Abstracts No.103 - Selywn Symposium 2012, The Great East Australia Geology Arm Waves by GSAustralia

The Geological Society of Australia

ABSTRACTS Number 103

2012 SELWYN SYMPOSIUM & MEMORIAL LECTURE The Great East-Australia Geology Arm Wave of 2012: Tectonics, modern analogues/events, mineralisation GAGA 2012 Thursday 27 September 2012 Fritz-Loewe Theatre, The University of Melbourne

Selwyn Symposium 2012 GSA Victoria Division GAGA 2012

Geological Society of Australia Abstracts 103

The Geological Society of Australia Extended ABSTRACTS Number 103

The Geological Society of Australia Victoria Division www.vic.gsa.org.au

SELWYN SYMPOSIUM 2012 The Great East-Australia Geology Arm Wave of 2012: Tectonics, modern analogues/events, mineralisation GAGA 2012 Thursday 27 September 2012 Fritz-Loewe Theatre, The University of Melbourne

Editor: Susan White Environmental Geoscience Latrobe University 3068

Selwyn Symposium 2012 GSA Victoria Division GAGA 2012

Geological Society of Australia Abstracts 103

September 2012 Geological Society of Australia Extended Abstracts Number 103 Geological Society of Australia Victoria Division Selwyn Symposium 2012

Editor: Dr Susan White ISSN 0729 01IX ©Geological Society of Australia Incorporated, 2012

Preferred Citation: White, S. (ed) The Great East-Australian Geology Arm Wave of 1012: Tectonics, modern analogues/events, mineralisation. (GAGA 2012), Selwyn Symposium of the GSA Victoria Division, September 2012, Geological Society of Australia Extended Abstracts No. 103, 71 pp.

Example Citation for papers in tliis volume: Musgrave, R.J. 2012. Contortions in the Tasmanides - A Geophysical View In: White, S. The Great East-Australian Geology Arm Wave of 1012:Tectonics, modern analogues/events, mineralisation. (GAGA 2012), Selwyn Symposium of the GSA Victoria Division, September 2012, Geological Society of Australia Extended Abstracts No. 103, p. 25 -31 Copies of this publication may be obtained from: The Executive Director Geological Society of Australia Incorporated Suite 61, 104 Bathurst Street Sydney NSW 2000 Australia

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The Great East-Australia Geology Arm Wave of 2012: Tectonics, modern analogues/events, mineralisation GAGA 2012 TABLE OF CONTENTS Organisation of Selwyn Symposium 2012

Alfred Richard Cecil Selwyn Biography

Foreward

Detailed Program

The Selwyn Medal

2012 Selwyn medalist: Dr Charles Lawrence

2012 Selwyn Memorial Lecture,

Professor Brian Kennett FAA, FRS (ANU) Exploiting earthquakes - probing subduction zones Plenary Address: Mike Sandiford (University of Melbourne) The slow divorce: tectonic signals in an ancient continent Nick Rawlinson (ANU) Passive seismic imaging beneath Southeast Australia: from the shallow crust to the base of the lithosphere

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Bob Musgrave (Geological Survey of NSW) Contortions in the Tasrnanides - A geophysical view

David Moore (Monash University) The Cambrian East Gondwanan Margin in Southeast Australia Ross Cayley (Geological Survey of Victoria)

Oroclinal folding in the Lachlan Fold Belt: consequence of southeast-directed Siluro-Devonian subduction rollback superimposed on an accreted arc assemblage in Eastern Australia John Foden (University of Adelaide) Rifts, Arcs, Backarcs and Rollback: Controls on the Late Precambrian to Early Palaeozoic Magmatic evolution of the S.E. Gondwanan Margin. Bill Collins (University of Newcastle) Evaluating Geodynamic Models of Tasmanides Evolution. Allan Rossiter (Consultant) Can granite chemistry constrain Palaeozoic tectonic models for Southeast Australia? Frank Bierlein (AREVA Group) Geodynamic controls on the metallogenic endowment of south-east Australia Andrew McNeill (CODES) High precision U-Pb chronostratigraphy of the Mount Read Voicanics, Tasmania: implications for mineral exploration and tectonic reconstructions

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The Great East-Australia Geology Arm Wave of 2012: Tectonics, modern analogues/events, mineralisation GAGA 2012

The Selwyn Symposium for 2012 has been organised by the Victorian Division Committee The Geological Society of Australia, Victoria Division The Geological Society of Australia (GSA) was established in 1952 as a learned nonprofit organisation. The Society's objectives have been expanded to promote, advance and support the Earth Sciences within the scientific and wider communities. The GSA Victoria Division holds scientific meetings on the last Thursday of each month at the University of Melbourne starting at 6.15 preceded by light refreshments from 5.30pm. It publishes geological books (such as the Geology of Victoria), and acknowledges geological leadership and accomplishment through various awards through the Selwyn Award and Symposia. Anyone interested is also welcome to attend the monthly committee meetings of the GSAV, which usually begin at 4 pm in a venue near the general meeting venue. For further information check our website: www.vic.qsa.org,au.

GSAV Committee 2009 OFFICE BEARERS Chair: Vice-chair: Secretary: Treasurer:

David Cantrill Vacant Adele Seymon Barbara Wagstaff

SUBCOMMITTEES Awards: Bicentennial Gold: Education: Heritage: Membership: Newsletter: Program: Publications: Promotions Webmaster: Geology of Victoria:

Ingrid Campbell Gerhard Krummel Noel Schleiger Susan White Vacant Matthew Bliss Vacant Vacant Vacant Ken McLean Bill Birch

COMMITTEE David Moore Noel Schleiger Susan White Matthew Bliss Helen Green Syed Amir Mahmud

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The Geological Society of Australia Victoria Division presents the

SELWYN SYMPOSIUM 2012 Thursday 27 September 2012 www.vic.gsa.or^.au

Fritz-Loewe Theatre, Earth Sciences University of Melbourne

Alfred Richard Cecil Selwyn (1824-1902) The Selwyn Memorial Lecture was introduced in 1984 to commemorate the work of A.R.C. Selwyn, the first Government Geologist of Victoria. This year represents the 155*^ anniversary of the Government Geological Survey in Victoria. In recent years the Lecture has followed a symposium, involving invited speakers on a significant theme for Australian Geology, to celebrate Selwyn's work. Alfred Selwyn was born in Somerset, England in 1824, and had a strong Church of England upbringing. He was educated in Switzerland where he developed an enthusiasm for geology. At the age of 21 he joined the Geological Survey of Great Britain under the direction of A. C. Ramsay in North Wales and was soon entrusted with independent mapping of large areas with key Silurian rocks. The discovery of Gold in Victoria in 1851 led Governor La Trobe to ask the Colonial Office in London for "a gentleman possessed of the requisite qualifications and acquaintance with geological science and phenomenon." Selwyn was awarded the post at an annual salary of 500 pounds and arrived in early December 1852. Almost immediately he commenced a series of wide ranging reconnaissance trips during which he studied the geology of Victoria with excursions into Tasmania and South Australia. Among many of the early records, the Geological Survey holds an original watercolour version of his 1856 map of the Yarra Basin and Western Port. Selwyn set up the Geological Survey proper in 1856 and gathered around him and trained a team of geologists including: Alpin, Ulrich, Wilkinson, Daintree, Taylor, Brown and Etheridge. They subsequently occupied senior positions in all mainland State Geological Surveys and became renowned in the early annals of Australian geology. Selwyn and his team established the 6-mile x 9-mile Victorian quarter sheet mapping programme of which 65 were produced. Selwyn was quiet and reserved man of indomitable disposition who carried out his plans despite all hindrances and achieved an amount of work that few could emulate. However, conflict arose from the overlapping activities of mining surveyors. The Survey often held views on scientific standards and policies opposed to those of the colonial officials and legistrators, in particular that of R. Brough Smyth, Secretary of Mines. Consequently, Selwyn resigned in 1869 and the Survey was disbanded for a year. Selwyn left Australia and was appointed the Director of the Geological Survey of Canada on retirement of Sir William Logan. He worked with distinction from 1869 until his retirement in 1894. Selwyn was awarded the prestigious Murchison Medal of the Geological Society of London in 1876. Other honours followed in Great Britain. The only recognition from Australia was his Clark Gold Medal from the Royal Society of New South Wales in 1884. He is commemorated in Victoria by an annual Geological Society of Australia Victoria Division lecture, symposium and medal.

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FOREWORD Just over 20 years ago, three important events in our understanding of the tectonic history of the Lachlan Orogen took place. The first was the publishing of the paper by Coney et al. (1990), which placed the Tasmanides in the wider context of our understanding of tectonic settings, particularly those seen and understood in North America. The second event was the LFB 91 Conference The tectonics and mineralisation of the Lachlan Fold Belt' at the Australian Academy of Science in Canberra. This conference had a wide range of speakers presenting the most diverse range of ideas about the formation and development of the Lachlan. Among other advancements, the first truly credible full-crust thickness cross-sections of Victoria and New South Wales were presented. We are fortunate to have two presenters here who also spoke at LFB 91. The Conference was followed by the third event, the publishing of these ideas in Volume 214 of Tectonophysics, which gave us a more expansive and widely available record than the LFB 91 abstracts volume, GSA Abstracts 29. Collectively these events caused a step change in our understanding of the processes that gave rise to and went on in the Lachlan Orogen. The topics covered in the present volume take the same subject, the tectonic history of the Lachlan Orogen and its surrounds, but reflect this greater understanding. Because of our location, we have chosen to take a broadly south-eastern Australian perspective. We now have new tools that allow us to see what we could have never seen before (Rawlinson). These new tools have given new understandings and/or hypotheses where previously applied tools had yielded only conundrums (Cayley, Moore, Musgrave). Like the LFB 91 Conference, we consider that the granites (Collins, Rossiter) and mineralisation (Bierlein, McNeill) present in the Lachlan Orogen are fundamental parts of it, and so understanding their significance is essential. Study at its margins can provide valuable information about the early Lachlan Orogen history (Foden). More than ever we understand that the present is the key to the past (Kennett, Sandiford). Yet as we look deeper, wider and more precisely than ever before, we remain with Julius Sumner Miller's eternal question "Why is it so?" Hopefully today will help you find out. Coney, P.J., Edwards, A., Mine, R., Morrison, F., Windrim, D., 1990. The regional tectonics of the Tasman orogenic system, eastern Australia. Journal of Structural Geology 12, 519-543.

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The Great East-Australia Geology Arm Wave of 2012: Tectonics, modern analogues/events, mineralisation GAGA 2012 DETAILED PROGRAM 8.00-9.00 Symposium registration 9.00-9.15 Introduction: David Cantrill 9.15-10.00 Plenary Address: Mike Sandiford University of Melbourne The slow divorce: tectonic signals in an ancient continent Nick Rawlinson Australian National University 10.00-10.30 Passive seismic imaging beneath Southeast Australia: from the shallow crust to the base of the lithosphere 10.30-11.00 Morning Tea Session 2: Tectonic models for Southeast Australia Chair of session: David Cantrill 11.00-11.30 Bob Musgrave Geological Survey of NSW Contortions in the Tasmanides - A geophysical view 11.30-12.00 David Moore Monash University The Cambrian East Gondwanan Margin in Southeast Australia Ross Cayley Geological Survey of Victoria Oroclinal folding in the Lachlan Fold Belt: consequence of southeast-directed 12.00-12.30 Siluro-Devonian subduction rollback superimposed on an accreted arc assemblage in Eastern Australia. 12.30-1.30 Lunch Session 3: Granites and their implications Chair of session: David Moore John Foden University of Adelaide 1.30-2.00 The Creation of the Source of the Tasmanian-Ferrar - Karoo Jurassic Large Igneous Province: Not a Plume Source? Bill Collins University of Newcastle 2.00-2.30 Evaluating Geodynamic Models of Tasmanides Evolution. Allan Rossiter Consultant 2.30-3.00 Can granite chemistry constrain Palaeozoic tectonic models for Southeast Australia? 3.00-3.30 Afternoon tea Session 4: Implications of tectonic setting Chair of session: Adele Seymon 3.30-4.00 Frank Bierlein AREVA Group Geodynamic controls on the metallogenic endowment of south-east Australia Andrew McNeill CODES 4.00-4.30 High precision U-Pb chronostratigraphy of the Mount Read Volcanics, Tasmania: implications for mineral exploration and tectonic reconstructions 4.30-5.00 Synthesis & discussion chaired by David Cantrill 5.00-6.00 Drinks & refreshments 6.00-6.15 Selwyn Medal presentation 6.15-7.00 GSA VICTORIA SELWYN LECTURE Brian Kennett FAA FRS Australian National University Exploiting earthquakes - probing subduction zones Selwyn Dinner 7.45 At Cafe Italia, 56-66 University Street, Carlton

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THE SELWYN MEDAL The Selwyn Medal is named in honour of Sir Alfred Selwyn, an eminent Victorian pioneering geologist and founder of the Geological Survey of Victoria. It is awarded, usually yearly, to recognise significant ongoing or former contributions of high calibre to any field of Victorian geology. A candidate for this medal should have made a major contribution to new knowledge of the geology of Victoria, or a significant reinterpretation of it based on critical observations, or has contributed importantly to a major mineral or oil discovery, or have produced important geological publications or have been involved successfully in the development of the geological profession.

THE 2012 SELWYN MEDALLIST CHARLES LAWRENCE

Charles Lawrence has made major contributions to Victorian geology, including the development and formalization of a stratigraphic framework for the geology of the Mallee of northwestern Victoria. This remains the basis for understanding and interpreting the geology and hydrogeology of the region and enabled the production of the geological map series of northern Victoria covering the Riverine Plain and the Mallee. He has contributed to major hydrogeological research and has been successfully involved over many years in the development and advancement of the geological profession, initially in the Geological Survey of Victoria, and later in the teaching and supervision of research in hydrogeology at the University of Melbourne. This has included the organisation and delivery of professional hydrogeology courses and mentoring of a great number of young hydrogeologists. He has been involved in research and teaching of hydrogeology in Chile and Argentina though the overseas work of Geoscience Australia. His hydrogeological research has covered a broad range of groundwater evaluation including the salinization of the Murray Basin, groundwater contamination, and desert studies in Australia, Argentina and Chile. He has collaborated with overseas organisations in Argentina and Mexico. He was the Chief Investigator in 2007 on the information and data exchange related to research into hydrogeology of the Otway Basin for the Corangamite Catchment Management Authority. Charles Lawrence has made an important contribution into the long neglected mapping, stratigraphy and the economic potential of the Victorian side of the Murray Basin. At a time in which water resource management had hardly begun to understand the potential of groundwater resources, his work established the baseline references against which modern understanding of those resources is still premised today. He built on that work to become a national and international ambassador for water conservation. He was inspirational in leading to successive stages defining the long misunderstood dynamics between surface and groundwater dynamics.

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THE 2012 GSAV SELWYN MEMORIAL LECTURE Brian Kennett FAA FRS

Brian Kennett is currently Professor of Seismology at the Research School of Earth Sciences, The Australian National University and was Director from September 2006 to January 2010. He received his Ph.D. in Theoretical Seismology from the University of Cambridge in 1973. He was a Lindemann Fellow at IGPP, University of California, San Diego and then a University Lecturer at the University of Cambridge. He moved to Australia in 1984, and was President of the International Association of Seismology and Physics of the Earths Interior from 1999-2003. His research has covered a very wide range of topics in seismology, from reflection seismology to studies of the deep Earth and from theoretical to observational studies. He has received recognition through many medals and awards including the Gold Medal in Geophysics from the Royal Astronomical Society, the Gutenberg Medal from the European Geosciences Union, the Murchison Medal from the Geological Society of London, and the Jaeger and Flinders Medals from the Australian Academy of Sciences. He is a Fellow of the American Geophysical Union, the Australian Academy of Sciences and the Royal Society (London). He has given many invited lectures and presentations including Union Plenary Lectures to the International Union of Geodesy and Geophysics in 1999 and 2011, and a Keynote Lecture at the International Geological Convention in 2012.

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EXPLOITING EARTHQUAKES - PROBING SUBDUCTION ZONES B.L.N. Kennett^ ^ Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia

Abstract Most of the world's earthquakes are associated with subduction zones where oceanic lithosphere descends into the Earth beneath an overriding plate. The growth of the sea floor through spreading at mid-ocean ridges is accommodated by recycling through the subduction process. The descending material is colder than its surroundings and the consequent variations in physical properties can be captured through their effects on the passage times of seismic waves between sources and receivers. Such seismic tomography is well suited to delineating the zones of faster wavespeed associated with the subducted plate.

earthquake that can be linked to changes in the physical properties of the subduction zone. More subtle features may well have controlled the source properties of the 2010 Mw 9.0 earthquake off the eastern coast of Japan that again produced a devastating tsunami.

By working with both compressional (P) and shear (S) waves we can develop multiple images of structure for each wavetype and so recognise changes within the subducting material. For the 2004 Sumatran-Andaman event (Mw 9.3) there is distinct segmentation of the

trap the energy and bring it to the surface from even events as deep as 500 km. Such high frequency waves will be disrupted if the properties of the slab change, and provide useful probes to understand features in tomographic images.

High frequency energy from deep earthquakes carried up the subduction zone can cause significant ground shaking well displaced from the source. We would expect such energy to be rapidly shed from the zone of fast wavespeeds, but internal heterogeneity elongated along the plate is sufficient to

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THE SLOW DIVORCE: TECTONIC SIGNALS IN AN ANCIENT CONTINENT Mike Sandiford ^ ^ University of Melbourne Australia is a continent of ancient cratonic rocks at least two-thirds of which formed more than 1400 million years ago. Australia is also the lowest and flattest of all the continents and is largely remote from active plate boundaries. So it is easy to see why Australia is often viewed as the most stable of continents. Of course, earthquakes such as the Newcastle (magnitude 5.6) earth quake on 28 December 1989, which led to 13 deaths and an estimated $4 billion total damage bill, indicate that Australia is not totally immune from tectonic activity. But it still comes as a surprise that, with an average of about 80 earthquakes of magnitude 3 or higher each year and one magnitude 6 or higher quake occurring about every 20 years, Australia releases signif-icantly more earthquake energy — on an areanormalised basis — than comparable cratonic regions in Canada, Brazil, northern Europe, and much of Africa. While the great majority of these earthquakes are natural, Australia also has an unusual preponder-ance of human-induced earth-quakes. These are mostly assoc-iated with filling of large reser-voirs, such as the 25 September 1996 quake associated with the Thompson Reservoir, east of Melbourne, which at magnitude 5 was Victoria's largest earthquake in the last 30 years. Globally only about 2% of large reservoirs have induced significant quakes. In Australia, five of the six largest dams have done so. Australia has a quite extraordinary historical record of surface fault ruptures. Surface ruptures accompanied three powerful earthquakes ranging from 6.3 to 6.7 in magnitude that occurred at Tennant Creek, in the Northern Territory in 1987 and the 1968

Meckering earthquake in Western Australia. These ruptures created fault scarps up to 35kms long with a maximum displacement of over one metre. Surface rupturing has also accom-panied at least five other quakes in recorded history. However, until recently there has been little understanding that the record of seismicity in Australia is also significantly expressed in its longer-term landscape evolution. A slow divorce Newer research indicates that there are literally hundreds of older neo-tectonic features, as now complied by Dr Dan Clark at Geosceince Australia. And the significance is not to be sneezed at! For example, strata deposited just a few kilometres from Adelaide as (geologically) recently as 30 million years ago are now overturned. Elsewhere in the Flinders Ranges and in the Snowy Mountains, there are numerous outcrops showing ancient basement rocks thrust several metres over river gravels and hill screes less than 100,000 years ago. Such features would not be out of place in the most active of mountain ranges. This slow, ongoing tectonic activity can be understood in the context of the evolution of the Indo-Australian Plate, one of the most dynamic and highly stressed of the major tectonic plates. In fact, Australian earth-quakes are just part of much broader distribution of intraplate earthquakes throughout the interior of our plate and reflect the gradual buildup of tectonic stress over the last five to 10 million years due to changes in the forces along the distant plate boundaries that both drive and resist our plate's northward motion. In tectonic terms, these quakes provide evidence that our plate is breaking up 11

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under progress-ively rising levels of stress. In effect, our 43-million-yearlong dalliance with India is coming to an end in a kind of grand plate tectonic 'divorce'. In a few million years, Australia and India will again be separate plates going their different ways as they were prior to 45 million years ago. But there is an even more intriguing picture of active tectonics that is beginning to emerge from studies of the Australian landscape. As it moves north, the continent is tilting, warping and bobbing up and down by several hundred metres. A striking asymmetry We see this most emphatically in the striking asymmetry in young marine sediments around the continental margin. For example, the southern onshore margin is literally plastered with young limestones, often ex-tending hundreds of kilometres inland. The Nullarbor Plain is the most striking example — a 2000-km-long relict submarine plain, exposed as seas retreated from the southern margin around 15 million years ago. In contrast, the onshore north-ern margin is virtually devoid of marine deposits of such age. Why is there no Nullarbor equivalent along the northern margin of the continent? The exposure of the Nullarbor can be attributed in part to eustatic proc-esses, which have seen the global sea levels fall by around 100 metres over the last 15 million years or so. However, in the western, inland part of Nullarbor, Eocene and Miocene palaeoshorelines show seas exten-ded to elevations now more than 250 metres above sea level, indicating the continent has actually risen by at least 150 metres since then. Moreover, these same shorelines can be traced to elevations more than 150 metres lower across the Nullarbor, implying a significant tilting of the Nullarbor since then. The absence of onshore marine deposits of comparable age along Aus-tralia's northern coastline implies that the seas are now as high as

at anytime during the last 65 million years. So the northern coastline must be sinking at least as fast as the longterm eustatic fall in sea level. The pattern of exposure in inundation reveals an extraordinary continental scale tilting. As it moves northward, the Australian continent is tilting, north side down, south-west side up and has been doing so at a rate of around 20 metres every million years for the last 15 million years. The pattern of young vertical motion is more complex than this simple tilting, as demonstrated by the profound enigma of Lake Eyre. The lowest point of Lake Eyre is some 15 metres beneath sea level, begging the question of how any part of a stable continent can remain below sea level without filling up with sediment. The answer is that it can't, and that Lake Eyre has not been at such a low point for more than few million years. We know this because we can see the remnants of an older, precursory palaeo-lake, equally as big as the modern Lake Eyre, now some 140 metres above Lake Eyre along its drainage divide with Lake Torrens. While we don't know the exact age of this palaeo-lake — termed lake Billa Kalina — it implies this landscape is going up and down by over 100 metres, gradually changing the position of the depocentre of the Eyre basin, on timescales of a few million to a few tens of million years. Today, the floor of Lake Eyre, and other large lake salt lakes of South Australia, has a southerly tilt several metres lower in the south than the north — with the lake floor seemingly acting as a kind of giant tilt meter! Similar features can be seen around Mt Gambier in south-east South Australia and in western Victoria, where fewmillion-year-old beach strand lines show the region has risen by 180 metres over at a rate of 65 metres per million years. In Tasmania, some beach ridges now 30 metres above sea level have been dated at only 120 000 years old. If correct, parts of Tasmania would appear to be literally rocketing out of the sea!

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The underlying issue

plate as it moves northward across the mantle beneath. When such instabilities detach from the plate above, the downward pull is released and the landscape above gently inverts leaving features such as Lake Billa Kalina improbably stranded on the present-day drainage divide.

The question becomes, what is causing this tilting, warping and bobbing up and down of our continent over the last few tens of millions of years? Most likely it is a response to the complex flow in the mantle hundreds of kilometres below the continent. At the largest scale, mantle flows as part of the circulation that accommodates subduction. Wher-ever there is vertical flow in the mantle, it pulls or pushes on the plates above. For example, the upward flow in the rising the plume beneath Africa explains the unusual elevation of much of that continent. In a similar way, as continents move into the realms of subduction, they get pulled downwards by the descending mantle flow. That is exactly what appears to be happening in northern Australia, as it encroaches on the Indonesian and Melanesian subduction realms. But what about the Lake Eyre and southern margin undulations? Can these be related to generation of smaller scale flow in the uppermost mantle, where drips and eddies form due to differential flow between the plate and the mantle beneath? If so, it presents the exciting possibility that we might be able to see which way the mantle is flowing relative to the plate above. Such flow instabilities would certainly help explain the strange coincidence of young basaltic volcanoes along the uplifting southern edge of Victoria, and south-east South Aust-ralia. Such instabilities would be consistent with the continent moving north across the mantle beneath, generating instabilities along its trailing edge. Such an interpretation may also provide a neat explanation for the apparent tilt on the floor of Lake Eyre and other salt lakes in South Australia — with the depo-centre tracking a transient mantle instability sweeping south-wards relative to continent. This is certainly my working hypo-thesis, with the transient nature of features such as Lake Eyre reflecting the growth of instabilities in the flow at the base of the lithosphere, gently pulling and on the

In fact, by virtue of its flat land-scapes, arid climate and its speed — Australia is the current speed champion of the continents travelling at around 6-7 cm/year northwards — Australia appears to preserve a unique record of the way the mantle flows deep beneath the continent. Nowhere else on any other continent can we get such an insight into the direction of flow beneath the continents. Far from being an inert continent irrelevant to active tectonic studies, we now understand that Australia has special attributes that preserves unique tectonic signals not available elsewhere. As a fast, flat and dry continent, it is helping us elucidate subtle surface responses that track change in distant plate boundary forces and the flow in the mantle deep beneath. In addition to these fundamental insights, this understanding is also important for a number of emerging, pragmatic issues. For example, understanding earthquakes and controlling them is becoming an increasingly important issue as we seek to deal with emerging energy issues, such as making enhanced geothermal sys-tem deliver significant, GW-scale, zero-emission electricity power, or for storing CO2 in the subsurface. The issue is not trivial, since perturbing pore fluid pressures in the crust is a key to inducing seismicity and can be easily done, as is testified by the Thompson Dam quake. Similarly, tracking the response of ancient drainage systems to subtle undulations helps provide one of the key controls on the dispersal and concentration of placer deposits such as uranium through time, and the dispersal and evolution of our freshwater faunas in our inland rivers systems. The suggestion that these 13

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systems must have changed significantly through time, at least in some parts of the continent, is crucial to efficient exploration targeting strategies and may be fundamental to understanding the biodiversity in our inland river systems. This work has been the subject of an ARC-funded research project including a professorial fellow-ship, during the period 2005-2009. The work has involved many collab-orations — those with Mark Quigley and Dan Clark are particularly worthy of mention. Further discussion of these issues can be found in the following papers. Sandiford, M. 2003, Neotectonics of southeastern Australia: linking the Quaternary faulting record with seismicity and in situ stress. IN Hillis, RR, & Muller, D. (Eds) Evolution and dynamics of the Australian Plate, Geological Society of Australia Special Publication 22: 101-113. Celerier, J. et al, 2005, Modes of active

intraplate deformation Flinders Ranges, Australia. Tectonics 24, TC6006, doi:10.029/2004&C001679 Quigley, M. et al, 2006, Quaternary faults of southern Australia: palaeoseismicity, slip rates and origin. Australian Journal of Earth Sciences 53: 285-301. Sandiford, M. 2007, The tilting continent: a new constraint on the dynamic topographic field from Australia. Earth & Planetary Science Letters 261:152-163. Demidjuk, Z. et al, 2007, U-series isotope and topographic constraints on mantle melting from the Newer Volcanic Province in South Australia Earth & Planetary Science Letters 261: 517-533. Sandiford, M. & Quigley, MC. 2009, TOPO-OZ: insights into the various modes of intraplate deformation in the Australian continent. Tectonophysics 474: 405-416.

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PASSIVE SEISMIC IMAGING BENEATH SOUTHEAST AUSTRALIA: FROM THE SHALLOW CRUST TO THE BASE OF THE LITHOSPHERE N. Rawlinson^ ^Research School of Earth Sciences, The Australian National University, Canberra ACT 0200 Abstract In this study, recent passive seismic imaging results from the WOMBAT transportable array experiment in southeast Australia are presented. Using data collected over a 14 year period from 14 sequentially deployed seismic arrays, high resolution images of the crust and mantle lithosphere have been constructed using both body wave teleseismic tomography and ambient noise tomography. Teleseismic tomography exploits relative delay times of distant earthquake signals to build a 3-D image of the upper mantle, while ambient noise tomography takes advantage of energy generated by oceanic microseisms and atmospheric disturbances to recover crustal structure. The 3-D teleseismic tomography images reveal a range of broad scale features, including an east dipping transition from high velocities in the west to low velocities in the east which is inferred to represent the eastern margin of the Delamerian Orogen underthusting the western subprovince of the Lachlan Orogen beneath the Stawell Zone. Other features of interest include a low velocity zone that extends to approximately 200 km beneath the Newer Volcanics Province in western Victoria that implies a hot spot source limited to the upper mantle, and a clear high velocity zone that extends to approximately 230 km beneath the Curnamona Province, which is a signature of its cratonic orogens. Group velocity maps of the shallow to mid-crust obtained from ambient noise tomography generally show a strong correlation with surface geology features; for instance, the Murray and Sydney basins are revealed as low velocity zones, while the outcropping

crystalline rocks of the Southern Highlands have a pronounced high velocity signature. The ambient noise results also provide constraints on azimuthal anisotropy; the most striking feature is a dominant alignment of the fast axis of anisotropy in the N-S direction with a magnitude that exceeds 4%. This orientation is consistent with a prevailing direction of compression that was perpendicular to the proto-Pacific margin of Gondwana. Introduction In 1998, an array of 40 short period seismic stations, spaced 40-50 km apart, was deployed in western Victoria for a period of 5 months to record seismic energy generated by distant earthquakes emanating largely from plate boundary regions. The principal goal of the deployment was to image 3D P-wave speed variations in the upper mantle beneath the Newer Volcanics province and southernmost Murray Basin (Graeber et al. 2002). In the following year, the array was moved northwest to span a large portion of the Murray Basin (Clifford et al., 2008), and in 2000, a third and final movement of the array to span the southern region of the Adelaide Fold Belt was made. Two years later, the Research School of Earth Sciences at The Australian National University made a decision to embark on an ambitious program of transportable array deployments with the goal of covering eastern Australia with a high density of passive seismic stations, with the initial three deployments noted above forming the nucleus of the evolving array cluster. To date, there have been 14 array deployments resulting in over 600 station locations, with two new arrays to 15

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be deployed between now and 2014 (see Figure 1). The transportable array experiment in southeast Australia, now known as the WOMBAT array, is the largest of its type in the southern hemisphere, and is second in size only to USArray in the world. It provides an unprecedented opportunity to illuminate structure at high resolution at a continuum of depths ranging between the shallow crust and the inner core using a variety of techniques. Due to the predominant use of short period sensors, the WOMBAT dataset mostly consists of relatively high frequency body waves (above -0.2 Hz) and surface waves (above -0.05-0.1 Hz). However, despite not recording the full natural wavefield, there is an abundance of information contained in the recorded data. For example, teleseismic body wave tomography can be used to image seismic P-wave velocity and attenuation structure in the upper mantle, ambient noise tomography can be used to image shear wave velocity in the crust, and teleseismic receiver functions can be used to recover detailed shear velocity structure beneath individual stations. The location of the WOMBAT array in southeast Australia makes it ideally placed to reveal a wealth of new constraints on the structure and evolution of the southern Tasmanides. This region of Australia is thought to have formed as a series of accretionary orogens outboard of the Pacific margin of eastern Gondwana from the Mid Cambrian through to the Mid Triassic (Glen, 2005). From west to east, the WOMBAT array overlies much of the Delamerian, Lachlan and New England orogenies, and is beginning to impinge on the Thomson Orogen to the north and the Gawler Craton to the west. Large tracts of the Palaeozoic basement in southeast Australia are masked by near surface cover of MesozoicCainozoic sedimentary and volcanic cover sequences such as the Murray Basin and Newer Volcanics. Seismic imaging is an ideal tool for penetrating beneath this surface cover and illuminating the structure beneath. As 16

such the results that are now being produced will help address various issues related to the tectonic evolution of the region, including the extent to which Precambrian continental lithosphere is incorporated within the Palaeozoic terrane. In this study, the focus will be on recent results from teleseismic P-wave tomography and ambient noise tomography. These techniques are complimentary in the sense that teleseismic tomography is limited to the upper mantle, whereas ambient noise tomography is largely limited to the crust. Thus, the application of both techniques allows constraints to be placed over the full depth range of the lithosphere. Data and Methods The dataset used here is sourced from the entire WOMBAT array shown in Figure 1. For the teleseismic tomography, distant earthquakes from surrounding plate boundary regions are used to construct a 3-D image of Pwave velocity structure beneath the array. Global phases such as direct P, PcP (reflection from the core-mantle boundary) and PKiKP (reflection from the inner core boundary) impinge subvertically on the WOMBAT array, and their relative arrival times provide valuable information on lateral variations in mantle structure. Figure 2 shows an example of a PKiKP phase clearly recorded by the EAL1 array. The coherency of the recorded waveform across the array allows crosscorrelation-type methods to be applied to obtain highly accurate relative arrival times. In this case we use the adaptive stacking method of Rawlinson and Kennett (2004) to automatically pick relative onset times, and the ak135 global reference model of Kennett et al. (1995) to obtain relative arrival time residuals which can be directly related to lateral variations in mantle structure beneath the array. In order to mitigate the effects of a heterogeneous distribution of earthquake sources and take advantage of data redundancy, summary rays are created using a sub-

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binning approach described in Rawlinson et al. (2011). Approximately 50,000 summary rays are used to constrain structure in this study. The method used for inverting the relative arrival time residuals for 3-D variations in P-wavespeed is described in detail in Rawlinson et al. (2006). Model structure is described by a regular grid of velocity nodes in latitude, longitude and depth, and cubic B-splines are used to define a smooth velocity continuum. The forward problem of traveltime prediction is solved using a finite difference solution of the eikonal equation (Rawlinson and Sambridge, 2004), and the inverse problem is solved using a subspace inversion method (Kennett et al., 1988), which includes smoothing and damping regularisation that is needed to account for solution non-uniqueness. The ambient noise dataset consists of seismic energy produced mainly from oceanic microseisms and atmospheric disturbances. Although indistinguishable from the background noise on a raw seismogram, the coherency of this energy is revealed through a process of long term cross-correlation of seismic records extracted from simultaneously recording stations (Benson, 2008). For each WOMBAT sub-array, several months of data are usually sufficient for producing high quality crosscorrelations. Ideally, these crosscorrelations reveal what is commonly referred to as the Empirical Green's function, which is related to the impulse response of the Earth between station pairs. For an array of N simultaneously recording stations, it is possible to construct N(N-1)/2 cross-correlations. Once the cross-correlations have been produced, standard dispersion analysis (e.g. Young et al. 2011) can be applied to create surface wave dispersion curves for either group or phase velocity over a range of periods for Rayleigh or Love waves. At a particular period, one can pick the interstation traveltime between every station pair and perform 2-D tomography to construct either

phase of group velocity maps. Since surface waves at different periods are sensitive to different depths, the construction of a series of phase or group velocity maps provides an approximate 3-D view of velocity structure. In this study, the surface wave tomography code described in Arroucau et al. (2010) is used to image group velocity. Results Figure 3 shows a 150 km depth slice through the 3-D P-wave tomography model produced by inversion of WOMBAT relative arrival time residuals. Two realisations of the solution are shown. In Figure 3a, the result obtained by using only the WOMBAT dataset is shown; in Figure 3b, the results of an inversion which uses an initial model derived from the surface wave tomography results of Fishwick and Rawlinson (2012) is illustrated. The reason for showing both plots is that the use of relative arrival time residuals, which eliminates source origin time errors and reduces the influence of deep structure beneath the model region, means that structure with wavelengths greater than the aperture of each array are effectively filtered out. Therefore, including a long wavelength initial model derived from surface wave tomography fills in this gap. However, the potential superiority of this result is somewhat reduced by the fact that conversion from S to P wavespeed assumes a constant VpA/s ratio. As such, it is better to make inferences from the two models side by side. Broad scale features resolved by the surface wave model can be clearly seen in Figure 3b; the first order reduction in velocity from the NW to the SE of the model reflects the transition from the shield region of central Australia to the Palaeozoic orogens in Victoria and NSW. Towards the continental margin, velocities decrease even further across the transition to oceanic lithosphere. Figure 3a, which does not include the long wavelength features, exhibits a detailed pattern of anomalies. The first order trend of the velocity perturbations 17

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suggests the presence of structure with a predominantly N-S strike at this depth, although the signature is not particularly strong. Checkerboard resolution tests indicate that the maximum horizontal spatial resolution of the results is of the order of 70 km between 50 and 300 km depth. Figure 4 shows a Rayleigh wave group velocity map that has been derived from the ambient noise dispersion curves via 2-D tomography. In this case, only seven sub-arrays of the WOMBAT experiment has been used, as the data processing required to extract dispersion information from the remaining arrays is ongoing. The period displayed in Figure 4 is 5s, which approximately corresponds to a vertical integration over a depth range of 3- 8 km. In addition to isotropic velocity, azimuthal anisotropy has also been recovered. The thin black lines superimposed on Figure 4 show the orientation and magnitude of the fast direction of anisotropy, which at this period are almost uniformly oriented in a N-S direction. Horizontal spatial resolution in Figure 4 is of the order of 100 km. Compared to the 50 km resolution achieved by standard isotropic tomography, this is a decrease by a factor of two, but is necessary since the inclusion of anisotropy greatly increases the number of unknowns in the inverse problem, which naturally results in a drop in resolution. However, since the results clearly reveal a strong anisotropic signature, the decision to include anisotropy is justified. Discussion and Conclusions In this study, recent results from teleseismic tomography and ambient noise tomography in southeast Australia have been presented. The use of data from the WOMBAT transportable array experiment, which has been recorded by over 600 stations separated by 50 km or less, has allowed high resolution images of the crust and lithospheric mantle to be produced. Compared to the previous study of Fishwick & Rawlinson (2012), data from an additional four arrays

(Gawler, Curnamona, EAL1 and EAL2 see Figure 1) has been included in the teleseismic tomography results. This has significantly extended the model northward, and it now spans most of NSW. Features that are clearly evident include a high velocity zone beneath the paleo-Proterozoic Curnamona Province, which was likely part of a larger cratonic region that included the Gawler Craton, but is now separated by NeoProterozoic sedimentary rocks of the Adelaide Rift Complex (Conor and Preiss, 2008). This separation is clearly observed in Figure 3b. Another prominent feature of the model is a low velocity zone beneath the northern part of the Newer Volcanics Province (NVP) in western Victoria (see Figure 3). This anomaly extends in depth from approximately 50 km to 200 km, and then appears to terminate. The NVP represents the youngest evidence of basaltic intraplate volcanism in the Australian continent, with the most recent eruptions dated less than 5 Ka (Price et al., 2003). Although one of many Cenozoic eruption centres to populate the eastern edge of the Australian mainland, the NVP is unique in that it is not obviously part of a hotspot chain. For example, the distribution of NVP eruption centres is elongated in the east-west direction, perpendicular to plate motion, and has essentially been fixed in space for about 7 Ma. Moreover, it appears that the NVP is the latest phase of an eruption cycle that has operated intermittently at least three times since the early Eocene when fast northern motion of the Australian continent commenced. Coupled with modest surface topographic responses (-100 m) and a relatively low eruption volume (-20,000 km^) for a continental basaltic province, researchers have begun to suspect that the source of the NVP does fit the mold of a traditional mantle plume model, but instead may be a phenomenon localised to the upper mantle (Demidjuk et al., 2007). One possible mechanism for hot spot localisation is edge driven convection (e.g. Demidjuk et al., 2007, Farrington et al., 2010), where plate motion coupled

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with a sharp change in depth of the base of the Lithosphere creates a convection cell that drives deeper material upwards. Although the results of this study cannot confirm the mechanism involved, it certainly provides strong evidence that there is no deeper mantle source for the NVP. The ambient noise tomography results provide a different view of the southeast Australian lithosphere compared to the teleseismic tomography, in that they are confined to the crust. The 5s group velocity map illustrated in Figure 4 shows a strong correlation between seismic wavespeed and the dominant surface geology, with the igneous and metamorphic rocks of the Southern Highlands producing higher velocities, and the sedimentary rocks of the Murray Basin and underlying infrabasins producing lower velocities. The fast direction of anisotropy at this depth is almost uniformly aligned in the N-S direction, which corresponds closely with the dominant geological fabric of the Palaeozoic orogens, and is consistent with a prevailing direction of compression that was perpendicular to the proto-Pacific margin of Gondwana. Future work with the teleseismic dataset includes using absolute traveltimes to constrain 3-D mantle velocities instead of relative traveltimes. This has the advantage that both short and long wavelength structure can be recovered in a consistent way. However, due to earthquake origin time uncertainty and deeper mantle structures, it will be necessary to embed the local model defined beneath WOMBAT within a global model that includes a large volume of complementary traveltime data so that both global and local structure can be simultaneously resolved. In the case of the ambient noise tomography, extending the results northward by including data from the more recent deployments is the main priority. Another goal is to compute phase velocity dispersion curves and perform an inversion for 3-D shear wave velocity, which will be much more useful than the group velocity maps.

References Arroucau, P., Rawlinson, N. and Sambridge, M., 2010. New insight into Cainozoic sedimentary basins and Palaeozoic suture zones in southeast Australia from ambient noise surface wave tomography. Geophys. Res. Lett. 37, L07303, doi:10.1029/2009GL041974 Bensen, G. D., Ritzwoller, M. H. and Shapiro, N. M., 2008, Broadband ambient noise surface wave tomography across the United States, J. Geophys. Res., 113, B05306, doi:10.1029/2007 JB005248. Clifford, P., Greenhaigh, S., Houseman, G. and Graeber, F., 2008. 3-D seismic tomography of the Adelaide fold belt. Geophysical Journal International, 172, 167-186. Conor, C.H.H. and Preiss, W.V., 2008. Understanding the 1720 - 1640Ma Palaeoproterozoic Willyama Supergroup, Curnamona Province, Southeastern Australia: implications for tectonics, basin evolution and ore genesis. Precambrian Research, 166, 297-317. Demidjuk, Z., Turner, S., Sandiford, M., George, R., Foden, J. and Etheridge, M. 2007. U-series isotope and geodynamic constraints on mantle melting processes beneath the Newer Volcanic Province in South Australia. Earth and Planetary Science Letters, 261, 517-533. Farrington, R., Stegman, D., Moresi, L.N., Sandiford, M. and May, D., 2010. Interactions of 3D mantle flow and continental lithosphere near passive margins, Tectonophysics, 483, 20-28. Fishwick, S. and Rawlinson, N., 2012. 3D structure of the Australian lithosphere from evolving seismic datasets. Australian Journal of Earth Sciences, 59, 809-826. Glen, R.A., 2005. The Tasmanides of eastern Australia. In: Vaughan, A.P.M., Leat, P.T., Pankhurst, R.J. (Eds.), Terrane Processes at the Margins of Gondwana. Geological Society, London, pp. 23-96. Graeber, F.M., Houseman, G.A., Greenhaigh, S.A., 2002. Regional 19

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teleseismic tomography of the western Lachlan Orogen and the Newer Volcanic Province, southeast Australia. Geophys. J. Int., 149, 249-266. Kennett, B.L.N., Engdahl, E.R. & Buland, R., 1995. Constraints on seismic velocities in the Earth from travel times, Geophys. J. Int., 122, 108 124. Kennett, B. L. N., Sambridge, M. S. and Williamson, P. R., 1988, Subspace methods for large scale inverse problems involving multiple parameter classes, Geophys. J., 94, 237- 247. Price, R.C., Nichols, I.A., Gray, C.M., 2003. Cainozoic igneous activity. In: W.D. Birch (Ed.), Geology of Victoria. Geo/. Soc. Aust. Spec. Publ. 23, 361375. Rawlinson, N. and Kennett, B. L. N., 2004. Rapid estimation of relative and absolute delay times across a network by adaptive stacking, Geophys. J. Int., 157, 332-340.

Rawlinson, N., Kennett, B. L. N., Vanacore, E., Glen, R. A. and Fishwick, S., 2011. The structure of the upper mantle beneath the Delamerian and Lachlan orogens from simultaneous teleseismic datasets. Gondwana Research,19, 788-799. Rawlinson, N., Reading, A. M. and Kennett, B. L. N., 2006. Lithospheric structure of Tasmania from a novel form of teleseismic tomography, J. Geophys. Res., I l l , B02301, doi:10.1029/2005 JB003803. Rawlinson, N. and Sambridge, M., 2004. Wavefront evolution in strongly heterogeneous layered media using the fast marching method, Geophys. J. Int., 156, 631-647. Young, M. K., Rawlinson, N., Arroucau, P., Reading, A. M. and Tkalcic, H., 2011. High-frequency ambient noise tomography of southeast Australia: New constraints on Tasmania's tectonic past. Geophysical Research Letters, 38, LI 3313,doi: 10.1029/2011GL047971

Key words: Southeast Australia, lithosphere, seismic tomography, crustal structure, upper mantle Email: nick@rses.anu.edu.au Figure 1: Location of all WOMBAT passive seismic arrays in southeast Australia

LF98{1998) MB99(1999) AFOO (2000) TIGGER (2001-2002) SEAL (2004-2005) MT. GAMBIER (2004-2006) EVA (2005-2006) SETA (2006-2007) SEAL2 (2007) SEAL3 (2007-2008) AUSCOPE-GW (2008) AUSCOPE~CU (2009) EAL1 (2009-2010) EAL2 (2010-2011) EAL3 (2011-2012)

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Figure 2: Example of a PKiKP phase recorded by the EAL1 array. Traces have been aligned using the moveout predictions of the ak135 reference model and adaptive stacking. The earthquake is located in Central America. asf.aq

13.51 - 8 9 . 9 1

56.0km

26/11/2009

19:26:52.15

time{s)

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Figure 3: Teleseismic tomography results from the inversion of WOMBAT data, (a) 150 km depth slice showing relative P-wave velocity perturbations, based on inversion of relative arrival time residuals and a laterally invariant starting model; (b) 150 km depth slice that shows absolute P-wave velocity, derived by incorporating a laterally variable initial model based on results from regional surface wave tomography.

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Figure 4: Ambient noise tomography results using a subset of the WOMBAT data. Rayleigh wave group velocity map at 5 s period illustrates both isotropic and azimuthally anisotropic components. In the latter case, fast axis of anisotropy is denoted by the short black lines superimposed on the velocity field.

1502.8

3.0

3.2

152740-

3.4

Rayleigh wave group velocity

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CONTORTIONS IN THE TASMANIDES - A GEOPHYSICAL VIEW R.J. Musgrave^ ^Geological Survey of New South Wales Abstract Mesozoic basins, and thick Cenozoic cover, obscure much of the Tasmanides west of the Great Dividing Range. Enhanced imaging of aeromagnetic data has allowed recognition of complex, curved structures in both the western Lachlan Orogen and the southern Thomson Orogen, which contrast sharply with the simple N - S strike of the eastern Lachlan Orogen. A novel suggestion that these reflect a Silurian orocline that folds the terranes of western Victoria around the core of the Selwyn Block is supported by a variety of geophysical evidence. Long-wavelength components of the magnetic field divide the Tasmanides into domains of oceanic versus continental/arc lower crust, matching the contrast between a ductile and deformation regime within the orocline and brittle deformation in the eastern Lachlan. Widely distributed palaeomagnetic poles from the pre-Devonian Tasmanides can be reconstructed to a much simpler polar wander path by removal of second-order megafolds related to the orocline. Oroclinal deformation after the middle Devonian migrated east to the New England Orogen - but more local vertical axis rotations may have continued to be a feature of the deforming belt that included not only the Tasmanides sensu stricto, but the Alice Springs Orogen as well. Paper Between the belts of outcrop defining the Palaeozoic Lachlan Orogen along the ridges and slopes of the Great Dividing Range to the east, and the Proterozoic to Cambrian backstop of the Delamerian Orogen, Adelaide Fold Belt, and cratonic Gondwana to the west, the subdued topography and thick sedimentary fill of the Mesozoic and Cenozoic basins obscures the underlying structural trends. Where it is exposed in NSW and Victoria, the relatively uniform N - S strike of the Lachlan Orogen gives little indication of any notable tectonic complexity: only the slightly divergent NNW-SSE trend of the Delamerian-Lachlan interface in Victoria (the Stawell and Stavely belts), and the right-angle 24

flexure of the Delamerian Koonenberry and Kars belts around the Curnamona promontory, hint at a more convolute structure. This apparent simplicity contrasts with the contortions clearly visible in the New England Orogen, where both mappable geology and geophysics attest to at least two, and arguably as many as four steeply plunging megafolds, defining a series of oroclines (Flood & Fergusson, 1982; Korsch & Harrington, 1990; Offler & Foster, 2008; Cawood et al., 2011; Glen & Roberts, 2012; Rosenbaum et al., 2012). Since the 1990s, the systematic acquisition of high-resolution, closely spaced aero-magnetic and gravity data by the state and federal government geological surveys has provided the basis for a substantial revision of this view of the Tasmanides. On the longer scale, accurate tie-line control provided by GA's Australia-Wide Airborne Geophysical Survey (AWAGS - Milligan et al., 2009) allows the division of the Tasmanides into domains with contrasting lower to middle crustal physical properties (Figure 1). West of a line defined by the Bootheragandra and Kiewa faults, the middle and lower crust of the western Lachlan Orogen is characterised by high magnetic susceptibility, consistent with oceanic crust, and by a relatively shallow Moho (Kennett et al., 2011). Both the eastern Lachlan Orogen and the DelamerianProterozoic backstop have a much less magnetic lower to middle crust of continental or arc affinity, and a deeper Moho. The Thomson Orogen also appears as a compound entity, with both oceanic and (extended) continental parts. Moho depth correlates better with inferred lower crust composition than with topography. Turning to shorter wavelength magnetic anomalies reveals the detailed structure of the western Tasmanides (Figure 2). Recognition of these features is made challenging by their low amplitude and by a variable depth of cover, which in places exceeds 5 km. Use of the tilt filter (Cooper & Cowan, 2006), a form of differential filter which is normalised to minimise the influence of depth of source.

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dramatically improves the visualisation of western Tasmanides structure (Musgrave & Rawlinson, 2010), and reveals a corollary between lower crust composition and differences in tectonic style between the two halves of the Lachlan Orogen. Brittle, faultdefined structure in the eastern Lachlan contrasts with ductile, semi-circular megafolding in the western Lachlan, where the extension of the Victorian Stawell Zone in NSW wraps smoothly around a core comprising the Hay-Booligal Zone, until terminated abruptly by the Bootheragandra Fault. The southern Thomson Orogen, notable also for a contrasting strike direction, is revealed as a series of arcuate thrust slices in the hanging wall of the Olepoloko Fualt, suggesting growth of the Thomson Orogen by accretion at the expense of the outer edge of the Delamerian Koonenberry Belt. Seizing on the new tilt-filter imagery, Cayley (2012) recognised its potential to explain the apparent repetition of Victorian structural terranes, and the reflection of the directions of structural vergence and sediment provenance, around the Melbourne Zone and underlying Selwyn Block. In place of a simple, linear displacement along a hypothesised extension of the Governor Fault (the "Baragwanath Transform", VandenBerg et al., 2000), the revised model depicts emplacement of the eastern Victorian terranes by rotation around an orocline defined by the Stawell Zone megafold, with strike-slip displacement along the N-S striking Bootheragandra Fault and NNW-striking Kiewa, Rookery and Gilmore Faults. Transpression in the fault system can be seen in the accretion at the Olepoloko Thrust. Growth of the orocline occurred during the Silurian Bindian Orogeny. Although largescale ductile behaviour was restricted to the western Lachlan Orogen, the eastern Lachlan experienced sympathetic deformation. Offset of NNW-SSW striking faults by the N-S Bootheragandra fault system suggests a progressive anti-clockwise rotation of the major transform fault relative to the stress field of about 30°, which may have reflected the rotation of the whole deforming system. More locally, the arcuate plan of the chain of large magnetic and gravity anomalies associated with the deep foundation of the Macquarie Arc suggests a clockwise vertical-axis rotation of about 60° as a drag fold on the Gilmore Fault.

The rotation of the limb of this megafold produced the deflection of the trend in the eastern Lachlan which marks the Lachlan Transverse Zone (Glen & Walshe, 1999). A similar megafold rotated the trend of the terranes in east Gippsland. Vertical-axis megafolding in the eastern Lachlan Orogen provides a mechanism to account for the observation of McElhinny et al. (2003) that palaeomagnetic poles from rocks of the Molong-Monaro terrane (Goleby, 1980), which provide most of the constraint on the Silurian part of the Gondwana apparent polar wander path (APWP), are dispersed over an apparently excessive angular spread (Figure 3). Sampling sites for these poles lie on the rotated limbs of the east Lachlan megafolds, and removal of the clockwise rotation around these folds, and related suspect megafolds in northern Queensland, restores the Silurian poles to a tight group at an appropriate point on the APWP (Musgrave, 2011). Further reduction in the path length of the Gondwana APWP, and the very high rates of continental drift it implies, follows from removal of the proposed overall anticlockwise rotation from the set of eastern Lachlan preMiddle Devonian poles. Subduction zone roll-back has been proposed as a mechanism for the development of smallradius oroclines in the Mediterranean (Rosenbaum & Lister, 2004) and Indonesia (e.g., Spakman & Hall, 2010), and is one of a number of mechanisms suggested for formation of the New England oroclines (Rosenbaum et al., 2012). A similar mechanism may have applied to the western Lachlan Orocline, and is consistent with the ductile deformation and Siluro-Devonian extension in the western Lachlan. Further oroclinal development within the Lachlan Orogen ceased with the advent of the Devonian Tabberabberan Orogeny: the next phase of orocline development migrated east to the New England Orogen in the Carboniferous or Permian. Palaeomagnetic data from this orocline also display rotations (Schmidt et al., 1994; Geeve et al., 2002; Klootwijk 2003), although disputes about the Devonian through Carboniferous track of the APWP for Gondwana have complicated the palaeomagnetic analysis of the orocline But had all the contortions in the rest of the Tasmanides really finished by the mid25

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Devonian? Although Australia west of the New England Orogen is commonly thought of as "cratonic" after this time, the Carboniferous Kanimblan Orogeny, which coincides with the peak of the central Australian Alice Springs Orogeny, was characterised by both eastwest shortening and strike-slip displacements (Glen, 2005). Aeromagnetic imagery dramatically illustrates transpressive displacement in the eastern Lachlan, and some palaeomagnetic data from supposedly cratonic Australia appears to be smeared around a vertical axis, which could reflect distributed strain. Some of the apparent complexity and ambiguity of the Australian and Gondwanan - Palaeozoic APWP may be attributable to continuing contortions in the "Greater Tasmanides" through the Palaeozoic. References: Cawood, P.A., Pisarevsky, S.A., & Leitch, E.G., 2011. Unravelling the New England orocline, east Gondwana accretionary margin. Tectonics, 30, TC5002, doi: 10.1029/2011TC002864. Cayley, R., 2012. Oroclinal folding in the Lachlan Fold Belt: Consequence of southeast-directed Siluro-Devonian subduction rollback superimposed on an accreted Ordovician arc assemblage in eastern Australia. This volume. Cooper, G.R.J., & Cowan, D.R., 2006. Enhancing potential field data using filters based on the local phase. Computers & Geosciences, 32, 1585-1591, doi:10.1016/ j.cageo.2006.02.016. Geeve, R.J., Schmidt, P.W., & Roberts, J., 2002. Paleomagnetic results indicate prePermian counter-clockwise rotation of the southern Tamworth Belt, southern New England Orogen, Australia. J. Geophys. Res., 107, 2196, doi:10.1029/2000JB000037 Glen, R., 2005. The Tasmanides of eastern Australia. In Terrane Processes at the Margins of Gondwana: Geol. Soc. Lond. Spec. Pub. 246, edited by A.P.M. Vaughan, P.T. Leat, & R.J. Pankhurst, pp. 23-96. Glen, R.A., & Roberts, J., 2012. Formation of oroclines in the New England Orogen, eastern Australia. In Oroclines, edited by S. Johnston and G. Rosenbaum, Journal of the Virtual

Explorer, 43, paper 10.3809/jvirtex.2012.00305.

doi:

Glen, R.A., & Walshe, J.L., 1999. Crossstructures in the Lachlan Orogen: The Lachlan Transverse Zone example. Aust. J. Earth Sc/., 46, 641-658. Goleby, B.R., 1980. Early Palaeozoic palaeomagnetism in south-east Australia. J. Geomagn. Geoelectr. 32 (Suppl. Ill), 11-21. Flood, P.G., & Fergusson, C.L, 1982. Tectono-stratigraphic units and structure of the Texas-Coffs Harbour region. In New England Geology, edited by P. G. Flood and B. Runnegar, pp. 71-78, Univ. of New England, Armidale. Hallett, M., Vassallo, J., Glen, R., & Webster, S, 2005. Murray-Riverina region: an interpretation of bedrock Palaeozoic geology based on geophysical data. Geological Survey of New South Wales, Quarterly Notes, 118, 1 - 1 6 . Kennett, B.L.N., Salmon, M., Saygin, E., & AusMoho Working Group, 2011. AusMoho: the variation of Moho depth in Australia. Geophys. J. Internat., 187, 946-958. Klootwijk, C.T., 2003. Carboniferous palaeomagnetism of the Werrie Block, northwestern Tamworth Belt, and the New England pole path. Aust. J. Earth Sci., 50, 865-902. Korsch, R.J., & Harrington, H.J., 1987. Oroclinal bending, fragmentation and deformation of terranes in the New England Orogen, eastern Australia. In Terrane Accretion and Orogenic Belts, Geodyn. Ser., vol. 19, edited by E.C. Leitch and E. Scheibner, pp. 129-139, AGU, Washington, D. C. McElhinny, M.W., Powell, C.McA., & Pisarevsky, S., 2003. Paleozoic terranes of eastern Australia and the drift history of Gondwana. Tectonophysics, 362, 41-65. Milligan, P.R., Minty, B.R.S., Richardson, M. & Franklin, R., 2009. The Australia-wide Airborne Geophysical Survey - accurate continental magnetic coverage. Preview, 138, p. 70. Musgrave, R.J., 2011. Oroclines, rotations, and orogenic history of the Tasmanides. In SGTSG 2012: Causes and Effects of Deformation of the Lithosphere, Geol. Soc.

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Aust Abstracts, No. 102, edited by P. Betts, C. Wilson, T. Rawling & C. Venn, pp. 95-96.

geochronology of early Permian granitoids. Tectonics, 31, TC1006, doi:10.1029/ 2011TC002960.

Musgrave, R., & Rawlinson, N., 2010. Linking the upper crust to the upper mantle: comparison of teleseismic tomography with long-wavelength features of the gravity and magnetic fields of southeastern Australia. Exploration Geophysics, 41, 155-162.

Schmidt, P.W., Aubourg, 0., Lennox, P.G., & Roberts., J., 1994. Palaeomagnetism and rectonic rotation of the Hastings Terrane, eastern Australia. Aust. J. Earth Sci., 41, 547560.

Offler, R., & Foster, D.A., 2008. Timing and development of oroclines in the southern New England Orogen, New South Wales, Aust. J. Earth Sci., 55, 331-340.

Spakman, W., & Hall, R., 2010. Surface deformation and slab-mantle interaction during Banda arc subduction rollback. Nature Geoscience, 3, 562-566, doi:10.1038/NGE0917.

Rosenbaum, G., & Lister, G.S., 2004. Formation of arcuate orogenic belts in the western Mediterranean region. Geo/. Soc. Am. Spec. Paper 383, pp. 41-56. Rosenbaum, G., Li, P., & Rubatto, D., 2012. The contorted New England Orogen (eastern Australia): New evidence from U-Pb

VandenBerg, A. H. M., Willman, C. E., Maher, S., Simons, B. A., Cayley, R. A., Taylor, D. H., Morand, V. J., Moore, D. H., & Radojkovic, A., 2000.7/76 Tasman Fold Belt System in Victoria, Geological Survey of Victoria Special Publication.

Key words: Tasmanides, Lachlan Orogen, New England Orogen, oroclines, Bindian Orogeny, Alice Springs Orogeny, palaeomagnetism, long-wavelength anomalies

Email: robert.musgrave@industry.nsw.gov.au

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Figure 1, Geophysical characterisation of the lower and middle crust of the Tasmanides. (A) Total magnetic intensity (TMI) - note broad divisions of the long-wavelength (regional) signal into blue (low) and green (high); (B) Moho depth (darker shades = deeper Moho) from Kennet et al. (2011); Semi-trapsarent overlay of TMI on Moho depth, showing correlation; (D) Division of the Tasmanides into crustal domains.

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Figure 2. First and second order megafolds in the central Tasmanides. Background image is tilt-filtered TMI. Boundaries of Victorian geological terranes, and their interpreted extension below cover into New South Wales (Hallett et aL, 2005), are shown by thin white traces. Major fault systems along the boundary between the eastern and western Lachlan Orogen in pin, red and orange. Thick white trace follows curved boundary of deep magnetic and gravity sources in the basement of the Macquarie arc; pink arrows indicate first order (the orocline defined by the megafold of the Stawell Zone) and second order vertical-axis rotations.

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Figure 3. (A) Ordovician to Devonian palaeomagnetic poles from the Tasmanides (blue) versus the "Y-type" apparent polar wander path for Gondwana (yellow), after McElhinny et al. (2003). Note very wide angular spread of the Tasmanides poles, particularly over the Silurian. (B) Tasmaindes poles after reconstruction for a simple model of orocline and second-order megafold rotation (Musgrave, 2011), showing reduced angular spread. (C) Detailed reconstruction of Silurian poles, allowing for imprecision in age assignment and rotation model, resulting in tight grouping.

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THE CAMBRIAN EAST GONDWANAN SOUTHEASTERN AUSTRALIA

MARGIN

D.H, M o o r e \ P.G. Betts^ & M. Hall^ ^Monash University, Clayton, Victoria Abstract Cambrian subduction zones in southeastern mainland Australia and Antarctica have been considered to be west directed, whilst those in Tasmania, between the two, are east directed. We have integrated field observations with magnetic and gravity data interpretation to resolve this paradox; the data show the Bambra Fault to have approximately 400 km of post-Cambrian right-lateral movement. This places Tasmania some 200 km east of the west-dipping subduction zone, and so resolves the apparent conflict. As well, the data help define several micro-continental ribbons that comprise the Selwyn Block, and explain the mafic source component and the high Ba content of the Tabberabberan granites in the Melbourne Zone. Paper The nature of the Cambrian east Gondwanan margin in southeastern Australia and the then-adjacent north Victoria Land is paradoxical. In Antarctica, the age of the Ross Orogeny is typically given as about 540 Ma, while in Tasmania the supposedly corresponding Tyennan Orogeny is from about 515 Ma to 500 Ma, and on mainland Australia it typically ranges from 510 Ma to 490 Ma. Even more paradoxical, is the direction of Cambrian subduction. In Antarctica and mainland Australia subduction was west-directed (Finn et al. 1999, Miller et al. 2005) whilst in Tasmania, it was east-directed (Berry and Crawford 1988). Integrating the magnetic and gravity data with field observations can better

resolve the Early Paleozoic relationships between the three regions. Field, magnetic and gravity data all show clear evidence of rifting between King Island and Tasmania, with breakup occurring at about 575 Ma (Calver et al. 2004). Reevaluation of the Tasmanian geology, in conjunction with the potential field data, suggests another crustal break west of the Tyennan Block in central Tasmania. Further west, the Arthur Lineament may also define another block margin, and yet another boundary may be present to the west of King Island. Other micro-continental slices are present in southeastern Australia and Antarctica. Examples of these microcontinents include the Stavely BeltDimboola Igneous Complex in western Victoria (Moore 2006) and crustal blocks to the east of the Curnamona Craton (Greenfield et al. 2011). Stump et al. (2006) suggested the presence of another microcontinental ribbon south of the Byrd Glacier in southernmost Victoria Land. Because the breaks have different magnetic and gravity responses to the sequences either side of them, the responses can be used to trace microcontinental strips under Bass Strait and so more directly connect the outcropping rocks in Tasmania with the substrate of the Melbourne Zone, the Selwyn Block (Cayley et al. 2002). Thus the mafic volcanic rocks on southeastern King Island can be correlated with those that crop out on Phillip Island, and the mafic Spinks Creek Volcanics in the Smithton Basin appear to cross Bass Strait and pass underneath much of the Melbourne Zone. This may account for the mafic igneous 'source' of the Mt Disappointment Granodiorite (Clemens and Benn 2010). The Smithton Basin

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sequence also contains both the Smithton and Black River Dolomite formations, and these may be the source of the high Ba contents of the Melbourne Zone granites recorded by Rossiter and Gray (2008). When correlating Cambrian features, any Benambran or Tabberabberan movement on faults should be reversed. This has a significant effect on the Moyston Fault, the proposed nearsurface location of the west-dipping subduction zone in western Victoria. This fault was truncated by later movements on the Bambra Fault, the northwestern edge of the Selwyn Block. The Bambra Fault has had approximately 400 km of right lateral movement in the Benambran and Tabberabberan Orogenies, and so displaced the Moyston Fault southwest to close to the present continent-ocean boundary. Removing this movement places the Moyston Fault some 200 km west of Tasmania. Thus, in the Victorian to Antarctic sections of Cambrian Gondwana, the east-dipping subduction zone in Tasmania lay outboard of any west-dipping subduction zone. This displacement also positions the Jamieson-Licola felsic volcanics in central Victoria, essentially along strike from the Mt Read Volcanics in western Tasmania. This interpretation is supported by their similar geochemistry and Late Cambrian ages (Crawford et al. 2003). A direct, along-strike correlation between the Stavely-Dimboola rocks and the Mt Read Volcanics is deemed highly unlikely. The presence of different subduction systems and different discrete continental ribbons invites comparison with the modern southwest Pacific, where similar features are common. These results show that combining structural geophysical and geological observations can provide a framework for resolving complex tectonic systems and bring unexpected insights into many disparate problems.

References BERRY R. F. & CRAWFORD A . J.

1988

The tectonic significance of Cambrian allochthonous mafic-ultramafic complexes in Tasmania, Australian Journal of Earth Sciences, 35, (4): 523533. CALVER C. R., et al. 2004 U-Pb zircon age constraints on late Neoproterozoic glaciation in Tasmania, Geology, 32, no. (10):893-896. CAYLEY R. A., et al. 2002 Proterozoic

Early Palaeozoic rocks and the Tyennan Orogeny in central Victoria: the Selwyn Block and its tectonic implications, Australian Journal of Earth Sciences, 49, (2), pp. 225-254. CLEMENS

BENN

2010

Anatomy, emplacement and evolution of a shallow-level, post-tectonic laccolith: the Mt Disappointment pluton, SE Australia, Journal of the Geological Society, London, 167: 9 1 5 - 9 4 1 . A. J., et al. 2003 Neoproterozoic and Cambrian. In BIRCH W. D. (ed). The geology of Victoria, pp. 73-93. Third ed. Melbourne: Geological Society of Australia (Victoria Division). CRAWFORD

FINN C., et al. 1 9 9 9 Aeromagnetic legacy of early Paleozoic subduction along the Pacific margin of Gondwana, Geology, 27, (12): 1 0 8 7 - 1 0 9 0 . GREENFIELD J. E., etal. 2011 The Mount Wright Arc: a Cambrian subduction system developed on the continental margin of East Gondwana, Koonenberry Belt, eastern Australia, Gondwana Research, 19: 650-669. MILLER J. M., et al. 2005 Evolution of a reworked orogenic zone: the boundary between the Delamerian and Lachlan Fold Belts, southeastern Australia, Australian Journal of Earth Sciences, 52, (6): 921-940. MOORE D. H. 2 0 0 6 The boundary of the Rodinian craton in southeastern Australia. In DENHAM D. ed. 2 0 0 6 Australian Earth Sciences Convention Melbourne: Geological Society of

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Australia and Australian Exploration Geophysicists.

Society

ROSSITER A. & GRAY C. M. 2 0 0 8 Barium contents of Victorian granites: key to understanding crustal architecture in the southern Lachlan Fold Belt?, Australian Journal of Earth Sciences, 55, (4) 433448.

Transantarctic Mountains during the Neoproterozoic - Early Paleozoic. In FUTTERER D. K., et al. eds. Antarctica Contributions to Global Earth Sciences Proceedings of the IX International Symposium of Antarctic Earth Sciences Potsdam 2003. pp. 181-190. Heildelberg Berlin: Springer.

STUMP E., GOOTEE B. & TALARICO F. 2 0 0 6 Tectonic model for development of

the Byrd Glacier discontinuity surrounding regions of

and the

Key words: Tasmania, Victoria, Antarctica, potential field interpretation, granite, Melbourne Zone Email: david.moore@monash.edu Figure 1, Total magnetic intensity image showing the Victorian and Tasmanian zone boundaries, the Bambra Fault (B), the Moyston Fault (M) and possible correlations of the Smithton Basin across Bass Strait to the Selwyn Block and the Mt Read Volcanics (R) to the Jamieson-Licola Volcanics (J). Magnetic data from Geoscience Australia and the GeoScience Victoria.

180

270

Kilometres 360

Spinks Ck Volcanics Zone boundary

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OROCLINAL FOLDING IN THE LACHLAN FOLD BELT: CONSEQUENCE OF SOUTHEAST-DIRECTED SILURODEVONIAN SUBDUCTION ROLLBACK SUPERIMPOSED ON AN ACCRETED ORDOVICIAN ARC ASSEMBLAGE IN EASTERN AUSTRALIA R.A. Cayley^ ^Geological Survey of Victoria Abstract A new model developed for the wide and complex Lachlan Fold Belt (LFB) of eastern Australia advocates mid- to Late Silurian clockwise oroclinal (mega-) folding and fault fragmentation during the Bindian Orogeny, superimposed onto a narrow, simple, accretionary Late Ordovician-Early Silurian LFB. A faultsegmented Z-shaped oroclinal fold of approximately 400km amplitude was initiated in the mid-Silurian by the propagation of large dextral strike-slip faults into central-west NSW from a newly-active plate boundary in North Queensland. The orocline developed to completion in the Late Silurian-Early Devonian within the advancing and fragmenting upper plate of a single continent-dipping subduction system in rollback through eastern Victoria and into north-eastern Tasmania. The new model fits new aeromagnetic data, resolves long-standing issues with paleomagnetic data, explains complex LFB palaeogeographic and stratigraphic relationships and structural vergence reversals (eg. the Tabberabbera Zone), and has allowed a link to be established between Bindian strike-slip deformation, the Alice Springs Orogeny, oroclinal folding, and a transition to crustal extension and granite genesis across the LFB in the Late Silurian/Early Devonian. Introduction: Recent tectonic models for the Lachlan Fold Belt (LFB) attribute geometry (structural vergence changes between west, central and eastern portions), sedimentary relationships, width and 34

magmatic history to several coeval subduction zones of opposing polarity (eg. Gray & Foster, 1998), or to largescale horizontal movements along strike-slip faults, such as the mid- to Late Silurian Baragwanath Transform (Willman etal., 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 (mega-) folding superimposed onto a narrow, simple Late Ordovician-Early Silurian LFB. Oroclinal folding was caused first by the propagation of large dextral strike-slip faults into central-west NSW from Queensland, and then by extension during south-directed subduction rollback which commenced in the Late Silurian and rolled through eastern Victoria into northeastern Tasmania. A fault-segmented, orogen-scale Zshaped fold superimposed over the accreted Ordovician Macquarie Arc system 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. Restoration to the simple Lachlan Fold Belt configuration of the Late Ordovician allows fore-arc and back-arc regions to the Macquarie Arc to be identified and characterised with confidence. This

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simplified configuration restores Eastern LFB Siluro-Devonian paleomagnetic poles to the Gondwanaland APWP (Musgrave & Cayley, 2011) and allows for closer ties with the coeval Ross Orogen elsewhere along the east Gondwanaland margin (Cayley, 2011). Following the new model through time: Late Ordovician-Early Silurian Benambran Orogeny: Formation of the Lachlan Fold Belt Mk. 1. Prior to the Late Ordovician, Tasmania 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). The western end of this apparently intraoceanic arc system may have terminated at a sinistral intraoceanic transform fault inferred to facilitate the northward migration of Vandieland' in the lower plate of the Macquarie Arc system throughout the Ordovician (Cayley, 2011). The protoTabberabbera and Mallacoota zones lay east and southeast of the Macquarie Arc in a fore-arc position, while the proto- Wagga-Omeo Zone lay in a back-arc position to the west and north. Farther south, the protoBendigo Zone was a region that apparently lay outside the influence of the Macquarie Arc - possibly a trappedplate segment of oceanic crust located west of the inferred sinistral intraoceanic transform. Geometrical restoration of paleocurrent data suggests that sediment supply into the proto-Lachlan region was probably via several overlapping submarine sediment aprons derived from the Ross Orogen via several different pathways. Some sediments appear to have travelled east and northeast directly off Gondwanaland, as in the case of the Bendigo and Wagga-Omeo zones. Other sediments appear to have travelled via submarine channels east of

Vandieland, sourced from Antarctic parts of Gondwanaland farther south, as in the case of the Tabberabbera and Mallacoota zones. Towards the end of the Ordovician a con ve rg ent conti ne nt-d i rected subduction system became established along the entire eastern edge of the LFB (eg. Miller & Gray, 1997), including outboard of Vandieland. This may have involved the migration of subduction systems from farther south (Cayley, 2011). The first hints of migration of outboard subduction into the region east of Vandieland and the Macquarie Arc may be recorded by the widespread disruption of sediment supply into the eastern Lachlan Fold Belt that began in the Late Ordovician. This is recorded as the dramatic change from thick Adaminaby Group turbidite sedimentation, to starved sedimentation of the Late Ordovician Bendoc Group which is dominated by black shale (VandenBerg et al., 2000). Disruption may have been caused by the growth of subduction trenches outboard of Vandiland, capturing sediments previously destined for the proto eastern LFB. Such a transition in sediment style is not seen in Ordovician strata of the western LFB, probably because sediment supplies to this region were via more direct routes less able to be influenced by subduction systems farther outboard. The Macquarie Arc system also began to become disrupted at this time, driven into convergence and ultimately accreted against the east Gondwanaland margin. This Late Ordovician-Early Silurian accretion event - the Benambran Orogeny - formed the LFB. In Victoria, parts of the Macquarie Arc appear to have been overthrust from both the back-arc and fore-arc sides along the Gilmore (Indi) and McLauchlan faults respectively at this time, downthrowing the arc, and allowing accumulation of sediments in overlying foreland basins; the Limestone Creek

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Graben and the proto-Deddick Zone with Early Silurian Yalmy Group clastics (VandenBerg et al., 1992). Developed into a thin-skinned thrust system at the culmination of the Benambran Orogeny, the Deddick Zone conceals the presence of the underlying Macquarie Arc in Eastern Victoria (Rawling et al., 2011). In the reconstruction presented here the LFB at the time of its formation and accretion to the eastern edge of Gondwanaland 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, and also including the Charters Towers Province farther north) accreted to, and perhaps partly superimposed upon, the eastern edge of the older Delamerian Orogen (Stawell and Kayrunnera zones); (2) the Macquarie Arc complex itself and; (3) a narrow fore-arc accretionary complex farther outboard characterised by predominantly east (ocean)-verging structures - the Mallacoota, Kuark and Tabberabbera zones. The Bendigo Zone lay southwest along strike from the Tabberabbera Zone at this time, more remote from subduction systems by virtue of its position west of the Selwyn Block (Cayley et al., 2002) and south of the Macquarie Arc, yet deformed in the same event and sharing the same gross fault geometries (Cayley, et al., 2011).

escape tectonics and the influence of subduction rollback. Translation and lateral escape in the NSW and Victorian parts of the Lachlan Fold Belt possibly followed the appearance of a new dextral transcurrent plate boundary along what is now recognised as the Tasman Line' in Northern Queensland (Figure 1). This system presumably developed in response to a Late Silurian change in regional plate kinematics along the whole east Gondwanaland margin, and propagated southwards into the Lachlan orogen. Beginning in northern Queensland, southward movements began to translate parts of the newly-accreted Macquarie Arc/LFB system southwards, beginning a process of fragmentation that continued into the Early Devonian. This event also heralded the onset of the Alice Springs Orogeny farther west. An Intraplate' orogeny that appears open to the east, the Alice Springs Orogeny was possibly driven by long-term stresstransfer across this new transcurrent plate-boundary into the interior of Gondwanaland.

Late Silurian Bindian Orogeny: subduction rollback, oroclinal folding, and formation of the Lachlan Fold Belt Mk. 2.

In NSW, the prominent salient of the Curnamona Province directed southerly LFB translation oceanwards, so that the southern parts of the orogen began to evolve into a fault-bounded southeastdirected lateral escape system. The main fault bounding the southern margin of this system is preserved today as the subvertical, dextral Kiewa Fault Zone in eastern Victoria (interpreted as lying beneath the Bootheragandra Fault in NSW).

The Late Silurian Bindian Orogeny appears to mark the beginning of an interval where the LFB began to become fragmented, with a portion remaining more-or-less 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. Overall, Bindian Orogeny movements in the LFB were southdirected and reminiscent of lateral

As parts of the accreted Macquarie Arc system adjacent to the Kiewa Fault began to be forced oceanward by lateral escape, adjacent segments of the downgoing paleo-Pacific slab farther east may have become point-loaded, a possible explanation for why the down-going slab here tripped into local rollback. Timing and location of the onset of rollback may be given by the sudden appearance, in the Late Silurian, of the Darling Basin behind the parts of the Macquarie Arc

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adjacent to the Curnamona salient. This basin, and its initial fill of Mt Daubeny Formation, may mark the point where southerly LFB translation switched from forced lateral escape to south-east directed extension and fragmentation chasing a retreating slab (Collins, 2002; Collins & Richards, 2008). Subduction zones in roll-back are almost invariably curved, and it is the natural curve of such systems that is envisaged as the primary control on the geometry of the giant oroclinal folds that subsequently developed in the LFB. The curve of systems in roll-back also reveals subduction polarity and, in this instance, confirms the continent-dipping geometry of the subduction systems active in the Tasmanides throughout the Ordovician-Early Devonian. The oroclinal folds in the LFB are asymmetrical. This is shown by the contrasts in structural style across the LFB between the largely intact megafold of the Mallacoota and Tabberabbera zones in the eastern LFB, the faultdisrupted Macquarie Arc and WaggaOmeo Metamorphic Complex, and the refolded northern Stawell Zone and regions that appear to have escaped any significant deformation at this time in the western LFB - the southern Stawell Zone, Bendigo Zone and Selwyn Block. Although inherited features such as the Curnamona salient undoubtedly imposed initial asymmetry upon the Lachlan Fold Belt system, it is the presence or absence of underlying active subduction zones in different parts of the fold-belt at the time of Late Silurian roll-back initiation that is interpreted here as the more profound influence. In the Ordovician, the Mallacoota and Tabberabbera zones apparently lay in a fore-arc position to the down-going Macquarie Arc slab. Consequently, these regions were strongly influenced by the geometry of the underlying subduction system when it evolved into rollback in the Late Silurian.

In contrast, there is little evidence supporting the existence of a downgoing slab beneath the Stawell or Bendigo zones or beneath the Selwyn Block at any time after the Cambrian (Cayley et al., 2011). Even if some continent-dipping subduction had occurred beneath the Bendigo Zone in the Ordovician (eg. Gray & Foster, 1998; Fergusson, 2009), the presence of the Selwyn Block continental craton farther east (Cayley et al., 2002; 2011) meant that no subsequent subduction-rollback could have occurred across this region. With no possibility of roll-back within the western Lachlan Fold Belt, this west Victorian part of the LFB remained accreted to the eastern edge of Gondwanaland as roll-back progressed farther north and east. The Victorian portion of the western LFB records only far-field effects such as minor dextral shearing along the Lachlan-Delamerian interface, probably responsible for wrench-dominated deformation of the overlying Grampians Group (Cayley & Taylor, 1997; 2001). Stawell Zone rocks that lay east and north of the Curnamona salient in NSW were more exposed to the influence of Late Silurian dextral strike-slip faults and rollback-induced extension, and were apparently peeled/sucked away from their original position of formation against the Wonnaminta Block of western NSW (where they were originally contiguous with the Kayrunnera Zone; Greenfield et al., 2011), and oroclinally folded 90° clockwise to extend east well into the interior of the extending Lachlan Fold Belt. This part of the Stawell Zone, together with the Hay-Booligal Zone and Tabberabbera Zone, represent paraautochthonous portions of the LFB, folded about a vertical megafold axis centred near the Hay-Booligal Zone and called the Boosey Orocline. Although folded, these regions were prevented from becoming fully detached from the western LFB because they were pinned northwest of the Selwyn Block. This 'pinning' is key to explaining

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why the large Late Silurian - Early Devonian dextral-strike slip faults that fragmented the eastern LFB did not propagate into the western LFB, and why the LFB oroclines are asymmetrical. In the eastern LFB, the Wagga-Omeo Metamorphic Complex and Macquarie Arc segments exposed in eastern NSW and in eastern Victoria all lie east of the major Kiewa-Kancoona dextral fault system, and this whole region was faultsegmented and translated hundreds of kilometres to the southeast within the advancing and fragmenting upper plate of a Late Silurian/Early Devonian subduction system in roll-back. This region was therefore virtually fully detached from Gondwanaland, and represents a fragmented, allochthonous portion of the Lachlan Fold Belt, transported south within the core of the growing orocline. South-directed Silurian subduction rollback superimposed upon an accreted Ordovician arc terrane provides a regional context for the sudden and widespread appearance of deep rifts (Tumut, Hill End troughs etc.; Fergusson, 2010), and Silurian arc magmatism - predominantly S-type within- and east of- the Ordovician Macquarie Arc (Collins, 2002), the development of related high-T terranes such as the Cooma Complex (Richards and Collins, 2002) and Kuark Metamorphic Complex within this part of the LFB, and the wrench-tectonics associated with magmatism documented in the vicinity of granites such as the Wyangala Batholith (Hobbs, 1965). Strike-slip faulting within the Macquarie Arc lead to its fragmentation, and juxtaposed rifts with back-arc chemistry (and likely initial location; eg. Tumut Trough) against arc and even fore-arc fault-segments to create a perplexing modern geometry. The region of crust east of the Kayrunnera Zone and north of the Stawell Zone in NSW was fragmented to form the irregular basement to much of the Darling Basin. However, a key difference to the eastern LFB is that the 38

Darling Basin region lay well behind the retreating slab, and so beyond the influence of arc-related magmatism. The only point of connection between the eastern and western LFB portions that survived the roll-back event intact was the Tabberabbera Zone which, instead of faulting, was able to fold ahead of advancing segments of the Macquarie Arc to form the central limb of a giant Z-shaped oroclinal fold. As the Kiewa-Kancoona Fault propagated southeast following the onset of roll-back, it cut across the northern Stawell Zone and then grew out into the interior of the Lachlan Orogen. Eventually, the Kiewa-Kancoona Fault cut across the southern Macquarie Arc, dividing it. To the east are the allochthonous fault slices of arc exposed in eastern NSW, and to the west lies a para-autochthonous arc segment pinned behind the Selwyn Block. Not previously recognised as such, this segment is the buried Hay-Booligal Zone. Although it cut and offset the arc, the Kiewa-Kancoona Fault foundered in the adjacent low-grade meta-sediment-rich fore-arc terrane (the Tabberabbera and Kuark/Mallacoota zones). Instead of faulting, this fore-arc region was able to accommodate strain by oroclinal folding, possibly facilitated partly by flexural slip reactivation of the multitude of large regularly-spaced thrusts already developed within this accretionary system (Willman et al. 2005). 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 ZoneTabberabbera Zone boundary as an intriguing array of sub-vertical strike-slip fault splays arranged in a horsetail (Willman et al., 2005), and as subvertical kink-folds and crenulation cleavages parasitic to a southerly

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oroclinal fold hinge, the giant Tambo Orocline. Eventually the Tabberabbera Zone underwent more than 120° clockwise rotation to form the middle limb of a Zshaped oroclinal fold couple of ~400km amplitude. One obvious consequence of so much rotation is the apparent reversal of pre-existing geometries within the rotated middle limb. Westdipping thrust faults developed in the Tabberabbera Zone during its accretion in the Benambran Orogeny in a fore-arc position east of the Macquarie Arc have been rotated to dip northeast in a position apparently west of the Macquarie Arc. Concomitant clockwise rotation within the Boosey Orocline to the west of the Kiewa-Kancoona Fault System explains the present-day east-west trending geometry of the Hay-Booligal Zone and the northern Stawell Zone and this, inturn, explains why the Kiewa-Kancoona Fault system itself, together with the allochthonous Macquarie Arc segments, would have also been forced to undergo progressive clockwise rotation throughout the Bindian Orogeny. As the oroclines developed, the portion of the Tabberabbera Zone that was once transitional with the northern Bendigo Zone began to separate within the core of the Boosey Orocline, and rotate progressively clockwise. As this part of the Tabberabbera Zone reached an east-west orientation, its proximal outer edge began to impinge upon, and then overthrust, the northern margin of the Selwyn Block, and the northern Bendigo Zone. East-west trending structures developed at this time include the Governor Fault at Dookie (Cayley et al., 2011), the Whroo Anticline, and other east-west striking faults and folds developed in the northern Melbourne Zone (Gray & Mortimer, 1996). As the orocline tightened further, end-loading of the Benambran-age, north-trending Heathcote Fault Zone caused internal and external buckling of this structure, manifest today as the Rochester Antiformal stack (VandenBerg et al..

2000) and Tooborac Megakink (Cayley et al., 2008), and subtle regional scale culminations and troughs superimposed on pre-existing Bendigo Zone fold structures farther west. Early Devonian basin formation, granite intrusion, Tabberabberan Orogeny Lachlan Fold Belt Mk. 3: In the Early Devonian rollback finally progressed to the point where Proterozoic crust of the Selwyn Block and western Tasmania ( Black et al., 2010) began to be drawn into the southwestern (ie. northeast-dipping) side of the subduction system. At this point, the western flank of the curved roll-back failed, and no further orocline rotation, driven by local southwest-directed subduction roll-back, was possible. The Tabberabbera Zone and, farther south, the Mathinna Terrane of NE Tasmania, finally docked against the eastern margin of Vandieland (ie. the eastern edge of the Selwyn Block and of western Tasmania), and the geometry of the Kiewa-Kancoona fault-system and of the eastern LFB orocline finally stabilised. Although the western side of the subduction system had now been forced to shut down, the eastern side of the rollback system still faced open ocean, and so south-east directed roll-back of the eastern part of the system was ongoing. This allowed large faults with more protracted dextral strike-slip movement histories (eg. the Cassilis Shear Zone) to develop within a stabilised south-east directed extensional stress field. This phase is marked by 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, predominantly l-type, granitic intrusions and eruptions, possibly generated by decompression melting accompanying distributed crustal extension. The region underlain by the Selwyn Block was one of the only regions not to experience decompression melting at this time. The older lower crust here had

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already been dehydrated by melt extraction during Cambrian-and olderorogenesis - as seen in Western Tasmania. With subduction rollback within the western limb of the orocline permanently extinguished by the ingestion of western Tasmania and the Selwyn Block, simple continent-dipping subduction reestablished farther outboard along the whole eastern margin of Gondwanaland at approximately 400 Ma. This system reverted to east-west directed convergence at approximately 390 Ma, generating the Early Devonian Tabberabberan Orogeny, and the final assembly of the Lachlan Fold Belt to virtually its present day complex and wide configuration. In the Tabberabberan Orogeny, eastwest directed convergence between the eastern and western LFB finally uplifted and deformed cover rocks upon the intervening Selwyn Block to evict the last major sea from the region, and to form the Melbourne Zone (Cayley et al., 2002). North-south trending folds and thrusts developed at this time overprinted the earlier east-west trending folds and thrusts along the northern margin of the Melbourne Zone to form intriguing fold and fault interference patterns (Gray & Mortimer, 1996). The pre-existing east-west trending portion of the Governor Fault along the northern Melbourne Zone margin was also refolded at this time, generating the sinuous fault-trace visible in magnetic data here. An episode of mid-Devonian extension related to renewed subduction rollback farther east (a precursor to the development of New England Fold Belt systems in the Carboniferous) caused the mid-crust beneath the newly thickened Melbourne Zone to melt, generating a suite of geochemically distinct granitic intrusions that are confined to central Victoria (adjacent regions did not re-melt at this time - they had already contributed to the Early Devonian magmatic event). Conclusion

This is a powerful new model. It does not require multiple subduction zones active in Ordovician-Early Silurian time to explain abrupt, coeval, reversals in structural vergence across the LFB (eg. the Tabberabbera Zone), or the observed complex distribution of sedimentary packages. Instead, it explains these complexities by interpreting oroclinal folds superimposed on a much simpler single system, the oroclines developed within the advancing and fragmenting upper plate of a single continent-dipping SiluroDevonian subduction system in rollback. 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 Late Silurian/Early Devonian. The model also explains the nature and origin of the enigmatic HayBooligal Zone as the fault-segmented western end of the Ordovician Macquarie Arc. The model has enormous implications for the Palaeozoic geological evolution and mineral prospectivity of all the eastern states of Australia, for example understanding the original context of the Charters Towers Province, and the potential for much greater diversity within the so-called Thompson Fold Belt'. The model establishes a possible regional framework for the Alice Springs Orogeny, the effects of which extend from Queensland across the Northern Territory into Western Australia. Two consequences for Victorian mineral prospectivity are 1: recognition that the Macquarie Arc extends into eastern Victoria beneath the Limestone Creek Graben and Deddick Zone (and Omeo Zone), providing a new exploration target, and also regional context for possible derivative deposits such as Benambra VHMS Cu, and Unicorn porphyry Mo; and 2: recognition that the northern-most Tabberabbera Zone -

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mostly under shallow cover - was, in the Benambran Orogeny, transitional to the Bendigo Zone, and therefore likely shares its Late Orodvician/Early Silurian orogenic gold prospectivity.

2002. Proterozoic-Early Palaeozoic rocks and the Tyennan Orogeny in central Victoria: the Selwyn Block and its tectonic implications. Australian Journal of Earth Science 49, 225-254.

Farther afield, the model raises questions about the development of plate boundaries in regions adjacent to Gondwanaland in the Late Ordovician Silurian, and the role of subduction rollback and consequent large-scale oroclinal fold rotations in generating contrasting geometries such as those seen today in related, but disconnected, regions such as New Zealand.

Cayley, R. A., Skladzien, P. B., Williams, B. & Willman, C. E. 2008. Redesdale and part of Pyalong 1:50 000 map area geological report. Geological Survey of Victoria Report 128. GeoScience Victoria, Depart-ment of Primary Industries.

References Black, L.P., Everard, J.L., McC.enaghan, MP, Korsch, RJ., Calver, C.R., Fioretti, A.M., Vrown, A.V., & Foudoulis, C., 2010. Controls on DevonianCarboniferous magma-tism in Tasmania, based on inherited zircon age patterns, Sr, Nd and Pb isotopes, and major and trace element geochemistry. Australian Journal of Earth Sciences 57, 933 - 968 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 - 6 4 9 . 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 2006 deep crustal reflection survey. Australian Journal of Earth Sciences 58, 113 - 156. Cayley, R. A. & Taylor, D. H. 1997. Grampians special map area geological report. Geological Survey of Victoria Report 107. Cayley, R. A. & Taylor, D. H. 2001. Ararat 1:100 000 map area geological report. Geological Survey of Victoria Report Cayley, R. A., Taylor, D. H., VandenBerg, A. H. M & Moore, D. H.

Collins, W.J., 2002. Hot Orogens, tectonic switching and creation of continental crust. Geology 30, 535-538 Collins, W.J. & Richards, S.W., 2008. Geodynamic significance of 'postcollisional' S-type granites in circumpacific orogens. Geology 36, 559-562. Fergusson, C.L., 2009. Tectonic evolution of the Ordovician Macquarie Arc, central New South Wales: arguments for subduction polarity and anticlockwise rotation. Australian Journal of Earth Sciences 56, 179-194. Fergusson, C. L., 2010. Plate-driven extension and convergence along the East Gondwana active margin: Late Silurian-Middle Devonian tectonics of the Lachlan Fold Belt, southeastern Australia. Australian Journal of Earth Sciences 57, 627 - 649. Glen, R.A., Crawford, A.J., Percival, I.G & Barron, L.M., 2007. Early Ordovician development of the Macquarie Arc, Lachlan Orogen of southeastern Australia. Australian Journal of Earth Sciences 54, 167 - 179. Gray D.R. & Foster D.A. 1998. Character and kinematics of faults within the turbidite-dominated Lachlan Orogen: implications for tectonic evolution of eastern Australia. Journal of Structural Geology M, 1691-1720. Gray, D.R. & Mortimer, L., 1996. Implications of overprinting deformations and fold interference patterns in the Melbourne Zone, Lachlan Fold Belt. Australian Journal of Earth Sciences 43, 103-114.

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Greenfield, J.E., Musgrave, R.J., Bruce, M.C., Gilmore, P.J. & Mills, K.J., 2011. The Mount Wright Arc: A Cambrian subduction system developed on the continental margin of East Gondwana, Koonenberry Belt, eastern Australia. Gondwana Research 19, 650 - 669. 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 118, 1-16.

Hobbs, B.E., 1965. Structural analysis of the rocks between the Wyangala Batholith and the Copperhannia Thrust, New South Wales. Journal of the Geological Society of Australia, 12 (1), 1 -24. Miller, J. Mc. L. & Gray, D.R., 1997. Subduction related deformation and the Narooma anticlinorium, eastern Lachlan Fold Belt. Australian Journal of Earth Sciences 44. 237-251.

Rawling, T.J., Osborne, C.R., Mc Lean, M.A., Skladzien, P.B., Cayley, R.A. & Williams, B., 2011. 3D Victoria Final Report. GeoScience Victoria 3D Victoria Report 14. Department of Primary Industries. VandenBerg, A. H. M., Nott, R.J. & Glen, R.A., 1992. Bendoc 1:100 000 Map geological report. Geological Survey of Victoria Report 90. VandenBerg, A. H. M., Willman, C. E., Maher, S., Simons, B. A., Cayley, R. A., Taylor, D. H., Morand, V. J., Moore, D. H. & Radojkovic, A. 2000. The Tasman Fold Belt System in Victoria. Geological Survey of Victoria Special Publication. Willman, C.E., Cayley, R.A., VandenBerg, A.H.M., Haydon, S.J., Osborne, C.R. Seymon, A.R. & Thom, J.L., 2005. Dargo 1:100 000 map area geological report. Geological Survey of Victoria Report 126. Department of Primary Industries. 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.

Musqrave,R.J. & Cavlev,R. A., 2011. Unfolding an Orocline Restores Australian Siluro-Devonian Poles to the Gondwana APWP. American Geophysical Union, Fall Meeting 2011, abstract #GP11A-0991 Key words: Lachlan Fold Belt; Oroclinal folding; Subduction roll-back; Bindian Orogeny; dextral trans-tension; Alice Springs Orogeny; Tambo Orocline; Boosey Orocline Email: Ross.Cayley@dpi.vic.gov.au

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Figure 1 Caption: Total Magnetic Intensity of Australia, showing the proposed evolution of the Lachlan Fold Belt and of the Australian portion of Gondwanaland from 440 - 310 Ma. At the end of the Benambran Orogeny, the Macquarie Arc had been accreted to the east-Gondwanaland nfiargin above the continent-dipping paleo-Pacific slab. Following initiation of a dextral transcurrent plate boundary within the upper plate at MSSMa in north Queensland, ocean-directed lateral escape began adjacent to the Curnamona Salient (near Broken Hill). Subduction roll-back initiated outboard at --430 Ma, and folded progressively south towards northeastern Tasmania, arriving at MOOMa. Euler Pole of orocline rotation matches Euler Pole calculated for restored Lachlan Silurian and Devonian paleomagnetic data (Musgrave & Cayley, 2011).The final configuration of the Lachlan oroclines is shown in the inset figure, together with crustal elements in central and north Queensland that are interpreted to have moved south throughout the Silurian - Early Devonian.

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THE CREATION OF THE SOURCE OF THE TASMANIANFERRAR- KAROO JURASSIC LARGE IGNEOUS PROVINCE: NOT A PLUME SOURCE? J . F o d e n \ P. Sossi^ ^ G. B a i n e s \ D. Segul^ ^University of Adelaide, School of Earth and Environmental Science, ^ RSES, ANU Abstract The --180 Ma Jurassic suite of low-Ti tholeiites forms a large igneous province (LIP) extending from the Karoo in South Africa to Tasmania and Kangaroo Island off South Australia. This LIP does not have OIB-related geochemical characteristics and is thus not plume-sourced. These mafic magmas are geochemically distinctive. Compared to oceanic basalts they are enriched in Si02, and depleted in Fe203(T), Na20, Ti02 and P2O5. These major element compositions imply derivation from a mantle that is more depleted than the source of MORB, and yet their isotopic and lithophile trace element signatures are highly anomalous, more typical of Proterozoic upper continental crust. We demonstrate that these characteristics are reconciled by partial melting of a refractory depleted harzburgite source contaminated by a small fraction (--3%) of upper continental crust at shallow mantle depths (-1.5 GPa). The depth of melting clearly indicates that this is a lithospheric mantle source. This Jurassic province exactly reflects the footprint of the prior Cambrian orogen (DelamerianRoss-Cape Saldarnian), which apparently along its length, was terminated by rapid buoyancy-driven exhumation, consistent with delamination of dense material from the sub-crustal lithosphere at the end of the Cambrian. Eclogite xenoliths are transported by South Australian Jurassic kimberlite. These record metamorphic pressures from 1 to >3 GPa and have geochemical and Nd-isotopic compositions that imply their protoliths were underplated, Late Neoproterozoic to Early Cambrian mafic magmas. An interpretation for these is as relics from

end of Cambrian delamination that shed the dense eclogite and entrained continental crustal material through the more buoyant refractory harzburgite peridotite lithospheric mantle. This delamination event could have contaminated (enriched) this residual lithospheric mantle with low temperature melts from entrained continental material. We have modelled the thermal evolution of a mantle peridotite layer enriched with 3% upper continental crust. Radiogenic heat production of this zone is about 30 times that of the refractory depleted mantle. However to achieve highly anomalous heating (to approach 1300°C) over the time period proposed (-300 Ma) requires that the uppermost lithospheric mantle has extremely (improbably?) low thermal conductivity. Our conclusion is that although the material source of the LIP is entirely within the lithospheric mantle with a substantial thermal component from radioactive heating, it also requires an asthenospheric heat source either due to lithosphere thinning or plume upwelling. Introduction The common assumption is that magma generation in the earth's mantle is due to one of three causes: near adiabatic decompression, hot mantle plume ascent and consequent decompressional melting or due to depression of solidus temperatures by subduction generated fluid flux. Because this paradigm is quite rigidly held-to it may have led to a failure to consider other mechanisms, and a consequent misconstrual of tectonic controls of some magmatic provinces. A case in point

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may be provided by the example the Jurassic Large Igneous Province (LIP) that stretched across Gondwana from the Karoo of Southern Africa to Tasmania and Kangaroo Island {Molzahn et al. 1996, Hergt et aL, 1989, 1991, 2001). The Ferrar-Tasmania-Kangaroo Island Low-Ti tholelite Suite. This -180 Ma. suite of magmatic rocks has a very wide distribution from southern Africa to Kangaroo Island (Fig. 1). It does not show the age-location progression to be expected for lithospheric plate migration across a fixed asthenospheric thermal anomaly. With some variation, the rocks across the entire province share similar distinctive and highly anomalous geochemical and isotopic characteristics. They are relatively siliceous mafic magmas. Compared to oceanic basalts (Fig. 2) they are enriched in Si02, and depleted in Fe203(T), Na20, CaO, Ti02 and P2O5. They also have relatively low concentrations of high field-strength incompatible trace elements. These characteristics are all consistent with a mantle peridotite source that is CPXpoor and relatively OPX-rich that is more refractory than the source of MORB. By contrast lithophile trace elements and the composition of Nd-, Sr- and Pbisotopes are very much more like continental crust. Compared to MORB they have very much higher Rb/Sr, Ba/Nb and much lower Ce/Pb and Nb/U. They have eNd-isotopic compositions of The Source and Conditions of Melting of the Ferrar-Tasmania Suite? By taking the most mafic, unfractionated compositions from the Ferrar literature and from our own Tasmanian and Ki data and reversing the low-pressure fractionation impact (OPX + PI and then 01), we have established a parental magma. Using both PMelts {Ghiorso et al. 2002) and the P-T dependency of Si02 activity {Lee et al., 2009) we have then calculated its melting conditions

from a depleted harzburgite source. Both calculation methods give similar results (Fig. 3), requiring very low pressures (1.4 GPa) and high T (1350°C). The indicated pressure is equivalent to a depth of ~ 50 km, clearly in the upper part of the lithospheric mantle. Constrained particularly by the Ndisotopic compositions, the lithophile trace element signatures of the suite are well modelled by the addition of -3% upper crustal material to the refractory depleted mantle. Note that the simultaneous satisfaction both of trace element and isotopic signatures can only be achieved by contamination of the source prior to melting rather than contamination of the rising mafic melts after melting. Discussion The data present an interesting dilemma in our attempts to understand the origins of this LIP suite. On the one hand we need a very refractory melt-depleted mantle and yet this needs to experience significant crustal contamination. It may appear that this is the same as the origin of subduction-related basalts and that the crustal component is added to a mantle wedge as a melt of subducted sediment. However there is not a single modern arc system where the Ndisotopic shift is to such low values, requiring a slab influence much larger than ever seen. We are also confronted with the need for an explanation for the shallow melting. An alternative explanation for the source of crustal contamination may be due to transport of entrained crustal material by delaminating eclogite from within the uppermost part of the mantle lithosphere. Along its whole length, the Pacific margin Cambrian orogenic belt was terminated by a phase of abrupt uplift, exhumation and erosion at the end of the Cambrian (--490 Ma). This phase exposed granites and metamorphic complexes and deposited proximal molasse and distal turbidite sediments.

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The most obvious explanation for this exhumation is by some delamination of dense material from the lithospheric mantle. In South Australia evidence for this proposed delamination and postorogenic exhumation is provided by a suite of (-470-480 Ma), isotopically primitive, high temperature granites (ferroan A-types) and associated contaminated mafic magmas. These imply influx and decompression of hot mantle to near Moho depths. However, as we have already argued that the substantive source for the LIP suite is shallow, refractory (and hence buoyant) harzburgite, it seems wholesale mantle delamination has not occurred. This would have replaced the lithospheric mantle with thermally stabilised MORBsource upper mantle. The occurrence also in South Australia, of a suite of eclogite and mafic granulite xenoliths transported by Jurassic kimberlite {Tappert, Foden et al, 2009, 2011) clearly record the occurrence of an mafic upper mantle zone with metamorphic pressures from 1 to 3 GPa still preserved to the north of the northermost edge of the LIP province. Is selective delamination of eclogite from the SCLM possible, leaving more buoyant peridotite in place a possible scenario? Modelling by Pysklywec, Percival and others (2007, 2009) have demonstrated that this is indeed a possibility (or even an inevitability) as eclogite can have a density approaching 10% greater than magnesian peridotite. Their modelling suggests that negative diapirs of eclogite collapsing through the SCLM will induce overturn of the remaining peridotite leading to a heating phase at the time . Our suggestion would be that the eclogite collapse may carry entrained crustal material to a depth where it melts (when T > 750°C) and that this melt would then rise and react yielding peridotite that is more orthopyroxene-rich. This establishes the LIP source.

Radiogenic heating? It is interesting to speculate whether the LIP source (refractory harzburgite + 3% sediment) has sufficient to radiogenic heat production to yield the required temperatures in the 300 My interval between the end of the Cambrian and the Jurassic. This requires the distortion of the geotherm at 50 kmby an elevation of the order of 500°C. Our proposed source has 30 times the heat production (~6 X 10"^ W/m^) of the normal depleted mantle. However this heat production coupled to what would be regarded as "normal" thermal conductivities (~4 W m" ^ K"'') will reach steady state in - 10^ years, with a temperature elevation of only ~ 80°C. It requires that we choose a much lower (improbable?) thermal conductivity (<0.5 W m"^ K"^) for the uppermost 10km of the SCLM (directly above the heat productive layer) to extend the time taken to reach steady state to 300 My or longer, and then the temperature elevations are still only of the order of - 150°C. In conclusion, though it seems that the radiogenic incubation may contribute to heating the intra-lithospheric LIP source, nevertheless this isn't enough. We need to invoke some additional source of heat, most probably due to lithospheric thinning leading to heat transfer from upwelling asthenosphere (but with insufficient decompression for this to produce asthenospheric melts). Email: john.foden@adelaide.edu.au

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Figure 1, Map showing the extent of the Karoo-Ferrar-Tasmania-KI Jurassic Large Igneous Province (after Hergt et al., 1989)

Karoo- Ferrar -Tasamania- Kangaroo Island Large Igneous Province (LIP) Jurassic- E. Cretaceous

Karoo Y ^ ^

Parana 130 Ma

Antarctica Kangaroo Is.

] I go M a

^rar G g ^ l ^stralia

1500 km

Pacific (Jurassic)

Figure 2. Comparison of the Jurassic LIP data from the Ferrar and Tasmania and Kl with the field of MORB . Ilustrating the more refractory character of the Jurassic LIP suite.

MORB

rsi

Oil

Ferrar

Tasmania 'KI 5

Wt % FeO (T)

\^ctor of Shift from MORB sourcc to more refractorv Fcrrar-Tas LIP source

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F i g u r e 3. PMelts modelling showing the co-variation of AI203 with Ti02 in the liquids produced by the melting of a depleted peridotite source, and the composition of the parental Jurassic LIP. PMelts Model: Generation of Parental Ferrar -Tasmanian - Kl basalt by by s h a l l o w m e l t i n g ( " ' l . A GPa = " ' 5 0 K m ! ) within the llthospherlc mantle at degrees!.17

1350

1350^C

Parental FerrarTas Dolerite Melt

15 fO

U75X

GPa

^ 12 11

1400^C

IGPa

01 + OPX + GXP + Sp

01 + OPX + Sp

9 8 0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

W t % TIO,

F i g u r e 4. P-T data from Jurassic eclogite and mafic granulate xenoliths from Jurassic kimberlite

T °C 0

1000

1200

Eclogite Garnet Granulite

(Pearson et al. 1990) 10

• iy 1

••

LIP SOURCE ZONE Eclogite-Garnet Granulite Xenoliths Garnet Peridotite Xenoliths

(Tappert et foden et al J Pet October 2011)

50 P (Kbars) 60

MAFIC GRANULITE AND ECLOGITE XENOLITHS AS RELICTS OF TERMINAL CAMBRIAN DELAMINATION? Base of Jurassic Lithospheric Mantle

1400

Mantle Adiabat Level of initial mafic intrusion

Are these lower pressure eclogite/ garnet granulites with high l v a l u e s recording the result of radiogenic heating since the terminal Cambrian delamination event?

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EVALUATING GEODYNAMIC MODELS OF TASMANIDES EVOLUTION WJ. Collins^ ^School of Environmental & Life Sciences, University of Newcastle, NSW , Australia A critical issue regarding the origin and evolution of the southern Tasmanides i^ whether the successive outboard orogens (Delamerian, Lachlan, New England) form by accretion of outboard fragments, as recently re-iterated by Aitcheson and Buckman (2012, Gondwana Research 22), or they formed behind a single outboard retreating subduction system (Collins, 2002, Geology 30). How can we distinguish between the two models? There are at least three ways the models can be distinguished: (1) Tectonostratigraphic relationships; If terrane accretion models apply, there is no link between the inboard and outboard orogens. If outboard retreat models exist, geodynamic models predict that a tripartite association should exist; from an inboard continental arc that undergoes extension ultimately to produce an outboard oceanic arc and intervening backac basin. In the Tasmanides, the inboard arc is dominated by S-type granites, produced by melting of a thickened metasedimentary pile formed from the inverted precursor backarc, e.g., the Ordovician turbidites behind the Macquarie arc. Four tripartite associations are apparently evident in the Tasmanides, reflecting four stages of outboard retreat; three of those potentially produced the Delamerian, Lachlan and New England orogens. (2) Petrological evolution of granitoids; the repeated successions of S- to I- to A-type granite throughout the Tasmanides, in the Delamerian, Lachlan and New England orogens respectively, are the expected result of successive subduction retreat episodes. S-types represent arc initiation after backarc closure (mentioned above), followed by l-types as the arc migrates outboard and the intervening (rifted) crust attenuates. Finally, A-types are produced in a distal backarc setting, well away from the subduction flux influence. This repeated petrological succession is not predicted by typical terrane accretion models, but occurs at least three times in the Tasmanides.

(3) Lu-Hf isotopic arrays: The Tasmanide Hf isotopic array, defined from magmatic zircons, is characterised by repeated rapid changes from positive to negative epilson Hf (8Hf) values then gradual reversions back to positive values. Wholerock epilson Nd arrays mimic the Hf arrays. Geochemically, the rapid change from positive to negative reflects a rapid crustal reworking phase, then the gradual reversion to positive values reflects a gradual increase in the mantle component. Geodynamically, this reflects a rapid crustal thickening phase, then a protracted extensional phase. The timing of the thickening phases coincides with the contractional orogenic events in the Tasmanides, confirming the arrays have geodynamic significance. It not only suggests that the thickening events were rapid, but the extensional events were prolonged. This is consistent with the short durations of S-type magmatism and much longer periods of I- to A-type magmatism. Taken together, the tectonostratigraphic, petrological and isotopic evidence embrace a consistent theme, which suggests that the orogens of the Tasmanides formed by semicontinuous retreat of an outboard arc system, intermittently interrupted by shortlived contractional events.

Key words: isotopes

Tasmanides,

granites,

Email: bill.collins@newcastle.ed

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CAN GRANITE GEOCHEMISTRY PALAEOZOIC TECTONIC MODELS EASTERN AUSTRALIA?

CONSTRAIN FOR SOUTH

A. G. Rossiter^ Rosscraft Minerals Pty Ltd

Abstract There is little doubt that the S-type granites of the Lachlan Fold (LFB) are derived at least partially from sedimentary material. However, the Ordovician sedimentary rocks that dominate surface exposures in the LFB are too low in CaO, Na20, or Sr to have played a dominant role in granite generation. This conclusion follows whether restite unmixing of partiallymelted sedimentary sources or magma mixing involving siliceous sedimentary endmembers is the preferred petrogenetic model. Zircon geochronology and Rb/Sr isotopic considerations rule out Proterozoic rocks as LFB granite sources: the presence of Proterozoic rocks beneath much of the LFB is geologically unlikely anyway. On the other hand, involvement of Cambrian sedimentary material is consistent with zircon geochronology and the fact that southeastern Australian granites exhibit a Rb/Sr isotopic relationship with their host rocks, where the latter are of demonstrated or apparent Cambrian age. The coincidence of Ba-rich granites and Ba-rich Cambrian sedimentary rocks in central Victoria further implicates Cambrian sedimentary rocks in LFB Stype granite petrogenesis. There is the possibility of a contribution from Ordovician material containing more CaO, Na20, and Sr than known Ordovician, but there is little evidence for this. Granite geochemistry implies that Cambrian (meta)-sedimentary rocks are abundant beneath the Ordovician rocks that dominate the LFB at the surface. Such rocks are exposed as the St Arnaud Group and equivalents and the high-grade metamorphic rocks of the Omeo-Wagga belt. The model of Coney et al. (1990) whereby the Ordovician turbidites were deposited on a quasi-

continental basement appears confirmed. Granite Ba contents confirm the existence of the Selwyn Block and show that its postulated boundaries need modification. A sharp discordance of granite Ba contents on either side of the Wonnangatta Fault suggests that it is a fundamental crustal break that should be emphasised in future tectonic models for Palaeozoic southeastern Australia. Introduction The relatively high K2O and Rb and low CaO, Na20, and Sr contents of the Stype granites of the Lachlan Fold Belt (LFB) indicate that sedimentary rocks have made a significant contribution to their petrogenesis. Clastic sedimentary rocks have low CaO, Na20, and Sr relative to igneous rocks because weathering of plagioclase to clays releases these components into solution to be concentrated in seawater, evaporites, and carbonate deposits (Chappell 1984). Low CaO and Na20 in S-type granites result in peraluminous chemistry and aluminium saturation index (ASI) values significantly greater than 1 (typically around 1.3 in siliceous S types): such values are relatively rare in igneous rocks. Comparatively high initial ®^Sr/®®Sr ratios and values and relatively low 8Nd values further suggest a sedimentary input to S-type granite petrogenesis. The role of sedimentary material in the origin of the S-type granites of the region is controversial, but there are two main hypotheses. One suggests derivation from purely sedimentary sources and restite unmixing to produce the geochemical variation within suites (e.g. Chappell & White 1974; Chappell 1984; Chappell et al. 1987; White & Chappell 1977, 1988a). The other suggests that S-type granites are the result of mixing of mafic magma and

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magma with a high sedimentary component (e.g. Gray 1984; Rossiter & Gray 1996; Collins 1996, 1998). It is not necessary to resolve this controversy to make sensible deductions regarding the composition of the sedimentary rocks involved. Few of the tectonic models so far proposed for the LFB address the problem of what was melted to generate the voluminous granites present. The restite-unmixing model implies that the composition of the source with respect to a particular component must lie somewhere on the (linear) trend seen when that component is plotted on a variation diagram against another component (say Si02). A substantial proportion of melt is required before wholesale mobilisation of the magma, transport over a significant distance, and intrusion as a granite pluton is possible. Hence the source composition must contain significantly less Si02 than the siliceous members of the granite suite. Magma mixing models imply a high-Si (sediment-derived) endmember for each suite that also lies on the granite trend on a variation diagram but at the siliceous end. Analyses of this type should ignore strongly siliceous granite compositions as these have almost certainly undergone fractional crystallisation. The main thrust of this study compares geochemical variation in S-type granite suites with LFB (meta)sedimentary compositions to try and work out which of the latter represent the best source materials. Cambrian Age for Granite Sources Conspicuous among the accessory minerals of many LFB granites are zircon crystals that have prominent (often rounded) inherited cores and euhedral igneous rims. The cores are unequivocal restite and provide directly datable samples of the granite protoliths. I. S. Williams and colleagues at the Australian National University have carried out ion-probe U/Pb dating of zircons from several plutons of the Bega, Berridale, and Kosciuzsko Batholiths. Most plutons for which data are available show a striking abundance of zircon

cores 500-600 Ma old. Zircon-core ages from the S-type Cooma pluton peak strongly at about 495 Ma and 565 Ma (Williams 2001). Anderson (1997) reported peaks in zircon-core ages at 525-550 Ma in the Pyalong granite (central Victoria) and 475-575 Ma in the Koetong granite (northeastern Victoria). Thus the granite sources must have been constituted after zircon crystallised in an igneous event at roughly 550 Ma. Gray (1990) established that LFB granites have initial ratios that are too low (<0.7200) for them to have been generated by simple melting of material similar to that occurring in any of the exposed Proterozoic terranes of eastern Australia. He further demonstrated that Ordovician sedimentary rocks in the low-grade area at Cooma and in the Lockhart terrane (northeastern Victoria) and the Mathinna Beds (Tasmania) have ®^Rb/®®Sr ratios too high to represent endmember compositions for the granites east of the Kiewa Fault and ®^Srr®Sr ratios too high to constitute endmembers for the granites west of the Kiewa Fault (central association of Gray 1990). Limited (n=2) Rb/Sr isotopic analyses suggest (but no more) that central Victorian Early Cambrian sedimentary rocks have isotopic compositions consistent with their being involved in the petrogenesis of the granites of central Victoria (Rossiter 2001). The correlation between the highest initial ratios observed in certain southeastern Australian granite groups and the ratios calculated for the local basement rocks at the time of granite formation (Gray 1990) further suggests a Cambrian age for the granite sources. This relationship holds for the Encounter Bay-type granites of southeastern South Australia (western association of Gray 1990) and the surrounding Kanmantoo Group and for granites east of the Kiewa Fault (eastern association of Gray 1990) and high-grade metasedimentary rocks near the Cooma pluton.

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The granites of central Victoria differ markedly from those occurring further west and further east in their higher Ba content (Chappell et al. 1988, White & Chappell 1988b, Rossiter 2003, Rossiter & Gray 2008). Rossiter (2001) showed that the Cambrian sedimentary rocks of central Victoria are also rich in Ba (averaging about 3000 ppm - Table 1). This is unlikely to be a coincidence. The existence of a Ba-rich Selwyn Block beneath central Victoria is confirmed by granite compositions. Cooma Supersuite Granites Several interesting granites in northeastern Victoria belong to the Cooma Supersuite. These include Yabba, Bethanga, House Creek, Cobungra (Willman et al. 1999), and a number of unnamed bodies. Like the well-known Cooma pluton, these are foliated, somewhat heterogeneous, Stype granodiorites and monzogranites emplaced into migmatitic host rocks. They contain abundant quartz, redbrown biotite, primary muscovite, cordierite, sillimanite, and sometimes a little garnet, but lack calcic plagioclase cores (about An4o maximum). Coomatype granites are characterised by an abundance of metasedimentary enclaves (indicating that they have not moved far from their sources); virtual absence of microgranular enclaves; low CaO, Na20 and Sr; and high initial ®^Sr/®®Sr ratios (all 0.718 or higher at 410 Ma) and ASI values (usually >1.5). They thus show stronger S-type tendencies than any other LFB granites and are the best candidates for derivation solely from sedimentary sources. Despite assertions to the contrary, Cooma Supersuite granites have too much CaO, Na20, and Sr to be derived from exposed LFB Ordovician sedimentary rocks. Cooma-type granites contain at least 0.84 percent CaOa, 1.22 percent Na20a, and 122 ppm Sra (Table 1). Average CaOa, Na20a, and Sra values calculated for LFB Ordovician sedimentary rocks vary from 0.08-0.37 percent, 0.42-1.79 percent, and 29-64 ppm respectively (Table 2). LFB Siluro-

Devonian sedimentary rocks are even lower in Na20, CaO, and Sr than their Ordovician counterparts (Wyborn & Chappell 1983, Hayes 1990) and represent even less likely Cooma-type granite sources. Table 1 shows various average compositions calculated for (meta)sedimentary rocks of known and possible Cambrian age from the Heathcote and Mount Wellington Fault Zones, the St Arnaud Group, the Gundowring terrane, and the Cooma area. The table also shows compositions for the various rock groups after the data have been regressed to 72 percent Si02a. The overall averages compare quite closely with the regressed compositions and both resemble means calculated for the Cooma, Yabba, Bethanga, and House Creek granites. In addition, initial ®^Sr/®®Sr ratios for metasedimentary rocks of the St Arnaud Group, the Gundowring terrane, and the Cooma area compare closely with those for Cooma Supersuite granites. Cooma Supersuite granites can thus be satisfactorily explained in terms of partial melting of (meta)sedimentary rocks with compositions similar to those of St Arnaud Group/Gundowring terrane rocks and mobilisation of the entire melt zone. More Typical S-type Granites Most of the S-type granites in the LFB do not belong to the Cooma Supersuite, but form large, often elliptical, plutons emplaced at relatively high levels and surrounded by contact metamorphic aureoles. They will be referred to here as the "large S types". Individual plutons are more homogeneous than Cooma Supersuite bodies and their compositional range is much more extensive. Some are significantly less siliceous than Cooma types while some fractionated examples are considerably more siliceous. Some large S types contain calcic plagioclase cores and mafic microgranular enclaves. Many have lower ASI values and initial

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ratios than Cooma Supersuite granites. The large S-type granites selected for study here are the Dalgety and Cootralantra Suites of southeastern New South Wales and the Strathbogie (coarse phase) and Warburton plutons of central Victoria. These were chosen because they were thought to be largely unfractionated (although the decrease in Ba as Si02 increases in the Strathbogie pluton could be partially the result of fractionation of potassium feldspar). More siliceous porphyritic Strathbogie rocks are definitely fractionated. Application of a magma-mixing petrogenetic model suggests that high-Si endmembers for the four suites/plutons contained 1.37-2.50 percent CaOa (at 72 percent Si02a). This compares with CaOa values of 0.56-1.18 percent (regressed to 72 percent Si02a) for LFB Cambrian (meta)sedimentary rocks (Table 1). Mafic igneous endmembers with high (but feasible) CaO are implied. All four granites have negative gradients on CaOa versus Si02a plots and employment of a restite-unmixing model suggests high CaO at the relatively low Si02 indicated for the sources if melting was extensive enough for wholesale mobilisation of the magmas to occur. If (for argument's sake) the sources contained 69 percent Si02a, then CaOa varied from 2.42-3.60 percent. The existence in the LFB of Cambrian sedimentary rocks that are relatively low in Si02 and so rich in CaO is problematical. Known Ordovician sedimentary rocks are far too low in CaO to constitute either high-Si endmembers or restite-unmixing sources (Table 2). High-Si endmembers deduced for the four suites/plutons using a magmamixing model must have contained about 2.07-2.80 percent Na20a (at 72 percent Si02a). This compares reasonably closely with the range of 0.90-2.93 percent Na20a seen in LFB Cambrian (meta)sedimentary rocks (Table 1). Trajectories on Na20a versus Si02a diagrams are fairly flat implying that any mafic igneous endmembers had similar

Na20 to the postulated high-Si endmembers. If restite unmixing produced the granites then the sources had similar Na20 to the melts. Again known Ordovician rocks contain too little Na20 to constitute either high-Si endmembers or restite-unmixing sources (Table 2). High-Si endmembers calculated for the four suites/plutons assuming magma mixing range from 104-193 ppm Sra. This compares reasonably closely with 56-137 ppm Sra (regressed to 72 percent Si02a) computed for LFB Cambrian (meta)sedimentary rocks. Application of the restite-unmixing model (assuming source Si02a of 69 percent) indicates Sra contents of 130233 ppm in the granite sources. Known Ordovician sedimentary rocks are far too low in Sr to constitute either siliceous endmembers or restiteunmixing sources (Table 2). Similar analysis of Baa versus Si02a plots suggests that the high-Si endmembers for Dalgety and Cootralantra contained less than 600 ppm Baa and those for Strathbogie and Warburton contained around 900 ppm (Table 1). The Baa contents of the Dalgety and Cootralantra high-Si endmembers compare closely with regressed (to 72 percent Si02) compositions for LFB Cambrian (meta)sedimentary rocks away from central Victoria (497-620 ppm). While the Strathbogie and Warburton high-Si endmembers are rich in Ba, they do not contain as much as the Early Cambrian sedimentary rocks from central Victoria shown in Table 1. The high Ba contents implied for the Strathbogie and Warburton mafic igneous endmembers are not a problem if shoshonitic compositions were involved (as suggested by Gray 1984). The Dalgety, Cootralantra, Strathbogie and Warburton Suites/granites have high-Si endmembers that resemble average (and regressed) Cambrian (meta)sedimentary compositions and mean Cooma Supersuite compositions. The only component not showing

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reasonable agreement is CaO, which is consistently higher in the high-Si endmembers. Loss of Ca-poor melt before additional melting to form the high-Si endmembers could conceivably have led to these having more CaO than their protoliths (assuming that the latter had St Arnaud/Gundowring-like compositions). Any melt lost would presumably have been Sr-poor and any increase in CaO produced by the above mechanism should have been accompanied by an increase in Sr. The calculated high-Si endmembers show similar Sr levels to the apparent protoliths, however. It is concluded, therefore, that the high-Si endmembers for Dalgety, Cootralantra, Strathbogie and Warburton have been derived from (meta)sedimentary rocks higher in CaO than those shown in Table 1. Gray & Kemp (2009) suggested that high-Si granite endmembers of southeastern Australia have compositions formed in a genetic spectrum from total source mobilisation to partial melt segregation from (meta)sedimentary rock. Similarity in the Ti02 contents of the high-Si endmembers and average (and regressed) Cambrian (meta)sedimentary compositions (Table 1) implies more-orless complete source mobilisation for the granite suites studied here. Conclusions Infiplications

and

Tectonic

There is little doubt that S-type granites are derived at least partially from sedimentary material. The Ordovician sedimentary rocks that dominate surface exposures in the LFB are too low in CaO, Na20, or Sr to have sourced either the Cooma Supersuite granites or the large S-type granites of the region, or to have acted as high-Si endmembers if magma mixing produced the latter. The Proterozoic (meta)sedimentary rocks cropping out in South Australia and Tasmania can be ruled out as (even partial) sources for LFB S-type granites on the basis of zircon geochronology and initial ®^Sr/®®Sr ratios. The presence of Proterozoic rocks beneath much of the

LFB is geologically unlikely anyway. The coincidence of Ba-rich granites and Ba-rich Cambrian sedimentary rocks in central Victoria and the fact that southeastern Australian granites also exhibit a Rb/Sr isotopic relationship with their host rocks, where the latter are of demonstrated or apparent Cambrian age, implicate Cambrian sedimentary rocks in LFB S-type granite petrogenesis. A series of oceanic island arcs and inter- and back-arc basins dominated the Cambrian geography of the LFB. It is reasonable to expect, then, that large amounts of detrital (including volcaniclastic) and pelagic sedimentary rocks, which were deposited in Cambrian inter- and back-arc basins, occur at depth beneath southeastern Australia. Pelagic rocks probably accumulated in the axial parts of interor back-arc basins and volcaniclastics near the margins. Craton-derived sedimentary material may well have accumulated in back-arc regions. A relatively high feldspar content would have made these Cambrian rocks susceptible to melting during subsequent heating events (unlike known Ordovician sedimentary rocks). There is the possibility of a contribution to granite petrogenesis from Ordovician material containing more CaO, Na20, and Sr than the known Ordovician, but there is no real evidence for this. It is highly likely that there is a thick layer of Cambrian (meta)sedimentary rocks beneath the Ordovician sequences that dominate the LFB. These are exposed as the St Arnaud Group and equivalents and the high-grade metamorphic rocks of the Omeo-Wagga belt. Remnants of this material are preserved as enclaves in the S-type granites of southeastern Australia. These are richer in CaO, Na20, and Sr than known Ordovician rocks and more strongly deformed (Anderson et al., 1998). The model of Coney et al. (1990) whereby the Ordovician turbidites were deposited on a quasi-continental basement appears confirmed. Granite Ba contents confirm the existence of the Selwyn Block and

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indicate that its boundaries may not be where conventional wisdom prefers. The western margin cannot be pinpointed as many granites in critical positions have had their Ba contents modified by fractionational crystallisation, but lies reasonably close to a line from Geelong to Swan Hill. The Tamar-Tiers terrane boundary in Tasmania may represent the southern extension of this structure. The eastern margin of the Selwyn Block lies unequivocally at the Wonnangatta Fault which is clearly a fundamental crustal break that should be emphasised in future tectonic models for Palaeozoic southeastern Australia. The Selwyn Block boundaries proposed here coincide well with the margins of the Hay-Booligal Block further north. Granite zircon geochronology and initial ®^Sr/®®Sr ratios indicate that the Selwyn Block consists of relatively young (Cambrian in a fairly loose sense) material quite unlike the Tyennan rocks of northwestern Tasmania or any of the other exposed Proterozoic terranes of eastern Australia (Gray, 1990). Selwyn Block material is similar to that comprising granite sources to the west and east, but is richer in Ba, has lower mean initial ®^Sr/®®Sr ratio and contains fewer inherited zircons. References ANDERSON, J. A. 0., 1997 A structural, metamorphic and isotopic geochemical study of metasedimentary enclaves from the southern Lachlan Fold Belt. Ph. D. thesis. La Trobe University (unpub). ANDERSON, J. A. 0., PRICE, R. 0., & FLEMING, P. D., 1998 Structural analysis of metasedimentary enclaves: implications for tectonic evolution and granite petrogenesis in the southern Lachlan Fold Belt. Geology 26, 119-122.

Philosophical Transactions of the Royal Society of London A310, 693-707. CHAPPELL, B. W., & WHITE, A. J. R., 1974 Two contrasting granite types. Pacific Geology S, 173-174. CHAPPELL, B. W., WHITE, A. J. R., & HINE, R., 1988- Granite provinces and basement terranes in the Lachlan Fold Belt, southeastern Australia. Australian Journal of Earth Sciences 35, 505-521. CHAPPELL, B. W., WHITE, A. J. R., & WYBORN, D., 1987 The importance of residual source material (restite) in granite petrogenesis. Journal of Petrology 1,111-1,138. COLLINS, W. J., 1996 S- and l-type granitoids of the eastern Lachlan fold belt: products of three-component mixing. Transactions of the Royal Society of Edinburgh, Earth Sciences 88, 171-179. COLLINS, W. J., 1998 Evaluation of petrogenetic models for Lachlan Fold Belt granitoids: implications for crustal architecture and tectonic models. Australian Journal of Earth Sciences 45, 483-500. CRAMER, J. J., 1979 Geological variation in some differentiated granitic magmas east of Melbourne, Victoria. Ph. D. thesis, La Trobe University (unpublished). GRAY, C. M., 1984 An isotopic mixing model for the origin of granitic rocks in southeastern Australia. Earth and Planetary Science Letters 70, 47-60. GRAY, C. M., 1990 A strontium isotopic traverse across the granitic rocks of southeastern Australia: petrogenetic and tectonic implications. Australian Journal of Earth Sciences 37, 331-349.

CARROLL, P. F., 1990 The structural, metamorphic and igneous geology of Indigo valley, northeast Victoria. B. Sc. (Hons) thesis, La Trobe University (unpublished).

GRAY, C. M., & KEMP, A. I. S., 2009 The two-component model for the genesis of granitic rocks in southestern Australia nature of the metasedimentary-derived and basaltic endmembers. Lithos 111, 113-124.

CHAPPELL, B. W., 1984 Source rocks of I- and S-type granites in the Lachlan Foldbelt, southeastern Australia.

LANYON, R., 1989 The structural, metamorphic and igneous geology of the Wodonga-lndigo valley area of

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Northeast Victoria. B. Sc. (Hons) thesis, La Trobe University (unpublished). MUNKSGAARD, N. C., 1988- Source of the Cooma granodiorite, New South Wales- a possible role of fluid-rock interactions. Australian Journal of Earth Sciences 35, 363-377. ROSSITER, A. G., 1973 The geology, petrology and geochemistry of the granitic rocks of Victoria. M. Sc. thesis. University of Melbourne (unpublished). ROSSITER, A. G., 2001 Granites of the Victorian section of the Lachlan Fold Belt: characterisation and origins. Ph.D. thesis. La Trobe University (unpublished). ROSSITER, A. G., 2003 Granitic rocks of the Lachlan Fold Belt in Victoria. In Birch W. D. (editor). Geology of Victoria, pp. 217-237, Geological Society of Australia (Victorian Division) Special Publication 23. ROSSITER, A. G., & GRAY, C. M., 1996 Magma mixing and the origin of S- and Itype granites. Geological Society of Australia Abstracts 41, 372. ROSSITER, A. G., & GRAY, C. M., 2008- Barium contents of granites: key to understanding crustal architecture in the southern Lachlan Fold Belt? Australian Journal of Earth Sciences 55, 433-448. STEELE, D. A., 1993- Petrological studies on gneisses and granites of the Tallangatta region, NE Victoria. Ph. D. thesis. La Trobe University (unpubl.). WHITE, A. J. R., & CHAPPELL, B. W., 1977- Ultrametamorphism and granitoid genesis. Tectonophysics 43, 7-22. WHITE, A. J. R., & CHAPPELL, B. W., 1988a Some supracrustal (S-type) granites of the Lachlan Fold Belt. Transactions of the Royal Society of Edinburgh: Earth Sciences 79, 169-181. WHITE, A. J. R., & CHAPPELL, B. W., 1988b Granites. In Douglas, J. G., & Ferguson, J. A. (editors), Geology of Victoria, pp. 427-439, Geological Society of Australia (Victorian Division), Melbourne.

WHITE, A. J. R., & CHAPPELL, B. W., 1989 Geology of the Numbia 1:100 ,000 Sheet 8624, New South Wales Geological Survey, Sydney. WHITE, A. J. R., WILLIAMS, I. S., & CHAPPELL, B. W., 1977 Geology of the Berridale 1:100 000 Sheet 8625, New South Wales Geological Survey, Sydney. WILLIAMS, I. 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, 557-580. WILLMAN, C. E., MORAND, V. J., HENDRICKX, M. A., VANDENBERG, A. H. M., HAYDON, S. J., & CARNEY, C., 1999 Omeo 1:100 000 map geological report. Geological Survey of Victoria Report WYBORN, L. A. I., & CHAPPELL, B. W., 1983 Chemistry of the Ordovician and Silurian greywackes of the Snowy Mountains, southeastern Australia: an example of chemical evolution of sediments with time. Chemical Geology 39, 81-92. Key words: Lachlan Fold Belt, S-type granites, Cooma Supersuite, Cambrian metasedimentary rocks, Ordovician turbidites, Selwyn Block, geochemistry, tectonics Email: allanrossiter@optusnet.com.au

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Table 1: Average and regressed compositions for LFB Cambrian? metasediments compared with means for Cooma Supersuite granites and high-Si endmembers calculated for the Dalgety, Strathbogie, Warburton and Cootralantra Suites/plutons.

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Table 2: Average compositions for LFB Ordovician metasedimentary rocks. GiriL-Glrilambone, Rossiter=Rossiter (2001)

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GEODYNAMIC CONTROLS ON THE METALLOGENIC ENDOWMENT OF SOUTH-EAST AUSTRALIA F.P. Bierlein^ ^School of Geosciences (Adjunct), Monash University, Melbourne VIC 3800 Abstract The Lachlan Orogen represents one of the world's most highly endowed Paleozoic mineral provinces. Because certain mineral-deposit types are diagnostic of specific tectonic settings, the metallogeny of the composite fold belt system can be directly related to the explicit tectonic framework and development of each of the three lithotectonic segments. The geodynamic evolution of the accretionary orogen within a complex oceanic setting also governed the location, size and preservation of a wide range of ore deposit styles typically associated with such a plate tectonic setting. These include orogenic and intrusion-related gold, porphyry Cu-Au and epithermal AuAg-Sb, volcanic-hosted massive sulphide and sediment-hosted Cu-Au, and granite-related Sn-W. Using the wellestablished correlation between metallogenic deposit type, tectonic setting and endowment provides additional constraints on the geodynamic framework of the Lachlan Orogen. Moreover, such a mineral-systemsdriven approach enables the assessment of the orogen's fertility and overall exploration potential. Introduction The metallogenic endowment of the Earth's crust is a direct consequence of geodynamic processes and plate tectonic evolution. This is because the location of any given (primary) mineral deposit is, at a fundamental level, controlled by processes that operate at the lithospheric scale. Conversely, the nature, style and magnitude of mineralisation present in, for example, a sedimentary basin, accretionary continental margin or exhumed metamorphic belt provide important

clues regarding the tectonic evolution of these geological domains. Combining the diametrical approaches therefore provides an extremely powerful tool for the conceptual assessment of a terrain's mineral resources 'fertility', as well the exploratory appraisal of its metallogenic endowment. Accretionary margins represent the Earth's foremost mineral deposit 'factories' (Fig. 1). This is due to the multitude and complexity of geological processes which occur along convergent and transpressive plate boundaries. These processes include subduction of hydrated oceanic crust, injection of fertile athenospheric material into the overlying crust, fluiddriven melting, metasomatism, recylcing and fractionation of crustal material, and the formation of new oceanic and/or continental-character crust. Under the right circumstances, these processes can give rise to the accumulation of vast amounts of metals within a range of settings associated with the assembly of accretionary orogens. Anatomy of the Lachlan Orogen South-East Australia (defined herein as the region to the east of the approximate limit of Phanerozoic deformation in central - eastern New South Wales, Victoria, and NE Tasmania) is dominated by the Lachlan Orogen (Fig. 2). This orogenic belt developed as part of an extensive Neoproterozoic Early Mesozoic accretionary system (the "Terra Australis Orogen") which extended for over 20,000km along the long-lived Pacific margin of Gondwana (e.g. Cawood, 2005). Because of favourable relative plate movements throughout the

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Phanerozoic and in the absence of a terminal collision between the Australian continent and its neighbours since the break-up of Gondwana, the Lachlan represents an exceptionally well preserved example of a composite Paleozoic accretionary orogen (e.g. Foster et al., 2009). The Lachlan Orogen formed outboard of a Proterozoic to Early Paleozoic passive continental margin rift succession which was inverted during the ca. 500 - 490 Ma Delamerian Orogeny (Cayley et al., 2011). Based on marked differences in lithology and structural make-up, the accretionary system is traditionally divided into three broad sub-provinces or structural zones, namely the western central and eastern Lachlan Orogen (Fergusson, 2003, 2010; Gray and Foster, 2004; Hough et al., 2007; Foster et al., 2009). The zones are separated by, and contain a series of polydeformed, crustal-scale and listric to steeply-dipping structures. Together, these zones define a composite fold belt system that extends for over 1,200km in N-S direction and reaches an E-W width of 700 - 800km (Gray and Foster, 2004). The western Lachlan in NE Tasmania and western-central Victoria is an east vergent thrust system with zones of NWSE- and N-S trending structures. The central Lachlan is dominated by a southwest vergent thrust system linked to the fault bounded Wagga-Omeo metamorphic complex in eastern Victoria and south-west New South Wales. Structures in the eastern sub-province of the Lachlan in central and eastern New South Wales are characterized by a north-south grain with east-directed faults, with an older, subductionaccretion complex preserved in the southeastern part. The lithological sequence of the orogen is dominated by a broadly eastward-younging, Ordovician - Devonian, deep- to shallow-marine and in excess of 10km thick, quartz turbidite succession. These are underlain by a Cambrian - Ordovician boninitic to tholeiitic assemblage with intervening but subordinate fragments of continental crust identified both in the western and 60

central portions of the orogen (Crawford and Keays, 1987). In addition, shoshonitic andesite, sub-aerial mafic to intermediate silicic volcanoclastics and limestone characterise the eastern Lachlan. Extensive felsic post-tectonic magmatism affected the entire orogen in the Silurian and Devonian, and gave rise to numerous shallow-level granitoids which make up over 20% of the total outcrop area (Gray and Foster, 2004). The nature and tectonic affinity of dominant lithologies across the Lachlan Orogen, lack of more extensive continental basement, and structural make-up are all indicative of an oceanic accretionary margin setting (e.g. Gray and Foster, 1998, 2004; Fergusson, 2003; Foster et al., 2009; Cayley et al., 2011). Although deep crustal reflection seismic data across much of the western Lachlan have significantly contributed to the understanding of the orogen's tectonic framework (c.f. Cayley et al., 2011, and references therein), the drivers for, and extent of subduction across the orogen during its geodynamic evolution remain less clear. Metallogeny of the Lachlan Orogen More than 22 mineral deposit types have been recognised in the Lachlan Orogen (Walshe et al., 1995; Bierlein et al., 2002). The majority of these deposits are concentrated in five metallogenic provinces (Fig. 3) and formed during peak intervals of globally recognised ore-forming periods (Hough et al., 2007). Because ore-forming processes require the conjunction of very specific geological conditions, each of these provinces is diagnostic of a prevalent setting within the overall tectonic framework of the developing accretionary Lachlan Orogen from the Cambrian through to the Early Mesozoic. 1) major Ordovician Silurian orogenic gold deposits including metamorphic (lode) Au (e.g. Stawell, Ballarat, Bendigo, Beaconsfield) and subordinate magmatic (intrusionrelated) Au and epizonal (disseminatedstockwork) Au-Sb dominate in the

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western Lachlan, with minor Devonian erogenic gold also developed in specific regions of the eastern Lachlan (e.g. Hill End); 2) Ordovician porphyry and porphyry-related Au-Cu (e.g. Cadia, Parkes, Cowal) prevail in the eastern zone of the orogen; 3) Late Silurian Early Devonian volcanic-hosted massive sulphide (e.g. Woodlawn, Currawong), and 4) contemporaneous sedimenthosted Cu-Au (e.g. Cobar Basin deposits) are developed in the central and eastern Lachlan; whereas 5) Silurian granite-related Sn-W±Mo deposits (e.g. Ardlethan, Beechworth, Anchor) occur almost exclusively in the central zone. The link between metallogeny and geodynamic evolution In a plate tectonic context (e.g. Kerrich et al., 2005; Groves and Bierlein, 2007), porphyry Cu-Au±Mo and geneticallyrelated epithermal Cu-Au-Ag and Au-Cu skarn deposits are intimately associated with the constructional development of the volcano-intrusive arc environment. Similarly, VMS deposits also form largely during the constructional to orogenic stages of accretion but in extensional back-arc settings in response to rifting and bimodal volcanism during crustal thinning. In contrast, orogenic gold deposits are typically deposited in forearc to back-arc settings during the lateto post-orogenic stage in the accretionary margin. Granite-related SnW deposits generally form from the melting of lower crust in suprasubduction far-back arc settings and on continental margins where they postdate the major compressional phase. The marked differences in metallogenic character of the western, central and eastern sub-domains of the Lachlan Orogen correspond closely to the long recognized variations in their deformational, metamorphic and magmatic character (Gray and Foster, 1997) and reflect distinct positions in the overall tectonic framework of the accretionary system. They also highlight the difficulty of explaining the evolution of the Lachlan Orogen in a 'simple' backarc setting inboard of a long-lived and

craton-directed 'master' subduction zone (Li et al., 1990; Cayley et al., 2011). Problematic aspects of such a model relate to the overall eastvergence of the upper crust, transfer of deformation across oceanic crust, two distinct zones of eastward migrating magmatism across the orogen, the presence of intervening fragments of continental crust (i.e. the 'Selwyn Block' of Cayley et al, 2002; Wagga-Wagga Omeo Metamorphic Complex), episodes of regionally occurring rifting and inversion, and the synchronous formation of subduction-related giant orogenic gold deposits (western Lachlan) and world-class porphyry CuAu systems (eastern Lachlan) at 'opposite ends' of the accretionary margin at ca. 440 Ma. Instead, the observed complexity of ore deposit patterns in the Lachlan Orogen is best explained by the interaction of microplates and up to three subduction systems in a relatively complex oceanic setting similar to that in the present-day SW Pacific, as proposed originally by Gray et al. (2002; Fig. 4). Role of the lithosphere The formation of giant orogenic gold deposits necessitates a short premineralisation crustal history, thin or destabilised sub-continental lithospheric mantle and high heat flux from asthenospheric upwelling (Bierlein et al., 2006). The occurrence of these processes can be linked, for example, to crustal overthickening due to underplating of slices of oceanic terranes to the accretionary wedge during seaward growth of the margin (Kerrich and Wyman, 1990), or the delamination of subducted oceanic lithosphere to enlarge the overlying mantle wedge (Collins 1994). While there is general consensus that westward subduction beneath the Gondwana margin occurred in the Cambrian, continuation of this process in the western Lachlan into the Ordovician Silurian remains contentious. On the basis of the interpreted 2006 seismic data, Cayley et al. (2011) reiterated the apparent

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validity of their Vice model' which postulates thickening of oceanic lower crust on either side of an intervening slab of continental-character basement in an overall intra-plate or back-arc setting. While this model elegantly explains the absence of significant orogenic gold mineralization in parts of the western Lachlan Orogen (e.g. Willman et al., 2010), it is at odds with accretionsubduction settings inferred for most, if not all. Late Archean - Phanerozoic orogenic provinces hosting giant gold deposits (Bierlein et al., 2006). Given the unsually short pre-gold history of the oceanic crust in the western Lachlan Orogen (i.e. <50 m.y.), it is conceivable that the subduction angle was shallow and did not produce melting at or after 440Ma. Gray and Foster (2000) postulated that a slow subduction rate was responsible for dehydration of the oceanic crust at shallow depths. The early onset of this process would have prevented partial melting at deeper levels, thus putting a stop to the development of a characteristic volcanic arc and inhibiting magmatism by up to 50 m.y. (e.g. Bierlein et al., 2001, Phillips et al., 2012). The virtual absence of any other kind of significant mineralisation in the western Lachlan Orogen, together with the distinct structural trend and crustal levels exposed all indicate that the pre-Devonian tectonic development of this part of the Lachlan Orogen was entirely separate from the sub-provinces to the east. The antithetic correlation between thick SCLM and gold endowment is also illustrated by the absence of more significant orogenic gold mineralization in the far-back arc and continental margin settings which dominate the central Lachlan. Geochemically distinct and intrusion-related gold (±Cu, Sb, W, Mo) occurrences formed in all three zones in the Devonian, and can be linked to the onset of post-collisional relaxation and/or incipient extension. These deposits are economically subordinate to, and postdate (by up to 80 m.y.) the metamorphic-associated phase of gold deposition in the western zone (e.g. 62

Bierlein and McKnight, 2005), where they are spatially related to the approximate extent of the Selwyn Block and the proximity of the Delamerian basement on the western margin of the Lachlan. Over 35 significant gold-rich porphyry copper systems and associated skarn, high-sulphidation and carbonate-based epithermal Au-Ag deposits have been identified in a series of volcanic belts (collectively referred to as the 'Macquarie Arc') in the northern part of the eastern sub-domain of the Lachlan Orogen (e.g. Smith et al., 2004). These occurrences formed in response to dehydration of a subducting oceanic slab and related high fluid flux into the overlying mantle wedge during the development of an intra-oceanic arc in the Ordovician. To account for their formation at approximately the same time when giant orogenic gold deposits developed in the spatially separated, distinct tectonic setting of the western Lachlan, Vos et al. (2007) invoked interplay of slab detachment and mantle upwelling along a portion of the subduction system outboard of the Gondwanan Pacific margin at ca. 440 Ma as the unifying driving mechanism. In contrast to orogenic gold deposits, which normally require thin or thinned continental lithosphere, VMS deposits form mostly in accretionary settings, such as extensional back-arcs and rifts in oceanic supra-subduction environments that are characterised by thinned oceanic lithosphere (e.g., Kerrich et al., 2005). Suitable conditions for the formation of Kuroko-style deposits prevailed in the central and eastern sub-provinces of the Lachlan Orogen during the Late Silurian to Early Devonian when roll-back of the underthrusted oceanic crust and/or oblique convergence gave rise to transient rift basins in the southwestern part of the eastern Lachlan (Gray et al., 2002; Fergusson, 2010). The formation of deposit styles so characteristic of the central subprovince is generally indicative of the

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deeper levels of a supra-subduction environment associated with peripheral accretion of an oceanic basin to tectonically thickened continental crust (Nelson 1996). The high geothermal gradient recorded in the central Lachlan Orogen, the elongate nature of the batholiths, the Pb-Pb and Sm-Nd signatures and their geochemical signatures are all consistent with the formation of these intrusives in crust above a subduction slab with some contributions from the asthenospheric wedge (Collins, 1998; Bierlein et al., 2002). If the central Lachlan Orogen is considered a Paleozoic analogue to the Tertiary Andean margin of the South American Plate, unroofing and the subsequent erosion of shallow crustal levels could readily account for the general lack of deposit types that are broadly associated with magmatic and hydrothermal processes at shallow levels in an differentiated arc system (e.g. Sillitoe, 2010). Implications for endowment fertility of the Lachlan Orogen

and

As was pointed out by Fergusson (2003), the Lachlan Orogen does not easily fit the generally accepted model of an oceanic accretionary complex as it lacks some of the features typically associated with well-studied - and metallogenically extremely fertile - accretionary systems in, for example, Alaska, Japan and the SW Pacific. This includes the lack of more extensive arc magmatism, limited structural dismemberment, lack of topography on the subducting plates, extent of accretion of mafic crust, and the dominance of turbidites across all three of the tectonic zones. On the other hand, slow convergence rates, alternating episodes of extension and contraction, a more 'simple' structural make-up and thicker-than-normal oceanic crust may imply that the Lachlan Orogen represents a mere 'end-member' of accretionary subduction complexes (Fergusson, 2010). Within this framework, the Lachlan Orogen consists of, from west to east, a Cambro-Devonian accretionary wedge

that was undergoing active erosion during progressive deformation, an Early Silurian magmatic arc, an Ordovician oceanic island arc and a Silurian developing fore-arc system. Ore-forming processes analogous to those in comparable volcano-magmatic accretionary systems operated within each of these litho-tectonic segments and therefore can provide important constraints on the tectonic model for the Lachlan Orogen. Consideration of metallogenic processes in a lithospheric context also has significant implications for the metallogenic potential and overall mineral endowment of the Lachlan Orogen. The tectonic make-up of the western Lachlan Orogen can explain why it is one of the world's most intensively mineralized and fertile orogenic gold provinces (Hough et al., 2007). Key factors include the unusually high abundance of primitive, hydrated and Au-rich oceanic crust beneath the turbidite pile, development of suitable crustal-scale conduits, overall lowT/high-P conditions typically associated with subduction-accretion settings, and the removal of between 2 and 6km of overburden during post-collisional uplift. Further discoveries of Ordovician worldclass gold deposits in the western Lachlan are likely, especially under cover and to the north of known feeder zones in the Bendigo and Stawell subzones (e.g. Lisitsin et al., 2007). Conversely, the evolution of the central and eastern zones was characterised, for the most part, by the development of small sub-basins and diachronous closure of rifted and accreted segments over thick continental lithosphere without extensive oceanic crust. These conditions favoured formation of volcanogenic-exhalative Cu-Au (Pb-Zn) deposits in some of the short-lived basins in the central and eastern Lachlan Orogen. Development of these deposits during the Palaeozoic peaked worldwide in the Cambro-Ordovician and Silurian. In all cases (e.g. Mt Morgan, Rosebery, Kuroko), mineralization formed as synvolcanic 63

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seafloor deposits above hydrothermal vents (c.f. black smokers at active oceanic rises) or as synvolcanic, subseafloor replacement deposits (Large et al., 2001). The preservation potential of VMS and SEDEX deposits typically is adversely affected by structural overprint associated with subsequent inversion and deformation; however, low uplift rates such as inferred for the eastern zone, as well as accretion to a stable margin in the central zone may favor their preservation, with as yet undiscovered occurrences in (concealed) structurally complex settings possible. The accretion of the Macquarie Arc to a stable continental block (i.e. the central zone of the Lachlan Orogen) permitted the preservation of Ordovician porphyry Cu-Au and genetically linked high- and low-sulphidation epithermal Au-Ag deposits with otherwise poor preservation potential beyond the Tertiary due to their formation at shallow crustal levels (Groves and Bierlein, 2007). The arc is relatively mature in terms of exploration but further discoveries are considered possible, especially at deeper levels under cover and in brownfields regions such as at Cadia (e.g. Wood, 2012). Uplift and removal of shallow crustal levels in the central Lachlan, combined with a restricted extent of accreted oceanic crust can account for the comparatively limited metallogenic endowment of this zone. However, the outboard portions of the central Lachlan might have considerable potential for intrusion-related gold (±Mo-Te-Cu-SbBi-W) deposits as these form in a similar tectonic setting to Sn-W deposits in far back arc or continental margins of convergent margin settings. The gold deposits postdate the major compressional phase in the hosting orogen and are related to the onset of extension due to shallow subduction close to the cratonic margin (Groves and Bierlein, 2007). Although the exposed portion of the Lachlan Orogen has to be considered relatively mature in terms of exploration

for up to 150 years, vast tracts of highly prospective but under-explored terrain remain concealed below younger sediments. There is no reason to believe that fertile lithosphere is entirely absent from buried regions situated along strike of the outcropping segments of the accretion-subduction complex Acknowledgements I am extremely grateful for countless discussions with, and input from a large number of former colleagues and collaborators who, over the past decade, have greatly contributed to my understanding of the metallogeny and tectonic evolution of SE Australia. These include D Groves, D Foster, D Gray, I Vos, M Hough, L Ailleres, B Collins, A Tomkins, P Betts, G Mark, A Wilde, A Crawford, S Maher, C Willman, F VandenBerg, R Cayley, D Phillips, R Keays, C Fergusson, J Miller, R Goldfarb, G Davidson, and S Cox. References Bierlein, F.P., McKnight, S. 2005. Geology and genesis of intrusionrelated gold systems in the Western Lachlan Orogen, SE Australia. Economic Geology, 100: 385-398. Bierlein, F.P., Arne, D.C., Foster, D.A., Reynolds, P.R. 2001. A geochronological framework for orogenic gold minera-lisation in central Victoria, Australia. Mineralium Deposita 36: 741-767. Bierlein, F.P., Gray, D.R., Foster, D.A. 2002. Metallogenic relationships to tectonic evolution the Lachlan Orogen, Australia. Earth and Planetary Sciences Letters 202: 1-13. Bierlein, F.P., Groves, D.I., Goldfarb, R.J., Dube, B., 2006. Lithospheric controls on the formation of giant orogenic gold deposits. Mineralium Deposita 40: 874-886. Cawood, P.A. 2005. Terra Australis Orogen: Rodinia breakup and development of the Pacific and lapetus margins of Gondwana during the

Selwyn Symposium 2012 GAGA 2012 Neoproterozoic and Paleozoic. Science Reviews 69: 249-279.

GSA Victoria Division Geological Society of Australia Abstracts 103 Earth-

rocks. Geological Society of London Special Publication 318: 329-349.

Cayley, R.A., Taylor, D.H., Vandenberg, A.H.M., Moore, D.H. 2002. Proterozoicearly Palaeozoic rocks and the Tyennan Orogeny in central Victoria: The Selwyn block and its tectonic implications. Australian Journal of Earth Sciences 49: 225-254.

Gray, D.R., Foster, D.A. 1997. Orogenic concepts— application and definition: Lachlan fold belt, eastern Australia. American Journal of Science 297: 859891.

Cayley, R.A., Korsch, R.J., Moore, D.H., Costelloe, R.D., Nakamura, A., Willman, C.E., Rawling, T.J., Morand, V.J., Skladzien, P.B., O'Shea, P. 2011. Crustal architecture of central Victoria: results from the 2006 deep crustal reflection seismic survey. Australian Journal of Earth Sciences 58: 113-156. Collins, W.J. 1994. Upper- and middlecrustal response to delamination; an example from the Lachlan fold belt, eastern Australia. Geology 22: 143-146. Collins, W.J. 1998. Evaluation of petrogenetic models for Lachlan fold belt granitoids: Implications for crustal architecture and tectonic models. Australian Journal of Earth Sciences 45: 483-500. Crawford, A.J., Keays, R.R. 1987. Petrogenesis of Victorian Cambrian tholeiites and implications for the origin of associated boninites. Journal of Petrology 2B: 1075-1109. Fergusson, C.L. 2003. OrdovicianSilurian accretion tectonics of the Lachlan Fold Belt, southeastern Australia. Australian Journal of Earth Sciences 50: 475-490. Fergusson, C.L. 2010. Plate-driven extension and convergence along the East Gondwana active margin: Late Silurian-Middle Devonian tectonics of the Lachlan Fold Belt, southeastern Australia. Australian Journal of Earth Sciences 57: 627-649. Foster, D.A., Gray, D.R., Spaggiari, C., Kamenov, G., Bierlein, F.P. 2009. Paleozoic Lachlan Orogen, Eastern Australia - accretion and construction of continental crust in a marginal ocean basin setting: evidence about the basement from Cambrian meta-volcanic

Gray, D.R., Foster, D.A. 1998. Character and kinematics of faults within the turbidite-dominated Lachlan Orogen: Implications for tectonic evolution of eastern Australia. Journal of Structural Geology 20: 1691-1720. Gray, D.R., Foster, D.A. 2004. Tectonic evolution of the Lachlan Orogen, southeast Australia: Historical review, data synthesis and modern perspectives. Australian Journal of Earth Sciences 51: 773-817. Gray, D.R., Foster, D.A., Bierlein, F.P. 2002. Geodynamics and metallogeny of the Lachlan Orogen. Australian Journal of Earth Sciences 49: 1041-1056. Groves, D.I., Bierlein, F.P. 2007. Geodynamic settings of mineral deposit systems. Journal of the Geological Society of London 164: 19-30. Hough, M.A., Bierlein F.P., Wilde A.R. 2007. A review of the metallogeny and tectonics of the Lachlan Orogen. Mineralium Deposita 42: 435-448. Kerrich, R., Wyman, D. 1990. Geodynamic setting of mesothermal gold deposits; an association with accretionary tectonic regimes. Geology 18: 882-885. Kerrich, R., Goldfarb, R.J., Richards, J. 2005. Metallogenic provinces in an evolving geodynamic framework. In: Hedenquist, J.W., Thompson, J.F.H., Goldfarb, R.J., Richards, J.P. (Eds.), Economic Geology 100th Anniversary Volume. Society of Economic Geologists, Littleton, Colorado, pp. 1097-1136. Large, R.R., McPhie, J., Gemmell, J.B., Herrmann, W., Davidson, G.J. 2001. The spectrum of ore deposit types, volcanic environments, alteration halos, and related exploration vectors in submarine volcanic successions: Some

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examples from Australia, Econ. Geo!. 96: Li, Z.V., Powell, C.M., Thrupp, G.A. 1990. Australian Palaeozoic palaeomagnetism and tectonics. II, A revised apparent polar wander path and palaeogeography. Journal of Structural Geology 12: 5 6 7 - 5 7 5 . Lisitsin, V., Olshina, A., Moore, D.H., Willman, C.E. 2007. Assessment of undiscovered mesozonal orogenic gold endowment under cover in the northern part of the Bendigo Zone. GeoScience Victoria Gold Undercover Report 2. Department of Primary Industries. Nelson, E.P., 1996. Suprasubduction mineralization: Metallo-tectonic terranes of the southernmost Andes. Subduction: Top to Bottom: American Geophysical Union Monograph, vol. 96, pp. 3 1 5 - 3 3 0 . Phillips, D., Fu, B., Wilson, C.J.L, Kendrick, M.A., Fairmaid, A.M., Miller, J.M. 2012. Timing of gold mineralisation in the western Lachlan Orogen, SE Australia: A critical overview, Australian Journal of Earth Sciences 59: 495-525. Sillitoe, R.H. 2010. Porphyry copper systems. Econ. GeoL 105: 3-41.

913-938.

Smith, S., Mowat, B., Sharry, M. 2004. Macquarie Arc porphyry Au-Cu systems; a review of the critical exploration features. Aust Geol Soc Abstr 7 A:5U62. Vos, I.M.A., Bierlein, F.P., Heithersay, P.S. 2007. A crucial role for slab breakoff in the generation of world-class mineral deposits: insights from central and eastern Australia. Mineralium Deposita42: 515-522. Walshe, J.L., Heithersay, P.S., Morrison, G.W. 1995. Toward an understanding of the metallogeny of the Tasman fold belt system. Econ Geo/90:1382-1401. Willman C.E., Korsch R.J., Moore D.H., Cayley R.A., Lisitsin T., Rawling T.J., Morand V.J., O'Shea P.J. 2010. Crustalscale fluid pathways and source rocks in the Victorian Gold Province, Australia: Insights from deep seismic reflection profiles. Economic Geology 105: 895915.

Wood, D. 2012. Discovery of the Cadia Deposits, NSW, Australia (Part 1). SEG Newsletter SS, ppl, 13-18

Key words: geodynamics, metallogeny, mineral deposits, Lachlan Orogen, exploration targeting Email: frank.bierlein@areva.com

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Figure 1: Schematic diagram showing the range of deposit types formed in accretionary margin settings. The deposit types are divided into those of the constructional, orogenic and late to post-orogenic stages. Arrows on subducting slabs indicate polarity of subduction, with the arc above migrating oceanward (i.e., right to left) in response to subvertically sinking oceanic lithosphere (from Bierlein et al., 2009).

B. Orogenic stage

Tmmported Cypms'type VMS

Archaean/Paimoproterozfm^ + CordiUenm'Styh omgenk gold ReUoHtre palaeophicer g(fid

Jiamifmg'Sfyh tmigenk gald

CmtraiAsi<t siyk gold

Fore^arc placer gtjld

Astlienospliere

'Normal' Corclilleran accretionary margin

Complex accretioii and subduction

C, Late orogenic to post-orogenic stage

MmcraliscU Occvinic

Bingham-iype Carlm-typegal4 ^^ porphyries Intrusion'related gold ^

Placer Au

Proteromic SEDEK m foreland basm

fore^arc MVT

Lkhospheric manllc

Andean-type margin (xnagniatlsm)

Cratonised arc (far backarc)

Andean-iype margin (no magmatlsm)

Continent-continent suture

GSA Victoria Division Geological Society of Australia Abstracts 103

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Figure 2: Simplified structural map of the Lachlan Orogen showing the defined subdivisions for the western, central and eastern parts, traces (heavy lines) and names (circled numbers) of the major fault zones, and the structural form defined by aeromagnetic trends (finer lines) (from Gray and Foster, 2004).

LACHLAN DIVISIONS/ MAJOR FAULTS

250 km Nannes of faults and fault zones 1.Moyston 2. Stawell-Ararat 3.Avoca 2 4.Heathcote 5 S.Waratah 6. Mt. Wellington/ M t Useful

__ fo ^ Q U

7. Governor 14.Yalmy 8. Wonnangatta E ISXombienbar S 16.Yarralaw 9. Kancoona-Klewa Jg 17.Razorback lO.lndi uj 18.Copperhannia ii.Ensay 19.Coolac- Narromine IZGilmore 20.P3rkes 13. Rookery 21.0lepolokoFault

Figure 3: (Opposite page) Structural trend map of the Lachlan Orogen in SE Australia, highlighting mineral deposit styles and their distribution in each of the three sub-divisions. Tectonic vergence and implied younging of deformation indicated by arrows (from Bierlein et al., 2002). WL = western Lachlan Orogen; CL = central Lachlan Orogen; EL = eastern Lachlan Orogen. W-OMC= Wagga-Omeo Metamorphic Complex. SZ = Stawell Zone; BZ= Bendigo Zone; MZ= Melbourne Zone; MWFZ= Mt Wellington Fault Zone; TZ = Tabberabbera Zone; CB = Cobar Basin (including Elura, CSA, Queen Bee and Cobar deposits); KF = Kiewa Fault; GIF = Gilmore-lndi Fault; WTB = Wagga Tin Belt; S = Stawell; B = Bendigo; BT = Ballarat; M = Maldon; C = Castlemaine; SC = Steels Creek; N = Nagambie; W = Walhalla; A = Ardlethan; K = Kikoira; ML = Mammoth Lode; MM = Mt Murphy; BW = Beechworth; MW = Mt Wills; CS = Cassilis; Wl = Wilga; CU = Currawong; CD = Cadia; PK = Parks; CH = Copper Hill; LC = Lake Cowal; PH = Peak Hill; CR = Clarkes Reef; BU = Buchan; CW = Cowarra; MC = Majors Creek; HE = Hill End; CF = Captains Flat; WL = Woodlawn.

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I 144'

148"

•< Lachlm

( Orogen

Y i / - ' WL-H

Lachlan Orogen Metaiiogenesis: Orogentc lode gold metamorphic Au (±As-Sb) magniatic Au{±Mo-Sb-Cti-Te) epkonal Au-Sb

^^ ^ ^^^

Granite-hosted Sn-W {t skam, greisen)

Sediment-hosted (cpigefietic) Cu-Au, VbZn Porphyry Cu-Au +Au-skarii, mmtm Pb-Zn and eptthermai Au-Ag Volcanc^enic-hosted inassive sulphides

M . ^

Figure 4: Sectional view of tectonic elements for the southern part of the Lachlan Orogen at ca. 440 - 445 Ma, combining multiple subduction zone scenario envisaged for the tectonic evolution of the orogen with metallogenic settings of the three portions of the orogen. Numbers (1, 2 and 3) refer to principal subduction zones (from Bierlein et al., 2002). 445-440 Ma

. ,

turbidite thrust wedge (wes^tern Lachlan)

porphyry Au-Cu, magmatic Sn-W-Mo sed-hosted Cu-Au, Pb-Zn

marginal ocean basin, sediment micrffinent f o l d - t e t w f d transport > <3

imbrication of upper oceanic crust

orogenic Au

Narooma complex

* fossil / Ordovician arc high!metamorphic/ (Eastem Lachlan) magmatic complex {Central Lachlan)

\/Ljiiiio r»u -7

VHMS Pb-Zn, OrogeniC Au

HIGH PRECISION U-PB CHRONOSTRATIGRAPHY OF THE MOUNT READ VOLCANICS, TASMANIA: IMPLICATIONS FOR MINERAL EXPLORATION AND TECTONIC RECONSTRUCTIONS A.W, M c N e i l l / J-K. Mortensen,^ a n d J.B. Gemmell^ ^CODES, ARC Centre of Excellence in Ore Deposits, University of Tasmania, Hobart ^Pacific Centre for Isotopic and Geochemical Research, University of British Columbia, Vancouver Abstract The Mount Read Volcanics (MRV) host six economically significant VHMS deposits, of diverse style, with a combined in-ground metal value of AU$70 billion (at September 2011). A major problem for ongoing exploration in the MRV remains the correlation of the texturally and compositionally diverse packages which host favourable mineralised horizons. Previous attempts to date the mineralisation and host volcanics using a range of techniques (K-Ar, Ar-Ar, U-Pb, and Re-Os) have mostly been unsuccessful, due to a combination of the effects of later deformation and metamorphism and poor precision of results (e.g., 1o errors of > ± 3.5 m.y. at 500 Ma for U-Pb SHRIMP dates), and previous regional stratigraphic correlations were based largely on biostratigraphic evidence.

This method can yield precise ages (with 2a errors of < ±1.0 m.y. at 500 Ma) and as a result allow fine scale correlation within the MRV. Results to date indicate that although the MRV were erupted/intruded over a period of at least 12.1 m.y., between 506.8 and 496.0 Ma (longer than previously suggested), the economically significant VHMS mineralisation in the central MRV is restricted to a break in volcanism, at the top of the Central volcanic Complex, and prior to emplacement of shallow rhyolitic intrusions in the overlying volcano-sedimentary sequences, at ~500±1 Ma. These results have implications for both mineral exploration and tectonic reconstructions.

We report the new results from U-Pb dating of zircons in 18 samples from throughout the MRV stratigraphy, and associated rock packages, using chemical abrasion ID-TIMS.

Key words: Tasmania, Mount Read Volcanics, geochronology Email: andrew.mcneill@utas.edu.