 9 - 7 3 6 .

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Investigating the structure of the Texas Orocline beneath the Surat and Bowen Basins (southeast Queensland) Samuel Brooke-Barnett - School of Earth Sciences, The University of Queensland, Brisbane 4072 Gideon Rosenbaum - School of Earth Sciences, The University of Queensland, Brisbane 4072 Corresponding author email: samuel.brooke-barnett@originenergy.com.au The Texas Orocline in southeast Queensland and northeast New South Wales is a large scale and tight curvature in the structural grain of the New England Orogen. The orocline is defined by the curvature of two different Devonian to Cretaceous tectonic elements, the forearc basin Tamworth Belt and the accretionary wedge Tablelands Complex. However, only - - 4 0 % of the area of the Texas Orocline observable using remote methods is expressed in outcrop in southeast Queensland.

been applied to key seismic sections using the 2D Move software suite in order to determine strain relationships along the fault.

The Leichhardt Fault is expressed as a fault propagation fold which broadens to the north and displays reactivated extensional features. Folding is also expressed on overlying Surat rocks indicating syn/post Jurassic reactivation.

To the south, the Goondiwindi Fault displays an eroded "harpoon" structure implying possible reactivation of an extensional fault.

However Jurassic cover is not affected by faulting, suggesting this fault was relatively dormant after Permian movement. Towards the north, the Moonie fault displays a complex multiple thrust system with apparent pop up structures which converge into a single fault to the north. The effect of this fault system is also clear in the Base Surat, indicating fault reactivation during/after the Jurassic was more prominent to the north. Between the Leichhardt and Moonie faults seismic interpretation reveals apparent extensional faulting and syn-rift Permian sedimentation. Dating of this sedimentation would provide a useful constraint on the initiation of Oroclinal bending assuming the extensional features are associated with limb rotation.

The aim of this work is to unravel the structure of the Texas Orocline under the sedimentary cover. The orocline is recognised in gravity and aeromagnetic data acquired over southeast Queensland. A curvature of magnetic fabrics parallel to the orocline can be observed within the Tamworth Belt and Tablelands complexes. On the western flank of the orocline, the bounding Moonie-Goondiwindi Faults correspond to pronounced magnetic highs. This study has utilised regional aeromagnetic and 2D seismic data acquired in southeast Queensland in order to identify the Texas Orocline under cover, and a depth map has been produced. The western flank of the Tamworth Belt, including the bounding Moonie-Goondiwindi Faults and the Leichhardt Fault are expressed in the dataset, and fault restorations have

17

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere The effect of data saturation on the variability of 3D geological Models.

Thomas Carmichael - Monash University Corresponding author email: thomas.carmichael@monash.edu Out technique involves creating numerous models from independent datasets representing discrete portions of a complete dataset. The metrics of the new models are compared to those of the models which represent 1 0 0 % data saturation, it is possible to show how the metrics are affected by an incremental decrease in data saturation.

There are three (3) different models which are tested with this technique, all with varying levels of geological complexity. It is possible to view this data purely as quantitative or use it qualitatively to observe which sections of the model are more likely to vary due to changes in data density.

All 3D Geological models are subject to a degree of uncertainty due to the nature of input data. With the approach addressed in this paper we analyse the affect of data saturation on the variability of a geological model.

Different metrics are used to measure the variability of a geological model, such as the Volume, Surface Area of lithological boundaries and the Dip of the geology and the concept of Roughness. Roughness is a metric which analyses a dip vector relative to its surrounding neighbours, allowing us to analyse areas of the model which have higher 'roughness' or are more complex.

18

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere South-directed oroclinal folding in the Lachlan Fold Belt: a solution to apparent Ordovician-Early Silurian complexity.

Ross Cayley - Geological Survey of Victoria Robert Musgrave - Geological Survey of NSW Wolfgang Preiss - PIRSA Minerals Corresponding author email: Ross.Cayley@dpi.vic.gov.au Recent tectonic models for the Lachlan Fold Belt (LFB) attribute geometry (structural vergence changes between west central and eastern portions), sedimentary relationships, width and magmatic history to several coeval subduction zones of opposing polarity (eg. Gray & Foster, 1998), or to largescale horizontal movements along the mid- to Late Silurian Baragwanath Transform (Willman et al., 2002).

Aeromagnetic data shows that the Baragwanath Transform does not occur in southwestern New South Wales (Hallett et al., 2005), but there remains compelling evidence that the Tabberabbera Zone was not contiguous with the Melbourne Zone until the Early Devonian. The model presented here provides a solution that involves mid- to Late Silurian clockwise oroclinal folding within the LFB adjacent to large dextral strike-slip faults in central-west NSW and eastern Victoria.

A faultsegmented, orogen-scale Z-shaped fold in Victoria and southern NSW solves the geometrical puzzle, vergence reversals, and apparent great width of the LFB, and greatly simplifies its Late Cambrian-Early Silurian configuration.

This simplified configuration restores Eastern LFB Siluro-Devonian paleomagnetic poles to the Gondwana APWP and allows for closer ties with the coeval Ross Orogen elsewhere along the east Gondwana margin.

and the Selwyn Block - 'Vandieland' (Cayley, 2011) - lay outboard of the Gondwana margin, separated by the intervening undeformed proto-Bendigo Zone ocean basin. Some distance north of this basin, and projecting out from the eastern Gondwanaland margin, lay the Macquarie Arc, active since the Early Ordovician (eg. Glen et al., 2007).

This system may have evolved as a hybrid arc, embedded into the Delamerian orogen at its southern end and grading to intraoceanic farther north. The proto- Tabberabbera and Mallacoota zones lay east and southeast of this arc in a fore-arc position, while the proto- Wagga-Omeo Zone lay in a back-arc position to the west and north.

Towards the end of the Ordovician a convergent continent-directed subduction system became established along the entire eastern edge of the LFB, including outboard of Vandieland. This may have involved the migration of subduction systems from farther south (eg. Cayley, 2011). The Macquarie Arc system became disrupted at this time, driven into convergence against the east Gondwanaland margin. This Late Ordovician-Early Silurian convergence the Benambran Orogeny - formed the LFB. In the reconstruction presented here the LFB at the time of its formation appears to have been an elongate and relatively skinny fold belt. From west to east it comprised; (1) a high-T back-arc region (the Wagga Omeo Zone) accreted to, and perhaps partly superimposed upon, the eastern edge of the older Delamerian Orogen; (2) the Macquarie Arc complex itself and; (3) a narrow fore-arc accretionary complex farther

Following the new model through time: Late Ordovician-Early Benambran Orogeny:

Silurian

Prior to the Late Ordovician, Tasmania

19

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere outboard characterised by predominantly east-verging structures the Mallacoota and Tabberabbera zones.

The Bendigo Zone lay south along strike from the Tabberabbera Zone at this time, more remote from subduction systems yet deformed in the same event and sharing the same gross fault geometries (Cayley, et al., 2011).

Lachlan Fold Belt. This part of the Stawell Zone, together with the HayBooligal Zone and Tabberabbera Zone, represent para-autochthonous portions of the LFB. The eastern LFB, including the Wagga-Omeo Metamorphic Complex and Macquarie Arc segments exposed in eastern NSW and in Victoria, lies east of the major Kiewa-Kancoona Fault system, was moved hundreds of km to the southeast along this fault system in the Bindian Orogeny, and was therefore detached fully from parts of the LFB west of the fault.

Late Silurian Bindian Orogeny: The Late Silurian Bindian Orogeny appears to mark the beginning of an interval where the LFB began to become fragmented, with a portion remaining attached to the east Gondwanaland margin as the western LFB, while other parts became progressively faulted and rotated away from the Delamerian margin to varying degrees. Bindian Orogeny movements in the LFB were south-directed and reminiscent of lateral escape tectonics, possibly driven by the onset of the Alice Springs Orogeny or by other events in the northern Tasman Fold Belt. Beginning in Queensland, southward movement and rotation of the Thompson Fold Belt began to impinge on the parts of the newly-accreted Macquarie Arc system that are today exposed in NSW.

The Kiewa-Kancoona Fault System is interpreted by us to underlie the Bootheragandra Fault in NSW. This dextral strike-slip fault system was initiated in Queensland at the onset of the Bindian Orogeny, possibly by southward movement of the Thompson Fold Belt. As the fault propagated southeast it cut across the northern Stawell Zone and then out into the interior of the Lachlan Orogen, focussed into the hot-weak Wagga-Omeo Metamorphic Complex. Eventually, the Kiewa-Kancoona Fault cut across the Macquarie Arc, dividing it into two parts: to the east are the allochthonous fault slices exposed in eastern NSW, and to the west lies a para-autochthonous arc portion, the Hay-Booligal Zone.

After cutting and offsetting the arc, the Kiewa-Kancoona Fault began to founder in low-grade metasediment-rich forearc terranes (the Tabberabbera and Kuark/Mallacoota zones).

Instead of faulting, these terranes were able to accommodate strain by oroclinal folding. As displacement on the KiewaKancoona Fault continued and offset of arc segments across it grew in magnitude, oroclinal folding of the Tabberabbera-Mallacoota zone region ahead of the fault network kept pace, forming a long-lived regional-scale lateral transition in structural style from faulting to folding. This is preserved along the southern Omeo Zone-Tabberabbera Zone boundary as an intriguing array of sub-vertical strike-slip fault splays arranged in a

In Victoria the portion of the western LFB that remained attached to the Gondwana margin during and after the Bindian Orogeny includes the southern Stawell Zone, the Bendigo Zone and the Selwyn Block (Melbourne Zone). This region was possibly protected from the effects of south-directed strike-slip faulting and oroclinal folding because it lay behind the prominent salient of the southeastern Curnamona Province.

Where Stawell Zone rocks extended east and north of this salient, aligned along the Delamerian margin, they were exposed to the influence of dextral south-directed strike-slip faulting propagating southeast from Queensland, were peeled away from their original position of formation, and oroclinally folded 90° clockwise to extend east well into the interior of the

20

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

horsetail, and as sub-vertical kink-folds and crenulation cleavages parasitic to a giant oroclinal fold hinge, the Tambo Orocline. Eventually the Tabberabbera Zone underwent more than 120° clockwise rotation to form the middle limb of a Z-shaped oroclinal fold of -'400km amplitude. One consequence is the apparent reversal, from east- to west-, of structural vergence within the Tabberabbera Zone.

As the allochthonous portions of the Omeo Zone and Macquarie Arc were carried south-east along the Kiewa-Kancoona Fault System, concomitant clockwise oroclinal rotation to the west of the fault explains the present-day geometry of the Hay-Booligal Zone and the northern Stawell Zone and in-turn provides a mechanism that can explain how the Kiewa-Kancoona Fault system itself, together with the allochthonous Macquarie Arc segments, may have been forced to undergo progressive clockwise rotation throughout the Bindian Orogeny.

accompanying crustal extension. This is a powerful new model. It does not require multiple subduction zones to explain reversals in vergence or the observed complex distribution of sedimentary packages, instead explaining them by subsequent oroclinal folding developed ahead of dextral strike-slip faults.

The new model fits new aeromagnetic data, resolves long-standing issues with paleomagnetic data, explains complex LFB palaeogeographic relationships, and has allowed a link to be established between Bindian strike-slip deformation, oroclinal folding, and a transition to crustal extension and granite genesis across the LFB in the early Devonian. The model also explains the nature and origin of the enigmatic Hay-Booligal Zone.

References Cayley, R.A., 2011. Exotic crustal block accretion to the eastern Gondwanaland margin in the Late Cambrian - Tasmania, the Selwyn Block, and implications for the Cambrian-Silurian evolution of the Ross, Delamerian and Lachlan Orogens.

Gondwana Research 19, 6 2 8 - 649.

Early Devonian basin formation, granite intrusion, Tabberabberan Orogeny:

Cayley, R.A., Korsch, R.J., Moore, D.H., Costelloe, R.D., Nakamura, A., Willman, C.E., Rawling, T.J., Morand, V.J., Skadzien, P.B. & O'Shea, P.J., 2011.

Crustal architecture of central Victoria: results from the 2 0 0 6 deep crustal reflection survey.

Australian Journal of Earth Sciences 58, 113 156.

In the Early Devonian the rotating Tabberabbera Zone finally docked against the eastern margin of the Selwyn Block, after which no further orocline rotation was possible. The geometry of the fault-system and of the eastern LFB orocline finally stabilised within a regional stress-field, allowing late faults to accumulate more protracted dextral strike-slip movement histories (eg. the Cassilis Shear Zone).

The orientation of the extension field associated with these late SEpropagating dextral strike-slip faults also stabilised, so that the Early Devonian marks the appearance of distributed north to northeast-trending sedimentary basins, including the Buchan, Bindi and Mitchell syncline successions and the appearance, across the southern LFB, of voluminous granitic intrusions possibly generated by decompression melting

Glen, R.A., Crawford, A.J., Percival, I.G & Barron, L.M., 2 0 0 7 . Early Ordovician development of the Macquarie Arc, Lachlan Orogen of southeastern Australia. Australian Journal of Earth Sciences 54,167 - 1 7 9 .

Gray D.R. & Foster D.A. 1998. Character and kinematics of faults within the turbiditedominated Lachlan Orogen: implications for tectonic evolution of eastern Australia. Journal of Structural Geology 12, 6 9 1 - 1 7 2 0 .

Hallett, M., Vassallo, J., Glen, R., & Webster, S., 2005. Murray-Riverina region: an interpretation of bedrock Palaeozoic geology based on geophysical data. Quarterly Notes of the Geological Survey of New South Wales 1 1 8 , 1 - 1 6 .

Willman C.E., VandenBerg A.H.M. & Morand V.J. 2002b. Evolution of the southeastern Lachlan Fold Belt in Victoria. Australian Journal of Earth Sciences 49, 271-289.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Three busted myths: 1) Pannotia existed; 2) Gondwana formed by the collision of two Neoproterozoic continents—East and West Gondwana; 3) There is a 'North Gondwana' Ediacaran petroleum superprovince Alan S Collins - Tectonics Resources and Exploration, School of Earth and Environmental Sciences, University of Adelaide Corresponding author email: alan.collins@adelaide.edu.au Gondwana grew throughout the latest Cryogenian, the Ediacaran and into the Cambrian by collision of many of the relics of Rodinia along a series of orogenic belts that lace Africa, India, South America and Australia. Recent work throughout these regions has highlighted the complexity and diversity of these orogens and advances in geochemistry, petrology and geochronology have allowed distinctions between the various orogenic events to be teased out. It is now clear that Gondwana amalgamated over approximately 150 Ma from largely extensional arc-margins on the periphery of the Congo-Sao Francisco in north Africa/Arabia and central South America to contractional accretionary orogens at --670-630 Ma in these regions and East Africa/Madagascar. In addition. West Africa appears to have collided with the Borborema/LATEA continent and both with the Saharan Metacraton and northern Congo-Sao Francisco craton between -'640 and 600 Ma. These early collisions evolved into three major latest EdiacaranCambrian collisional orogens that finally stitched Gondwana together. These

orogens are located in Gondwana 1) between India and Western Australia and extended into Wilkes Land of East Antarctica, 2) in areas now in the eastern Mediterranean and the Middle East, through Arabia, eastern Africa and Madagascar this orogen then bifurcates with a branch extending through southern India and Sri Lanka to Dronning Maud Land (Antarctica) and a second heading through the Zambezi Belt, the Damara orogen to between South America and southern Africa. 3) A third orogenic system can be traced from the Rokelides of West Africa, through the Araguaia and Paraguay Belts of Brazil to the Pampean region of Argentina.

These vast collisional orogenic systems form the final Cambrian collisions that went to cement Gondwana. In this talk I shall focus on the middle one, the East African Orogen, and discuss recent work in Arabia, Madagascar, South India and Sri Lanka that we have done in focussing on the nature and timing of Gondwana amalgamation in this region debunking a number of widely held myths.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Phanerozoic global geodynamics revealed by hafnium isotopes in zircons WJ. Collins - University of Newcastle, Newcastle, Australia EA. Belousova - Macquarie University, Sydney, Australia J.B. Murphy - St Francis Xavier University, Canada A.I.S. Kemp - University of Western Australia, Perth, Australia Corresponding author email: bill.collins@newcastle.edu.au Two fundamentally different subduction systems presently exist on Earth - the dominantly accretionary circumPacific system and the dominantly collisional AlpineHimalayan-lndonesian system. The two types of subduction systems have produced contrasting orogens throughout the Phanerozoic, and can be grouped into two long-lived (--550 Ma) orogenic systems, internal and external.

Internal orogens form during a Wilson cycle and following collisional orogenesis, become stranded within continental interiors, whereas external orogens are accretionary, and have formed around the Pacific rim throughout the Phanerozoic.

symmetry, which controls the nature of the lower plate, orogen dynamics and potential magma sources. Within the external (circumPacific) system, ancient lower crust and subcontinental lithospheric mantle (SCLM) are progressively removed or isolated, primarily by thermal erosion and subduction retreat respectively, and replaced with juvenile crust. Wi±in the internal (including Alpine-Himalayan) system, they are replaced by ancient crust and SCLM usually similar to that already removed during or following terminal continental collision. Thus, typically the internal orogens of Asia involve progressive northward transfer of Gondwanan fragments into the nextforming supercontinent. Australia is but the latest Gondwanan fragment to arrive. This model introduces a new method for reconstructing supercontinental cycles and provides a different basis for unraveling the global geodynamic evolution of the ancient Earth.

Both systems have contrasting roles in the generation and reworking of continental crust, as shown by a global compilation of zircon Hf isotope analyses from the Phanerozoic. The contrasting Hf isotope arrays for both orogenic systems are caused by differing global-scale subduction

23

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Variable uplift from Quaternary deformation along the northern coast of East Timor, based on U-series age determinations of marine terraces Nicole L Cox - University ofBallarat, Vic, Australia Ron A. Harris - Brigham Young University, Provo, UT, USA Chuan-Chou Shen - National Taiv\/an University, Taipei, Taiwan Dorothy Merritts - Franklin and Marshall College, Lancaster, PA, USA Corresponding author email: n.cox@ballarat.edu.au The island of Timor is an emergent orogenic product of the arc-continent collision between the Banda Arc and the Australian continental margin.

The accretionary wedge, composed of thrust sheets from both Australian and Banda Arc derived material, is estimated to have risen above sea-level 3.1 Ma ago with uplift rates as high as 3 mm/yr.

Evidence suggests a decreased uplift rate of 1.5 mm/yr for the PlioceneQuaternary, and possibly as low as 0.5 mm/yr for the last <1 Ma. A current evolutionary model for Timor associates these waning vertical displacement rates to post-collision lithospheric delamination and isostatic rebound, but is crustal shortening completely ruled out? Low seismic activity and regional GPS measurements raise questions about how strain is partitioned in the region. Marine terraces throughout the collision zone are an additional horizontal data set, used to quantifying rates of Quaternary uplift, which offers insight into the later stages of orogenic development.

between --100 and 600 meters above sea-level. To identify the neotectonic processes controlling these surface expressions and quantify minimal uplift rates, two primary variables were measured for 10 coastal locations along orogenic strike: (1) terrace altitudes by means of terrace profiles, built with high-resolution GPS field data and detailed Quaternary maps from aerial photos, topographic maps, and Landsat images. (2) U-series age analyses for coral samples from the lower terraces to determine terrace ages. Comparison of inner margin (shoreline angle) altitudes with eustatic sea-level curves, in conjunction with U-series ages for the lowest terrace, shows a non-linear increase in vertical displacement rates that vary between < 0.1 to 1.6 mm/yr.

Along an east-west transect a mean surface uplift rate of 0.5-0.6 m/yr can be estimated over a 170 km distance, likely related to isostatic rebound from lithospheric delamination.

However, there are anomalous regions of higher uplift superimposed on this mean uplift.

The peak surface uplift rate is 1.6 mm/yr, with a mean of 1.2 mm/yr over a <15 km distance. This anomalous high uplift is likely associated with active movement along a fault, possibly a retro-wedge thrust fault at depth. The results in this study suggest that continued crustal shortening in the Timor region should be factored into the neotectonic evolution presently affecting the outer Banda arc.

Uplifted marine terraces are documented along --180 km of the north coast of East Timor.

The occurrence and number of marine terraces is not uniform along the coastline. Terraces never stretch over distances greater than 20 km; more commonly less than a few kilometers because of wide stream valleys and slumps. Numbers vary between 2 to 25, thus reaching altitudes anywhere

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Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Explanation of Symbols Thrust fault (teeth on upper plate) Transform fault Structural antiform Plate motion velocity vector •

i

Quaternary emergent material

1

Terrace profile locations (see caption)

Study Area

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Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

A geophysical investigation of deformation and reactivation of a major intraplate shear zone Brenton Crawford - School ofGeosciences, Monash University Peter Betts - School ofGeosciences, Monash University Laurent Ailleres - School ofGeosciences, Monash University Corresponding author email: brenton.crawford@monash.edu.au margin processes that are commonly associated with the development of major linear shear zones on the modern Earth.

The Mundrabilla shear zone represents a major crustal-scale discontinuity that appears to have been first active after 1140 Ma, and is now buried beneath the Officer and Eucla basins. During its subsequent geological evolution this structure appears to have been preferentially reactivated several times and now separates crustal blocks that display contrasting geophysical and structural characteristics.

The first event recognised along the Mundrabilla shear zone SZl, is associated with the rotation of earlier NE-trending fold axis that is consistent with a major phase of sinistral shearing (with probable east over west thrusting). This event is interpreted to be high-strain with a significant sinistral offset that was responsible for producing parallel mylonite zones up to 40km wide that decrease in size and in intensity from south to north.

This study employs detailed structural analysis of new high resolution aeromagnetic data collected over the region to unravel the complex kinematic and structural history of deformation associated with a major intraplate zone of weakness.

SZ2 represents a subsequent major sinistral reactivation of the existing SZl architecture, involving the generation of several parallel but discrete shear zones that are extremely linear. These structures overprint interpreted SZl mylonites, and also show a decrease in the intensity of deformation from south to north.

Inverse modelling of the magnetic and regional gravity data and detailed sensitivity analysis were performed in order to understand and quantify the geophysical differences between crustal blocks on either side of the Mundrabilla shear zone, and the 3D geometry of the structure.

Beyond SZ2 there is evidence for at least two additional episodes of reactivation of the Mundrabilla shear zone. A NWtrending system of brittle/ductile sinistral faults and shear zones are rotated into a major SZ2 shear zone and define a period of dextral displacement assigned SZ2+(a) . NE-NNE trending brittle/ductile sinistral faults and shear zones SZ2+[b) turn into and merge with a major SZ2 structure. These SZ2+(b) structures may have formed during SZ2 deformation, but given there lower grade and relatively minor displacements are probably a later reactivation.

The Mundrabilla structure represents a major shear zone that overprints the NE-trending grain of the Grenville-aged (ca. 1345-1140) orogenic belts in the Albany-Fraser and Musgrave provinces that are associated with the assembly of Rodinia in Australia. Further, post tectonic granites that are interpreted to mark the end of the intraplate Stage 2 deformation of the Albany-Fraser Orogeny were deformed and rotated during early deformation along the shear zone after 1140±8 Ma. These observations suggest this major crustal discontinuity initiated in an intraplate setting, distant from plate

26

Geological Society ofAustralia, Abstract No. 102 SGTSG2012: Cause and Effects of Deformation in the Lithosphere Mundrabilla structure is proposed to have originated as a terrane boundary between the Madura Complex and the Forrest basement province to the east.

During its early evolution, this structure was likely oriented NE to NNE and was utilised as major dextral thrust during the early stages of the Albany-Fraser Orogeny, resulting in the exhumation of the mid-crustal rocks of the Madura Complex.

may be the intraplate response inboard of the relatively rigid Yilgarn craton of a major period of shearing along the parallel Neoproterozoic Pinjarra Orogen.

In either scenario, the locus of the Mundrabilla boundary and the relatively large amounts of strain that have been repeatedly transmitted through the structure are probably related to the alignment of the ProtoMundrabilla boundary with the eastern margin of the relatively rigid Yilgarn craton. The combination of these two factors provided a favourably oriented weak zone that accommodated far field stresses that would be focused along the margins of the rigid Yilgarn craton.

A tectonic scenario is proposed for the genesis and tectonic evolution of the Mundrabilla shear zone, whereby SZl reflects a change in convergence direction between the Mawson continent and West Australian craton from the N W to the N N W during the intraplate stage 2 of the Albany-Fraser Orogeny. This resulted in a change from dextral transpression along NE trending structures, to the sinistral transpression along N-trending structures. In this scenario SZ2 could either represent a later pulse of deformation associated with the same tectonic regime as SZl, or

szj >Ca. 1140Ma « High strain i Development of parallel < sinistral shear lones over 40 km wide Magnetite enrichment «

The Mundrabilla shear zone represents a Proterozoic example of a persistent zone of weakness in the lithosphere that has been exploited during successive periods of intraplate deformation operating within an assembled yet internally active Rodinian supercontinent.

sz,|

sz^

Post SZl High/Moderate < Development of anastomosing < sinistral shear zones over 20km wide Magnetite depletion '

. Post SZ2 PostSZ2 • Low strain Moderate strain Rotation of NW-trending sinistral shear • Development of system of zones into S22 structure sinistral Faults and shears Dextral shear reactivation of existing SZ2 • Kinematica«v linked shear rones to major SZ2 shear tones

Strain Partitioning

Displacement, Strain

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-

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Complex >

Fon-est Province

Deformed ca.-1140 Ma Esperance granite

I

27

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Advances in materials and vizualisation techniques for applications in experimental tectonics Alexander Cruden - School of Geosciences, Monash University David Boutelier - School ofGeosciences, Monash University Christoph Schrank - School of Earth and the Environment, University of Western Australia Ulrich Riller - School of Geography and Earth Sciences, McMaster

University

Corresponding author email: sandy.cruden@monash.edu of model materials and power-law creep of rocks is now feasible. The adoption of 2D and 3D Particle Imaging Velocimetry (PIV) imaging techniques, combined with digital photogrammetry and laser scanning gives experimentalists the ability to quantify progressive deformation and surface topography evolution with high temporal and spatial resolution. This in turn opens up comparisons between model results and GPS velocity fields and (dynamic) topography in regions of active tectonics.

Temperaturedependent materials and digital stress monitoring mean that fully dynamic, thermo-mechanical laboratory experiments are now possible with enormous potential for modelling largescale geodynamic processes in 3D. In this presentation w e will review these new techniques and approaches, highlighting recent applications in studies of deformation localization, crust-mantle interaction and modelling of arc-continent collisions.

Since its beginning in the early 1800's experimental tectonics (a.k.a., analogue modelling) has been a powerful tool for testing hypotheses in structural geology, tectonics and geodynamics.

Despite enormous advances in numerical modelling techniques, laboratory experiments are still important for the geodynamics toolbox because of their inherent 3D nature and high spatial and temporal resolution.

However, unlike their numerical counterparts the results of analogue models have traditionally been evaluated in terms of geometrical and kinematic similarity to nature with less emphasis on rheological and dynamic similarity and temperature-time dependence. Recent advances within the experimental tectonics community mean that closer comparisons between analogue and numerical models and with nature are now possible.

The introduction of new analogue materials combined with sophisticated rheological measurements means that rheological similarity between the flow

28

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Lower oceanic crust exhumation within a Miocene slow-spreading ridge: Macquarie Island, Southern Ocean Nathan R Daczko - Macquarie University Ryan A Partner - MBARI Corresponding author email: nathan.daczko@mq.edu.au UpHft, exhumation, and denudation of the lower oceanic crust are recorded by sedimentary rocks of Macquarie Island (54°30'S, 158°54'E), which were deposited within the slow-spreading proto-Macquarie spreading ridge between ca. 9 and 12 Ma. Measured stratigraphic sections typically contain basal basaltic breccia lithofacies that are overlain by a thick sequence of enriched mid- ocean ridge basalt (E-MORB) with thin intercalations of gabbroic sedimentary lithofacies.

Basaltic detritus has zeolite to lowergreenschist metamorphic grades typical of the upper oceanic crust, and gabbroic detritus has upper-greenschist to amphibolite metamorphic grades typical of the lower oceanic crust.

Breccia clast counts and sedimentary structures indicate that basaltic lithofacies were locally derived from the footwalls of adjacent spreading-related faults. Sedimentary structures, detrital clinopyroxene major- and trace-element geochemistry, and 206Pb/238U zircon geochronology indicate that the

gabbroic lithofacies were more distally derived from a Paleogene-aged tholeiitic MORB source.

Detrital zircon populations of ca. 27 and ca. 33 Ma correspond to oceanic magnetic anomalies 8o and 13o, respectively, and exclude ca. 8.5 Ma gabbroic rocks of Macquarie Island as a potential source.

Geodynamic reconstructions show that anomaly 8o crust from the Southeast Indian Ridge was juxtaposed against the active proto- Macquarie spreading ridge when sedimentary rocks of Macquarie Island were deposited and was a likely source for the gabbroic lithofacies.

The proto-Macquarie spreading ridge and Southeast Indian Ridge were connected by the Jurru longoffset transform, which has undergone significant transpression since 27 Ma. This transpression formed a bathymetric bathymetric transverse ridge that was composed of structurally isolated blocks of heterogeneously aged Paleogene source crust, which provided the source for Macquarie Island's gabbroic sedimentary lithofacies.

29

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

The generation of sheath folds in viscously stratified materials: an analogue and numerical modelling approach Damiano DeU'Ertole - School ofGeosciences, Monash University Wouter P. Schellart - School of Geosciences, Monash University Corresponding author email: Damiano.DellErtole@monash.edu frequency sweep test provides indications about the prevailing behaviour (viscous vs. elastic) of the compound in defined oscillation frequency domains (10-3 - 102 Hz), at constant temperature (+20°C). The following step involved the design of a new simple shear box apparatus, capable to deform the silicones and to reach high shear strains (y « 8). The dimensions of the sample are 6 8 x 1 2 0 x 8 0 0 mm, and the experiments are conducted at room temperature (+20o). The shear box is composed of three Perspex walls (base and lateral walls), two stacks of mobile plates (16 plates for each stack) and a mobile lid.

Lid and plates are pulled with different speeds by a set of wires linked to a gearbox, in order to generate a planar Couette flow, in which the horizontal velocity increases linearly from the bottom (the base doesn't move) to the top (plate with maximum speed) of the tank. The friction along the two sidewalls is minimized by lubricating these walls with a very low viscosity soap. An electronic controller unit is used to set the velocity of the lid. The rheological layering is variable, and the simplest case is a thin, single pink putty layer between two thick PDMS layers.

The pink layer is deformed prior to shearing into a non-cylindrical deflection, and forms the starting point in the sheath fold's nucleation. The first set of experiments is focussed on the effects of the high-viscosity layer thickness. The initial configuration includes a 2 mm thick silicone putty layer lying at intermediate depth in the model. This thickness is then increased in the next tests by 2 mm increments, ending with a 10 mm silicone putty layer. The second set of experiments

Sheath folds have been identified in a variety of outcrops around the globe, in different kinds of rocks and tectonic settings (Skjernaa 1989; Mies 1993; Alsop 2004; Alsop and Carreras 2007; Searle and Alsop 2007; Marques, Guerreiro et al. 2008; Srivastava 2010).

The formation processes of sheath folds in rocks with high viscosity contrasts, in simple shear conditions, is still an open debate. This work shows a combined approach to the problem, using both analogue and numerical modelling to investigate the importance of some of the influencing parameters involved in sheath folds generation.

The investigation is focussed on the effects of differences in the relative thickness of the different layers, the effects of differences in viscosity between the layers and the interferences patterns in case of multiple folds. The materials used as analogue models are a PDMS (Rhodorsil Gum FB) and a silicone putty (Dow Corning 3176 Dilatant compound). The transparent PDMS represents the low viscosity layers, while the pink putty acts as high viscosity layer(s); the viscosity ratio between the pure silicone putty and the pure PDMS is ^^ 13. The initial step in this workflow concerns the analysis of the rheology of the materials used, in order to define their suitability as rock analogues: the materials properties have been analysed by simple shear, temperature ramp and frequency sweep tests. The simple shear test defines the behaviour of the material among a defined range of shear rates, at constant temperature (+20°C); the temperature ramp test defines the changes in the sample's viscosity for different temperature (-10°C +40°C), at constant shear rate (10-2 s-1); the

30

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere includes a fixed thickness for the viscous layer [2 mm), which is made by a mix of pink putty and PDMS: the greater the percentage of PDMS is added to the mix, the lower the viscosity of the layers becomes. The viscosity ratio is decreased from 13 (pure materials) to 2. The last experimental set includes a multilayered geometry, in which 3 - 5 high viscosity layers (with a deflection in each one) alternating with PDMS layers are deformed together.

The two main limitations imposed by

the analogue materials are the minimal thickness of the high viscosity layer and the maximum viscosity ratio between the silicone putty and the PDMS; those limitations can be overcome by the numerical model, in which both parameters can assume a broader range of values. The numerical models are run using the Underworld modelling framework, and it is benchmarked with the results of the tests with the analogue models.

31

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Tectonics of the Thomson Orogen, northeastern Australia: Grenville connection, passive and active margin C. L Fergusson - School of Earth & Environmental Sciences, University ofWollongong, South Wales 2522, Australia

Nev^

R. A. Henderson - School of Earth Sciences James Cook University, Townville 4811, Queensland, Australia Corresponding author email: cferguss@uow.edu.au The Thomson Orogen was originally distinguished as a substantial part of the Tasman Orogenic Zone in Queensland based on the apparent northeast structural grain, as shown by gravity anomalies. Although considerable new data are available for the Thomson Orogen, it is largely concealed and therefore its history has been difficult to establish. Radiometric ages from central Queensland have shown that the Thomson Orogen has been affected by the Delamerian Orogeny and therefore represents a northeastern Australian equivalent of the Ross-Delamerian orogenic belt which developed in the late Neoproterozoic to Ordovician along the ancient Pacific margin of Gondwana.

In contrast, the Thomson Orogen in northern New South Wales is a curving east-west belt that has been affected by the Benambran Orogeny.

continuation of the Musgrave Province into northeastern Australia within the Thomson Orogen. Thus the makeup of the Thomson Orogen appears to include a late Mesoproterozoic orogenic belt as well as equivalents of the Delamerian Orogen.

The proposition that a Mesoproterozoic suture zone crosses Australia connecting the North and South Australian Cratons is consistent with these data.

Development of a '-600Ma volcanic passive margin has been proposed for the western margin of the Tasmanides in southeastern Australia based on the geology of the Koonenberry belt of northwestern New South Wales, the Glenelg Zone of western Victoria, and King Island of northwestern Tasmania.

Rifting at this time is associated with renewed intracontinental basin development in central Australia including formation of late Neoproterozoic rifts in the Georgina Basin. In the northern Thomson Orogen, deposition of siliciclastics and associated N-MORB-like igneous activity are consistent with passive margin formation. No evidence has been found for rifting and breakup of Rodinia at -SOOMa in the Thomson Orogen. Thus Rodinian rifting is likely to have been outboard from the present location of the Tasman Line with subsequent rifting of a microcontinent away from the East Gondwana margin to form the western margin of the Tasmanides.

In the northern Thomson Orogen, the structurally lowest metasedimentary rocks contain detrital zircons with ages commonly in the range 1100-1250 Ma indicating derivation from a late Mesoproterozoic orogenic belt with abundant igneous and metamorphic rocks. These samples contain only limited detrital zircons with ages indicative of derivation from the North Australian Craton showing that the Mesoproterozoic source was much more dominant and most likely locally developed.

A similar late Mesoproterozoic source is indicated by samples from the early Cambrian succession of the Koonenberry belt and the late Neoproterozoic succession of the northern Adelaide Rift Complex.

These data are consistent with a

In the early Cambrian the northern Thomson Orogen, as for the RossDelamerian orogenic belt further south, was the site of deposition of siliciclastic

32

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

sediments containing 600-500 Ma detrital zircons characteristic of the Pacific-Gondwana margin. These rocks are related to renewed extension along the former passive margin in a backarc setting involving rollback on the outboard Delamerian active margin, as has been proposed for the Kanmantoo Trough in South Australia. The middle to late Cambrian Delamerian Orogeny was marked by strong contractional deformation, presumably in a backarc setting. Backarc settings are sites of thermal weakening and are therefore preferentially affected by contractional deformation during overriding plate advance.

Group of the Greenvale Province. Thus in the Ordovician, the Thomson Orogen was in a backarc setting developed inboard from an active margin. Several SHRIMP zircon ages from volcanic rocks and one granite sample show the widespread development of Early Ordovician silicic backarc igneous activity in the northern Thomson Orogen that post-dated Delamerian orogenesis.

In the Late Ordovician, the Lachlan, -Thomson and Mossman Orogens were affected to various extents by the Benambran Orogeny. In the Lachlan Orogen, the Benambran Orogeny was associated with the last major phase of igneous activity in the Macquarie arc, and development of the Wagga-Omeo Zone and the accretionary Stawell and Ballarat Zones in western Victoria. In the Thomson Orogen, the Benambran Orogeny had the most intense effects in northern New South Wales, the southern Anakie Inlier, and parts of the Greenvale Province. In the Mossman Orogen adjacent to the Greenvale Province, the Benambran Orogeny was associated with accretion of the Late Ordovician Carriers Well island arc assemblage followed by deposition of thick conglomeratic wedges at the base of the Silurian succession of the Graveyard Creek Subprovince.

Extensional tectonics was widely developed in the late Cambrian to Ordovician in Australia including intracontinental rifting associated with the Larapintine seaway and the Larapinta event in central Australia and development of the Macquarie arc in the Lachlan Orogen. Structures in the northern Thomson Orogen related to the Delamerian Orogeny are overprinted by intense late Cambrian to Early Ordovician deformation and metamorphism related to extensional deformation, which has formed lowangle foliation and associated recumbent folds. These structures are well developed in the Argentine and Cape River Metamorphics of the Charters Towers Province. Downwardly increasing metamorphism, consistent with extensional tectonics, is indicated for the Cape River Metamorphics.

Extension associated with backarc rifting is reflected in formation of the Mt Windsor and Trooper CreekVolcanics in the Seventy Mile Range Group of the Charters Towers Province and the correlative Balcooma Metavolcanic

From '-540Ma to the mid Triassic the Thomson Orogen developed in a backarc setting. Orogenic events were associated with accretion of island arcs as in the Delamerian and Benambran Orogenies. Latest Devonian accretion of a Devonian island arc within the New England Orogen is a continuation of this pattern.

33

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Extension overprinting shortening in an active convergent setting: the South Tibetan Detachment System, N W Himalaya Melanie Finch - School of Geosciences, Monash University, Clayton, 3800, Victoria, Australia.

Roberto Weinberg - School of Geosciences, Monash University, Clayton, 3800, Victoria, Australia.

Pavlina Hasalova - School of Geosciences, Monash University, Clayton, 3800, Victoria, Australia.

Corresponding author email: melanie.finch@monash.edu The Himalaya are the result of the N-S collision of the Indian and Asian continental plates at about - 55 Ma. As the type example of collisional orogenesis, the structures which accommodated shortening and thickening have been well documented.

Large scale thrust faults with remarkable lateral continuity strike parallel to the orogen, the Main Central Thrust (MCT) is one such example. To the north of and parallel to the MCT lies the South Tibetan Detachment System (STDS), an extensional system that extends along the entire length of the 2,400 km Himalaya. The MCT and STDS shared a period of broadly coeval movement in the early Miocene which caused extrusion of the High Himalayan Crystalline Series (HHC), the metamorphic core of the Himalaya. In the N W Himalaya lies the Zanskar Range where the STDS is known as the Zanskar Shear Zone (ZSZ) and overprints an earlier stage of thrusting.

Here, w e report on our study of the relationships between different generations of structures using field mapping, structural analyses, fabric analysis, and metamorphic petrography.

The results indicate that the ZSZ is a 1 km thick normal shear zone which overprints an earlier thrust shear zone

(Figs. 1, 2). The expression of normal shearing was found to be dependent on structural position and lithology. Less competent lithologies that were at deeper structural positions during normal movement record ductile normal movement only (Fig. 2a). More competent lithologies at shallower positions preserved earlier thrust shear structures overprinted by normal shear on sharp planes (Fig. 2b). Relationships between melt intrusion and shear planes indicated that melting started during thrust shearing and continued during and after ductile normal shearing. A kilometre scale leucogranite intrusion zone, the Gumburanjon Dome, was found to have exhumed during this normal movement. Additionally, the thrust and normal shear planes were found to be coaxial but with opposite transport directions (Fig. 1). This suggests that the normal shear zone reactivated earlier thrust planes and the change in shear direction was not a result of change in the tectonic regime.

The temporal relationship between shearing and melt intrusion indicates that the switch from thrust to normal shearing was a result of crustal weakening due to melting and consequent orogenic collapse.

34

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Smith liiKjivw

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hmiirc I Geological map of the studied area. Based on structural observations, four domains were distinguished (black rectangles). In all map figures leucogranite svwbology used where the proportion of leucogranite intrusion > 50°'o of ihc lithology. Data within - one kiloinetrc of tlic Kargyak River is based on tlic field work in this study. Other lithological boundaries were taken from Dezes et al. (1999) and landsat images, Stereographic projections are lower hemisphere, equal area of the foliation and stretching lineations for each domain, the mean plane (.¥) indicated with a dashed great circle and mean pole w ith a grey pole.

Figure 2. T^-pical outcrop structures of tlie Zanskar Shear Zone: a) top-to-NE ductile normal shearing in domain one; b) top-to-west ductile thrusting overprinted by top-to-E shaip normal shear plane in domain two.

35

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Ultramylonites of the El Pichao Shear Zone, Sierra de Quilmes, NW Argentina Melanie Finch - School of Geosciences, Monash University, Clayton, 3800, Australia.

Victoria,

Roberto Weinberg - School of Geosciences, Monash University, Clayton, 3800, Australia.

Victoria,

Pavlina Hasalova - School of Geosciences, Monash University, Clayton, 3800, Australia.

Victoria,

Corresponding author email: melanie.finch@monash.edu Ultramylonites are most commonly found in greenschist facies rocks where they appear as cm-thick bands contained within rocks of lower strain.

Higher grade examples are rare and are generally found within major orogenic zones, such as the Striding - Athabasca mylonite zone in Canada (e.g., Hanmer et al, 1995) and the Main Central Thrust in the Himalaya (e.g., Singh and Thatkur, 2001). The research presented here introduces a previously undescribed, kilometres-thick ultramylonitemylonite shear zone, part of the Famatinian orogeny, exposed in a region that has received little attention, the Sierra de Quilmes in NW Argentina.

The Sierra de Quilmes is part of the Sierras Pampeanas which is a range of approximately twelve mountain blocks uplifted in the foreland of the Andean Orogeny. The Sierra de Quilmes is located in the northern Sierras Pampeanas and consists of a 140 km long metamorphic complex that underwent deformation and anatexis during the Famatinian orogeny at approximately 470 Ma. The Sierra de Quilmes consists of two distinct metamorphic sequences: the lower grade schists of the Agua del Sapo complex in the south, and the higher grade migmatites of the Tolombon complex in the north. There is limited structural data for this region, but published work suggests that the Sierra de Quilmes records Famatinian extension (Biittner, 2009). Contrary to this, our field work suggests that the

Sierra de Quilmes consists of a zone of thrusting at least 16 km thick. The high strain zone is at least 5 km thick and consists of intercalated, decametrethick bands of ultramylonite and mylonite, designated here the El Pichao Shear Zone (Figs. 1, 2). The El Pichao Shear Zone is one of two major ultramylonite shear zones in the Sierras Pampeanas.

The other lies approximately 700 km south of the Sierra de Quilmes (Whitmeyer and Simpson, 2 0 0 3 ) and is markedly similar to the El Pichao Shear Zone, suggesting that the Sierras Pampeanas preserves a previously unrecognised major orogenic front.

References Buttner, S. H., 2009. The Ordovician Sierras Pampeanas-Puna basin connection: Basement thinning and basin formation in the ProtoAndean back-arc. Tectonophysics 4 7 7 ( 3 - 4 ) , 278291.

Hanmer, S, Williams, M., Kopf, C., 1995. Striding Athabasca mylonite zone: implications for the Archean and Early Proterozoic tectonics of the western Canadian Shield. Canadian Journal of Earth Science 3 2 , 1 7 8 - 1 9 6 .

Singh, K., Thakur, V. C., 2001. Microstructures and strain variation across the footwall of the Main Central Thrust Zone, Garhwal Himalaya, India. Journal of Asian Earth Sciences 1 9 , 1 7 - 2 9 .

Whitmeyer, S. J., Simpson, C., 2003. High strainrate deformation fabrics characterize a kilometers-thick Paleozoic fault zone in the Eastern Sierras Pampeanas, central Argentina.

Journal of Structural Geology 25(6], 9 0 9 - 9 2 2 .

36

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Ultrainylonite

a) X ' , :

Figure 1, a) Mylonite grading into ultramylonite in the El Pichao Shear Zone; b) Pegmatite d>'ke sheared into discontinuous lenses in mylonitised granodiorite, note quartz poiphyroclast percentage20%; c) Protomylonitic granodiorite with > 50% quartz porphyroclasts. All photographs parallel to tlie stretching hneation.

Figure 2. Sheared pegmatite dyke in mylonitic gi-ancdiorite. Dykelets are rotated into approximate parallelism with tlie C plane. Fold s&ymmetxy, S-C planes and asymmetrically sheared dykelets indicate top-to-the-SW, thrust shear. Photograph is parallel to the stretching lineation.

37

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Dating movement using 40Ar/3 9Ar geochronology M. A. Forster - The Australian National University, Research School of Earth Sciences G. S. Lister - The Australian National University, Research School of Earth Sciences Corresponding author email: marnie.forster@anu.edu.au (or introduction of) argon into the microstructural reservoirs within a sample.

To understand the cause and effects of deformation in the hthosphere structural geologists need innovative methods that allow the timing of movement to be ascertained, in terms of when it starts, when in stopped, and how long it endured. This outcome will not be achieved without reform in terms of methodology and concepts.

Hence here we introduce the theory of argon geochronology in the partial retention zone. We show that concepts such as "closure temperature" are largely irrelevant, and could better be discarded. We compare and contrast methodologies, and show that a carefully designed step-hearing experiment is analogues to etching a microstructure. Examples show how the microstructure of a sample can be analysed, and the methodology to carefully design schedules for stepheating experiments of white mica in vacuo. Carefully designed step-heating experiments significantly increase the likelihood that the measured apparent age spectra can be interpreted in terms of the distribution of radiogenic argon in different microstructural reservoirs.

Laser fusion on such rocks does not have the capacity to resolve the variation at a point because multi-scale (or fractal-) diffusion ensures invalidates the widely held concept that there is an age at any particular point in a dated mineral. Laser techniques provide inaccurate results because there is chaotic mixing and poor to nonexistent temperature control.

Argon geochronology has traditionally focussed on cooling or crystallisation ages, with theory developed that applies only during monotonic cooling of a microstructure that does not change and remains invariant throughout the entire history of the sampled area. Scant attention is paid to the complex interaction of microstructural processes, and as to where and how radiogenic argon is able to be retained during such processes. Yet, for the most part, the rocks of most interest to structural geology and tectonics reside in the argon partial retention zone, where diffusion occurs at a rate sufficient to ensure that not all radiogenic argon is lost to the environment, nor all of it retained. To resolve issues such as how to utilise argon geochronology to determine the duration and timing of movement in shear zones we must develop an understanding of microstructural process and how it affects the release of

38

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Geology of the Hat Trick Prospect, Tropicana Region, Northern Foreland of the Albany Fraser orogeny, WA LJFoX' AngloGoMAshanti Ltd, Level 13, St Martins Tower, PO Box Z5046, Perth WA 6831, WA 6000, Australia T G Blenkinsop - Economic Geology Research Unit, School of Earth and Environmental Science, James Cook University, Townsville QLD 4811, Australia M G Doyle - AngloGoMAshanti Ltd, Level 13, St Martins Tower, PO Box Z5046, Perth WA 6831, WA 6000, Australia Corresponding author email: lfox@anglogoldashanti.com.au The Hat Trick prospect is 4 km NE of the Tropicana gold deposit, located in the Northern Foreland of the Albany Fraser orogeny, on the southeastern margin of the Yilgarn craton. Detailed structural mapping of surface exposures at scales of 1: 5000 and 1: 100 established a deformational history in an area with little previous geological work due to its remoteness and limited outcrop.

open, and have NS hinges coaxial with Fl; the onset of amphibolitic retrogression (M2) may have begun during this stage. Dl and D2 are both characterised by EW contraction, with a strong NS extension in Dl. NE-trending tight to open folds (F3) represent D3, most obviously shown by refolding the orientation of LI. The dacite dyke is kinked in a NE orientation indicating it was likely late D3 in timing.

These features are consistent with NWSE shortening in D3. D4 produces E-W orientated open, metre scale folds refolding N-S trending F2 folds, and forming dome and basin interference fold patterns.

Greenschist facies retrogression (M3) most likely began during D4 and affects all rock types.

Three stages of shearing affect all previous folding events, including common NE-trending dextral-normal shear zones dipping moderately to the SE with m-scale offsets.

Three main rock types outcrop in the study area: banded iron formation (BIF), granite/monzonite, and mafic granulite, distributed within eight structural domains. Two intrusive units are also mapped: pegmatites in a NS orientation , and a SE-trending dacite dyke with a flow foliation.

Four main penetrative deformation events (D1-D4) are recorded, and the metamorphic evolution encompasses three stages (M1-M3). D1 produced centimetre scale isoclinal to open asymmetric folds with generally gently N-plunging hinges within the BIF, during peak M l metamorphism at granulite facies. A strong SI fabric, in places associated with a strong mineral lineation (LI), occurs in all rocks. The granite/monzonite appears gneissic in hand specimen, but exhibits classic igneous textures in thin section, indicating syn-deformational (Dl) emplacement. The pegmatites have variable degrees of SI foliation, perhaps also consistent with intrusion at varying stages during Dl. F2 folds, on scales of several to 100 metres, are upright and

Structural domains at Hat Trick are distinguished by the prevalence of different deformation phases, creating a pronounced spatial heterogeneity in structure at a 100 m scale. Combined with the non-continuous outcrop, this causes difficulties in evaluating the larger scale structure. However, in combination with geophysical data, large scale W to NW verging thrusting may be inferred, probably during D3.

Both Hat Trick and Tropicana show peak metamorphism at granulite facies

39

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

with later retrogressive phases, with an SI fabric dipping moderately to gently to the E to SE; graphite micro veins also occur in southern Hat Trick, which are similar to those occasionally hosting mineralisation at Tropicana. Unlike Hat Trick no post peak Ml igneous units are known to occur at Tropicana, and NWtrending folds that may occur in the Tropicana deposit are not found at Hat Trick.

has also been suggested for the host rocks at the Tropicana deposit, and is consistent with the location of the area in the central part of the Northern Foreland of the Albany Eraser orogeny.

The protracted history of deformation at Hat Trick may thus extend from the Archean, through Biranup orogeny events at 1760-1650 Ma, possibly to the Albany Eraser orogeny (1345-1140 Ma).

Multiple overprinting deformations were localised in domains on a scale of less than 1 km, making correlations of deformation events along strike difficult, which is likely to be a general feature of the Northern Eoreland.

Similarities between the rock assemblages at Hat Trick hill, and those of the Yilgarn craton, support geochronological arguments for an Archean protolith at Hat Trick, which

LOCALITY MAP

Hat Trick Prospect

cS^Boston Shaker Tropicana Havana

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I Hat Trick Propect Outline

I

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1

Havana South

kilometres

40

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Lithosphere scale 3D Model of Southwest Turkey Reveals Role of Slab Edge in Tectonics, Ore Deposits and Geothermal Systems Klaus Gessner - Centre for Exploration Targeting, Western Australian Geothermal Centre of Excellence, School of Earth and Environment, The University of Western Australia, 35 Stirling Highv^ay, Crawley Luis Gallardo Delgado - Centre for Exploration Targeting, School of Earth and Environment, The University of Western Australia, 35 Stirling Highway, Crawley Vanessa Markwitz - Centre for Exploration Targeting, School of Earth and Environment, The University of Western Australia, 35 Stirling Highway, Crawley Corresponding author email: klaus.gessner@uwa.edu.au Lithosphere discontinuities that are not at the scale of plate boundaries can control structural evolution and the location of hydrothermal activity in Earth's crust. As part of the Tethyan orogen, western Anatolia and the Aegean Sea region share some of their geological history but also show significant variations. The Menderes Massif represents the deepest tectonic unit of the Tethyan orogen in western Turkey, an Eocene to Oligocene nappe stack including Precambrian and Phanerozoic tectonic units. At a similar tectonic position in the Aegean we find the External Hellenides, a Mesozoic meta-sedimentary sequence.

This change in structural evolution along strike of the Alpine orogen requires the existence of a structure at depth that separates the Aegean and Anatolia.

sinistral wrench zone, and separated Anatolia from the Aegean since the Miocene. While this discontinuity does not represent an obvious feature in the crustal architecture, it can be inferred from gravity anomaly models, seismic tomography data, the distribution of Miocene to Recent hydrothermal fluid flow systems, the location of earthquakes, and the age and chemistry of igneous rocks. We argue that following Alpine crustal shortening the wrench zone stretched, exhumed, denuded and fragmented one of Earth's largest metamorphic core complexes, the Menderes Massif, in a kinematic framework that is consistent with an edge in the slab, and partial removal of the lithosphere.

Our model addresses some of the open questions in the geology of the Eastern Mediterranean and shows how the 3D visualisation and integration of geoscience datasets can be used to better understand regional tectonics.

Here we present a three-dimensional structural model of southwest Turkey to show that a N-trending lithosphere discontinuity forced the formation of a

41

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Rift geometry and fracture zone development along Australia's southern continental margin: influence of basement structure and implications for reconstructions of the Australian and Antarctic conjugate margins Gibson, G.M. - Geoscience Australia,

Canberra, ACT,

Australia

Totterdell, J.M. - Geoscience Australia,

Canberra, ACT,

Australia

Goncharov, A. - Geoscience Australia,

Canberra, ACT,

Australia

Mitchell, C.H. - Geoscience Australia,

Canberra, ACT,

Australia

Stacey,A.R.

- Geoscience Australia,

Whitaker,

A.]. - Geoscience Australia,

Canberra, ACT,

Australia

Canberra, ACT,

Australia

Morse, M.P. - Geoscience Australia,

Canberra, ACT,

Australia

Nayak, G.K. - Geoscience Australia,

Canberra, ACT,

Australia

Corresponding author email: george.gibson@ga.gov.au Paleogeographic reconstructions of the conjugate Australian and Antarctic rifted continental margins based on geological versus plate tectonic considerations are rarely, if ever, fully compatible. Possible exceptions include a recently published plate tectonic reconstruction combining ocean floor fabrics and magnetic anomalies with revised rotational poles for successive extensional events in the region that coincidently brings about a match between the Kalinjala Mylonite Zone in South Australia and Mertz Shear Zone in Antarctica (Whittaker et al., 2007). A match between these two crustal-scale shear zones has been previously proposed on isotopic and geological grounds (Di Vincenzo et al., 2007; Goodge and Fanning, 2010). However, whereas the Mertz Shear Zone marks the western limits of ca. 500 Ma magmatic activity in Antarctica (Delamerian-Ross Orogen), the Kalinjala Mylonite Zone lies well to the west of this magmatic front and is bounded either side by rocks of the Mesoarchean-Mesoproterozoic Gawler craton. An alternative geological match for the Mertz Shear Zone in Australia is the hitherto unrecognised Coorong Shear Zone in South Australia (Fig. 1), tracts of which have been intruded by gabbro and granite of Delamerian-Ross

age and west of which such rocks are either completely absent or greatly reduced in volume. The north-southtrending Coorong Shear Zone lies directly along strike from the (Spencer-) George V Fracture Zone and is clearly visible in aeromagnetic images and offshore deep seismic reflection data as a steep to subvertical crustalpenetrating basement structure across which there is an abrupt change in the orientation of magnetic fabrics and sedimentary basin fault geometries. An equally conspicuous change of direction is evident in ocean floor fabrics immediately offshore, inviting speculation that the along-strike George V Fracture Zone originated through reactivation of the older Coorong Shear Zone and shares the same orientation as the original basement structure.

Correlation of this basement structure with the Mertz Shear Zone leads to a reconstruction of the Australian and Antarctic continental margins in which Antarctica and the entrained Mertz Shear Zone are located farther east than some recent restorations allow (Fig. 1).

These restorations commonly fail to take into account an episode of NE-SW to NNE-SSW-directed extension preserved in the sedimentary and seismic record of the neighbouring 42

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Otway Basin and which is intermediate in age between initial NW-SE directed rifting in the Bight Basin and later N-S rifting that affected all of the continental margin and produced most of the ocean floor fabrics, including all of the major oceanic fracture zones. The Coorong basement structure was briefly reactivated as a sinistral strike-slip fault during this phase of NE-SW extension, but failed to evolve into a continental transform fault as was the case farther east off the southwest coast of Tasmania. There, an analogous preexisting north-south-trending basement structure identified as the Avoca-Sorell Shear Zone was optimally oriented for reactivation as a strike-slip faulting during north-south rifting (Gibson et al., 2011). This reactivated structure is continuous along strike with the Tasman Fracture Zone and shares many similarities with the Coorong Shear Zone, separating not only basement domains with opposing magnetic fabrics but sedimentary rift basins with differently oriented sets of normal

faults. Together, these two basement structures constitute an important first order constraint on palaeogeographic reconstructions of the Australian and Antarctic margins, and serve as a critical test of future palaeogeographic reconstructions based on ocean floor fabrics and plate tectonic considerations.

References Di Vincenzo, G., Talarico, F., and Kleinschmidt, G., 2007: Precambrian Research, Volume 152, p. 93118.

Gibson, G. M., Morse, M. P., Ireland, T. R., and Nayak, G. K., 2011: Gondwana Research, v. 19, no.

3, p. 608-627.

Goodge, J. W., and Fanning, C. M., 2010: Geological Society of America Bulletin, v. 122, no.

7-8, p. 1135-1159.

Stagg, H. M. J., and Reading, A. M., 2007: USGS Open File Report 2007-1047; U.S. Geological Survey and The National Academies, p. 109-114.

Whittaker, J. M., Muller, R. D., Leitchenkov, G., Stagg, H., Sdrolias, M., Gaina, C., and Goncharov, A., 2007: Science, v. 318, no. 5847, p. 83-86.

43

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Development of extensional shear bands in the protomylonites and mylonites of the Alpine Fault Zone, Central Southern Alps, New Zealand.

Ben Gilam - Victoria University of Wellington, New Zealand Timothy A. Little - Victoria University of Wellington, Nevi^ Zealand Euan G. C. Smith - Victoria University of Wellington, New Zealand Toy, Virginia A. - University of Otago, New Zealand Corresponding author email: timothy.little@vuw.ac.nz uniformly to the SE at - 6 3 ± 2° SE. Because the foliation did not deflect as a result of the finite strain gradient of the Alpine shear zone, we infer that the foliation was predisposed parallel to the Alpine mylonite zone's boundary (the "fabric attractor"). The attitude of the bulk shear zone boundary (SZB) is thus independently determinable from the attitude of the amphibolite-facies foliation. It's dip (63 ±2°) is >10° steeper than previous estimates of the dip of the Alpine Fault zone based on data from other areas, as is consistent with the maximum rates of rock uplift and exhumation being present in this central part of the Southern Alps.

Despite the abundance of extensional (C'3 shear bands in natural ductile shear zones, few quantitative studies have been undertaken regarding their attitude, spacing, slip, and finite rotation (if any) as a function of increasing bulk deformation, especially in shear zones whose motions are young and well understood. The protomylonitic to mylonitic parts of the 1 - 1.5 km-thick Alpine mylonite zone are well exposed in the lower gorge of Tartare Steam near Franz Josef Glacier in the central Southern Alps of New Zealand. Here, the Alpine Fault appears to be a single, steeply NW-dipping dextral-reverse fault without the added complexity of any near surface segmentation or splaying. Starting with the outer nonmylonitic rocks of the (garnet zone) Alpine Schist, and proceeding northwestward towards the Alpine Fault, LS-tectonite fabric intensity and ductile strain demonstrably increase into the outer protomylonite zone, which is about 600 m thick. This zone is characterized by pervasively developed C shear bands in micaceous quartzofeldspathic schists. Farther to the NW, the main, inner part of the mylonite zone, abutting the Alpine Fault, is - 1 2 0 0 m thick, but only the upper --250 m of the zone (including the mylonite-protomylonite transition) is here exposed. Remarkably, across this variably sheared section of nonmylonitic to protomylonitic to strongly mylonitic rocks a single, continuous dominant foliation (S, inherited from the non-mylonitic protolith rocks) is present, and this foliation dips

Abundant layer and shear band boudinage of m-thick mafic layers, foliation boudinage of the mylonitic quartzofeldspathic rocks, and the occurrence of antithetic (conjugate, C") shear bands in the protomylonites record significant thinning of the Alpine shear zone orthogonal to its boundaries together with up-dip stretching of the zone in a direction subparallel to the ductile shearing vector.

In transects across the shear band arrays at outcrop and thin-section scale, in which we observed >1000 extensional shear bands, apparent dips on variably oriented exposures or sections were used to measure mean planar attitudes precisely. The mean C shear band strikes 064 ±4° and dips 38 ±3° SE (± 2a), intersecting the foliation at a dihedral angle of 30 ±2°. The less abundant antithetic (C") extensional

44

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

shears strike on average 205 ±23° and dip 79 ±1 ° NW, and intersect the fohation at a dihedral angle of 135 ± 3°.

The C and C" shears do not change their dips or dihedral angles across the late Cenozoic ductile strain gradient. We use the intersection of the shear bands with the mylonitic foliation (shear boundary) to calculate a mean azimuth of late Cenozoic shearing that is 090 ±6°. This is discordant to the NE trend of the mylonitic lineation, pitching 30-40° more steeply in the foliation plane than the lineation. By contrast, the azimuth of late Cenozoic Pacific-Australia motion was -070°. In other words, contrary to previous suggestions (including by some of the coauthors of this paper) slip on the Alpine Fault zone was not perfectly "unpartitioned" and it was not parallel to the plate motion, but was down-dip of it.

observational scales) is circa 0.5 - 1.0, and it contributed <1 km of slip to overall ductile the plate motion "budget". Based on a simple model of progressive exhumation of the Alpine Fault's hangingwall, this amount of slip is far less that what would be "required" to be accommodated by plate tectonics as the rocks were exhumed through the greenschist-facies. We infer that the shears were late-stage overprints on an already well-developed mylonitic foliation (S). Based on a simple kinematic model of rotating, spaced shears separated by rigid microlithons embedded in a constant-area, thinning and stretched shear zone, the mm-scale offsets per-shear are calculated to have resulted in at most - 3 ° of finite backrotation of the C shears and up to 15° of forward rotation of the C" shears.

Restoring the shear bands to their original attitude, we infer that the conjugate C and C" shear bands nucleated approximately orthogonal to one another on planes of maximum instantaneous shear-strain-rate in the mylonite zone. Their angles to the mylonitic foliation imply a kinematic vorticity number of - 0 . 5 ( - 6 2 % pure shear) in a stretching-and-thinning type of monoclinic shear zone at the time of shear band activity. This accords with the boudinage structures and the distortional rotation of inherited (Alpine Schist object) lineations towards more downdip directions on the on the foliation plane across the protomylonite-mylonite transition. It also reinforces suggestions that extensional shear bands may be particularly diagnostic of thinning shear zones, as has been previously suggested.

Finally, we conclude that the pure shear part of the late Cenozoic ductile deformational imprint in the mylonite zone was here largely responsible for the (much discussed) variable down-dip scattering of (in part inherited) lineations in the mylonite zone.

Although affected by lithology at the outcrop scale of observation (where the shears or shear band boudins are spaced on average at 1-3 cm), the mean spacing between the C bands at the thin-section scale is 3-4 mm in the protomylonite zone, decreasing to 1-2 mm in the main mylonite zone. In both zones (and at both observation scales) the shears accommodate an internal finite shear strain of - 1 0 - 15. Some shear bands show evidence for localized greenschist-facies retogression. Even though the shear bands would seem to require a strain softening rheology in order to initiate and localize, the uniformity of finite shear strain and the increased density of similar-offset shears in the mylonite zone suggests a strain hardening rheology during their finite development, such that old shear bands died and new ones nucleated.

This process presumably led to an increased density of shear bands in the higher strain (rate) mylonite zone. Bulk finite shear strain accomplished by accumulated slip on the C shear band arrays (summed from both

45

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

The role of inherited structures during deformation-propagation: A thermochronological case study on the Kyrygz South Tien Shan basement Stijn Glorie - MINPET Group, Dept. Geology & Soil Science, Ghent University, 281 -S8 Krijgslaan, 9000, Ghent, Belgium Johan De Grave - MINPET Group, Dept. Geology & Soil Science, Ghent University, 281'S8 Krijgslaan, 9000, Ghent, Belgium Fedor Zhimulev - Institute of Geology & Mineralogy, SB-RAS, 3 prosp. Akad. Koptyuga, 630090, Novosibirsk, Russia Mihael Buslov - Institute of Geology & Mineralogy, SB-RAS, 3 prosp. Akad. Koptyuga, 630090, Novosibirsk, Russia Peter Van den haute - MINPET Group, Dept. Geology & Soil Science, Ghent University, 281S8 Krijgslaan, 9000, Ghent, Belgium Marlina Elburg - MINPET Group, Dept. Geology & Soil Science, Ghent University, 281-S8 Krijgslaan, 9000, Ghent, Belgium; Present address: School of Geological Sciences, University ofKvi^aZulu-Natal, Durban, South Africa Corresponding author email: stijn.glorie@ugent.be the Tien Shan basement architecture, we performed a multichronological study on granitoid basement samples along the Kyrgyz (Atbashi-Inylchek) segment of the STSs. Zircon U/Pb data indicate similarities between the Tien Shan and Tarim Precambrian crust.

Caledonian (-440-410 Ma) and Hercynian (-^-310-280 Ma) zircon U/Pb ages were found at the edge of the STSs, related to subduction and closure of the Turkestan Ocean and the formation of the suture itself Permian-Triassic (--280-210 Ma) titanite fission track and zircon (U-Th)/He data record the first signs of exhumation when the STSs evolved into a shear zone and the adjacent Tarim basin started to subside.

Low-temperature thermochronological (apatite fission track, zircon and apatite (U-Th)/He) analyses reveal three distinct cooling phases, becoming younger towards the STSs centre: (1) Jurassic - Cretaceous cooling ages provide evidence that a Mesozoic South Tien Shan orogen formed as a response to the Cimmerian orogeny; (2) Early Paleogene (--60-45 Ma) data indicate a renewed pulse of STSs reactivation during the Early Cenozoic; (3) Neogene

The Tien Shan represent a composite intracontinental orogen, build on an amalgamated basement of Precambrian microcontinental fragments and Caledonian and Hercynian subductionaccretion complexes at the southern margin of the Palaeo-Kazakhstan continent (Windley et al., 2007). Its current mountainous topography and active structures mimic the main NW-SE striking Palaeozoic basement architecture. The South Tien Shan suture zone (STSs) is an important example of such a reactivated basement structure in the Tien Shan edifice. This ophiolite-bearing suture zone formed during the Late Palaeozoic closure of the Turkestan ocean between Tarim and Palaeo-Kazakhstan (Seltmann et al., 2011). After the closure of this oceanic basin, the ancestral Tien Shan was built.

Its Palaeozoic suture zones however, were reactivated multiple times throughout the Mesozoic and Cenozoic as a far-field effect of collisions at the Meso-Cenozoic southern Eurasian margin (De Grave et al., 2007).

In order to constrain the chronology of formation and tectonic reactivation of

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

ages constrain the onset of the modern Tien Shan mountain building to the Late OHgocene (-30-25 Ma), which intensified during the Miocene (--10-8 Ma) and Pliocene (-3-2 Ma). The Cenozoic signals may reflect renewed responses to collisions at the southern Eurasian border, i.e. the Kohistan-Dras (-80-70 Ma) and India-Eurasia collisions (-55 Ma). This progressive rejuvenation of the STSs demonstrates that deformation has not migrated steadily into the forelands, but was focused on pre-existing basement structures.

Seltmann, R., Konopelko, D., Biske, G., Divaev, F., Sergeev, S., 2011. Hercynian post-collisional magmatism in the context of Paleozoic magmatic evolution of the Tien Shan orogenic belt. Journal of Asian Earth Sciences, 42(5), 821-838.

De Grave, J., Buslov, M.M., Van den haute. P., 2007. Distant effects of India-Eurasia convergence and Mesozoic intracontinental deformation in Central Asia: Constraints from apatite fission-track thermochronology. Journal of Asian Earth Sciences 29,188-204.

Further reading Glorie, S., De Grave, J., Buslov, M.M, Zhimulev, F.I., Stockli D.F., Batalev, V., Izmer, A., Van den haute, P., Vanhaecke, F., Elburg, M. Tectonic history of the Kyrgyz South Tien Shan (Atbashi-Inylchek) suture zone: the role of inherited structures during deformation-propagation. Tectonics, doi:10.1029/2011TC002949, in press.

References

Windley, B.F., Alexeiev, D, Xiao, W., Kroner, A., Badarch,G., 2007. Tectonic models for accretion of the Central Asian Orogenic Belt. Journal of the Geological Society of London 164, 31-47.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Deformation of post-collisional/subduction architectures within continental lithosphere Weronika Gorczyk - University of Western Australia Bruce Hobbs - University of Western Australia, CSIRO Klaus Cesser - University of Western Australia Alsion Ord - University of Western Australia Taras Gerya - ETHZ Corresponding author email: weronika.gorczyk@uwa.edu.au Geodynamic theories of the deformation and metamorphism of continental lithosphere tend to be dominated by considerations of the effects of subduction and collision zones upon the deformation of the margins of continental lithospheric blocks. Yet it becomes increasingly apparent that fossil subduction and collision zones, occurring far form continent boundaries play a key role in intra-continental deformation.

addition, the perturbation of the interface must be larger than a critical value that depends upon the elastic shear modulus. Thus in a lithosphere of uniform thickness undergoing shortening with a through-going weak zone, this zone yields first and preferentially thickens introducing a perturbation at the base of the lithosphere. Ultimately an instability develops; this is the only place such an instability occurs until another part of the lithosphere yields and begins to thicken.

Most geodynamic investigations of the Rayleigh-Taylor instability derive from the work of Houseman and Molnar and consider only viscous materials. Such materials require a significant perturbation in lithospheric thickness (>10%) to nucleate instability. The present study differs in adopting an elastic-plasticviscous constitutive law; the result is that large thickness perturbations and density variations are no longer necessary to nucleate the instability.

Examples are given for models of the Gawler Craton, the Albany-Fraser belt and the Musgrave area.

Generally speaking postcollisional/subduction zones are defined by different rheological and chemical chraracteristics from surrounding continental cratons. The crust is strongly sheared and mantle lithosphere metasomatised. Reworking of such settings reveals a surprisingly large range of instabilities that develop in a compressed/extended lithosphere with lateral heterogeneities that are inherited from the subduction settings.

Unexpected structural complexity arises which is quite sensitive to the geometry and rheological properties. This has dramatic effects on melting and devolatilisation within the lithosphere and hence in the localisation of mineralisation in the crust. The effects arise from Rayleigh-Taylor instabilities that are nucleated by plastic yielding of weak zones. One can show that in an elastic-plastic material a necessary condition for a Rayleigh-Taylor instability to develop is that the material be at the yield point; in

The numerical code used in this study is based on a conservative finitedifference, multigrid, marker in cell method developed by Gerya at ETHZurich. Dehydration reactions and melting can affect the physical properties of rocks and are incorporated in a self-consistent manner. We use a petrological-thermo-

48

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere mechanical modelling approach with all rock properties including mechanical properties calculated in the Lagrangian scheme using the PERPLEX code developed by Connolly at ETH-Zurich

for rock markers at every time step based on Gibbs free energy minimization as a function of the local pressure, temperature and rock composition.

49

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere The relationship between microstructure, metasomatism and hydrogen incorporation in deformed mantle olivine Gray, E. - The Institute for Geoscience Research (TIGeR), Curtin University of Technology, Perth, Australia Reddy, S. - The Institute for Geoscience Research (TIGeR), Curtin University of Technology, Perth, Australia Evans, K.E. - The Institute for Geoscience Research (TIGeR), Curtin University of Technology, Perth, Australia Timms, N.E. - The Institute for Geoscience Research (TIGeR), Curtin University of Technology, Perth, Australia Mclnnes, B. - fohn de Laeter Center for Isotope Research, Curtin University, GPO Box U1987, Perth WA 6845 Australia Corresponding author email: erin.gray@postgrad.curtin.edu. accommodated into the olivine crystal structure along low angle boundaries, with petrographic evidence preserved in the form of very fine fluid inclusion trails parallel to these. IR absorbance at a number of wavelengths is observed to correlate to the presence of syn-mantle intragrain structures including low angle boundaries, fractures and fluid inclusions. This correlation also exists for absorbance at --3355 cm-1 indicating that the fluids are oxidising in nature. It is usually assumed that the 3355 cm-1 peak does not represent 'normal mantle conditions' and thus is not generally included when calculating the water content of olivine from FTIR (Berry et al. 2007). Additionally, magnesium isotope analyses show a large variation in both 525 and 626 Mg, which has been suggested to record mantle metasomatism events (Pearson etal,2006).

Water plays a significant role in the deformation of the Earth's mantle, directly affecting olivine rheology and fabric development, and resulting in the formation of hydrous phases such as serpentine, which are also important for mantle deformation. Through its' effect on mantle deformation, water strongly influences the seismic properties of the mantle, and in recent years there has been significant controversy surrounding the link between water and seismic anisotropy. Thus understanding how the presence of water in nominally anhydrous mantle minerals such as olivine affects deformation is critical for the development of models for mantle geodynamics, particularly in regions of the mantle where water is abundant, such as the supra-subduction zone mantle.

Here we present a detailed study that integrates fourier transform infrared analysis (FTIR), magnesium isotope analysis, detailed petrography and electron backscatter diffraction (EBSD) of peridotite xenoliths from the Tubaf Seamount (Lihir, PNG) in order to shed light on the mechanisms involved in the incorporation of water into olivine. FTIR analyses reveal that water is being

Our results highlight a link between olivine deformation microstructure, hydroxide incorporation and magnesium isotope composition, and have implications for the quantification of water in olivine as well as models for mantle flow.

50

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Analyses of natural gold alloy microstructures

Angela Halfpenny - CSIRO Robert Hough - CSIRO Corresponding author email: angela.halfpenny@csiro.au In the last decade micro-analytical techniques have been developed which allow accurate measurement of structural and geochemical properties of individual crystals and textures. This has opened up the microstructural scale for quantifiable analyses.

A mineral such as gold in part due to its economic significance, but also its variable properties is an important metal to understand. Most geological research on gold has been performed to constrain the macro-scale location and timing of gold deposition by understanding the structural framework and controlling fluid pathways. The research presented here focuses upon characterizing the microstructures of natural gold alloys by quantifying the crystallography and composition of the visible gold.

Studying gold at this small scale will improve our understanding of how gold nucleates, crystallizes and deforms. This work also studies mechanisms that help control the formation of large visible gold accumulation and why it is so unevenly distributed. Another aim of the research is determining if there are any mechanisms which cause gold modification after crystallization and what changes can be attributed to each mechanism.

located in the carbonate dominated vein material between the breccias clasts (Figure 1).

(2) Visible gold in the shear hosted veins occurs mostly as veinlets which are long and thin and always have smaller thinner veinlets linked to them (Figure 2).

(3) Visible gold in the shear hosted veins also occurs as discrete zones with undulatory edges which can vary in size from being a f e w millimeters in diameter up to a centimeter in diameter.

The boundaries between the gold zones and the surrounding phases are extremely undulatory and the gold appears to be replacing the existing phase or is in filling a void which has been left by dissolution of the existing phase, in this example tetrahedrite (Figure 3).

(4) The final visible gold style has a fractured appearance and each fractured piece of gold is roughly triangular in shape (Figure 4).

The full crystallographic orientation of each gold crystal was measured using electron backscatter diffraction (EBSD) and chemical variations using energy dispersive spectra (EDS), on a scanning electron microscope (SEM). Chemical variations were measured by wavelength dispersive spectra (WDS) using an electron microprobe, to capture changes which are beyond the detection of the EDS technique.

The gold crystals in all samples show no crystallographic preferred orientation, the crystals have random orientations.

Most gold crystals contain twin boundaries, which are not always planar (coherent and incoherent

W e have studied visible gold hosted by quartz carbonate veins from lode gold deposits in Western Australia. The samples vary in structural style from weak breccias to discrete veins hosted in shear zones. The visible gold styles vary but four main types have been noted so far: (1) Visible gold in breccia veins occurs as aggregates of subhedral grains which vary in grain size from tens of microns to hundreds of microns in size and are

51

Geological Society ofAustralia, Abstract No. 102 SGTSG2012: Cause and Effects of Deformation in the Lithosphere boundaries) and exhibit weakly curved grain boundaries. The gold grains also exhibit a weak misorientation of the crystal lattice. Possible causes for this lattice misorientation could be due to deformation of the crystal or replacement of Au atoms (Cu is - 12% smaller in atomic size, which leads to distortions in the crystal lattice when it replaces Au) or thermal alteration.

none of the crystals measured contained Cu, only Au and Ag. This indicates that the lattice variations detected via EBSD are not due to substitution of Au atoms.

The EBSD data also show that the smaller gold crystals contain more twin boundaries than the large gold crystals.

A thermally annealed gold microstructure would be characterised by straight grain boundaries and twins and there should not be a vast difference in the number of 2 3 twin boundaries a crystals contains independent of size, which is not the case here. Therefore w e conclude that the original microstructure may have been representative of thermal annealing after crystallisation but has since been modified by plastic deformation.

Measurements of the gold's composition

by EDS show negligible variations across each gold crystal with a 1% detection limit. The composition of the gold crystals ranged between 90-98% Au and 2-10% Ag. The WDS analysis show the gold alloy composition undulates a little but does not vary significantly across the crystals and

Carbonate

^^

+ Quartz

% L

^

Vf 2.5 m m Figure 1

Figure 2

52

i

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Figure 4

53

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Applying Principles of Fracture and Fluid Mechanics to Plastically Deforming Rocks: examples from the Musgrave Ranges, central Australia Lachlan Hallett - Northern Territory Geological Survey Corresponding author email: lachlan.hallett@nt.gov.au bulging recrystallisation, sub-grain rotation and high-temperature grain boundary migration are able to reduce the internal disturbances of the crystals in the rock caused by creep (Passchier & Trouw, 2005). Rock experiencing these strain rates and modes of deformation give the meso- to macro-scopic appearance of a viscous fluid. As a result the behaviour of the rock and its effect on any rigid objects within the rock can be modelled using principles of fluid mechanics.

The lower crust deforms by creep. An understanding of creep is crucial as it controls many of the structures we see in high-grade gneiss terranes. Principles of fracture and fluid mechanics can be used to understand how creep is resolved in the lower crust. This knowledge can be used to determine the specific creep mode affecting the terrane. The creep mode is dependent on rock type, temperature and the differential stress. Thus identifying the mode allows some constraints to be placed on the likely regional temperature and differential stress regimes experienced by the rocks. Here examples from the Musgrave Ranges, central Australia, are presented, showing evidence for the development two domains experiencing different thermal and stress regimes: One deforming dominantly via dislocation creep and the other diffusion creep. This has implications for the tectonothermal regime placed on the rocks during the Neoproterozoic Petermann Orogeny.

There are two main viscous flow types, Newtonian and non-Newtonian (Passchier, 1994). The strain rates of Newtonian fluids are linearly related to applied stresses whereas the strain rates of non-Newtonian fluids are not linearly related to applied stresses (Davis & Reynolds, 1996).

Creep can occur as diffusion creep or as dislocation creep. Dislocation creep tends to occur at higher differential stresses than diffusion creep. Rocks experiencing diffusion creep have strain rates that are linearly related to applied stresses and hence they can be modelled as Newtonian fluids. Rocks experiencing dislocation creep do not have strain rates that are linearly related to applied stresses and hence they can be modelled as non-Newtonian fluids.

Deformation modes Solids are able to respond to external stresses through a variety of failure modes. These modes include elastic shape change, brittle fracturing, mechanical twining, pressure solution and creep.

Elastic and brittle mechanisms tend to dominate in the upper crust. The higher temperatures of the lower crust allow the intragranular crystallographic deformation mechanisms that facilitate mechanical twining, pressure solution and creep failure modes to dominate.

Behaviour of viscoplastics

rigid

objects

in

When rigid objects, i.e. porphyroclasts, occur in deforming viscoplastic substances they can form specific symmetric and asymmetric flow paths.

The shape of these flow paths is dependent on the sense of shear and on

Rocks can accommodate high strains over long time scales if dynamic recrystallisation processes such as

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Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

regimes during the Petermann Orogeny.

whether the flow is Newtonian or nonNewtonian.

References Eye-shaped flow patterns develop in rocks experiencing Newtonian flows [Passchier & Sokoutis 1993). The presence of the porphyroclast causes two stagnation points to develop. These stagnation points occur at either end of a lens of material containing the porphyroclast that is isolated from the main flow. The relative size of this lens compared to the porphyroclast determines whether non-stair stepping 0-, 6- or 0-type objects form (Passchier, 1994).

Davis GH and Reynolds SJ, 1996. "Structural Geology" Second Edition, John Wiley & Sons, Inc., USA. Passchier CW, 1994.

Mixing in flow perturbations: a model for development of mantled porphyroclasts in mylonites. Journal of Structural Geology, 16, 7 3 3 - 7 3 6 .

Passchier CW and Sokoutis D 1993. Experimental modelling of mantled porphyroclasts. Journal of Structural Geology, 1 5 , 8 9 5 - 9 0 9 .

Passchier CW Microtectonics.

Germany.

Bow-tie shaped patterns develop in rocks experiencing non-Newtonian flows (Passchier & Sokoutis 1993). The presence of the porphyroclast causes two saddle points to develop, allowing the flow to rotate through 180° on either side of the porphyroclast.

Between these saddle points a lens of material containing the porphyroclast occurs which is isolated from the main flow. The relative size of this lens compared with the porphyroclast determines whether non-stair stepping 0-type objects, stair stepping 6-type objects or stair stepping a-type objects form (Passchier, 1994).

Forty-two sites containing porphyroclasts comprise this study, located between the Mann Fault and the Woodroofe Thrust in the Musgrave Ranges along the SA/NT border. Of these, 22 were identified as displaying characteristics of Newtonian flow and 14 displayed characteristics of nonNewtonian flow. The sites displaying non-Newtonian flow tended to occur in the northwestern part of the investigated area. This suggests that dislocation creep was more common in this area indicating that the area was subjected to greater differential stresses that than the other sites. Identifying the different failure modes of rocks in this area of the Musgrave Ranges can be used to identify regional variations in temperature and differential stress

55

and Trouw RAJ, 2005.

Second Edition, Springer,

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Tectonic Evolution of the Arkaroola Basin: Implications for the development of the Adelaide Rift Complex Rowan Hansberry - Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Science, University of Adelaide Ashleigh Job - Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Science, University of Adelaide David Giles - Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Science, University of Adelaide Alan S Collins - Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Science, University of Adelaide Corresponding author email: alan.collins@adelaide.edu.au The Neoproterozoic to Cambro-Ordovician sediments of the Adelaide Rift Complex (formerly Adelaide Geosyncline) have been the focus of extensive investigation. Despite this, comparatively little is known about the Earliest Adelaidean Callanna Group sediments, due to their sparse preservation in outcrop geology. Exposure of the Callanna Group, and structures related to early Cryogenian graben formation at Arkaroola, in the northern Flinders Ranges, provides a unique opportunity to unravel the local geometries of rift initiation. These rocks have been subjected to multiple intracontinental deformations, most notably the Delamerian Orogeny. Through detailed structural mapping and analysis it is possible to propose models of tectonic evolution for this area. Previous regional scale mapping of the northern Flinders Ranges has identified a disparity between the tectonic history of the Arkaroola Basin and broader northern Flinders Ranges. The nature of the rifting and orogenic evolution of the Arkaroola Basin is determined though analysis of field data, rock samples in thin section and EBSD analysis.

Graben

formation accommodated an initial period of clastic and evaporitic deposition, followed by rift-related basalt extrusion.

This was followed by several phases of localised rifting and deposition, controlled by evolving fault geometries. Broad-scale orthogonal folding has folded an earlier composite fabric in conjunction with bedding. This initially planar fabric, most notable in the Woodnamoka Phyllite, formed during peak metamorphism of at least 500° C and approximately 3 kbars and is primarily attributed to burial beneath a thick pile of rift and sag phase sediments, coupled with a change in horizontal stresses. This is loosely constrained to postrift cessation and before a previously indentified thermal pulse, ca 440 Ma. A set of NE-SW trending faults in the basin have been identified as En echelon stepovers of the Paralana Fault system, responsible for the formation of the pull-apart geometries.

This system of faults details a strike-slip duplex, the reactivation of which, coupled with an anomalously high-heat producing basement, has controlled and localised deformation of the Arkaroola Basin,

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Local controls on rifting and inversion in the Northern Flinders Ranges Rowan Hansberry - Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Sciences, The University ofAdelaide.

Ashleigh Job- Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Sciences, The University ofAdelaide.

Alan Collins - Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Sciences, The University ofAdelaide.

David Giles - Centre for Mineral Exploration Under Cover, School of Earth and Environmental Sciences, The University ofAdelaide.

Corresponding author email: david.giles@adelaide.edu.au thicken to the east toward NE-SW trending and N-S trending faults (Arkaroola Bore Fault and Paralana Fault respectively) with apparent normal and dextral offset. In the southeast of the study area, the Woodnamoka Phyllite passes conformably upward into the Blue Mine Conglomerate, whereas in the central southern part of the study area the two units are juxtaposed, and the Blue Mine Conglomerate is truncated against, an E-W trending structure that we refer to as the Sitting Bull Fault.

We interpret this structure as a grabenbounding normal fault active during deposition of the Blue Mine Conglomerate, with the footwall crest (to the NW) uplifted and eroded prior to deposition of the overlying Opaminda Formation.

We report the results of field work, structural analysis, detrital zircon and Ndisotope chemistry from the lowermost Adelaidean just north of Arkaroola Wilderness Resort in the Northern Flinders Ranges.

Metasedimentary and volcanic rocks of the Arkaroola Subgroup and Emeroo Subgroup overlie Mesoproterozoic basement of the Mt Painter Inlier and are exposed in a complex map pattern influenced by multiple episodes of riftrelated faulting during the Neoproterozoic and overprinted by Paleozoic folding and fabric development. The Paralana Fault appears to have been an important control throughout the history of the basin, with local dextral transtension during the riftphase (regional N-S extension) being overprinted by sinistral transpression during folding (regional N-S shortening).

We interpret the rift geometry of the lower Emeroo Subgroup in terms of a localised basin in which trans-tensional strain resulted in NW-SE extension within a regional scheme of broadly NE-SW extension. A significant dextral strike-slip component was accommodated on the N-S trending section of the Paralana Fault and NW-SE extension was accommodated on second order NE-SW trending faults such as the Arkaroola Bore Fault. Rapid deepening of the basin was accompanied by the deposition of locally-derived, coarsegrained detritus in alluvial fans and thereafter by an upward fining sequence of immature sands and silts of the Woodnamoka Phyllite. Rejuvenated rifting, marked by deposition of the Blue Mine

During the earliest stages of basin development mature sands of the Pandurra formation and overlying calc-silicate rocks of the Wywyana Formation were deposited as sheet-like units of relatively uniform thickness. Active extension was heralded by the shallow intrusion and eruption of the Wooltana Volcanics, discontinuous exposures of which are controlled by N-S and NE-SW trending growth faults. The lower most units of the Emeroo Subgroup (Humanity Seat Formation and Woodnamoka Phyllite, incorporating a coarse-grained basal unit that we refer to as the Arkaroola Creek Breccia Member) have wedge-shaped stratal geometries that

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Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Conglomerate, resulted (at least locally] in N-S extension, prior to a period of relative tectonic quiescence during which the upper most units of the Emeroo Subgroup (the Opaminda Formation and Wortupa Quartzite) were deposited as continuous sheets blanketing the pre-existing rift geometry.

these folds can be traced through the Blue Mine Conglomerate and into the overlying Opaminda Formation, where they rotate toward parallelism with the metasedimentary layering and die out. We interpret that strain was accommodated by layer parallel movement within the Opaminda formation, consistent with, although not diagnostic of, large-scale flexural slip during the formation of regional folds with wavelengths of 10s of kilometres. In the central west of the study area the NE-SW trending folds combine with a localised op^n fold with a NW-SE axial trace to produce a bowl-shaped interference pattern. From the apparent deflection of the axial traces, we infer that the NW-SE fold set post-dated the NE-SW fold set, however the two fold orientations may have formed during progressive deformation. We interpret the overall fold geometry to have resulted from inversion of the rift basin during sinistral transpression in a regional environment of N-S shortening.

The Paralana Fault accommodated a significant component of sinistral strike slip motion, whereas shortening was accommodated by roughly orthogonal fold sets with variations in the orientation and intensity of strain imposed locally by preexisting, rift-phase structures.

An early period of ductile deformation within the Adelaidean volcano-sedimentary sequence is represented by a composite shear fabric, broadly synchronous with amphibolite facies metamorphism. This fabric comprises a combination of elements including a ubiquitous schistosity at low angle to bedding and more localised S-C fabrics and asymmetric crenulations, which we interpret to have formed during progressive shearing.

Low angle intersection lineations and crenulations associated with this fabric plunge SW and NE, depending on their location with respect to later folding. The boundary of the Mesoproterozoic basement where it is overlain by Paralana Quartzite is heterogeneously sheared within 5 to 10 m of the contact, generally with increasing strain toward the contact. Deformation involved a combination of flattening strain and simple shear with variable kinematics and an extension lineation parallel with intersection lineations in the overlying metasedimentary rocks.

We cannot unequivocally constrain the relative timing of this fabric with respect to the early fabric in the metasediments, however crystallographic preferred orientation analyses carried out using the EBSD technique are consistent with a transition from medium to high-temperature deformation and thus, potentially with prograde amphibolite facies metamorphism.

Early fabrics in the basement and metasedimentary rocks are folded by a series of upright folds with NE-SW trending axial traces and a SW plunge. These folds increase in intensity adjacent to the NE-SW and E-W trending faults, with localised overturned bedding, consistent with strain partitioning by buttressing against preexisting normal faults. The axial traces of

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere The tectonic evolution of the Ongole Domain, India: A metamorphic and geochronological approach

Bonnie Henderson - Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Science, University of Adelaide Alan S Collins - Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Science, University of Adelaide Caroline Forbes - Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Science, University of Adelaide Justin Payne - Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Science, University of Adelaide Dilip Saha- Indian Statistical Institute, Kolkata Corresponding author email: bonnie.henderson@student.adelaide.edu.au The Ongole Domain of India, situated in the southern Eastern Ghats Belt, exposes an assemblage of granulitefacies metasedimentary and metaigneous rocks that preserve fundamental evidence for the Paleoproterozoic-Mesoproterozoic reconstruction of the supercontinent Nuna. LA-ICP-MS detrital zircon data from metasedimentary rocks constrain the timing of deposition for the sedimentary precursors, to between ca.

1850-1750 Ma. Lu-Hf isotopic data from detrital zircons provide a wide range of £Hf values between -18 and +10, and TDM of ca. 3.2- 2.6 Ga. The Mesoarchean to Paleoproterozoic detrital components display geochemical similarities with the Napier Complex, the North Australian Craton and to a lesser extent, the North China

Craton. U-Pb zircon and monazite geochronology have identified three episodes of metamorphism in the Ongole Domain; at ca. 1750, 1640 and 1590 Ma. Peak P-T estimates of 900 910°C and 9 - 9.2 kbar are calculated for metamorphism associated with collisional orogenesis, between ca.

1640-1590 Ma. Ti-in-zircon thermometry independently constrains the UHT conditions, yielding estimates of 935 ± 55°C. U-Pb geochronology and trace element analysis of zircon grains from metaigneous rocks confirm syntectonic magmatism occurred in the Ongole Domain between ca. 1640-1570 Ma. The results provide support for paleogeographic reconstructions that link the southern Eastern Ghats Belt and East Antarctica during the late Paleoproterozoic.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Cross-gradient joint inversion and structural interpretation of gravity and magnetic data from the Tennant Creek mineral field, Northern Territory, Australia.

Matthew P. Hill - Centre for Exploration Targeting, University of Western Australia Luis A. Gallardo - Earth Science Division, CICESE, Mexico Corresponding author email: matt@mhgeoI.com Interpreting the location and geometry of poorly exposed crustal structures in the Tennant Creek mineral field of the Northern Territory is critical for understanding the deformation history of the Palaeoproterozoic rocks of the Tennant Creek Inlier in the North Australian Craton.

Aiming at determining the location of these structures and targeting new areas for mineral exploration, Emmerson Resources Ltd has recently collected high-resolution gravity and aeromagnetic data over the region. We undertook the modelling of these data using the emerging cross-gradient joint inversion technique of Gallardo (2007).

The resulting structurally-coincident cross-sections of magnetisation and density have allowed us to make an integral interpretation of structural and lithological features of the upper-crust in three-dimensions, which is a powerful tool in understanding the geological history of the area studied.

below these profiles have been represented by a grid of rectangular cells corresponding to density and magnetisation, which extend to a depth of 12 km. The resultant estimated property values were then combined to produce 21 geospectral images which were imported into 3D modelling software for structural and lithological interpretation.

The final ensemble of geospectral images permitted a distinctive association of density and magnetisation that matches the rock types in outcrop and diamond-drill hole core comprising the various lithostratigraphic units in the region.

After delineating these associations with the known geology we confidently continued our geological interpretation into areas with poor exposure and drill hole data. The final identification of regional faults, lithological boundaries and igneous intrusions has enabled us to create a 3D geological model of the Tennant Creek mineral field, which provides a solid framework for ongoing research and prospectivity campaigns of the area around Tennant Creek.

The task was carried out by inverting 21 north-trending and 60 km long profiles of Bouguer and TMI data distributed over an 80 x 60 km region around the Tennant Creek mineral field. Sections

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Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Strain-rate Dependence of Power-law Creep and the Development of Localised Folds in Metamorphic Rocks.

Bruce Hobbs - Centre for Exploration Targeting, University of Western Australia; CSIRO Alison Ord - Centre for Exploration Targeting, University of Western Australia Corresponding author email: bruce.hobbs@csiro.au are always strictly sinusoidal. Such classical solutions are therefore relevant to linear viscous and elastic materials. If the embedding materials are nonlinear, as is the case for powerlaw viscous materials or nonlinear elastic materials then sinusoidal solutions are no longer possible and a rich variety of localised, non-sinusoidal solutions become possible. We give examples of such localised fold systems.

The stress and strain-rate dependence of power law parameters results in even more complex behaviour including strongly localised folding and the development of axial plane shear fabrics. These structures resemble natural ones more than predicted by the Biot theory and even more than those that arise from simple power-law behaviour. We explore the influence of metamorphic reactions on the nonlinear nature of constitutive relations and link such influence to the development of localised fold systems and the development of axial plane fabrics.

It is common practice to extrapolate experimentally determined power laws for creep to geological strain-rates.

However several studies indicate that power-law relations observed experimentally for deforming rocks may be different for geological strain-rates, in that rate laws may become relatively strain-rate insensitive at low strainrates. The proposal is that the power laws determined experimentally at laboratory accessible strain-rates are limited at high strain-rates by the observation that dislocations cannot travel faster than sound in the crystal and at low strain rates by thermally activated drift. This means that the power law exponent for a given material is stress and strain-rate dependent. We explore the implications for folding of linear viscous single layers embedded in such materials. Biot's theory of folding assumes that the force exerted on the folding layer by the embedding medium is linear in nature.

The solutions to the relevant equations

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Caught between a rock and a hard place: Microplate dynamics on the northern Australian plate margin.

Robert Holm - James Cook University Simon Richards - James Cook University Corresponding author email: rob.holm@my.jcu.edu.au Tectonics in the southwest Pacific is often used as an analogue for complex accretionary orogens. Such complexity involves with micro-plate rotation and jamming, subduction reversals, and strain regime partitioning.

The boundary between the northern Australian plate and the Pacific plate, but more significantly, the Australian continent and the Ontong Java Plateau (OJP), includes the Bismarck Sea, Solomon Sea and Woodlark Basin of Papua New Guinea (PNG) and Solomon Islands, and comprises some of the youngest and most active tectonic elements of the southwest Pacific.

Furthermore, it also hosts some of the world's largest subduction-related porphyry deposits including Lihir, Bougainville, Ok Tedi and Grasberg, therefore, this region presents an ideal location to examine the tectonic and geodynamic processes associated with subduction, accretion, basin formation and mineralization. Unraveling the evolution of microplates in response to imposed proximal and distal plate interactions is critical in advancing our understanding of complex tectonic processes at convergent plate boundaries.

text provides the narrative for the reconstruction steps and context for tectonic evolution and microplate organisation.

Cessation of the Pocklington-Aure trough at approximately 12 Ma in response to collision of the Australian continent resulted in a northward trench-jump and initiated northward subducton of the Solomon Sea plate beneath the Outer Melanesian Arc at the New Britain and San Cristobal trenches.

Trench retreat relative to Australia resulted in progressive convergence between the Outer Melanesian Arc and the PNG mainland. Collision of OJP at by 8 Ma with the Solomon Islands (Outer Melanesian Arc) ceased the highly oblique southwest-directed subduction at the West Melanesian trench and North Solomon trench (Mann and Taira, 2004).

This primary soft-docking resulted in trench retreat at the San Cristobal trench but more importantly increased westward convergence sufficiently through the Solomon Sea plate to initiate west-directed subduction at the Trobriand trough.

Rifting and sea floor spreading from at least 6 Ma, also suggested by Taylor et al. (1999), opens the Woodlark Basin to accommodate the imposed northwestdirected subduction of the Solomon Sea plate at the New Britain trench and Trobriand trough.

We present a new reconstruction highlighting the role of far-field tectonic processes in localised, small-scale changes in sea floor spreading and subduction dynamics. Figure 1 shows selected snapshots in time from tectonic reconstructions in the PNG-Solomon Islands region. Bold arrows denote plate motion direction of Australian and Pacific plates; minor arrows representative of plate motion vectors, overprinting arrows reflect components of imposed plate motion. The following

Collision of the Adelbert-Finisterre Terrane with the PNG mainland at approximately 4 Ma (Abbott, 1995) prevented further west-directed shortening resulting in a redistribution of stress related to the oblique convergence. Hard-docking of OJP with the Solomon Islands at approximately 4 Ma (Mann and Taira, 2004) increased

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere convergence and subduction rates at the New Britain and San Cristobal trenches leading to increased magmatism on New Britain, Bougainville and Guadalcanal; and formation of the New Georgia islands from approximately 3.6 Ma. Further convergence subsequently fragmented the Bismarck plate into the North Bismarck and South Bismarck plates. The North Bismarck effectively coupled with the Pacific and Caroline plates, adopted the Pacific plate motion (e.g. Tregoning, 2002). The South Bismarck plate, however, is described by more complex clockwise rotation (e.g. Tregoning, 2002); as the Australian continent advances an impinges on the western South Bismarck plate, the east is decoupled along the New Britain trench forcing trench retreat.

Differential motion between the North Bismarck and South Bismarck plates is accommodated by sinistral slip on the Bismarck Sea seismic lineation, rifting and asymmetric sea floor spreading at the Manus Spreading Center (Martinez and Taylor, 1996). The southwest Pacific has experienced both subduction initiation and termination in a very short time period (<20 Ma). Although localised to the microplates of the PNG-

Solomon Islands region, these changes are driven by far-field processes. This has implications for subduction systems around the world where short-term subduction initiation and termination may be driven by processes well removed from the area of interest.

References Abbott, L. D., 1995, Neogene tectonic reconstruction of the Adelbert-Finisterre-New Britain collision, northern Papua N e w Guinea: Journal of Southeast Asian Earth Sciences, 11, 33-51.

Mann, P., and Taira, A., 2004, Global tectonic significance of the Solomon Islands and Ontong Java Plateau convergent zone: Tectonophysics, 389,137-190.

Martinez, F., and Taylor, B., 1996, Backarc spreading, rifting, and microplate rotation between transform faults in the Manus basin: Marine Geophysical Researches, 18, 203-224.

Taylor, B., Goodliffe, A. M., and Martinez, P., 1999, How continents break up: Insights from Papua New Guinea: Journal of Geophysical Research, 104, 7497-7512.

Tregoning, P., 2002, Plate kinematics in the western Pacific derived from geodetic observations: Journal of Geophysical Reseach, 107, Bl, 2020.

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Geological Society of Australia, Abstract SGTSG2012:

Cause and Effects of Deformation

64

No. 102 in the

Lithosphere

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Tectonic reconstruction of the Labrador Sea with a focus on continental deformation Maral Hosseinpour - The University of Sydney R. Dietmar MuUer - The University of Sydney Simon Williams - The University of Sydney Corresponding author email: maral.hosseinpour@sydney.edu.au both margins.

In traditional plate models, the continents are represented as rigid blocks that overlap in full-fit reconstructions. However, well-mapped conjugate passive margins, such as those in the North Atlantic region, provide geological and geophysical data to identify the landward limit of significant extension and crustal thinning. Here we use the conjugate margins of the Labrador Sea, a region well-covered by geophysical data, to apply a recently developed methodology to build a deforming plate model and derive full-fit rotation poles, based on restoring the pre-rift geometries of the extended continental lithosphere. This region is also an attractive test-bed for such a methodology because of major controversies around the extent of thinned continental crust, and about whether or not the oldest Late Cretaceous magnetic lineations along the margins in fact reflect sea floor spreading. There are major debates over weather the relatively wide domain of so-called "transitional crust" reflects stretched continental crust, exhumed mantle or oceanic crust with a very slow seafloor spreading.

We derive individual motion histories for each point on the conjugate COBs. Joined together, these COB points form the topological boundaries of deforming domains in which each vertex moves independently. The deforming domains represented by topological meshes extend as the major rigid plates either side diverge.

In our tectonic reconstruction with deforming plates, the timing and the intensity of continental extension is imposed by the progressive, diachronous breakup and initiation of seafloor spreading for each major margin segment.

Restoration of the COB along each margin is accomplished using stage poles of relative motion.

This restoration is tested using alternative stage rotations and recalculated by changing initial assumptions about crustal thickness, switching between different types of continental margins and applying alternative directions of extension. This process leads to deriving different sets of restored COBs which are then used as an input for computation of "total fit" Euler poles using the quantitative least-squares "Hellinger" methodology. We then choose the scenario which shows the best compatibility with geological observations and basin-scale extension directions, and which results in the best fit in a least-squares sense (i.e. minimal residuals).

Such a reconstruction can then linked with geodynamic models to investigate the effects of mantle flow on surface heat flow and topography through time.

We address this question by locating the boundaries of the extended continental region using potential field data and sediment thickness maps together with very well documented seismic profiles in this region and create a crustal thickness map using a well-established gravity inversion method. This part of our analysis plays a major role in setting up COB and UCCL (unstretched continental crust limit) boundaries in

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Strain partitioning in heterogeneous geological systems Mark Jessell - IRD; Universite de Toulouse III; UPS (SVT-OMP); LMTG; 14 Av. Edouard Belin, F-31400 Toulouse, France Paul Bans - Institute for Geosciences, Eberhard Karls University Tubingen, Germany Lynn Evans - School of Geosciences, Monash University, Australia Corresponding author email: mark.jessell@ird.fr The localisation of deformation has been studied extensively in the past from the perspective of the triggers of localisation, both internal and external.

We present results of two-phase deformation experiments in 2D using Elle [Finite Element-based calculations) and Gale (particle in cell calculations) and in 3D using Gale. In order to provide a base-line for the analysis of more complex materials, we assign fixed mechanical properties to the materials, so that all evolution is the result of the evolution of strucure. These apparently

simple mechanical systems reveals a remarkable range of behaviours, with both spatial and temporal variations in strain rate and vorticity.

The 3D models show an expected evolution towards elongate shapes for more viscous bodies, followed by the "rolling up" of certain of these objects at high strain as they rotate out of their orientation of stability. These structures resemble rootless folds early on in their formation, and eventually result in an apparent steady state elongation.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Geodynamic Implications of Deep Seismic Reflection Profiling in the Georgina Basin-Arunta Inlier region, southeastern Northern RJ. Korsch - Geoscience Australia, GPO Box 378, Canberra, ACT2601, Australia R.S. Blewett - Geoscience Australia, GPO Box 378, Canberra, ACT2601, Australia D.F. Close - Northern Territory Geological Survey, GPO Box 3000, Darwin, NT 0801, Australia LR. Scrimgeour - Northern Territory Geological Survey, GPO Box 3000, Darwin, NT 0801, Australia D.L Huston - Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia N. Kositcin - Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia JA. Whelan - Northern Territory Geological Survey, GPO Box 3000, Darwin, NT 0801, Australia LK Carr - Geoscience Australia, GPO Box 378, Canberra, ACT2601, Australia J. Duan - Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia Corresponding author email: Richard.Blewett@ga.gov.au In 2009, as part of its Onshore Energy Security Program, Geoscience Australia, in conjunction with the Northern Territory Geological Survey, acquired 373 km of vibroseis-source, deep seismic reflection, magnetotelluric and gravity data, in the southeastern Northern Territory, along a single north-south traverse from about 30 km north of the Sandover Highway in the north to the Todd River in the south.

This traverse, 09GA-GA1, referred to as the Georgina-Arunta seismic line, extends from the southernmost Davenport Province, across the Georgina Basin, Aileron and Irindina Provinces and the Casey Inlier of the Arunta Region, to the northeastern Amadeus Basin. The Georgina-Arunta seismic line was designed, in part, to evaluate the crustal architecture of this part of central Australia and its implications for the geodynamics of the region, and in particular, the architecture of the intracratonic Neoproterozoic to Cambrian Irindina Province.

approximation, beneath the Neoproterozoic-Devonian sedimentary basins, the crust can be divided into four distinct regions, namely, the Aileron, Irindina and Davenport Provinces, and the Ooratippra Seismic Province. Each of these regions is separated from each other by major, crustal-scale faults. The observed crustal architecture has implications for geodynamic models describing the evolution of the region, implying amalgamation of these crustal blocks in the Paleoproterozoic and major shortening and basin inversion in the Paleozoic.

Relationship between Davenport Province and Arunta Region Based on seismic reflectivity, the crust below the Georgina Basin, north of the Atuckera Fault, can be divided into three layers. The uppermost layer is only weakly reflective, and we interpret it to consist of the Warramunga Formation, Ooradidgee Group and Hatches Creek Group. The middle package is moderately reflective, and is probably basement to the Davenport Province, on which the Warramunga

The Georgina-Arunta deep seismic reflection line (09GA-GA1) has provided an image of the entire crust in this part of central Australia. At a first

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Formation and younger units were deposited. The lowermost package, forming the lower crust to the Moho, extends from the Atuckera Fault in the south to beyond the limit of the seismic line in the north, and is termed the Ooratippra Seismic Province. It is highly reflective and, thus, is distinct from all other parts of the seismic section. In general, the Aileron Province of the Arunta Region is only weakly reflective and it has not been possible to subdivide it based on its reflectivity. The Aileron Province is interpreted to extend to the north, beneath a thin cover of Neoproterozoic to Paleozoic rocks of the Georgina Basin, to the Atuckera Fault. Thus, there is a marked change in the nature of the seismic reflectivity across the Atuckera Fault, which w e interpret to represent the boundary between the Aileron Province of the Arunta Region and the Davenport Province. Because of its crustal-scale, and contrasting reflectivity across it, w e infer that this fault is an ancient suture between two different crustal blocks.

W e consider that this suture is the same structure as the Willowra Suture interpreted from the 2005 Tanami seismic line, nearly 500 km to the westnorthwest.

has the geometry of a doubly-vergent orogen, controlled by a north-dipping master thrust which connects to the Moho. The thickest crust in the seismic section occurs almost entirely under the Irindina Province, where the Aileron Province is nearly 60 km thick, compared to 40-45 km at the northern and southern ends of the seismic section. This shortening can not be attributed solely to thrusting of the Irindina Province southwards over the Aileron Province, as this would imply that the Aileron crust was originally anomalously very thick in this region only. Therefore, additional thrusting within the Aileron Province, such as the crustal-scale Atnarta Imbricate Fault Zone, to the south, is likely to have also played a significant role in thickening the crust.

Significance Orogeny

of

the

Alice

Springs

The Alice Springs Orogeny (450-300 Ma) has different manifestations in different parts of the seismic line, depending on the rheological properties of the rocks which it has affected. The Irindina Province has the geometry of a thin-skinned,- doubly-vergent orogen, with a master thrust cutting the crust through to the Moho. The Atnarta Imbricate Fault Zone has shortened the Aileron Province, and is thicker skinned.

The Neoproterozoic to Devonian Amadeus Basin, north of the Casey Inlier, has the geometry of a classic, south-directed, foreland fold-thrust belt, including the involvement of a basement nappe. To the north, there has been minor reactivation of the Atuckera Fault, a Paleoproterozoic crustal suture beneath the southern Georgina Basin, which caused uplift and folding, but did not cut the stratigraphy in this locality.

Geometry and development of the Irindina Province Metasedimentary rocks in the Irindina Province were deposited during the Neoproterozoic to Cambrian in a deep, fault-controlled basin surrounded by the Paleoproterozoic Aileron Province, with significant crustal thinning occurring during extension. The rocks underwent high-grade metamorphism, up to granulite facies during the 480460 Ma Larapinta Event. The granulites formed in the lower crust at a depth of ---SO km during extension-related metamorphism, and subsequent inversion of the original basin brought these rocks to the surface during several phases of the Alice Springs Orogeny (450-300 Ma). The seismic section shows that the Irindina Province now

In summary, new deep seismic data in the eastern Northern Territory has elucidated the crustal architecture in this region, with implications for current geodynamic models for its evolution.

68

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Geodynamic implications of the Capricorn deep seismic survey: Crossing the Capricorn Orogen from the Pilbara Craton to the Yilgarn Craton

RJ Korsch - Minerals and Natural Hazards Division, Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia SP Johnson - Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australia IM Tyler - Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australia AM Thome - Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australia HN Cutten - Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australia J Goodwin - Minerals and Natural Hazards Division, Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia BIN Kennett - Research School of Earth Sciences, The Australian National Canberra ACT 0200, Australia

University,

Corresponding author email: ian.tyler@dmp.wa.gov.au In April and May 2010, 581 kilometres of vibroseis-source, deep seismic reflection and gravity data were acquired along three traverses (lOGACPl, 10GA-CP2 and 10GA-CP3) through the Capricorn Orogen funded jointly by AuScope and GSWA's Exploration Incentive Scheme. The lines started in the southern part of the Pilbara Craton and crossed the Gascoyne Province, ending in the Narryer Terrane of the Yilgarn Craton. The aim of the survey was to image the crustal architecture in the region, and to examine the relationships between the three tectonic units. The Capricorn Orogen deep seismic public release workshop extended abstracts (Johnson et al., 2011), and geological plates showing the location of the lines and the interpreted seismic profiles, can be viewed at http://www.dmp.wa.gov.au/14219.asp

subdivided into at least two discrete crustal blocks, with the interpretation of a suture between them at the Lyons River Fault. Finally, the seismic interpretation has confirmed previous interpretations that the crustal architecture between the Narryer Terrane of the Yilgarn Craton and the Glenburgh Terrane consists of a southdipping structure in the middle to lower crust, where the Glenburgh Terrane has been thrust southward beneath the Narryer Terrane, with the Moho duplicated by thrust faulting along the Cardilya Fault. The Errabiddy Shear Zone is an upper crustal thrust system, where the Glenburgh Terrane has been thrust over the Narryer Terrane, possibly as a backthrust.

Several crustal-scale terranes and provinces can be recognised, based on the nature of the seismic reflectivity, with sedimentary basins occupying the upper crust along the northern half of the transect. The region to the north of the Baring Downs Fault is dominated by north-dipping crustal scale structures, whereas, to the south of it, the majority of the transect is dominated by south-

X.

Interpretation of the seismic lines indicates that there is a suture between the Pilbara Craton and the newlyrecognised Bandee Seismic Province.

The Gascoyne Province can be

69

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere dipping structures, many of which cut the entire crust through to the Moho.

faults imaged in the seismic sections showing evidence of post-collisional reactivation, during one of more of the later orogenies recognised in the craton For example, the Baring Downs Fault shows significant displacement of units in the Ashburton Basin, including the youngest Ashburton Formation. The Lyons River Fault was reactivated as an extensional fault during deposition of the Edmund Group, and later inverted as a thrust to produce hangingwall anticlines. Splays to the north off the Taiga Fault show a thrust sense of displacement, but near the surface the Taiga Fault itself appears to have extensional movement during deposition of the Edmund Group, with later inversion as a positive flower structure. Crustal scale faults within the Bandee Seismic Province and Glenburgh Terrane, such as the Godfrey Fault and Chalba Shear Zone, also show evidence of late reactivation.

The West Australian Craton has been built through the progressive accretion of continental slivers onto the southern margin of the Pilbara Craton (and its basement forming the Carlathunda Seismic Province). Initially, the Bandee Seismic Province was sutured to the Pilbara Craton, most likely before 2775 Ma. Suturing of the Glenburgh Terrane to the amalgamated Pilbara-Bandee Craton occurred most likely at about 2215 Ma along the Lyons River Fault, during the Ophthalmian Orogeny. Final assembly of the West Australian Craton occurred when the Narryer Terrane of the northern Yilgarn Craton was sutured to the composite PilbaraBandee-Glenburgh continental block at about 2005 Ma along the Cardilya Fault and the Errabiddy Shear Zone during the Glenburgh Orogeny.

Reference Johnson, SP, Thome, AM and Tyler, IM (eds) 2011, Capricorn Orogen seismic and magnetotelluric (MT) workshop 2 0 1 1 : extended abstracts: Geological Survey of Western Australia, Record 2 0 1 1 / 2 5 , 1 1 4 p .

Following the final assembly of the West Australian Craton to form a single continental mass, reactivation in an intracontinental setting occurred at several discrete times, with most major

70

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Fault Control on the Architecture of the CSA Cu-Ag Mine, Cobar Raisin Kyne - CODES, University of Tasmania Ron Berry - CODES, University of Tasmania Bruce Gemmell - CODES, University of Tasmania Derek Webb - CSA Copper Mine Corresponding author email: rkyne@utas.edu.au explain the high strain features within the ore bodies. The faults crosscut folds and some ore bodies but other ore bodies truncate faults of the same geometry (Fig. 1, 2). The mineralization has been intensely deformed consistent with its position within the faults. Rocks including mineralized veins outside the fault zones show much lower strain features.

We conclude that mineralization was focused into fault intersection while the faults were still active.

The CSA Cu-Pb-Zn-Ag mine, 11km NNW of Cobar in central NSW, is one of Australia's highest grade copper deposits (Fig. 1). Historically workers have attributed its nearly vertical pipelike ore body geometry to the intersection of two major regional faults, the Cobar and the Plug Tank.

While the position of the CSA mine is in the hanging wall of the Cobar fault and directly along strike from the Plug Tank Fault there is no evidence that these faults intersect within the mine area. It is possible these faults control the far field focusing of fluids that form CSA but they do not host the mineralization.

Underground mapping and drill core analysis within the CSA mine, indicate that faults striking 000-010 and 340360 have mutually cross cutting relationships and the vertical ore bodies lie along the intersection between these minor faults. The fault zone within the mine is 500 m above the Cobar Fault and is probably a synthetic splay off this regional scale fault. Detailed structural analysis based on level plans and core logging has shown the faults within the deposit form a cross-cutting orthorhombic fault array (Fig. 2).

The strain in the ore bodies is high.

Quartz veins are isoclinally folded with quartz grains exhibiting undulatory extinction, deformation bands and grain size reduction. Chlorite occurs as both pervasive and vein style alteration replacing minerals such as biotite.

Calcite grains show intense twinning that has been used to calculate a palaeostress of 200MPa for the deposit.

Sulfide minerals, such as sphalerite and chalcopyrite, exhibit deformation twinning.

In order to understand the CSA deposit's spatial location a regional stress model was constructed using the boundary elements method (BEM). The effect of the two regional faults (the Plug Tank and the Cobar) on stress fields surrounding the location of the CSA Mine were modeled by applying a horizontal a l = 1 Bar on a azimuth of 120^. Dilation was achieved along the lines of intersection of the orthorhombic fault arrays in this model.

The CSA deposit formed epigenetically from hydrothermal fluids migrating up

A number of lines of evidence can be used to suggest that the ore bodies at CSA are syn-kinematic with respect to the folding and faulting. The folds within the mine are offset along faults but show a compatible cleavage orientation. Folds within the ore body veins have a similar axial plane to the regional folds. Only a weak deformation event overprints the folding within the CSA mine and this deformation cannot

71

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere along areas of high strain. The high strain zone formed as a result of regional shortening which produced an orthorhombic fault array in the hanging wall of the Cobar Fault. The deposit is localized 0.8 kilometers to the east of

H

Upper Amphitheatre Group

the termination of the mapped Plug Tank Fault (Fig. 1). Boundary element modeling has shown that the spatial location of the CSA deposit is likely the result of the geometry and movement of the Plug Tank and Cobar faults.

^ CSA Siltsione Q Great Cobar Shale •—' '—' Okm

CSA Ore Lenses

— — «. Faults

3 km

Figure 1. Map of the Cobar region including pertinent faults and location of the CSA mine. Cross section A-A' showing the location of the CSA ore bodies in relation to surrounding regional faults. Stereonet of faults within the CSA deposit showing four major fault orientations congruent with a orthorhombic fault array.

CSAMine Level 8980

Figure 2. Plan view of level 8980 showing cross-cutting orthorhombic fault array.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere A fold nappe in the NW Indian Himalaya Jia-Urnn Lee - Research School of Earth Sciences, the Australian National University, Canberra.

Mamie Forster - Research School of Earth Sciences, the Australian National University, Canberra.

Gordon Lister - Research School of Earth Sciences, the Australian National University, Canberra.

Corresponding author email: jia-urnn.lee@anu.edu.au before the mid-late Oligocene. These structural fabrics are overprinted by static Barrovian metamorphic minerals which occurred prior to the OligoMiocene ( - 2 4 Ma). The preservation of these static minerals is inconsistent with published Main Central Thrust ages that suggest thrusting occurred in the Miocene.

Kilometre-scale recumbent folds in the Himalaya have been described as classic fold nappes, whereby rocks and stratigraphy have been overturned as a result of thrusting above a basal detachment thrust. In the NW Indian Himalaya, one such fold nappe is the Phojal fold within the Greater Himalaya.

It is thought to have been formed as a result of deformation along the Main Central Thrust, a major Himalayan ductile shear zone that was active in the Miocene-Pliocene.

Simple fold nappes caused by thrusting are insufficient to explain this data. An alternative hypothesis is that the Phojal fold is within the upper plate of a laterally extensive normal fault or shear zone. Therefore the Main Central Thrust in this environment may be a much younger structure than previously thought.

Microstructurally-focussed argon geochronology has been undertaken on rocks from the lower limb of the Phojal fold in the Greater Himalaya. Results suggest that structural fabrics related to folding and shearing were formed

73

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Does the Manning Orocline exist?

Dr Paul Lennox - School of BEES, The University of New South Wales Associate Professor Robin Offler - Discpline of Earth Sciences, University of Newcastle Mr Jie Yan - School of BEES, The University of New South Wales Corresponding author email: p.lennox@unsw.edu.au subduction complex around an orocline (Figure 1 Cross section).

These observations therefore do not support the existence of the Manning Orocline.

The southern New England Orogen from west to east, consists of a buried volcanic arc, concave to the east forearc (Tamworth Belt) and a subductionaccretion complex (Tablelands Complex) associated with a westdipping subduction zone. It was deformed into a megafold in the north (Texas-Coffs Harbour Orocline) and a possible smaller scale orocline (Manning Orocline) in the south between 275-260Ma. The Texas-Coffs Harbour Orocline shows rotation of bedding and early foliations about a vertical axis and thus is a classical orocline.

It can also be argued that the Manning Orocline in the southern New England Orogen did not form at the same time as the Texas-Coffs Harbour Orocline, because the Hastings Block was already emplaced before deposition of Early Permian sediments of the Nambucca Block. Subsequently, as a result of the formation and movement south of the Texas-Coffs Harbour Orocline, the Early Permian sequences in the Nambucca Block were strongly deformed. Those in the northern margin of the Hastings Block were less deformed because the rocks were more competent.

The presence of the Manning Orocline is based on the distribution of forearc basin sequences in the southern Tamworth Belt and the Hastings Block (Figure 1). The centre of the orocline is near Mt George in the Southern Hastings Block. However rotation of bedding within the Tamworth Belt around this structure does not occur nor in the subduction-accretion complex sequences. Further, orientation of S and C structures observed in serpentinites from the Mt George area are not consistent with ductile bending of the serpentinites. Studies by Phillips (pers.com. 2011) of the subductionaccretion complex sequences west of the Hastings Block show coherent rock packages younging to the west. This is not consistent with rotation of the

The southern Tamworth Belt consists of four blocks (Rouchel, Gresford, Myall and Hastings) bounded by major faults.

Palaeomagnetic studies in these blocks indicate progressively increasing degrees of rotation of these blocks, with the Hastings Block recording the greatest rotation. Rotation of the Rouchel, Gresford and Myall blocks is thought to have occurred between the mid-Namurian and latest Carboniferous.

This argues against the suggestion that rotation of these blocks and formation of the Texas-Coffs Harbour and Manning oroclines were at the same time.

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Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

SUBDUCTIONACCRETION COMPLEX

TAMWORTH i BELT

COFFS NAMBUCCA/ HARBOUR BLOCK ( ASSOCIATION

, HASTINGS ' BLOCK

ROUGHED BLOCK GRESFORD BLOCK

MYALL BLOCK

Q. T C ^ . 'CHA-.TC H B ,

NB X'

OVERALL STRATIGRAPHIC YOUNGING YOUNGING IN PACKAGES

Figure 1: Map of the Southern New England Orogen (modified after Korsch & Harrington 1987) and cross section. Arrows in the cross section indicate the younging direction for individual rock packages and overall assuming that the Manning Orocline existed. Abbreviations: TC = Tablelands Complex, CHA = Coffs Harbour Association, HB = Hastings Block and NB = Nambucca Block.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Deformation history of the Texas Orocline, Eastern Australia Pengfei Li - School of Earth Sciences, The University of Queensland, Brisbane 4072, Queensland, Australia Gideon Rosenbaum - School of Earth Sciences, The University of Queensland, Brisbane 4072, Queensland, Australia Paul J.T. Donchak - Geological Survey of Queensland, Department of Employment, Economic Development and Innovation, Brisbane 4000, Queensland, Australia Corresponding author email: lpf3039165@gmail.com The Texas Orocline, with a half wavelength of '-120km, is the largest and most obvious orocline in the southern New England Orogen and is clearly recognized in geological maps and geophysical images. In the area of the orocline, there is a major unconformity between DevonianCarboniferous metasedimentary rocks (Texas beds) and the overlying Early Permian rift-related basins. Detailed structural mapping shows that units both above and below the unconformity are folded around the orocline, indicating that at least part of the oroclinal deformation has occurred after the deposition of the Early Permian rocks.

In addition, the alignment of Early Permian (298-290 Ma) granitoids parallel to the oroclinal structure, is also consistent with this interpretation. Pre-oroclinal isoclinal folds ( F l ) and related axial plane cleavage ( S I ) are well developed in the Texas beds and are curved around the

oroclinal structure.

Syn-oroclinal structures are characterised by minor kink folds and disharmonic folding (F2) in the core of the orocline. A later phase of kink folding corresponding to a --N-S shortening direction does not seem to be related to the orocline, and is interpreted as post-oroclinal deformation. A secondary penetrative fabric parallel to the axial plane of the orocline was not observed, indicating low contractional shortening across the orocline (<30%). W e demonstrate that the observed strain is too low to account for oroclinal bending during dextral transpression, as previously proposed. W e suggest an alternative model involving an initial curved structure, probably related to subduction rollback or a pre-existing curvature in the paleomargin of eastern Australia, which was amplified by dextral transpression and subsequent E-W contraction.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Triassic asymmetric subduction rollback in the southern New England Orogen: implication to the earliest phase of the Mesozoic rifting of eastern Australia Pengfei Li - School of Earth Sciences, The University of Queensland, Brisbane 4072, Queensland, Australia Gideon Rosenbaum - School of Earth Sciences, The University of Queensland, Brisbane 4072, Queensland, Australia Daniela Rubatto - Research School of Earth Sciences, Australian National University, Canberra, 0200 ACT, Australia Corresponding author email: lpf3039165@gmail.com southern New England Orogen, suggest that magmatism during the Hunter Bowen Orogeny was spatially distributed along a NE-SW belt that was likely associated with a west-dipping Andean-type subduction zone.

In contrast, younger magmatism (235-210 Ma) is aligned along a N-S belt that is offset to the east, indicating eastward arc migration, possibly in response to slab rollback. W e propose that this phase of tectonic activity marked the transition from contractional deformation in the continental margin to extensional tectonism during the earliest phase of the Mesozoic rifting of eastern Australia.

The New England Orogen is the youngest subduction-related component in the Australian continent, recording a prolonged history of westdipping subduction from the Devonian to the Triassic. Here w e focus on the late Permian to Triassic tectonic history, a period that included a significant contractional event at --265-235 Ma (the Hunter Bowen Orogeny) and widespread I-type calc-alkaline magmatism. New zircon U-Pb ages, ranging from 255 to 215 Ma, were obtained for seven granitic samples from key positions across the magmatic system. The SHRIMP U-Pb data, combined with previous geochronological

data

from

the

77

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Using uncertainty simulations and geodiversity to identify structural endmembers in three-dimensional geological model suites Mark Lindsay - School ofGeosciences, III

Monash University and GET, Universite de

Stephane Perrouty - GET, Universite de Toulouse III and School of Geosciences, University

Toulouse Monash

Mark Jessell - IRD, GET, Universite de Toulouse III Laurent Ailleres - School of Geosciences, Monash

University

Eric de Kemp - Geological Survey of Canada, Ottawa, Peter Betts - School of Geosciences, Monash

Canada

University

Corresponding author email: mark.lindsay@monash.edu Introduction

elements are represented is directly related to their usability in analysis. If a 3D model inaccurately represents aspects of study area, the relevance of geometrical analysis is negatively affected. It is therefore critical that 3D models are made to be as accurate and reliable as possible. This requirement relies heavily on the quality of input data. Field observations, geophysical interpretation and the prevailing tectonic evolution hypothesis are typical forms of data use to construct these models. In particular field observations, consisting of orientation measurements and lithological observations, are often supported by interpreted geophysics in covered terranes.

The tectonic evolution hypothesis describes the timing of important geological events so has a large influence on the stratigraphic column, fault networks and interactions between the modelled elements. Any field observations, be they observed and measured in the field or taken from interpreted geophysics introduce ambiguity as it is sometimes difficult to explicitly identify lithological and structural fabric orientations in covered or weathered terranes (Fig.la and c).

Some measured fabrics may not best represent the surface at depth (Fig. lb).

Error removal is performed at much effort and cost to mitigate the effects of uncertainty with the aim to produce a single, optimised data set that

Detailed three-dimensional (3D) geological models can be produced to benefit studies in many geoscientific disciplines. The varied and large amounts of data types available to the geoscientist can be rendered meaningless when presented as tables and charts, so it is often necessary to represent different geoscientific data in 3D in order to determine any spatial and property relationships through visualisation.

Geostatistics have traditionally been the common form of analysis performed in 3D geological space, for example, kriging is typically used to determine spatial trends in assay data and is particularly wellsuited for use in three dimensions. Less widely used is geometrical analysis for 3D model analysis and comparison. 3D models are containers of volumetric and polygonal objects representing geological elements.

Considered analysis of these containers and internal geometries can reveal useful information about the geological study area. Relationships between faults, lithologies and folds can aid resource estimates and mineral distribution to generate 3D prospectivity maps that can aid exploration activity.

3D model construction The accuracy by which these geological 78

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

represents geology in the most accurate manner.

subsequently required for analysis.

Identi^ng important geometrical attributes of fault hosted placer deposits will require a different approach than those focussing on predominantly volumetric attributes of an oil and gas field.

A different approach has been taken in this study by assuming that error exists but does not make any attempt to remove it (Jessell et al., 2010). A series of models are generated from the same input data set by simulating uncertainty on the orientation observations.

Different geometries can result from the combination of varied measurements as data is varied prior to model input.

These variations in geometry can produce models that vary considerably in terms of morphology and depth (Fig. 2) revealing what may be geologically possible from the input data set.

Uncertain regions within the data set can be identified by performing model comparisons (Wellmann et al., 2010).

Measures such as strat-igraphic variability (Lindsay et al., 2011) and information entropy (Wellmann and Regenauer-Lieb, 2011) have been developed to quantify and visualise the results of these comparisons. Different geometrical relationships can also be iden-tified and quantified by comparing these models. While there are many methods in which geometrical relationships can be described, the challenge in this area is to identify geological, or 'geodiversity', metrics that adequately describe these relationships in a relevant manner to the geoscientific community.

Further analysis on the model suites has been performed to identify which geometrical metrics are most important when describing model suite variability.

Principal Component Analysis (PCA) has been chosen to perform this task as it is able to (1) identify these important metrics obtained from different model suites and (2) effectively visualise the model suite to identify suite outlier and barycentre models (Fig. 3). Variability in influential metrics has a large effect on the geometry of the entire 3D model.

Particular information can then be obtained if it is desired that this variability is to be constrained, further reducing the potential for uncertainty within the model suite. Identifying outlier and barycentre models allows a large model suite to be sampled effectively for further processing by time and computationally-intensive techniques such as geophysical inversion.

References

Jessell, M.W., Ailleres, L., de Kemp, EA, 2010.

Towards an integrated Inversion of geoscientific data: What price of geology? Tectonophysics 490, 294-306.

Geodiverisity metrics

Lindsay, M., Ailleres, L., Jessell, M.W., de Kemp, EA, Betts, P., 2011. Locating and quantifying geological uncertainty in three-dimensional models: Analysis of the Gippsland Basin, southeastern Australia. Tectonophysics In review.

We present some geodiversity metrics in a comparative case study area. The cratonic palaeoproterozoic Ashanti Green-stone Belt, southwestern Ghana and the Gippsland Basin, southeastern Australia provide two different geological ter-ranes that test the efficacy of these techniques. Each of these regions have different resource targets: orogenic and placer gold deposits in the Ashanti Greenstone Belt versus oil and gas in the Gippsland Basin. Different analytical methods are

Wellmann, F.J., Horowitz, F.G., Schill, E., Regenauer-Lieb, K., 2010. Towards incorporating uncertainty of structural data in 3D geological inversion. Tectonophysics 490,141-151.

Wellmann, J.F., Regenauer-Lieb, K., 2011.

Uncertainties have a meaning: Information entropy as a quality measure for 3-D geological models. Tectonophysics 490,141-151.

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Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Closing Tethys Gordon Lister - Research School of Earth Sciences, Australian National

University

Simon McClusky - Research School of Earth Sciences, Australian National

University

Mamie Forster - Research School of Earth Sciences, Australian National

University

Robert Hall - SEARG, RHUL Corresponding author email: gordon.lister@anu.edu.au The Tethys Ocean once separated Eurasia and the dispersing fragments of Gondwana. When this ocean began to close, from about 120-90 Ma, subduction drove these fragments northward. A sequence of relatively strong indentors ploughed through a complex palaeogeography defined by marginal basins on the northern Tethyan margin.

factors that determined its deceleration during the subsequent sequence of accretion events as India ploughed into and through Tethyan arcs and continental ribbons.

There are many uncertainties as to how orogens behave, however. It is critical that w e refine the geochronological data as to the timing of individual events.

From the point of view of rheology there is controversy as to whether orogens are plate-like or fluid-like, and w e must also resolve this issue. Is the upper-crust detached from a more plastic or more viscous substrate. Do the internal zones of orogens manifest channels in which hot rocks flow, carrying the brittle upper crust in a more or less passive fashion. The 4D dynamics required by indentation, rollback and orogenic collapse imply interactions that are within the capabilities of modelling and simulation software.

One hypothesis is that the lithosphere beneath the impacted basins was thus torn, and some of this lithosphere foundered (i.e., sank holus bolus back into the mantle). Did such foundering cause the Tethys-wide continental stretching that commenced at the Eocene-Oligocene transition (--35 Ma) and continued until - 3 0 Ma? Does such a mechanism allow general explanation of continental stretching observed adjacent to all indentors? Are the dynamics of this process explained by slab roll-back [involving migration of the hinge of the subducting lithospheric slab as it falls into the mantle) or by the attendant uplift in adjacent regions?

How far field do the dynamic effects related to the collision of India with Asia extend during this process? Are the potential effects of the many other events that took place at this time equally deserving of our attention (e.g., those marking the progress of the Arabian indentor) and what underlies the temporal coincidences with such events, e.g., the opening of the Gulf of Aden or the Red Sea? In respect to the closure of Tethys there are many such dynamics problems that deserve explanation, e.g., the acceleration of the northward motion of India as it approached the Deccan plume and the

A half-century has gone by since Carey attempted to explain the many arcuate zones (including curved mountain belts or oroclines) that characterise the -12,000 km strike length of the AlpineHimalayan orogen (i.e. the oncethickened and often later stretched welt of relatively mobile crust that found itself involved in the collisions between Eurasia and the northward migrating Gondwana fragments as the Tethys Ocean closed). Can w e close this debate with convincing physics-based dynamic models that are generally applicable, rather than geological explanations that are qualitative, ad hoc, and different in each individual circumstance?

80

Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Linking plate reconstructions with deforming lithosphere to mantle convection models R. Dietmar MuUer - The University of Sydney Corresponding author email: dietmar.muller@sydney.edu.au and buoyancy based on the tectonothermal age of the continents: Archean, Proterozoic and Phanerozoic.

In the fourth isostatic column, the oceans, the thickness of the thermal lithosphere is assimilated using a halfspace cooling model. We also define the thickness of the thermal lithosphere for different continental types, with the exception of the deforming areas that are fully dynamic. Finally, we introduce a "slab assimilation" method in which the thermal structure of the slab, derived anal3^ically, is progressively assimilated into the upper mantle through time. This method not only improves the continuity of slabs in forward models with imposed plate motions, but it also allows us to model flat slab segments that are particularly relevant for understanding dynamic surface topography. When it comes to post-processing and visualisation, GPlates allows the user to import timedependent model output image stacks to visualise mantle properties (e.g. temperature) at a given depth through time, with plate boundaries and other data attached to plates overlain. This approach provides an avenue to simultaneously investigate the contributions of lithospheric deformation and mantle flow to surface topography. Currently GPlates is being used collaboratively in conjunction with the codes CitcomS, Terra, BEMEarth and the adaptive mesh refinement code Rhea; a GPlates python plugin infrastructure under development will make it easy to extend interoperability with other geodynamic modelling codes.

While global computational models are rapidly advancing in terms of their capabilities, there is an increasing need for assimilating observations into these models and/or ground-truthing model outputs. The open-source and platform independent GPlates software fills this gap. It was originally conceived as a tool to interactively visualize and manipulate classical rigid plate reconstructions and represent them as time-dependent topological networks of editable plate boundaries. The user can export time-dependent plate velocity meshes that can be used either to define initial surface boundary conditions for geodynamic models or alternatively impose plate motions throughout a geodynamic model run. A new version of GPlates is now being developed that allows an embedding of deforming plates into topological plate boundary networks.

We use geophysical and geological data to define the limit between rigid and deforming areas, and the deformation history of non-rigid blocks. The velocity field predicted by these reconstructions can then be used as a time-dependent surface boundary condition in regional or global geodynamic models, or alternatively as an initial boundary condition for a particular plate configuration at a given time. For time-dependent models with imposed plate motions (e.g. using CitcomS) we incorporate the continental lithosphere by embedding compositionally distinct crust and continental lithosphere within the thermal lithosphere. We define three isostatic columns of different thickness

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Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

The Late Jurassic to present evolution of the Andean margin: drivers and the geological record Kayla Maloney - Earthbyte Group, School of Geosciences, University of Sydney Geoffrey Clarke - Earthbyte Group, School of Geosciences, University of Sydney Keith Klepeis - Department of Geology, University of Vermont Leonardo Quevedo - Earthbyte Group, School of Geosciences, University of Sydney Christian Heine - Earthbyte Group, School of Geosciences, University of Sydney Nicolas Flament - Earthbyte Group, School of Geosciences, University of Sydney Corresponding author email: kayla.maloney@sydney.edu.au Uncommonly long-lived subduction and variable plate geometry along the South American Andean plate margin resulted in diverse relationships between magmatic flux, divergent and convergent deformation, as recorded by the over-riding continental plate.

Convergence velocities and absolute overriding plate velocities, both trench normal and trench parallel, and subducting slab ages were resolved along the trench from 170 Ma to the present using a recently developed kinematic plate model, in an attempt to define how subduction conditions are related to deformation and magmatic features in the overriding plate. Several key correlations reflect the dependence of macroscopic crustal strain on subduction mechanism and relative plate vectors.

Extensional basins involving mafic/oceanic crust developed only when the overriding plate velocity of South America was directed away from the trench and the age of the subducting slab was older

than 50 Ma. The development of fold and thrust belts and uplift of major plateaus occurred when the trench normal convergence velocity was greater than 4 cm/yr and the age of the subducting slab was older than 50 Ma. The parameters investigated in this study revealed no correlation with the timing of major magmatic events, nor was any correlation observed with the structural style developed in fold and thrust belts. Furthermore, a model incorporating the deformation of the margin is being developed for the southernmost portion of South America in an attempt to resolve discrepancies revealed in the initial analysis. The inclusion of changes in geometry provides a more realistic framework for the investigation of the relationship between subduction conditions and geological events in the overriding plate in regions, such as southernmost South America, where the margin has undergone significant deformation.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere The Interplay between Fracture and Flow in the Localization of Crustal Deformation Neil Mancktelow - Department of Earth Sciences, ETH Zurich, Switzerland Giorgio Pennacchioni - Department of Geosciences, University of Padua, Italy Corresponding author email: neil.mancktelow@erdw.ethz.ch The Earth's crust is generally considered to consist of distinct brittle and viscous (or "ductile") rheological layers, corresponding to NavierCoulomb failure or viscous flow, with a "brittle-ductile transition" occurring over a specific and relatively limited depth interval. Depending on the assumed geothermal gradient, a compositionally layered crust could have several such brittle-ductile transitions, but the model still implies that large regions of the crust deform exclusively by either brittle fracture or viscous crystal-plastic flow. However, it is becoming increasingly clear from field observation that, in reality, there is an intimate interplay in space and time between precursor heterogeneities (either structural or compositional), brittle fracture, fluid-rock interaction and more distributed "ductile flow". In particular, there are now several welldocumented examples of brittle precursors localizing subsequent ductile deformation under high grade metamorphic conditions ranging from upper amphibolite (Mancktelow and Pennacchioni 2005, 2007) to even eclogite facies (Austrheim and Boundy 1994). This interplay between fracture and crystal-plastic creep and/or diffusion occurs over a wide range of scales, from lOO's of kilometres down to individual grains.

variation in local pore-fluid pressure.

These relatively planar and discrete faults and shear zones are commonly observed to cross-cut layering and foliation at a small angle. Small-scale examples from the field, as well as numerical models, show that viscous localization is strongly controlled by existing compositional and rheological heterogeneity (such as bedding, dykes, veins etc), whereas fractures may crosscut such compositional layering at small angles. This suggests that major crosscutting faults, which may now be dominated by mylonitic fabrics characteristic of crystal-plastic flow (e.g., the Periadriatic Fault in the European Alps), could also have had a large-scale, brittle precursor that controlled subsequent ductile localization.

On a smaller scale, flanking structures (Passchier 2001; Grasemann and Stiiwe 2001) developed around brittle fractures of limited length are particularly clear examples of interacting brittle-ductile deformation, because their geometry can only be explained if discrete slip occurred synchronously with more distributed surrounding ductile flow (Exner et al.

2004). Flanking structures that formed in calcite marbles under amphibolite facies conditions (e.g., on the island of Naxos, Greece) demonstrate that brittle fracturing can play an important role even in weak rocks at high temperatures - conditions generally taken to imply exclusively ductile or viscous behaviour.

Such flanking structures are common in mylonitic shear zones (e.g., in mylonites in the footwall of the major Simplon low-angle normal fault in the central Alps) and

Localization of strain in the crust can lead to the development of zones of very large relative displacement (such as low-angle thrusts and detachments, and steep strike-slip faults). The mechanics of this localization on a narrow zone and its repeated reactivation can only be considered in terms of a cyclical interaction between fracture, flow, and

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Pseudotachylytes subsequently act as rheologically weak layers that strongly localize ductile shearing under dry upper amphibolite facies conditions (Pennacchioni and Cesare 1997).

demonstrate the delicate balance between fracture and flow in such high strain zones, with switches back and forth varying locally in space and through time.

References

This behaviour is not totally unexpected. The reduction of bulk porosity and permeability in rocks with depth raises the local pore fluid pressure from hydrostatic to near lithostatic (as usually assumed in metamorphic petrology), with the result that rocks are generally critically stressed and close to failure. Only minor local changes in the controlling parameters (strain rate, pore fluid pressure, dynamic or "tectonic" pressure) can cause a switch between fracture and flow. In natural examples, the interplay between fracture and flow is observed in middle to lower crustal rocks irrespective of whether they are weak ("wet") or strong ("dry").

Excellent examples of interacting fracture and flow from glacier-polished outcrops of granodiorite in the Neves area of the eastern Alps developed under wet conditions, with very common quartz vein development and marked fluid-rock interaction along fractures. The deviatoric stress during both flow and fracture was low (<10 MPa), as demonstrated by little deformed calcite porphyroclasts in quartz mylonites, which did not even significantly twin during crystal plastic flow of the matrix quartz under upper amphibolite fades conditions (Mancktelow and Pennacchioni 2010).

In contrast, in dry lower crust, such as from the Mont Mary area of the western Alps, stresses were high (as indicated by very small recrystallized quartz grain sizes; Fitz Gerald et al. 2006) and seismic fracture was associated with pseudotachlyte development.

Austrheim, H., Boundy, T.M., 1994.

Pseudotachylytes generated during seismic faulting and eclogitization of the deep crust.

Science 265, 82-83.

Exner, U., Mancktelow, N.S., Grasemann, B., 2004.

Progressive development of s-type flanking folds in simple shear. Journal of Structural Geology 26, 2191-2201.

Grasemann, B., Stuwe, K., 2001. The development of flanking folds during simple shear and their use as kinematic indicators. Journal of Structural Geology 23, 715-724.

Fitz Gerald, J.D., Mancktelow, N.S., Pennacchioni, G., Kunze, K., 2006. Ultraflne-grained quartz mylonites from high-grade shear zones: Evidence for strong dry middle to lower crust. Geology 34, 369-372.

Mancktelow, N.S., Pennacchioni, G., 2005. The control of precursor brittle fracture and fluidrock interaction on the development of single and paired ductile shear zones. Journal of structural Geology 27, 645-661.

Mancktelow, N.S., Pennacchioni, G., 2010. Why calcite can be stronger than quartz. Journal of Geophysical Research 115.

Passchier, C.W., 2001. Flanking structures.

Journal of Structural Geology 23, 951-962.

Pennacchioni, G., Cesare, B., 1997. Ductile-brittle transition in pre-Alpine amphibolite facies mylonites during evolution from water-present to water-deficient conditions (Mont Maiy Nappe, Italian Western Alps). Journal of Metamorphic Geology 15, 777-791.

Pennacchioni, G., Mancktelow, N.S., 2007.

Nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions. Journal of Structural Geology 29,1757-1780.

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A new mobile application for viewing 3D geospatially located geological maps and data Dylan Matherson - School of Business, Disciplin of IT James Cook University, Queensland, Australia.

Simon Richards - School of Earth and Environmental Sciences, James Cook University, Townsville Australia.

Trina Myers - School of Business, Disciplin of IT, James Cook University, Queensland, Australia.

Corresponding author email: dylan.matherson@my.jcu.edu.au Visualizing the 3D geometry of structures and other geological features in the field has, until recently, been extremely expensive, difficult to obtain and use and do not typically incorporate geological structures and objects beneath the surface. Newly developed applications that incorporate geological data are being developed to overcome these aforementioned issues. New geospecific visualisation tools will allow geologists to visualise geological structures and other geological data in real time in the field. Visualisation methods applied in field work are still typically constrained to portable computers running the conventional 3D modelling packages. New methods for the visualisation of 3D geological maps in the field are now possible with the advances in mobile computing power and the advent of augmented reality on consumer grade devices. Furthermore, the incorporation of GPS, accelerometers and digital compasses into these portable devices means that the user can now become geospatially integrated into the 3D map.

visual information from a variety of mobile devices.

For a geologist, 3D models showing features such as strike and dip of strata, fault zones, geophysics and interpretations of geophysical data are overlayed or "augmented" so they can be explored while in the field.

Geologists then have the ability to see the data as though it were part of the environment.

This is analogous to giving geologists x-ray vision in the field where they can literally see through fixed objects such as fault zones, plutons, even the walls of open cut mines as long as the geological map has been constructed and imported into the device. The effect while only pseudoreal can be used to visualise pre captured data stitched into the environment to provide a realistic view of sub-surface features. This ability to instantly reference and visualise geological data sets within the localised environment without the requirement to use landmarks or build mental images is a powerful advantage of augmented reality. The model simply fits into the environment and allows the user to see how its intended representation fits within that environment.

Augmented reality denotes a technologically supported hybrid between the virtual world and the real world. Information can be positioned in 3 dimensions and overlayed into the real world.

The user can see information augmented into the real world from their location and perspective. This technology has many applications in structural geology where geologists can attain rapid and effective

Standard smart phones or tablets are all that is required to apply this augmented reality technology. The software is designed to run on any device with a standard mobile operating system such as android. 3D models can be exported

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Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Potential users of the system range from geology students learning to visualise geology at depth to pit geologists who require geospatially located and interactive geological maps of the subsurface geology.

Applications and Demo models available at http://fieldvis.com

from the 3D package as a DXF (Drawing Interchange Format) and be automatically converted to run on the device. The prototype device is capable of rendering any geological models exportable in DXF format and has been tested with models created in GOCAD(TM).

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Geological Society of Australia, Abstract No. 102 SGTSG2012: Cause and Effects of Deformation in the Lithosphere

Origin of Barite Veins in the Central Flinders Ranges of the Adelaide Fold Belt, South Australia Premalal J Mendis Corresponding author email: prelal_mendis@yahoo.com.au Intro: The Central Flinders Ranges (CFR) in South Australia comprise of Neoproterozoic to Cambrian sedimentary sequences of the Adelaide Fold Belt (AFB). The CFR hosts a number of mineable barite [Ba(Sr)S04] vein deposits. These veins show cross cutting relationships with the gently dipping and locally steep host sequences. Barite veins are generally 3m wide and 300 m long and are being mined at the Oraparinna Barite mine of the CFR, the oldest barite mine in Australia that commenced production since 1910 (Figure 1).

The mineralization at the mine is hosted by the siltstones of the Brachina Formation. It is widely believed that basinal brines provided much needed Ba for the barite vein formation which is reexamined in this study.

increase from median layer to border (Figure 4).

Host contribution for Ba: Out of the lithologies of the AFB, the siltstone shows high Ba concentrations of 546 ppm (25 analyses). The siltstone mixed with other lithologies host 72% of known barite deposits containing more than 2000 tonnes (production + estimated reserves).

Whole rock analyses show Ba concentrations of the hosts deplete from 883 ppm (5 samples) to 532 ppm (11 samples) from distant, non-mineralised areas to adjacent areas of the barite rich Oraparinna Barite mine. Presence of trace barite grains in the rock samples collected may increase whole rock Ba concentrations therefore, grain scale analyses were carried out using microprobe. K-feldspar and Na-feldspar grains in siltstones of Brachina Formation from non-mineralised areas away from the mine showed Ba concentrations of 0.4 wt% (24 samples) and 0.04 wt% (15 samples) respectively, which were decreased by 50% at the mine. The lost Ba is believed to have contributed towards vein formation.

Vein architecture: Barite veins in the CFR are generally unconnected to each other. The veins are commonly earthywhite and may contain a thin median layer, defined by slightly different properties than rest of the vein. Median layer may be colourless to light grey, exhibit a glassy amorphous texture with conchoidal fracturing having more hardness than rest of the vein. It may consist clasts of host rock inclusions. At the margins of the veins, there may also be millimetre to metre scale wide inclusions ripped off from the host rock (Figure 2). The veins exhibit some fibrous nature with which fibres continuing across the median layer towards borders, which was confirmed in microprobe studies (Figure 3). The Ba concentration increases from median layer to border while Sr concentration decreases, leaving high Sr concentrations along median layer (Figure 4). The 87Sr/86Sr isotope ratios

Ba migration: Detailed grain-mapping shows Ba concentrations within Kfeldspar grains of the Brachina Formation 11km NNW of the mine increase towards the grain's order closest to a barite grain being formed, as much as up to 1.18 wt% (Figure 5). This increase may not be explained by Ba migration from barite grains to feldspars grains due to strong covalent bonds in BaS04, and instead, it is suggested that there was Ba migration within feldspar grains towards barite

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

formed

grains being deformation.

similar lithologies instead of the previous understanding of the source of Ba from basement brines. On this basis, 87Sr/86Sr isotope ratios showed that the time span to form a vein of 2 m thickness was 3.4 Ma.

during

In conclusion, both structural and geochemical evidence suggest barite veins in the CFR have formed following the antitaxial mechanism of fibrous vein formation. It is suggested that the dominant source of Ba for the veins was from feldspars of host siltstones and

Acknowlegements: Dr Ross A Both & Prof Patrick R James (former staff, University of Adelaide)

QUO WA

Australia SA NSW

Great POWERED BvAuslrallan

VIC

"^ACT

G o c g l C data ©2011 - Terms of Use Latitude: SIMS'SrS Longitude: 13S'51'57"E

Figure 1. Location map of Oraparinna Barite Mine, CFR, South Australia Figure 2. Remnant of a mined Barite vein with host inclusions (Carey Hill - CFR). Width of remnant: Im.

Figure 3. Left: A barite fibre of 150 micron long from a minor vein. Right: A barite fibre of 500 micron long, grown continually across the median line defined by a trail of fluid inclusions (black dots, see along arrow)

Figure 4. X axis: Sample positions in veins, 1-median line, 2-between median line and border, and 3-adjacent to border Vs Sr wt% and 87Sr/86Sr isotope ratios.

Figure 5. K feldspar grains (K) showing Ba (lighter colours) migrate towards forming barite grain (B). Scale bar 20 microns.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Halmahera Island, Indonesia - Subduction Polarity Reversal, Complex Slab Interactions and Rapid Changes to Arc Extension S Micklethwaite - Centre for Exploration Targeting, School of Earth and Environment, University of Western Australia & CODES, School of Earth Sciences, The University of Tasmania, Hobart, TAS 7001. Australia.

Corresponding author email: steven.micklethwaite@uwa.edu.au The active island arc of Halmahera, is located in a unique tectonic setting at a polarity reversal between the subducting Molucca and Philippines Sea Plates. The majority of the active volcanic arc runs parallel with the strike of the underlying subducting Molucca Sea Plate but, in the northern arm of Halmahera, the arc diverges markedly towards the tip of the Philippines subduction zone. The surface geology of Halmahera also shows differences from north to south, with thrust faulting and folding of Oligocene to Miocene rocks in south and central Halmahera, but a northern arm dominated by Quaternary and Pliocene volcanic rocks and no obvious thrusting and folding. We examine the complex tectonic geometries and evolution of the region by integrating geochemical, isotopic and geophysical data. The locations of earthquake hypocentres, seismic tomography, bathymetry and digital elevation models were analysed using GIS and three-dimensional visualization software. It was found that seismic activity extends to 300 km, is concentrated in both slabs at the northern end of Halmahera and describes an unusual C-shaped geometry to the Philippines Sea Plate.

Geomorphological analysis of the northern arm confirms the presence of a large linear-drainage valley representing the surface expression of an active strike-slip fault. Halmahera's volcanic arc changes from parallelism with subduction of the Molucca Sea Plate, to parallelism with the strike-slip fault. In addition, arc-parallel mountain ranges in the northern arm are bound by active to recently active normal faults. The U-Pb zircon geochronology, trace element ratios and Sr-Nd-Pb

isotopic values were analysed for Pliocene rocks of the northern arm and compared to data from Quaternary volcanic rocks of the same latitude and data from other formations in south and central Halmahera. Nd isotope values indicate the mantle wedge beneath north Halmahera is depleted, and is consistent with a shift in mantle wedge composition between the Miocene and Quaternary.

Furthermore, Pliocene volcanic rocks were erupted and extension began, soon after convergence and thrusting of central and southern Halmahera. Taking these observations together, we interpret that Pliocene volcanism occurred when subduction of the Philippines Sea Plate initiated and interacted with the subducting Molucca Sea Plate. A megastepover formed between the two plates, representing a transtensional transform zone, with associated thinning of the lithosphere. Corner flow around the lateral tip of the descending Philippines Sea Plate has led to upwelling of Pacific-type mantle beneath north Halmahera.

Rapid Communication of magma to the surface is partially controlled by the presence of lithospheric-scale transform faults, while at depth the Philippines Sea Plate may be colliding with the Molucca Sea Plate and folding over. It is within this tectonic setting that a collection of the world's highest grade epithermal gold deposits formed, in the Gosowong goldfield. There also remains a pressing need to examine the seismic and related tsunamigenic hazard of the Halmahera region, given that large active faults are present in the north arm and a significant strain gradient is implied by the distribution of seismicity.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Subduction Polarity Reversal, Complex Slab Interactions and Rapid Changes to Arc Extension: Halmahera Island Arc, Indonesia S Micklethwaite - Centre for Exploration Targeting, School of Earth and Environment, University of Western Australia & CODES, School of Earth Sciences, The University of Tasmania, Hobart, TAS 7001. Australia Corresponding author email: steven.micklethwaite@uwa.edu.au The active island arc of Halmahera, is located in a unique tectonic setting at a polarity reversal between the subducting Molucca and Philippines Sea Plates. The majority of the active volcanic arc runs parallel with the strike of the underlying subducting Molucca Sea Plate but, in the northern arm of Halmahera, the arc diverges markedly towards the tip of the Philippines subduction zone. The surface geology of Halmahera also shows differences from north to south, with thrust faulting and folding of Oligocene to Miocene rocks in south and central Halmahera, but a northern arm dominated by Quaternary and Pliocene volcanic rocks and no obvious thrusting and folding. We examine the complex tectonic geometries and evolution of the region by integrating geochemical, isotopic and geophysical data. The locations of earthquake hypocentres, seismic tomography, bathymetry and digital elevation models were analysed using GIS and three-dimensional visualization software. It was found that seismic activity extends to 300 km, is concentrated in both slabs at the northern end of Halmahera and describes an unusual C-shaped geometry to the Philippines Sea Plate.

Geomorphological analysis of the northern arm confirms the presence of a large linear-drainage valley representing the surface expression of an active strike-slip fault. Halmahera's volcanic arc changes from parallelism with subduction of the Molucca Sea Plate, to parallelism with the strike-slip fault. In addition, arc-parallel mountain ranges in the northern arm are bound by active to recently active normal faults. The U-Pb zircon geochronology,

trace element ratios and Sr-Nd-Pb isotopic values were analysed for Pliocene rocks of the northern arm and compared to data from Quaternary volcanic rocks of the same latitude and data from other formations in south and central Halmahera. Nd isotope values indicate the mantle wedge beneath north Halmahera is depleted, and is consistent with a shift in mantle wedge composition between the Miocene and Quaternary. Furthermore, Pliocene volcanic rocks were erupted and extension began, soon after conver-gence and thrusting of central and southern Halmahera. Taking these observations together, we interpret that Pliocene volcanism occurred when subduction of the Philippines Sea Plate initiated and interacted with the subducting Molucca Sea Plate. A mega-stepover formed between the two plates, representing a transtensional transform zone, with associated thinning of the lithosphere.

Corner flow around the lateral tip of the descending Philippines Sea Plate has led to upwelling of Pacific-type mantle beneath north Halmahera.

Rapid Communication of magma to the surface is partially controlled by the presence of lithospheric-scale transform faults, while at depth the Philippines Sea Plate may be colliding with the Molucca Sea Plate and folding over. It is within this tectonic setting that a collection of the world's highest grade epithermal gold deposits formed, in the Gosowong goldfield. There also remains a pressing need to examine the seismic and related tsunamigenic hazard of the Halmahera region, given that large active faults are present in the north arm and a significant strain gradient is implied by the distribution of seismicity.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Resolving the Ross-Tyennan-Delamerian subduction paradox David H Moore - School of Geosciences, Monash University, Clayton VIC 3800 Peter G. Betts- School of Geosciences, Monash University, Clayton VIC 3800 Laurent Ailleres - School of Geosciences, Monash University, Clayton VIC 3800 Corresponding author email: david.moore@monash.edu The early Gondwanan margin of eastern Australia and Antarctica is poorly understood. In Antarctica, the timing of the Ross Orogen in North Victoria Land is typically given as about 540 Ma, whilst in Tasmania the age of the apparently coincidental Tyennan Orogeny is about 515 Ma and on the south eastern Australian mainland it ranges from about 510 Ma to 490 Ma. Even more paradoxical is the direction of subduction; in Antarctica and south eastern mainland Australia it was westdirected, whilst in Tasmania it was dominantly east-directed.

Crustal slices are also present in south eastern mainland Australia in the Stavely Belt-Dimboola Igneous Complex area of western Victoria and outboard of the Curnamona Craton in western New South Wales (Greenfield et al.

2011).

Another micro-continental ribbon has been suggested south of the Byrd Glacier in Antarctica (Stump et al

2006).

The presence of back-arc rifting and several discrete continental ribbons implies a similar tectonic setting at the end of the Rodinian breakup to that in the modern south west Pacific.

An inevitable consequence of this setting is that there would be different back-arc basin inversion events and, quite likely, different subduction directions.

The interpreted subduction zone in Tasmania west of the Tyennan Block was probably east-directed and was active outboard of the Gondwanan continental margin. Much closer to the Gondwanan margin, west-directed subduction took place in what is now south eastern mainland Australia, west of Tasmania and in Antarctica. These results show how combining structural geophysics with geological observations can provide a framework for resolving complex tectonic systems.

Integrating the latest magnetic and gravity compilations with field observations to examine the early Paleozoic relationships between the Australian mainland and Tasmania can resolve these inconsistencies. The data show clear evidence of rifting between eastern King Island and Tasmania, with rift failure at about 580 Ma. They also suggest the presence of another rift on the western side of King Island.

Furthermore, reinterpreting the geology along the north coast of Tasmania suggests the presence of other narrow crustal slices (continental ribbons) in western Tasmania west of the Tyennan Block, and perhaps also within the Arthur Lineament.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Constraints on the evolution of the Fila Costena Thrust Belt in western Panama using deposits associated with Pleistocene sector collapse of Volcan Baru Kristin D. Morell - The University of Melbourne, Melbourne, Victoria, Australia Thomas W. Gardner - Trinity University, San Antonio, TX, USA Donald M. Fisher - Penn State University, University Park, PA, USA Corresponding author email: kristin.morell@unimelb.edu.au We present new geologic, stratigraphic and geomorphic data along with age constraints to characterize Late Pleistocene to Holocene deposits associated with edifice failure and sector collapse of Volcan Barii (alt. 3 3 7 4 m) in westernmost Panama. This stratigraphic control provides constraints on the timing of thrusting associated with the southeastern termination of the Fila Costena thrust belt, located in the inner forearc of the Central American convergent margin.

The Fila Costena, which contains at many as five thrust faults that root at the basement-cover contact, is migrating to the southeast coeval with the Panama Triple Junction, along with the associated variations in subducting plate geometry that occur on either side of the triple junction. Radiocarbon dating of organic material, together with Ar/Ar ages of clasts, identify at least three debris avalanche events (DAI, DA2 and DA3) associated with Baru Volcano, located directly inboard of the triple junction.

10, 345 + / - 225 cal. YBP and 12, 845 + / " 55 cal. YBP. DA2 is the least exposed of the three debris avalanches, yet it is distinguished from other units by its brown soil color (7.5 YR), and also, its age. A radiocarbon date of wood within this deposit yields an age of 4 2 , 8 0 0 + / 5 0 0 YBP. The oldest deposit (DAI) contains a soil profile that appears significantly more developed than younger deposits and yields radiocarbon ages > 4 4 , 0 0 0 YBP. Ar/Ar dating of plagioclase feldspar crystals extracted from clasts within the DAI deposit, however, yield dates as young as 2 3 1 + / - 32 ka, constraining the lower age boundary.

These stratigraphic constraints yield new insights into the timing and evolution of the Fila Costena Thrust Belt given that nearly all of the thrusts displace Barii volcanic units. The ages and distribution of debris-avalanche deposits indicates active thrust sheet propagation and development of topography in the hanging wall of thrusts since deposition of DAI. The upper surface of DAI is offset by the rearmost thrust of the Fila Costena by as much as 200-m, and suggests a minimum uplift rate of '-0.7 m/ka since deposition of the DAI unit. Together, these analyses reveal a rich history of Quaternary volcanic collapse and deposition followed by uplift for the region surrounding Baru Volcano.

Preliminary volume estimates for the youngest deposit (DA3) range from a minimum of --30 km3 to a maximum of - 6 0 km3, a result that has led to the suggestion that it is the largest Holocene deposit of this type in the western hemisphere. Radiocarbon analysis of wood and other organic material extracted from this debris avalanche (n=3) yields ages that range between

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Geological Society ofAustralia, Abstract No. 102 SGTSG2012: Cause and Effects of Deformation in the Lithosphere Rapid tectonic mode switching recorded by porphyroblast-matrix relationships Mark A Munro - School of Earth & Environmental Sciences, James Cook University, Townsville, QLD 4811, Australia Simon W. Richards - School of Earth & Environmental Sciences, James Cook University, Townsville, QLD 4811, Australia Corresponding author email: mark.munro@my.jcu.edu.au "Rapid tectonic mode switching recorded by porphyroblast-matrix relationships"

Eastern structural domain of the fold belt during the mid-Ordovician to Silurian periods was generally associated with low (greenschist) regional metamorphic grades, but with localized zones of high-temperature, low-pressure (HTLP) metamorphism produced within this. These include the Cooma, Cambalong, Kuark and Jerangle Metamorphic Complexes, each of which generally preserve a more intricate structural history than the surrounding, colder, crust which composes the majority of the domain.

Alternating sub-horizontal and subvertical foliation development is a feature preserved within a number of orogens and metamorphic complexes world-wide. The fabrics are typically preserved as inclusion trails within porphyroblast or locally within the bulk matrix.

Inclusion trail orientations within porphyroblasts are often used to infer shortening directions during tectonic events but such interpretations should be made with consideration that such porphyroblasts may rotate in response to subsequent strain during younger deformation events. Where detailed examination of metamorphic rocks has been conducted at the microand meso-scales, successive orthogonality in foliations is interpreted as indicating episodic changes in the principal shortening direction throughout the region's tectonometamorphic history.

Evidently the structural evolution of these HTLP metamorphic complexes developed whilst the Eastern Lachlan Fold Belt constituted part of a marginal ocean basin, as represented by coeval, widespread deposition of deep-marine facies sediments and volcanics (Foster & Gray, 2000; Foster et al, 2009). Thus, owing to it's topographic profile existing well below sea level, the lithosphere possessed no gravitational potential energy, despite the influence of horizontal bulk shortening, consequently lacking the fundamental requirement for the occurrence of gravitational collapse. Moreover, the attainment of such high temperatures at low pressures (i.e. the character of metamorphism) is inconsistent with the applicability of gravitational collapse, being incompatible with a significant thickening of the crust, which generally gives rise to it.

Two principal models have been proposed for the origin of horizontal fabrics during orogenesis: orogenic collapse and horizontal bulk extension.

The former may be significant where bulk shortening generates lateral variations in gravitational potential energy between adjacent sections, following which the crust extrudes laterally to alleviate [Rey et al, 2001; Bell & Newman, 2006).

Alternatively, the repeated generation of horizontal fabrics and intervening vertical fabrics is thus attributed to the control of subduction zone geodynamics. The transient subduction of bathymetric highs such as seamounts

The Lachlan Fold Belt of S.E. Australia is an extensive palaeozoic subductionaccretionary orogen located on the Eastern margin of the Gondwana supercontinent.

Deformation in the

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Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere or oceanic plateaus temporarily disrupts long-term subduction operation (Collins, 2002). This results in fundamental changes in the stress field experienced within the over-riding plate. Accordingly, phases of extension are generally associated with slab hinge retreat while horizontal bulk shortening is linked to slab hinge advance (Lister & Forster, 2008).

mechanisms between the thermal perturbation of the system, resultant metamorphic reactions and the accommodation of deformation through pressure solution, recrystallisation and crystallographic re-orientation. These factors act in conjunction with each other to render the crust subsequently more deformable. Moreover, where abundant porphyroblasts are being nucleated, there is a significantly enhanced potential to record and preserve these events, preventing them being removed from the structural record via obliteration from the bulk matrix throughout later events.

Therefore, whilst the HTLP complexes may be deemed local perturbations within the regional context, these thermally-enhanced portions of the crust may preserve a more representative structural and metamorphic evolution of the terrane than is readily available elsewhere.

Absolute age brackets (Williams, 2001) for the deposition of the turbidite protoliths (ca.

450Ma) and emplacement of the Cooma Granodiorite pluton at the end of the Cooma Complex's structural history (435.2 ±6.3) constrain the deformation into a maximum ~21My time window.

Six discernable tectono-metamorphic events influenced the rocks of the complex during this time: three horizontal bulk shortening and three bulk extension.

This constrains the maximum mean duration of tectonic mode switching within the fold belt during this time as --3.5 My. This signifies the potential for subductioninduced tectonic mode switching to occur over far shorter intervals than is generally inferred, more commonly in the order of 6-lOMy (Collins, 2002).

References Bell, T. H. and Newman, R., 2006. Appalachian orogenesis: the role of repeated gravitational collapse. In: R. Butler and S. Mazzoli (Eds.), Styles of Continental Compression. Geol. Soc. Amer.

Spec. Papers, v.414, pp.95-118.

Collins, W. J., 2002.

Hot orogens, tectonic switching, and creation of continental crust.

Geology, 30, 535-538.

Although tectonic mode switching is recorded in many fold belts, evidence for extension is less common, or entirely absent. The Eastern Lachlan Fold Belt presents a prime example of this. Whilst both steep and flat-lying fabrics are manifested in the HTLP complexes, only a restricted number of the steep fabrics are developed throughout the majority of the fold belt.

The explanation for this derives from the contrast in relative thermal state evident between the complexes and their surroundings. Thermally softened, higher-temperature zones are more readily susceptible to strain accumulation, resulting in preferential strain localization within them.

Significantly elevated geothermal gradients resulting from regional-scale processes instigate positive feedback

Foster, D. A. & Gray, D. R., 2000. Evolution and structure of the Lachlan Fold Belt (Orogen] of Eastern Australia. Annual Review of Earth And Planetary Sciences, 28,47-80.

Foster, D. A., Gray, D. R., Spaggiari, C., Kamenov, G., & Bierlein, F. P., 2009. Palaeozoic Lachlan Orogen, Australia; accretion and construction of continental crust in a marginal ocean setting: isotopic evidence from the Cambrian metavolcanic rocks. Journal of the Geological Society of London, Spec. Pub, 318, 329-349.

Lister, G. & Forster, M., 2008. Tectonic mode switches and the nature of orogenesis. Lithos, 113,274-291.

Rey, P., Vanderhaeghe, 0., & Teyssier, C., 2001.

Gravitational Collapse of Continental Lithosphere: Definition, Regimes and Modes.

Tectonophysics, 342,435-449.

Williams, L S., 2001. Response of detrital zircon and monazite, and their U-Pb isotopic systems, to regional metamorphism and host-rock partial melting, Cooma Complex, southeastern Australia.

Australian Journal of Earth Sciences, 48, 557580.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Oroclines, rotations, and orogenic history of the Tasmanides Robert Musgrave - Geological Survey of NSW Corresponding author email: robert.musgrave@industry.nsw.gov.au The sensitivity of the palaeoremanence "compass" to vertical axis rotations gives palaeomagnetism great power in recognising and quantifying both oroclines and shear rotations. Within the Tasmanides, however, palaeomagnetic analysis of rotations has been limited to studies of the New England oroclines. Controversy that has attended even this relatively straightforward case (e.g. Klootwijk, 2003, 2009 and Cawood et al, 2011) reflects a wider uncertainty about Palaeozoic palaeomagnetism in the Tasmanides, which has centred on disputes between contending apparent polar wander paths (see McElhinny et al., 2003), and which has discouraged applications of palaeomagnetism to interpretation of the internal tectonics of this orogenic system.

Restoring the Silurian rotations does not resolve the issue of the large gap (60°) between the Snowy River Volcanics pole at 405 Ma and the Gondwana APWP for the late Ordovician: indeed, restoring the Silurian oroclinal rotation pulls Ordovician poles from the MolongMonaro terrane further away from the Gondwana path.

Devonian to Carboniferous poles from eastern and northern Australia - the "SLP path" of Schmidt et al. (1990) - roughly describe another small circle about a rotation pole in southern Australia, representing about 30-40° of anticlockwise motion (time-forward). This rotation would restore both the SRV pole and the Ordovician Molong-Monaro poles to positions corresponding to a relatively simple path linking the well-defined Cambro-Orodovician and Permian segments of the Gondwana APWP.

A new model for the Silurian-Early Devonian tectonics of the Delamerian and Lachlan orogens invokes a substantial orocline, folding the Stawell Zone around the Hay-Booligal Zone, and rotating the southern part of the Macquarie Arc (Cayley, 2010; Cayley, Musgrave & Price, in preparation).

Support for Silurian clockwise rotation within the Lachlan Orogen comes from the set of pre-400 Ma poles from the Molong-Monaro terrane identified by McElhinny et al. (2003) as allochthonous (but proximal to the craton), which are restored close to the 400 Ma pole (the Snowy River Volcanics pole) by anticlockwise rotations of up to 90° around an Euler pole in NSW (Figure 1).

Silurian rotation in the Molong-Monaro terrane appears to have involved smaller-scale oroclines and rotations accompanying distributed shear, visible today as arcuate structures in deeply sourced magnetic anomalies and in cross-cutting fault sets.

If this anticlockwise rotation corresponds to a real tectonic event, what could that be? The Carboniferous timing suggests that it corresponds to the terminal stages of the Alice Springs Orogeny, and the geographic span of poles involved suggests a block rotation of all of Australia east of the Flinders Ranges and north of the Amadeus Basin.

Li & Evans (2011) have proposed a similar, 40° anticlockwise rotation of northern Australia relative to the southern and western Australian cratons to explain similar offsets between corresponding pairs of Proterozoic poles, but they associate this with the Petermann Orogeny. It may be that most of the rotation they recognised was actually accounted for by a similar transpressional convergence between a mobile block comprising northern and eastern Australia, and cratonic southern and

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western Australia as part of Gondwana.

This would reduce the amount of shortening or crustal extrusion needed in the Petermann Orogeny, but would add a substantial element of shortening to the Alice Springs Orogeny. Correction for a 30° anticlockwise rotation in the Carboniferous reorients the Koonenberry (Mount Wright Arc) and Stavely belts to a north-south south alignment, parallel to other elements of the Ross-Delamerian orogen.

Klootwijk, C.T., 2003.

Carboniferous palaeomagnetism of the Werrie Block, northwestern Tamworth Belt, and the New England pole path. Aust. J. Earth Sci., 50, 865902.

Klootwijk, C., 2009. Sedimentary basins of eastern Australia: paleomagnetic constraints on geodynamic evolution in a global context. Aust. J. Earth Sci, 56, 273-308.

Li, Z-X, & Evans, D.A.D., 2011. Late Neoproterozoic 40° intraplate rotation within Australia allows for a tighter-fitting and longerlasting Rodinia. Geology, 39, 39-42.

References

Cawood, P.A., Pisarevsky, S.A., & Leitch, 2011.

Unraveling the New England orocline, east Gondwana accretionaiy margin. Tectonics, 30, TC5002, doi:10.1029/2011TC002864.

McElhinny, M.W., Powell, C.McA., & Pisarevsky, S.A., 2003. Paleozoic terranes of eastern Australia and the drift history of Gondwana.

Tectonophysics, 362,41-65.

Cayley, R., 2010. South-directed monoclinal folding in the Lachlan Fold Belt: unravelling the Mid-Late Silurian fold belt assembly to solve apparent Ordovician-early Silurian complexity.

20th AESC, GeoL Soc. Australia Abstracts, 98, 5859.

Schmidt, P.W., Powell, C.McA., Li, Z.X., & Thrupp, G.A., 1990. reliability of Palaeozoic palaeomagnetic poles and APWP of Gondwanaland. Tectonophysics, 184, 87-100.

n

A

/ Veil

V I-

Figure 1: Australian Palaeozoic poles and APWP, from McElhinny et al. (2003). Grey arc illustrates Silurian clockwise oroclinal rotation of Macquarie Arc around local vertical axis.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere South America goes straight - unbending the Bolivian orocline T.P. O'Kane - Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia S. Hart - Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia G.S. Lister - Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia Corresponding author email: tomas.okane@anu.edu.au There is evidence, from both geodesy and seismology, that laterally variable vertical axis rotations take place in convergent zones. The sense of rotation can be different on either side of arcuate pinning points, for example as is the case today across the Arica Bend in the Central Andes, South America. Such variable vertical axis rotations also appear to have occurred in the past, and the magnitude of these rotations can be ascertained by obtaining spatially and temporally distributed palaeomagnetic data. Palaeomagnetic data in this case illustrate that the bend was, at least in part, produced by diverging rotations of the limbs (--40° counterclockwise north of the bend and --30° clockwise south of it).

another forming triangular mesh faces.

Motion data can be added to individual nodes while the node-connecting springs can be kept rigid or be used to distribute stress and strain among the mesh nodes. This deformation then applies to any data carried by the mesh.

For this reconstruction of the Andean Margin of continental South America we utilize the meshes to carry the NOAA ET0P02 data, and apply simple isostatic balance equations in order to monitor predicted variations in crustal thickness. We calculate the pattern of stress and strain required by the boundary constraints, and changes in surface elevation required by isostatic balance.

The simulations suggest the prior existence of intermontane basins, and that the Bolivian Orocline had attained its present geometry by the midMiocene. In deconstructing the orocline it appears that the palaeomagnetic data cannot be completely explained by simple bending alone. In situ block rotations may have contributed to observed vertical axis rotations.

Here we use the Pplates tectonic reconstruction software to assimilate available information regarding the magnitude and age of tectonic shortening, and reverse engineer the formation of the Bolivian Orocline from the mid-Eocene to Present. Pplates uses deformable and tearable meshes (irregularly tessellated polyhedrons) consisting of nodes connected to one

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The role of serpentinization in fault stress and long-term deformation in the JavaSumatra subduction system.

Craig O'Neill - Macquarie University Juan Carlos Afonso - Macquarie University Corresponding author email: craig.oneill@mq.edu.au Subduction in the Sunda-Java trench, driven by the northward progression of the Australian plate, exhibits marked contrasts in seismicity and gravity over the length of this active system. Whilst the Sumatran length of the trench exhibits high background seismicity and the ability to generate megathrust (M>8) earthquakes, the Javan trench exhibits comparatively low seismicity. Previous work has attributed the difference in gravity signatures between the two arc segments to intruding asthenosphere under the Javan overriding plate and reduced length of the seismic coupling zone.

Other differences between these two trench segments include subduction of topographic features such as the Roo rise (eastern Java), and the Investigator Fracture zone and Wharton Ridge (Sumatra). The age of the subducting lithosphere varies from 40Ma (extinct Wharton spreading ridge, northern Sumatra), to over ISOMa east of Java.

The serpentinization of peridotite into hydrous phases can drastically affect the rheology of fault systems, and play an important part in the long-term evolution of subduction zones. Here we explore the extent to which the gravity signature of the Sumatra-Java subduction system is affected by serpentinization, and assess the importance of this in governing longtimescale geodynamic responses. We use LithMod - an integrated code which solves the thermodynamic relations for stable mineral assemblages, and simultaneously models the geophysical response of these systems, to calculate serpentinite pervasiveness in subducting material. We couple these constraints with thin-shell finiteelement models of the plate system to assess the effect weak serpentinite fault rheology has on plate kinematics and seismicity.

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Brecciation in mineralising systems Alison Ord - The University of Western Australia Bruce Hobbs - The University of Western Australia; CSIRO Daniel Lester' CSIRO Corresponding author email: alison.ord@uwa.edu.au Brecciation is common, in many hydrothermal mineralising systems.

Typical are the spectrum of lOCG deposits (Hunt et al. , 2007) and orogenic gold deposits (Oreskes and Einaudi, 1990). This paper treats hydrothermal mineralising systems as open flow chemical reactors and examines the progressive development of such systems from inception to final quenching.

Such systems are maintained far from equilibrium as long as the input of heat and mass is maintained but are forced to proceed through a number of stages as either (i) reactants within the system are exhausted and/or (ii) as the input flow conditions (temperature and fluid composition) evolve. Although the transition from one stage to another is transient the system ultimately adjusts to a steady state once a new stage in development is reached. The principles involved in defining these steady states are outlined and depend primarily on whether the processes involved are exothermic or endothermic but are also very sensitive to the local permeability structure. Alteration such as the development of hydrous phases, carbonates and iron oxides are exothermic processes whilst the precipitation of sulphides and silicates is commonly endothermic. An additional highly exothermic process is brecciation (Wu et al., 2006a,b).

Exothermic processes (such as alteration) are important since the rates involved are thermally activated and once they begin they are self-enhancing.

Endothermic processes (such as mineralisation) tend to quench the system.

A fundamental process in open flow controlled systems is the maintenance

and evolution of permeability. Darcys Law for fluid flow in flow controlled systems says that the fluid pressure gradient is very sensitive to changes in fluid density and fluid viscosity. Thus in flow controlled systems any change in fluid temperature or composition is reflected in a mandatory and rapid change in permeability. This paper explores the development of breccias as a means of achieving these changes in permeability.

In lithostatically pressured systems subjected to regional stress regimes (such as orogenic gold and lOCG systems) the controls on permeability are complicated. If the fluid pressure gradient is close to hydrostatic as in most hydrothermal systems near to the surface of the Earth, and the fluid flow is driven by gradients in hydraulic potentials arising from topography or buoyancy then the presence of a relatively impermeable layer in the pathway of the fluid flow results in ponding of the fluid until some new pathway is developed. This is the process involved in the trapping of oil and gas in hydrocarbon systems.

However if the fluid flux is fixed, as in fluids released by a devolatilising reaction front and the fluid pressure is near to lithostatic then the permeability must adjust to accommodate the imposed flux. In stressed material this can lead to fracturing, brecciation or the opening of vein systems. Ponding in such systems can only be transient and last until the permeability adjusts.

Thus it is of fundamental importance to distinguish between open flow systems that are flow controlled (constant flux) and those that are hydraulic-potential

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere References

controlled (constant hydraulic potential). The evolution of these two types of flow systems is quite different.

The flow-controlled systems (Mikula et al. , 2007) must adjust their permeability structure (and hence porosity structure; (Ord et al., 2007)) to account for changes in the physical characteristics (density, viscosity) of the fluid. This can promote widespread fracturing, veining and brecciation. The hydraulic head-controlled systems must adjust their permeability structure to account for clogging of their porosity by new minerals if the system is to continue to operate and such adjustments may be solely chemical in nature.

Hunt JA, Baker T, Thorkelson DJ. A review of iron oxide copper-gold deposits, with focus on the Wernecke Breccias, Yukon, Canada, as an example of a non-magmatic end member and implications for lOCG genesis and classification.

Exploration and Mining Geology. 2007;16:20932.

Mikula S, Sheldon H, Ord A, Hobbs B. Particle modelling of brecciation. In: Andrew CJea, editor.

Proceedings of the Ninth Biennial SGA Meeting.

Dublin2007. p. 1331-4.

Ord A, Hobbs B, Mikula S, Malcolm J, Sheldon H. Veins and breccias.

In: Blenkinsop TG, Duckworth R, editors. Breccia Symposium 2007. p. 24.

Oreskes N, Einaudi MT. Origin of rare earth element-enriched hematite breccias at the Olympic Dam Cu-U-Au-Ag deposit, Roxby Downs, South Australia. Economic Geology. 1990;85:1-

W e explore the successive development of minearalising systems such as Olympic Dam and the Yilgarn gold deposits within the above nonequilibrium framework for open flow systems and emphasise the fundamental thermodynamic role that brecciation plays in the overall development of these mineralising systems.

28.

Wu L, Liu S, Wu Y, Wang C. Precursors for rock fracturing and failure—Part I: IRR image abnormalities. International Journal of Rock Mechanics and Mining Sciences. 2006a;43:47382.

Wu L, Liu S, Wu Y, Wang G. Precursors for rock fracturing and failure—Part II: IRR T-Gurve abnormalities. International Journal of Rock Mechanics and Mining Sciences. 2006b;43:48393.

100

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Crustal evolution of some Peri-Tyrrhenian blocks deduced from Lu-Hf and U-Pb isotopic studies on single zircon grains.

Pavanetto Pamela - University ofCagliari (Italy) Matteini Massimo - University of Brasilia Funedda Antonio - University ofCagliari (Italy) Loi Alfredo - University ofCagliari (Italy) Corresponding author email: p.pavanetto@unica.it The reconstruction of peri-Tyrrhenian terranes in the Mediterranean area, and the relationships between SardiniaCorsica block (SCB) and CalabrianPeloritain Arc (CPA), have fascinated geologists in the last decades. No choral draft has been reached about the relative movements of these terranes and their Pre-Variscan linkages.

the Tyrrhenian Sea Shimabukuro, 2009).

(Alvarez

&

The SA-ALB sample represents the Cuccuru 'e Flores Conglomerate (CFC) in Eastern Sardinia, the CO-SOL sample represents the Solaro Flysch (SF) in the Eastern Corsica; CA-PAL sample represents the Stilo Capo d'Orlando Fm. (SCOF) part of the Stilo Unit in the Southern Calabria (Aspromonte subregion); the CAV-05 sample represents the Paludi Fm (PF) in the Northern Calabria (Sila subregion).

These deposits represent the early stage of the tectonic event developed in the Tyrrhenian region during late Oligocene-Lower Miocene in a broader regional tectonic context dominated by the convergence of S-Europe and Adria plate(?) (Oggiano etal., 2009).

In order to contribute to better understand the pre-Variscan evolution of southern Europe, combined U-Pb and Lu-Hf analyses on zircons from tertiary detrital sediments sampling of CPA and SCB have been performed.

The SCB and CPA are considered belonging to a branch of Southern Variscan Realm and separated each others during the Miocene with the opening of Southern Tyrrhenian Basin.

During this period the CPA moved southeastward, with respect to the SCB, driven by a progressive roll-back of the subducted slab.

Combined U-Pb and Lu-Hf analyses on detrital zircons were performed using a Thermo-Fisher Neptune MC-ICP-MS coupled with a Nd:YAG UP213 New Wave laser ablation system, at the Laboratory of Geochronology of the University of Brasilia. The analyses have been carried out following the analytical procedures described by Biihn et al.

(2009) and Matteini et al. (2010).

However is still ambiguous if the CPA in the early time was formed by an amalgamation of two or more continental "terranes" that collided during the Tertiary or was a single terrane during the middle and late Tertiary. The "two-terrane model" considers the CPA made up by two terranes: a northern terrane (labelled Sila), in continuity with SCB, and a southern terrane (Serre, Aspromonte, Peloritani) subjected to different evolution before his amalgamation with the Sila, shortly before the opening of

Sample SA-ALB shows an important Panafrican African zircon populations (n=28) with peaks at 670 and 850 Ma and a Grenville population (n=18) with a peak at --1015 Ma. Scattered paleoProterozoic and Archean zircons are also present. Panafrican zircons (£Hf from +10 to -16 and TDM from 0.9 to

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Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere 2.8 Ga) and Grenville zircons [zHf from +14 to -19 and TDM from 1.0 to 2.8 Ga) have both juvenile and crustal signature.

Ga) Grenvillian zircons (eHf from +4.4 to -11.7 and TDM from 1.5 to 2.4 Ga) have both juvenile and crustal signature.

Sample CO-SOL shows a Variscan population (n=27) with a peak at - 3 4 0 Ma, a main Cambro-Ordovician population (n=10) with a peak at --490 Ma, and minor "Grenville" and Archean populations.

Variscan, CambroOrdovician and Grenville zircons (eHf from +6 to -30 and TDM from 0.9 to 3.0 Ga) show mainly crustal isotopic signature with some grain having juvenile characteristics.

Neoarchean zircons have strong juvenile signature (£Hf from +8 to +6 and TDM of --2.7 Ga).

The main difference between the two blocks is about the Grenvillian data. This age were recognized only on zircons from the SCB samples and from the Northern sector of CPA. W e suggest that the Southern sector of CPA, lacking of Grenville input, during the Paleozoic time did not occupy the same position of the others blocks, probably was part of Cadomian terranes, classically characterized by the lacking of Grenvillian age (Kober et al., 2004). The SCB together with the Northern sector of CPA was direct linked to Grenvillian sources as demonstrated by the juvenile isotopic signature. For the negative £Hf values w e have to take in account or crustal reworking or a mixing between juvenile and crustal components. The provenence of Grenvillian zircon could be linked to Amazonian Craton (Friedl et al., 2000) or marginally to the North Africa sources (Avigad, et al. 2011) even if here the Grenvillian basement rocks are usually not exposed.

Sample CA-PAL shows an important late Neoproterozoic-early Cambrian population (n=45) with the main peaks at - 6 5 0 and at - 530 Ma and a minor Variscan population (n=5) with a peak at - 3 5 0 Ma. Scattered Neoarchean zircons are also present. Neoproterozoic and Cambrian zircons (eHf from +1.5 to - 3 and TDM from 1.3 to 1.5 Ga) have mainly a crustal isotopic signature.

Carboniferous zircons (sHf values from -2.2 to 0.3 and TDM from - 1 . 1 to 1.3 Ga) have both juvenile and crustal isotopic signature. A Neoarchean zircon analyzed ( e H f - 5.8 and TDM 3.2 Ga) has a crustal signature.

References Alvarez & Shimabukuro, 2009. Italian Journal of Geosciences, 128.

Avigad, Gerdes, Morag & Bechstadt, Gondwana Research (in press).

Sample CAV-05 shows an important Variscan population (n= 24) with a main peak at 290 Ma, a secondary Panafrican zircon population [n= 23) with peaks at - 5 0 0 Ma, 650 and 800 Ma and a Grenville population (n=6) with a peak at -1000 Ma. Scattered Paleoproterozoic and Archean zircons are also present. Variscan zircons (sHf from -1.4 to - 35 and TDM from 1.3 to 3.2 Ga) show a strong crustal isotopic signature. A Panafrican zircons (eHf from +8 to -2 and TDM from 1.1 to 1.4

2011.

Buhn, Pimentel, Matteini & Dantas, 2009. An Acad. Bras. Cienc. 81.

Friedl, Finger, McNaughton & Fletcher, 2000.

Geology 28.

Kober, Kalt, Hanel & Fidgeon, 2004.

Contributions to Mineralogy and Petrology 147.

Matteini, Junges, Dantas, Pimentel & Buhn, 2010.

Gondwana Research, 17.

Oggiano, Funedda, Carmignani & Pasci, 2009.

Boll.Soc.Geol.It., 128.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere The recognition of multiple magmatic events and pre-existing deformation zones in metamorphic rocks as illustrated by CL signatures and numerical modelling: Examples from the Ballachulish contact aureole, Scotland Sandra Piazolo - GEMOCARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Australia Helena Bergman - Department of Geological Sciences, Stockholm University, Sv^eden Corresponding author email: sandra.piazolo@mq.edu.au The combination of T-t modelling, numerical modelling of the microstructural evolution and detailed CL analysis allows us to derive the detailed time-resolved evolution of a contact aureole in terms of temperatures, activity of processes at the microscale, fluid influx and magmatic events. In particular, the Appin Quartzite from the Ballachulish aureole (Scotiand) shows evidence of two distinct magmatic events and associated fluid infiltration.

Three microstructurally and spatially defined regions can be identified. Region A, 0400 m of contact (663°C < Tmax < 714°C), is characterized by a three-stage grain boundary migration (GBM) evolution coupled with fluid 1 and 11 associated with magma event 1 and 2, respectively, and late stage rest fluids (fluid 111). Region B, 400-700 m of contact (630°C < Tmax < 663°C), carries evidence of one GBM stage only, inferred to be associated with magma event 2. In this region indications of high-strain zones are observed. Region C, >700 m of contact (Tmax < 630°C), is characterized by brittle fracturing since temperatures were too low for any significant GBM to occur.

different signatures of strain induced GBM and grain growth to determine whether or not a rock was highly deformed before the annealing stage.

Grain growth produces straight or slightly curved lines closely associated with the present-day boundaries (Fig. l a ) , while strain induced GBM results in wavy bands that often fill up the interior of the grains (Fig. l b ) .

Figure 1: Examples of CL signatures; scale bar is lOOjim; a) microstructure produced by grain growth showing alternating, slightly bent to straight, bright and dark lines commonly which accompany the present-day grain boundaries (white dotted lines); b) microstructure produced by strain induced grain boundary migration showing within the whole grain linear to slightly curved, highly luminescent elongate areas (black arrow) in the centre of wavy thin lines (white arrow); black dotted lines show present day grain boundaries.

In our samples grain boundary migration is initiated at c. 620°C. In medium grained, initially largely anhydrous rocks, grain boundary migration only causes significant grain size increase and development of equilibrium microstructures such as 120° triple junctions if temperatures are 620°C or higher for more than 5 million years. In the studied area the grain boundary network approached energy equilibrium only if rocks were for 5-60 ka above 620°C.

Importantly, our study shows that careful CL analysis can help to "seethrough" high temperature events within metamorphic terrains.

We suggest that alternating bright and dark banding and bright rims can form when grain boundary rates are fast and slow relative to CL activator replenishment rates, respectively. It is possible to study a now completely annealed microstructure and use the significantly

Temperature estimates using the Tigeothermometer, TitaniQ, agree with

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the rocks at the time of quartz formation, necessitate a careful evaluation of data obtained.

those derived from T-t modelling and previous studies. Analytical limitations coupled with re-equilibration issues and difficulties to determine Ti activity in

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Tectonic Evolution of the Andes in the Rio Blanco-Los Bronces district, Central Chile: controls on stratigraphy, magmatism and mineralization Jose Piquer - Centre of Excellence in Ore Deposits, University of Tasmania Jorge Skarmeta - Gerencia de Exploraciones, CODELCO-Chile Corresponding author email: Jose.PiquerRomo@utas.edu.au Introduction: The high Andes of central Chile and Argentina (32-35°S) can be divided into two major geological domains. The eastern domain exposed close to and to the east of the international border, is composed of strongly deformed marine and continental sedimentary rocks of Jurassic to Early Cretaceous age which constitutes the Aconcagua fold and trust belt (Ramos, 1996). The western (Chilean) domain is composed of volcanic rocks of Eocene to Pliocene age which were erupted during the evolution and inversion of an intra-arc basin. They have been grouped in the syn-extensional Abanico Formation and the syn-inversion Farellones Formation (Charrier et al., 2002 and references therein). Our study has focused on the evolution of the western domain, with an emphasis on the district of the giant Rio Blanco-Los Bronces porphyry CuMo cluster. We have completed four structural E-W cross-sections across the district, with the southernmost one passing through the mineral deposits.

They provide the basis for our new model for the tectonic evolution of this part of the Andes, which aims to clarify the first-order controls on stratigraphic changes, magmatic activity and associated mineralization.

sedimentary facies, both factors indicative of the presence of various sub-basins and depocenters, which are bounded by NNW and NE-oriented internal normal faults. The ascent of magma to the surface was favoured by the existence of several deep-tapping extensional structures. From 37 to 22 Ma as much as 5 km of volcanic rocks were deposited in the basin, with no coeval intrusive bodies recognized.

Tectonic inversion and plutonism since the Lower Miocene: During this period, the high angle (60-65°) NNW and Ntrending faults were reactivated in reverse mode, folding the rocks of the Abanico Formation nearby. This implies that, before their movement, supralithostatic pressures were achieved.

Abundant sills of Miocene age crop out in the study area, evidencing the presence of dilatational sub-horizontal lenses associated with this stress field.

In this compressional tectonic regime, NE-trending faults were reactivated mainly as dextral strike-slip faults, with variable although generally minor dipslip reverse movements. This selective reactivation of pre-existing normal faults with different orientations has produced the present-day structural architecture, whereby sub-basins are bounded by high-angle faults, each one with its own thickness of local volcanosedimentary facies, intensity of folding and exhumation level. By correlating data from the Argentinean flank of the Andes (Ramos, 1996; Giambiagi, 2003) with earthquakes hypocenters and the inferred location of Mesozoic evaporites, we propose the existence of three main Miocene detachment levels beneath the Tertiary volcanic rocks.

Tectonic inversion was coeval with the

Upper Eocene Lower Miocene extension: This period is associated with the development of an intra-arc volcanotectonic basin. The main basinmargin normal faults (Pocuro and Alto del Juncal - El Fierro faults, fig. 1) are Noriented, and the area within them was completely covered by the products of the Abanico Formation, but our crosssections show strong changes in thickness (from 2 to 5 km) and volcano-

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere deposition of the Farellones Formation, which differs markedly from the Abanico Formation in that it is restricted to specific volcanic centres and reaches a maximum thickness of only 1.5 km. The basal units of the Farellones Formation were deposited in progressive unconformities over the Abanico Formation, and have been dated at 22.7±0.4 Ma (U-Pb SHRIMP age; Piquer, 2010). Plutonic activity was contemporaneous with Farellones Formation volcanism.

The main intrusive complex in the area, the Rio Blanco-San Francisco Batholith, was emplaced between 20.1 and 3.9 Ma (Deckart et al., 2005). The units dated between 20.1 and 8.4 Ma are coarse equigranular plutonic rocks, while those with ages between 6.3 and 3.9 Ma are subvolcanic rocks directly associated with hydrothermal activity and mineralisation. The host rocks of these subvolcanic complexes are the older equigranular plutons. This implies that between 8.4 and 6.3 Ma this area was subject to a violent exhumation event, unroofing the older, plutonic rocks and exposing them to the subvolcanic environment, with porphyries and breccias being fed by a deeper, unexposed magma chamber. Given the characteristics and erosion level of the Rio Blanco deposit, this magma chamber is inferred to have been localized between 5-7 km below the present surface (e.g., Proffett, 2009; Sillitoe, 2010). This depth coincides well with the uppermost of the three detachment levels and also with a notable area of low Vp/Vs in seismic tomography (fig. 1), which w e speculate

correlates with the very young (<4 Ma) crystalline rocks of the deep magma chamber that solidified after volatile exsolution and formation of the Rio Blanco-Los Bronces deposit. Intrusive contacts, porphyry dikes, hydrothermal breccias and mineralized veins, all show clear NNW and NE preferred orientations, indicating that pre-existing normal faults inherited from the extensional period channelled the ascent and emplacement of magma and hydrothermal fluids during the compressive stage.

References Charrier, R., 0. Baeza, S. Elgueta, J. J. Flynn, P. Cans, S. M. Kay, N. Munoz, A. R. Wyss, and E. Zurita (2002), Evidence for Cenozoic extensional basin development and tectonic inversion south of the flat-slab segment, southern Central Andes, Chile (33 degrees-36 degrees SL), Journal of South American Earth Sciences, 15(1), 117-139.

Deckart, K., A. H. Clark, C. Aguilar, R. Vargas, A. Bertens, J. K. Mortensen, and M. Fanning (2005), Magmatic and hydrothermal chronology of the giant Rio Blanco porphyry copper deposit, central Chile: Implications of an integrated U-Pb and Ar-40/Ar-39 database. Economic Geology, 100(5), 905-934.

Giambiagi, L. B., V. A. Ramos, E. Godoy, P. P. Alvarez, and S. Orts (2003), Cenozoic deformation and tectonic style of the Andes, between 33 degrees and 34 degrees south latitude. Tectonics, 22(4).

Piquer, J. (2010), Geologia del Distrito Andina, escala 1:25000, edited, CODELCO-EMSA. Ramos, V. A. (1996), Evolucion Tectonica de la Alta Cordillera de San Juan y Mendoza, in Geologia de la Region del Aconcagua, edited by V. A. Ramos, pp. 447-460, Subsecretaria de Mineria de la Nacion.

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Figure 1: bedding planes and intrusive contacts, faults, Vp/Vs tomography and distribution ofhypocentres in the cross-section through the Rio Blanco-Los Bronces (RB-LB) deposit

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Putting together pieces of a jigsaw puzzle: the nature and fit of the main tectonic domains using age and provenance studies of charnockites and metasediments from across the Southern Granulite Terrane (South India) Diana Plavsa - Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Science, University of Adelaide Alan S Collins - Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Science, University of Adelaide Justin Payne - Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Science, University of Adelaide John Foden - Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Science, University of Adelaide Chris Clark - The Institute for Geoscience Research (TIGER), Dept of Applied Geology, Curtin University of Technology M. Santosh - Kochi University, Japan Corresponding author email: diana.plavsa@adelaide,edu.au Structurally placed at an orogenic triple junction, the Southern Granulite Terrane (SGT) of India plays an important role in unravelling the reconstruction of the amalgamation of the supercontinent Gondwana. The tectonic domains making up the Southern Granulite Terrane have been subdivided based on the age, isotopic character and structural evidence. From the north and towards the south, these include the Salem Block, the Madurai Block and the Trivandrum Block. The main suture representing the closure of the Mozambique Ocean, is believed to be the Palghat-Cauvery shear system (PCSS), between the Salem and Madurai Blocks. The contact between the Madurai and Trivandrum Blocks (named the Achankovil Shear Zone) is believed to be of a structural nature.

However, the true nature of the contacts between the above mentioned tectonic domains remain controversial to this day. The most controversial and most debated of all is the isotopic boundary

within the Madurai Block defined in the literature as the Karur-KimbamPainavu-Trichur (KKPT) shear zone.

This boundary marks a transition between dominantly Archaean igneous province to the north-west of the KKPT zone and largely Mesoproterozoic metasedimentary sequences to the south-east.

To better understand the nature of the contacts between the different tectonic domains, we present the U-Pb geochronology and Hf-isotopic analysis of zircons and provenance studies from a range of rock types including orthopyroxene-bearing gneisses (charnockites) and granulite-facies metasedimentary rocks from across the Southern Granulite Terrane. This study brings us one step closer to putting the pieces of a jigsaw puzzle together and understanding the role each tectonic domain played in the amalgamation of Gondwana.

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Geological Society of Australia, Abstract No. 102 SGTSG2012: Cause and Effects of Deformation in the Lithosphere A TALE OF T W O OROGENIES Wolfgang V. Preiss - Geological Survey of South Australia, PIRSA Corresponding author email: Wolfgang.Preiss@sa.gov.au T w o orogenies more than a billion apart are parts of much larger orogenic systems that extended far beyond their remnants now exposed in South Australia. The 1.6 Ga Olarian and 0.5 Ga Delamerian Orogenies both verged onto western cratonic forelands and involved polyphase deformation, under high-T low-P metamorphic conditions, of rifted sedimentary basins with long depositional histories before onset of orogenesis.

Overall NW-directed tectonic transport in both resulted from far-field effects of distant plateboundary interactions.

of Olarian orogenesis runs across Yorke Peninsula to northern Eyre Peninsula, where an east-dipping thrust cut the Corunna Conglomerate (Gawler Craton seismic transect 08GA-G1: Fraser et al., 2010) before extrusion of the 1.59 Ga Gawler Range Volcanics. This may be the westernmost foreland thrust of the Olarian Orogen.

Within the Curnamona Province, earliest metamorphism (monazite growth at 1.62 Ga: Forbes et al., 2007), allows 20 m.y. for deposition, above the outcropping Willyama Supergroup, of a thick, ?pelitic blanket as imaged on the N-S Curnamona seismic transect 08GAC1 (Korsch et al., 2010). This insulating cover allowed for metamorphism of the Willyama Supergroup under a high geothermal gradient, reaching granulite fades in the SE of the Curnamona Province. There is little consistency in identification and correlation of deformation events in the Willyama Supergroup.

The intensity of deformation and orientation of structural elements are highly variable, due to both primary sheath morphology and fold interference (e.g. Forbes and Betts, 2004). Deformation was probably continuous and progressive under an overall NW-directed stress regime.

Stretching lineations such as those measured by Clarke et al. (1986) may provide some evidence for the overall direction, if not sense, of tectonic transport. Earliest deformation and heating produced bedding-parallel foliation and strata-bound leucosomes that were deformed by thrusts and isoclinal, recumbent and sheath-style folds, greatly thickening the sedimentary pile. Later more upright folds produced the arcuate Olarian trends. Grade varies from greenschist facies in the north to granulite in the SE,

First recognised in the 1.72-1.64 Ga Willyama Supergroup (Conor and Preiss, 2008), the Olarian Orogeny was also felt outside the Curnamona Province. Before Neoproterozoic rifting and sedimentation in the Adelaide Geosyncline, the Curnamona Province and Gawler Craton were parts of a single, continuous Precambrian shield.

The Olarian Orogen extends SW from the Curnamona Province, forming the basement under the southern part of the Adelaide Geosyncline; 1.6 Ga highgrade metamorphism is recorded in deep crustal xenoliths in the Calcutteroo kimberlite (Chen et al., 1994) and in basement inliers of the Mount Lofty Ranges (Preiss, 1993; Szpunar et al., 2007). 1.6 Ga orogenesis is also recorded in the N W Gawler Craton and in N W Queensland (Isan Orogeny).

In the southern Gawler Craton, the variably deformed and metamorphosed 1.75 Ga Wallaroo Group of northern Yorke Peninsula was intruded by syntectonic to post-tectonicl.60-1.58 Ga granites (Conor et al., 2010). Gneisses on southern Yorke Peninsula show no evidence of orogenesis <1.85 Ga (Reid and Hand, 2008). Thus the western limit

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Neoproterozoic corridors (Preiss and Conor, 2001).

The NW-trending MacDonald Fault is a Delamerian dextral oblique-slip reactivation of a Sturtian growth fault that separated a glaciomarine basin to the SW from mountainous basement topography, with glacial valleys and fjords, to the NE. In the Mt Lofty Ranges, arcuate NEtrending early Cambrian extensional faults controlling deposition in the Kanmantoo Trough were probable precursors of the basement-rooted, Delamerian thrusts that dominate the structure.

yet always affecting the same, relatively thin (?3-5 km) package of the known Willyama Supergroup. This implies stacking of detached nappes, followed by differential exhumation. Ninnerie Supersuite felsic magmas were generated during late stages of the Olarian Orogeny, as deeply tectonically buried Willyama Supergroup underwent melting in response to mantle-derived heating (1.59-1.58 Ga "Hiltaba Event").

Latest Olarian conjugate, ENE and WSW retrograde shears segmented the Curnamona Province and were responsible for differential exhumation.

Associated regional N-S compression caused upright E-W folds of varying intensity, possibly accompanied by oroclinal bending of originally more linear fold trends. At the N end of the Curnamona Province Seismic Survey 08GA-C1, a Sdipping thrust separates relatively gently deformed Willyama Supergroup from upward-coarsening clastic metasediments of the Radium Creek Group. These possibly accumulated in a syn-Olarian foreland basin and were cut by slightly younger Mesoproterozoic felsic intrusives (Cowley et al., 2011).

Delamerian metamorphism did not involve stacking of nappes. Grade exceeded greenschist facies only close to local heat sources. Isograds in the eastern Mount Lofty Ranges are crosscutting with no clear relationship to stratigraphy, but those in lower Adelaidean metasediments overlying the Mount Painter Inlier are broadly concordant with stratigraphy. The concordance of Olarian and Delamerian arcuate fold trends might suggest postDelamerian oroclinal flexure. However, straight unconformities and rift faults that truncate bends in the Olarian trends demonstrate that the Olarian bending pre-dates Neoproterozoic rifting and deposition. Although the Delamerian arcs (Nackara and Fleurieu Arcs) are not oroclines, they have partly inherited the trends of Olarian structures, but they were primarily controlled by both rift geometry of the Adelaide Geosyncline and the NWdirected Delamerian stress field. A combination of Neoproterozoic and early Cambrian rifts left a southeastern promontory of the Gawler Craton against which Delamerian deformation was accommodated. The adjacent Fleurieu Arc underwent the greatest shortening in the Delamerian Orogen. In the northeast, Delamerian structures similarly wrap around the Curnamona Province, the central portion of which remained cratonic during the Delamerian Orogeny while the margins were folded and sheared together with

The Delamerian Orogeny post-dated Neoproterozoic and early Cambrian rifting and deposition of very thick sediments in the Adelaide Geosyncline.

Cambrian magmatic arcs were probably accreted to the passive margin early in the Delamerian Orogeny, but associated deformation extended west as far as the rifted eastern edge of the Gawler Craton. Earliest deformation (515-510 Ma) was NW-directed, with deep detachments emerging at the western thrust front. Early structures were overprinted by upright F2 folds developed under sinistral transpression as deformation propagated northwestward, oblique to Neoproterozoic growth faults. There was extensive inversion of rift faults of various ages.

Blocks rotated on Sturtian growth faults at the SW edge of the Curnamona Province were the precursors of the present basement inliers and

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the Adelaidean cover.

Hill Block: implications for strain partitioning across a detachment during the Olarian Orogeny.

Australian Journal of Earth Sciences, 51:173-188.

References Chen, Y.D., O'Reilly, S.Y., Kinny, P.O. and Griffin, W.L., 1994. Dating lower crust and upper mantle events: an ion microprobe study of xenoliths from kimberlitic pipes. South Australia. Lithos, 32:77-94.

Eraser, G.L., Blewett, R.S., Reid, A.J., Korsch, R.J., Dutch, R., Neumann, N.L., Meixner, A.J., Skirrow, R.G., Cowley, W.M., Szpunar, M., Preiss, W.V., Nakamura, A., Fomin, T., Holzschuh, J., Thiel, S., Milligan, P.R. and Bendall, B.R., 2010. Geological interpretation of deep seismic reflection and magnetotelluric line 08GA-G1: Eyre Peninsula, Gawler Craton, South Australia. In: Korsch, R.J. and Kositcin, N. (Eds). South AustraUan Seismic and MT Workshop 2010, Extended Abstracts.

Geoscience Australia, Record 2010/10:81-95.

Clarke, G.L., Burg, J.P. and Wilson, C.J.L., 1986.

Stratigraphic and structural constraints on the Proterozoic tectonic history of the Olary Block, South Australia. Precambrian Research, 34:107137.

Conor, C.H.H. and Preiss, W.V., 2008.

Understanding the 1720-1640 Ma Palaeoproterozoic Willyama Supergroup, Curnamona Province: implications for tectonics, basin evolution and ore genesis. Precambrian Research, 166:297-317.

Korsch, R.J., Preiss, W.V., Blewett, R.S., Fabris, A.J., Neumann, N.L., Fricke, C.E., Eraser, G.L., Holzschuh, J. and Jones, L.E.A., 2010. Geological interpretation of deep seismic reflection and magnetotelluric line 08GA-C1: Curnamona Province, South Australia. In: Korsch, R.J. and Kositcin, N. (Eds). South Australian Seismic and MT Workshop 2010, Extended Abstracts.

Geoscience Australia, Record 2010/10:42-53.

Conor, C., Raymond, 0., Baker, T., Teale, G, Say, P. and Lowe, G., 2010 Alteration and Mineralisation in the Moonta-Wallaroo CopperGold Mining Field Region, Olympic Domain, South Australia; in Porter, T.M., (ed.), Hydrothermal Iron Oxide Copper-Gold Related Deposits: A Global Perspective, v. 3 - Advances in the Understanding of lOCG Deposits; PGC Publishing, Adelaide, pp 147-170.

Preiss, W.V., 1993. Basement inhers of the Mount Lofty Ranges. In: Drexel, J.F., Preiss, W.V. and Parker, A.J. (Eds). The Geology of South Australia.

Volume 1 The Precambrian. Geological Survey of South Australia. Bulletin, 54:102-105.

Preiss, W.V. and Conor, C.H.H., 2001. Origin and nomenclature of the Willyama Inliers.

MESA Journal 21:47-49.

Cowley, W.M., Hore, S.B., Preiss, W.V., Sheard, M.J. and Wade, C.E., 2011. A revised stratigraphic scheme for the Mount Painter and Mount Babbage Inliers. In: Forbes, C.J. (Ed.) Unravelling the Northern Fhnders and Beyond. GSA Abstracts, 100 (in press).

Reid, A.J. and Hand, M.P., 2008. Aspects of Palaeoproterozoic orogenesis in the Gawler Craton: the c. 1850 Ma Cornian Orogeny. MESA Journal, 50:26-31.

Forbes, C.J., Giles, D., Betts, P.G., Weinberg, R. and Kinny, P. 2007. Dating prograde amphibolite and granulite metamorphism in the Broke Hill Block, NSW, using in situ monazite U-Pb SHRIMP analysis. Journal of Geology, 115:691-705.

Szpunar, M.., Wade, B., Hand, M.P. and Barovich, K.M., 2007. Timing of Proterozoic high-grade metamorphism in the Barossa Complex, southern South Australia: exploring the extent of the 1590 Ma event. MESA Journal, 47:21-27.

Forbes, C.J. and Betts, P.G., 2004. Development of Type 2 fold interference patterns in the Broken

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What do deformation microstructures tell us about rheology?

David J Prior - University ofOtago Corresponding author email: david.prior@otago.ac.nz whereas GSS mechanisms have more subtle microstructural effects; consequently strain-rate contributions of GSS mechanisms can be underestimated and w e gain a GSlbiased view of rock rheology. Electronbackscatter diffraction (EBSD) enables full quantification of rock microstructures; misorientation

It has long been recognized that the microstructures of rocks and constituent rock forming minerals can record the history of a rock's deformation and may put constraints upon conditions of deformation and rheology. The general approach is to use microstructures to constrain the controlling deformation mechanisms, so that constitutive flow laws from laboratory experiments can be applied.

Empirical relationships between microstructures and stress, strain-rate and temperature are important, as are independent constraints [e.g. petrological) on deformation conditions, as real rheologies are not linear-viscous and have Arrhenius temperature dependencies.

analysis (based on EBSD data) provides a new tool to identify the activity of processes associated with GSS mechanisms. Misorientation data reveal GSS activity in a wide range of samples, some of which have no other indication of GSS mechanisms. In order to constrain fully the rheologies that pertain to deformed rock samples, the next challenge is to develop tools that enable the quantification of the strainrate contributions of GSS and GSI mechanisms. In this talk I will outline the data that are needed from studies of naturally deformed rocks and laboratory experiments to achieve these goals.

In high-temperature creep the strainrate cumulates from component strainrates related to grain size insensitive (GSI) and grain size sensitive (GSS) micro-mechanisms. Dislocation creep, the main GSI mechanism, leaves a strong microstructural signature.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere The structural anatomy of New Zealand active faults, from surface to seismogenic depths.

Mark C Quigley - University of Canterbury, Department of Geological Sciences, Christchurch, New Zealand

Corresponding author email: mark.quigley@canterbury.ac.nz The Canterbury (New Zealand) earthquake sequence, beginning with the Mw 7.1 Darfield earthquake in Sept 2010 and including three aftershocks of Mw >= 6 over the last 17 months, provides important insights into fault behavior and earthquake triggering.

This earthquake sequence is one of the best-recorded globally; having occurred in a low-relief agricultural and urban landscape in close proximity to dense seismometer and geodetic networks.

The Darfield earthquake generated a 29.5 ± 0.5-km-long surface rupture and high-accuracy measurements of coseismic slip yield values of maximum (5.3 ± 0.5 m) and average (2.5 ± 0.1 m) net slip. These values are anomalously high for a fault of this length and earthquake of this magnitude when compared with other historical earthquakes globally.

The surface rupture morphology of the Greendale Fault includes en echelon fault arrays

with fault step-overs up to 1.1 km wide.

Fault mapping to depths of 1-2 km has been undertaken using seismic reflection and relocated aftershocks provide insights into fault geometry to 10-15 km depths. The "geologically derived" static stress drop of 13.9 ± 3.7 MPa is comparable to seismologicallyderived estimates and provides a context with which to compare this earthquake rupture to interplate and intraplate ruptures of similar Mw. The absence of discrete surface rupturing below threshold lateral displacements of --1 m along the Greendale Fault, together with the absence of surface rupturing in the large (M>6) aftershocks suggests that these potentially catastrophic events would not leave easily identifiable surface geologic manifestations. Statistical methods and subsurface investigations provide important datasets to assess seismic hazard in this instance and globally.

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Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere An insight to geology of Iran 5. Rajabi - The Australian National University, Research School of Earth Sciences Corresponding author email: s.rajabi.sdvr@gmail.com Iran has a long and rather complex tectonic evolution related to the multistage history of the Tethys domain.

The accretion of small continental blocks of Gondwanian affinity to Eurasia has resulted from successive opening and closure of large oceanic domains or narrow back-arc and marginal basins.

The active tectonics of Iran are controlled by the northward motion of Arabia with respect to Eurasia, which has a velocity of - 2 8 - 4 0 mm yr-1 at longitude 60°E. Shortening within Iran is accommodated in the seismically active Zagros mountains in the south of the country and the Alborz-Kopeh Dagh ranges in the north.

The Kopet Dagh sedimentary basin has formed after Middle Triassic orogeny in the northeastern Iran and started to sink along the major roughly NW-SE trending faults.

Structurally, it represents the margin of Central Iran at the edge of the Turan Plate. Aghdarband klippe in the eastern Kopet Dagh is the unique area where the Triassic Kopet Dagh basement is outcropped. During Paleogene, the Cretaceous layers have been folded by a NE-SW compressional stress and a N-S compressional regime which generated strike-slip fault systems in Paleogene and Quaternary rock units of central Kopet Dagh is the youngest stress regime during the tectonic evolution of studied area.

The Alborz is an excellent example of coeval strike-slip and compressional deformation, and as such can be an analogue for inactive fold and thrust belts thought to involve a component of oblique shortening (transpressional deformation). The Alborz is a stack of thrust sheets, produced by late Cenozoic compressional deformation. The trend of the main folds and thrusts varies along the length of the range, changing from an ENE strike in the east to a WNW strike in the west. Prominent left-lateral strike-slip faults occur along the length of the Alborz, trending parallel to the thrusts and folds in each region.

The Zagros is a northwest-trending fold and thrust belt made up of a 6 to 15 km thick sedimentary pile which overlies a Precambrian metamorphic basement.

GPS measurements estimated that 10±4mmyr-l of shortening is currently absorbed by the Central Zagros deformational belt. The basementinvolved active fold-thrust belt of the Zagros in southwest Iran is underlain by numerous seismogenic blind basement thrust faults covered by the folded Phanerozoic sedimentary rocks.

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Integrated application of structural geophysics, regional 3D geological modeling and numerical simulation to minerals and energy exploration.

Tim Rawling - AGOS - University of Melbourne Corresponding author email: trawling@unimelb.edu.au Recent advances in 3D geological modeling software and 2.5D and 3D geophysical forward and inversion modeling algorithms have made these tools ubiquitous in most aspects of geological endeavor. The usability of the newer software interfaces have also blurred the lines between the geologist, geophysicist and 3D modeler, leading to the development of fields such as structural geophysics. However, whist usage workflows for these tools are well embedded within exploration companies, they are typically used mostly at the "pointy" end of the exploration cycle - ie once some level of well or drilling control has been established - from prospect scale exploration to resource development.

The use of integrated regional-scale 3D geological, geophysical and numerical models, at the area selection stage of the exploration cycle, is growing but still

limited in part due to the lack of robust workflows allowing these less well constrained datasets to be used in a meaningful way.

Here we present two new workflows for the application of geophysically constrained regional 3D geological models to target generation problems for both the minerals and energy sectors in Victoria. We show how the identification of geometry of regional structures can be used to develop targets and how understanding the connectivity between different flow systems is critical to making informed resource management decisions. We also consider what the requirements of the next generation of regional 3D geological models and maps may be for both exploration and resource system management.

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Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Serpentinite Deformation in Subducting Plates and Implications for Seismic Anisotropy in Supra-Subduction Zone Environments Steve Reddy - Department WA 6845, Australia

of Applied Geology, Curtin University, GPO Box U1987, Perth,

Jed Bridges - Department 6845, Australia

of Applied Geology, Curtin University, GPO Box U1987, Perth, WA

Mary-Alix Kaczmarek - Department Perth, WA 6845, Australia Chris Clark - Department 6845, Australia

of Applied Geology, Curtin University, GPO Box U1987,

of Applied Geology, Curtin University, GPO Box U1987, Perth, WA

Dave Healy - University of Aberdeen Corresponding author email: s.reddy@curtin.edu.au which poles to {100} and { 0 0 1 } lie parallel to the X and Z directions of the finite strain ellipse respectively. In the high-pressure antigorite serpentinites poles to {100} and {010} are less defined and the pole to {010} sometimes lies parallel to X. These microstructural data have been integrated with antigorite elasticity data to investigate the seismic anisotropy of the Zermatt-Saas serpenitintes. This modelling shows a strong Vp anisotropy that is aligned parallel to X direction, inferred to be aligned with dip of the slab, and a strong AVs anisotropy in the XY plane, inferred to be the plane of the slab. These data have been used to model the effect of serpentinite anisotropy on the orientation of seismic velocity fast direction that would be recorded above the subducting slab, to assess the potential role of slab serpentinites in explaining commonly observed trench-parallel fast directions.

Significantly, comparison of the data from the high-pressure antigorite serpentinites and the mylonitic serpentinites indicates that the exhumation process does not weaken the seismic anisotropy but may increase it. Hence it is likely that serpentinites in the slab provide a significant seismic anisotropy component at both peak metamorphic conditions and during subsequent exhumation.

The Zermatt-Saas Unit of the western Italian Alps preserves high-pressure, eclogite facies mineral assemblages of Eocene age formed by subduction of Jurassic oceanic lithosphere.

Serpentinites within this high-pressure unit are characterised by antigorite ± clinohumite ± forsterite ± talc assemblages that define a strong foliation (XZ plane) and associated mineral elongation (X direction). Welldeveloped kinematic indicators within the serpentinites define a top-to-SE sense of shear that has previously been linked to the early stages of exhumation of the Zermatt-Saas rocks. A localised zone of mylonitic serpentinite 10m thick) marks the contact of the ZermattSaas serpentinites with overlying greenschist facies rocks of the Combin Zone; a zone of high-strain extensional deformation. Structural data from the mylonitic serpentinites are also consistent with extensional deformation and the mylonitic serpentinites are interpreted to represent localised highstrain reworking of the Zermatt-Saas serpentinite during exhumation of the high-pressure footwall.

Electron Backscatter Diffraction data from both the high-pressure serpentinites and serpentinitic mylonites indicate the development of a strong lattice preferred orientation in

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Quartz and olivine microstructures and textures: How do crystallographic preferred orientations and shape preferred orientations relate?

Zee Reid Lindroos - University ofOtago Virginia Toy - University ofOtago David Prior - University ofOtago Corresponding author email: zoe.reidlindroos@gmail.com Microstructures developed during creep deformation are related to physical conditions such as differential stress, strain (rate), temperature, confining pressure and fluid content (expressed as fH20). Microstructures are also related to mineral properties such as crystal system and symmetry.

Deformation conditions influence glide on slip systems within the crystals and the formation of new grains both by increases in systematic lattice misorientation and through strain induced bulging associated with grain boundary migration.

represented. Type 1 microstructures have large, flattened, relict grains that show sweeping to patchy undulose extinction, fine-scale grain boundary suturing and have smaller recrystallised grains around their edges. These are interpreted to be relatively 'unrecovered'. Type II microstructures have semi-equant grain size and large scale grain boundary sutures. These are interpreted to be relatively wellrecovered microstructures.

In the Alpine Fault metacherts the change from type 1 to type II microstructure occurs over less than a metre within individual metachert layers. These variations cannot logically be attributed to temperature differences during deformation, although stress/strain rate variations are possible as the metachert is hosted in a quartzo-feldspathic lithology and may have behaved in a rheologically heterogeneous way.

Furthermore, secondary phases such as micas and garnet are present in varying quantities throughout the metachert layer; these would have variably pinned quartz grain boundaries, preventing growth during recrystallisation. They may also have facilitated grain boundary sliding therefore relaxing the geometric constraints on slip system operation.

This research investigates the microstructures and textures developed during creep deformation within both olivine and quartz over a range of naturally-generated microstructures collected from sites in New Zealand's South Island. Of particular interest is the relationship between the crystallographic preferred orientation (CPO) and the shape preferred orientation (SPO) of grains. Quartz samples are derived from metachert layers within the mylonite zone of the dextral reverse Alpine Fault while olivine samples are from within the Red Hills Ultramafic Massif (RHUM); part of the Permian Dun Mountain Ophiolite Belt. These two field areas provide the opportunity to study the deformation that has occurred at midcrustal and mantle lithosphere levels.

Preliminary results show that the metachert layers display single girdle patterns, indicating glide occurred on the basal system, or Y-maxima, indicating glide occurred on the prism system. The RHUM samples display CPOs that indicate glide occurred on (010)[100] or (0kl)[100]. Further data on grain size, and relationship of the 3D shape of grains to the CPO will be presented at the time of the meeting.

Data were collected from three mutually orthogonal sections of each sample via two methods; computer integrated polarisation microscopy (CIP) for quartz, and electron backscatter diffraction (EBSD) for olivine and quartz (for comparison with CIP results).

Within both mineralogies, two member microstructures

end are

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Nature and origin of the Hindu Rush indentor Simon Richards - School of Earth and Environmental Sciences James Cook University, Tov\/nsviUe Australia Lloyd White - Research School of Earth Sciences, Australian National University, Canberra, Australia Gordon Lister - Research School of Earth Sciences, Australian National University, Canberra, Australia Corresponding author email: simon.richards@jcu.edu.au The Amu-Darya and associated sediments of the Afgahan-Tajik Basin and Tajik Depression lie on the western side of the Pamir (Fig. 1). The age of sediment fill within the basin indicates that it shares similar sedimentary and deformation cycles as the Tarim basin.

The Amu-Darya basin initiated as a Permian to Mesozoic rift basin (Dyman et al., 1999) then later developed into a Mesozoic to Cenezoic aged sag basin.

The deepest parts of the basin are located near its southern margin where the axis trends approximately E-W, parallel to the trend of the Kopet-Dag fold belt, the western equivalent to the Kunlun Shan. The asymmetric structure of the basin is indicative of its late-stage evolution as a foreland-style basin formed during the north-directed advance of the Kopet-Dag Fold Belt during the late Oligocene at ca. 25 Ma ago. The eastern margin of the AmuDarya basin is continuous with but overlain by sediments of the AfgahanTajik Basin. Sedimentary fill within this overlying sequence is up to 12 kilometers deep adjacent to the Hindu Kush (Nikolaev, 2002). Deepening of the Tajik Basin at 25 Ma is coincident with the transition from passive to foreland sedimentation in response to a change in the geodynamics between 25 and 30 Ma. Similarities in the age, type and distribution of sedimentation in the two basins on opposite sides of the Pamir Orogen suggest a genetic link between the two and similar geodynamic controls on their evolution.

During the India-Asia colHsion the Hindu-Kush indentor parted the palaeoTajik-Tarim Tethyan Basin, ramming northward bounded to the east by the Karakorum Fault and to the west by the Shaman Fault and equivalent linked systems. Today, the Pamir Orogen is undergoing primarily north-directed plate motion and now separates the two main basins of the region, the Tarim and Tajik basins to the east and west respectively (Fig. 1). The Tarim Basin is a rhomboidal-shaped basin that trends approximately E-W (Fig. 1) for approximately 1000 kilometers through western China. The deepest Tertiaryaged sedimentation (over 7 km) occurs in the southwestern part of the basin where it abuts the Altyn Tagh fault and the south-dipping Kundi Suture (Metievier at al., 1999; Wittlinger et al., 2004). Cenezoic foredeep sediments between southern Tien Shan and the western Kunlun indicate that the Tien Shan and Hotan thrust belts were initiated by at least 25 Ma (Sobel and Dumitru, 1997; Wittlinger et al., 2004).

The asymmetry of the basin and the thickened sedimentary sequence adjacent to the marginal fold belts, particularly along the SW margin, are interpreted here to have occurred in response to tectonic loading of the Kunlun Shan. This occurred during north-directed compression associated with the India-Asia collision in a foreland-type setting by 35 Ma. However, localised basin deepening and thrusting along the leading edge of the Kunlun Shan occurred after 25 Ma.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere The northern Pamir and Hindu Kush is characterised by unusual and relatively deep (<500 km), intra-plate seismicity.

In plan view, the distribution of seismicity beneath this region defined a broadly sinusoidal shape with two distinct zones as shown. The northeastern limit of the seismogenic belt terminates at the western margin of the Tarim Basin while the south western limit coincides with the eastern margin of the Tajik Basin (Fig. 1). W e have separated the two separate seismogenic zones based on their contrasting geometries. Successive cross-sections through the Pamir together with analysis of the data in 3D reveals that the Pamir hypocenter cluster defines south-dipping surface that is continuous to over 200 kilometers depth (Fig. 2).

This seismogenic zone extends eastward and terminates at the Kunlun Suture. This south-dipping section of the seismogenic zone links at the surface with the south-dipping Northern Pamir Thrust (NPT). W e propose that the seismicity here defines the south-dipping surface along which the once continuous crust between the now separate Tarim and Tajik basins has subducted. Prior to 25 Ma, sedimentary material for the two basins was shed from the bounding mountain belts located along the northern and southern margins of the basins. At 55 Ma there was a slight increase in the rate of sedimentation within the Tajik Basin but by 25 Ma the direction of sedimentation in both the Tarim Basin and the Tajik Basin switched to easterly and westerly sources respectively. This rapid increase in sedimentation could be accounted for by either uplift or northward motion of the Pamir -Hindu Kush. Based on our reconstructions of the Tarim plate, the Pamir Orogen must also move northwards some 400

kilometers, and the indentor has thus resulted in the closure of a once semicontinuous, E-W trending sequence of basins that include the Tarim, Tajik and Amu Darya basins. Reconstructions of subducted slab components below India are interpreted to represent the detached fragments of the Tethyan oceanic crust.

The intrusion of the Hindu-Kushindentor has thus led to the formation of a complex area of orogenesis during the collision between India and Asia, now marked by intra-plate seismicity, basin termination and recent basin development, and splitting of the once continuous Tarim and Tajik Basins. A review of basin sedimentation combined with seismic and tectonic analysis suggests a significant change in the dynamics of the Indian plate collision occurred between 30 and 25 Ma when rapid deepening of the two basins occurred as a result of northward advance of the indentor.

References Whittlinger, G. et al., 2004. Teleseismic imaging of subducting lithosphere and Moho offsets beneath western Tibet. Earth and Planetary Science Letters, 221,1-4, 30,117-130.

Metivier, F. et al. 1999. Mass accumulation rates in Asia during the Cenozoic. Geophysical Journal International, 137, 280-318.

Dyman, T.S., et al., 1999, Geology and natural gas potential of deep sedimentary basins in the former Soviet Union. USGS open file report. 99381.USGS. Issn. 0094-9140.

Nikolaev, V.G., 2002. Afghan-Tajik depression: Architecture of sedimentary cover and evolution.

Russian Journal of Earth Sciences, 4, 6, 399-421.

Sobel, E.R., and Dumitru, T.A., 1997, Thrusting and exhumation around the margins of the western Tarim basin during the India-Asia collision. Journal of Geophysical Research. 02, B3, 5043-5063.

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40N

38N

36N

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

New England Oroclines: The most contorted orogen in the world revealed by U-Pb geochronology of early Permian granitoids Gideon Rosenbaum - The University of Queensland Pengfei Li - The University of Queensland Daniela Rubatto - The Australian National University Corresponding author email: g.rosenbaum@uq.edu.au A series of sharp bends (oroclines) are recognized in the Paleozoic to early Mesozoic New England Orogen of eastern Australia. The exact geometry and origin of these bends is obscured by voluminous magmatism and is still debated. We present zircon U-Pb ages that confirm the lateral continuation of early Permian (296-288 Ma) granitoids and shed new light on the oroclinal structure. Orogenic curvature is defined by the alignment of early Permian granitoids parallel to the structural grain of the orogen, as well as the curved geometry of sub-vertical deformation fabrics, forearc basin terranes, and serpentinite outcrops.

Alternative geometrical interpretations may involve two bends (Texas and Coffs Harbour Oroclines), three bends (+Manning Orocline), or even four

bends (+Nambucca Orocline). We argue that the model involving four bends is most consistent with available data, although further kinematic constraints are required to confirm the existence of the Manning and Nambucca Oroclines. A subsequent phase of younger magmatism (<260 Ma) cuts across the curved structural grain, providing a minimum age constraint for orocline development. Assuming a structure of four oroclines, we suggest a tentative tectonic model that involves an early stage of subduction curvature during slab rollback at 300-285 Ma, followed by bending associated with dextral transpression. A final tightening of the curved structures was possibly obtained

by E-W shortening during the late Permian to Triassic (265-230 Ma) Hunter-Bowen orogeny.

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Simulating damage, changes in permeability and fluid flow in rocks: New numerical techniques for understanding mineralising processes Peter Schaubs - CSIRO Earth Science and Resource Engineering, Minerals Down Under Flagship, ARRQ Bentley, WA 6102, AUSTRALIA Thomas Poulet - CSIRO Earth Science and Resource Engineering, Minerals Down Under Flagship, ARRC, Bentley, WA 6102, AUSTRALIA Ali Karrech - CSIRO Earth Science and Resource Engineering, Minerals Down Under Flagship, ARRC, Bentley, WA 6102, AUSTRALIA Yanhua Zhang - CSIRO Earth Science and Resource Engineering, Minerals Down Under Flagship, ARRC, Bentley, WA 6102, AUSTRALIA Corresponding author email: Peter.Schaubs@csiro.au utive behaviour of crustal materials and is implemented using Abaqus. The resulting code builds on the modular architecture of escriptRT to solve sequentially these coupled THMC mechanisms and a number of feedback mechanisms including shear heating and damage. Damage in the host rock is linked to porosity evolution which in turn affects permeability. In this way w e are able to simulate thermal, fluid flow, geochemical and deformational process within one framework.

We apply these numeric techniques and codes to simple generic geologic scenarios involving unconformities and dilational jogs which are important for understanding the processes important for unconformity-related uranium deposits and many orogenic gold deposits, respectively.

Geochemical reactions are not considered in these particular models which focus on the effects of deformation on permeability.

The role that deformation plays in localising many ore-deposits and controlling mineralising systems is widely recognised.

The effects that deformation has on permeability creation and resultant flow are also generally accepted as critical to the formation of many ore-deposits. Many current numeric techniques for simulating permeability changes involve mohr-coulomb, elastic-plastic rheologies and changing the permeability as function of dilation, shear strain and/or as a function of plastic failure.

W e present a new method for simulating geologic processes which couples the thermal (T), hydraulic (H) and fluid-rock chemical (C) interaction capabilities of escriptRT with a rate-dependent mechanical (M) formulation of the finite element method including a continuum damage mechanics algorithm that considers the temperature and time dependent elastic-visco-plastic constit-

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Three-dimensional subduction models of overriding plate deformation and mantle flow WouterP. Schellart - School of Geosciences, Monash University, Melbourne, VIC 3800, Australia Corresponding author email: wouter.schellart@monash.edu Subduction zones form along convergent plate boundaries where an oceanic plate is thrust underneath an overriding plate into the Earth's mantle.

Some overriding plates are characterized by extension and backarc basins, such as found in the Southwest Pacific, while others are characterized by shortening and mountain belts like the Andes in South America. The variability in overriding plate deformation at different subduction zones has been attributed to different physical parameters. Three parameters that have frequently been discussed are subducting plate age, overriding plate velocity and subduction zone friction.

For subducting plate age it has been suggested that old oceanic lithosphere preferentially rolls back, causing backarc extension, while young oceanic lithosphere does not and corresponds to overriding plate shortening [Molnar and Atwater, 1978]. For overriding plate motion it has been suggested that trenchward motion causes shortening, while motion away from the trench causes extension [Jarrard, 1986]. For subduction zone friction it has in particular been suggested that high friction causes mountain building such as in the Andes [Lamb and Davis, 2003].

A global statistical investigation of active subduction zones, however, indicates that none of these physical parameters can explain the wide variety of overriding plate deformation and deformation rates as observed in nature, as all of these parameters show low correlations (|R| = 0.07-0.25) that are statistically insignificant at the 9 5 % confidence level [Schellart, 2008a]. The only correlation of statistical significance is the one that relates trench migration and overriding plate

deformation. This correlation shows that relatively rapid trench retreat corresponds to backarc extension, while a relatively stationary trench or a slowly advancing trench corresponds to backarc shortening. This correlation implies that the negative buoyancy force of the slab is the primary driver of trench migration, backarc basin formation and subduction zone orogeny [Schellart, 2008b].

Notwithstanding the statistical support for the slab's involvement in controlling overriding plate deformation, a physical basis for this hypothesis is lacking. The kinematics and dynamics of the subduction process have been investigated extensively in threedimensional models of progressive subduction over the last ten years, but many of these models lack an overriding plate [e.g. Schellart, 2004; Funiciello et al., 2006].

Three-dimensional subduction models that do include an overriding plate generally have applied kinematic boundary conditions, and are thus not fully dynamic [e.g. Heuret et al., 2007], or they are instantaneous models [e.g. Billen et al., 2 0 0 3 ] and thus do not provide insight into the progressive evolution of subduction and deformation. The current work presents fully dynamic laboratory and numerical experiments of progressive subduction in 3D space illustrating how a subduction zone slab interacts with the overriding plate and the ambient mantle at depth. The models involve a relatively narrow ( 7 5 0 - 8 0 0 km) subducting plate and overriding plate, and both plates are free to move laterally.

The laboratory models show that narrow slabs retreat and that the style

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere of subduction, plate velocities and trench velocity are not significantly affected by the presence of an overriding plate as long as significant decoupling occurs at the plate boundary interface. Indeed, models with and without an overriding plate show the same geometrical and kinematic evolution. The overriding plate is stretched considerably in the direction normal to the trench with several hundred km of extension after ---ZOOO km of subduction. The slab, trench and overriding plate develop an arc shaped geometry that is concave towards the mantle wedge. For subduction models with a narrow slab subduction is accommodated predominantly by trench retreat (--60%) and to a lesser extent by trench-directed subducting plate motion (--40%).

deviatoric tensional normal stress. The dynamic models thus imply that slab rollback, and the flow associated with a retreating slab, is the principal driving mechanism for overriding plate extension and backarc basin formation.

References Billen, M., M. Gurnis, and M. Simons (2003), Multiscale dynamics of the Tonga-Kermadec subduction zone, Geophysical Journal International, 153, 359-388.

Funiciello, F., M. Moroni, C. Piromallo, C. Faccenna, A. Cenedese, and H. A. Bui (2006), Mapping mantle flow during retreating subduction: Laboratory models analyzed by feature tracking, Journal of Geophysical Research, 111, B03402, doi:03410.01029/02005JB003792.

Heuret, A., F. Funiciello, C. Faccenna, and S. Lallemand (2007), Plate kinematics, slab shape and back-arc stress: A comparison between laboratory models and current subduction zones.

Earth and Planetary Science Letters, 256, 473483.

In the numerical models the narrow slab predominantly retreats but also shows a short phase of slow trench advance in a late stage of subduction. As in the laboratory models, the numerical models with and without an overriding plate show the same geometrical and kinematic evolution. However, plate velocity and trench velocity are reduced with the presence of an overriding plate.

The overriding plate is stretched considerably in the direction normal to the trench, except for the fore-arc region where shortening and deviatoric compression is observed.

For subduction models with a narrow slab subduction is accommodated approximately equally by trench retreat and trench-directed subducting plate motion, except during the late phase of slab folding.

Jarrard, R. D. (1986), Relations among subduction parameters. Reviews of Geophysics, 24,217-284.

Lamb, S., and P. Davis (2003), Cenozoic climate change as a possible cause for the rise of the Andes, Nature, 425, 792-797.

Molnar, P., and T. Atwater (1978), Interarc spreading and Cordilleran tectonics as alternates related to the age of subducted oceanic lithosphere. Earth and Planetary Science Letters, 41,330-340.

Schellart, W. P. (2004), Kinematics of subduction and subduction-induced flow in the upper mantle. Journal of Geophysical Research, 109, B07401, doi:07410.01029/02004JB002970.

Schellart, W. P. (2008a), Overriding plate shortening and extension above subduction zones: A parametric study to explain formation of the Andes mountains. Geological Society of America Bulletin, 120, 1441-1454, doi: 1410.1130/B26360.26361.

Both laboratory and numerical models show a strong correlation between trench retreat and overriding plate deformation, where the trench retreat velocity corresponds with the trenchnormal extensional strain rate and the magnitude of the trench-normal

Schellart, W. P. (2008b), Subduction zone trench migration: Slab driven or overriding-platedriven?, Physics of the Earth and Planetary Interiors, 170, 73-88, doi:10.1016/j.pepi.2008.1007.1040.

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Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere The ups and downs of Africa's largest copper deposit: The structural and metamorphic history at Lumwana, Mwombezhi Dome, NW Zambia Robert Scott - CODES, ARC Centre of Excellence in Ore Deposits, University of Hobart, Tasmania

Tasmania,

Jacqueline Halpin - CODES, ARC Centre of Excellence in Ore Deposits, University of Tasmania, Hobart, Tasmania David Selley - CODES, ARC Centre of Excellence in Ore Deposits, University of Hobart, Tasmania; now atBHR-Billiton, Singapore

Tasmania,

Mark Duffett - CODES, ARC Centre of Excellence in Ore Deposits, University of Tasmania, Hobart, Tasmania; now at Mineral Resources Tasmania, Rosny Park, Tasmania Mike Richards - Equinox Minerals Ltd, now Barrick (Australia Pacific) Limited, Perth, WA Corresponding author email: Robert.Scott@utas.edu.au With a combined resource of over 922 Mt at 0.68% Cu and 99 ppm Co, the Lumwana copper deposits (Malundwe and Chimiwungo) form the largest copper deposit in the Central African Copperbelt (CACB, 190 Mt Cu). Unlike the majority of CACB deposits, which are largely hosted by Neoproterozoic sedimentary rocks of the Katangan Supergroup (Selley et al., 2005), the Lumwana deposits occupy shear zones in underlying Palaeoproterozoic basement of the Mwombezhi Dome in the Domes Region, NW Zambia. The Domes Region is the most intensely deformed and metamorphosed of four broadly acuate structural belts, which together comprise the latest Neoproterozoic to early Palaeozoic (Pan-African) Lufilian Fold Belt in northern Zambia, southern DRC and western Angola (Selley et al., 2005).

Basement rocks and Katangan Supergroup strata in the Domes Region (including the Mwombezhi Dome) are mostly metamorphosed to amphibolite facies. However, very high pressure metamorphic assemblages (including talc-kyanite whiteschists) are locally preserved inside and on the flanks of the basement-cored domes (Cosi et al., 1992; John et al., 2004). These assemblages are interpreted to record burial to depths > 4 0 - 5 0 km (P = 13±1 kb) at ca. 530 Ma (Lufilian Orogeny),

along a suture formed during collision between the Congo and Kalahari cratons (Cosi et al., 1992; John et al., 2004).

Basement rocks of the Mwombezhi Dome are overlain by a thick package of largely undivided Katangan metasedimentary rocks that has a relatively thin, but distinctive, kyanite-bearing micaceous quartzite unit at its base.

This unit, termed the Rimming Quartzite, is generally correlated with quartz-rich sandstones of the Lower Roan Group (basal Katangan Supergroup). The nature of the contact between the Rimming Quartzite and basement rocks is controversial.

Although locally termed the Main Decollement, this contact has generally been interpreted as a modified (by deformation) basal unconformity to the Katangan succession (Cosi et al., 1992; John et al., 2004). The Malundwe and Chimiwungo copper deposits, within the core of the dome, are hosted by basement-derived chalcopyrite»bornite-bearing, quartz + muscovite + biotite/phlogopite + porphyroblastic kyanite schists, locally termed the Ore Schist. At Malundwe, a single Ore Schist horizon up to 20 m wide is developed. It is mineralized to ore grade over a strike-length >4 km, and up to 1.4 km down dip. At Chimiwungo, multiple sub-parallel Ore Schist horizons, separated by 125

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Copperbelt, 100-150 km to the north, are mostly sub-greenschist facies, and no major structural breaks between these areas have been recognised. Given the great depth of burial, (tectonic) extensional unroofing appears the most likely means of returning these rocks to shallow crustal levels, yet no evidence for significant post-collisional extension has been reported from the CACB.

unmineralized basement gneiss, occur over an interval up to 70 m wide. At Chimiwungo, both the top and bottom of the Ore Schist package display abrupt transitions to feldspathic basement gneiss or schistose gneiss. The hanging wall geology at Malundwe is similar to that at Chimiwungo, however at Malundwe the Ore Schist is underlain by a - 5 0 m thick package of unmineralized, texturally heterogeneous and variably porphyroblastic (e.g. plagioclase, scapolite, epidote ± relict kyanite) basement-derived schist termed the Footwall Schists. The Footwall Schists overlie much finer grained, kyanitebearing, micaceous quartzites similar in composition and appearance to the Rimming Quartzite and also correlated with the Katangan Lower Roan Group.

In turn the micaceous quartzites structurally overlie a heterogeneous schist package dominated by Katanganderived units, with strongly deformed basement slivers present locally (Cosi et al., 1992). Katangan strata inside the Mwombezhi Dome have previously been interpreted as occupying the synclinal cores of large, nappe-like recumbent folds affecting both basement and Katangan rocks (Cosi et al., 1992).

Our studies at Lumwana were part of a larger ARC and industry-funded project (AMIRA P872) investigating the age, origin, stratigraphic distribution and geochemical signatures of copper deposits of the CACB. The main goals of the Lumwana study were to determine the timing of copper mineralization relative to movement on the host shear zones, and to relate the movement history of the shear zones to the overall structural and metamorphic development of the Domes Region.

Related problems concerned the nature of the Main Decollement (major fault or modified unconformity?), and mechanisms for burial and exhumation of the very high pressure rocks. The latter is problematic because equivalent Katangan strata in Congolese

Our studies indicate four major phases of metamorphism and foliation development at Lumwana: • Dl: Initial thrust imbrication and high pressure (HP > 12 kb, T -'750°C) metamorphism of basement and Katangan rocks in the core of the Mwombezhi Dome (ca. 530 Ma, John et al., 2004).

• Die: Locallized extensional reactivation of earlier thrusts, decompression and retrogression of HP metamorphic mineral assemblages.

Formation of, or last major movement on, the Main Decollement of the Mwombezhi Dome.

• D2: Renewed shortening and recumbent folding at moderately high pressure (6-8 kb) and temperature (600-700°C). Effects widespread in core of the Mwombezhi Dome, but most obvious in structurally-intercalated Katangan rocks in the footwall of the Malundwe deposit. Earliest preserved metamorphic event in Katangan succession above the Main Decollement (e.g. Cosi et al., 1992, this study).

• D3: Widespread top-to-N shearing and retrogression to greenschist facies. Most likely associated with extensional exhumation of the Domes Region (in western Zambia) from beneath the southern margin of the Congolese Copperbelt.

These events were followed by one or more episodes of upright low-amplitude folding and local cleavage development, without significant new mineral growth.

Near isothermal decompression of HP rocks (Cosi et al., 1992; John et al., 2004) is supported by our metamorphic studies, and interpreted to reflect twostage extensional exhumation following 126

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

References

pulses of compressional deformation and crustal thickening. Fabric development in the Ore Schist is interpreted to reflect the D1 C±D2) and D3 deformation phases. At the hand specimen scale, the distribution of Cusulfides in the Ore Schist is most strongly influenced by D3 fabrics; however, local inclusions of chalcopyrite and bornite in syn-Dl kyanite porphyroblasts suggest mineralization occurred during or before initial thrust imbrication. The Footwall Schists, which form a sharp lower boundary to the copper mineralized zone at Malundwe, are interpreted to mark the top of a major Die extensional shear zone.

Cosi, M., De Bonis, A., Gosso, G., Hunziker, J., Martinotti, G., Moratto, S., Robert, J. P. and Ruhlman, F., 1992, Late Proterozoic thrust tectonics, high-pressure metamorphism and uranium minerahzation in the Domes area, LufiHan Arc, northwestern Zambia. Precambrian Research, v. 58, p. 215-240.

John, T., Schenk, V., Mezger, K. and Tembo, F., 2004, Timing and PT evolution of whiteschist metamorphism in the LufiHan Arc-Zambezi Belt orogen (Zambia): implications for the assembly of Gondwana. Journal of Geology, v. 112: p. 7190.

Selley, D., Broughton, D., Scott, R., Hitzman, M., Bull, S., Large, R., McGoldrick, P., Croaker, M., Pollington, N. and Barra, F., 2005, A new look at the geology of the Zambian Copperbelt.

Economic Geology 100th Anniversary Volume, 965-1000.

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Transfer of strain between rift segments: an example from the Paredones accommodation zone in Baja California, Mexico Christian Seller - School of Earth Sciences, The University of Melbourne, Victoria 3010 Barry Kohn - School of Earth Sciences, The University of Melbourne, Victoria 3010 Andrew Gleadow - School of Earth Sciences, The University of Melbourne, Victoria 3010 Corresponding author email: seilerc@unimelb.edu.au Rifts are commonly segmented into several hundred kilometer long zones of opposing upper-plate transport direction with boundaries defined by accommodation and transfer zones. A number of such rift segments have been recognized in the northern Gulf of California; a youthful oceanic basin that separates the Baja California peninsula from mainland Mexico and which is currently undergoing rift-drift transition. As rift segments are defined by obvious reversals in rift polarity their extent and structural style is relatively well known, however the nature of the intervening accommodation zones remains poorly understood. Based on regional fault patterns, for example, it is known that some accommodation zones must play a major role in the transfer of strain between different strands of the rift breakaway fault (Axen, 1995). Yet, detailed field observations (e.g. Nagy, 2000) failed to identify suitable structures that could accommodate the obvious strain gradients, so the nature of strain transfer between rift segments remains somewhat enigmatic. The situation is even less clear in central and southern Baja California, where a number of rift segments have been hypothesized based on changes in regional topographic or structural grain (Axen, 1995).

Such segmentation obviously requires accommodation zones to facilitate the transfer of strain, but it is unknown whether these zones represent true reversals in the upperplate transport direction, or whether they simply accommodate differences in the timing, style or magnitude of deformation.

The Paredones accommodation zone (PAZ) in central Baja California is a linear, WNW-ESE striking structural discontinuity separating two rift segments of opposite transport direction, and different magnitudes and styles of extensional deformation. North of the PAZ, the Libertad fault is part of the Main Gulf Escarpment, which represents the breakaway rift fault that separates the Gulf of California Extensional Province to the east from the relatively stable western portion of the peninsula. The NNW striking Libertad escarpment developed during the Late Miocene and exhibits a topographic relief of ca. 1,000m along a strike-length of ca. 50km. The steep, eastward concave shape of the escarpment and the westward tilt of volcanic strata in its hanging wall suggest a listric fault geometry that roots into a regionally important detachment horizon at depth. The Libertad fault shows marked displacement gradients with maximum fault slip in its central portion and decreasing displacement towards both the northern and southern fault terminations. In the hanging wall of the Libertad fault, a series of W-tilted horsts are bound along their eastern margin by two large-magnitude normal faults that probably sole into the shallow/flat continuation of the Libertad fault. South of the PAZ, rifting was much more distributed on a series of W- and Edipping normal faults.

Finite displacement across individual faults is much less than further north, which explains the more subdued topographic expression and generally shallower tilt

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Geological Society ofAustralia, Abstract No. 102 SGTSG2012: Cause and Effects of Deformation in the Lithosphere of pre- to syn-rift strata in this area. The PAZ itself is characterized by two W N W ESE striking dextral-obhque transfer faults with a significant down-to-theNNE extensional component. Strain is transferred from the Libertad breakaway fault onto the transfer faults over a distance of >2 0km through a network of interacting normal, oblique and strike-slip faults. Transfer faults at the termination of the major normal faults were active as two parallel, spatially and kinematically separate oblique-slip faults over a distance of at least 10km before merging into a single fault strand. Compared to other, less well-understood accommodation zones

in the Gulf of California rift, the PAZ shows a distinct lack of volcanic activity, which may help explain the different exposure and structural expression of the various accommodation zones.

References Axen, G., 1995. Extensional segmentation of the Main Gulf Escarpment, Mexico and United States: Geology, 23: 515-518.

Nagy, E.A., 2000. Extensional deformation and paleomagnetism at the western margin of the Gulf extensional province, Puertecitos Volcanic Province, norteastern

Baja California, Mexico:

Geological Society of America Bulletin, 112: 857870.

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Kinematic framework of Neoarchean gold mineralization in Finland - constraints from bedrock mapping and numerical simulations Peter Sorjonen-Ward - Geological Survey of Finland, PO Box 1237, Kuopio, 70211, FINLAND Peter Schaubs - CSIRO Earth Resources and Engineering Science, Minerals Down Uner Flagship, ARRC, Bentley, WA 6102, AUSTRALIA Yanhua Zhang - CSIRO Earth Resources and Engineering Science, Minerals Down Uner Flagship, ARRC, Bentley, WA 6102, AUSTRALIA Corresponding author email: Peter.schaubs@csiro.au of lithic units and structures to be further clarified.

Structurally enhanced permeability during orogenic deformation is widely invoked and accepted as a critical factor in the formation of lode gold deposits, driven by feedback between rock material properties and structural architecture, fluid pressure and orientation of far-field stresses. The purpose of this study has been to use numerical simulations to test the influence of systematic variations in stress fields and mechanical contrasts between supracrustal rock units in granitoids, based on known structural architectures from the Neoarchean Hattu schist belt in eastern Finland.

The structural architecture of the Hattu schist belt is characterized by upwardfacing, generally steeply dipping structures and it is possible to establish a close, sequential relationship between tightening of folds, attenuation of fold limbs, development of shear zones with strike-slip displacements, and the propagation of new folds due to strain incompatibilities between shear zones.

Refold interference patterns or attenuation and excision of certain units at outcrop and map scale are therefore more likely to represent progressive deformation of initially upright structures with strain becoming more partitioned within discrete narrow zones. The kinematic histories of these zones suggest the importance of regionally coaxial and vertical constrictional strains, although evidence for more localized local strikeslip deformation is certainly present.

Rather than invoking separate deformation episodes, a progressive continuum interpretation is preferred, in which younger structures record the partitioning of deformation into discrete, high-strain zones, with an increasing component of constrictional strain related to continuing granitoid intrusion. Some granitoids intruding the schist belt, such as the Kuittila Suite, form distinct asymmetrical elongate plutons aligned within the structural trend of the schist belt and appear to have been constructed by coalescing

The Hattu schist belt has been systematically explored for gold over several decades and the Fampalo mine commenced operations during the latter half of 2010. The supracrustal sequence is notable for the relative abundance of felsic volcanic and epiclastic deposits.

Isotopic data indicate that deposition, deformation and granitoid intrusion were very closely related in time, the ages of the earliest supracrustal units, at 2754±6 Ma, effectively overlapping with those of syntectonic granitoids. All exposed contacts between the Hattu schist belt and these granitoids are intrusive, or else tectonically modified, and hence the granitoids cannot represent depositional basement to the greenstone belt. In spite of locally intense and complex deformation, the Hattu schist belt has retained a high degree of stratigraphical coherence, which has enabled the overall topology

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere steeply dipping en echelon sheets intruded during highly partitioned transpressive deformation.

Microstructural evidence clearly indicates dynamic recrystallization of alteration assemblages during deformation under upper-greenschist to lower-amphibolite conditions and that the thermal metamorphic peak was synchronous with, or outlasted most of the deformation. The presence of gold mineralization in the syntectonic Kuittila tonalite(2745±10 Ma), while concordant monazite and titanite ages around 2700 Ma are considered to constrain the onset of cooling following post-mineralization metamorphic recrystallization.

contrasts between rock units and interaction with large scale structures.

This is most apparent in the distribution of disseminated mineralization in the hanging wall above the western margin of the Kuittila Tonalite and in the location of the Pampalo gold deposit within the back-rotated toe of a strike slip duplex recording a progressive transition from contractional to oblique extensional behaviour.

Numerical simulations of regional structural patterns with FLAC3D are intended to assess the importance of variations in far-field stress configuration in contolling rock failure and localizing mineralization, in particular addressing the issue of whether the inferred kinematics represent a local response to orthogonal shortening and compression, rather than deformation within a regional strike-slip regime

Structural controls on alteration and mineralization are a consequence of strain partitioning due to rheological

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Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

Deep Australia: Understanding plate architecture and its evolution - An example from SE Mount Isa Inlier Giovanni Spampinato - School of Geosciences, Monash University, Clayton VIC 3800 Peter Betts - School of Geosciences, Monash University, Clayton VIC 3800 Laurent Ailleres - School of Geosciences, Monash University, Clayton VIC 3800 Corresponding author email: Giovanni.Spampinato@monash.edu The negative gravity anomalies characterizing the Thomson Orogen reflect mostly the geometries of the superimposed basins.

However sometimes there is no obvious correlation between the gravity response and post - Permian (Basins) structural trends.

The Mount Isa Terrane has a protracted Proterozoic tectonic evolution recording multiple basin and orogenic events, and intra-plate magmatism (Giles et al., 2006; O'Dea et al., 1997).

These rocks form the basement to the Early Jurassic to Late Cretaceous sedimentary rocks constituting the Eromanga Basin cover the south eastern portion of the Mount Isa Domain and overlie basins such as the Devonian Avadale Basin and the Permian - Triassic sequences of the Cooper and Galilee Basins (Finlayson et al., 1988).

The Thomson Orogen has been suggested to have developed in a continental margin which graded to an oceanic crust in its eastern limit (Kirkegaard, 1974) or to be occupied either by a small ocean basin or a backarc marginal sea (Harrington, 1974).

Other models envisage either a PreCambrian basement underlying the superimposed basins (Henderson, 1980) or invoke a Neoproterozoic partial continental break-up which formed a pericratonic basin (Veevers et al., 1982).

Potential field analyses are a valuable tool in the comprehension of regional geology and large-scale characterization of tectonic systems, particularly in regions where rocks are poorly exposed.

This analysis is being undertaken to assess the basement structure and evolution along the southern margin of the Mount Isa terrane.

Tomographic studies indicate that the transition from the Proterozoic to Phanerozoic domain generally occurs at least 200 km east of any of the recent "Tasman Line" inferred by several authors (Fichtner et al., 2009). The heterogeneity displayed by the geophysical responses could be the expression of a complex interaction between several geological elements rather than a simple 'Line'.

The north trending geophysical domain of Mount Isa extends for at least 200 km north and 250 km south of the exposed Inlier showing the highest magnetic and gravity amplitude signature compared to the surrounding regions (Wellman, 1992). In the south-eastern margin of the Mount Isa Inlier, there is an abrupt change in the trend of geophysical anomalies; the Mount Isa Domain is truncated by the Cork Fault, interpreted to be part of the Tasman Line (Powell et al., 1994).

The structural grain of the Phanerozoic Eromanga Region doesn't reflect entirely the more complex architecture of the underlying magnetic basement as

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

inferred from sedimentological and seismic studies (Finlayson, 1982,1993).

While magnetic data highlight mostly the structural setting of the magnetic basement at depth missing the overlying sedimentary and non magnetic bodies, the gravity signal gives indication about the geometries of bodies with different density with respect to the surrounding rocks losing the source faster than the magnetic signal at depth.

Finlayson, D.M., 1993. Crustal architecture across Phanerozoic Australia along the EromangaBrisbane Geoscience Transect: evolution and analogues. Tectonophysics 219, 191-200,205211.

Finlayson, D.M., Leven, J.H., Etheridge, M.A., 1988.

Structural styles and basin evolution in Eromanga region, eastern Australia. American Association of Petroleum Geologists Bulletin 72, 33-48.

Giles, D., Ailleres, L, Jeffries, D., Betts, P., Lister, G., 2006. Crustal architecture of basin inversion during the Proterozoic Isan Orogeny, Eastern Mount Isa Inher, Australia. Precambrian Research 148, 67-84.

If the basement underlying the Thomson Orogen is of Proterozoic age.

Pre - 1710 Ma subduction between the Mount Isa Inlier and the Numil terrane could have been continuous southward and could have been recorded in the Northern and Central Thomson Orogen.

Several earlier tectonic events such as the Delamerian, Benambran and Bindian orogenies - being in a favorable location and orientation - could have been recorded within the Thomson Fold Belt.

Giles, D., Betts, P.G., Lister, G.S., 2004. 1.8-1.5-Ga links between the North and South Australian Cratons and the Early-Middle Proterozoic configuration of Australia. Tectonophysics 380, 27-41.

Harrington, H.J., 1974. The Tasman Geosyncline in Australia, The Tasman Geosynchne; A Symposium. Geol. Soc. Aust. Inc., Queensl. Div., Brisbane, Queensland, pp. 383-409.

Henderson, R.A., 1980. Structural outline and summary geological history for northeastern Australia, in: Henderson, R.A., Stephenson, P.J. (Eds.), The geology and geophysics of northeastern Australia. Geol. Soc. Aust, Queensl.

Div., Brisbane, Queensl., Australia, pp. 1-26.

The application of multi-scale potential field analysis and interpretation along with recent deep seismic surveys will contribute to develop a more comprehensive understanding of the architecture and the kinematic evolution of the Inlier and the surrounding regions in a modern synthesis of the Proterozoic evolution and the crustal architecture.

Kirkegaard, A.G., 1974. Structural elements of the northern part of the Tasman Geosyncline, The Tasman Geosyncline; A Symposium. Geol. Soc. Aust. Inc., Queensl. Div., Brisbane, Queensland, pp. 47-63.

O'Dea, M.G., Lister, G.S., Maccready, T., Betts, P.G., Oliver, N.H.S., Pound, K.S., Huang, W., Valenta, R.K., 1997. Geodynamic evolution of the Proterozoic Mount Isa terrain, pp. 99-122.

References

Powell, C.M., Preiss, W.V., Gatehouse, C.G., Krapez, B., Li, Z.X., 1994. South Australian record of a Rodinian epicontinental basin and its midneoproterozoic breakup (-700 Ma] to form the Palaeo-Pacific Ocean. Tectonophysics 237, 113140.

Betts, P.G., Giles, D., Mark, G., Lister, G.S, Goleby, B.R., Ailleres, L., 2006. Synthesis of the proterozoic evolution of the Mt. Isa Inlier.

Australian Journal of Earth Sciences 53,187-211.

Fichtner, A., Kennett, B.L.N., Igel, H., Bunge, H.P., 2009. Full seismic waveform tomography for upper-mantle structure in the Australasian region using adjoint methods. Geophysical Journal International 179,1703-1725.

Veevers, J.G., Jones, J.G., Powell, C.M., 1982.

Tectonic framework of Australia's sedimentary basins. The APEA Journal 22, Part 1, 283-300.

Wellman, P., 1992. Structure of the Mount Isa region inferred from gravity and magnetic anomalies. Detailed studies of the Mount Isa Inlier, 15-27.

Finlayson, D.M., 1982. Seismic crustal structure of the Proterozoic North Australian Craton between Tennant Creek and Mount Isa. Journal of Geophysical Research 87,10 569-510 578.

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Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Localisation of fault-controlled fluid flow during growth of a normal fault network, Jebel Akhdar Dome, Oman.

Paul Stenhouse - Research School of Earth Sciences, Australian National Canberra, ACT 0200, Australia

University,

Stephen Cox - Research School of Earth Sciences, Australian National University, ACT 0200, Australia Simon Virgo - Structural Geology, Tectonics and Geomechanics, Germany

RWTH Aachen,

Canberra, Aachen,

MaxArndt - Structural Geology, Tectonics and Geomechanics, Germany

RWTH Aachen,

Aachen,

Janos Urai - Structural Geology, Tectonics and Geomechanics, Germany

RWTH Aachen,

Aachen,

Corresponding author email: paul.stenhouse@anu.edu.au associated with fault-valve behaviour.

Delta 1 8 0 compositions of vein calcite in the major normal fault zone vary from host-rock buffered (ca. 27%o) to strongly fluid-buffered (13%o). Fluidbuffered vein calcite, and hence domains of high fluid flux, are localised in structural complexities such as jogs, splays and termination zones. Veins from planar segments of the fault tend to be more host-rock buffered. The network of smaller faults, adjacent to the main fault, hosts a similar range of calcite compositions (15-27%o). This similarity of oxygen isotope compositions between the main fault and the associated, lower displacement network highlights the role of both the high displacement and low displacement components of fault networks in localizing crustal fluid redistribution.

Field and stable isotope studies of calcite vein systems are combined to explore the localization of fluid flow during the growth of an exceptionally well-exposed normal fault system in the Jebel Akhdar dome of Oman. The Dar A1 Bayhda fault is one of the largest faults in the system at over 25km long and has a maximum displacement of 800m. It is associated with numerous other faults with strike lengths ranging from metres to many kilometres. During its growth, the fault network breached an overpressured fluid reservoir. Fluid redistribution through the fault network in response to episodic fault slip and permeability enhancement has developed large fault-fill veins, breccia complexes and extension vein arrays in the fault zones. The exposed levels of the fault system formed at depths of at least 5km. Episodic fluid flow was

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Fluid Flow and extensional detachments in continental and oceanic core complexes Christian Teyssier - Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA. Corresponding author email: teyssier© umn.edu

On a planetary scale, the Earth cycles water dynamically from the outer layers to the deep interior such that, over time, water reservoirs may be in nearly steady state. The interaction of water with rock is potentially profound during this cycling and plays back into the Earth's dynamic systems on various temporal and spatial scales. The plate tectonic system generates large-scale fluid exchange and may also be sustained by the mechanical and geochemical consequences of the presence of water (hydrolitic weakening, dehydration melting, partitioning into melt, etc.). Fluids are also present in each subset of Earth's main tectonic systems; here I focus on water in extensional tectonic settings, particularly in regions of high heat flow, where water circulation and water-rock exchange are optimal: Mid-ocean ridges and extended/collapsed orogens.

In the oceans, the major sites of fluidrock interaction are mid-ocean ridges as represented by intense hydrothermal activity and alteration of oceanic crust and mantle. The close proximity of hightemperature magma chambers and erupted lava with ocean water has traditionally been seen as the main condition that promotes hydrothermal activity. However, the relation of the fluid plumbing system to fracturing, faulting, and deeper shearing is not well understood. The discovery of oceanic core complexes and associated detachment and transform systems, particularly at slow spreading or magma-restricted centers, provides a context in which deformation and fluid flow are intimately linked. Hightemperature deformation produces mylonite zones of gabbro and peridotite

that are exhumed in the footwall of detachments in oceanic core complex and also in segments of transform faults. These shear zones show serpentinite but also commonly higher temperature assemblages where olivine is stable, hydrous minerals are present (high-T amphibole such as kaersutite), and olivine contains fluid inclusions.

In order to address this problem we have run a series of high-temperature deformation experiments of olivine aggregates in torsion, in the presence of excess water. At 1200C, olivine and water are not reacting and the excess water occupies fluid inclusions. During torsion to maximum shear strain gamma = 3, fluid inclusions become aligned along a planar surface that dips shallowly (--20 degrees) in the direction of the shear. This alignment persists even though olivine grain boundaries migrate and interact with fluid inclusions and a strong crystallographic fabric of olivine develops (dislocation creep).

Results indicate that a permeability anisotropy may develop, such that fluid can move readily according to pressure gradients. The next stage of experiments will consist in growing synkinematic hydrous phases to better understand the dynamic feedback that likely exists between deformation and the growth of hydrous minerals.

Oceanic core complexes are not unlike continental metamorphic core complexes (mcc) that form in regions of extension or collapse of thick and hot crust. In first-order, both display a seismogenic brittle layer, a detachment zone in which brittle/ductile processes result in the formation of breccia and mylonite, and an underlying viscous

135

Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere In detail, the absolute deltaD values that are preserved in mylonite hydrous minerals, such as mica fish, are extremely negative in the northern part of the North American Cordillera (British Columbia) and become less negative toward the south. These data are consistent with companion studies of sedimentary basins in which this isotopic trend suggests a wave of uplift southward, from Eocene to Miocene time. Combined investigations of uplift history, extension history, and volcanism/magmatism indicate that the cause of this wave of uplift and collapse corresponds to mantle dynamics beneath the Cordillera, and not simply a response to crustal thickening.

Surface fluids, such as ocean and meteoric waters, circulate to deep levels of the crust or lithosphere in extensional settings; fluid flow in these settings has thermal and mechanical implications. In the oceans, hydration of high-T shear zones may be an important mechanism by which water becomes prisoner of the lithosphere, only to be released during subduction. Therefore, deformation-induced hydration at or near mid-ocean ridges, particularly at high T, is potentially a first-order process in the cycling of water on the scale of the planet.

layer that deforms in a solid, partially molten, or magmatic state.

On continents, these regions are represented by extension of previously thickened crust, where orogenic collapse creates high permeability and high heat flow; cool upper crust is juxtaposed with deep, hot, and commonly partially molten crust to form mcc.

W e have now studied a suite of North American mcc from British Columbia to Arizona/ California, with the goal of understanding fluid flow in these systems. On the basis of hydrogen isotope values (deltaD), we demonstrate that meteoric water permeated the upper crust and participated in the growth of synkinematic minerals in the thick mylonite zones that form the footwall of extentional brittle detachments; this is the case in all the mcc w e have studied and is therefore a first-order characteristic of orogenic extension.

Meteoric water was involved in vigorous circulation from the Earth's surface to the ductile crust owing to (1) continued fracturing in extension that maintains a high permeability of the brittle crust, and (2) continued heat advection from the hot ductile crust owing to thinning and emplacement of granite bodies and/or migmatite domes.

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Deformation of the Lithosphere by Impact Events: What we can learn from zircon microstructure Nicholas E. Timms - Dept. of Applied Geology, Curtin University, Perth, WA 6845, Steven M. Reddy - Dept. of Applied Geology, Curtin University, Perth, WA 6845,

Australia Australia

David Healy - School of Geosciences, King's College, University of Aberdeen, Aberdeen, UK Alexander A. Nemchin - Dept. of Applied Geology, Curtin University, Perth, WA 6845, Australia Marion L. Grange - Dept. of Applied Geology, Curtin University, Perth, WA 6845, Robert T. Pidgeon - Depat of Applied Geology, Curtin University, Perth, WA 6845, Robert Hart-Centre

for Materials Research, Curtin University, Perth, WA 6845,

Australia Australia Australia

Corresponding author email: n.timms@curtin.edu.au Impact events cause a series of phenomena that extend into the lithosphere far beyond the eventual development of an impact crater. In the early stages of an impact event, strong stress (elastic, elastic-plastic and shock) waves propagate through the target rocks, causing elevated 'shock' temperatures and plastic damage. This is closely followed by excavation of a transient crater and a wake of incipient melting, vapourization and ejection of debris, which happen in the order of a few minutes, depending on the scale of impact. Zircon from impact sites can preserve a variety of impact-related microstructures and localized resetting of U-Pb system in shocked zircon can provide a means of dating impact events. However, some deformation microstructures in terrestrial zircon could also form through tectonic activity. To isolate the processes of shock-related deformation, the microstructures of lunar zircon grains from breccia samples collected during the Apollo 17 mission have been characterized via optical microscopy, cathodoluminescence imaging and electron backscatter diffraction mapping. These zircon grains preserve deformation microstructures that show a wide range in style and complexity.

Planar deformation features [PDFs) are documented in lunar zircon for the first

time, and occur along {001}, {110}, and {112}, typically with 0.1-25 \im spacing.

The widest PDFs associated with { 1 1 2 } contain micro-twin lamellae with 65° / <110> misorientation relationships.

Deformation bands parallel to {100} planes and irregular low-angle (<10°) boundaries most commonly have <001> misorientation axes. This geometry is consistent with a dislocation glide system with <100>{010} during dislocation creep. Non-planar fractures, recrystallised domains with sharp, irregular interfaces, and localized annealing textures along fractures are also observed. No occurrences of reidite were detected. Shock deformation microstructures in zircon are explained in terms of elastic anisotropy of zircon.

PDFs form along a limited number of specific {hkl} planes that are perpendicular to directions of high Young's modulus, suggesting that PDFs are likely to be planes of longitudinal lattice damage. Twinned {112} PDFs also contain directions of high shear modulus. A conceptual model is proposed for the development of different deformation microstructures during an impact event. This 'shock deformation mechanism map' is used to explain the relative timing, conditions and complexity relationships between impact-related deformation microstructures in zircon.

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Revealing faults in the Perth metropolitan area, Western Australia: Implications for groundwater systematics Nicholas E. Timms - WA Geothermal Centre of Excellence, Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845, Australia Paul G. Wilkes - WA Geothermal Centre of Excellence, Perth, PO Box 1130, Bentley, WA 6102

Dave Healy - School of Geosciences, King's College, University of Aberdeen, Aberdeen, AB24 SUE, UK Soazig Corbel - WA Geothermal Centre of Excellence, Perth, PO Box 1130, Bentley, WA 6102

Frank G. Horowitz - WA Geothermal Centre of Excellence, Perth, PO Box 1130, Bentley, WA 6102

Corresponding author email: n.timms@curtin.edu.au technique (sometimes called 'worming') that involves multiscale edge detection.

Linear gravity worms sourced from different depths have been used to determine the position and dip of several new major faults. These results, combined with the fluvial drainage network, land surface topography and surface and subsurface geological boundaries has been used to interpret a new fault network in the Perth metropolitan area that includes at least five differently oriented sets of faults.

Hough transformation is used to compare fault orientations picked from different data sources. Our results suggest that many faults are at least as young as Pleistocene in age and that many of the prominent landforms, including the Swan and Canning Rivers are fault-controlled. The significance of the newly recognised faults in controlling the modern subsurface fluid flow is discussed.

Knowledge of the structural architecture and bedrock geology under Perth is a vital prerequisite for understanding subsurface fluid flow for targeting geothermal prospects and for aquifer management. Perth is situated on the Perth Basin which is filled by up to 10km thick Permian to Holocene sediments and is associated with the separation of Greater India from Australia. The only previously identified fault in the metropolitan area is the Darling Fault, which is eastern boundary with the Yilgarn Craton is the Darling Fault. This study has integrated new and existing geophysical data with Shuttle Radar Topographic data (SRTM), geomorphology and surface geology to provide a new interpretation of the subsurface geology of Perth and its setting within the Perth Basin. New ground gravity data have been acquired for the Perth metropolitan area with a maximum spacing of 1.5 x 1.5 km have been processed with a wavelet

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Extensional arc flare-ups: An example from Iran Charles Verdel - School of Earth Sciences, University of Queensland Corresponding author email: c.verdel@uq.edu.au Arc flare-ups are relatively short magmatic pulses that are uncorrelated with subduction rate and which account for a disproportionately large fraction of the total magmatic production of an arc.

Flare-ups may be linked with either shortening or extension. The existence of flare-ups suggests that simple fluxmelting models of arc volcanism may not fully account for magma generation processes that operate at some convergent margins.

One of these processes could be mantle upwelling, which is linked with volcanism in backarc basins and may generate melting beneath volcanic arcs as well.

Paleocene-Eocene volcanic rocks erupted during this flare-up are intercalated with marine sediments and have major and trace element characteristics that are typical of continental arc magmatism, whereas the chemical composition of limited Oligocene basalts in the UrumiehDokhtar belt and the Alborz Mtns., which were erupted after the flare-up ended, are more consistent with derivation from the asthenosphere.

Together with the recent recognition of Eocene metamorphic core complexes in central and east-central Iran, stratigraphic evidence of Eocene subsidence, and descriptions of Paleogene normal faulting, these geochemical and geochronological data suggest that the Late Paleocene-Eocene magmatic flare-up was extensionrelated.

A proposed tectonic model attributes the flare-up to decompression melting of lithospheric mantle hydrated by slab-derived fluids, followed by Oligocene upwelling and melting of enriched mantle that was less extensively modified by hydrous fluids.

Paleogene magmatism and extension may have been driven by an episode of slab-rollback following a Cretaceous period of flat slab subduction.

The general features of this model are similar to other geodynamic models for extension-related volcanism from numerous locations around the world.

A profound arc flare-up occurred across Iran during the Paleogene. This event is under-reported in the geological literature, but by virtue of its tectonic configuration, size, and relative youth, it may ultimately stand as a particularly noteworthy example of flare-up magmatism. Volcanism across Iran is dominated by a Paleogene pulse, despite protracted and presumably continuous subduction along the northern margin of the Neotethyan ocean for most of Mesozoic and Cenozoic time. U-Pb and 40Ar/39Ar data from volcanic arcs in central and northern Iran constrain the duration of the pulse to - 1 7 Myr, roughly 10% of the total duration of Neotethyan subduction beneath Iran.

Late

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Calibration of low-grade metamorphic indicators with low- and medium-T thermochronometers: 40Ar/39Ar and (U-Th)/He test cases from the western U.S. Charles Verdel - School of Earth Sciences, University of Queensland Corresponding author email: c.verdel@uq.edu.au some frequently used low-grade indicators and thermochronology methods. First, 40Ar/39Ar step-heating data from pelites and metapelites were used to calilDrate the Ar closure temperature of illite/clay-size muscovite with one of the most commonly used low-grade indicators: illite crystallinity [IC). Findings from this study suggest that resetting of the Ar illite/muscovite thermochronometer occurs at approximately the anchizone/epizone boundary, or roughly 300 °C. Second, (U-Th)/He data from detrital zircons in sandstones and quartzites collected along the same transect were used to calibrate the zircon (U-Th)/He thermochrometer with the IC scale. Initial data suggests that the zircon (U-Th)/He partial retention zone, which has a temperature range of -^-130-200 °C, lies within the deep diagenetic metapelitic zone.

Both the illite/muscovite 40Ar/39Ar and zircon (U-Th)/He empirical results are consistent with previous estimates for the temperature range of illite crystallinity variations.

Though empirical examples are lacking, these results are expected to correlate with other low-grade metamorphic indicators, such as vitrinite reflectance, conodant alteration index, and the newly-developed illite spectral maturity scale. In addition to being useful for regional tectonic studies, these calibrations have implications for evaluating basin maturity and may find utility in petroleum exploration and studies of coal rank.

In the vernacular of low-T thermochronology, sedimentary rocks are considered "reset" when they contain detrital grains (typically apatite or zircon) that have (U-ThVHe or fission-track ages younger than the depositional age. In studies focused on cooling histories, thermochronology data are frequently considered useful only if they come from reset samples, but in some cases it may be difficult to predict in advance whether a particular stratigraphic unit is reset. Quantitative indicators of low-grade metamorphism, which typically involve analyses that are less time-consuming and less costly than thermochronology analyses, may be useful for estimating peak temperatures experienced by sedimentary strata and thus whether those strata are reset, but currently there are few empirically-based calibrations between these indicators and the various low-T thermochronometers.

These tools complement each other: low-grade indicators are estimates of paleotemperature but provide no age information, while thermochronometers can record the time since cooling below a given temperature but do not independently provide paleotemperature estimates.

The combination of these tools is particularly relevant to subgreenschistfacies thermal histories of sedimentary basins.

Samples of Cambrian strata collected along a 500 km-long transect in the western US have been used to calibrate

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere J Criteria used to define the timing of the India-Asia collision Lloyd White - Research School of Earth Sciences, The Australian National

University

Mamie Forster - Research School of Earth Sciences, The Australian National

University

Talat Ahmad - The University of Kashmir Corresponding author email: lloyd.white@anu.edu.au Continent-continent collision is defined as the time when two plates of continental affinity come into contact with one another. However, it is difficult to define when this process occurs as geologists are limited by the observations that they can make at the surface, or the inferences that they can draw from geophysical data. Here w e review the different geological and geophysical criteria that have been used to infer when India and various microcontinents collided with Asia.

Other workers use plate tectonic reconstructions to infer when India and Asia collided, where a significant deceleration of the Indian plate at --50 Ma is taken as evidence of India colliding with Asia (e.g. Patriat and Achache 1984). However, a review of the India-Eurasia plate circuit, and a new reconstruction of India's motion relative to Eurasia indicate that India accelerated and decelerated several times during its northward progression (Figure 1) (White and Lister, in press).

It therefore follows that each deceleration is indicative of an accretion event, or that variations in India's plate velocity are not indicative of collision.

Many consider that India-Asia collision is marked by: (1) the change from marine to continental sedimentation in the Indus-Tsangpo Suture Zone; (2) the end of 1-type magmatism in the Gangdese Batholith and beginning of Stype anatectic granites in the Lhasa Terrane, and: (3) the first development of south-facing folds and thrusts in the Indus-Tsangpo Suture Zone (e.g. Searle et al., 1987). These criteria are typically used to suggest that India collided with Asia between 55 and 50 million years ago (e.g. St-Onge et al., 2010). However, other workers have used the same criteria and found evidence of a collision event at 35-30 Ma (Aitchison et al., 2007; White et al., in press). Those who advocate an older collision (e.g. 5550 Ma) must invoke various models to explain the younger magmatic and thermal events (e.g. Channel Flow). A younger India-Asia collision is much simpler to explain, as the older evidence for collision can be attributed to the accretion of an island arc or microcontinent to India or Asia. W e must take stock and consider if these geological criteria are appropriate to define the timing of continent collision.

Other workers use tectonic reconstructions and arbitrary plate boundaries (e.g. "Greater India" or "Greater Eurasia") to argue when India and Asia collided. However, these usually do not account for the crustal shortening and extension that is associated with collision and slabrollback as Tethys closed. W e therefore use the Didactic Tree concept (White and Lister, in press) to review the supposed evidence for when India and Asia collided.

We show that the arguments surrounding the time when India and Asia collided are dependent on the geological criteria that are adopted, or the assumptions that are made in plate tectonic reconstructions. It is likely that the arguments about when India and Asia collided will continue, unless w e stop to consider the strengths and weaknesses of the criteria that are used to define when continent collision

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occurred. These concepts have ramifications for the interpretation of other colhsion systems around the world.

St-Onge M. R., Rayner, N. & Searle, M. P. 2010.

Zircon age determinations for the Ladakh bathohth at Chumathang (Northwest India): Implications for the age of the India-Asia collision in the Ladakh Himalaya.

Tectonophysics, doi:10.1016/j.tecto.2010.09.010.

References

Aitchison, J. C., J. R. Ali, and A. M. Davis (2007), When and where did India and Asia colhde?, J. Geophys.

Res., 112, B05423, doi:10.1029/2006JB004706.

White, L. T., Ahmad, T., Lister, G. S., Ireland, T. R. and Forster, M. A. (in press) Is the switch from Ito S-type magmatism in the Himalayan orogen indicative of the collision of India and Eurasia?

Australian Journal of Earth Sciences, 59 (3).

Patriat, P., Achache, J., 1984. India-Eurasia collision chronology has implications for crustal shortening and driving mechanism of plates.

Nature, 311, 615-621.

White, L. T. and Lister, G. S. (in press). The collision of India with Asia. Journal of Geodynamics (doi: 10.1016/j.jog.2011.06.006).

Searle, M. P., B. F. Windley, M. P. Coward, D. J. W., Cooper, A. J. Rex, D. Rex., L. Tingdong, X. Xuchang, M. Q. Jan. V. C. Thakur, and S. Kumar (1987). The closing of Tethys and the tectonics of the Himalaya. GSA Bulletin, 98, 678-701.

250 Velocity of IND-EU calculated at 0.001 Ma increments

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Figure 1: Velocity vs. time plot of the Indian plate relative to Eurasia according to White and Lister (in press). This shows that the Indian plate accelerated and decelerated several times during the past 100 million years. Each deceleration could be interpreted to reflect the timing of an accretion event if we use the same logic that has been used by some authors (e.g. Patriat and Achache 1984).

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Ice an analogue used for rock textural development Christopher J. L Wilson - School of Geosciences, Monash University, Clayton, VIC, 3800, Australia Mark Peternell - Dept. of Earth Sciences, University of Mainz, 55099 Mainz, Germany Sandra Piazolo - Dept. of Earth and Planetary Sciences, Macquarie University, NSW, Australia Vladimir Luzin - ANSTO, Kirrawee DC, Lucas Heights, NSW2232,

Australia

Corresponding author email: Chris.Wilson@monash.edu Advances in our understanding the plasticity of ice, and its application to quartz-rich rocks, have come from experimental investigations.

In polycrystalline ice it has been clearly shown that the plastic deformation is produced by intermittent dislocation bursts, triggered by long-range interaction of dislocations, which play an essential role in primary creep and dynamic recrystallization processes during tertiary or steady state creep. To support this w e describe deformation in polycrystalline 'standard' water-ice and 'heavy' water-ice bulk samples of both layered and polycrystalline ice that will be compared with results from in situ experiments where c-axis orientations were recorded using a fabric analyser.

asymmetry in the direction of the shear, and a secondary maximum inclined at - 4 5 - to the plane of shearing. An initial c-axis preferred orientation plays a critical role in the initial mechanical evolution. In contrast to established ideas, a strong alignment of basal planes parallel to the plane of easy glide inhibited deformation and there was an increased component of strain hardening until recrystallization processes become dominant.

The initiation of the recrystallization process involves: (1) Migration of boundaries in initial randomly oriented grains and appears to be the main strain accommodation process during the stage of primary creep. (2) Once a sample reaches the minimum (secondary creep) there appears to be nucleation of a discrete population of small new recrystallized grains that are preferentially parallel to the compression direction. In all samples examined close to the minimum octahedral strain rate there appears to be the nucleation of discrete new grains with a random c-axis preferred orientation. (3) During the tertiary creep stage there appears to be a further episode of grain boundary migration that contributes to the final recrystallization history. Above -15^C the distinction between a grain boundary migration versus a discrete grain nucleation stage becomes more difficult. These observations need to be taken into account in future numerical modelling studies linking fabric evolution to microstructure in both ice and quartz-rich rocks.

As the temperature of ice is increased from -20 to -2^C there is at least a two fold enhancement in octahedral shear strain rate, which coincides with the onset of extensive dynamic recrystallization and a change in grain size distribution at -15-C. Between IS^C and -lO^C the c-axis preferred orientation rapidly evolves with the initiation of two maxima fabrics. From lO^C to -2^C there is a progressive evolution of a final c-axis pattern with episodes of random grain nucleation. In pure shear two maxima symmetric patterns are produced after a random preferred orientation is produced. In a general shear the fabric initially randomizes before it becomes asymmetric with respect to the direction of shortening with a strong maximum at ^ S - to the pole of the imposed zone of high strain, a sense of

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Geological Society of Australia, Abstract No. 102 SGTSG2012: Cause and Effects of Deformation in the Lithosphere

Structural and Kinematic Analysis on the Jinshan gold deposit, West Qinling Orogen, Central China Qingtao Zeng - Center for Exploration Targeting, University of Western Australia, WA, 6009 Graeme Eraser - Dragon Mountain Gold Company, West Perth. WA. 6011 Corresponding author email: zengqingtao@gmail.com The Jinshan gold deposit is located in the Lixian County, south of the Gansu Province in Central China. Geologically, it is hosted in the Xihanshui Group, mainly low metamorphosed mudstone and siltstone. Middle Devonian at the West Qinling Orogen, which is sandwiched between the North and South China cratons. The No. 30s and 40s orobodies constitute the major resource of the deposit, and 1.52 Moz gold resources under JORC standard from No. 30s have been achieved.

relationship and dipping angles between Bedding and SI cleavage, we interpret that the Dl event deformed the entire exploration area when it was deeply buried, forming km-scale wavelength E-W striking folds. The field area is located at the northern limb of this anticline.

Since following deformations overprinted this area significantly, the best guess of the orientation of the Dl fold axial plane is dipping to South. Large quartz veins or pods (Vl, whose width usually over 0.5 m) with limited volume of sulfide mineralization is interpreted to be associated with this deformation, this can be seen by that the VI is cut or offset by later deformation and veinlets, and the host rocks crenulations observed when approaching the boundaries of the quartz pods.

The style of gold mineralization in the Jinshan gold deposit has strongly similarities with orogenic gold system.

The gold is carried by pyrite and arsenopyrite, quartz-calcite stockworks with chlorite and sericite alterations are dominating in high grade zone, and mineralization is strictly structurecontrolled. Detailed mapping was conducted to understand the major control on the localization of the gold orebodies. A structural and kinematic history has been established to accommodate the field observation and structural measurements.

D2 deformation. D2 structures are characterized by ENE (most) or ESE (minor)-trending meter-scale folding and reverse faults. The shortening was conducted both vertically and along NW-SE direction.

The S2 folds preferentially located at the areas where bedding (or S I ) is sub E-W-trending, which make Dl and D2 sub-parallel to each other.

The mineralization is interpreted to strike NE, and plunge east at over 45°, following the hinges of D2 folds, which is superposed upon the limbs of the Dl fold. Therefore, gold mineralization can be interpreted to have participated during the D2 event, when the trend of the maximum principal stress a l switched from sub North-south in Dl to NW-SE in D2. This is likely to be the wanning stage of this NW-SE

The deformation history has at least three periods of penetrative deformation, Dl, D2, and D3. The D1 deformation is likely to be a regional sub North-south compression event, represented by universal SI cleavage, which presents in every lithology except Triassic intrusive in the exploration area. Throughout the field area, the angle between Bedding and SI regardless the rock property are narrow within 25 appx. 10° at most circumstances. Dl fold has been recognized by comparing the rotation

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Geological Society of Australia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere

compression. Quartz veins with pyrite, arsenopyrite, enveloped by chlorite, sericite and carbonate alterations, are termed V2 in this study. This vein system contains significant volume of gold when it is hosted by spotted slate, and the veins filled into kinematic weaknesses of the host rocks, like the S2 cleavage or along bedding or SI cleavage.

planes can be observed in the Triassic diorite dykes. This is interpreted as a regional tectonic relaxation. Calcite (with lesser quartz) vein, V3, penetrated into these fracture planes and openings with limited gold participation.

This mapping program pointed out that the mineralization is strongly associated with D2 deformation event, and hinges of D2 fold are mineralization-favored structures. This understanding will substantially advance our targeting ability in this polyphase terrene.

D3 deformation postdates mineralization. D3 is characterized by ubiquitous 320-340° striking and west dipping fracture planes. This fracture

Fault

Limestone

Inferred fault

Diorite dyke

Ore body and its number

Granodiorite dyke

The major lithology is grey low-metamorphosed, strongly foliated mudstone, siltstone and sandstone in the map

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Geological Society ofAustralia, Abstract No. 102 SGTSG 2012: Cause and Effects of Deformation in the Lithosphere Syndeformational granite crystallization along the Mount Magnet Greenstone Belt, Yilgam Craton: evidence of large-scale, magma-driven strain localization during Neoarchean times.

Ivan Zibra - Geological Survey of Western Australia Corresponding author email: ivan.zibra@dmp.wa.gov.au The Neoarchean is characterized by a global-scale period of crust formation, extensive mafic to granitic magmatism, crustal reworking and intracrustal partial melting. As a result of extensive mantle melting and high internal heat from radioactivity, Neoarchean lithosphere is commonly regarded as hot and weak and thus unable to sustain the pronounced topography that characterizes most of post-Archean orogenic belts. Since a dramatic strength drop is associated with the presence of melt in crystallizing or melting rocks, Neoarchean continental deformation is thought to have been largely accommodated by shearing of high-grade gneisses and by syntectonic granitic magmatism. However, direct field examples of such strain localization in hot and partially molten granites are relatively scarce. The Archean Yilgarn Craton, which is made up of volumetrically dominant granites

and granitic gneiss, represents a natural laboratory to study the effect of melting on continental deformation. This talk illustrates evidence of large-scale, synmagmatic deformation recorded during crystallization of a Neoarchean heterogeneous granitic complex.

Outcrop-scale key features include a well-defined magmatic foliation concordant with a compositional layering, commonly overprinted by synmagmatic folds and shear zones.

Moreover, microstructures testify that deformation occurred under meltpresent conditions and during the transition to high-temperature solidstate flow. The studied complex represents a natural example of strain localization within a low viscosity corridor that may have accommodated large amounts of strain during the Neoarchean, as predicted by numerical experiments and tectonic models.

Figure 1. Dextral synmagmatic shear zone, highlighted by asymmetrically folded schlieren layering, in equigranular monzogranite. No important solid-state deformation is detectable, even at thin section scale.

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