Issuu

Geological Society of Australia

ABSTRACTS Number 63

2000 SELWYN SYMPOSIUM and MEMORIAL LECTURE

PLATE TECTONICS AT THE END OF THE MILLENNIUM Thursday 26th October 2000 Fritz Loewe Theatre University of Melbourne

Geological Society of Australia ABSTRACTS No. 63

2000 Selwyn Symposium and Memorial Lecture PLATE TECTONICS AT THE END OF THE MILLENNIUM

Editor:

David Giles

GEOLOGICAL SOCIETY OF AUSTRALIA INC, ABSTRACTS No. 63 2000 Selwyn Symposium and Memorial Lecture, Plate Tectonics at the End of the Millennium, Melbourne, October 2000

ISSN 0729-01IX © Geological Society of Australia Inc (Year) Copies of this publication may be obtained from: Geological Society of Australia Inc, Suite 706, 301 George Street, SYDNEY NSW 2000

Example citation for papers in this volume: Le Grand, HE., 2000. Scraping plates and my back garden, in Giles, D., (ed.), 2000 Selwyn Symposium and Memorial Lecture, Plate Tectonics at the End of the Millennium, Geological Society of Australia Inc, Abstracts No. 63, pp 3 - 17. Printed by: Monash University Print Services

This important event in the Victorian Division calendar would not have been possible without the generous support of the following organisations:

Mountains^ and

An A u s t r a l i a n

Crustal

THE AUSTRALIAN

Research

CRUSTAL RESEARCH

MOUNTAINS AND METALS

DIAMOND

FOREVER

STOCKDALE PROSPECTING

Initiative

CENTRE,

INITIATIVE

DE BEERS A

Centre

JOHNSONS WELL MINING

LIMITED

THE GEOLOGICAL SURVEY OF

THE GEOLOGICAL SOCIETY OF

VICTORIA

AUSTRALIA (VICTORIA DIVISION)

iii

ALFRED RICHARD CECIL SELWYN (1824 -1902) The Selwyn Memorial Lecture was introduced in 1984 to celegrate the life and work of A.R.C Selwyn, the first Government Geologist of Victoria. This year is the 145th anniversary of the Government Geological Survey in Victoria. In recent years the lecture has followed a symposium, involving invited speakers, on a significant theme of Australian geology. Alfred Selwyn was a silent and reserved man of indomitable disposition who carried out his plans in spite of all hindrances and achieved an amount of work that few could emulate.

He was

born in Somerset, England in 1824 and educated in Switzerland where he developed an enthusiasm for geology. At the age of 21 he joined the Geological Survey of Great Britain. He began work under the direction of A C . Ramsay in North Wales and was soon entrusted with the independent mapping of large areas containing 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 and arrived in early December 1852. Almost immediately he commenced a program of reconnaissance which took him throughout Victoria, with excursions into Tasmania and South Australia. Selwyn set up the Geological Survey proper in 1856.

He gathered and trained a team of geologists

(including Alpin, Ulrich, Wilkinson, Daintree, Taylor, Brown and Etheridge) who subsequently occupied senior positions in all mainland state geological surveys and became renowned in the annals of Australian geology. Selwyn and his team established the 6 x 9 mile Victorian mapping progamme of which 65 maps were produced. Despite these achievements, conflict arose due to the overlapping interests of the Geological Survey and the state's mining surveyors. The Survey often held views on scientific standards and policies opposed to those of the colonial officials and legislators, in particular those 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. He filled this office with distinction from 1869 until his retirement in 1894. The Murchison Medal of the Geological Society of London was awarded to Selwyn in 1876 and the Clarke Gold Medal from the Royal Society of New South Wales in 1884. He is commemorated in Victoria by the annual Selwyn Symposium and Memorial Lecture and the Selwyn Medal.

2000 SELWYN SYMPOSIUM AND MEMORIAL LECTURE

PLATE TECTONICS AT THE END OF THE MILLENIUM Fritz Loewe Theatre, School of Earth Sciences University of Melbourne Thursday 26th October

SCHEDULE

Registration

1:00 - 2:00 pm

Symposium Homer Le Grand 2:00 - 2:40 pm Mike Sandiford 2:40 - 3:20 pm Patricia Vickers-Rich 3:20 - 4:00 pm Afternoon tea Mark Barley 4:20-5:00 pm Andrew Gleadow 5:00-5:40 pm Drinks and Nibbles Medal Presentation and Selwyn Memorial Lecture Dinner at the Melbourne University Staff Club

6:30-7:30 pm 8:00 pm

TABLE OF CONTENTS

2000 Selwyn Medallist

- Mr John Neilson

2000 Selwyn Memorial Lecturer

- Professor Chris Powell

Scraping plates and my back garden

- Professor Homer E. Le Grand

Topography and tectonics

- Professor Mike Sandiford

2000 Selwyn Symposium

Environments, polar dinosaurs and moving continents in the Early Cretaceous - Professor Patricia Vickers-Rich of East Gondwana

- Associate Professor Mark Barley

Plate tectonics and mineral deposits

Exploring the thermal record of continental - Professor Andrew Gleadow extension in plate tectonics

2000 SELWYN MEDALLIST - JOHN NEILSON John Neilson joined the Geological Survey of Victoria in 1954 after graduating from the University of Melbourne with a major in geology and post-graduate work in geomorphology. following a period of regional mapping

In 1958,

with the Geological

Survey he was asked to provide geological

assistance during

construction of the King Street Bridge. His secondment focussed attention on the geological and construction needs at the time and the lack of available expertise in engineering geology. With the increasing number of Victorian development projects and growing demand for geological input he took charge of the Engineering Geology Section when it was set up by the Geological Survey in 1963.

In those early days, several German geologists provided

valuable assistance to the Engineering Geology Section and John particularly remembers the contributions of Dr Josef Niedermeyer from Hamburg and Professor Fritz Reuter of Freiberg University. Director of the Geological Survey at the time, Dr D. E. Thomas is remembered by John as one of the Victorian pioneers of engineering geology through his involvement with planning and construction of the Eildon Dam project. During 25 years of responsibility for engineering geology specific major projects that came under his control in the Melbourne area included: the West Melbourne railway works, a major sewer tunnel under the Yarra River at Spotswood, foundation studies for the West Gate Bridge, the South-Eastern Trunk Sewer project and the Melbourne underground rail loop. He was often called on to provide advice to other governmental and private bodies and was responsible for overseeing development of geological mapping services to meet the needs of engineering and regional planning, cemetery site selection, coastal cliff stability studies, beach erosion, mining subsidence and other civil engineering works. Following questionable changes in government policy that led to closure of the Engineering Geology Section in 1990 John became a consultant and has been consulting in engineering geology since 1992.

He is actively involved with the GSA, AGS and IAEG, where he is

highly regarded for his friendship and technical contributions.

John has contributed to the

education of many engineers and geologists through his presentations and published seminar papers to GSA and AGS and guest lectures delivered at universities. He is a member of the GSA geomorphology special interest group and is currently an editor and contributor to the revised version of the "Geology of Victoria". John gratefully acknowledges the help and encouragement of Melbourne colleagues, too numerous to be mentioned individually, in both geology and engineering.

2000 SELWYN MEMORIAL LECTURER PROFESSOR CHRIS McA. POWELL

Chris Powell is the Professor of Geology at The University of Western Australia.

He graduated

with a first-class BSc(Hons) degree from the University of Queensland in 1964, and a PhD in Geology from the University of Tasmania in 1968. After a NASA Postdoctoral Fellowship at Northwestern University, Illinois, in

1967-68

working on the iron formations of

northern

Michigan, and an Associate Professorship at the University of Cincinnati from 1968 to 1970, he joined Macquarie University as a Lecturer in 1970, becoming Associate Professor in 1979.

In 1987, he became Head of the School of

Earth Sciences, a position that he held until he took up the Chair of Geology at The University of Western Australia in 1990.

Chris Powell's research is in structural geology,

basin analysis and tectonics, with a strong field orientation. Recently, he has been involved in palaeomagnetism and global palaeogeography. He is the currently Director of the Tectonics Special Research Centre, a research centre that was established in 1997 for nine years by the Australian Research Council with the mission of discovering the supercontinents of which Australia has been a part. He is the author of more than 120 publications in refereed journals and books, including a book on Deformational

and Metamorphic

Rock Fabrics.

Professor

Powell was awarded the Royal Society of New South Wales Clarke Memorial Medal in 1995, and the Australian Academy of Science Mawson Medal in 2000.

Professor Powell will be presenting the 2000 Selwyn Memorial Lecture titled: "THE SEARCH FOR SUPERCONTINENTS".

The 2000 Selwyn Memorial Lecture is proudly brought to you by:

Australian

Crustal

Research 2

Centre

Initiative

SCRAPING PLATES AND MY BACK GARDEN Homer E. Le Grand Dean, Faculty of Arts and Interim Dean, Faculty of Science: Monash University, Victoria 3800, Australia.

Homer.Legrand@arts.monash.edu.au

SUMMARY The "modern revolution" in the earth sciences is associated with the emergence of plate tectonics in the late 1960s. Plate theory marked the culmination of a half-century of debates over crustal mobilism and drew together in a grand synthesis of developments in many branches of the earth sciences. The rapid and widespread acceptance of plate theory, J. Tuzo Wilson forcefully argued in 1976, "has transformed the earth sciences from a group of rather unimaginative studies based upon pedestrian interpretations of natural phenomena into a unified science that holds the promise of great intellectual and practical advances". Has this promise been fulfilled? The assumption that the crust of the Earth was composed of a small number of rigid, non-deformable, mobile plates enabled a quantitative, kinematic description of current geological processes and reconstructions of past plate interactions. The simple model of plate theory ca. 1970, for example its depiction of a subduction zone, is readily grasped but has since undergone considerable refinement. Moreover, many structural geologists and others judge plate theory even in its current form to be of limited value in addressing questions of continental tectonics. Geologists concerned with regional problems are often NIMBYs on this question: plate tectonics may be a grand generalisation, but it is not particularly useful in my back garden.

The "modern revolution" in the earth sciences is associated with the emergence of plate tectonics in the late 1960s.1 Plate theory marked the culmination of a half-century of debates over crustal mobilism and, in a grand synthesis, drew together developments in many branches of the earth sciences. The rapid and widespread acceptance of plate theory, J. Tuzo Wilson forcefully argued in 1976 (vii), "has transformed the earth sciences from a group of rather unimaginative studies based upon pedestrian interpretations of natural phenomena into a unified science that holds the promise of great intellectual and practical advances." Over the past thirty years, it could be argued with some justice, a major feature of the "revolution" has been a gradual shift toward a more physical and quantitative geology. However, many structural and regional geologists judge plate theory in its current form to be of limited value in addressing questions of continental tectonics. One response to the perceived shortcomings 1 This story has been told in varying ways by Marvin (1973), Hallam (1973), Menard (1986), Le Grand (1988), Stewart (1990) andOreskes (1999).

Le Grand

of plate tectonics, especially with respect to problems of regional geology, is the concept of accreted, exotic, suspect or allochthonous terranes. Well into the early 1960s, there was no reason for geologists to assume that explanatory frameworks based on the study of the continents over two centuries could not be readily applied to processes and structures beneath the seas. For most geologists in the Englishspeaking world, the crust of their Earth was relatively stable. North and South America, for example, were each though to be composed of an ancient core to which mountain belts had accumulated through the geosynclinal cycle. Mountain chains might be elevated or eroded, the continents might grow slowly on their margins but, broadly speaking, the continents and ocean basins had been essentially permanent features of the surface of the globe since their formation. Few took seriously the notion of continental drift. Extrapolating from the relatively well-known continents to the little-known ocean basins, it seemed obvious that granitic continents could not possibly move through the unyielding basaltic oceans. The "modern revolution in geology" was to reverse strikingly this situation. By the early 1970s, the revolution was essentially over. A vast amount of new and unexpected data had been harvested from the ocean floors. Their history, structure, volcanicity, magnetisation, heat flow and other characteristics were different from anything known about the continents. Along with this new data had come both new theories of, and new evidence for, great lateral motion of the continents. Plate tectonics, the triumphant version of continental drift, was developed largely by geophysicists and geologists to make sense of this deluge of novel geophysical and geological data from the sea floor, much of which had no continental counterpart. By the early 1970s plate tectonics formed for most earth scientists the theoretical framework for understanding and describing the workings of the Earth's outer shell or, as one influential textbook (Wyllie, 1976) was titled, The Way the Earth Works."2 From one perspective, it constituted only the culmination of a series of theories of global mobilism originating with Alfred Wegener in the early years of the Twentieth Century. That is to say, it provided a credible mechanism for, and thereby removed a major conceptual objection to, Wegener's theory of " continental displacement" . Wegener, Alexander du Toit and others over a period of half a century

had

gathered

and

marshalled

palaeontological, stratigraphic

and

geophysical evidence, taken mostly from the continents, to support the view that the continents had once been joined together but had been broken apart and moved to their current locations. Ironically, for many early plate theorists the continents were merely uninteresting excrescences on a fascinating sea floor. Could scientists apply this new framework to the continents to solve or re-solve problems that had bedevilled generations of land-based researchers?

C.f. W. Glen, 1975.

Scraping plates and my back garden

For plate theorists, their rigid, non-deformable plates and plate boundaries were almost geometrical entities which one could use to calculate the kinematics of plate motions and interactions over time. Dan McKenzie's Earth, for example, was a geometrical construct on which transform faults were arcs of circles defined by the poles of rotation of idealized plates; ridges and trenches were merely " lines along which crust is produced and destroyed" (McKenzie and Parker, 1967: 1276). Jason Morgan offered his version as " a geometrical framework with which to describe present-day continental drift" (Morgan, 1968: 1959). The third member of the early plate triumvirate, Xavier Le Pichon, put forward " a geometrical model of the surface of the earth" (Le Pichon, 1968: 3661) which, though necessarily involving " great simplifications and generalizations"

(3679) enabled him to give " a

mathematical solution which can be considered a first-approximation solution to the actual problem of earth's surface displacements" (3674). Field geologists concerned with the problems of continental geology and its history could not easily begin to use ocean-derived plate theory to solve them. They were in possession of an enormous store of knowledge, including detailed maps, of the geology of the continents but knowledge of ocean-floor geology consisted of a growing fund of widely scattered data. Only very coarse models were available to indicate how oceanic crust might be connected to and interact with continental crust.3 Once one moved away from the geometrician's globe, the local expressions of past and present plate movements were conjectural, diverse and different from place to place.

How might one infer from this large-scale, general and

coarsely-grained framework, solutions for the finer-grained, specific problems of local geology that, though well-known and well-mapped, had seemed heretofore intractable? If this could be done, then the resistance of land-locked earth scientists to the new sea-born theory could be overcome. From a cognitive perspective, the challenge lay in the necessarily uncertain and speculative extrapolation from processes thought to occur in the relatively youthful sea floor to explain the much more ancient structures of the continents. But, I suggest that challenge lay too in the scientific and social interests of most land-based geologists. They rapidly gave assent — or at least lip service — to plate theory at a general level4 but controversies abounded over its applications to regional and local geological problems.

3 As early as the 1930s, the National Research Council among the several major geophysical research problems for the community to address, included that of the nature of the "join" between the oceanic and continental crusts, particularly at what we now know to be a passive margin; eg. the Atlantic Ocean floor joining the North American and South American continents. In spite of enormous advances in several branches of geophysics that could be brought to bear on this problem, more than a half-century later, a detailed cross-section of that join is still extremely tenuous.

As is common with novel, over-arching conceptual frameworks, plate theory was assimilated at different rates and to different depths in different specialties and in different regions. At a functional level it could be said that different groups of earth scientists were operating with different versions of plate theory depending on their backgrounds and the problems that they were trying to s o l v e (Glen, unpublished; Le Grand, 1988: 75, 80-99, 163-164).

Le Grand

There was considerable initial resistance to the very idea of global mobilism, especially from more senior earth scientists who had invested their careers in a " fixist" approach to continental problems. The social interests of those who had achieved positions of authority through their work on continental features might well lead them to oppose extrapolations from the sea floor to the continents. These land-based earth scientists had invested years in meticulous local mapping, fieldwork, and analysis or ever more refined synthesizing and theorizing. They had thereby achieved positions of authority in their chosen period or region or technique or structure.5 "Teddy" Bullard incisively commented (1975: 5): "Such a group has a considerable investment in orthodoxy.... To think the whole subject through again when one is no longer young is not easy and involves admitting a partially misspent youth." Mason Hill, for example, the " founder" of the previously accepted view of the San Andreas system, " used to shake with rage when somebody would get up and talk about the San Andreas transform" (Jones, 1990a). " The new global tectonics" was the agenda for the Penrose Conference, organised by Bill Dickinson, held 15-20 December 1969 at the Asilomar Conference Grounds in Pacific Grove, California. It marked a major turning point in attempts to apply plate tectonics to the continents. 6 Among the participants were John Dewey, Jack Bird, Seiya Uyeda, Clark Burchfiel, Clark Blake, Greg Davis, Tanya Atwater, Peter Coney and Warren Hamilton. Dewey and Hamilton were already formulating approaches to continental geology based upon plate tectonics and their first papers bracketed the Conference. Atwater at the Conference gave a presentation that inspired many of the participants to try their own hand at plate tectonicsbased interpretations. Several of those present were also to take part in the later development of, and debates about, the terrane approach to regional geology. Dewey, soon after the adumbration of plate tectonics, and just before the Penrose at Asilomar, had begun to apply the new tectonics to construct an overview of orogeny on convergent Atlantic-type plate margins. Pursuing a suggestion of Tuzo Wilson (1966), he proposed that the Appalachians and other mountains bordering the Atlantic had been pushed up through collisions resulting from the openings and closings of the Atlantic; eg. a second convergence of the Atlantic and African plates had thrust up mountains in Virginia and Pennsylvania (Dewey, 1969b). Subsequent to the Conference, Dewey and Bird, who had also been a participant, extended this view to other convergent plate margins including the North American Cordillera, the Andes, and the Himalayas in a broad-brush paper that was to be quite influential. They believed, contrary to some at the time, that " plate tectonics is too powerful and viable a mechanism in explaining modern mountain belts to be disregarded in

Forthe roles of technical and social interests in scientific controversies, see, inter alia, Bourdieu, 1975; Latour, 1987; Le Grand, 1988: 80-99; McAllister, 1992. 6

One rule for the Penrose is that proceedings are not published as such nor are formal minutes kept; the emphasis is upon frank and free-ranging discussion initiated by a few speculative, provocative presentations. The organiser, W. Dickinson, published a report and overview of the Conference (1970a) and has kindly supplied considerable information about it.

Scraping plates and my back garden

favour of ad hoc, actualistic models for ancient mountain belts," and that understanding of all mountain belts could come only from the new global tectonics (Dewey and Bird, 1970: 2626). Their presentation included many sketches of cross-sections of crust presenting in simplified form their ideas. Warren Hamilton was one of the very few North American geologists in the early 1960s to advocate large-scale crustal mobility as a solution to regional geological problems. In 1961, for example, he proposed as a " speculation" and a " radical explanation" for geological correlations that Baja California had once been part of that mainland but had been both shifted 100 miles to the west and transported northward some 250 miles along the San Andreas Fault to its present location (Hamilton, 1961: 1307). By late 1967, Hamilton (1990), was " aware of this great surge in plate tectonics, but didn't comprehend it...." In the fall of 1968, he visited the Scripps Oceanographic Institute where he encountered a group of graduate students including Tanya Atwater and Jean Francheteau who were " totally up to speed on plate geometry" . In his words (1990) he was "led by the hand" by them through plate tectonics. The new global tectonics "meshed beautifully with my ... background in descriptive global geology. All of sudden here was a framework for it." He set out to write a synthetic paper on the geology of California. 46 Mesozoic California and the Underflow of Pacific Mantle" (Hamilton, 1969) appeared the same month as the Asilomar Conference. In it he proposed that much of California was made up of island arcs, oceanic crust, abyssal hills and other sea floor materials that had been scraped off more than 2000 kilometers of Pacific floor which had been subducted beneath the North American plate.

Dewey (1988) recalls

Hamilton saying at Asilomar," My God, we must be able to explain things like the Franciscan and the Coast Ranges, all those things, in terms of plate tectonics" . Hamilton's interpretation was certainly a radical one at the time (1990): " ...it was totally contrary to the way practically every Californian geologist looked at it...and there was quite a bit of resentment among the natives" . The most notable event at Asilomar was the presentation by Tanya Atwater. She proposed an elegant solution of the San Andreas Fault in terms of plate kinematics guided by sea floor magnetics as an age control. She drew together both continental geology and oceanography in a quantitative way and provided refinements in the geometrical kinematics of plate motions. She thought that plate movement could be related to many of the features of continental geology (1970:3513) and provided models which were designed to provide "testable predictions for the distribution of igneous rocks" and also the timing and amount of deformation. Although she treated only schematically configured, not geologically specific, crustal units, her approach made an immediate and profound impression on many of the participants (eg. Hamilton, 1990; Burchfiel, 1990; Coney; 1990; Davis, 1990; Dewey, 1988). Davy Jones, who was to be an architect of the terrane program, describes his reaction when he learned of her work as follows (1990b): " That was the first application of plate tectonics to a real setting and she was able to show people who had been fussing with the San Andreas Fault

Le Grand

all of their lives that they were completely missing the story. It was a marvelous paper and that's what convinced us that plate tectonics was the way to go." For a few years after Asilomar, there seems to have been an almost euphoric belief, or at least an incautious optimism, that problems of continental geology would quickly yield to the new global tectonics. The initial successes of Atwater, Dewey, Bird, Hamilton, Dickinson (1970b) and others seemed to show the way forward. In the first few years of the 1970s, there was a revolutionary fervour: many geologists rushed into print with redescriptions of their patches of ground with reference to so-called plate tectonics corollaries; those who forbore such descriptions were regarded as troglodytes. More than one earth scientist refers to that era as being cluttered with premature, simplistic, cursory, or naive interpretations. Plate tectonics was not to be confused with continental tectonics. The marine magnetic record that had proved so critical for Atwater represented only a small fraction of the geological timescale. Dewey and Bird's sketches were only that, and drawn to a very large scale. Hamilton's syntheses, though highly suggestive, were grand generalisations. Indeed, Hamilton (1995: 3) himself recently commented that the complex nature of plate interactions and their boundaries " invalidates many of the tectonic and magmatic models which clutter the literature" and even n o w " few of the geologists and petrologists who work with the structures...produced by convergent-plate interactions, and few of the geophysicists who model subduction, have familiarized themselves with the characteristics of actual plate systems." How helpful would this global theory be in explaining this outcrop or that group of hills? The task of applying global tectonics to a field geologist's " patch" is not unlike trying to explain the flight of a baseball using general relativity theory. One has to make certain simplifying assumptions. For example, may one properly treat the plates as absolutely rigid, knowing full well that the continents, which presumably record previous plate movements, also record considerable deformation? Dan McKenzie, one of the pioneer plate theorists, himself sought to " modify plate tectonics to describe continental, as well as oceanic, tectonics." But, as he has recently remarked (pers. com. 2000), " [T]his problem...is much harder than plate tectonics. ...Plate tectonics was clearly defined as a kinematic theory: one that is concerned with geometry. It is not a dynamic theory.... I myself do not describe continental tectonics as plate tectonics, because continental deformation occurs in wide zones where the idea of rigidity is of limited use." Molnar, a noted tectonician, takes a similar view. For him, the major importance of plate tectonics for most geologists in the 1970s was that it convinced them that continental drift had occurred. However, though it was a useful framework on a global scale, in terms of unravelling problems of continental tectonics, " plate tectonics is a poor approximation for the tectonics of many continental regions" (1988: 131-33). Plate tectonics may account well for the behaviour of oceanic lithosphere but continental lithosphere differs in buoyancy, thickness and rheology. In particular he holds (133) that " The broad, diffuse deformation of the western United States... is much more complex than the rigid-body displacements of a 8

Scraping plates and my back garden

small number of large plates, and finding a simple and accurate way to represent the deformation of continents remains a major task." He bluntly concludes (137), "The tectonics of continents has found plate tectonics an inadequate paradigm." He muses more recently (pers. com. 1999) that it is " no wonder field geologists had not discovered plate tectonics, for diffuse deformation and widespread strain makes recognizing rigid plates within continents difficult" . Dewey's work in the mid-1970s (1988) similarly brought home to him the complexities of applying plate tectonics to smaller-scale geological problems at plate boundaries: "What I found out from this work was that not only is plate tectonics not too simple to understand the geology of rock masses at plate margins, in fact it's bound to lead to such bloody enormous complexity that we may never work it out....

Plate tectonics is a simple concept but the

kinematics tends to some immense complications. You can build wonderfully complicated models as I did in that paper...but taking the results [of field work] and working backwards to a model, a unique model, whew! Very hard! The value of models is not that they give us solutions but give us an idea of how to proceed." In this respect, Tanya Atwater's model remains a " unique masterpiece" . In a similar vein McKenzie (pers. com. 2000) comments that " Geologists such as John Dewey and Jack Bird recognised that the geological continental record contains structures and stratigraphy produced by plate boundaries, and have sketched plate geometries that could generate the features concerned. But, they are unable to show that the motions involved were those of rigid plates, and in many cases I suspect, but cannot yet prove, that they were not." It is in this context that controversy over what earth scientists call variously accreted, suspect, exotic, allochthonous or tectonostratigraphic terranes erupted in the 1970s and continues today. Geologists — not rocks or other forms of evidence — open, sustain, and close geological controversies. Geologists make extensive use of field observations and other data, preferred techniques, and information presented in journals, books, reports, maps and so forth. However, neither the rocks nor other facts speak for themselves: it is the geologist who makes "the mute stones speak" for one or another side in a controversy. It is only after a controversy is settled that the "facts of the matter" are agreed.

What is at the center of

arguments over the terrane concept is not a clash between rival theories but rather preferred means of extending the global theory of plate tectonics to address problems of local geology. There is a perhaps inevitable tension in this respect between divergent and convergent thinkers. Convergent thinkers; that is, earth scientists aiming at a global or regional synthesis, especially an explanatory one, involves making various potentially treacherous simplifying assumptions, generalizing from myriad particulars of varying quality and entering less familiar subjects that strain the synthesizer's understanding. Divergent thinkers; that is, earth scientists who are intimately familiar with a wide range of the particulars of a patch of ground, and indeed may have spent much labour collecting them, may well mount heated objections to the effect that various details have been ignored, distorted, misunderstood, or

Le Grand

misinterpreted. Such, for example, was part of the negative reaction of specialists to Wegener's promulgation of continental drift. 7 The terrane concept 3 and research programs were initially developed in the 1970s to account for various specific features of the North American Cordillera by a loose-knit group of geologists. Most of them were then working for the United States Geological Survey in Menlo Park, California — the so-called Menlo Park Mafia. The " Godfather" of the group, Davy Jones, was a palaeontologist with the Survey and had worked in such key regions as the Franciscan and Alaska. In the early years of the terrane program he provided not only much of the drive but also considerable cognitive and social resources.9 Porter Irwin, also at the Survey, first used the term " terrane" in a special sense to characterise formations in the Klamath Mountains. David Howell joined the group and helped both to codify the terrane concept (1989a; 1995) and extend the program worldwide (1985; 1988). They, together with others including James Monger of the Canadian Survey, Alan Cox, and Peter Coney proposed that most of western North America is made up of a chaotic assortment of rock packages that have come from elsewhere, sometimes from thousands of miles to the south and west, and then been plastered onto the older North American craton. To underpin this approach, terranists used not only published and unpublished information on stratigraphy, palaeontology, palaeomagnetism, structural geography, petrology and geochemistry gathered by others, but also conducted their own fieldwork. Coney (1990), for example, recalls " one year when we were working in Alaska Davy put together a team - two palaeomagicians, geochemists, biostratigraphers, a structural geologist, a sandstone penologist - all working together and the interdisciplinary character of it was very exciting because you realised there was no way you were going to solve it by yourself." 10 The value of such collaborative endeavours is well illustrated by the first major success of the terrane program: the identification in 1977 of the Wrangellia terrane (Jones et

al1977).

James Monger, who liaised with members of the Mafia in the 1970s, and Charles Ross (1971) had, on the basis of palaeontological and other evidence, earlier suggested that the Palaeozoic rocks of the coastal region of south-eastern Alaska and British Columbia might be a fragment of North America that had undergone significant dislocation: they described it as a fragment of oceanic crust; not a continental fragment (1971). Wilson (1968) and Danner (1970) had speculated that it might be a fragment of Asia left behind after a collision between the two continents. Jones and his collaborators (Jones et al., 1977: 2565) now proposed that this large

See, for example, Frankel (1976); Le Grand (1988: 55-99).

Terranists prefer the term "concept" to "theory" or "hypothesis" in part because they regard it as subsumable within plate tectonics and in part because they consider it to be an empirical generalization.

For a more detailed study of Jones's contributions, see Le Grand and Glen (1993).

The range and sophistication of methods has grown steadily over the past two decades. For example, a recent reassessment of terranes in Scotland and northern England (Stone et al., 1999) is based upon the analysis of a large database of systematic geochemical analyses of stream sediments.

Scraping plates and my back garden

crustal unit which they named Wrangellia was a large "coherent" terrane extending along the Pacific margin of North America from the Wrangell Mountains in Alaska to Vancouver Island and that it had come from far to the south of its present location. Their claim was reinforced by the combination of data from the three specialties of the authors. Jones was a palaeontologist and able to draw upon data from the Radiolarian Lab he had recently established at Menlo Park.11 Silberling was a stratigrapher and also a palaeontologist. Hillhouse was a palaeomagnetist. The combination of palaeomagnetic with more traditional data was of key importance. One might argue endlessly about whether or not fossils indicate distant origin or compressed facies change, but the consilience of tropical fossils with a tropical palaeolatitude proved to be very persuasive. The "hard" empirical evidence was telling: even geologists who were in general opposed to the claims of the "terranists" accepted that Wrangellia" had come from afar. In 1978, the year following the identification of Wrangellia, the Pacific section of the Society of Economic Palaeontologists and Mineralogists (S.E.P.M.) organized a symposium at Sacramento, California on the Mesozoic palaeogeography of the western United States (Howell and McDougall, 1978). It provided an opportunity for the terranists to present their case. The tone was set by the editors (Howell and McDougall, 1978: viii) who stated that the border of the western United States was a passive margin in the Palaeozoic (and subject to tensional strain). In the Mesozoic (late Devonian and Mississippian) an episode of mountain building occurred as the result of plate convergence (compression). They claim this marked "the beginning of a protracted series of continental rifting, accretion, and island arc construction that persisted through the Mesozoic." They noted the difficulty of making a palaeogeographic reconstruction for the Cordilleran region because of the allochthonous nature of many "terranes". They also stated, "In most instances the allochthonous aspect of specific terranes is documented, but conclusions regarding the original setting remain unknown or equivocal." The Symposium was also significant for the change of rhetoric visa-vis

allochthonous terranes. There is little evidence of the tentative nature of earlier papers;

it seemed, at least for most of the people involved in the symposium that there was no argument about whether there were such things as allochthonous terranes.

They were an

accepted geologic fact. Indeed, for North American terranists virtually all of the vast lands west of longitude 111° from the imposing Brooks Range in Alaska south down the Cordillera through California and then almost all of Mexico are " suspect" as to their birthplace and history. It is no accident that the terrane concept was put forward — and is defended most stoutly — by geologists facing the Pacific rather than the Atlantic. The Pacific and Atlantic borders of the Americas differ greatly in their history and in the processes that now shape them. The Wilson cycle; i.e., the successive openings and closings of such ocean basins as the Atlantic, did not lend itself

For a detailed study of the role of the Rad Lab in buttressing the terrane program, see Le Grand and Glen,

1993.

Le Grand

to solutions for the problems faced by West Coast geologists. Their problems differed from those facing their East Coast counterparts concerned with Atlantic problems. The West Coast for the last 600 million years had, in plate theory, been an active margin. The San Andreas, the Rockies and the Andes are youthful examples of tectonic activity. The East Coast, in contrast, had been a passive margin for the last few hundred million years. Over the next decade and a half, terranists, including many new recruits, extended the approach to other regions of North America; eg. the Appalachians and even to the North American craton itself. Conferences in other countries further promoted the terrane approach (eg. Hashimoto and Uyeda, 1983; Howell et al., 1984; Howell and Wiley, 1988), collaborative projects were undertaken to construct world-wide maps of terranes (eg. Howell, 1985), and the terrane program begun by the Menlo Park Mafia is actively pursued in other regions around the Pacific Rim, including Japan, China, Australia, South America and New Zealand as well as in Europe. The aims of this ambitious undertaking were four-fold according to the public statements of leading terranists (Jones et al., 1983: 32): " (1) to identify, characterize, and portray on terrane maps all major allochthonous terranes; (2) to relate their faunal and floral characteristics through time to major paleobiogeographic provinces; (3) to establish paleolatitudes through time; (4) and finally, in the case of the Cordillera and Pacific region, to attempt paleogeographic reconstructions of the paleo-Pacific Ocean (Panthalassa) and surrounding cratonal regions." More informally, such activities were in part aimed at proselytising (Howell, 1989b): " ...our purpose was to go to a variety of places and in part use the conference as a vehicle to explain to the indigenous community where we were with the whole [terrane] concept" and in part at gaining familiarity with new areas to which the terrane approach could be applied.12 As was the case in North America, there were arguments over the novelty and utility of the terrane concept as well as skirmishes between the more enthusiastic converts to terranes and both plate theorists and committed field geologists. For terranists, this expansion in domain, if successful, would generate additional cognitive and social power. Clearly, the terrane program and its supporters would become more significant if the program could be shown to apply to regions beyond the Cordillera. To date, over 3000 publications, maps and abstracts have appeared which invoke or critique terranes since that term took on the specific connotation of " fault-bounded geologic entities of regional extent, each characterised by a geologic history that is different from the histories of contiguous terranes" (Jones et al., 1983: 22). Almost all earth scientists today acknowledge that some accretion has occurred. To many, it seems that the terrane concept is a natural outgrowth and extension of plate tectonics theory; indeed, that it is merely an anticipatable corollary of plate tectonics. Yet, from the outset, 12 This is a twist on the usual "centre-periphery" relationship in which the data is gathered in the "provinces" and sent to the "centre" for processing; eg. Jones's RadLab. Howell, Jones and others at the "centre" travel to the "periphery" to gain support and acquire data.

Scraping plates and my back garden

considerable controversy has surrounded this approach. The controversy does not involve a clash between fundamentally incompatible theories; rather, more typical of much everyday science, it turns on and throws into sharp relief differing judgments of, and disagreements over matters of approach, style, preferred methods and techniques, and aims within an shared, over-arching theoretical framework. Conflict over facts and their interpretations is conjoined with the conflicting specialist and social interests of the geologists themselves. One might explicate the terrane controversy in many ways. I emphasize the tension between analysers or divergent thinkers who seek increasingly fine-grained information about, and an explanatory account of, a piece of the crust; and synthesisers or convergent thinkers who seek large-scale patterns and over-arching conceptual schemes. For the former, maps may often be ends in themselves; for the latter, means to ends. This tension is certainly evident in the relationships between terranists and plate theorists and, perhaps ironically, between terranists and those specialising in the detailed study of smaller crustal units. 13

On the one hand, some terranists have been criticised by plate theorists aiming at large-scale regional or global syntheses. Many proponents of terranes believe, not altogether without reasons, that their work highlights the inadequacies of plate tectonics as applied to smallerscale regional and local problems {eg. Howell, 1989a: 51) and serves as a useful, empiricallybased corrective. Jones and others refer disparagingly to "naive plate tectonics models" and "Deweygrams." For example, the famous Franciscan Formation literally surrounds Berkeley and is on the doorsteps of Stanford and Menlo Park. In a general way, the Franciscan could now be interpreted as a subduction complex. However, at the level of enormous quantities of specific data and problems accumulated over the years, no specific guidance or highly detailed explanations could yet be extracted from plate theory. Rather than providing hasty generalisations, terranists argue that a more prudent course of action is to identify and characterise all the pieces of the puzzle, the terranes, before trying to interpret their emplacement and subsequent history in terms of plate tectonics. This is reflected in the very terminology of terranes, which by USGS convention are named simply after some geographical feature associated with the terrane, thus avoiding any implied theoretical context or interpretation and any suggestion of genetic relationships with other terranes. 14

Those critical of the terrane approach deny significant value or originality to the terrane concept and relegate what some of them term " terrane theory" — although few, if any, Glen, 1994, pp. 39-91, shows the influence of disciplinary specialization on theory choice and the selection and application of differing standards of appraisal in a continuing scientific debate. 13

The term refers to idealized representations of plate interactions, together with cross-sections of crust such as those pictured in Dewey and Bird, "Mountain Belts". David Howell, a member of the Menlo Park Mafia andsystematiser (1989a) of the terrane program, coined the phrase. He recalls (1989b) "I in a public meeting once referred to those as Deweygrams andl didn't mean to be insulting, I meant that they were very lucid and simplistic renderings of a perception of how, for instance, ... an island arc accretes. But when you then go to an area you don't findjust a big hunk of island arc with a knife sharp boundary which is the suture zone and then the adjoining terrane or whatever it was. It clearly was much more complicated than that." 14

Le Grand

terranists themselves have claimed theoretical status for their conceptual structure15 — to the wastebin of rehashed, conventional, descriptive geological methods and techniques. These critics often regard it as just a corollary or conceptual adjunct of plate tectonics and decry what they perceive to be a "terrane bandwagon" while denying to terranes any novelty or heuristic power. Critics of the terrane program argue that terranists themselves have simplistic and outmoded understandings of plate theory; that is, they are not conversant with contemporary plate theory as opposed to the cartoons of the 1970s. Such a view is forcefully expressed by Hamilton (1990): [T]he California group in general have a two-dimensional view of plate tectonics. They think of motion as either more or less perpendicular subduction or more or less strike-slip. ...[S]o most of the Californians are still thinking of California as being

assembled

strictly

Andean-type,

steady-state

subduction,

...of

subduction as a one sided-process picturing subduction as the rolling over a [fixed] hinge of a subducting plate, whereas in fact the hinge always migrates back into the subducting plate [with extension occurring in the over-riding plate. They just haven't learned how modern systems work, so I think much of their work is going to have to be done again" There is of course, exaggeration on both sides of the fact-theory divide. Were one to take literally the first of the " f o u r steps"

of the terranist "manifesto"; i.e., " t o identify,

characterize, and portray on terrane maps all major allochthonous terranes, that itself appears to be a never-ending task" which would mean that the next three steps would never be undertaken. First, as a methodological

point, one would have to describe in

some

considerable detail, without any preconceptions, every conceivable crustal unit in non-theoryladen language. This is in accord with the formal presumption that every crustal unit is "suspect" until proven "innocent; that is, that the crustal unit should be presumed to be allochthonous until proved otherwise and, moreover, that it is legitimate to infer fault boundaries even if they have not been directly observed. Moreover, there are social and cognitive mechanisms for generating more and more and smaller and smaller terranes. Each time one remaps an area, each time one uses new techniques, one tends to produce a finer and finer grained study and therefore more and more detail and to find new, previously unmapped features. In a very real sense there is an unspoken mandate to produce a more finely divided taxonomy than previous workers, thereby implying a more profound understanding. If one were to remap an area and find nothing new, that could well be construed as a failure. Thus, every time an area is mapped, there are delineated finer and finer subdivisions and greater and greater specificity of components and structures. Only partly tongue in cheek, one might predict that the number of terranes is inversely proportional to 15 Peter Coney, amongst others, explicitly (1990) makes the point that the terrane approach "was never a theory. It never was. It was a method of analysis...to know what youv'e got, so you can talk about what's there...."

Scraping plates and my back garden

the distance from major centers of geological study and fieldwork and directly proportional to the number of geologists who have worked in the region. On the other hand, terranists venturing new interpretations of crustal units have been harshly criticised by some field geologists for trespassing on the pieces of crust to which they have devoted their careers. Howell observes (1989b) that most geologists work in only one or two areas for good reason: " It's a lot of work to learn all the stratigraphies, the names, the places and you are never satisfied and there are no artificial boundaries to a given area - you can expand it as much as you can." Many were expert in, and had in fact mastered, the finely detailed geology of the crustal units and domains that the terranists sought to subsume in their newly defined taxonomy. Such a conceptual re-ordering, re-interpretation, redefinition and reclassification of what was regarded as well-established and defined geology appeared to turn bodies of knowledge won through strenuous efforts of mapping and study by teams of distinguished geologists into mere grist for the terranists' mill. Howell freely acknowledges (1989b) this source of criticism from some Alaskan " old hands" : " guys who have worked an awful lot, sweated blood, witnessed deaths, been threatened themselves to extract this information...take a certain amount of umbrage at some dandy coming in and spending a summer reinterpreting all the rocks.... We were bold and at various times may have given the impression of arrogance; frankly, we were just enthusiastic." Although the terrane program's detractors may be correct in their claim the terrane program offers nothing theoretically or methodologically new, the terranists' claim of new solutions has mobilized efforts at reassessment of long-standing, intractable problems and led to the generation of new, refined data and their reinterpretation. It has brought large numbers of researchers from the ether of plate theory abstraction down to the empirical, fine-scale reassessment of previous and

long-established

maps, data, fossils, monographs, and

interpretations in terms of the standards that define the terrane program. 16 It may also have provided impetus to its detractors further to refine alternative explanations couched in plate tectonics terms.

The terrane program may be a bandwagon, but as Coney

(1990)

refreshingly once put it," it was a bandwagon...and in part deliberately orchestrated, but it's been a great ride and a hell of a good way to do regional geology." Though John Dewey has occasionally put spokes in the wheels of that bandwagon, he too asserts the fundamental importance of field geology and mapping for, as he (pers. com.) puts it, 44 Rocks, fossils and minerals are immensely complicated systems but they, with geophysical and geochemical data and the ideas that stem from them...are the substance of our science" .

16 These are captured in what is sometimes referred to as the "Manifesto" i.e., Jones, Howell, Coney and Monger, 1983 or, more recently, Howell, 1989a and 1995.

Le Grand

REFERENCES Glen, W., 1982. The Road to Jaramillo: Critical Years of the Revolution in Earth Science. Stanford University Press: Stanford

Atwater, T., 1970. Implications of plate tectonics for the Cenozoic tectonic evolution of western North America. Geological Society of America Bulletin, 81: 3513-3536

Glen, W., 1994. The Mass Extinction Debates: How Science Works in a Crisis. Stanford University Press: Stanford

Bourdieu, P., 1975. The specificity of the scientific field and the social conditions of the progress of reason. Social Science Information, 14, No. 6: 19-47.

Hal lam, A., 1973. A Revolution in the Earth Sciences. Clarendon Press: Oxford

Bullard, E.C., 1975. The emergence of plate tectonics: A personal view. Annual Reviews of Earth and Planetary Science, 3: 1-30.

Hamilton, W.B., 1961. Origin of the Gulf of California. Geological Society of America Bulletin, 72: 1307-1318.

Burchfiel, B.C., 1990. Interview taped by H.E. Le Grand on 26 April 1990 at Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge.

Hamilton, W.B., 1969. Mesozoic California and the underflow of the Pacific mantle. Geological Society of America Bulletin, 80: 2409-2430.

Coney, P.J., 1990. Interview taped by H.E Le Grand on 15 February 1990 at Department of Geosciences, University of Arizona, Tucson.

Hamilton, W.B., 1990. Interview taped by H.E. Le Grand on 22 January 1990, U.S.G.S., Denver, CO.

Davis, G.A., 1990. Interview conducted by telephone by H.E. Le Grand on 14 May 1990 at the University of Southern California, Department of Geological Sciences.

Hamilton, W.B., 1995. Subduction systems and magmatism, in Smellie, J.L., ed, Volcanism Associated with Extension at Consuming Plate Margins, Geological Society Special Publication No. 81, pp. 3-28.

Danner, W.R., 1970. Paleontologic and stratigraphic evidence for and against sea floor spreading and opening and closing oceans in the Pacific Northwest. Geological Society of America Abstracts with Programs, 2, no. 2: 8485.

Hashimoto, M. and Uyeda, S., 1983. Accretion terranes in the circum-Pacific region. Proceedings of the Oji International Seminar on Accretion Tectonics, September, 1981, Tomakomai Japan. Terra Scientific Publishing Company: Tokyo.

Dewey, J.F., 1969a. Structure and sequence in paratectonic British Caledonides, in Kay, M., ed, North Atlantic Geology and Continental Drift, AAPG Memoir 12, pp. 309-335.

Howell, D.G., 1985. Tectono strati graphic terranes of the circum-Pacific region. Circum-Pacific Council for Energy and Mineral Resources: Houston.

Dewey, J.F., 1969b. Evolution of the Appalachian/Caledonian Orogen. Nature, 222: 124-129.

Howell, D.G., 1989a. Tectonics of Suspect Terranes: Mountain Building and Continental Growth. Chapman and Hall: London.

Dewey, J.F., 1988. Interview taped by H.E. Le Grand on 21 December 1988 at Department of Geology, Oxford University.

Howell, D.G. 1989b., Interview taped by H.E. Le Grand on 19 January 1989 at Palo Alto, CA. Howell, D.G., 1995. Principles of Terrane Analysis: New Applications for Global Tectonics. Chapman and Hall: London (2 ed of Howell, 1989)

Dewey, J.F. and Bird, J.M., 1970. Mountain belts and the new global tectonics. Journal of Geophysical Research, 75: 2625-2647.

Dickinson, W.R., 1970a. 2 Penrose Conference: The New Global Tectonics. Geotimes, 15, 4: 1820 and22. d

Howell, D.G., Jones D.L., Cox A. and Nur A., eds. 1984. Proceedings of the Circum-Pacific Terrane Conference (Stanford, 1983). Stanford University.

Dickinson, W.R., 1970b. Relations of andesites, granites, and derivative sandstones to arc-trench tectonics. Reviews of Geophysics and Space Physics, 8: 813-860.

Howell, D.G. and McDougall, K.A., eds. 1978. Mesozoic Paleogeography of the Western United States: Pacific Coast Paleogeography Symposium 2. Pacific Section, Society of Economic Paleontologists and Mineralogists: Los Angeles.

Frankel, H., 1976. Alfred Wegener and the specialists. Centaurus, 20: 305-324. Glen, W., 1975. Continental Drift and Plate Tectonics. Charles E. Merrill: Columbus.

Scraping plates and my back garden McKenzie, D.P. and Parker R.L., 1967. The North Pacific: An example of tectonics on a sphere. Nature, 216: 1276-80.

Howell, D.G. and Wiley T.J., eds. 1988. Proceedings of the 4 International Tectonostratigraphic Terrane Conference. Nanjing University: Nanjing. th

Menard, H.W., 1986. The Ocean of Truth. Princeton University Press: Princeton.

Jones, D.L., 1990a. Interview taped by H.E. Le Grand on 18 January 1990 at Berkeley. Jones, D.L., 1990b. Interview taped by H.E. Le Grand on 15 May 1990 at Berkeley.

Molnar, P., 1988. Continental tectonics in the aftermath of plate tectonics. Nature, 335: 131137

Jones, D.L., Silberling, N.J. and Hillhouse, J., 1977. Wrangellia - A displaced terrane in northwestern North America. Canadian Journal of Earth Sciences, 14: 2565-2577.

Monger, J.W.H. andRoss C.A., 1971. Distribution of fusulinaceans in the western Canadian Cordillera. Canadian Journal of Earth Sciences, 8:259-278.

Jones, D.L, Howell D.G., Coney P.J. and Monger J.W.H., 1983. Recognition, character, and analysis of tectonostratigraphic terranes in western North America, in Hashimoto M. and Uyeda, S. eds., Accretion Tectonics in the Circum-Pacific Regions, Terra Scientific: 2135.

Morgan, J., 1968. Rises, trenches, great faults and crustal blocks. Journal of Geophysical Research, 73: 1959-1982. Oreskes, N. 1999. The Rejection of Continental Drift: Theory and Method in American Earth Science. Oxford University Press: New York.

Latour, B., 1987. Science in Action. Harvard University Press: Cambridge, MA.

Stewart, J.S., 1990. Drifting Continents and Colliding Paradigms. Indiana University Press: Bloomington.

Le Grand, H.E., 1988. Drifting Continents and Shifting Theories. Cambridge University Press: Cambridge.

Stone, P., Plant J.A., MendumJ.R. and Green P.M., 1999. A regional geochemical assessment of some terrane relationships in the British Caledonides. Scottish Journal of Geology, 35: 145-156.

Le Grand, H.E. and Glen W., 1993. Choke-holds, radiolarian cherts, and Davy Jones's locker. Perspectives on Science, 1: 24-67.

Wilson, J.T., 1966. Did the Atlantic close and then re-open. Nature, 211:676-81.

Le Pichon, X., 1968. Sea-floor spreading and continental drift. Journal of Geophysical Research, 73: 3661-97.

Wilson, J.T., 1968. Static or mobile Earth: Hie current scientific revolution. Proceedings of the American Philosophical Society, 112: 309319.

Marvin, U.B., 1973. Continental Drift: The Evolution of a Concept. Smithsonian Institution Press: Washington D.C.

Wilson, J.T., 1976. Continents Adrift and Continents Aground Readings from Scientific American. W.H. Freeman: San Francisco.

McAllister, J.W., 1992. Competition among scientific disciplines in cold nuclear fusion research. Science in Context, 5, no. 1: 17-49.

Wyllie, P.J., 1976. The Way the Earth Works. John Wiley and Sons: New York.

TOPOGRAPHY AND TECTONICS Mike Sandiford Professorial Fellow, School of Earth Sciences, University of Melbourne, Victoria 3010, Australia. m.sandiford@earthsci.unimelb.edu.au http://jaeger.earthsci.unimelb.edu.au

SUMMARY The Earth is a hot, dense planet in a cold, sparse universe. Many of Earth's psychoses, such as volcanism, earth-quaking and other forms of anti-social behaviour, can be understood in terms of a competition between heat loss, which strives to disperse the thermal energy anomaly, and self-gravitation which holds the mass anomaly together. This competition is manifest as tectonics and is perhaps most spectacularly observed through the topography of the Earth's surface. As is well understood, plate tectonics provides a framework that accounts for most of the large-scale surface topographic features. Increasingly it is able to account for subtle features!

Topography of the ocean basins (pinks are high, blues are low) as measured and estimated from gravity data derived from satellite altimetry and shipboard depth soundings.

Sandiford

Plate tectonics provides a framework for understanding the connection between the processes that facilitate heat loss and the forces that drive plate motion. One of the great successes of plate tectonics lies in its explanation of most of the major surface topographic features of the Earth, particularly in the ocean basins (Figs. 1 and 2). In recent years, this success has extended to many subtle aspects of topography at the continental-scale that potentially provide important insights into the dynamics of the hot interior of the planet.

This talk

discusses the links between topography and tectonics, highlighting recent advances that help relate surface topography to dynamic processes in the mantle.

In recent years large-scale digital databases have revolutionised our approach

to Earth

sciences. This is particularly true for tectonics where global topographic and gravity datasets provide a level of analysis hitherto unimaginable. This talk will use visualisations based on a number of different datasets including digital terrain models (DTMs) such as GTOP030 1 , the GEOSAT synthetic topography and gravity datasets. These images are available at my web site: http://jaeger.earthsci.unimelb.edu.au/msandifo/Talks/2000/Selwvn/Selwvn.html.

The many topographies

of the plate tectonic world

The lithospheric plates represent the surface manifestation of large-scale flow in the mantle. Much of the surface topography reflects the density distribution within the plates themselves, related to the way in which they grow and age, and contributes to what we term the "isostatic topography \

The isostatic response of the lithosphere reflects the way in which the

lithosphere accommodates lateral variations in density by creating topography, not only at the

Figure 1. Shaded relief map of the world from the E T 0 P 0 5 dataset

1 GTOP030, U.S. Geological Survey, EROS Data Center, see also the GLOBE dataset from the NGDC web site http://www.ngdc.noaa.gov/, GEOSAT, http://topex.ucsdedu/marine_topo/mar_topo.html, For a catalogue of digital elevation data http://www.geo.edac.uk/home/ded.html

Topography and tectonics

surface, but also on internal density interfaces such as the crust-mantle boundary.

Density

variations in the lithosphere represent internal loads and, at length-scales less than several hundred kilometers, such loads flex the lithosphere, resulting in the production of "flexural topography \ with the characteristic flexural response depending on the size and age of the load and the strength of the lithosphere. In the deep mantle, the large-scale convective flow involving both upwelling and downwelling deforms the Earth's surface thereby "dynamic topographyDynamic

creating

topography is best seen in the "residual topographic"

field

obtained by removing the isostatic contribution from the observed topography (Fig. 3). Isostatic

and dynamic

topography

in the ocean

basins

Plate tectonics finds its greatest success in the ocean basins. The ocean floors are dominated by the mid-ocean ridges, with their symmetrical, gently curved ridge flanks descending into the abyssal plains (Fig. 2). To a very good approximation the bathymetry of the ocean floors increases with square root of the age of the ocean floor, at least for ocean lithosphere younger than about 80 Ma 2 . A corresponding decrease in the heat flow with the square root of age gives rise to the remarkable age-bathymetry-heatflow

relationship for the ocean floor. This

relationship is probably the most profound observation pertaining to the behaviour of the Earth and provides one of the cornerstones of plate tectonics: the ocean lithosphere forms by the conductive cooling of hot mantlel

The increase in density as the lithosphere cools causes

it to "sink" giving rise to variations in topography that are essentially isostatic in nature. The resulting topography developed around the mid-ocean ridges generates horizontal buoyancy forces, helping to drive flow away from the ridges and consequently stabilising the plate divergence at the ridge axis.

Figure 2. Topographic field of the ocean basins. Note the anomalously low segment of the mid-ocean ridge on the Australian-Antarctic-discordance to the south of Australia. 2

Debate continues as to the nature of the age-bathymetry relation in oceanic lithosphere older than about 80 Ma. The perception that this relation breakdown at about 80 Ma has lead to the thermal plate model e.g. Parsons & Sclater, (1978). For an alternative view see Marty &Cazenave (1989), Davies andPribac (1993).

Sandiford

Figure 3. The residual topography of the ocean basins is obtained by subtracting the isostatic contributions from the topographic field in Figure 2. This effectively removes the age-dependence of oceanic bathymetry, such that mid-ocean-ridges disappear. Variations in the residual topography reflect dynamic and flexural contributions at long at short wavelengths, respectively. Note the low in the residual topography on the Australian-Antarcticdiscordance to the south of Australia, and the high in the western Pacific. Australia is currently in transit from a dynamic topographic low to a dynamic topographic high.

The age-bathymetry-heatflow relationship implies that the ocean lithosphere has a simple structure that changes in a very predictable way with time. Therefore we can easily unroll the effects of time, allowing us to separate the "isostatic topography" from the "residual topography". Variations in residual topography reflect dynamic processes associated with convection deep within the Earth. The buoyancy flux associated with upwelling of hot mantle contributes to broad swells around many hot-spot or plume-related volcanic islands, where the contributions of dynamic and flexural topography are relatively obvious (Fig. 4). One of the most dramatic, large-scale residual topographic features occurs south of Australia on the Australian-Antarctic discordance (Fig. 3). The anomalously deep nature of the residual topography in this region suggests the possibility of mantle downwelling, while the apparent slope in the residual topography from the Antarctic to the Australian margin points to large-scale density anomalies in the sub-lithospheric mantle (Fig. 5).

The geoid and continental flooding The geology of the ocean crust is very simple, consisting of a thin layer of sediment (siliceous ooze and red clay) overlying a 5-7 km thick mafic igneous crust consisting mainly of basalt, "sheeted" dyke complexes and gabbro. Simply on the basis of the age we can predict the major geophysical features of the ocean such as its bathymetry and heat flow. In comparison, the continents are much more heterogeneous, and plate tectonics sensu stricto has found more difficulty in accounting for the subtleties in continental topography. Many subtle features are however explicable in terms of the links between dynamic topography and sea level as the continents circuit above a "lumpy mantle" (Gurnis et al., 1998).

Topography and tectonics

•

swell

Figure 4. Line profiles of topography and free-air gravity across Hawaii, showing the flexural response of the lithosphere to the seamount.

60 50 40 30 20 10

residual V.

North

South

Figure 5. Bathymetric profile from Antarctica (left) to Australia (right) across the Southern Ocean. Top panel shows age of ocean floor. Bottom panel shows the observed and residual topography. The residual topography is anomalously low (~ -1 km), with the suggestion of a slight gradient across the Southern ocean.

Sandiford

Variations in the density structure at depth create variations in gravitational potential inducing anomalies in the equipotential surfaces relative to a spherically symmetric Earth. The geoid is a reference equipotential surface that equates to the mean sea level. There is a general correlation between the geoid anomalies and the large-scale pattern of dynamic topography (Figs. 3 and 6). If the geoid height changed at exactly the same rate as dynamic topography, then the "lumpy mantle" would have no effect continental sea level. On the other hand, when geoid changes at a different rate to the dynamic topography, the extent of continental flooding should change as the continents transit across the mantle. The ratio of the amplitude of the variation in the geoid to the amplitude in the variation in the dynamic topography (the so-called admittance) is still uncertain but is estimated to be about 0.3 - 0.5 (Gurnis, 1990). The range in the geoid anomalies on the modern Earth is about 150m, suggesting that dynamic topography can readily account for 150-300 m of relative sea-level change. Over the last 150 Ma, Australia has moved northward over the dynamic topographic low now centered beneath the Australian-Antarctic discordance. It is now moving towards a dynamic and geoid high centered over the western Pacific (Fig. 7). The impact of this dynamic topography has left its imprint in the flooding record and drainage systems on the Australian continent. For example, the flooding of the Mesozoic inland seas can be attributed to the passage of the Australian continent over a developing dynamic topographic low associated with downwelling, the remnants of which are now located beneath the Australian-Antarctic discordance (Gurnis et al., 1998). The re-emergence of the continent and the development of the -1000 km scale west-flowing drainage systems of the Murray-Darling basin can be linked to the changing topography of the eastern sea-board as it exited this dynamic topographic low.

Figure 6. Global geoid map. There is general correlation between the geoid highs (light colours) and the residual topographic highs in Figure 3. The total amplitude in the geoid is about 150m from the low of Sri Lanka, to the high near Papua-New Guinea. He ghosted outlines of the continents reflect the fact that sources due to lithospheric density variations are subordinate to deep mantle density anomalies.

Topography and tectonics

Figure 7. The" lumpy mantle" effect illustrated by draping a shaded relief image of the Indian Ocean on the geoid surface. The view is to the east, with north to the left. A broad geoid trough extends from India (lower left) to southern Australia (upper right). Papua-New Guinea (top center) is riding the crest of the western pacific geoid high. The admittance between the geoid and the dynamic topography I estimated to be -0.3 -0.5 implying that transit across a lumpy mantle will modulate relative sea level.

The role of dynamic topography in the Australian Neogene sea-level record is yet to be satisfactorily elucidated. Nevertheless, it is clear from the current setting of Australia that dynamical processes are likely to have helped shaped this record. The elucidation of the dynamic topographic signal of the continents as distinct from ice, and sea-floor spreading rate, controlled sea-level changes remains a major challenge for the Earth sciences, in part because it will help unravel the dynamic story of the Earth's interior.

REFERENCES Marty, J. C., and Cazenave, A., 1989, Regional variations in subsidence rate of oceanic plates: A global analysis, Earth and Planetary Science Letters, 94 301-315.

Davies, G.F. and Pribac, F. Mesozoic seafloor subsidence and the Darwin Rise, past and present. In The Mesozoic Pacific, (M.S. Pringle, W.W. Sager, W.V. Sliter and S. Stein, eds.) American Geophysical Union Geophysics Monograph 77, 39-52.

Parsons, B., and Sclater, J.G, 1977, An analysis of the variation of ocean floor bathymetry and heat flow with age. Journal of Geophysical Research, 32, 803-827.

Gurnis, M., 1990, Bounds on global dynamic topography from Phanerozoic flooding of continental platforms, Nature, 344, 754-756. Gurnis, M., Muller, R.D., and Moresi, L., 1998, Dynamics of Cretaceous to the Present Vertical Motion of Australia and the Origin of the Australian-Antarctic Discordance, Science, 279, 1499-1504.

ENVIRONMENTS, POLAR DINOSAURS AND MOVING CONTINENTS IN THE EARLY CRETACEOUS OF EAST GONDWANA

Patricia Vickers-Rich* with Thomas H. Richt A. Constantine*, and Anusuya Chinsamy

^Professor of Vertebrate Palaeontology, Department of Earth Science and the Monash Science Centre, Monash University, Victoria 3800, Australia ^Department of Vertebrate Palaeontology, Museum of Victoria, P.O. Box 666E, Melbourne, Victoria 3001, Australia Earth Science Division, South African Museum, Cape Town, South Africa 8000; Zoology Department, University of Cape Town, Private Bag, Rondebosch, South Africa 7701 #

The ornithomimosaur Timimus hermani is shown hibernating in the foreground, while three hypsilophodontids, including Leaellynasaura amicagraphica and Qantassaurus intrepidus, actively forage during an Early Cretaceous polar winter in southeastern Australia. Artist: Peter Trusler 27

Rich, Rich, Constantine and

Chinsamy

SUMMARY Palaeomagnetic evidence places southeastern Australia well inside the palaeo-Antarctic Circle in the Early Cretaceous. Over the past two decades a small but modestly diverse dinosaur assemblage has been recovered from a variety of sediments laid down in a broad rift valley that was forming as Australia and Antarctica began separating at the beginning of the Cretaceous. The primary source of the sediments was volcanic vents that lay to the east on the Lord Howe Rise. A diverse flora of more than 150 species are associated with the dinosaur material. Analysis of this flora suggests a mean annual air temperature (MAAT) of 7°C to 10°C. Oxygen isotope investigations suggest a lower MAAT (-2°C to 5°C). Because of the discrepancy in the temperature estimates (the plants indicating a MAAT similar to that of modern day Denver or Chicago whereas the oxygen isotopes suggest conditions similar to present day Fairbanks, Alaska) finding a third, independent way of assessing the MAAT has been a primary goal of the southeastern Australian polar dinosaur project. A third method of estimating temperature was found in 1996 when cryoturbation structures were recognized at four localities in the Strzelecki Group to the southeast of Melbourne, suggesting the former presence of seasonally frozen ground. These cryoturbation structures suggest MAAT of -6°C to +3°C. The lower limit is set by the absence of sand- or ice-wedges, which form only at a MAAT < -4°C. In regions today with a similar MAAT, the coldest month of the year has an average temperature of-32.3°C and -23.9°C. The first hint of a special adaptation of any of the dinosaurs from southeastern Australia to polar conditions was pointed out nearly a decade ago. Comparison with the few available endocasts of the dorsal surface of the brains of hypsilophodontids from lower palaeolatitudes showed that the optic lobes of the hypsilophodontid Leaellynasaura amicagraphica were noticeably enlarged. This enlargement was interpreted as possibly allowing the animal to see under the low light conditions of a polar winter because of enhancement of the lobes ability to process the visual signals received from the eye. This interpretation implied activity during the polar winter. Histological analysis of femoral specimens of Leaellynasaura amicagraphica and two other hypsilophodontids from southeastern Australia shows that all taxa lack Lines of Arrested Growth, LAGs, in the compacta. LAGs, denser, darker layers of bone, form when the rate of growth is reduced such as when the animal is under food stress or hibernates. Animals that lack LAGs consequently are interpreted as not having undergone such phases. While LAGs occur in most dinosaurs examined, hypsilophodontids at both low and high palaeolatitudes are a noticeable exception. The absence of LAGs in all hypsilophodontids coupled with the enlarged optic lobes in Leaellynasaura amicagraphica but not in lower palaeolatitude hypsilophodontids, suggests that the family was pre-adapted for year around activity and then acquired greater visual acuity when they entered higher latitudes, if they, in fact, first evolved at lower palaeolatitudes. The diversity of hypsilophodontids in the southeastern Australian dinosaur assemblage, where they constitute half the approximately dozen taxa known, would suggest that they lived in a habitat particularly suitable for them. Some, but certainly not all, polar dinosaur sites show this pronounced bias towards hypsilophodontids. The one other southeastern Australian dinosaur examined for LAGs was the ornithomimosaur Timimus hermani. This animal has strongly developed LAGs. Presumably, it may have survived the Winters by hibernating, unlike the hypsilophodontids, which evidently did not.

Introduction During the Early Cretaceous, southeastern Australia lay well inside the palaeo-Antarctic Circle of the day - a fact established by extensive palaeomagnetic studies carried out by M. Whitelaw (formerly working with Neil Updyke University of Florida) involving more than 400 samples across the Otway and Strzelecki Ranges of the south Victorian coast.

A significant number

of dinosaur bones and associated flora and fauna have been recovered from this area over the

Environments, polar dinosaurs and moving

continents

past two and a half decades, and because of its high latitude location, it is of some marked biogeographic and evolutionary interest. Polar terrestrial biotas, particularly polar dinosaur faunas, are rare in the fossil record, the two most diverse being recovered from southeastern Australia and the North Slope of Alaska. The faunas and floras of this area have been extensively described in a number of papers, summarized by T. Rich (1996) P. V. Rich., et al (1988), T. H. Rich, et al (1997), VickersRich & Rich (1997), Warren et al (1997), Gaffney et al (1998) and Drinnan & Chambers, 1986, and references therein). This paper summarizes new data that has come to light in the last few years which sheds further light on the kind of environment that typified this polar area and the possible survival stategies employed by some of the vertebrates inhabiting this Early Cretaceous marginal environment. This environment apparently sheltered some groups by excluding others and may have been the source of novelty that then later was exported to other parts of the world. New sedimentary evidence for cool temperatures and frozen ground in Australia's Early Cretaceous southeast Sedimentary structures in the Early Cretaceous Wonthaggi Formation of southeastern Australia appear to have formed in the presence of seasonally frozen ground, indicating a frigid climate (Constantine et al,

1998).

Histological analysis of a hypsilophodontid

dinosaur bone found 3 meters stratigraphically above one of the seasonally frozen ground horizons shows that this individual never experienced a period of slow growth as would have been the case had it not been active during the polar winter (Chinsamy et al, 1998). Another of the southeastern Australian hypsilophodontids had enlarged optic lobes of the brain, perhaps an adaptation for enhanced visual acuity during the winter darkness, facilitating active foraging at that time (Rich et al, 1988).

Figure 1. Position of southeastern Australia relative to Antarctic during the Aptian-Albian (Early Cretaceous).

Rich, Rich, Constantine and

Chinsamy

Cryoturbation structures are deformation structures seen associated with the seasonal freezing and thawing of soils (French, 1976; Vandenberghe, 1988; Washburn, 1980). These structures are formed in cold-climate, non-glacial environments in which cryogenic, or frost-action processes dominate (French, 1976; Vandenberghe, 1988), and can be subdivided into six types based on their size and shape (Vandenberghe (1988). v

Such environments are termed

periglaciar and are, to a large part, underlain by permafrost.

In the Gippsland Basin of

southeastern Australia, cryoturbation structures have been identified in fluvial-lacustrine sediments of the Wonthaggi Formation, which is Aptian-Albian in age (122-98 Ma) (Fig. 1). The presence of such structures in this unit is significant because it, together with its temporal equivalent in the adjacent Otway Basin, the Eumeralla Formation, has yielded a diverse vertebrate fauna including monotremes, a tribosphenic mammal, birds, dinosaurs, plesiosaurs, lepidosaurs, labyrinthodont amphibians, turtles and fish (Currie et al., 1996; Rich & Vickers Rich, 1994; Rich and Rich., 1989; Gaffney et al1998;

Rich, T.H. et al, 1998; Warren, Rich

& Vickers-Rich, 1997). The cryoturbation structures occur at four sites; three of which occur within 1 km of each other in coastal cliff exposures at Flat Rocks, southwest of Inverloch, Victoria. The three sites (labelled A, B, C in Fig. 2) occur in two fault-bounded blocks within fine-grained overbank sediments. The fourth site is located near the Kilcunda-B coal mine. All four sites have been palynologically dated as Aptian (122 - 113 Ma).

channel sediments overbank sediments —

cryoturbation site vertebrate fossil site

fault

Flat Rocks-C

Flat Rocks-B

Flat Rocks-A

30-

Kilcunda-B coal mine

. p 30-

20-

10-

Figure 2. Simplified stratigraphic columns showing the Flat Rocks cryoturbation sites A-C, and the earth hummock cryoturbation site at the Kilcunda-B coal mine. Flat Rocks cryoturbation site-A: 38° 39.7' S, 145 0 40.8' E; Flat Rocks cryoturbation site-B: 38° 39.6* S, 145° 40.9* E; R a t Rocks cryoturbation site-C: 38° 39.2' S, 145° 41.2' E; Kilcunda-B coal mine (38° 33.1 S, 145 0 27.6' E).

Environments, polar dinosaurs and moving continents

backpack [•.*/.VJ fine- to medium-grained sandstone channel complex [

j very fine-grained sandstone | mudstone

overbank sediments

shore platform littered with cobbles and boulders

coal band

Figure 3. Flat Rocks-A cryoturbation site. A. View of Type 3a cryoturbation horizons. B. Line interpretation of Fig. 3 A. showing main features. L = lower horizon, U = upper horizon. Bone-bearing conglomerate occurs in channel complex approximately 3 meters above the upper horizon.

At the Flat Rocks-A site (Fig. 3), two cryoturbation horizons containing Type 3a involutions of Vandenberghe (1988) are present, with one overlying the other. Each horizon consists of a basal fine-grained sandstone overlain by a mudstone. In both cases, the contact between the sandstone and the mudstone is spectacularly deformed with tear-shaped drops of mudstone up to 20 cm in depth sinking into the sandstone. Drop width is variable with the majority ranging between 10 to 15 cm in width. These typically have bulbous forms, whereas larger drops up to 70 cm wide have flat-bottomed bases. None of the drops, though, penetrate more than 20 cm into the underlying sandstone. Similar shaped features have also been observed 31

Rich, Rich, Constantine and Chinsamy

at the Flat Rocks-B and -C sites, but they are nowhere near as spectacularly developed as those visible at Site A. The Type 3a involutions observed at the Flat Rocks-A, -B, and -C sites are interpreted as reverse density gradient structures associated with the thawing of seasonally frozen sands below mud-filled depressions. The involutions form at the start of the thaw season (Fig. 4) as the result of water moving laterally through the active layer towards the center of the depression where it is trapped by the overlying, relatively impermeable mud. thawing of ice at the top of the active layer increases pore-fluid pressure in this area creating reverse density gradients in the active layer. This causes the overlying mud to sink into the sand. At the Kilcunda-B coal mine site, a series of involutions pass laterally over a distance of about 8 meters into an earth hummock horizon (Fig. 5). The involutions (Fig. 6) consist of mudstone sinking into a fine-grained sandstone, but are more symmetrical than those at Flat Rocks-A, B, and -C, and are classified as Type 3b according to the scheme Vandenberghe (1988). The earth hummock horizon (Fig. 7) is a 30 cm thick fine-grained cryoturbated sandstone whose hummocky upper surface is mantled by a 1 to 2 cm thick coal band. The same coal band is visible in Fig. 6 just above the Type 3b involutions.

Isand

I silt

Figure 4. (left) Development of a cryoturbation structure, a drop soil associated with the melting of seasonally frozen ground below a shallow pool 2.

Y/\ frozen

20 m Earth hummocks

coal band

mudstone

sandstone Type 3b involutions

Figure 5. (above) Line diagram of Kilcunda-B coal mine site, showing locations of Figs 6 and 7 (facing page). 32

Environments, polar dinosaurs and moving continents

Figure 6. Close-up of Type 3b cryoturbation structure in the Strzelecki Group near the Kilcunda-B coal mine site. 50 cm long rod for scale.

Figure 7. Close-up view of earth hummock horizon in the Strzelecki Group near the Kilcunda-B coal mine site. 8 cm long penknife for scale. Note disruption of bedding below the coal band in the sandstone. Photo is approximately 8 meters to right of cryoturbations structures illustrated in Fig. 6.

The occurrence of cryoturbation structures and earth hummocks in the Wonthaggi Formation is useful from a palaeoclimatic perspective, because it allows us to place some constraints on the Mean Annual Air Temperature (MAAT) at the time these structures formed. According to French (1976), modern periglacial environments in which frost-action processes dominate (i.e. active periglacial) have MAATs < -2°C, while regions where frost-action processes do occur, but to a far lesser extent (i.e. marginal periglacial), have MAATs between -2°C and +3°C. Earth hummocks, incomparison, are primarily restricted to periglacial environments 33

Rich, Rich, Constantine

and

Chinsamy

with MAATs < -4°C, although there are two reports of them occurring in regions with MAATs of +3°C (i.e. Mt. Kosciusko, Australia) and +6°C (Hokkaido, Japan) (Costin & Wimbush, 1973; Koaze et al., 1974). Both of these occurrences, though, are in mountainous regions which suggests MAAT may not be the controlling factor. The complete absence of ice- and sand-wedges in the Wonthaggi and Eumeralla formations is considered significant, because these structures only form at MAATs much lower than cryoturbation structures and earth hummocks form at.

Ice wedges form today in fine-

grained sediments with high ice content in areas with a MAAT less than or equal to -6°C (Pewe et al., 1969; Karte, 1983), while sand wedges occur in sand and gravels with low ice content in regions with a MAAT less than or equal to -12°C (Karte, 1983).

The apparent

absence of structures in the Eumerella and Wonthaggi formations which could be interpreted as fossil ice- or sand-wedges is considered very significant, as it indicates that the MAAT during the Aptian-Albian never dropped below -6°C for any significant period of time. The co-occurrence of this evidence suggests that the MAAT in southeastern Australia during the Aptian was probably somewhere on the order of -6°C to +3°C. The presence of earth hummocks at one site, however, suggests that MAAT did occasionally get below -4°C. However, the fact that we have not found more cryoturbation structures elsewhere in the Wonthaggi and Eumeralla formations strongly suggests frost-action processes were not that dominant, and the MAAT was probably somewhere on the order of -2°C to +3°C.

Figure 8. Mean monthly temperatures over a period of one year for a continental weather station at Fort Smith with a MAAT = -2.6°C, maximum temperature = + 16.5°C, minimum temperature = -25.3°C, and temperature range = 41.8°C. Temperature ranges over a period of one year for eight other continental weather stations with MAATs of between -6.2°C and +5.1°C: A. St. Cloud, MAAT = +5.1°C, max. =+21.1°C, min. = -14.9°C, range = 36.0°C, B. Qiqhar, MAAT = +3.3°C, max. = +23.1°C, min. = -19.4°C, range = 42.5°C, C. Winnipeg International, MAAT = +2.2°C, max. = +19.6°C, min. = -19.5°C, range = 39.1°C, D. Blagovescensk, MAAT = +0.5°C, max. = +21.4°C, min. = -23.9°C, range = 45.3°C, E. Fairbanks, MAAT = -2.9°C, max. = +17.2°C, min. = -24.3°C, range = 41.5°C, F. Bomnak, MAAT = -4.8°C, max. = +17.4°C, min. = -32.3°C, range = 49.7°C, G. Norman Wells, MAAT = 5.8°C, max. = +17.3°C, min. =-27.1°C, range = 44.4°C, H. Aldan, MAAT = -6.2°C, max. = +15.6°C, min. = -27.3°C, range = 42.9°C. GNEP (1995), GNTP - Global Network of Isotopes in Precipitation, IAEA / WMO Isotopes-inPrecipitation network database available on the Internet at: http://www.iaea.or.at/programs/ ri/gnip/gnipmain.html

O £ 13 2 CD Q_

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Environments,

polar dinosaurs

and moving

continents

Analysis of the ratio of l s O to 1 6 0 ratios in carbonate concretions from both the Wonthaggi and Eumerella formations of southeastern Australia has suggested a MAAT there of -2°C to 5°C for the entire Aptian and Albian (Gregory et al, 1989). On the other hand, analysis of the flora from the same units has suggested a warmer MAAT of +7°C to +10°C (Hallam, 1985; Spicer & Corfield, 1992). The presence of sedimentary structures interpreted as reflecting events that took place because of the presence of seasonally frozen ground at temperatures between -6°C and +3°C, supports the oxygen isotope results. However, continuous sampling has not been carried out over the entire 3 kilometer thickness of the Wonthaggi and Eumerella formations, and the various catagories of evidence relevant to the palaeotemperature estimates were not collected at the same sites. Until at least the latter is done, the matter of the palaeotemperature history of the entire time span of the Wonthaggi and Eumerella formations cannot not considered satisfactorily settled. What the mean annual range in temperature (MART) was at the cryoturbation sites during the Aptian can be estimated by comparison with modern sites that have similar MAATs (Fig 8). The data indicate that the MART during their formation would have been on the order of 41.5°C to 49.7°C, with a warmest month mean of between +15.6°C and +21.4°C and a coldest month mean of between -32.3°C and -23.9°C.

Such climates are clearly seasonal.

This estimate hinges strongly on an assumption of Substantive Uniformitarianism with its inherent weaknesses as detailed by Gould (1965). There is, unfortunately, no other more exact way of estimating the MART. The contribution of vertebrate histology Small, but diverse, tetrapod faunas are known from the Eumerella Formation (Currie et al., 1996) and the Wonthaggi Formation (Rich & Rich, 1989; Rich & Vickers-Rich, 1994; Rich et al1988).

Histological analysis has now been carried out on four of the dinosaurs from

these assemblages ornithomimosaur amicagraphica

(Chinsamy Timimus

et al,

hermani

1998). (Fig.

9),

Sections the

from

the

femora

hypsilophodontid

the

Leaellynasaura

(Fig. 10A), a second as yet unnamed hypsilophodontid all from the early

Albian site at Dinosaur Cove (Rich & Vickers, Rich, 1994); Chinsamy et al, 1998) together with a third hypsilophodontid also as yet unnamed from the Aptian site at Rat Rocks (Fig. IOC) have been so examined.

The latter specimen occurs just 3 meters stratigraphically

above the Flat Rocks-Site A cryoturbation site (Fig. 2). Timimus hermani shows clear evidence in the presence of distinct lines of arrested growth (LAGs) in the compact bone that regular episodes of rapid growth alternated with ones of little or none (Fig. 9). Such a pattern is found in all dinosaurs examined thus far except some bipedal ornithischians, e.g. hypsilophodontids, Iguanodon but not hadrosaurs.

Rich, Rich, Constantine and Chinsamy

Figure 9. Transverse section of the femur of Timimus hermani, NMV P186317. Arrows indicate lines of arrested growth (LAGs) in the compact bone. Scale bar = 93 microns.

All three hypsilophodontids from southeastern Australia that have been

histologically

examined completely lack LAGs, indicating they grew at a continuous rate (Fig. 10).

This

would mean that these hypsilophodontids were active all year around in order to maintain a constant level of food intake.

Year around activity would necessitate activity during the

winter period of continuous darkness when the monthly mean temperature may have reached as low as -30°C, unless these small terrestrial animals migrated a significant distance. Such a low temperature is particularly likely for the hypsilophodontid from the Flat Rocks locality to have experienced owing to its being from only 3 metres above a seasonally frozen ground horizon. The palaeolatitude of the southeastern Australian Aptian-Albian tetrapod localities was about 78° to 75°S16. At 73°S, the palaeo-Antarctic Circle would have been at minimum 600 km away, a distance too great for the smaller terrestrial dinosaurs such as hypsilophodontids to have reached easily in annual migrations. Even there, the temperatures are unlikely to have been sufficiently high to have made migration a viable strategy to avoid the stress of polar winter conditions.

Despite the apparent harshness of the southeastern Australian Early

Cretaceous polar climate, hypsilophodontids seem to have thrived there. Their known record is far more diverse at any one time there than anywhere else on Earth, with as many as three coexisting genera (Rich & Rich, 1989).

That polar hypsilophodontids

should display

evidence for continuous growth is not altogether surprising in light of the fact that three other small bipedal ornithischians examined from significantly lower palaeolatitudes (Dryosaurus lettow-vorbecki

from Tendaguru, Tanzania, Hypsilophodon

foxii from the Isle of Wight,

Environments, polar dinosaurs and moving continents

England, and an as yet unnamed genus and species from the Proctor Lake site, Texas, U.S.A.) also have a similar pattern; i.e. the absence of LAGs in compact bone Rich & Rich, 1989). In one of the Dinosaur Cove hypsilophodontids, Leaellynasaura

amicagraphica,

of the dorsal aspect of the brain has been preserved (Rich & Rich, 1989).

an endocast

The optic lobes of

this specimen are quite hypertrophied in contrast to those of the small, bipedal ornithischians known from Tendaguru, the Isle of Wight, and Proctor Lake as the only three examples of lower palaeolatitude hypsilophodontids for which an endocast of the brain is known (Rich & Rich, 1989). This difference has been interpreted (Rich & Rich, 1989) as reflecting the enhanced ability of L. amicagraphica to process visual signals, presumably an adaptation that would have boosted the ability of this animal to forage for food during the period of prolonged, continuous darkness each polar winter, as well as at night in more benign parts of the year. If the low palaeolatitude ancestors of L. amicagraphica were adapted for a uniform growth rate, a evolutionary response to the light regime at polar latitudes by an increased visual acuity selected for to maintain that growth pattern is plausible. If this interpretation of the function of the enlarged optic lobes of

Leaellynasaura

amicagraphica is correct, it follows that if endocasts of the dorsal surface of the brains of the other dinosaurs histologically examined ever become available, the two other southeastern Australian hypsilophodontids already examined histologically and found to have been active year around should also have enlarged optic lobes, while Timimus hermani would be expected to have optic lobes no larger than ornithomimosaurs from lower latitudes.

Figure 10. Transverse sections of three hypsilophodont femora. Note the azonal nature of the compact bone in all three examples. A. Leaellynasaura amicagraphica, Dinosaur Cove, NMV PI79561. Scale bar = 98 microns. B. A second, unnamed hypsilophodontid from Dinosaur Cove, NMV P177935. Scale bar = 328 microns. C. Unnamed hypsilophodontid from Flat Rocks, 3 metres stratigraphically above the Flat Rocks-Site A cryoturbation site, NMV P199180. Scale bar = 234 microns. The bone has been altered by diagensis. Photograph taken under polarised light.

Rich, Rich, Constantine

and

Chinsamy

Modern reptiles do reach the Arctic Circle, but they are not active during the winter months. It would seem that in order to do so at temperatures in the range suggested for southeastern Australia during the Aptian-Albian, endothermy

would have been advantageous

for

hypsilophodontid dinosaurs. Such endothermy may have been fundamentally different in dinosaurs than in modern day mammals and birds as suggested by the recent work of Ruben et al. (1996). They demonstrated that the dinosaurs sampled lacked turbinal bones in the nasal passages. The presence of these bones is a nearly universal element in living mammals and birds related to the regulation of air temperature and water retention in the nasal passages at the higher metabolic rates associated with endothermy. The sample of Ruben et al (1996) did not include hypsilophodontids. To determine whether hypsilophodontids did have turbinal bones is be an obvious next step to see whether they may have differed from other dinosaurs in this regard and, therefore, possibly in their mechanism of endothermy. Conclusions The evidence reported here suggests that seasonally frozen ground occurred in the late Mesozoic.

It must be emphasized, though, that the presence of cryoturbations and earth

hummocks in the Wonthaggi Formation does not necessarily mean that the sites at which these structures occur was underlain by permafrost, as both types of structures can occur in non-permafrost regions. All we can say at the moment is that the sediments in which they occur must have have been seasonally frozen.

This is significant in itself, as it is widely

accepted that the polar regions during the Early Cretaceous were not frigid (Hallam, 1985; Spicer & Corfield, 1992). This it would appear, is clearly not the case, and lends support to the idea that there were cold places on the Earth during this period (Gregory et Frakes et al1995;

al1989;

Stoll & Scrag, 1996), and that these areas were inhabited by some

dinosaurian groups which had more than one strategy that allowed them to live in such environments. In regions today with a similar MAAT to that of Early Cretaceous southern Victoria, the coldest month of the year has an average temperature of -32.3°C and -23.9°C. The first kind of a special adaptation of any of the dinosaurs from southeastern Australia to polar conditions was pointed out nearly a decade ago (Rich & Rich, 1989). Comparison with the few available endocasts of the dorsal surface of the brains of hypsilophodontids from the lower palaeolatitudes showed that the optic lobes of the hypsilophodontid amicagraphica were noticeably enlarged.

Leaellynasaura

This enlargement was interpreted as possibly

enhancing the ability of the dinosaur to see under low light conditions imposed by the polar position of this area of the world. This interpretation favoured activity during the polar Winter.

Histological analysis presented by Chinsamy et al (1998) of the femora of

Leaellynasaura amicagraphica and two other hypsilophodontids from southeastern Australia shows that all taxa lack Lines of Arrested Growth (LAGs), in the compacta. 38

LAGs, denser,

Environments, polar dinosaurs and moving continents

darker layers of bone, form when the rate of growth of the bone is reduced by environmental stress - such as when the animal is under food, temperature or hibernation stress. Animals which lack LAGs consequently are interpreted as not having undergrone such stress. While LAGs occur in most dinosaurs examined, hypsilophodonteds at both low and high palaeolatitudes are a noticeable exception. The absence of LAGs in all hypsilophodontids coupled with the enlarged optic lobes in Leaellynasaura amicagraphica, but not in lower palaeolatitude hypsilophodontids, suggests that the family was quite preadapted for year round activity and then acquired greater visual acuity when they entered the higher latitudes if, in fact, they first evolved at lower palaeolatitudes! The diversity of hypsilophodontids in the southeastern Australian dinosaur assemblages, where they constitute half of the approximate dozen taxa known, would suggest that they lived in a habitat particularly suitable for them. Some, but certainly not all, polar dinosaur sites show this pronounced bias towards hypsilophodontids (Rich, Gangloff & Hammer, 1997). The one other southeastern Australian dinosaur examined for LAGs was the ornithomimosaur Timimus hermani. This animal has strongly developed LAGs. Presumably, Timimus may have survived the Winters by hibernating, unlike the hypsilophodontids, which evidently did not. Such new information about growth strategies and brain development in dinosaurs as well as new palaeomagnetic and sedimentological interpretations clearly support a mobile earth and not a stable arrangement of the continents as they are today.

Acknowledgments We thank the Committee for Research and Exploration of the National Geographic Society, the Australian Research Council, Atlas Copco Australia and Orica for their long term support for this work. Material allowing this paper to be written was collected under Natural Resources and Environment of Victoria permit number NP978/111 and others.

REFERENCES Currie, P. J., Rich, T. H. & Vickers-Rich, P., 1996. Possible oviraptorosaur (Theropoda, Dinosauria) specimens from the Early Cretaceous Otway Group of Dinosaur Cove, Australia. Alcheringa 20: 73-79.

Chinsamy, A., Rich, T. H., & Vickers-Rich, P., 1998. Polar dinosaur bone histology. Journal of Vertebrate Paleontology 18(2): 385-390. Constantine, A., Chinsamy, A., Vickers-Rich, P. & Rich, T. H., 1998. Periglacial environments and polar dinosaurs. South African Journal of Science 94: 137-141.

Drinnan, A. H. & Chambers, T. C., 1986. Flora of the lower Koonwarra fossil bed (Korumburra Group), South Gippsland, Victoria. Association of Australasian Palaeontologists Memoir 3: 177.

Costin, A. B. & Wimbush, D. J., 1973. Frost cracks and earth hummocks at Kosciusko, Snowy Mountains, Australia. Arctic Alpine Research 5: 111-120.

Frakes, L.A., Alley, N.E., & Deynoux, M., 1995. Early Cretaceous Ice Rafting and Climate

Rich, Rich, Constantine and

Chinsamy

Zonation in Australia. Review 37:567-583.

International

French, H. M., 1976. The Periglacial Longmann, London.

Rich, P.V., Rich, T.H., Wagstaff, B., McEwen Mason, J., Douthitt, R.T., Gregory, R.T. & Felton, A., 1988. Evidence for low temperatures and biologic diversity in Cretaceous high latitudes of Australia. Science 242: 1403-1406.

Geology

Environment.

Rich, T. H. & Vickers-Rich, P., 1994. Neoceratopsians & ornithomimosaurs:dinosaurs of Gondwana origin? National Geographic Research and Exploration 10: 129-131.

Gaffney, E. S., Kool, L., Brinkman, D. B., Rich, T. H. & Vickers-Rich, P., 1998. Otwayemys, a new cryptodiran turtle from the Early Cretaceous of Australia. American Museum Novitates 3233: 1-18.

Rich, T. H., Vickers-Rich, P., Constantine, A., Flannery, T. F., Kool, L. & van Klaveren, N., 1997. A tribosphenic mammal from the Mesozoic of Australia. Science 278: 14381442.

Gregory, R. T., Douthitt, C. B., Duddy, I. R., Rich, P. V. & Rich, T. H. , 1989. Oxygen isotopic composition of carbonate concretions from the lower Cretaceous of Victoria, Australia; implications for the evolution of meteoric waters on the Australian continent in a palaeopolar environment. Earth and Planetary Science Utters 92: 27-42.

Vandenberghe, J. , 1988. Cryoturbations. In: M. J. Clark (ed.) Advances in Periglacial Geomorphology: 179-198. John Wiley and Sons Ltd., Chichester.

Gould, S. J., 1965. Is Uniformitarianism necessary? American Journal of Science 263: 223-228.

Washburn, A. L., 1980. Geocryology. A Survey of Periglacial Processes and Environments. John Wiley and Sons Ltd., New York.

Hallam, A., 1985. A review of Mesozoic climates. Journal of the Geological Society, London 142, 433-445.

Rich, T. H. & Rich P. V., 1989. Polar dinosaurs and biotas of the Early Cretaceous of southeastern Australia. National Geographic Society Research Reports 5: 15-53.

Karte, J., 1983. Periglacial phenomena and their significance as climate and edaphic indicators. Geojour. 7: 329-340.

Ruben, J. A., Hillenius, W.J., Geist, N.R : , Leitch, A., Jones, T.D., Currie, P.J., Homer, J.R., Espe III, 1996. The metabolic status of some Late Cretaceous dinosaurs. Science 273: 1204-1207.

Koaze, T., Nogami, M. & Iwata, S., 1974. Palaeoclimatic significance of fossil periglacial phenomena in Hokkaido, northern Japan. Quaternary Research 12: 177-191.

Spicer, R. A. & Corfield, R. M., 1992. A review of terrestrial and marine climates in the Cretaceous with implications for modelling the "Greenhouse Earth". Geological Magazine 129: 169-180.

Parris, J. T., Spicer, R. A., Doublas, J. G., Rich, T. H. & Vickers-Rich, P., 1991. Continental climate near the Albian South Pole and comparison with climate near the North Pole. Geological Society of America:, Abstracts with Programs 23: A302.

Stoll, H. M. & Schrag, D. P., 1996. Evidence for glacial control of rapid sea level changes in the Early Cretaceous. Science 272: 1771-1774.

Pewe, T. L., Church, R. E. & Andresen, M. J., 1969. Origin and palaeoclimate significance of largescale patterned ground in Donnelly Dome Area, Alaska. Geological Society of America Special Papers 103.

Veevers, J. J., McA Powell, C. & Roots, S. B., 1991. Review of seafloor spreading around Australia. I. Synthesis of the patterns of spreading. Australian Journal of Earth Sciences 38: 373-389.

Rich, T. H., 1996. Significance of polar dinosaurs in Gondwana. Memoirs of the Queensland Museum 39 (3): 711-717.

Vickers-Rich, P. & Rich, T. H., 1997. The polar dinosaurs of southeastern Australia. Dinofest International: 253-257.

Rich, T. H., Gangloff, R. A. & Hammer, W., 1997. Chapter 102. Polar dinosaurs. In Padian, K. & Currie, P. J. (eds),. The Dinosaur Encyclopedia. Adademic Press, New York.

Warren, A., Rich, T. H. & Vickers-Rich, P., 1997. The last last labyrinthodonts? Palaeontographica 247: 1-24.

PLATE TECTONICS AND MINERAL DEPOSITS

Mark E. Barley Associate Professor, Centre for Global Metallogeny, Department of Geology and Geophysics, The University of Western Australia, Nedlands WA 6907. mbarley@cgm.uwa.edu.au

SUMMARY

Metallogeny is the study of the origin of mineral deposits, with emphasis on their relationship in space and time to regional (tectonic) features of the Earth's crust. Modern Earth science emphasises the interdependence of Earth's biological, chemical and tectonic cycles. Knowledge of this interdependence is critical to understanding global metallogeny. This is because the abundance of many metal deposits reflects interactions between biological and chemical cycles, tectonic settings and plate-tectonic cyclicity. In this talk I will first look at the relationship between mineral deposits and tectonic settings, something that has been relatively well known since the 1970s (eg. Mitchell and Garson, 1972; 1981). I will then illustrate and discuss the relationship in time and space between gold and copper deposits and plate-tectonics in the magmatic arcs of Southeast Asia using a mineral deposit database (Rak 1999) overlain on Robert Hall's (1996) animated plate-tectonic model. This will show that although there is a clear spatial and temporal relationship between the tectonic evolution of the region and the distribution of mineral deposits, currently accepted static models that link these deposits to their tectonic setting are either wrong or too simplistic. In this century the most exciting challenge for metallogenic research will be to better understand the link between mineral deposits and global tectonics and stages in the evolution of the biosphere to aid the search for much needed resources.

Metal deposits and tectonic settings

Tectonic settings exert a major control on mineral deposits, their deformation and preservation potential in several ways. Firstly types of magmatism, each associated with particular types of hydrothermal alteration are characteristic of one or more tectonic setting. For example calc-alkaline volcanic and plutonic rocks with their associated porphyry and epithermal Cu-Au mineralisation are erupted and intruded in subduction-related magmatic arcs such as those that surround the 41

Barley

Pacific Ocean. There reasons for the major peak in the abundance of these deposits in the last 25 million years will be examined in the second half of this talk.

Secondly, the nature of the Earth's major sedimentary successions, their geometry, thickness, composition and facies, and associated formation of mineral deposits during or after deposition is controlled by the tectonic setting. For example, extensive prisms of shallow marine carbonates with associated lead-zinc sulphide deposits are characteristic of the continental seas that formed behind major Phanerozoic orogenic belts. Deposits of this type were most abundant during the final stages of the amalgamation of Pangea and first stages of its breakup

Thirdly the rocks in many tectonic settings undergo subsequent deformation and in some cases metamorphism, uplift and erosion, the nature and extent of which is dependent on their tectonic setting. Although some metal deposits such as the gold-bearing veins of many convergent margin orogenic belts form as a result of deformation and metamorphism, and placer gold deposits form within and adjacent to orogenic belts, many other deposit types may be uplifted, eroded and destroyed by such processes. The preservation potential of rocks and their mineral deposits is hence dependent on their tectonic setting.

Fourthly the geothermal gradient is dependent on tectonic setting. This is not only important for the generation and maturation of hydrocarbons, but also exerts a major control on the circulation of hydrothermal fluids. The movement of hot waters and in some cases hydrocarbons is essential to the formation of many types of sediment-hosted metal sulphide deposit.

Finally, because major faults can control the location of igneous intrusions and the circulation of hydrothermal fluids they can also exert a major tectonic control on the location of mineral deposits. For example some of the most important porphyry and epithermal Cu-Au deposits of the Pacific Rim are situated adjacent to major transfer faults.

Plate tectonics and metal deposits

Although relatively few types of mineral deposit can be observed forming in modern tectonic settings at the Earth's surface, the association of many mineral deposits with volcanic, intrusive and sedimentary rocks identical to those of modern tectonic settings is well established. Based on the relationships I have just described it is possible to classify mineral deposits in terms of the tectonic environments in which they form and/or are preserved. This information which is summarised in textbooks by Mitchell and Garson (1981) and Sawkins (1990), is very useful in targeting under-explored parts of the world for mineral exploration. However, because the plates move, the present configuration of continents and the terranes that they contain does not necessarily reflect the original tectonic controls on mineral deposit distribution. Mineral deposits and moving plates

Plate tectonic cyclicity can be viewed as the cyclic breakup, dispersal and assembly of supercontinents such as Pangea, or as the cyclic contraction and expansion of the superocean (Panthalassa). With modern plate-tectonic reconstructions it is now possible to examine the spatial distribution of metal deposits on Pangea and postulated Precambrian supercontinents and test the hypothesis that peaks in the distribution of certain metal deposits reflect tectonic events during their breakup and amalgamation (eg. Barley and Groves, 1992). It is also now possible to examine the distribution of mineral deposits in space and time in more detail, taking into account plate motion, by overlaying mineral deposit databases on animated tectonic models. We have done this for the porphyry, epithermal and skarn Cu-Au deposits of the magmatic arcs of Southeast Asia, using Hall's (1996) tectonic model for evolution of that area during the past 50 million years. Tectonic controls on gold in the magmatic arcs of Southeast Asia

The Cenozoic magmatic arcs that surround the Pacific Ocean are richly endowed with magmatichydrothermal gold deposits such as high- and low-sulfidation epthithermal Au deposits, porphyry Cu-Au deposits and skarn Cu-Au deposits. Although a spatial and temporal link between these types of metal deposit and subduction-related magmatism has been recognised for some time (Mitchell & Garson, 1972; Sillitoe, 1989), the deposits are most abundant within specific arc sectors and during specific periods. This strongly suggests that large-scale tectonic factors are 43

Barley

important to the formation and localisation of magmatic-hydrothermal gold deposits in magmatic arcs.

The magmatic arcs of Southeast Asia contain some of the world's richest and largest magmatichydrothermal gold deposits and formed during the Cenozoic as a result of the convergence of the Indian-Australian, Philippine, Pacific and Eurasian plates (Fig. 1). This area has experienced relatively rapid changes in plate configuration and rates of tectonic processes during the Cenozoic. It is thus a key area for assessing the effects of plate tectonics on the spatial and temporal distribution of magmatic-hydrothermal gold deposits.

Examination of the spatial and temporal distribution of gold deposits relative to the regional tectonic model shows that all principal deposits formed after 25 Ma (Sillitoe, 1989) with the majority from the Late Miocene to Pleistocene (6 to 1 Ma) (Fig. 2). Most gold deposits formed during major tectonic reorganisations (Fig. 2). The first of these followed the entry of the Australian craton into the Sunda-Banda subduction zone at around 25 Ma. This was followed by a Middle Miocene period of mineralisation that followed the maximum rotation or extrusion of Indochina, cessation of spreading in the South China Sea and collision of the Ontong-Java Plateau with the Solomons arc at around 17 Ma. However, the majority of deposits formed in a broad belt from Taiwan through the Philippine arc, Irian Jaya and Papua New Guinea, to the Solomons arc following a major change in relative plate motion between the Pacific and adjacent plates at ~5 Ma. This change in relative plate motion was coincident with the ~5 Ma collision of the Philippine arc with the Eurasian plate in Taiwan (Hall, 1996) and the change from transtension to transpression along the Alpine Fault in New Zealand (Norris et al., 1992).

It has been suggested (Sillitoe, 1989) that the preponderance of young deposits is due to the increased likelihood of erosion with increasing age. Though erosion will act to de-emphasise older near surface metal deposits, pre-Pliocene volcanic and high-level intrusive rocks are relatively common with little evidence that they were as richly mineralised. Hence it is likely that the dominance of Pliocene gold deposits relative to other periods is not caused solely by the removal by erosion of older deposits, but may be related to the tectonic evolution of the arcs.

Plate tectonics and metal deposits

EURASIA

Red River sJFault _

TAIWAN Luzon is.

1 I

500 Kms \ Oblique Mercator Projection x ~ 5.45°/pa

f 'PHILIPPINES ^ ) pole at \ PHILIPPINE > Masbate is>\ SEA PLATE x Negros is. /

suiu y^ Masbate^C.

trenches/* v \ PENINSULAR v \ \MAliAYSIA 0

\ Mindanao Wi is. CAROLINE SEA PLATE Kilinailau

\ N. Sulawesi < trench^. '

irt \ PAPUAN tNEW _

New^s

Bougainville

AUSTRALIAN CRATON

'INDIANA.

PLATE

Figure 1. Map of Southeast Asia, showing tectonic plates (Eurasian, Indian-Australian and Pacific) in pale grey, and Cenozoic island arcs in dark grey. Gold districts are numbered: 1 = Camarines Norte, 2 = Batangas, 3 = Baguio, 4 = Mankayan, 5 = Isabella Didipio, 6 = Ertsberg-Grasberg, 7 = Tombolilato-Gorontalo, 8 = Kotamobagu-Ratatotok.

Gold deposit tonnage v Age

r 2000

at c 5 1500 r-

Age (Ma)

C.w. Rotation of Borneo Subduction under SW Borneo Negros and Suiu Trench activity Subduction along Eastern Luzon. PHUppine trench Pr< Proto-Philippine trench Subduction at Manila trench _ Extrusion of Indochina South China Sea 5.45V Ma 1.7° / Ma Philippine Sea plate rotation 5 Ma Collision of Philippine plate 25 Ma Collision of Philippine plate and Eurasian plate in Taiwan and Australian craton in PNG. Late Late I Middle j Early Oligocene Miocene Plei. Plio. Figure 2. Graph of gold tonnage against deposit age, compared to major tectonic events in the region. The grey shaded columns indicate periods of plate reorganistion.

Barley

At around 25 Ma, the arrival of the Australian craton at the Sunda-Banda subduction zone caused major tectonic reorganisations in the Southeast Asian island arc region (Hall, 1996). The direct effects were a change in the tectonic regime in the Papua New Guinea region from near orthogonal subduction to sinistral strike-slip and the commencement of clockwise rotation of the Philippine Sea plate. The Philippine Sea plate is central to the region, this change in motion affected neighbouring terranes. These effects included a reversal of subduction beneath the Philippines from eastward subduction on the proto-Manila trench to westward subduction on the proto-Philippine trench. This period coincided with gold deposits in the Isabella-Didipio district (Luzon) and in New Britain (Mitchell and Leach, 1991). During the Middle Miocene, the island arcs experienced more rapid convergence and plate rotation following the rotation, or extrusion, of Indochina and the opening of the South China Sea and (Fig. 3b). An early tectonic response to the spreading in the South China Sea was the initiation of subduction under southeast Borneo. This is may be related to the formation of Central Kalimantan gold deposits, dated between 24 and 18 Ma (Fig. 1 and 3c). As maximum spreading of the South China Sea was reached at around 17 Ma, subduction reversal occurred in the Philippines, from the proto-Philippine trench in the east to the modern Manila and SuluMasbate trenches in the west (Hayes and Lewis, 1984). This subduction reversal coincided with gold deposits in the Camarines Norte district, eastern Philippines, and the Negros, Masbate, Batangas and Baguio districts along the western side of the archipelago (Fig. 3b). Middle Miocene gold deposits (15 to 12 Ma) also formed in the Maramuni arc of northern New Guinea. This period of mineralisation followed impingement of the Philippine-Caroline arc with the extended margin of New Guinea and collision of the Ontong-Java Plateau with the Solomons arc (Hill, 2000).

Plate tectonics and metal deposits

Figure 3. Tectonic maps of Southeast Asia at 5, 15 and 24 Ma (adapted from Hall, 1996) with superimposed gold deposits. 3 A and 3C include all gold deposits formed between 0 and 6 Ma, and 23 to 25 Ma respectively. 3B shows all gold deposits formed between 12 and 15 Ma as circles and those formed between 16 and 22 Ma as squares.

Barley

The last major tectonic event to affect Southeast Asian magmatic arcs was the collision of the Philippine arc with the Eurasian plate in Taiwan at 5 Ma (Hall, 1996) and contemporary change in the relative movement of the Pacific and adjacent plates. These events increased the rate of rotation of the Philippine Sea plate from 1.7 to 5.4° per Ma and changed the pole of rotation from 15°N 160°E to 48°N 157°E (Hall, 1996). The consequent effects on neighbouring terranes were widespread. In the Philippines, subduction appears to be transferring from the Manila trench, where subduction has slowed since 5 Ma, to the modern Philippine trench, where subduction commenced at around 5 Ma (Mitchell and Leach, 1991)(Fig. 1). Significant gold deposits formed in the eastern Philippines, and particularly the Baguio and Mankayan districts (Fig. 3a). Further afield, the changes in plate movement affected Sulawesi, Irian Jaya and New Guinea. Subduction was initiated in the Pliocene on the north Sulawesi trench, also associated with subduction reversal. Associated gold deposits include those in the Tombulilato/Gorontalo and Ratatotok/Kotamobagu districts (Fig. 3a). In Irian Jaya and Papua New Guinea subduction was initiated on the New Guinea trench and Pliocene magmatism was associated with significant gold deposits, including Ertsberg-Grasberg in the New Guinea Orogen. Subduction was also initiated on the Eastern New Britain trench. The Panguna deposit on Bougainville and deposits in the New Ireland arc formed at this time. There is a clear relationship between the age of gold deposits in SE Asian magmatic arcs and the reorganisations caused by changes in the regional tectonic regime (Figs 2 & 3). The most recent and important of these resulted in an intensely mineralised belt extending from Taiwan through the Philippines, Sulawesi, Iryan Jaya, Papua New Guinea to the Solomon Islands and Fiji. However, the nature of the link between subduction and magmatic-hydrothermal gold mineralisation and how, the tectonic reorganisations control the location of mineralised districts are less certain. Early models that directly link mineralisation and magmatism to steady-state subduction and explain the scarcity of mineralisation in older arcs as resulting from erosion are clearly too simplistic.

Plate tectonics and metal deposits

Sillitoe (1989) observed that all mineralised districts occur in magmatic arcs with evidence for active extension or uplift at the time of mineralisation. In such environments volatile- and metalrich magmas may be emplaced at high levels in the crust before Au is lost to early crystallising oxide or sulphide phases. Further many mineralised districts with high gold content occur in volcanic arcs following a reversal in subduction polarity (Solomon, 1990). In such a setting, extension may result from slowing subduction (and possibly roll-back of the slab) on one side of the volcanic arc and incipient subduction on the other. Magmatism may result from either dehydration or melting of the slab, or inflow of hot mantle as subduction slows or during slab roll-back. Solomon (1990) suggested that this would induce melting of sub-arc mantle that had been both modified and previously melted by earlier episodes of subduction and that such magmas may be intrinsically gold rich. Gold deposits that follow subduction reversal occur in Late Oligocene, Middle Miocene and Pliocene Luzon and Pliocene Mindanao, north Sulawesi and Bougainville. Further possible districts with this setting are Romang-Wetar islands in Indonesia and New Britain and New Ireland in Papua New Guinea. However, although subduction reversal appears to be a common regime for magmatic-hydrothermal gold deposit formation, it is unlikely that the Celebes Sea slab had penetrated to sufficient depth to provide a source for magmas associated with Pliocene mineralisation in Mindanao (Macpherson and Hall, 1999). Also the Pliocene magmas associated with deposits of the New Guinea Orogen in Irian Jaya and northern Papua New Guinea, including the prodigious Grasberg-Ertsberg district, can not be directly linked to coeval subduction. Thermal or tectonic reactivation of subduction-modifled mantle beneath a thickened arc or orogen, with melting in the mantle and lower crust seems a more likely cause for magmatism in both cases. There are clearly still a lot of questions to be answered before we can adequately explain the tectonic causes of the super abundance of magmatic-hydrothermal gold in Southeast Asia and the western Pacific rim. My current preferred view is that the change in relative plate motion between the Pacific and adjacent plates at ~5 Ma more or less coincides with the impingement of island arcs against old Australian and Eurasian lithosphere. It has thus resulted in widespread

Barley

reactivation of fertile sub-arc mantle reservoirs with previous or prolonged histories of subduction. It has also created regional and local stress fields that allowed rapid emplacement of volatile- and metal-rich magmas to high levels in the crust.

REFERENCES Barley, M.E. and Groves, D.I., 1992. Supercontinent cycles and the distribution of metal deposits through time. Geology, 20: 291-294.

Mitchell, A.H.G. and Garson, M.S., 1981. Mineral Deposits and Global Tectonic Settings. Academic Press: London.

Hall, R., 1996. Reconstructing Cenozoic SE Asia, in Hall, R. and Blundell, D. eds, Tectonic Evolution of Southeast Asia, Geological Society of London Special Publication, 106: 153-184.

Mitchell, A.H.G. and Leach, T.M., 1991. Epithermal Gold in the Philippines. Academic Press: London. Norris, R.J., Kooons, P.O. and Cooper, A.L., 1992. The obliquely converging plate boundary in the South island of New Zealand: implications for ancient collision zones. Journal of Structural Geology, 12: 715-725.

Hayes, D.E. and Lewis, S.D., 1984. A geophysical study of the Manila Trench, Luzon, Philippines; 1, crustal structure, gravity, and regional tectonic evolution. Journal of Geophysical Research, B89: 9171-9195.

Rak, P., 1999. The relationship between gold deposit distribution and major tectonic events in Southeast Asia. BSc (Honours) Thesis, The University of Western Australia.

Hill, K.C., 2000. Mesozoic-Tertiary rvolution of the northern Australian margin. Geological Society of Australia, Abstracts 59: 223.

Sawkins, F.J., 1990. Metal Deposits in Relation to Plate Tectonics. Springer-Verlag.

Macpherson, C.G. and Hall, R., 1999. Tectonic controls of geochemical evolution in arc magmatism of SE Asia. Proceedings Parcim 99, Australasian Institute of Mining and Metallurgy: 359-368.

Sillitoe, R.H., 1989. Gold deposits in western Pacific island arcs: the magmatic connection. Economic Geology, Monograph 6: 274-291.

Mitchell, A.H.G. and Garson, M.S., 1972. Relationships of porphyry copper and circum-Pacific tin deposits to palaeo-Benioff zones. Transactions of the Institute of Mining and Metallurgy, 81: 10-25.

Solomon, M., 1990. Subduction, arc reversal, and the origin of porphyry copper-gold deposits in island arcs. Geology, 18: 630-633.

EXPLORING THE THERMAL RECORD OF CONTINENTAL EXTENSION IN PLATE TECTONICS Andrew Gleadow Professor of Earth Science and Head of School, School of Earth Sciences, University of Melbourne, Victoria 3010, Australia. a.gleadow@earthsci.unimelb.edu.au

136'

144-

152'

136*

144'

152"

Fission Track Age (Ma)

Figure 1. Interpolated images of fission track age (left) and mean track length (right) for apatites in eastern Australia. The apatites are mainly from granitic rocks of Palaeozoic age.

Gleadow

SUMMARY The basic premise of plate tectonics, that the earth's surface is made up of essentially rigid lithospheric plates that are in constant motion relative to each other, is now abundantly demonstrated by satellite-based geodetic surveying. The same data however, have shown that the plates are not completely rigid, as indeed much geological evidence has long suggested, and that plate boundaries, especially convergent boundaries, are more complex than originally anticipated. Extensional tectonic settings are somewhat simpler than convergent environments, but even here the picture has become more complex as new ways of looking at the continental record have emerged. One important new record has been that derived from low temperature thermochronology using the apatite fission-track (FT) system. Many rifted continental margins are now known to be characterised by a coastal zone of low apatite FT age. These FT age anomalies tend to show a general coincidence in time with the onset of sea-floor spreading in the adjacent ocean basin and were quickly associated with the processes of continental rifting. However, there is now general agreement that the dominant control on the observed apatite FT age patterns is the history of surface denudation on crustal blocks of differing scales - a history that is linked to processes occurring before and during continental breakup rather than with any direct thermal effect of the onset of sea-floor spreading. These complications demonstrate that there is still much work to be done in applying what we have learned from plate tectonics to the continental geological record. New methods and approaches will enhance our ability to tackle these important issues.

Over the last 30 years plate tectonics has provided the unifying paradigm which explains the way our planet works and has profoundly changed our understanding of the large scale processes within the earth, as well as the inter-relationships between these and surface processes. In that time the major tenets of the original theory have been spectacularly confirmed by mounting bodies of evidence from many sources, particularly from the ocean basins. Part of the early success of plate tectonics derived from the relative simplicity of structure and composition, and youthfulness of age inherent in the oceanic lithosphere. Even in these regions, however, new global data sets, such as the sea-floor topographic imaging derived from satellite gravity data, are now revealing details and complexities which were previously unknown. The basic premise of plate tectonics, that the earth's surface is made up of essentially rigid lithospheric plates that are in constant motion relative to each other, is now abundantly demonstrated and most strikingly confirmed by satellite-based geodetic surveying. Global Positioning System measurements are now able to measure actual plate movements in only a few months, generally confirming the integrated estimates over millions of years based on geological evidence The earth is indeed a mobile planet and all of our understanding of its workings and history must be moderated and informed by this fact. These direct measurements and other modern datasets are also leading to additional elaboration and modification of the simple principles of plate tectonics. They show that plates are clearly not completely rigid, as indeed much geological evidence has long suggested, and that plate boundaries, especially convergent boundaries, are even more complex than originally anticipated. 52

Thermal record of continental

extension

Extensional tectonic settings are somewhat simpler than convergent environments, but even here the picture has become more complex as new ways of looking at the continental record have emerged.

One important new record has been that derived from low temperature

thermochronology using the apatite fission-track (FT) system.

Many rifted continental

margins are now known to be characterised by a coastal zone of low apatite FT age (Fig. 1). The indicated ages are much lower than the actual ages of the host rocks. Apatites from these areas of anomalously low age are typically also characterised by long fission track lengths, which indicate that there is no record of track accumulation prior to the indicated apatite age. This means that the apparent FT age can be taken as having geological significance in such samples. This is explained either by a heating event sufficient to erase all pre-existing tracks, or by a rapid cooling from temperatures above the fission track annealing zone (i.e. above about 100-110°C). The first indications of this fission track age anomaly along rifted continental margins were found around Bass Strait (Gleadow and Lovering, 1978, Foster and Gleadow, 1992,) and have been followed by studies adjacent to many rifted margins, for example: the Tasman Sea (Moore et al, 1986; Dumitru et al., 1991), the Red Sea (Kohn and Eyal, 1981; Kohn et al, 1993, 1997; Menzies et al., 1997; Bohannon et al., 1989; Omar et al., 1989) and the South Atlantic (Brown et al., 1990, 2000; Gallagher et al., 1994, 1995). These FT age anomalies tend to show a general coincidence in time with the onset of sea-floor spreading in the adjacent ocean basin and, for this reason, were quickly associated with the processes of continental rifting. Further studies have showed that the patterns are more complex than first thought from simple transects and the limited regional studies first carried out. Fission track data are now available from the entire eastern margin of Australia (Fig. 1) and over wide areas of other rifted margins, as well as from active extensional environments in the Basin and Range Province (eg. Foster et al., 1993), and the East African Rift System (eg. Foster and Gleadow, 1993, 1996). The larger perspectives now available from these studies show that significant regional variations in thermal history

occur along

extensional

continental margins and within extensional tectonic environments in general. Interpretations of the thermal information contained within these data sets are still developing but there is now a general agreement that the dominant control on the observed apatite FT age patterns is the history of surface denudation on crustal blocks of differing scales. Thus, the link is most strongly made with the processes occurring before and during continental breakup, rather than with any direct thermal effect of the onset of sea-floor spreading (Gallagher et al., 1998).

The fission track patterns also give important indications of the structure of

continental blocks in environments where other structural markers are absent (eg. Foster and Gleadow, 1996). Many researchers have also now made it clear that extensional and compressional tectonics are not separated from each other but frequently coexist, or alternate with each other, within

Gleadow

an orogenic belt. When we look at the astonishing mobility and complexity of modern orogenic belts, such as that stretching from SE Asia to the SW Pacific, it is not surprising that plate tectonic reconstructions of older orogenic belts have been difficult and often controversial. Indeed, the application of plate tectonics to past Earth history is, in many ways, the least successful aspect of the development of the theory and the one that still holds out the greatest challenges for the future. Surprisingly, our models of the tectonic evolution of the continents often remain very limited, usually non-unique and all too often staunchly 'fixist' in perspective. Clearly there is still much work to be done in applying what we have learned from plate tectonics to the continental geological record of even the Phanerozoic, let alone the Precambrian Earth. Plate tectonics will therefore continue to develop as our understanding of modern tectonic environments and our knowledge of past configurations of the Earth increases. New methods and approaches will enhance our ability to tackle these important issues.

REFERENCES Foster D.A. and Gleadow A.J.W., 1993. Episodic denudation in East Africa - a legacy of intracontinental tectonism. Geophysical Research Letters, 20/21: 2395-2398.

Bohannon R.G., Naeser C.W., Schmidt D.L., Zimmermann RA., 1989. The timing of uplift, volcanism and rifting peripheral to the Red Sea: a case for passive rifting? Journal of Geophysical Research, 94: 1683-701.

Foster D.A. and Gleadow A.J.W., 1996. Structural framework and denudation history of the flanks of the Kenya and Anza Rifts, East Africa. Tectonics, 15: 258-271.

Brown R.W., Rust D.J., Summerfield M.A. and Gleadow A. J. W., 1990. Temporal and spatial patterns of Cenozoic and Late Mesozoic deposition and erosion along the western margin of southern Africa. American Association Petroleum Geologists Bulletin: 74, 620.

FosterD.A., Gleadow A.J.W, Reynolds, S.J. and Fitzgerald P.F., 1993. The denudation of metamorphic core complexes and the reconstruction of the Transition Zone, west central Arizona: constraints from apatite fission track thermochronology. Journal of Geophysical Research, 98: 2167-2185.

Brown R.W., Gallagher K., Gleadow A.J.W. and Summerfield M.J., 2000. Morphotectonic evolution of the South Atlantic margins of Africa and South America., in Summerfield M.J. ed., Geomorphology and Global Tectonics, Wiley: 255-281.

Gallagher K, Brown R.W. and Johnston C., 1998. Fission track analysis and its application to geological problems. Annual Reviews of Earth and Planetary Sciences, 26: 519-572.

Dumitru T.A., Hill K.C., Coyle D.A., Duddy I.R., Foster D.A., Gleadow A.J.W., Green P.F., Laslett G.M., Kohn B.P. and O'Sullivan A.B., 1991. Fission track thermochronology: application to continental rifting of southeastern Australia. Australian Petroleum Exploration Association Journal, 31: 131-142.

Gallagher K, Hawkesworth C, Mantovani M. 1994. The denudation history of the onshore continental margin of S.E. Brazil inferred from fission track data. Journal of Geophysical Research, 99: 18117-118145.

Foster D.A. and Gleadow A.J.W., 1992. Reactivated tectonic boundaries and implications for the reconstruction of southeastern Australia and northern Victoria Land, Antarctica. Geology 20: 267-270.

Gallagher K, Hawkesworth C, Mantovani M., 1995. Denudation, fission track analysis and the longterm evolution of passive margin topography: application to the S.E. Brazilian margin. Journal of South American Earth Science, 8: 65-77. 54

Thermal record of continental extension

Gleadow A.J.W. and Lovering J.F., 1978. Fission track geochronology of King Island, Bass Strait, Australia: Relationship to continental rifting. Earth and Planetary Science Letters, 37: 429-437.

Moore M.E., Gleadow A.J.W. and Lovering J.F., 1986. Thermal evolution of rifted continental margins: new evidence from fission track dating of apatites from southeastern Australia. Earth and Planetary Science Letters, 78: 255-270.

Kohn B.P. and Eyal M., 1981. History of uplift of the crystalline basement of Sinai and its relation to the opening of the Red Sea as revealed by fission track dating of apatites. Earth and Planetary Science Letters, 52: 129141.

Menzies MA, Gallagher K, Hurford AJ, Yelland A. 1997. Red Sea and Gulf of Aden rifted margins, Yemen: denudational histo-ries and margin evolution. Geochimica Cosmochimica Acta, 61: 2511-2527. Omar G.I., Steckler M.S., Buck W.R., Kohn B.P., 1989. Fission-track analysis of basement apatites at the western margin of the Gulf of Suez rift, Egypt: evidence for synchroneity of uplift and subsidence. Earth and Planetary Science Letters, 94: 316-328.

Kohn, B.P., Wagner, M.E., Lutz, T.M., and Organist, G., 1993. Anomalous thermal regime during Mesozoic rifting, central Appalachian Piedmont: evidence from sphene and zircon fission track dating. Journal of Geology, 101: 779-794. Kohn, B.P., Feinstein, S., Foster, D.A., Steckler, M.S., and Eyal, M., 1997. Thermal history of the eastern Gulf of Suez: II Reconstruction from apatite fission track and AT/39 AT K-feldspar measurements. Tectonophysics, 283: 219-239. 40