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The Assembly and Breakup

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Workshop Proceedings Perth, 7-8 September, 1998 Australia The University of Westerm

GEOLOGICAL SOCIETY OF AUSTRALIA

ABSTRACTS, Number 50

Geological Society of Australia

ABSTRACTS No. 50

THE ASSEMBLY AND BREAKUP OF RODINIA

Editor: Robert T. Bird

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

ISSN 0729-01IX © Geological Society of Australia Incorporated 1998 Copies of this publication may be obtained from the Geological Society of Australia Incorporated, 301 George Street, Sydney, NSW, Australia 2000 Tectonics Special Research Centre — established and supported under the Australia Research Council's Research Centres Program Example citation for papers in this volume: Pirajno, F., and Cooke, A., 1998, Metallogeny of the Rodinia supercontinent, in Bird, R.T., (ed.), The Assembly and Breakup of Rodinia, Geological Society of Australia, Abstracts No. 50, p. 45-48. Cover design: Michael T. D. Wingate Printed by: Fineline Print & Copy Service, 11 Bramall St., East Perth 6004 WA

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

THE ASSEMBLY AND BREAKUP OF RODINIA Proceedings of a workshop sponsored by the

TECTONICS SPECIAL RESEARCH CENTRE Department of Geology and Geophysics The University of Western Australia School of Applied Geology Curtin University of Technology University of Texas at Austin

in association with the Geological Society of Australia Western Australian Division and the Centre for Ore Deposit Research School of Earth Sciences University of Tasmania

WORKSHOP CONVENORS: Robert T. Bird Christopher McA. Powell Michael T. D. Wingate

KEYNOTE SPEAKERS: Ian W. D. Dalziel Raphael Unrug

Workshop held at the Department of Geology and Geophysics The University of Western Australia Nedlands, Western Australia 7-8 September 1998

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

TABLE OF CONTENTS (In alphabetical order of first authors) ANOTHER LOOK AT RODINIA RECONSTRUCTIONS: USING CONTINENTAL AEROMAGNETIC AND GRAVITY DATA Robert T. Bird and Walter R. Roest 1 THE CONCEPT OF TRIPLE JUNCTION STABILITY IN QUESTION: EPISODIC TRIPLE JUNCTION MIGRATION BY RIFT PROPAGATION AND MICROPLATES Robert T. Bird, Sarah F. Tebbens, Martin C. Kleinrock and David F. Naar 6 WAVELETS TO BROWSE Robert T. Bird, Thomas A. Ridsdill-Smith, R. Dietmar Miiller and Mark Pilkington 7 TEMPERATURE-TIME (T-t) PATH FOR THE TICKALARA MET AMORPHICS OF THE HALLS CREEK OROGEN, WA: IMPLICATIONS FOR TECTONIC MODELS Simon Bodorkos, Peter A. Cawood and Nicholas H. S. OHver 8 INITIATION OF SUBDUCTION IN THE PROTO-PACIFIC OCEAN: CONSTRAINTS FROM EASTERN GONDWANA Peter A. Cawood and Evan C. Leitch 11 LATE PROTEROZOIC — EARLY PALAEOZOIC EVOLUTION OF THE EASTERN ADELAIDE FOLDBELT AND WESTERN LACHLAN FOLDBELT — THE REAL BREAKUP, AND SUBSEQUENT CONTINENTAL CRUSTFORMING EVENTS Anthony J. Crawford and Nicholas G. Direen 15 ^''Ar/^^Ar GEOCHRONOLOGY AND THE NEOPROTEROZOIC TECTONICS ALONG THE NORTHERN MARGIN OF THE EASTERN GHATS PROVINCE IN NORTH ORISSA, INDIA Warwick A. Crowe, Michael A. Cosca and Lyal B. Harris 17 EXTENSIONAL SEDIMENTARY REGIMES IN THE NEOPROTEROZOIC OF THE ADELAIDE GEOSYNCLINE BobDalgamo 19 REFINING RODINIA lanW. D. Dalziel 23 EVIDENCE FOR THE TIMING OF CONTINENTAL BREAKUP AND VOLCANIC PASSIVE MARGIN FORMATION, SOUTHEASTERN GONDWANA: AN ACTUALISTIC PERSPECTIVE N. G. Direen and A. J. Crawford 24 THE BREAKUP UNCONFORMITY, SALT TECTONICS AND DEVELOPMENT OF MINI-BASINS IN THE NEOPROTEROZOIC SUCCESSION OF THE ADELAIDE GEOSYNCLINE Ian A. Dyson 29 SUPERCONTINENTAL CYCLES AND TRUE POLAR WANDER David A. D. Evans 34 LOWER-CRUSTAL FLOW AND ITS EFFECT ON EXTENSIONAL STYLES A.P. Gartrell 36

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

INITIATION OF RODINIA BREAKUP IN SOUTHEAST AUSTRALIA BY SWNE EXTENSION: EVIDENCE FROM SHEAR ZONE KINEMATICS AND DYKE INTRUSION George M. Gibson 37 CRUSTAL-SCALE EXTENSIONAL COLLAPSE OF THE ELZEVIR OROGEN, GRENVILLE PROVINCE, S.W. QUEBEC IMAGED BY LITHOPROBE Lyal B. Harris, Benoit Rivard and Louise Corriveau 39 RECOGNITION OF REGIONAL DETACHMENTS IN THE SOUTHEASTERN HAMERSLEY PROVINCE, WESTERN AUSTRALIA D. A. Hollingsworth, P. A. Cawood, R. I. Hackney and C. McA. Powell 40 Ca. 825 Ma MAFIC TO FELSIC IGNEOUS ACTIVITIES IN SOUTH CHINA: PART OF PLUME-MDUCED RIFTING EVENTS THAT LED TO THE BREAKUP OF RODINIA? Z.X. Li, X.H. Li, P.D. Kinny and J. Wang 42 METALLOGENY OF THE RODINIA SUPERCONTINENT Franco Pirajno and Alasdair Cooke 45 ASSEMBLY AND BREAK-UP OF RODINIA LEADING TO FORMATION OF GONDWAN ALAND C. McA. Powell 49 DEFORMATION MECHANISMS AND ISOTOPE SYSTEMATICS: IMPLICATIONS FOR THE ABSOLUTE DATING OF DEFORMATION Steven M. Reddy and Graham J. Potts 54 STRUCTURAL RELATIONSHIPS AND THE SYSTEMATIC ASSESSMENT OF DEFORMATION HISTORIES Steven M. Reddy and Graham J. Potts 56 THE CENTRAL AFRICAN COPPERBELT: TECTONIC OVERVIEW, METALLOGENY AND ANALYSIS OF CURRENT RESEARCH John Simmonds 57 EFFECTS OF SUBSIDIARY FAULTS ON THE GEOMETRIC RECONSTRUCTION OF LISTRIC NORMAL FAULT SYSTEMS Tingguang Song and Peter A. Cawood 60 OBLIQUE RIFTS: BASIN DEVELOPMENT AND TRAPS FOR HYDROCARBON ACCUMULATION IN WESTERN AUSTRALIA AND CHINA Tingguang Song and Peter A. Cawood 62 THE GEODYNAMIC MAP OF GONDWANA SUPERCONTINENT ASSEMBLY (IGCP PROJECT 288: GONDWANALAND SUTURES AND FOLD BELTS) Raphael Unrug, Christian Castaing, Jean-Louis Feybesse, Pieter G. Gresse, C. McA. Powell, Georg R. Sadowski and Luc Tack 63 SUPERCONTINENT RECONSTRUCTIONS: LESSONS LEARNED FROM THE GEODYNAMIC MAP OF GONDWANA SUPERCONTINENT ASSEMBLY APPLICABLE TO RECONSTRUCTION OF RODINIA Raphael Unrug 65 ISOTOPIC AND PALEOMAGNETIC CONSTRAINTS ON THE TIMING OF NEOPROTEROZOIC BREAKUP OF RODINIAN AUSTRALIA Michael T.D. Wingate 70 NEW LATE PRECAMBRIAN PALAEOMAGNETIC RESULTS FROM NORTH CHINA AND THEIR PALAEOGEOGRAPHIC IMPLICATIONS Shihong Zhang and Zheng-Xiang Li 72

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

TECTONOTHERMAL EVOLUTION OF THE MA YUAN TECTONIC UNIT IN THE CATHAYSIA BLOCK AND ITS BEARING ON THE ASSEMBLY OF RODINIA Guochun Zhao and Peter A. Cawood 75 AUTHOR INDEX

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

ANOTHER LOOK AT RODINIA RECONSTRUCTIONS: USING CONTINENTAL AEROMAGNETIC AND GRAVITY DATA Robert T. Bird

Tectonics Special Research Centre, Department of Geology and Geophysics University of Western Australia, Nedlands WA 6907

Walter R. Roest

Geological Survey of Canada Ottawa ON K1A 0E9, Canada

SUMMARY Compelling geologic and paleomagnetic evidence has been presented by a number of researchers (e.g., Bell and Jefferson, 1987; Dalziel, 1991; Hoffman, 1991; Young, 1992; Powell et al., 1993; Park et al., 1995) suggesting that the paleo-continents of East Gondwanaland (Australia, Antarctica and India) and Laurentia (North America and Greenland) were once juxtaposed in the Proterozoic (-1050-725 Ma, Powell et al., 1993) supercontinent Rodinia (McMenamin and McMenamin, 1990). It also has been suggested that this supercontinent included cratonic blocks from Siberia (e.g., Hoffman, 1991) and South China (Li et al., 1995), among others. Controls on paleogeographic reconstructions before Pangea are limited (Dalziel, 1995), however, and proposed models for thefitof the continents are not well constrained. Recently, continent-wide compilations of magnetic and gravity measurements have allowed large-scale interpretation and correlation within old cratonic blocks, and aeromagnetic and gravity data covering Australia, Canada, Eurasia and the United States have become available. These data reveal characteristic signatures of major geologic features in the Precambrian basement that may be correlated across reconstructed continental boundaries. Therefore, the principal objective of this new study is to use these potential field data to test the various possible Rodinia fits. INTRODUCTION

Significant advances have been made in recent years in understanding the processes of plate tectonics, the birth and death of the oceans, and the continents' history of rifting, drifting and assemblage (e.g., Hoffman, 1988; Myers, 1993). The magnetic reversal history recorded in today's ocean basins provides a relatively tight constraint on continental reconstructions as far back as the Cretaceous (e.g., Larson et al, 1985; Scotese et al, 1988). In addition to these marine geophysical data, a wealth of data have been compiled to further constrain paleogeography into the Precambrian, although alternative reconstructions are often permissible. In many cases, the evidence is enough to hypothesize that two cratonic blocks were once together, but the orientations or relative positions of these cratons remains obscure. Such is the case for the Proterozoic supercontinent of Rodinia (McMenamin and McMenamin, 1990), where only the assemblage of East Gondwanaland is fairly well estabhshed.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

MODELS OF RODINIA At the centre of the Rodinia debate is Laurentia. Jefferson (1978) first suggested a physical connection between Laurentia (northwestern Canada) and East Gondwanaland (in South Australia) during the Proterozoic. Bell and Jefferson (1987) expanded on this hypothesis with stratigraphic and metallogenic comparisons, as well as a discussion of paleomagnetic constraints, although no reconstruction was offered. Moores (1991) presented the first reconstruction of western North America and the Australian-Antarctic shield; the southwest U.S.— East Antarctic (SWEAT) hypothesis, complete with a schematic map of Precambrian features which may extend from Laurentia (North America) into East Gondwanaland (eastern Antarctica, Australia and India). Dalziel (1991) supported this hypothesis and produced a computer-generated reconstruction of Gondwanaland and Laurentia at 570 Ma. Hoffman (1991) followed with a complete reconstruction of Rodinia at -700 Ma which included Siberia, Baltica and the Congo, Kalahari, West Africa and Amazonia cratons. He suggested that late Proterozoic fragmentation of Rodinia was centred on Laurentia, and that it was followed by the fan-like collapse of the (present) southern continents and the consolidation of East and West Gondwanaland by -500 Ma (Hoffman, 1991). The fundamental concept of the Rodinian supercontinent in these models (Moores, 1991; Dalziel, 1991; Hoffman, 1991) is derived from the rifted nature of the continental margins of Laurentia, Australia and Antarctica, the correlation of Grenville-age rocks, and the age equivalence of passive margin sedimentary sequences. In addition to these pioneering reconstructions, other models have been suggested recently. Brookfield (1993) proposed another configuration, with a different relative position of Laurentia in Rodinia, by correlating interpreted paleotransform faults in the southwestern U.S. and eastern Australia. Condie and Rosen (1994) developed a new model for the orientation of Siberia and Laurentia, and most recently. Frost et al (1998) presented yet another model which depicts Siberia rotated about 180* when compared to the model of Hoffman (1991). The Frost et al. (1998) model is based on a geological traverse across the Aldan Shield and subsequent thermobarometry, whole rock and isotope geochemistry, and geochronology. To address the question of South China, Li et al (1995) used stratigraphic correlations and tectonic analysis to suggest that the Yangtze block of South China may have been wedged between Australia and Laurentia in Rodinia. TESTING RODINIA HYPOTHESES Each of the above reconstructions may not claim to provide a precise fit of the continents, but each does imply that a number of prominent geologic features should have conjugate features across the continental boundaries, and these models may be tested independently. Moores (1991) suggested four possible tests of his hypothesis which may be extended in principle to any of the proposed Rodinia models: 7. Detailed magnetic and gravity surveys to reveal geophysically prominent sutures along province boundaries, 2. Careful stratigraphic comparisons of late Precambrian and early Paleozoic age rocks between continents, 3. Comparative petrologic and tectonic studies of ophiolitic and other mafic-ultramafic rocks, and 4. More and better paleomagnetic data to help resolve the relative positions of the continents between 500 Ma and 1000 Ma. In this regard. Young (1992) found similarities in glaciogenic successions of Canada and Australia, and Stump (1992) found the late Proterozoic geology of the Transantarctic Mountains

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

and the southwestern U.S. to be similar. Powell et al (1993) interpreted paleomagnetic data for East Gondwanaland and Laurentia to illustrate that these two continents were indeed juxtaposed in Rodinia by 1050 Ma, and that their separation began after 725 Ma. By 580 Ma, Laurentia had moved to polar latitudes, with these continents separated by a wide Pacific Ocean. Borg and DePaolo (1994) attempted to analyze the fit of Precambrian provinces on the basis of isotopic and geochronologic maps. This analysis led to the suggestion of an alternative reconstruction to that of Moores (1991), with Laurentia more-closely fit to East Gondwanaland provided the allochthonous provinces from the Transantarctic Mountains and southeastern Australia are removed (Borg and DePaolo, 1994). Using paleomagnetic data. Park et al (1995) proposed that mafic dykes and sheets in three areas of western North America are part of a giant radiating dyke swarm which included the Gairdner dyke swarm in Australia. Each of the tests cited here lends support to the hypothesis of a Rodinian supercontinent, although a compelling argument for the fit of its constituent continents has yet to be developed. POTENTIAL FIELD DATA The data which have great promise to constrain a model for the Rodinia fit are magnetic and gravity data. These data can reveal prominent geologic features in the Precambrian basement even if these rocks are covered with a thick accumulation of sediments. Prey et al (1983) exploited this ability in an attempt to reassemble the continents of Pangea. They found that POGO and MAGS AT data could be used to correlate long-wavelength magnetic anomalies across now-rifted continental margins. These satellite data are of broad areal extent, but they are of insufficient resolution to test Rodinia fits. However, high-resolution aeromagnetic and gravity data now cover many of the former Rodinian constituents. These data have not been readily available to the scientific community at large, and they have not been used to test any of the models mentioned above. The Precambrian basement of the continents in question has been relatively wellstudied geologically (e.g., Hoffman, 1988; Myers, 1993), and to a lesser extent geophysically (e.g., Wellman, 1988; Thomas, 1992). Most geophysical programs have concentrated on basic mapping (to which we owe the wealth of available data) or regional studies. From these studies we know that suture zones between two adjacent structural provinces often exhibit a distinctive, paired positive/negative gravity anomaly due to density and crustal thickness constrasts across the boundary (e.g., Thomas, 1992). Magnetic signatures at structural boundaries may also be distinctive. For example, the Thelon magmatic arc, which may be attributed to subduction prior to collision of the Slave and Rae Provinces (Canada), exhibits a significant positive magnetic anomaly. This magnetic high is flanked to the west by a belt of negative anomalies straddling the Thelon Front. In some cases, such as along part of the Grenville Front, the polarity of the paired magnetic signature follows the sense of the paired gravity signature and the respective anomalies may be approximately coincident. Because these structural boundaries exhibit significant contrasts in their geophysical signature, enhanced derivative maps may also exhibit a characteristic signature which may help to better delineate these features. For example, a paired gravity signature will be divided by an extreme change in the gravity signal which should be clearly identifiable on the horizontal gravity gradient map. In many cases, these enhanced products provide an effective tool highlighting geophysical anomalies that may not be as obvious in the raw data.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

In this project, we intend to examine all characteristic geophysical signatures (keeping in mind the geology and age of their source rocks), especially where they intersect the estimated location of a paleo-continental margin. The anomalies to be given attention initially will be those catalogued during the preliminary literature review. For example, we will interpret the aeromagnetic data from Siberia in order to determine the geophysical expression of the Akitkan fold belt and Anabar Shield (and others) for comparison with the (present) northern margin of Laurentia (Taltson-Thelon zone; e.g.. Frost et al, 1998). Dyke swarms also may provide a valuable geometric constraint, and therefore we shall examine the expression and geometry of all resolvable dykes swarms of appropriate age such as those in Laurentia which may relate to the Gairdner swarm in Australia (Park et al, 1995). We shall examine the Grenvillean belts which may have been continuous originally in Rodinia (Hoffman, 1991), although segments such as the Albany-Fraser belt appear to be abruptly terminated. The Wopmay Orogen of western Canada, which includes the Fort Simpson and Great Bear magmatic arcs, may have possible connections with the Capricorn Orogen of Western Australia or perhaps it once continued into southeastern Australia (Moores, 1991). Other prominent geophysical features in western Canada are the Snowbird tectonic zone between the Rae and Heame Provinces and the Great Slave Lake shear zone, between the Bear, Slave and Rae Provinces. Both of these features may be traced geophysically beneath the Phanerozoic cover to intersect the Laurentian paleo-continental boundary (e.g., Ross et al., 1995), and thus they may serve as important piercing points should conjugate features on another continent be positively correlated. The abundant, identifiable geophysical anomalies on the (present) northern and western margins of Laurentia provide us with an excellent data set to investigate where this piece of the Rodinia puzzle fits relative to other cratonic blocks. Through collaborative ties, aeromagnetic and gravity data have become available for this proposed research project from the Geological Survey of Canada and discussions relating to collaboration and access to the Australian data are in progress with the Australian Geological Survey Organisation. Gravity data cover much of Australia, Canada, and the United States, and aeromagnetic data cover Australia, Canada, Eurasia and the United States. Via qualitative and quantitative geophysical interpretation and plate tectonic reconstructions (e.g., Verhoef et al, 1990; Bird et al, 1996), these data will be used to test the various Rodinia fits for Australia, Laurentia, Siberia and South China. A new or revised model is expected to be developed based on these data. As the project advances, more data may become available and will be added to the study at that time. REFERENCES Bell, R., and Jefferson, C.W., 1987, An hypothesis for an Australian-Canadian connection in the Late Proterozoic and the birth of the Pacific Ocean, PacRim Congress 1987, Parkville, Australia, p. 3950. Bird, R.T., Roest, W.R., Pilkington, M., Ernst, R.E., and Buchan, K.L., 1996, The application of digital geophysical data to the restoration of crustal deformation in the Canadian Shield, Ontario, in Current Research 1996-C, Geological Survey of Canada, p. 117-124. Borg, S.G., and DePaolo, D.J., 1994, Laurentia, Australia, and Antarctica as a Late Proterozoic supercontinent: Constraints from isotopic mapping. Geology, v. 22, p. 307-310. Brookfield, M.E., 1993, Neoproterozoic Laurentia-Australia fit. Geology, v. 21, p. 683-686. Condie, K.C., and Rosen, O.M., 1994, Laurentia-Siberia connection revisited. Geology, v. 22, p. 168170. Dalziel, LW.D., 1991, Pacific margins of Laurentia and East Antarctica-Australia as a conjugate rift pair: Evidence and implications for an Eocambrian supercontinent, Geology, v. 19, p. 598-601. Dalziel, LW.D., 1995, Earth before Pangea, Scientific American, January, p. 58-63.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998 Frey, H., Langel, R., Mead, G., and Brown, K., 1983, POGO and Pangaea, Tectonophysics, v. 95, p. 181-189. Frost, R. B., Avchenko, O. V., Chamberlain, K. R., and Frost, C. D., Evidence for extensive Proterozoic remobilization of the Aldan shield and implications for Proterozoic plate tectonic reconstructions of Siberia and Laurentia, Precambrian Research, v. 89, p. 1-23. Hoffman, P.F., 1988, United plates of America, the birth of a craton: Early Proterozoic assembly and growth of Laurentia, Annual Review of Earth and Planetary Sciences, v. 16, p. 543-603. Hoffman, P.F., 1991, Did the breakout of Laurentia turn Gondwanaland inside out?. Science, v. 252, p. 1409-1412. Jefferson, C.W., 1978, Correlation of middle and upper Proterozoic strata between northwestern Canada and south and central Australia, Geological Association of Canada, Program with Abstracts, v. 13, p. 429. Larson, R.L., Pitman III, W.C., Golovchenko, X., Cande, S.C., Dewey, J.F., Haxby, W.F., and Labrecque, J.L., 1985, The Bedrock Geology of the World, Freeman, New York. Li, Z.X., Zhang, L., and Powell, C.McA., 1995, South China in Rodinia: Part of the missing Hnk between Australia-East Antarctica and Laurentia?, Geology, v. 23, p. 407-410. McMenamin, M.A.S., and McMenamin, D.L.S., 1990, The Emergence of the Animals. The Cambrian Breakthrough, Columbia University Press, New York, 217 pp. Moores, E.M., 1991, Southwest U.S.-East Antarctic (SWEAT) connection: A hypothesis. Geology, v. 19, p. 425-428. Myers, J.S., 1993, Precambrian history of the West Australian Craton and adjacent orogens. Annual Review of Earth and Planetary Sciences, v. 21, p. 453-485. Park, J.K., Buchan, K.L., and Harlan, S.S., 1995, A proposed giant radiating dyke swarm fragmented by the separation of Laurentia and Australia based on paleomagnetism of ca. 780 Ma mafic intrusions in western North America, Earth and Planetary Science Letters, v. 132, p. 129-139. Powell, C.McA., Roots, S.R., and Veevers, J.J., 1988, Pre-breakup continental extension in East Gondwanaland and the early opening of the Indian Ocean, Tectonophysics, v. 155, p. 261-283. Powell, C.McA., Li, Z.X., McElhinny, M.W., Meert, J.G., and Park, J.K., 1993, Paleomagnetic constraints on the Neo-proterozoic breakup of Rodinia and the mid-Cambrian formation of Gondwanaland, Geology, v. 21, p. 889-892. Ross, G.M., Milkereit, B., Eaton, D., White, D., Kanasewich, E.R., and Burianyk, M.J.A., 1995, Paleoproterozoic collisional orogen beneath the western Canada sedimentary basin imaged by Lithoprobe crustal seismic-reflection data. Geology, v. 23, p. 195-199. Scotese, C.R., Gahagan, L.M., and Larson, R.L., 1988, Plate tectonic reconstructions of the Cretaceous and Cenozoic ocean basins, Tectonophysics, v. 155, p. 27-48. Stump, E., 1992, The Ross orogen of the Transantarctic Mountains in ligh of the Laurentia-Gondwana split, GSA Today, v. 2, p. 25-27, 30-31. Thomas, M.D., 1992, Ancient collisional continental margins in the Canadian Shield: Geophysical signatures and derived crustal transects, in Basement Tectonics 8: Characterization and Comparison of Ancient and Mesozoic Continental Margins-Proceedings of the 8th International Conference on Basement Tectonics (Butte, Montana, 1988), Bartholomew, M.J., Hyndman, D.W., Mogk, D.W., and Mason, R., eds., Kluwer Academic PubHshers, Dordrecht, p. 5-25. Verhoef, J., Usow, K.H., and Roest, W.R., 1990, A new method for plate reconstructions: The use of gridded data. Computers and Geosciences, v. 16, p. 51-74. Wellman, P., 1988, Development of the Australian Proterozoic crust as inferred from gravity and magnetic anomalies, Precambrian Research, v. 40/41, p. 89-100. Young, G.M., 1992, Late Proterozoic stratigraphy and the Canada-Australia connection. Geology, v. 20, p. 215-218.

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THE CONCEPT OF TRIPLE JUNCTION STABILITY IN QUESTION: EPISODIC TRIPLE JUNCTION MIGRATION BY RIFT PROPAGATION AND MICROPLATES Robert T. Bird Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Australia, Nedlands WA 6907 Sarah F. Tebbens Department of Marine Science University of South Florida, St. Petersburg FL 33701, U.S.A. Martin C. Kleinrock Department of Geology Vanderbilt University, Nashville TN 37235, U.S.A. David F. Naar Department of Marine Science, University of South Florida, St. Petersburg FL 33701, U.S.A. We describe a model for episodic, open-ocean triple junction migration based on observations of actual triple junction evolutions. The migration model consists of repeated episodes of rift propagation, microplate formation, and microplate accretion to an adjacent, larger plate. These episodes may be highly variable in space and time depending on triple junction geometry, velocity triangle and other factors affecting local thermal and rheological conditions. Resulting tectonic features may include an abandoned transform fault, straight and potentially curving pseudofaults, sheared and potentially rotated abyssal hill fabric, and a paleomicroplate with no associated failed rift. The potential structures are dependent on the duration of microplate activity and detailed plate motion history. This model, developed mainly from the evolution of the Pacific-Antarctic-Nazca triple junction, may be relevant for other types of triple junctions such as the Bouvet and Azores triple junctions and ridge-ridge-ridge triple junctions in the Indian and Pacific oceans. Episodic migration is found to occur even when the triple junction is apparently kinematically stable. This model highlights the difference between predicted kinematic stability of triple junctions and observations of their true tectonic history.

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1998

WAVELETS TO BROWSE Robert T. Bird Tectonics Special Research Centre, Department of Geology and Geophysics University of Western Australia, Nedlands WA 6907 Thomas A. Ridsdill-Smith Centre for Strategic Mineral Deposits, Department of Geology and Geophysics, and Department of Mathematics, University of Western Australia, Nedlands WA 6907 R. Dietmar Miiller Department of Geology and Geophysics The University of Sydney, Sydney NSW 2006 Mark Pilkington Geological Survey of Canada Ottawa ON K1A 0E9, Canada In this paper, we demonstrate the utility of the discrete wavelet transform in the quantitative interpretation of aeromagnetic data. In particular, we analyze synthetic data and real high-resolution data acquired over the Browse Basin on the North West Shelf of Australia. Potential field data such as these are essential to mineral resource exploration and their application to petroleum exploration is continually increasing. This rise in use coincides with significant technological advances in the acquisition, processing, and interpretation of magnetic data. For example, the production and display of horizontal and vertical derivatives of the magnetic field are common and quantitative interpretation methods, such as Euler deconvolution (e.g., Thompson, 1982) and analytic signal calculation (e.g., Nabighian, 1972), incorporate these derivatives. Typically, derivative calculations are conducted in the Fourier domain although the resulting data may suffer from the corruption of high-frequency noise. Calculation of derivatives higher than first order will accentuate this noise. Therefore, subsequent use of these derivatives may be compromised. As an alternative, we use the discrete wavelet transform (DWT) method of Ridsdill-Smith and Dentith (1998) which provides an efficient means for the calculation of horizontal and vertical derivatives by the diagonalization of these operators in the wavelet domain. The algorithm is stabilized in the presence of noise with wavelet de-noising techniques. The results may be incorporated into subsequent applications. In particular, we are interested in quantitative methods that retrieve magnetic source positions and depths (e.g., Euler deconvolution) thus providing additional constraints on geologic interpretations. This approach is demonstrated for profile data and future work will advance the method to a threedimensional analysis using gridded data sets. REFERENCES Nabighian, M., 1972, The analytic signal of two-dimensional magnetic bodies with polygonal crosssection: Its properties and use for automated anomaly interpretation. Geophysics, v. 37, p. 507517. Ridsdill-Smith, T.A., and Dentith, M.C., 1998, The wavelet transform in aeromagnetic processing, submitted to Geophysics. Thompson, D., 1982, EULDPH: A new technique for making computer-assisted depth estimates from magnetic data. Geophysics, v. 47, p. 31-37.

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TEMPERATURE-TIME (T-t) PATH FOR THE TICKALARA METAMORPHICS OF THE HALLS CREEK OROGEN, WA: IMPLICATIONS FOR TECTONIC MODELS Simon Bodorkos Tectonics Special Research Centre, School of Applied Geology Curtin University of Technology, GPO Box U1987, Perth WA 6845

Peter A. Cawood Tectonics Special Research Centre, School of Applied Geology Curtin University of Technology, GPO Box U1987, Perth WA 6845

Nicholas H. S. Oliver Economic Geology Research Unit, School of Earth Sciences James Cook University, Townsville QLD 4811

Detailed field, petrological and geochronological data from the Tickalara Metamorphics, in the Central zone of the Halls Creek Orogen provides a framework within which a temperature-time (T-t) path can be constructed. The resulting thermal history has implications for both orogenesis in the region and our understanding of Proterozoic tectonics. The Tickalara Metamorphics are the oldest rocks exposed in the Central zone and the unit is dominated by clastic metasedimentary rocks, with minor carbonates and mafic volcanics. Metamorphic grade varies from upper-greenschist facies in the south to granulite facies in the north, where the study area is located. Detrital zircon populations are dominated by grains with U-Pb SHRIMP ages in the range -1880-1860 Ma, providing a maximum depositional age for the protoTickalara sedimentary rocks. Youngest detrital suites have been dated at 1865 ± 2 Ma (Page et aU 1995a), 1864 ± 4 and 1867 ± 4 Ma (Bodorkos et al, 1998) from low- and high-grade metasedimentary rocks throughout the unit. The northern Tickalara Metamorphics were then intruded by a number of volumetrically minor granitoid sills at -1850 Ma, such as the Fletcher Creek Granite, a gamet-bearing Stype pluton with a U-Pb SHRIMP igneous crystallisation age of 1850 ± 2 Ma (Page et aly 1995b). These bodies place important constraints on the thermal history of the unit. Despite the close correspondence of this granite to the ternary minimum melt composition (estimated solidus temperature -700* C on the basis of modal quartz and feldspars), it has a prominent pyroxene-homfels facies contact metamorphic aureole -100 metres wide. This is interpreted as evidence that the metasedimentary host rocks were still cold (<300-400* C) at the time of intrusion. The presence of a well-developed solid-state foliation and pervasive recrystallisation within the Fletcher Creek Granite is also consistent with emplacement prior to the onset of HTLP metamorphism and associated deformation. Peak regional metamorphism in the study area reached temperatures of -700750® C at pressures of 300-400 MPa, resulting in widespread development of gamet-biotite and gamet-cordierite±spinel assemblages in metapelites. Leucosomes in stromatic migmatites define the regional foliation. Timing of peak metamorphism is constrained by a U-Pb SHRIMP age of 1845 ± 4 Ma from overgrowths on zircon from an upper-amphibolite facies metapelite (Bodorkos et 8

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

al, 1998) and an identical conventional U-Pb monazite age of 1845 ± 3 Ma from a nearby granulite facies stromatic migmatite (Oliver et al, in press). A minimum metamorphic age is provided by the Sally Malay layered mafic-ultramafic intrusion (U-Pb SHRIMP age 1841 ±3 Ma, Trudu and Hoatson, 1996), which intruded lowergranulite facies stromatic migmatites, cross-cutting the regional gneissosity. The pluton (with an estimated solidus temperature of ~ 1100-1200* C) has superimposed a diatexitic contact aureole on local metapelitic gneisses. Repeated mafic magma injection has resulted in multiple generations of contact/sheath migmatites (Oliver and Barr, 1997), from which U-Pb SHRIMP ages for metamorphic zircon are as young as 1835 ±3 Ma (Oliver et al, in press). East of the Sally Malay intrusion, emplacement of the Mabel Downs Tonalite (U-Pb SHRIMP age 1832 ± 3 Ma, Page et aU 1995b) was synchronous with the onset of D3 deformation, which folded the pre-existing regional gneissosity in the metapelitic wallrocks. Extreme compositional heterogeneity in the pluton and mutual cross-cutting relationships between folding, intrusive phases and ductile shear zones suggest that the Mabel Downs Tonalite was emplaced under amphibolite-facies conditions. Nearby younger plutons in the same granitoid suite (e.g., Sally Downs Tonalite, U-Pb SHRIMP age 1821 ±4 Ma, Sheppard et aU 1995) have undergone significantly less D3-related ductile deformation. However, all structures are folded by macroscopic F4 folds and cut by retrograde shear zones parallel to the F4 fold axes. This latest deformation is bracketed in the range 18201810 Ma, with the lower age limit provided by a -1810 Ma granitoid that "stitches" an S4 shear zone (Sheppard et a/., 1995). This well-constrained geochronological framework suggests that the Tickalara Metamorphics in the northern Central zone represent a section of the middle crust that underwent a protracted thermal event, with temperatures remaining above -500* C in the interval ~ 1850-1820 Ma. The voluminous intrusion of intermediate and mafic plutons during this period suggests that advective heat transfer played a major role in maintaining the elevated crustal geotherm. In addition, the constructed T-t path infers that the sequence of clearly overprinting deformation events evident in the field are superimposed on a single prolonged episode of crustal heating, in contrast with the interpretation of Tyler and Page (1996) involving the existence of two separate orogenic events: the Hooper Orogeny at -1850 Ma and the Halls Creek Orogeny at -1830 Ma. Although aspects of crustal evolution in the Halls Creek Orogen remain unclear, the substantial presence of (i) hightemperature, low-pressure metamorphic rocks; (ii) voluminous 1860-1830 Ma layered mafic bodies, and (iii) 1830-1820 Ma tonalitic and granodioritic plutons with mantle-derived geochemical signatures suggest extensive asthenospheric upwelling within this overall collisional zone. REFERENCES Bodorkos, S., Cawood, P.A, and Oliver, N.H.S., 1998, Thermal evolution of the central Halls Creek Orogen, W.A: field, petrological and geochronological constraints, Geological Society of Austraha, Abstracts No. 49, p. 46. Oliver, N.H.S., Bodorkos, S., Nemchin, A.A., Kinny, P.D., and Watt, G.R., in press, Relationships between zircon U-Pb SHRIMP isotope patterns and migmatite type in the Halls Creek Orogen, Western Australia, Journal of Petrology. Page, R.W., Hoatson, D.M, Sun, S., and Foudoulis, C., 1995a, High-precision geochronology of Palaeoproterozoic layered mafic-ultramafic intrusions in the East Kimberley, AGSO Research Newsletter 22, p. 7-8. Page, R.W., Tyler, I.M., and Blake, D.H., 1995b, Geochronology of magmatism and high-grade metamorphism, Kimberley region, W.A., Australian Conference on Geochronology, Abstracts 3,

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998 p. 25. Sheppard, S., Griffin, T.J., and Tyler, I.M., 1995, Geochemistry of felsic igneous rocks from the southern Halls Creek Orogen, West Australian Geological Survey Record 1995/4, 81 pp. Trudu, A., and Hoatson, D., 1996, Depths of emplacement of Precambrian layered intrusions in the East Kimberley, AGSO Research Newsletter 25, p. 10-12. Tyler, I.M., and Page, R.W., 1996, Palaeoproterozoic deformation, metamorphism and igneous intrusion in the Central zone of the Lamboo Complex, Halls Creek Orogen, Western Australia, Geological Society of Australia, Abstracts No. 41, p. 450.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

INITIATION OF SUBDUCTION IN THE PROTO-PACIFIC OCEAN: CONSTRAINTS FROM EASTERN GONDWANA Peter A. Cawood

Tectonics Special Research Centre, School of Applied Geology Curtin University of Technology, GPO Box U1987, Perth WA 6001

Evan C. Leitch

Department of Applied Geology University of Technology, Sydney, PO Box 123, Broadway NSW 2007 The proto-Pacific ocean formed through Neoproterozoic rifting of Rodinia and was bounded by the conjugate Laurentian and Gondwanan margins. Despite a long history of plate convergence this ocean has never subsequently closed, and is manifest today in the Pacific ocean. The record of inception of convergent plate interaction in the proto-Pacific is preserved in a Neoproterozoic to late Paleozoic orogenic belt here termed the Terra Australis orogen. The orogen forms a fundamental crustal element along the Pacific margin of Gondwana and is preserved in the now dispersed Tasman orogen of east Australia, Ross orogen of the Transantarctic Mountains, and Tuhua orogen of New Zealand. Older rocks of the Terra Australis orogen are divisible, from west to east, into three main stratotectonic assemblages, a continental margin assemblage, an ocean margin assemblage and an intra-oceanic assemblage. The continental margin assemblage developed on continental crust stabilized within Rodinia, whereas the latter two developed in an oceanic realm formed following breakup of Rodinia and are differentiated on their relative isolation from the continental margin. The ocean-margin assemblage is linked to cratonic Gondwana by widespread siliciclastic detritus, but the intraoceanic elements show no evidence for a continental influence until the late Paleozoic. Ocean margin and intra-oceanic elements were progressively incorporated onto the Gondwana margin during Paleozoic orogenic movements (e.g. Tabberabberan, Tuhua) culminating in the Permian Hunter-Bowen event which marks the termination of the Terra Australis orogen and a stepping out of the plate margin to a Permian to Mesozoic orogenic tract extending through New Guinea, New Zealand, Marie Byrd Land, the Antarctic Peninsula and into South America (Cawood, 1984). Continental margin sequences of the Terra Australis orogen are preserved in the Adelaide fold belt of eastern South Australia and its continuation in western New South Wales and western Tasmania, the Ross orogen of the Transantarctic Mountains, and the Anakie High in central Queensland. They consist of a Mesoproterozoic or older crystalline basement overlain by a Neoproterozoic to early Paleozoic mixed siliciclastic and carbonate cover locally intercalated with mafic and felsic volcanic rocks. Development of the margin involved a cycle of Neoproterozic rifting (-830 - 750 Ma), leading to continental breakup, generation of the protoPacific and establishment of a passive margin sequence along the East Gondwana margin. Stratigraphic, structural and geochronological data suggest the continental margin sequences were deformed during a protracted phase of end Neoproterozoic to early Paleozoic tectonism, termed the Ross/Delamerian orogeny. This event resulted in termination of sedimentation within the continental margin sequences. In the central Transantarctic Mountains and in western Tasmania Neoproterozoic 11

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Early Cambrian deformation, metamorphism and magmatism have been recognised within rocks included in the continental margin sequence. In South Australia these disturbances may be reflected in unconformities within the Adelaide Fold Belt rocks and inboard, within the Gondwana craton, by overthrusting of the Musgrave Block. In the Anakie Inlier, Fergusson et al (1998) on the basis of U/Pb monazite ages have suggested that initial high grade metamorphism and associated deformation occurred at 583 ±10 Ma and was followed by regional deformation and greenschist facies metamorphism at around -540 Ma. The earlier event they ascribed to convergence and subduction of the proto-Pacific. Ocean margin sequences occur within the Lachlan fold belt of southeastern Australia, within the Northern Victoria Land (Robertson Bay terrane) and Marie Byrd Land segments of the Ross orogen, and in the Buller terrane of the Tuhua orogen in New Zealand. The oldest rocks of the ocean margin sequences are basefaulted belts of mostly altered mafic and ultramafic rocks of early Cambrian age found in central Victoria and western Tasmania. They are structurally disrupted and their age is not well constrained but in Victoria they are locally conformably overlain by Middle Cambrian shale and tuff, and in Tasmania ultramafic detritus occurs in late Middle to early Late Cambrian sedimentary rocks. The presence of boninites and the overall geochemical composition of the mafic rocks indicate generation in a supra-subduction zone environment. Voluminous Ordovician quartzrich turbidites and black shale characterise the ocean margin sequences. Igneous enclaves within I-type granites intrusive into the ocean-margin sequences record ages as old as approximately 570 Ma (Ian Williams, pers. comm.) Several problematic terranes lie close to the boundary between the ocean margin and the continental margin assemblages. They are characterised by the presence of Middle Cambrian - Early Ordovician volcanic rocks of convergent margin character associated with shallow marine or terrestrial siliciclastic strata. Despite the latter, which suggest deposition on continental crust, some of the volcanic rocks show an oceanic signature. Apart from thrusting of possible Cambrian age in the Bowers terrane, these assemblages are further demarked by the general absence of evidence for Ross/Delamerian deformation. Included in this group are the Mount Windsor province of northeast Queensland, the Mt. Wright volcanics of western New South Wales, the Mount Stavely belt of western Victoria, the Bowers terrane, and the Takaka terrane of the Tuhua orogen. Crawford et al (1997) have determined a U/Pb zircon age for calc-alkaline extension related igneous activity within the Mt Wright region at 586 ±7 Ma. Intraoceanic sequences are restricted to the New England fold belt of eastern Australia where a series of fault-bounded convergent plate margin elements are exposed. Early Paleozoic elements include an inferred magmatic arc and associated an arc-flanking sedimentary basin which contains volcaniclastic detritus as old as Middle Cambrian, subducted oceanic crust represented by middle Ordovician high P/T metamorphic phacoids embedded in serpentinite melange, and Early Cambrian (c. 530 Ma) ophiolitic rocks of supra-subduction zone character. Although contacts between exposed elements are faulted, their character and distribution suggests development in an east-facing intraoceanic arc. Recently, Watanabe et al (1998) have reported a U/Pb zircon age of 571 ±22 Ma for an isolated block of eclogite along the Peel Fault. This fault separates the arc-flanking sedimentary basin and subduction complex assemblages preserved within the intraoceanic sequence. Relations within and between the continental margin, ocean margin and intraoceanic assemblages indicate that the end Neoproterozoic marks a fundamental 12

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change in tectonic style along the Terra Australis orogen. We believe this corresponds with the propagation of a plate boundary through the orogen and the initiation of subduction of the proto-Pacific. Available but limited data suggest that initial disruption of the continental margin sequences and subduction initiation occurred toward the end Neoproterozoic at around 590-570 Ma. This is based on the timing of metamorphism and deformation within the Anakie Inlier, the timing of igneous activity preserved at Mt. Wright, the age of enclaves associated with granites intrusive into the ocean margin sequences, and on the age of eclogite in the intra-oceanic sequences. Between 570 and 530 Ma the orogen contains evidence for deformation, at least locally in a transpressional regime (Goodge et al, 1993), and for tensional or transtensional deformation associated with carbonatite emplacement (Hall et aL, 1995). The main phase of subduction along the margin commences at around 530 Ma. In the Transantarctic segment of the margin, 530 Ma corresponds with the commencement of emplacement of granitoids of inferred magmatic arc character. In the ocean margin and intra-oceanic segments this time period corresponds with the age of generation of supra-subduction zone ophiolites. Although the relative paleogeographic position of individual ophiolitic bodies in the ocean margin and intra-oceanic assemblages is unconstrained, their overall position outboard of the Gondwana margin, their comparable age, and their supra-subduction zone character suggest their generation during the same tectonic episode and along the same plate margin. The presence of magmatic arc rocks within the continental margin assemblage as well as the presence of siliciclastic strata in stratigraphic continuity with the oceanic substrate in the ocean margin sequences indicates that at least this main phase of convergent plate margin likely formed close to the Gondwana continent where relatively old, dense oceanic lithosphere was susceptible to subduction. Disruption and deformation of the continental margin successions and the initiation of subduction and convergent plate margin activity within the Terra Australis orogen between 590 and 530 Ma corresponds with a period of global plate reorganization. This involved opening of the lapetus Ocean, final assembly of Gondwana, and rifting of Siberia off northem Laurentia. The temporal equivalence of this series of end Neoproterozoic Wilson cycle stages suggests they may be interdependent and we consider that their far-field effects could have influenced subduction initiation in the proto-Pacific. The margins of the Mozambique Ocean were aligned approximately orthogonal to the trend of the East Gondwana margin preserved in the Terra Australis orogen. With collision of East and West Gondwana along the Mozambique Belt, continued plate motion may have been transferred to the East Gondwana - proto-Pacific ocean margin resulting in strike slip deformation and margin decoupling. The initiation of subduction along the Pacific margin of Gondwana by -590-570 Ma argues against models which suggest breakup of Rodinia and generation of conjugate Laurentian and Gondwana margins did not occur until the end Neoproterozoic at around 560 Ma (e.g. Veevers et al, 1997). The initiation of subduction in the Early Cambrian marks the inception of the Pacific 'ring of fire', yet throughout the Phanerozoic the Pacific has remained a major ocean basin. This indicates that the longevity of the Pacific and its antecedents is a result of continued production of oceanic lithosphere throughout the Phanerozoic, rather than a delayed onset of subduction. Although the Pacific has been cited as an example as the declining stage of the Wilson cycle of ocean basins, its protracted history of ongoing subduction, and by inference oceanic crust 13

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generation, contrasts with the clear evidence for opening and closing of oceans preserved in the lapetus/Atlantic and Tethyan realms.

REFERENCES

Crawford, A.J., B.P.J. Stevens, and M. Fanning, 1997, Geochemistry and tectonic setting of some Neoproterozoic and Early Cambrian volcanics in western New South Wales, Australian Journal of Earth Sciences, v. 44, p. 831-852. Fergusson, C.L., Green, T.J., Fanning, C.M., Carr, P.F., Withmall, I.W., and Crouch, S.B.S., 1998, Neoproterozoic age for deformation and metamorphism in the Anakie Inlier, Central Queensland: Implications for opening of the Pacific Ocean and timing of the Delamerian-Ross Orogeny, Geological Society of AustraHa, Abstracts No. 49, p. 141. Goodge, J.W., Walker, N.W., and Hansen, V.L., 1993, Neoproterozoic-Cambrian basement-involved orogenesis within the Antarctic margin of Gondwana, Geology, v. 21, p. 37-40. Hall, C.E., Cooper, A.F., and Parkinson, D.L., 1995, Early Cambrian carbonatite in Antarctica, Journal of the Geological Society, London, v. 152, p. 721-728. Veevers, J.J., Walter, M.R., and Scheibner, E., 1997, Neoproterozoic tectonics of Australia-Antarctica and Laurentia and the 560 Ma birth of the Pacific Ocean reflect 400 m.y. Pangean supercycle. Journal of Geology, v. 105, p. 225-242. Watanabe, T., Fanning, C.M. and Leitch, E.C., 1998, Neoproterozoic Attunga eclogite in the New England Fold Belt, Geological Society of Australia, Abstracts No. 49, p. 458.

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LATE PROTEROZOIC — EARLY PALAEOZOIC EVOLUTION OF THE EASTERN ADELAIDE FOLDBELT AND WESTERN LACHLAN FOLDBELT — THE REAL BREAKUP, AND SUBSEQUENT CONTINENTAL CRUST-FORMING EVENTS Anthony J. Crawford Centre for Ore Deposit Research, School of Earth Sciences University of Tasmania, GPO Box 252-79, Hobart TAS 7001 Tony.Crawford@utas.edu.au Nicholas G. Direen Centre for Ore Deposit Research, School of Earth Sciences University of Tasmania, GPO Box 252-79, Hobart TAS 7001 On-going geological and geochemical evaluation of meta-igneous rock associations along the transition zone between the Adelaide and Lachlan Foldbelts have provided new constraints on the tectonic setting of eruption of these rocks, on their significance in foldbelt evolution, and on the timing of breakup and subsequent deformation. Key elements are the following: 1. Break-up Associations: Shortly after 600 Ma, rift tholeiites were erupted along western NSW-Vic-Tas, and record the first stage of a Wilson cycle that eventually led to the formation of the Lachlan Foldbelt. These rocks vary from transitional alkaline packages dated at 586 ± 7 Ma in the Mt Arrowsmith region of western NSW, through undated picritic rocks drilled near the Vic-SA border in far western Victoria (our unpublished data), to tholeiite - picrite sequences on King Island, and rift tholeiite piles and transitional alkaline dolerite dyke swarms (the latter K-Ar dated at -590-600 Ma) in numerous areas in western Tasmania. The geochemical associations represented, and the important presence of voluminous high-temperature picritic lavas in shelf facies sequences at several of these localities, invites comparison with the plume-triggered Mesozoic rifting of the northern and southern Atlantic regions. There can be little doubt that this rifting commenced around 600Ma, and was probably generating oceanic crust by 580Ma. 2. The Collider Associations: Subduction of crust in this latest Proterozoic ocean may have begun shortly before 525 Ma (as recorded by SHRIMP ages for ophiolitic low-Ti lava - plagiogranite associations in northern NSW basement of the New England Foldbelt), but was unambiguously recorded by boninite - low Ti tholeiite associations in allochthons in Victoria, Tasmania, and NW Nelson in New Zealand, dated at -515-510 Ma. This association characterises several unusual forearc sequences in modem western Pacific arcs (e.g., northern Tongan forearc, BoninMariana forearc) and may record either subduction initiation, or subduction of an active (or still hot) spreading centre. Whatever the case, these allochthonous rocks formed in forearc positions above an east-dipping subduction zone, and were emplaced as major thrust sheets onto attenuated passive margin crust probably around 510 Ma, when the 600 Ma passive margin rode into the west-facing subduction zone. A key facet of this collision is that it was relatively passive, and did not produce a major mountain range, since sedimentation continued with little interruption along this margin, and post-collisional volcanic suites (see below) are submarine. 3. The Post-Collisional Suites: Suites of orogenic-type volcanics with mediumto high-K and shoshonitic signatures occur in western Victoria (Stavely Volcanic 15

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Complex and correlates), Tasmania (Mt. Read Volcanics and correlates) and New Zealand (Devil River Volcanics). Their eruption is well constrained at 503-495 Ma. A post-collisional setting for their eruption is strongly implied by the occurrence of distinctive refractory boninitic chromites in interbedded sedimentary rocks, indicating that the boninitic ophiolites had been emplaced (the collision) and were being erupted at this time. Soon after emplacement, extensional collapse of the new crustal collage is recorded by the Henty (Tasmania) and Gendarme (NZ) dyke swarms, that are Late Cambrian in age, and by the exhumation of overthrust passive margin rocks to form the exposed Precambrian Tyennan block in Tasmania. Although several key problems remain, not the least of which is the significance of the Kanmantoo Group and its -510 Ma tholeiitic dykes, in South Australia and western Victoria, the general model proposed above for the Late Neoproterozoic and Early Palaeozoic evolution of the Lachlan Foldbelt appears to be valid. There can be no doubt that rifting at the start of this Wilson cycle commenced around 600 Ma, and we note the similar aged flood basalts (Antrim Plateau Basalts) in northern Australia strengthen this proposal. It is highly unlikely, therefore, that this rifting event could be related to the Rodinia breakup event dated around 700-800 Ma. If the Rodinia event 700-800 Ma did spall off a major block of continental crust now located in China or western North America, that portion (the western part) of the rifted margin that stayed in Australia must have been removed and reworked by the 600Ma event described above. Tectonism associated with formation of the Lachlan Foldbelt's Mid- to Late Devonian Tabberabberan deformation event involved eastdirected listric faulting, possibly implying collision with a major crustal block east of the foldbelt. This mysterious block, which may have underthrust part of the Lachlan to provide the source for the widespread S-type granites, should carry a record of the Rodinian rift event in the form of 700-800 Ma rift tholeiites and associated igneous rocks, similar to those of the 600 Ma rift event now preserved in western NSW-Victoria-Tasmania.

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"^Âr/^Âr GEOCHRONOLOGY AND THE NEOPROTEROZOIC TECTONICS ALONG THE NORTHERN MARGIN OF THE EASTERN GHATS PROVINCE IN NORTH ORISSA, INDIA Warwick A. Crowe Tectonics Special Research Centre, Department of Geology and Geophysics University of Western Australia, Nedlands WA 6907 Michael A. Cosca Institut de Mineralogie Universite de Lausanne, BFSH-2, CH-1015 Lausanne, Switzerland Lyal B. Harris Tectonics Special Research Centre, Department of Geology and Geophysics University of Western Australia, Nedlands WA 6907 Only recently have Neoproterozoic and early Paleozoic thermal events been shown to have widely affected the high grade Eastern Ghats belt on the eastern Indian margin (Takigami et al, 1995, Kovach et aU 1997, Shaw et a/., 1997). Despite an increasing volume of geochronological information for the Eastern Ghats Province, the tectonic significance of this data is poorly understood. It is apparent that there has been a widespread ngh-grade Grenvillian metamorphic event which was followed by younger Pan African events. Here we present ^Âr/^Âr data for schist and gneiss samples from the northern margin of the Eastern Ghats belt to elucidate the post-Grenvillian cooling history. The Eastern Ghats Province (EGP) is a granulite facies metamorphic belt which comprise metasediments, quartzo-feldspathic gneiss, chamockite, subordinate mafic granulite, and anorthosite and alkaline intrusive lithologies (Naqvi and Rogers, 1987, Mahalik, 1996). The western and northern contacts of the EGP are sheared margins against the Archaean granite-gneiss association of the Bhandara Province in the west and northwest and the Rengali Province in the north. Widespread amphibolite facies retrogression in the north of the Province is associated with large intra-province shear zones. The Rengali Province (RP) is a fault-bound WNW trending amphibolite facies belt between the granite volcanosedimentary association of the Singhbhum Province and the Eastern Ghats and Bhandara Provinces comprising intercalated metavolcanosediments and orthogneiss, banded gneiss and migmatite. The southern fault contact of the WNW trending belt truncates unconformable Gondwana coal sequences within the EGP. The Ar data show a common cooling history for the EGP and RP and indicate that the EGP and RP were juxtaposed at similar crustal levels by ca. 700 Ma. Two data groups define distinct cooling trends common in both the EGP and RP. An older group (group-A) data show a slow cooling path while a younger group (groupB) define a relatively higher cooling rate. Spatially group-B data is from the margin of the RP and EGP and within the EGP along the NW margin of the Bhandara Province. Group-B data is interpreted to represent a transient thermal event at ca. 500 Ma associated with reactivation of major shear zones within the EGP and RP and possibly associated with extensive graphitisation in the west of the EGP. Dextral activation at amphibolite facies conditions along the WNW bounding fault zones of the RP occurred between ca. 950 and 700 Ma and is correlated with a 17

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major displacement of the Singhbhum Province. A N-S to NW-SE shortening at ca. 500 Ma was associated with reactivation of major shear zones and localised ingress of high temperature fluids up to ~500-600°C in the northern EGP and RP, resetting and or disturbing Ar/Ar systems. Reconstructions of the Rodinian Supercontinent and the subsequent dispersal and amalgamation of Gondwanaland from the late Mesoproterozoic to Cambrian have an East Gondwana nucleous comprising India, Antarctica and Australia as an intact continental block throughout this period (Powell et al, 1993, Powell et al, 1994, Unrug, 1994). Correlation of the new post-Grenvillian tectonic picture of the northern EGP with the defined ca. 500 Ma events in East Antarctica provide a new perspective for intracontinental deformation during the Pan-African. REFERENCES Kovach, V.P., Salnikova, E.B., et al., 1997, Pan-African zircon age from apatite-magnetite veins of Eastern Ghats granulite belt, India, Journal of the Geological Society of India, v. 50, p. 421-424. Mahalik, N.K., 1996, Lithology and tectonothermal history of the Precambrian rocks of Orissa along the eastern coast of India, Journal of Southeast Asian Earth Sciences, v. 14(3/4), p. 209-219. Naqvi, S.M., and Rogers, J.J.W., 1987, Precambrian Geology of India, Oxford University Press, Oxford. Powell, C.McA., Li, Z.X., et al., 1993, Paleomagnetic constraints on timing of the Neoproterozoic breakup ofRodinia and the Cambrian formation of Gondwana, Geology, v. 21, p. 889-892. Powell, C.McA., Preiss, W.V., et al., 1994, South Australian record of a Rodinian epicontinental basin and its mid-Neoproterozoic breakup (-700 Ma) to form the Paleo-Pacific Ocean, Tectonophysics, V. 237, 113-140. Shaw, R.K., Arima, M., et al., 1997, Proterozoic events in the Eastern Ghats granulite belt, India: evidence from Rb-Sr, Sm-Nd systematics, and SHRIMP dating. Journal of Geology, v. 105, p. 645-656. Takigami, Y., Sakai, K., et al., 1995, 40Ar-39Ar ages and paleomagnetic data for metamorphic and igneous rocks from Mahanadi and Godavari grabens. East India, NIPR Symposium on Antarctic Geoscience. Unrug, R, 1994, The assembly of Gondwana supercontinent: contrasting histories of East and West Gondwana, Gondwana Nine, Ninth International Gondwana Symposium, Geological Survey of India, Oxford and IBH Publishing Co. Pvt. Ltd., v. 2, p. 989-998.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

EXTENSIONAL SEDIMENTARY REGIMES IN THE NEOPROTEROZOIC OF THE ADELAIDE GEOSYNCLINE Bob Dalgarno 19 Rubens Grove, Canterbury VIC 3126 SUMMARY Tectonism in the Australian Neoproterozoic basins is defined by the stratigraphic record of unconformities, major phases of deposition, growth faulting, slumping, oUstostrome development, etc., and apparently by phases of syndepositional diapir development and erosion. Early diapir growth occurred prior to the Sturtian glaciation, coincident with a significant extensional phase which has been invoked as the onset of continent separation. Rapid subsidence with associated depositional features continued through the Sturtian, notably in the Adelaide Geosyncline. In the Ediacaran widespread synchronous diapiric uplift and emergence is related to a compressional phase and its regional adjustments while mild extension prevailed in the central Flinders Ranges with similar diapir response during the Lower Cambrian. There is no record of syndepositional diapir activity subsequent to Lower Cambrian in the Flinders Ranges apparently reflecting the onset of compressional tectonism. The Neoproterozoic synrift sediments of the Adelaide and Amadeus basins are responsible for widespread diapirism and salt tectonics which were initiated by extension in the Centralian basins dating from the Sturtian glaciation. Prior to the development of this style of deformation in cover rocks the distribution of rift sediments appears to have been restricted by parallel fault systems. In the Flinders and Willouran Range the facies which included the incompetent units sourcing the diapirs appear to have been restricted by craton to the northeast and east (existing Cumamona block) and to the west by the present topographic trends defined by the Norwest Fault and the western margin of the ranges south to Port Augusta (Preiss et ai, 1993b). Northwest trending controls paralleling the Gairdner Dyke Swarm influenced early diapir growth in the Flinders Ranges and extrapolation of this trend from Port Augusta to the Nackara Arc broadly limits diapirism to the south. The northwest element is indicative of the major extension direction to the northeast in the northern Adelaide rift. The Burra Group in the Willouran Range demonstrates graben development with thickening controlled by these trends. Southeast of Port Augusta the aulocogen broadens and may have been open early to a seaway, hence limiting evaporite development and subsequent diapir development. NNW trending structures remain conspicuous from Port Augusta to the region of Adelaide and also on the western margin of the Cumamona Craton indicating the importance of easterly directed extension. The traditional inter-regional "Grand Unconformity" predates the Sturtian glacials. Interplay of sealevels and glacioeustacy complicate interpretation but important examples, particularly around the Cumamona Craton attest to its significance as an indicator of breakup. Arkaroola, Olary and Broken Hill regions have long been used as examples of overlap of the glacial units on to basement in the east. Within the aulocogen the major faults developed basement topography prior to the Sturtian which initiated accumulation stmctures and diapirism. This

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup of Rodinia, Perth, September 1998

early reactive phase of diapirism is taken to indicate the period of maximum extension (Dyson, 1996). In the Willouran Range the Norwest Fault and the parallel structure a few kilometres to the northeast (Bungarider Fault) were active prior to the deposition of the Sturtian glacials as evidenced by angular unconformities. The Bungarider structure near Chintapanna Dam appears to have a compressional fold truncated by the glacials. Diapirs intruding allochthonous synclines to the north show upturning of the glacials on their flanks and apparent thinning of the Burra Group. The diapiric structures show clear growth by the close of the Sturtian glacial phase. Basin directed slumping recurred through the Sturtian in this region indicating rapid subsidence and sediment supply with diapir growth. Coats (1965) recorded pre-Sturtian diapir growth in the northern Flinders Ranges (Burr Diapir) and Preiss (1985) provided very detailed mapping of the Worumba Anticline near Hawker in the Central Flinders Ranges which delineated major olistostrome development in the Burra Group and clear growth phases of the anticline suggesting features of evaporite accumulation, breccia development and transition to active diapirism during glacial deposition. The significance of extension on this northwest trend in this period is again evident on examination of the detail of the structures aligned with the Worumba Anticline. The Yednalue Anticline to the southeast shows the Sturtian glacials truncating a classic sequence of tilted Torrensian (Burra Group) fault blocks. At the northern limit a contemporaneous half graben hosts the first appearance of the Holowilena Ironstone facies of the glacial sequence. A later passive stage of the Yednalue Diapir resulted in complete decollement of this sequence. The Worumba Anticline mapping by Preiss reveals a similar complex interplay of sedimentation and faulting related to incipient diapirism in what is the most westerly known occurrences of the haematite diamictite facies. These features attest to widespread basement structuring and reflect the continued phase of regional extension. Complexities within this part of the Sturtian glacial sequence have been demonstrated to result from large scale olistostromes related to normal listric faults. These features associated with the Warcowie Dolomite relate to rapidly thickening units of the Wilyerpa Formation and are likely to be widespread through the Flinders and Willouran Ranges (Dyson, pers. com.). The next structure to the northwest of the Worumba Anticline is the Arkaba Diapir in the heart of Wilpena tourist country. This body lies higher in the stratigraphy and intrudes a ramp at this point in the tightly folded northeast trending Chase-Druid Cambrian syncline. It has a satellite diapir core to the northwest immediately juxtaposed to the Wilpena "basin". Further to the northwest again on this same trend is the Moralana Diapir described by Dyson (1996) as having attained a piercement stage with boulders and small rafts of diapiric detritus in the immediate post glacial phase corresponding to the base of the Tapley Hill Formation. Erosion of diapiric core material in this interval is observed at the adjacent Oraparinna Diapir (so too at Worumba). The basal Tapley Hill Formation (Tindelpina Shale) has long been recognised as a marker unit above the Sturtian glacials throughout the Centralian Basins and on the Stuart Shelf to the west of the Flinders Ranges. The distinctively laminated thin dolomitic interbeds in finely laminated anoxic shale are readily identified and provide a key to stratigraphy in South Australian mapping. The interval marks a prominent phase of extension defined by the Serle Conglomerate in the Arkaroola region and by numerous erosional breaks and an active diapirism adjacent to many 20

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

of the diapirs exposed at this level in the Flinders and Willouran Ranges. The "homed" nature of the basin at this time taken together with the regular isopachs of the late Sturtian interglacial tend to suggest a sag phase following the earlier development of deep troughs adjacent to the Cumamona Craton. Clearly there is need to interpret the regional section east to the Willyama Ranges and Caloola Syncline in the context of developing ideas on crust delamination. Studies of modem passive continental margin development suggest that the "break up" unconformity may not be a unique event. There is the possibility for a pull apart migrating in time such that the deeper glacial basins which developed somewhat later in the east, viz. the Yudnamutana and Baratta Troughs, may reflect an eastward trend for extension and consequent sag. Widespread sediment instability and down to basin slumping continued in the Flinders and Willouran Ranges to the early Marinoan. Examples of relevant diapir emergence are the eroded west flank of the Enorama Diapir, and the Pinda and Beltana Diapirs. A close analogue of the Enorama stmcture is provided by the phases of erosion recorded on the eastem flank of the Ringwood diapir in the Eastem Amadeus Basin. Both this and other southem examples in the early Marinoan may be interpreted as diapir growth reflecting sediment loading in a sag type depositional setting. In the Amadeus Basin, Oaks et al (1991) demonstrated that growth of salt stmctures was episodic and began by passive flow of salt to regional anticlinal cores and was then accelerated during the Petermann movements with the formation of local unconformities adjacent to salt cored stmctures involving units of the Pertatataka, Julie formation and Ammbera Sandstone, eg., Goyder Pass stmcture, Gardiner Range and Waterhouse anticlines. The movements continued into the Cambrian. A parallel history has been documented by various workers for the Ediacaran of the Flinders Ranges in South Australia. This is a period of uplift of the Cratonic region to the west and non-deposition on the Stuart Shelf and Cumamona Craton. The carbonate platform cover and ramp/shelf limestones of the Lower Cambrian of the Arrowie and eastem Officer and Amadeus Basins appear to be mature sag sequences and tend to indicate a significantly earlier breakup event. Diapir activity in the central Flinders Ranges at this time is related to mild extension as evidenced by growth faulting and development of mini basins. Regional unconformity of the nature of the pre-Sturtian phase of extension is not a feature of the late Neoproterozoic or Cambrian. Early Cambrian growth faults and diapir activity represent passive and collapse phases during carbonate shelf and slope environments in the Flinders Ranges and are not characteristic of significant extension. Notable thickness and facies changes develop across syndepositional transfer faults which parallel the Paralana stmcture. The notable change to redbed facies reflects fault inversion, and uplift of sediment sources related to the Delamerian front in the southeast in the late Lower Cambrian. In summary depositional features tend to support the concept of (Powell et a/., 1994) for the rift/drift transition about the time of the Sturtian glaciation. Significant events are the widespread uplift and erosion of the craton margin in the earliest Sturtian with subsequent onlap and the development of unconformity by faulting within the rift itself. The sag type history of the later Sturtian and younger sequences is supported by the appearance of sedimentary features characteristic of passive margin sequences, vis., abundant terrigenous clastics, listric growth faults. 21

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olistostromes, syndepositional diapirism, canyons and carbonate shelf and ramp deposits. The apparent negative feature in relation to continental break-up in the early Sturtian, that of very limited indication of volcanics in the section, is not unique to the Neoproterozoic Centralian Basins. On the other hand the King Island volcanics in Bass Strait, which follow a diamictite with striated quartzite boulders capped by a cream to buff dolomite (a post Marinoan glacial cap dolomite look-alike) might suggest a second, immediate post Umberatana Group separation for that element of the puzzle. REFERENCES

Coats, R. P., 1965, Diapirism in the Adelaide Geosyncline, APEA Journal, 1965, p. 98-102. Dyson, LA., 1996, A new model for diapirism in the Adelaide Geosyncline, South Austraha. Geological Note, MESA Journal, v. 3, p.41-48. Oaks, R.Q., Jr., Deckelman, J.A., Conrad, K.T., Hamp, L.P., Phillips, J.O. and Stewart, A.J., 1991, Sedimentation and tectonics in the northeastern and central Amadeus Basin, central Australia, in Korsch, R.J. and Kennard, J.M., eds.. Geological and geophysical studies in the Amadeus Basin central Australia, Bureau of Mineral Resources, Australia, Bulletin 236, p.73-90. Powell, C.McA., Preiss, W.V., Gatehouse, C.G., Krapez, B., and Li, Z.X., 1994, South Australian record of a Rodinian epicontinental basin and its mid-neoproterozoic breakup (-700 Ma) to form the Palaeo-Pacific Ocean, Tectonophysics, v. 237, p. 113-140 Preiss, W.V., 1985, Stratigraphy and tectonics of the Worumba Anticline and associated intrusive breccias. South Australian Geological Survey, Bulletin 52. Preiss, W.V., Belperio, A.P., Cowley, W.M. & Rankin, L.R., 1993b, Neoproterozoic, m Drexel, J.F., Preiss, W.V. and Parker A.J. (editors), The geology of South Australia, vol. 1, The Precambrian, South Australian Geological Survey, Bulletin 54, p. 170-203.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

REFINING RODINIA Ian W. D. Dalziel Tectonics Special Research Centre University of Texas at Austin and University of Western Australia Rodinia, the supercontinent assembled at the time of global "Grenvillian" orogenesis at the end of Mesoproterozoic times, appears to have been "Laurentiacentric." Rather than supporting the Wilsonian concept that protoAppalachian Laurentia was juxtaposed with NW Africa in the latest Precambrian to earliest Paleozoic, geologic data seem to favor reconstructions of the early Neoproterozoic supercontinent with the ancient rifted Pacific and Atlantic margins of the core of present-day North America located between fragments of East and West Gondwanaland respectively. Siberia and Baltica were most likely juxtaposed with the present Arctic islands and NE Greenland margins. These relative positions are generally consistent with the paleomagnetic data and with global time-space relations that require opening of the Pacific and assembly of Gondwanaland by earliest Paleozoic times. There are at present, however, no "smoking guns" demanding particular craton-to-craton relationships. The present southern margin of the Laurentian craton, neglected in considerations of Paleozoic and Precambrian geography prior to the 1990's, has become perhaps the most critical area for understanding Laurentia-Gondwana relations in the mid-late Proterozoic and early-mid Paleozoic time interval. Four recent hypotheses central to Rodinia reconstructions all emerged from consideration of this previously neglected "southern cone": 1. The Yavapai and Mazatzal belts of Arizona and New Mexico and the Grenville belt of Texas might continue into East Antarctica (Moores, Geology, 1991; Dalziel, Geology, 1991); 2. The Taconic orogen might have had a continuation into southern South America (Dalla Salda et al. Geology, 1992a,b; Dalziel et aL, GSA Bulletin, 1994); 3. The Precordillera of Argentina was derived from the vicinity of the Ouachita embayment (Dalla Salda et al. Geology, 1992b; Astini et al, GSA Bulletin, 1995); and 4. The Precordillera might have been part of a marginal plateau outboard of the Ouachita trough that rifted off southernmost Africa and East Antarctica (Dalziel, GSA Bulletin, 1997). These suggested relationships may provide relative paleolongitudinal control for Laurentia and Gondwanaland back to the end of Precambrian times, when they may fleetingly have formed the Pannotia supercontinent, and for Laurentia relative to the component cratons of Gondwanaland back to the amalgamation of Rodinia at the close of the Mesoproterozoic Era (Dalziel, GSA Bulletin, 1997). The position of the Kalahari craton within Rodinia has been enigmatic. Did it belong in East or West Gondwanaland? This problem may now have been resolved, again by examination of the geologic history of the present southern margin of Laurentia. The paleomagnetically permissible reconstruction that I am proposing with my colleagues Sharon Mosher and Lisa Gahagan (Dalziel et a/., abstract submitted for GSA Annual Meeting, Toronto, Ontario, 1998) leads not only to refinement of the paleogeography of Rodinia with a geologically reasonable model of Kalahari-Laurentia relations, but also to a model for the assembly of that supercontinent along discrete segments of Grenvillian orogen. Further, recognition of the major cratonic entities that came together along these segments to form Rodinia should facilitate understanding of global paleogeography well back into Mesoproterozoic times. 23

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

EVIDENCE FOR THE TIMING OF CONTINENTAL BREAKUP AND VOLCANIC PASSIVE MARGIN FORMATION, SOUTHEASTERN GONDWANA: AN ACTUALISTIC PERSPECTIVE N. G. Direen Centre for Ore Deposit Research, School of Earth Sciences University of Tasmania, GPO Box 252-79, Hobart TAS 7001 A. J. Crawford Centre for Ore Deposit Research, School of Earth Sciences University of Tasmania, GPO Box 252-79, Hobart TAS 7001 The release of the Australian National Geoscience Datasets (http://www.agso.gov.au/map/njds) has drawn attention to a system of orthogonal gravity and magnetic lineaments in southeastern Australia. For approximately 95% of its area, this feature is obscured by sedimentary rocks of the Mesozoic Murray Basin. The orthogonality of this system has been considered to be the expression of a Proterozoic rift margin, dating from the --700 Ma Rodinian breakup. But is it? This presentation critically examines this concept. In this anomaly system there are three major NW trending anomalies which have common characteristics vis. Anomaly Max. Width (km) Bacannia 40 Dimboola-Stavely 45 East King Island 50

Length (km) Max. Amplitude (nT) 200 350 240 450 200 500

We have investigated the physical properties of outcrop and drillholes at these locations, and modelled the sources that give rise to these anomalies, as outlined below. Figures 2, 3 & 4 illustrate 2.5D gravity and magnetic forward models for the three anomalies. The Bancannia anomaly is modelled as a maximum 4 km thick east-dipping wedge of Mt. Arrowsmith Volcanics (MAV). At Mt. Arrowsmith, 50 km north of the section, the MAV outcrop as a 4.5 km thick steeply east-dipping pile of high Nb alkaline basalt to alkali rhyolite lavas, pillow lavas, plugs sills and dykes, interbedded with volcanic breccias and hyaloclastites. The MAV have been dated as Late Neoproterozoic (586 ± 7 Ma). Outcrop-derived densities and susceptibilities scaled for weathering and appropriately weighted for lithological combinations have been used in the model. Peebinga-1, drilled on the axis of the northern termination of the Dimboola anomaly, intersected high Nb alkaline metabasalt lavas, interbedded with tuffs, agglomerates, micritic metalimestone and phyllitic black calcareous shale. Magnetic forward modelling of the anomaly using weighted properties from Peebinga-1 indicates a 1100 m thick irregular slab dipping 30^ east. This is concealed beneath a sedimentary basin containing up to 4000 m of the low density, non-magnetic Grampians Group. The southernmost anomaly outcrops between Bold Head and Naracoopa, King Is. Previous workers showed three distinct volcanic packages dipping and younging 24

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup of Rodinia, Perth, September 1998

east: a series of pyroxene-phyric pillow and massive low-Ti tholeiitic lavas, tuffs and agglomerates; an overlying picritic pillow and pahoehoe lava, hyaloclastite breccia and volcanogenic sediment sequence; and an upper sequence, comprising high-Ti plagioclase- and clinopyroxene-phyric, tholeiitic to transitional alkaline lavas interbedded with volcanogenic conglomerates. The King Is. sequences are correlated with the Crimson Creek Group of mainland Tasmania. This latter unit is a series of low- and high-Ti tholeiites and picrites, including rare pillow lavas. Crimson Creek Group dykes have been K-Ar dated at 588 ± 8 and 600 ± 8 Ma. Modelling using properties from King Is. predicts a total thickness of up to 14252 m for this pile. This extreme thickness is likely to be due to duplication by Devonian thrusting. The true thickness probably ranges from 6000 to 7000 m. Both slices dip east at -80^. The modelled geometry of magnetic sources in southeastern Australia bear striking similarity to Seaward Dipping Reflector Sequences (SDRS). Known SDRS have wedge geometries with dimensions 40 and 100 km wide, maximum thicknesses of 2 to 7 km, and strike lengths of 400 to 2400 km. The volcanics drilled in SDRS typically include picrites, olivine-, clinopyroxeneand plagioclase-phyric basaltic volcanics. These form massive or compound flows, with subaerial (pahoehoe, aa) and submarine (pillow lavas) eruptive phases. Flows are typically interbedded with ash-derived volcanogenic sediments and volcanogenic breccias. Geochemically, these have transitional alkaline to tholeiitic affinities, often with elevated Nb and Zr values. These features are all found within the -600 Ma mafic volcanics associated with magnetic anomalies in southeastern Australia, leading us to the conclusion that they represent a volcanic passive margin associated with a major breakup event in the Late Neoproterozoic. For 18 years, the Normanville and Kanmantoo Groups of South Australia (SA) have been thought to represent a marine transgression and continental shelf turbidite blanket above a breakup unconformity event near the base of the Cambrian. Our recent work in New South Wales and SA confirms a prediction that units of the Koonenberry Fold Belt correlate with the Normanville and Kanmantoo Groups, in particular the Early to Late Cambrian Gnalta Group, parts of the informal "Kara beds", and the Middle?-Late? Cambrian Teltawongee Group. In addition, postbreakup magmatism in the form of tholeiitic dykes sills and plugs is found in both packages. In SA, this activity has been dated at -510 Ma. In contrast to the SDRS forming event, late dykes have typical MORE character. The pattern of massive outpourings of early alkaline-tholeiitic volcanics as SDRS, followed by posttransgression MORB-like dyke swarms is well documented on the Atlantic seaboard of eastern North America. In conclusion, there are very convincing geophysical, lithological and geochemical grounds to support the development of a volcanic passive margin in southeastern Gondwana between 600 and 580 Ma. This event was followed by a breakup unconformity, marine transgression and post-breakup magmatism to form a segmented passive margin shelf system. It cannot be conclusively demonstrated that this continental scale event was the Rodinian breakup; however, this episode stands in contrast to postulated models for Rodinian breakup at -700 Ma, which rely only on evidence from dyke swarms for support. 25

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

Figure la: Eastern Australian TMI

Figure lb: Eastern Australian Gravity 26

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

DlsUnce (m)

Figure 2 Bancannia Model

r J 5000

10000

Distance (m)

Figure 3 Dimboola Model

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

Figure 4 King Island Model

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup of Rodinia, Perth, September 1998

THE BREAKUP UNCONFORMITY, SALT TECTONICS AND DEVELOPMENT OF MINI-BASINS IN THE NEOPROTEROZOIC SUCCESSION OF THE ADELAIDE GEOSYNCLINE Ian A. Dyson National Centre for Petroleum Geology & Geophysics University of Adelaide, Adelaide SA 5005

SUMMARY Dyson (1992a) suggested that breakup of the Neoproterozoic supercontinent occurred after deposition of the Sturtian glacials. Recent research reported herein suggests that other breakup unconformities may occur at the base of the SeacUff Sandstone and Bonney Sandstone. Alternatively, either or both unconformities may correspond to postrifting megasequence boundaries. Large-scale erosional unconformities associated v^^ith olistostromes also occur at the base of the Warcowie Dolomite, Tindelpina Shale and Enorama Shale and are related to major extensional events. They are interpreted as major sequence boundaries and are laterally correlative with deep-water dolostone or are overlain by transgressive units that are capped by deep-water dolostone. The dolostones are widespread marker beds in the Adelaide GeosyncHne, and signal the turn around from transgressive to regressive sedimentation within individual depositional sequences. Two of these, namely the Nuccaleena Formation and Wearing Dolomite, are well developed in the northern Adelaide Geosyncline where they were deposited in mini-basins. The mini-basins formed during salt withdrawal following major extension on a regional scale. Another dolostone, referred to as the Wearing Dolomite and overlying a deep-water sequence boundary, was deposited in mini-basins during a very high rate of basin subsidence. Slumping at this stratigraphic level was related to a phase of passive diapirism and independent of extension. A demarcation between diapirs associated with anticlinal cores and mini-basins runs approximately northwest-southeast from Blinman to Martins Well. North of this line, minibasins are dominant. This may reflect a regional change that accompanied further breakup of Rodinia. It was most likely initiated after deposition of the Marinoan glacials, an interpretation based, in part, on deposition of Wilpena Group and basal Cambrian sediments in the mini-basins. The Umberatana and Wilpena Groups, previously referred to as the post-rift or drift succession of the Adelaide Geosyncline, are punctuated by several sequence boundaries and accompanying development of thick siliciclastic units. This is at odds with previous models for passive margins in which subsidence decayed with time following the breakup unconformity. The base of the Umberatana Group was interpreted previously to represent the breakup unconformity within the Neoproterozoic succession of the Adelaide Geosyncline (Preiss, 1983). This level corresponds to the base of the Sturtian glacials. Dyson (1992a) was the first to suggest that the base of the Wilyerpa Formation represented the breakup unconformity. Further work in the Willippa Anticline (Dyson, 1996a) supported placement of a major unconformity at the base of the Warcowie Dolomite. This unconformity truncated large-scale extensional faults that permeated the underlying 29

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Burra Group and Sturtian glacials. Coats (1965, 1973) demonstrated the existence of an angular unconformity between the Burra Group and overlying Wilyerpa Formation at the Burr and Lyndhurst diapirs, and suggested that diapirism preceded formation of this unconformity. This was supported by Dyson (1992a, 1996b) who suggested that the first phase of diapirism was the result of a major extensional event at the base of the Umberatana Group, prior to breakup of the Neoproterozoic supercontinent. Major unconformities of the Umberatana and Wilpena Groups are best developed adjacent to diapirs in the Adelaide Geosyncline (Coats, 1973). Dyson (1995, 1996b) suggested that the formation of sequence boundaries was related to salt tectonics in an extensional regime. Dyson and Dalgamo (1998) further suggested that olistostromes and large-scale slumping overlie major unconformities in the Adelaidean succession. Olistostromes are common in extensional settings, and are characteristic of the lowstand prograding complex or early phase of the transgressive systems tract. Olistostromes in the Callanna and Burra Groups were interpreted to be associated with graben development and listric faulting (Dyson and

Dalgamo, 1998). Major incisions at the base of the Warcowie Dolomite and

Tindelpina Shale are associated with olistostromes. The olistostrome at the base of the Warcowie Dolomite was interpreted to be adjacent to an exposed fault scarp in the Willippa Anticline (Dyson, 1996a, 1996b). A large channel at the base of the Serle Conglomerate is filled with conglomerates and olistoliths, and passes laterally into the base of the Tindelpina Shale (Dyson, 1996a). An olistostrome overlying the sequence boundary at the base of the Enorama Shale resulted from slumping adjacent to the subsiding Enorama Diapir during salt withdrawal. In the Willouran Ranges, slumping at the base of the Amberoona Formation (Coats and Dalgamo, 1983) is correlated with erosional unconformity at the base of the Enorama Shale. These unconformities are further interpreted to represent deep-water sequence boundaries. Sequence analysis of the Umberatana and Wilpena Groups in the Adelaide Geosyncline (Dyson, 1992a, 1995, 1996a, 1996b) identified five dolostones or dolomitic units interpreted to be of deep-water origin (Dyson, 1996c, 1997). These are referred to as the Warcowie Dolomite, Tindelpina Shale, Artipena Dolomite Member of the Enorama Shale, Nuccaleena Formation and the Wearing Dolomite. Each is associated with former anoxic deposition on a maximum flooding surface below storm wave base (Dyson, 1992a, 1996c) that corresponded to periods of maximum accommodation in the basin. The dolostones attest to higher subsidence rates that were characterised by enhanced transgression at the maximum flooding surface and followed a major hiatal break at the sequence bounding unconformity. They mark the turn around from transgressive to regressive sedimentation within individual depositional sequences. Furthermore, deposition of each dolostone postdated major extensional events. The Neoproterozoic dolostones are regional stratigraphic markers in the Adelaide Geosyncline. The Nuccaleena Formation and Wearing Dolomite are well developed in several large synclines on the COPLEY 4-mile sheet. There are no shallow water unconformities associated with these dolostones. However, local slumping is widespread in these synclines. They are interpreted as mini-basins that formed during salt withdrawal, possibly as a result of continued extension. Diapirs are found as peripheral ridges between the mini-basins. Outcrop of the Beltana Diapir represents a cross-section through this style of mini-basin. It formed by evacuation of a salt stock during vertical subsidence. Local slumping into the mini30

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basin occurred as Brachina sediments on the basin flanks were uplifted and tilted. The sequence boundary at the base of the overlying Wilcolo Sandstone (Dyson, 1996c) most likely formed during subsequent uplift and rotation of the southern flank. Large-scale slumping at the base of the Wonoka Formation was first identified by Coats (1964). These slumps were deposited in a submarine environment and von der Borch et al (1982) interpreted channelling at the base of the Wonoka Formation to represent submarine canyons. Submarine canyons of the Wonoka Formation are enigmatic. Numerous interpretations have been suggested for their formation, ranging from subaerial to subaqueous, or a combination of both. Also controversial is the timing of canyon incision. Dyson (1996a, 1996b) suggested that incision occurred just prior to deposition of the Wearing Dolomite. An angular unconformity is observed at the base of the Wearing Dolomite at Pinda Diapir (Dyson, 1998), suggesting that slumping of the underlying allochthonous salt sheet within the upper Bunyeroo Formation resulted from passive diapirism. However, most theories suggest a transgressive fill for the kilometre-deep Wonoka canyons. This suggests that a significant amount of accommodation was available, and possibly as a result of subsidence associated with mini-basin development. Another excellent example of slumping associated with mini-basin development is the olistostromes at the base of the Billy Springs Formation in the Umberatana Syncline. It suggests that this stratigraphic level may be correlatable with the unconformable base of the Bonney Sandstone. On COPLEY, it represents a major extensional event. Nested synclines mapped near Witchelina in the southeastern Willouran Ranges may represent mini-basins that are underlain by deformed evaporitic sediments of the Callanna Group. They were most likely developed after deposition of the Sturtian glacials (C.R. Dalgamo; pers. comm, 1997), and contain the Amberoona slumps of Coats and Dalgamo (1983). These slumps were interpreted as olistostromes that overlie a major sequence boundary (Dyson and Dalgamo, 1998). A number of diapirs (e.g., Moralana, Mucatoona, Oraparinna, Pinda and Wirrealpa) are overlain by fault-bounded grabens. These crestal grabens were interpreted by Dyson (1996b) as salt withdrawal features as the reactive diapir began to subside with subsequent depletion or restriction of the salt supply due to continued extension. Wilpena Group and basal Cambrian sediments were deposited in these grabens. Further south in the central Flinders Ranges (PARACHILNA 4mile sheet), diapirs are found in anticlinal cores and reflect movement after deposition of the Tapley Hill Formation. However, the dolomitic Tindelpina Shale is commonly found in the intervening synclines, and may even rim the diapir (e.g., at Wommba). A demarcation between the diapirs associated with anticlinal cores and minibasins mns approximately northwest-southeast from Blinman to Martins Well. North of this line, mini-basins are dominant. This may reflect a regional change that accompanied further breakup of Rodinia. It was most likely initiated after deposition of the Marinoan glacials, an interpretation based, in part, on deposition of Wilpena Group and basal Cambrian sediments in the mini-basins. Sedimentation of allochthonous salt sheets on COPLEY is associated with passive diapirism during periods of slow sedimentation in mini-basins. Allochthonous salt sheets have been identified in the Tindelpina Shale and lower Bonney Sandstone, as well as the upper Bunyeroo Formation. Passive diapirism is the result of sedimentary loading and is independent of regional extension. 31

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However, it follows a major extensional event, which in turn was followed by a high rate of basin subsidence and therefore high accommodation. The large increase in accommodation was followed by deposition of thick Tapley Hill Formation, Wonoka Formation and the Bonney Sandstone. These stratigraphic units are well developed in mini-basins on COPLEY. Major sequence boundaries associated with development of incised valley fills occur at the base of the Seacliff Sandstone and Bonney Sandstone. In both cases, the incised valleys are filled with shallow water facies. Incised valley fills of the Seacliff Sandstone (Dyson and von der Borch, 1994) are too thick (>100m) to have been the result of eustasy alone. The sequence boundary at the base of the Seacliff Sandstone resulted from a major extensional event, perhaps representing renewed rifting in the Adelaide Geosyncline. It was followed by high subsidence resulting in deposition of the Sandison Subgroup. The great thickness of the Sandison Subgroup was accompanied by active faulting immediately east of the Torrens Hinge Zone. The Sandison Subgroup is also well developed in mini-basins of the northern Adelaide Geosyncline where its base is represented by the Nuccaleena Formation of deep-water origin. A lesser extensional event at the top of the ABC Range Quartzite was superimposed on this high rate of subsidence. Transgressive sedimentation of the Bunyeroo Formation was concomitant with passive diapirism and canyon development at the base of the Wonoka Formation (Dyson, 1996b, 1998). The base of the Bonney Sandstone is the major Ediacaran sequence boundary. Large-scale extensional faults are found on the eastern side of the Mount Frome Diapir (Dalgamo, 1983), where metre-thick shoreface sandstones onlap the unconformity at the top of the Wonoka Formation. These sandstones were deposited during active growth of the Mount Frome Diapir. A high-angle unconformable relationship with the underlying Wonoka Formation is also observed at the Pinda and Beltana Diapirs (Dyson, 1998). Powell et al. (1994) and Powell (1998) suggested that a phase of renewed rifting corresponded to the upper part of the Neoproterozoic succession but were unable to identify the stratigraphic level. The sequence boundaries at the base of the Seacliff Sandstone and Bonney Sandstone were most likely cut during major extensional events associated with renewed rifting. They may be interpreted as breakup unconformities or megasequence boundaries. The development of mini-basins in the northern Adelaide Geosyncline resulted from salt withdrawal, most likely as a result of an increase in extension after deposition of the Marinoan glacials. The synform of the mini-basins better reflects syndepositional tectonics associated with diapirism during the Neoproterozoic, and not the effects of the Delamerian Orogeny.

REFERENCES

Coats, R.P., 1964, Large scale Precambrian slump structures. Flinders Ranges, Quarterly geological Notes, Geological Survey of South Australia, v. 11, p. 1-2. Coats, R.P., 1965, Diapirism in the Adelaide Geosyncline., APEA Journal, 1965, p. 98-102. Coats, R.P., 1973, COPLEY, South Australia, Explanatory Notes, 1:250 000 geological series. Geological Survey of South Australia. Coats, R.P. and Dalgarno, C.R., 1983, Large scale slumping in the Umberatana Group, Willouran Ranges, Geological Society of Australia, Abstracts No. 10, p. 63-64. Dalgamo, C.R., 1983, Marinoan mud islands. Mount Frome, Geological Society of Australia, Abstracts No. 10, p. 34-35 Dalgamo, C.R. and Dyson, LA., 1998, Ediacaran diapirism. Geological Society of Australia, Abstracts No. 51, p. 43.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998 Dyson, LA., 1992a, Geology of the Upalinna Diapir, central Flinders Ranges, Quarterly Geological Notes, Geological Survey of South Australia, v. 124, p. 2-19. Dyson, I.A., 1992b, Stratigraphic nomenclature and sequence stratigraphy of the lower Wilpena Group, Adelaide Geosyncline: the Sandison Subgroup, Quarterly Geological Notes, Geological Survey of South Australia, v. 122, p. 2-13. Dyson, LA., 1995, Sedimentology and stratigraphy of the Neoproterozoic Sandison Subgroup: a storm-dominated shallow marine sequence in the Adelaide Geosyncline, South Australia, unpub. Ph.D. thesis, Flinders University of South Australia. Dyson, LA., 1996a, Sequence stratigraphy of the Burra and Umberatana Groups, Willippa Anticline, central Flinders Ranges, Quarterly Geological Notes, Geological Survey of South Australia, v. 129, p. 10-26. Dyson, LA., 1996b A model for diapirism in the Adelaide Geosyncline, MESA Journal, v. 3, p. 41-48. Dyson, LA., 1996c, Stratigraphy of the Neoproterozoic Aruhna and Depot Springs Subgroups, Adelaide Geosyncline, Transactions of the Royal Society of South Australia, v. 120, p. 101-115. Dyson, LA., 1997, Definition of the Artipena Dolomite Member, Enorama Shale, MESA Journal, v. 6, p. 33-35. Dyson, LA., 1998, The Christmas tree diapir and salt glacier at Pinda Springs, central Flinders Ranges, MESA Journal, v. 10, p. 34-38. Dyson, LA. and Dalgamo, C.R., 1998, Salt glaciers and slumps in Neoproterozoic successions of the Adelaide Geosyncline, Geological Society of Australia, Abstracts No. 51, p. 48-49. Dyson, LA. and von der Borch, C.C., 1994, Sequence stratigraphy of an incised valley fill: the Neoproterozoic Seacliff Sandstone, Adelaide Geosyncline, South Australia, m Dalrymple, R., Zaitlin, B. and Boyd, R. (eds.). Incised valley systems: origin and sedimentary sequences, SEPM Special Publication, v. 51, p. 209-222. Powell, C. McA., 1998, Assembly and breakup ofRodinia, Geological Society of Australia, Abstracts No. 49, p. 360. Powell, C. McA., Preiss, W.V., Gatehouse, C.G., Krapez, B. and Li, Z.X., 1994, South Australian record of a Rodinian epicontinental basin and its mid-Proterozoic breakup (-700 Ma) to form the Palaeo-Pacific Ocean, Tectonophysics, v. 237, p. 113-140. Preiss, W.V., 1983, Depositional and tectonic contrasts between Burra and Umberatana Group sedimentation. Geological Society of Australia, Abstracts No. 10, p. 13-16. von der Borch, C.C., Smit, R. and Grady, A.E., 1982, Late Proterozoic submarine canyons of Adelaide Geosyncline, South Australia, AAPG Bulletin, v. 66, p. 332-347.

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SUPERCONTINENTAL CYCLES AND TRUE POLAR WANDER David A. D. Evans Tectonics Special Research Centre, Department of Geology and Geophysics University of Western Australia, Nedlands WA 6907 True polar wander (TPW) is the reorientation of an entire solid planet relative to its rotational axis, the natural result of mass redistributions on and beneath its surface. Theoretical calculations of TPW allow sustained shifts as fast as 100 cm/yr, a rate which dwarfs those of typical between-plate motions. Pre-Mesozoic TPW, if rapid enough through a given interval of time, can only be estimated by a common component of every continent's palaeomagnetically-determined apparent polar wander (APW) paths. Recent interpretations of the global palaeomagnetic dataset for two intervals in early-middle Palaeozoic time have invoked TPW to account for large and rapid swings in APW (Van der Voo, 1994; Kirschvink et al., 1997). When plotted relative to Gondwanaland, the Palaeozoic Era's largest continent and thus perhaps the slowest moving over the mantle, the hypothesized TPW paths oscillate nearly along the same great-circle. Such oscillatory TPW might be expected from a planet whose long-lived mass anomalies concentrated along two antipodal and equatorial regions, yielding a prolate (rugby-ball shaped) nonhydrostatic geoid. Today, prolateness of the nonhydrostatic geoid can be correlated with a time-integrated system of mantle upwelling inherited from the breakup of Pangaea and the slow retreat of its marginal subduction zones. TPW during the last 200 Myr is almost insignificant, perhaps because of very stable loci of specific peri-Pacific subduction zones. In the case of Palaeozoic Earth, the long axis of the "rugby ball" is determined as the pole to the hypothesized TPW great-circles, to lie within the Australian sector of Gondwanaland. This curious position dangling on the side of a supercontinent makes more sense if we consider that it nearly coincides with the center of the former Rodinia supercontinent as reconstructed via the "SWEAT" hypothesis. By this speculation, the old Rodinia supercontinent isolated its underlying mantle from subduction for so long that an upwelling was induced, which would migrate to the Equator via TPW. The upwelling would eventually disaggregate the dying supercontinent at -700 Ma, and the first-order pattern of mantle convection (two antipodal upwellings, one beneath the fragmented supercontinent) would have persisted even hundreds of Myr later. Gondwanaland assembled during this time, but was confined within a late Neoproterozoic to early Palaeozoic sectoral girdle of mantle downwelling, oscillating over the southern hemisphere via TPW. By Mesozoic time, Gondwanaland (or Pangaea) was old enough so that a new pattern of circum-supercontinental subduction zones had induced a sub-Pangaean mantle upwelling that eviscerated the supercontinent's Gondwanaland core. The supercontinental cycle continues today as Super-Asia assembles within the present "Ring of Fire." Although Rodinia's reconstruction and, even more, the preceding and following cratonic configurations remain speculative, several observations of Neoproterozoic palaeogeography appear to conform to the above model. First, the model predicts that old supercontinents will lie on the Equator. This appears true for Rodinia at -700 Ma, as it is for Pangaea at 200 Ma. Second, the model predicts that a young supercontinent will assemble at any latitude within a sectoral ring of fire (inherited from its predecessor) and be subjected to oscillatory TPW, rapidly shifting latitudes. 34

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

This appears true for Gondwanaland—nascent Pangaea—and perhaps also for Rodinia, as indicated by oscillatory APW motion during the interval 1100-900 Ma (Weil et al, 1998). Third, the model predicts that oscillatory TPW may predominate during the intervals prior to supercontinental assembly. This is a plausible explanation for the great-circle distributions of palaeomagnetic poles from nearly every well-investigated late Neoproterozoic craton, including Laurentia, East Gondwanaland, South China, and Baltica. Finally, the model allows for rapid motions of the continents because the entire mantle is assumed to rotate in the TPW response to planetary mass redistribution. The late Neoproterozoic and early Palaeozoic interval is indeed noted for several discrete intervals of rapid continental motions (Meert et al, 1993), all of which are consistent with the model of oscillatory TPW inherited from Rodinia.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

LOWER-CRUSTAL FLOW AND ITS EFFECT ON EXTENSIONAL STYLES A. P. Gartrell

Tectonics Special Research Centre, Department of Geology and Geophysics University of Western Australia, Nedlands WA 6907 Geological, geophysical and laboratory evidence all suggest that the continental lithosphere is rheologically stratified. This layering reflects changes in mechanical behaviour and flow process of continental lithospheric rocks as determined by depth-dependant physical (temperature, pressure) and chemical (mineralogy, presence of water) environment. Rheological modelling predicts a range of rheological configurations, depending mainly on geothermal gradient and rock composition. The lower-crust is shown to be very weak under all but the coolest geothermal gradients. Such weak layers could mechanically decouple stronger layers, allowing layer parallel slip. Numerical modelling (e.g. Westaway, 1992; Melosh, 1990) show that shear traction at the base of the strong middle-crust can to rotate the principal stress axes up to 45° from the vertical, thus allowing low-angle faults to develop. A relationship between the magnitude of the shear couple developed the magnitude of pressure of the overburden is shown. A large basal shear stress relative to lithostatic pressure results in a significant rotation of the stress axes, whereas a relatively small shear stress will have little effect on the orientation of stress axes. However, although major fault styles may be implied (low-angle detachment vs high-angle faulting), the numerical modelling only predicts stress conditions and is unable to demonstrate the formation and evolution of faults and shear zones during deformation. Here, analogue modelling techniques from Gartrell (1997) are used to physically demonstrate a relationship between the thickness of rheological layers and rift basin styles. Three-layer crustal systems, comprising a brittle upper-crust (sand), a strong transitional or semi-brittle middle crust (strong silicone putty) and a weak ductile lower-crust (weak silicone putty) were extended. Listric detachment fault margins, with highly rotated hangingwalls, develop when the combined thickness (and therefore weight) of the upper- and middlecrustal layers is low. Where the upper layers are thicker, high-angle planar fault margins develop, resulting in distictly different basin architectures. In both cases the basins produced may be symmetrical or asymmetrical depending on whther extension was bidirectional or unidirectional.

REFERENCES

Gartrell A. P. 1997, Evolution of rift basins and low-angle detachments in analog models, Geology, v. 25, p. 615-618. Melosh H. J. 1990, Mechanical basis for low-angle normal faulting in the Basin and Range province. Nature, v. 343, p. 331-335. Westaway R. 1998, Dependence of active normal fault dips on lower-crustal flow regimes, Journal of the Geological Society, London, v. 155, p. 233-253.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

INITIATION OF RODINIA BREAKUP IN SOUTHEAST AUSTRALIA BY SW-NE EXTENSION: EVIDENCE FROM SHEAR ZONE KINEMATICS AND DYKE INTRUSION George M. Gibson

Australian Geological Survey Organisation, Canberra ACT 2601 The Broken Hill Inlier, comprising mainly high-grade metamorphic rocks of the Paleoproterozoic Willyama Supergroup and locally overlain by sediments of the late Neoproterozoic Adelaide Supergroup, is bounded to the south and east by the Tasman Line, a major structural and geophysical boundary formed during late Neoproterozoic continental rifting and representing the eastern limits of known Proterozoic continental crust in mainland Australia. This line has both northwest and northeast-trending elements, and is the single most obvious feature in eastern Australia dating from the breakup of Rodinia. Many major shear zones in the Broken Hill region share these same northwest and northeast trends, and several of them probably owe their origin to the same late Neoproterozoic extensional processes which gave rise to the Tasman Line (e.g.. Redan Fault); others more likely represent older Mesoproterozoic structures reactivated during the course of continental extension. Irrespective of their origin, displacement on these shear zones places important constraints on the initial direction of extension accompanying continental breakup. Mafic and ultramafic dykes intruded into some shear zones provide a further constraint on the direction of extension but more importantly place the onset of extension at around 827 Ma. This paper describes the tectonic history of the shear zones and shows that a combination of dyke intrusion and shear zone kinematics can be used to define the direction of extension uniquely. Northeast-trending shear zones are widely developed throughout the Broken Hill region and have been variously interpreted as: (1) post-metamorphic shear zones effecting little or no displacement; (2) strike-slip faults; and (3) reactivated thrust faults which originated during the earlier high-grade regional metamorphism. Most dip steeeply southeast (50° or more in seismic images) and some preserve shallowplunging lineations in keeping with a strike-slip origin. More importantly, several of these shear zones (e.g., Lakes Creek discontinuity) have effected km-scale sinistral offsets in the regional stratigraphy. However, almost all have been overprinted by younger deformational events, including the early Paleozoic Delamerian Orogeny during which time many of these shear zones were reactivated as west or northwest-directed thrust faults. Many northeast-trending shear zones have also been overprinted by a conjugate system of later east-west and north-south striking structures which has further masked the early displacement history of these shear zones. This is particularly evident in the case of the Stephens Creek Shear Zone which in its type locality has been reoriented into an east-west trend by this later deformation. Kinematic indicators and other minor sructures in this segment of the Stephens Creek Shear Zone (shear bands, rotated quartz veins, and asymmetric boudins) support dextral, mainly strike-slip movement, whereas offsets in the regional stratigraphy require an earlier and even greater amount of sinistral displacement. It is this earlier displacement history which is equated with late Neoproterozoic rifting. Northwest-trending shear zones truncate the regional stratigraphy at high angles and generally dip steeply eastward. They too have been overprinted by later deformational events, m^ing assessment of their origin and early displacement

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history equally difficult. A history of early normal faulting is inferred from the fact that parallel, and presumably related, structures controlled deposition of the Adelaide Supergroup which was largely laid down in northwest-trending fault-angle depressions and half-graben. Independent information in support of early normal faulting on these structures comes from the Stirling Hill Shear Zone where early mylonitic fabrics with a prominent down-dip lineation are preserved within sericitic schists formed during later reactivation (? early Paleozoic). Shear bands and other kinematic indicators (asymmetric boudins, rotated quartz veins) developed during this later reactivation indicate that the latest movement on the Stirling Hill Shear Zone and other northwest-trending shear zones was sinistral and dominantly transpressive in character. The Stirling Hill Shear Zone is host to a swarm of strongly magnetic ultramafic dykes equated with a string of variably sheared ultramafic intrusions locally intruded by the 827 Ma Little Broken Hill Gabbro. Little Broken Hill Gabbro is uniformly coarse grained and shows no evidence of chilled margins adjacent to the ultramafic rocks, indicating that the gabbro was either intruded contemporaneously with the latter or at the very least was emplaced into the ultramafic rocks before they had undergone significant cooling. It is concluded that the ultramafic intrusions and Stirling Hill dyke swarm are of comparable age to the Little Broken Hill Gabbro and thus were similarly intruded around 827 Ma. Based on this age, intrusion of the Stirling Hill dyke swarm must have occurred contemporaneously with deposition in the Adelaide Supergroup which commenced around 830 Ma. A common age for dyke intrusion and the start of Adelaidean sedimentation is not unexpected given that the Adelaidean Supergroup was deposited in basins sharing the same northwest trend as the dykes. Basaltic rocks preserved near the base of the Adelaidean sequence in the Broken Hill region may be related to a suite of younger non-magnetic metadolerite dykes trending WNW. A structural control on emplacement of the various mafic and ultramafic intrusive rocks is evident from the fact that the Little Broken Hill and its associated ultramafic rocks are located in a northeast-trending shear zone; an ultramafic mass at Red Hill has intruded the northwest-trending Rockwell Shear Zone. Less obvious is whether intrusion of these rocks was simply facilitated by the presence of older shear zones or whether it occurred through the introduction of melt into actively deforming shear zones. Almost all mafic and ultramafic rocks are sheared to some degree but it is not immediately clear whether this shearing occurred contemporaneously with intrusion or was induced by much younger deformational events. Unequivocal evidence for syn-deformational intrusion has not been found although shear fabrics in ultramafic rocks associated with the Little Broken Hill Gabbro appear to have been truncated by the gabbro which has itself been locally sheared along similarly oriented structures. Shearing evidently occurred before and subsequent to intrusion of the Little Broken Hill Gabbro. It would thus appear that dyke intrusion, deposition of the Adelaidean Supergroup, and displacement on the major shear zones described above are all related; they are all expressions of late Neoproterozoic continental extension. Together they point to development of the Broken Hill region as part of a lower plate rifted continental margin. Moreover, parallelism between the dykes, shear zones, and sedimentary basins points to a SW-NE direction for initial extension thereby indicating that the northwest-trending segment of the Tasman line in the Broken Hill region most likely has a normal fault geometry whereas the northeasttrending section of the Tasman Line more likely developed as a transfer fault. 38

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup of Rodinia, Perth, September 1998

CRUSTAL-SCALE EXTENSIONAL COLLAPSE OF THE ELZEVIR OROGEN, GRENVILLE PROVINCE, S.W. QUEBEC IMAGED BY LITHOPROBE Lyal B. Harris

Tectonics Special Research Centre, Department of Geology and Geophysics University of Western Australia, Nedlands WA 6907

Benoit Rivard

Department of Earth and Atmospheric Sciences University of Alberta, Edmonton T6G 2E3 AB, Canada

Louise Corriveau

Centre geoscientifique de Quebec Commission geologique du Canada, C.P.7500, Sainte-Foy QC G1V 4C7, Canada Interpretation of LITHOPROBE deep seismic data (Abitibi-Grenville Transect Lines 52 and 53) across the Grenville Province of southwestern Quebec shows an upper deck comprising the Central Metasedimentary Belt over-thrust by the Morin Terrane overlying a lower deck upon a basal detachment. This package overlies an Archaean parautochthon upon the Baskatong Ramp, a major ductile normal shear zone that represents a reactivated accretionary thrust boundary. The Reservoir Cabonga Terrane overlies the parautochthon in the NW of the profile. Spectacular examples of normal shear zones, block rotation between shear zones and necking of the lower deck indicate crustal-scale extension followed terrane amalgamation and assembly to the Superior Craton during the Elzevirian Orogeny. Block rotation between normal shear zones is seen to be an important mechanism for regional folding and exhumation of high grade rocks as elongate gneiss domes. In the Central Metasedimentary Belt and Morin Terrane, conjugate transcurrent ductile shear zones, E-W normal shear zones, and N-S flattening zones and minor reverse faults and thrusts (observed in the field and on earth sensing imagery) postdate 1165 Ma plutonism and indicate E-W contraction and N-S extension following terrane assembly and AMCG magmatism. These structures, however, appear to be insignificant at the crustal scale; in SW Quebec, there is therefore no evidence for a major tectonothermal event during the ca. 1 Ga Ottawan Orogeny, an important event in other parts of the Canadian Grenville Province.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

RECOGNITION OF REGIONAL DETACHMENTS IN THE SOUTHEASTERN HAMERSLEY PROVINCE, WESTERN AUSTRALIA D. A. Hollingsworth Tectonics Special Research Centre, School of Applied Geology Curtin University of Technology, GPO Box U1987, Perth WA 6001 Hollingd@lithos.curtin.edu.au P. A. Cawood Tectonics Special Research Centre, School of Applied Geology Curtin University of Technology, GPO Box U1987, Perth WA 6001 R. I. Hackney Tectonics Special Research Centre, Department of Geology and Geophysics University of Western Australia, Nedlands WA 6907 C. McA. Powell Tectonics Special Research Centre, Department of Geology and Geophysics University of Western Australia, Nedlands WA 6907 The Hamersley Province along the southern half of the Pilbara craton of Western Australia contains a fold-and-thrust belt formed during the Paleoproterozoic. There are two major phases of folding evident in the southeast of the province (Powell and Horwitz, 1994), the first (F2) is characterized by east-west (~090''-270°) trending folds that are generally north-verging and tight, and upright to overturned. The second phase of folding (F3) also trends roughly east-west (~100°-280^) with upright and commonly open folds. Detailed mapping along a 90 km transect to the west of Newman has failed to reveal major detachments exposed at the surface. The only thrusts previously mapped in the Hamersley Province occur in the southeastern comer, adjacent to the Sylvania Inlier; an overthrust basement block. Our mapping along the transect suggests that detachments are present at depth within shale units at various stratigraphic levels. The difficulty associated with identifying bedding parallel and intraformational detachments as well as poor exposure of the shale units throughout the southeastem part of the Hamersley Province may contribute to the lack of evident thrusts. Evidence for detachments includes: a) fold geometry, b) outcropscale detachments, and c) large variations in shortening between adjacent layers. The overall north vergence of the F2 folds is indicative of formation above a detachment and both fault-propagation folds and fault-bend folds are recognised in the field. Large-scale folds on the southern side of Coondawanna Ridge are characterised by long, shallow south dipping limbs and short, steep to overturned northem limbs. They interpreted to represent a fault-propagation fold developed above the tip line of a detachment. Many smaller scale folds associated with detachments are evident in the Wonmunna Anticline, at The Governor and on the northem side of the Weeli Wolli Anticline. Outcrop scale detachments and associated folds are present throughout the Bee Gorge Member of the Wittenoom Formation, the Mount Sylvia Formation and the Mount McRae Shale. Within the Wittenoom Formation the detachments have a ramp-flat geometry, with up to three ramps exposed along a single detachment. The flats associated with the 40

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detachments are often difficult to recognise as they are bedding parallel and are only positively identified where ramps cut the bedding. In the Bee Gorge Member at West Angela Hills, a series of splays propagate off a detachment generating a series of fault-propagation folds. Similar detachment splays are also evident in the Mount Sylvia Formation at The Governor. Splays in the detachment surface are evident on the ramp of a sample collected from the Bee Gorge Member. Within the shale units, the deformation has been dispersed into a series of detachments and associated folds. The displacement on each detachment is small, however the cumulative displacement may be large. The number of detachments with small displacements may also be in response to a nearby unexposed large-scale detachment. Detachments observed within the Brockman Iron Formation are largely related to bedding-parallel slip and accommodation of bed length during folding. However, on the southern side of Coondawanna Ridge there is evidence to suggest that a major thrust related detachment is present within the Whaleback Shale Member. The detachment is inferred because the Joffre Member appears to have undergone significantly more shortening than the underlying Dales Gorge Member. This relationship requires the intervening Whaleback Shale Member to accommodate the difference in shortening by acting as a detachment. Such variations in shortening also exist between the Marra Mamba Iron Formation and the Brockman Iron Formation, indicating a detachment within the intervening Wittenoom Formation.

REFERENCES

Powell, C.McA., and Horwitz, R.C., 1994, Late Archaean and Early Proterozoic tectonics and basin formation of the Hamersley Ranges, Geological Society of Australia (WA Division) Excursion Guidebook, v. 4, 57 pp.

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Ca. 825 Ma MAFIC TO FELSIC IGNEOUS ACTIVITIES IN SOUTH CHINA: PART OF PLUME-INDUCED RIFTING EVENTS THAT LED TO THE BREAKUP OF RODINIA? Z. X. Li Tectonics Special Research Centre, Department of Geology and Geophysics University of Western Australia, Nedlands WA 6907 X. H. Li Institute of Geochemistry Chinese Academy of Sciences, PO Box 1131, Guangzhou, China P. D. Kinny Tectonics Special Research Centre Curtin University of Technology, Perth WA 6845 J. Wang Institute of Geology and Mineral Resources Chinese Academy of Geological Sciences, Chengdu, China Neoproterozoic mantle plume-related dyke swarms in Australia (Zhao et al, 1994), and ca. 780 Ma mafic intrusives in westem North America, have been interpreted as part of a giant plume-related radiating dyke swarm formed just before the breakup of supercontinent Rodinia (Park et al, 1995), thus supporting the SWEAT hypothesis. The plume centre is believed to be just east of the Adelaide Fold Belt (Zhao et al, 1994; Park et al, 1995). However, as pointed out by Wingate et al (1998), the age difference between the dyke swarms in Australia (827 ± 6 Ma and 824 ± 4 Ma for the Gairdner and Amata Dyke Swarms respectively — Glikson et al 1996; Wingate et al 1998) and the ca. 780 Ma mafic intrusives in North America make them unlikely to be part of the same radiating dyke swarm. On the other hand, Li et al (1995, 1996) suggested that South China could have been between Australia and Laurentia during Rodinia time, very close to the proposed plume centre. If both the plume-origin for the Australian dyke swarms (Zhao et al, 1994) and Li et a/.'s reconstruction are accepted, one would expect to find ca. 825 Ma mafic intrusives in South China. No major ca. 825 Ma mafic intrusive has been reported from South China. However, there were widespread granite intrusions of similar age. These granites have commonly been interpreted as of post-orogenic origin, although Li et al (1995, 1996) recognised that they intruded at the time of rift development possibly related to Rodinia breakup. During a field trip to the northern Guangxi region (section Y3 in figures 1 and 3 of Li et al 1996) we sampled mafic intrusives that intruded the tightly-folded Mesoproterozoic Sibao Group. They are possibly coeval with some ultramafic intrusives also intruding the Sibao Group, and both have been mapped as part of the Sibao Group by local geologists. The Sibao Group and the mafic-ultramafic intrusives were cut by granite bodies dated as 819 ± 9 Ma, 826 ±10 Ma and 824 ± 4 Ma (Li, in press, zircon U-Pb). All these rocks are unconformably covered by the Neoproterozoic Danzhou Group, a volcanoclastic rifting succession (Li et al, 1995; 1996). 42

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

SHRIMP zircon dating of four mafic intrusives all gives ages of ca. 825 Ma, which are in remarkable agreement with the ages of the Gairdner and Amata Dyke Swarms in Australia. These ages are also indistinguishable from those of the granites, although field relationships suggest that the mafic intrusives probably occurred slightly earlier than the granites. Although we can not yet rule out the possibility of a early Neoproterozoic orogeny between the Yangtze and Cathaysia Blocks in South China (further field and laboratory work are in progress), our preferred interpretation of the new results is that the mafic-ultramafic intrusives in South China are part of the same plumerelated giant radiating dyke swarm as the one suggested for the Australian dykes. The bimodal magmatic activities in South China, closely followed by a continental rifting event, are consistent with such an interpretation. If geochemical work (in progress) and palaeomagnetic work (planned for 1999) can further verify such an interpretation, that would leave little room to move about the close relationship between South China and Australia during the early Neoproterozoic. This would also suggest that the rifting which eventually broke apart Rodinia started at about 825 Ma. No ca. 825 Ma igneous activity has been reported for western Laurentia. This could mean that either such activity existed but is yet to be reported, or the relative position between Laurentia and Australia in the Rodinia configuration should not be as close as in the classical SWEAT connection (Dalziel, 1991; Hoffman, 1991; Moores, 1991). On the other hand, similar aged mafic-ultramafic rocks have recently been reported from both the southern (Izokh et al, 1998) and western (Volobuyev, 1994) margins of the Siberia Craton, which are believed to be adjacent to the northern end of Laurentia during the Neoproterozoic (e.g., Hoffman, 1991; Condie and Rosen, 1994; Pelechaty, 1996; Frost et aU 1998). If these rocks can be proved to be related to the same plume break-out, the favourable position for Siberia in Rodinia would be the one proposed by Frost et al (1998). ACKNOWLEDGMENTS This work was supported by the Australian Research Council through the Tectonic Special Research Centre and a QEII Fellowship and a small grant to Z.X. Li, and grants to X.H. Li from both the National Natural Science Foundation of China and the Chinese Academy of Sciences. We thank Dr S.S. Sun for discussions and providing valuable literature, and Dr J.E. Glover for proofreading the paper. REFERENCES

Condie, K. C. and Rosen, O. M., 1994, Laurentia-Siberia connection revisited. Geology, v. 22, p. 168170. Dalziel, I. W. D., 1991, Pacific margins of Laurentia and East Antarctica-Australia as a conjugate rift pair: evidence and implications for an Eocambrian supercontinent. Geology, v. 19, p. 598-601. Frost, B. R., Avchenko, O. V., Chamberlain, K. R., and Frost, C. D., 1998, Evidence for extensive Proterozoic remobilization of the Aldan shield and implications for Proterozoic plate tectonic reconstructions of Siberia and Laurentia, Precambrian Research, v. 89, p. 1-23. Glikson, A. Y., Stewart, A. J., Ballhaus, C. G., Clark, G. L., Feeken, E. H. J., Leven, J. H., Sheraton, J. W., and Sun, S. S., 1996, Geology of the western Musgrave Block, central Australia, with particular reference to the mafic-ultramafic Giles Complex, AGSO Bulletin 239, 206 pp. Hoffman, P. F., 1991, Did the breakout of Laurentia turn Gondwanaland inside-out?. Science, v. 252, p. 1409-1412. Izokh, A.E., Gibsher, A. S., Zhuravlev, D. Z., and Balykin, P. A., 1998, Sm-Nd dating of the ultramafic-mafic massifs of the eastern branch of the Baikal-Muya ophiolite belt, Doklady Earth Science, v. 359, p. 525-529. Li, X.H., in press, U-Pb zircon ages of granites from the southern margin of Yangtze Block and the

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998 timing of Neoproterozoic Jinning Orogeny in SE China: termination of Rodinia assembly?, Precambrian Research. Li, Z. X., Zhang, L., and Powell, C.McA., 1995, South China in Rodinia: part of the missing link between Australia-East Antarctica and Laurentia?, Geology, v. 23, p. 407-410. Li, Z. X., Zhang, L., and Powell, C.McA., 1996, Positions of the East Asian cratons in the Neoproterozoic supercontinent Rodinia. In Breakup of Rodinia and Gondwanaland and Assembly of Asia, Z. X. Li, I. Metcalfe, and C. McA. Powell (editors), Australian Journal of Earth Science, V. 43, p. 593-604. Moores, E. M., 1991, Southwest U.S. - East Antarctic (SWEAT) connection: a hypothesis, Geology, V. 19, p. 425-428. Park, J. K., Buchan, K. L., and Harlan, S. S., 1995, A proposed giant radiating dyke swarm fragmented by the separation of Laurentia and Australia based on paleomagnetism of ca. 780 Ma mafic intrusions in western North America, Earth and Planetary Science Letters, v. 132, p. 129139. Pelechaty, S. M., 1996. Stratigraphic evidence for the Siberia-Laurentia connection and early Cambrian rifting. Geology, v. 24, p. 719-722. Volobuyev, M. I., 1994, Riphean ophiolite complex of Yenisey Range, Geotectonics, v. 27, p. 524528. Wingate, M. T. D., Campbell, I. H., Compston, W., and Gibson, G. M., 1998, Ion micropobe U-Pb ages for Neoproterozoic basaltic magmatism in south-central Australia and implications for the breakup ofRodinia, Precambrian Research, v. 87, p. 135-159. Zhao, J.-X., Malcolm, M. T., and Korsch, R. J., 1994, Characterisation of a plume-related -800 Ma magmatic event and its implications for basin formation in central-southern Australia, Earth and Planetary Science Letters, v. 121, p. 349-367.

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METALLOGENY OF THE RODINIA SUPERCONTINENT Franco Pirajno Geological Survey of Western Australia, 100 Plain Street, East Perth WA 6004 Alasdair Cooke BHP Minerals, 3 Plain Street, East Perth WA 6004 INTRODUCTION The Rodinia Supercontinent was assembled between 1.3 and 1.0 Ga and fragmented between 750 and 600 Ma (Hoffman, 1991). The assembly of Rodinia involved the formation of juvenile crust, followed by intense orogenic activity and reactivation of pre-existing Archaean and Proterozoic terranes. METALLOGENY OF SUPERCONTINENT ASSEMBLY AND BREAKUP The origin of mineral deposits in relation to supercontinent assembly and breakup has been discussed by Barley and Groves (1992). These authors suggested that phases of continental breakup and active rifting are characterised by sedimenthosted base metal deposits, Mississippi Valley-type Pb-Zn deposits, and deposits related to anorogenic magmatism (e.g., Cu-U-REE-Au of Olympic Dam-style, or Cu-Ni-PGE, Ti-V in layered intrusions). During phases of continental assembly, which involve subduction, collision tectonics and island arc accretion, mesothermal or shear zone-hosted lode Au deposits, epithermal-porphyry systems and volcanogenic massive sulphides are also formed. In Rodinia between ca 1.3 and 0.6 Ga, we identify: 1) mineralisation in tectonic settings related to assembly of the supercontinent, and 2) mineralisation in tectonic settings related to breakup of the supercontinent. MINERALISATION ASSOCIATED WITH ASSEMBLY OF RODINIA In the Rodinia configuration suggested by Hoffman (1991), an intracontinental collisional and accretionary orogenic zone (nearly 20 000 km long) was formed during assembly of the supercontinent. This orogenic zone, informally named Rodinides (Fig. 1), includes the southern and eastern margins of the Yilgam Craton in Western Australia (Albany-Eraser orogen, Musgrave Block, Central Australian Terranes; Tyler et al, 1998), the eastern margin of the Dharwar craton (Eastern Ghats, India), Sri Lanka, Madagascar, the northern margin of East Antarctica, the margins of the Kalahari province (Namaqua-Natal orogen), the eastern side of the Congo Craton, the southern margins of Laurentia (Grenville orogen), and the northern margin of Amazonia (Rio Branco, Parecis, Sierra Pampeana, Patagonia, Pre-Cordillera orogenic zones). The Rodinides were largely the result of continental collisions and re-activation of pre-existing basement materials. Concomitant with or soon after collision, rifting occurred in a number of places, almost perpendicular to the collision line. An example of this is the Mid Continent Rift System in North America (Fig. 1), which is of Rodinian age (ca 1.2 Ga). The Belt Supergroup basin in Canada (Fig. 1), on the western margin of Laurentia, and its possible correlative in North Queensland (Blewett et al, 1998) may have formed in the same way. Mineral deposits that formed in the Rodinides include sedimentary-rock-hosted polymetallic massive sulphides (e.g.. Trilogy in the Albany-Eraser orogen and the Zn-Pb-Ag Sullivan deposit of the Belt Supergoup in Canada), the magmatic Cu-Ni45

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PGE in layered mafic-ultramafic intrusions (e.g., Voisey's Bay in Canada) and FeTi-V deposits in anorthosites. The latter are associated with a belt of 1.5-1.3 Ga anorogenic volcanic and plutonic rocks, that extends for thousands of km from California to Labrador, southern Greenland and Scandinavia (Windley, 1993). Metalliferous deposits in the Mid Continent Rift include Cu-Ag in tholeiitic lava flows of the Coppermine River Group in NW Canada and in the Lake Superior region (e.g White Pine). Also related to this rift are L29-1.27 Ga layered intrusions that host Cu-Ni-PGE mineralisation (e.g., Duluth and Muskox complexes). Uranium, Th, REE mineralisation in the Ilimaussaq L3-1.0 Ga alkaline complexes in South Greenland is related to a similar extensional episode. We speculate that the Giles Complex, Musgrave Block, in Western Australia contains most of the features common to intracontinental layered complexes and therefore has good potential to host Cu-Ni-PGE mineralisation. We conclude that most metallic mineral deposits associated with assembly were predominantly related to extensional settings. Large resources of W, U, REE and Mo are contained in the huge pegmatite belts in western South Africa-and Namibia (Orange River pegmatite belt; Namaqua orogen), Sri Lanka and Madagascar. In contrast to rift-related settings, these deposits were formed in continental collisional settings. METALLOGENY ASSOCIATED WITH THE BREAKUP OF RODINIA The breakup of Rodinia at ca 700 Ma was heralded by a number of rift zones (Fig. 1). A major rift zone split Laurentia, Baltica, Siberia and the South America blocks from East Antarctica, India, and Australia (Li et al, 1996). In Australia, the Adelaide Geosyncline is possibly the failed arm of this major rift zone (Fig. 1). This is supported by the NW-trending ca 800 Ma Gairdner dyke swarm (Zhao et ai, 1994). The Mt. Gunson Cu mineralisation in the Adelaide geosyncline is associated with carbonate and shale. Stratiform Cu mineralisation of the Yeneena Basin in Western Australia probably belongs to the same rift setting. Another failed rift system that could be linked to the breakup of Rodinia is the Lufillian orogen in central Africa, host to the stratiform copperbelt deposits (Cu-Co) of Zambia and the Republic of Congo. Similarly, the NE-trending intracontinental troughs and basins that form the Irumide belt, could also be related to the breakup. The Irumide belt extends from south-central Namibia, from where it may connect with the Namaqua Province, across northern Botswana to the northern margin of the Zimbabwe craton. In southcentral Namibia, the Irumide belt hosts the Cu-Ag Klein Aub and Witvlei stratabound deposits. In northern Botswana, a similar style of mineralisation is present at Lake N'Gami. Large mineral sands deposits were formed along several Rodinian rift margins. For example, along the east coast of South Africa (e.g., Richards Bay), Mozambique and eastern India. CONCLUSIONS The assembly and breakup of supercontinents constitute major tectono-magmatic episodes that span several hundred million years. Metallogeny is linked with these tectono-magmatic episodes, resulting in wide range of mineral deposits types, such as Cu-Ni-PGE in continental layered intrusions and hydrothermal sedimentary rockhosted polymetallic deposits. Geotectonic and supercontinent reconstructions enable linkage and comparisons of metallogenic provinces. The implications of 46

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these possible intercontinental links, in terms of mineral exploration can be farreaching. ACKNOWLEDGMENTS FP publishes with the permission of the Director of the GSWA. We thank Dr. S. Sheppard (GSWA) for his constructive criticism of this contribution. REFERENCES

Barley, M. E. and Groves, D. L, 1992, Supercontinent cycles and the distribution of metal deposits through time, Geology, v. 20, p. 291-294. Blewett, R. S., Black, L. P., Sun, S-S, Knutson, J., Hutton, L. J. and Bain, J. H. C., 1998, U-Pb zircon and Sm-Nd geochronology of the Mesoproterozoic of North Queensland: implications for a Rodinian connection with the Belt Supergroup of North America, Precambrian Research, v. 89, p. 101-127. Hasten and Fyon, 1992, Metallogeny of the Grenville province, Ontario Geological Survey Special Volume 4, p. 1217-1252. Hoffman, P. F., 1991, Did the breakout of Laurentia turn Gondwanaland inside-out?. Science, v. 252, p. 1409-1412. Li, Z. X., Zhang, L. and Powell McA, 1996, Positions of the East Asian cratons in the Neoproterozoic supercontinent Rodinia, Australian Journal of Earth Sciences, v. 43, p. 593-604. Myers, J. S., 1990, Albany-Fraser orogen, Western Australia Geological Survey, Memoir 3, p. 255264. Tyler, I. M, Pirajno, F., Bagas, L., Myers, J. S. and Preston, W. A., 1998, The geology and mineral deposits of the Proterozoic in Western Australia, AGSO Journal of Geology and Geophysics, v. 17, p. 223-244. Unrug, R. (ed.), 1996, Geodynamic map of Gondwana supercontinent assembly, Council for Geoscience, South Africa and Bureau des Recherches Geologiques and Minieres, France, Bureau des Recherches Geologiques and Minieres, Orleans. Windley, B. F., 1993, Proterozoic magmatism and its orogenic connections. Journal of the Geological Society, London, v. 150, p. 39-50. Zhao, J-X, McCulloch, M. T., Korsch, R. J., 1994, Characterisation of a plume-related --800 Ma magmatic event, and its implications for basin formation in central-southern Australia, Earth and Planetary Science Letters, v. 21, p. 349-367.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

Col It iQiaiaid Ktb lari/'o roge i f (Rodli ldef;:i Rrrt Bre^ip rrrt

Figure 1: Rodinia supercontinent assembly showing collisional and accretionary orogens, rift zones and margins: Amz Amazonia, Bal Baltica, Bs Belt Supergroup, Cng Congo, D Dharwar Craton, Ea East Antarctica, Ga Gawler, Kg Kaapvaal Craton, Lau Laurentia, Mcr Mid Continent Rift, Na North Australia, Nc North China, Nn Namaqua-Natal, Sib Siberia, Wa Western Australia, Waf West Africa; Based On Unrug (1996).

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

ASSEMBLY AND BREAK-UP OF RODINIA LEADING TO FORMATION OF GONDWANALAND C. McA. Powell Tectonics Special Research Centre, Department of Geology and Geophysics University of Western Australia, Nedlands WA 6907 By the early Mesoproterozoic, Australia had three broad, relatively stable regions, the North Australia, the Pilbara-Yilgam and the Gawler cratons, with active orogenesis along the eastern margins of the North Australia and Gawler cratons. This northerly-trending orogenic belt contained the Mt. Isa and Broken Hill metallogenic provinces, and formed through a complex series of basin openings and closures in the 1.7 to 1.4 Ga interval, during which world-class, base-metal deposits were formed. Precambrian Australia was probably largely assembled in its present configuration by 1.4 Ga, so that the Grenvillian orogenesis in Australia was more one of reactivation of zones of crustal weakness than closure of wide oceanic basins. By 1.07 Ga, movement on the Albany-Fraser-Musgrave belt had ceased, and mafic dykes were intruded in central Australia (Fig. 1). Final assembly of the Rodinia supercontinent took place during the late Mesoproterozoic, as recorded by 1300 to 1100 Ma orogenic belts which can be traced from northeastern Laurentia through west Antarctica and thence through India and Antarctica into the Albany, Fraser and Musgrave belts in Australia. The eastern margin of Australia in the Rodinia supercontinent lay along the Tasman Line, recently defined so clearly by AGSO aeromagnetic and gravity compilations. In the Rodinia supercontinent, Laurentia lay to the east of Australia, possibly with South China in between (Li et aU 1995; 1996). Palaeomagnetic data indicate that the Rodinian configuration of LaurentiaAustralia endured until -750 Ma, but that around this time Laurentia rotated away from from Australia-Antarctica (Fig. 2). In southeastern Australia, the time of the breakup is interpreted to coincide with the end of the Sturtian glaciation, after which a broad sag-phase transgression occurred (Powell et al, 1994). Before Rodinian breakup, the interior of Australia was covered by a broad epicontinental basin. Rifting and volcanic activity in northwesterly- and northerly-trending basins along what was to become the eastern margin of Neoproterozoic Australia occurred from -830 Ma until breakup. The 827-Ma NW-trending Gairdner dyke swam was emplaced during this early extension (Wingate et al, 1998). During breakup the extension direction was oriented NE-SW, implying that the rectilinear margin of Precambrian comprises a series of NE-trending transform and NW-trending rift margins (Fig. 3b). Analysis of the directions preserved suggests that opening was about a rotation pole at 126°E, 7°S, approximately 25° from the Adelaidean region where the rift margin is best preserved. The preservation of a wide rift margin in South Australia, which formed the base on which younger Adelaidean sediments were deposited, indicates that the South Australian segment of the Rodinian margin had lower-plate geometry. In Laurentia, the orientation of the rifting was NNW-SSE (Stewart, 1972), implying that a third continental fragment lay between Australia and Laurentia (Fig. 3b). Li et al. (1995, 1996) showed that on stratigraphic and preliminary palaeomagnetic grounds this block could have been South China. Mid-Neoproterozoic breakup along the Tasman Line truncated the Mesoproterozoic metallogenic zones, which raises the question of where missing 49

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pieces are. An 800-km extension of the Mt. Isa Belt is projected to have been lost along the northeastern South Australian rift margin, and a further 600 to 1000 km was lost along the rift segment south of Broken Hill. At least three possibilities for where these fragments are can be considered: the first is that the they are located in the conjugate margin of Laurentia, most likely to be in western Canada, the second possibility is that they are located in South China, and the third is that they lie in a ribbon of thinned continental crust (c.f., the present Lord Howe Rise) now embedded in a Phanerozoic fold belt. If the conjugate margin lies in western Canada, the missing pieces could lie buried beneath Phanerozoic deposits of the Rocky Mountains. In South China, the conjugate margin could be concealed under the widespread younger Yangtze carbonate platform. If the metallogenic zones are part of a thinned continental ribbon, they could lie in the basement to the Delamerian Fold Belt, having first separated from Australia in the late Neoproterozoic and later having been brought back during the Late Cambrian contractional phase. Palaeomagnetic work indicates that the Kalahari craton and Queen Maud Land of Antarctica lay adjacent to the eastern margin of Rodinia at -1,100 Ma (Gose et al, 1997). The question of where the Congo block lay at the end of the Mesoproterozoic is, however, an open question, as there are no palaeomagnetic constraints from the Congo at that time. Geological evidence suggests that there might have been an interval of continental extension leading to breakup in the Congo craton (Meert et al, 1995) and adjacent cratonic areas of Kenya around 800 Ma. The palaeolatitude derived from the -743 Ma Mbozi gabbro-syenite complex in the Congo craton lies well away from that predicited from the corresponding poles for India-Australia at that time, implying that the Mozambique ocean was at least latitudinally 30° to 40° wide at that time (Fig. 2b). Recent petrological and geochronological work in Madagascar has identified a number of -790-775 Ma gabbroic complexes with sleeves of coeval granite (Tucker et al, 1997), which might be high-level intrusions related to extension associated with the breakup of Congo from the Madagascar-India margin. A similar timing for continental breakup of the Kenya margin of the Congo craton is implied by Stem (1994). In this tectonic arrangement, the Congo could have been connected to Madagascar-India until -780 Ma, and then separated from that continent by 30° to 40° of latitude by 740 Ma, the time of emplacement of the Mbozi complex. At least some pelites in the Madagascar part of Mozambique ocean were deposited after 720 Ma, as shown by the discovery of -720 Ma detrital cores in zircons from granulite facies metapelites in southern Madagascar (Kroner et al, 1996). This part of the Mozambique Ocean may not have closed finally until -600550 Ma, the time of widespread metamorphism (Kroner et al, 1996; Tucker et a/., 1997). Gondwanaland assembled in the latest Precambrian or Early Cambrian by the closure of two major ocean basins (Rogers et al, 1995). The eastern one, the Mozambique Ocean, separated East Gondwanaland and the Congo-Kaapvaal craton, and closed along a late Neoproterozoic zone running through Madagascar and Sri Lanka into East Antarctica. In Antarctica, Queen Maud Land was part of the Congo-Kaapvaal craton to the west. The western line of oceans, the BrazilidePharusian Ocean, runs between the Amazonia-West African craton and Congo-San Francisco-Kaapvaal craton, and may not have closed until the end of the Neoproterozoic or earliest Cambrian. Africa and South America were thus not assembled as the continents we know today until Gondwanaland formed; instead.

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they were a series of continental blocks attached either to the eastern Laurentian margin and the western margin of India-Madagascar, or, possibly, as independent continental blocks separate from any other major continental blocks. REFERENCES Gose, W.A., Helper, M.A., Connelly, J., Hutson, P., Dalziel, LW.D., 1997, Paleomagnetic results and U-Pb isotopic ages from Coats Land, Antarctica: A test of the SWEAT hypothesis, Journal of Geophysical Research, v. 102(B4), p. 7887-7902. Kroner, A., Braun, L, and Jaeckel, P., 1996, Zircon geochronology of anatectic melts and residues from a high-grade pelitic assemblage at Ihosy, southern Madagascar: evidence for Pan-African granulite metamorphism. Geological Magazine, v. 133, p. 311-323. Li, Z.X., Zhang, L. and Powell, C. McA., 1995, South China in Rodinia: Part of the missing link between Australia-East Antarctic and Laurentia?, Geology, v. 23, p. 407-410. Li, Z.X., Zhang, L. and Powell, C. McA., 1996, Positions of the east Asian cratonic blocks in the Neoproterozoic supercontinent Rodinia, Australian Journal of Earth Science, v. 43, p. 593-604. Meert, J.G., Van der Voo, R. and Ayub, S., 1995, Paleomagnetic investigation of the Neoproterozoic Gagwe lavas and Mbozi complex, Tanzania and the assembly of Gondwana, Precambrian Research, v. 74, p. 225-244. Powell, C. McA., Li, Z.X., McElhinny, M.W., Meert, J.G., and Park, J.K., 1993, Paleomapetic constraints on timing of the Neoproterozoic breakup of Rodinia and the Cambrian formation of Gondwana, Geology, v. 21, p. 889-892. Powell, C. McA., Preiss, W.V., Gatehouse, C.G., Krapez, B. and Li, Z. X., 1994, South Australian record of a Rodinian epicontinental basin and its mid-Neoproterozoic breakup to form the PalaeoPacific Ocean, Tectonophysics, v. 237, p. 113-140. Rogers, J.J.W., Unrug, R., and Sultan, M., 1995, Tectonic assembly of Gondwana, Journal of Geodynamics, v. 19, p. 1-34. Radhakrishna, T. and Mathew, J., 1996, Late Precambrian (850-800 Ma) palaeomagnetic pole for the south Indian shield from the Harohalli alkaline dykes: geotectonic implications for Gondwana reconstructions, Precambrian Research, v. 80, p. 77-87. Stewart, J.H., 1972, Initial deposits in the Cordilleran Geosyncline: Evidence of a Late Precambrian (<850 m.y.) continental separation. Geological Society of America Bulletin, v. 83, p. 1345-1360. Stem, R.J., 1994, Arc assembly and continental collision in the Neoproterozoic East African orogen Annual Reviews of Earth and Planetary Science, v. 22, p. 319-351. Tucker, R.D., Ashwal, L.D., Haudke, M.J., and Hamilton, M.A., 1997, A geochronologic overview of the Precambrian rocks of Madagascar: a record from the Middle Archean to the Late Neoproterozoic, Proceedings of the UNESCO-IGCP-348/368 International Field Workshop on Proterozoic Geology of Madagascar, Gondwana Research Group, Osaka, Miscellaneous Publication No. 5, p. 99. Wingate, M.T.D., Campbell, I.H., Compston, W. and Gibson, G.M., 1998, Ion microprobe U-Pb ages for Neoproterozoic basaltic magmatism in south-central Australia and implications for the breakup of Rodinia, Precambrian Research, v. 87, p. 135-159.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup of Rodinia, Perth, September 1998

LATE M E S O P R O T E R O Z O I C ( 1 . 2 - 1 . 1 G a ) [J Graben ^ Trend line ^ Thrust * Late tectonic silicic magmatism

Georgetown Inlier

? Greater India

# #

Epicratonic Basin 1 . 3 - 1 . 1 Ga mobile belt Graton

Fig. 1. Tectonic configuration of Australia at the end of the Mesoproterozoic

1200

1100

1000

900 800 Age (Ma)

700

600

500

1200

1100

1000

900 800 Age (Ma)

700

600

500

Fig. 2. Palaeolatitudes for a point (50^N, 97^W) projected onto continental blocks from the Rodinia fit of Powell et al (1993), from 1,200 to 500 Ma. (a) Laurenda and Australia, and (b) Laurentia, Australia, India (Radakrishna and Mathew, 1996) and Congo. 52

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup of Rodinia, Perth, September 1998

Arabia

/^p

I \\ \

Fig. 3. Australasian palaeogeography (a) ~ 1,000 Ma, (b) ~ 750 Ma, and (c) -530 Ma

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

DEFORMATION MECHANISMS AND ISOTOPE SYSTEMATICS: IMPLICATIONS FOR THE ABSOLUTE DATING OF DEFORMATION Steven M. Reddy Tectonics Special Research Centre, School of Applied Geology Curtin University of Technology, Perth WA 6845 Graham J. Potts Department of Earth Sciences University of Liverpool, Liverpool L69 3BX, United Kingdom Absolute ages of individual deformation events are fundamental to our understanding of the rates of deformation processes and tectonic histories. However, in deformed materials, isotope systematics usually are difficult to interpret because the migration and consequential resetting of isotope systems may be a function of both deformation and temperature. The migration of isotopic species by diffusion is a strongly temperature-dependent process governed by an Arrhenius relationship. The extent of diffusive equilibration in a mineral is also related to the grain size. Deformation must therefore be an important consideration in the interpretation of isotopic data because it provides a means of changing grain size and therefore affecting isotope systematics. Relationships between diffusion, temperature and grain size in geological materials may be considered in terms of the mathematical approximation known as the "closure temperature" (Tc). Tc is the mean age of the age profile across a grain in a simple cooling system and corresponds to a temperature below which significant daughter isotopes accumulate in the sample. However, this approach does not take account of grain size modifications due to deformation or the implications that such changes would have for the correct data interpretation. Here we look at the ways in which different deformation mechanisms may modify grain size and thereby effect isotope systematics. We consider three different thermal scenarios with deformation taking place during a) the prograde heating path, b) at temperatures close to Tc during cooling and c) at temperatures below Tc. We restrict our discussion to the ^^Ar/^^As dating technique and have placed emphasis on the behaviour of argon in white mica. We have modelled these simple systems using a finite difference algorithm that simulates argon diffusion profiles and bulk ages. Although we have restricted our analysis to a specific isotope system and mineral, our conclusions are general and will be equally applicable to other isotopic and mineral systems. Our results show that the range of deformation mechanisms in white micas has significant implication for grain size modification and the resetting of isotope systems during deformation. Simple modelling illustrates that in deforming rocks in which there is no change in grain size, it is difficult to establish true deformation ages. In the cases where deformation causes a change in grain size, it is important to characterise the temperature of deformation and the closure temperature of grains formed during the deformation. Without this information, distinction between cooling and deformation ages is equivocal. The development of grains with Tc greater than the deformation temperature may record a deformation age. This situation will arise in the following cases: i) neocrystallisation, ii) when grain size 54

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

reduction occurs at temperatures below Tc of the reduced grain size and the deformation mechanism has opened the grains, or iii) when deformation-induced grain coarsening has occurred. Since closure temperatures are also dependent upon cooling rate and diffusion parameters, these must also be known for a correct interpretation of the radiometric data.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

STRUCTURAL RELATIONSHIPS AND THE SYSTEMATIC ASSESSMENT OF DEFORMATION HISTORIES Steven M. Reddy

Tectonics Special Research Centre, School of Applied Geology Curtin University of Technology, Perth WA 6845

Graham J. Potts

Department of Earth Sciences University of Liverpool, Liverpool L69 3BX United Kingdom The systematics of deformation histories have been investigated using basic combinatorial mathematics. This approach enables the number of possible relationships associated with a given number of structures and the range of possible deformation histories to be calculated. Two different cases are considered: noncyclic and cyclic deformation histories. The former considers the case when structures develop only once through time. Cyclic deformation is the repeated development of structures, for example during a progressive deformation where structural elements are transposed, rotated and reworked. Our data allows the ambiguous nature of cyclic and non-cyclic deformation histories to be recognised and highlights the impact of unobserved relationships on the confidence that can be placed on any particular deformation history. Ambiguities in the assessment of deformation histories arise from two sources: firstly the difficulty in observing all structural relationships within an area, and secondly the fact that structural relationships in non-cyclic deformation histories are subsets of more complex deformation histories involving cyclic or repeated structural histories. The number of possible deformation histories that are compatible with a collection of age relationships is used as a crude measure of confidence in those histories. Finally we describe a systematic procedure for the construction of deformation histories. The advantages of this approach are that it is systematic, repeatable and rapid and also allows easy recognition of ambiguities in the structural data set.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

THE CENTRAL AFRICAN COPPERBELT: TECTONIC OVERVIEW, METALLOGENY AND ANALYSIS OF CURRENT RESEARCH John Simmonds XPLOR Pty Ltd, GPO Box 206, Floreat WA 6014 TECTONIC OVERVIEW The Central African Copperbelt (including the Zambian Copperbelt) comprises a 500 km long, 100 km wide arc (the 'Lufilian arc') of NeoProterozoic rocks in northcentral Zambia and the Katanga Province of the Democratic Republic of Congo. Formed initially within a major zone of intra-continental extension (the Katanga rift) that can be identified over >1500 km, the Copperbelt sequence developed its present fold and thrust belt character during development of the transcontinental Damara-Lufilian-Zambezi orogen [Hanson et al, 1998]. This complex orogenic system separates the Zaire, Bangweulu and Kalahari Cratons, overprinting the 13001100 Ma Kibaran and Irumide orogenic belts. Separated by the major -550 kmoffset Mwembeshi Shear Zone, the Lufilian-Zambezi Belt components have been termed the Katanga Orogen by Porada [1989]. Unrug [1987] suggests that the southern arm of the Katanga rift system lay in the Zambezi belt, the central arm constituted the nuclei of the Lufilian arm, while the northern arm of the rift, which extended NE at a high angle from the reentrant of the Lufilian arc, developed into the Kundelungu aulacogen. To the SW, the orogenic system shows increasing incorporation of an oceanic crustal component in the Damaran Belt. As an alternative interpretation of the relationship between the Zambezi and Lufilian arc segments, Hoffman [pers.comm.] suggests the potential for the Roan extensional basins to have developed within orogenic collapse structures related to the 0.8 Ga Zambezi Belt and its extensions: This scenario would be analogous to that noted from a number of foreland basin regions, such as between the Grenville Orogen and the Midcontinental Rift extension on the SW borders of Laurentia at 1300 Ma. The timing of incipient continental breakup recorded by the rift-fill sedimentation, followed by the extended, multi-phase orogenesis of the rift- and sag-basin sediments, is consistent with inclusion of the terrane into the broad system of Rodinia breakup. KATANGAN SEDIMENTATION AND SUBSEQUENT DEFORMATION Starting from 880-870 Ma, up to 10,000m of Katangan Sequence sediments were deposited within a system of linked, differentially-subsiding rift basins and subsequent sag basin. Differences in the original rift morphology and subsequent tectonic evolution of the Lufilian arc accounts for the variations in stratigraphy between regions and structural domains in the orogenic belt. Unconformably overlying deformed Proterozoic basement in Zambia , the Roan Supergroup grades from basal continental clastics, through a marine transgression to accumulations of shallow marine clastics and a mixed carbonate platform-hypersaline lagoon evaporitic sequence. In Katanga, the base of the thick Roan sequence is not generally seen; the thicker Roan sequence here is interpreted from stratigraphic data and the presence of significant intraformational breccias, to represent tectonic stacking of north-thrusted sheets [eg., Wendorff, 1998]. The Roan Group is unconformably overlain by carbonaceous shales, iron formations and pyroclastics of

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

the Mwashya Group. Within the Copperbelt, the Upper Roan and Mwashya Group sediments contain significant volumes of irregular sill-like gabbro ('amphibolite'). It is probable that these gabbros comprise part of a major bimodal magmatic province, localised by rift faults during deposition of the Kundelungu Supergroups. The overlying Lower Kundelungu and Upper Kundelungu Supergroups both have glacial sequences at their bases, terminated by cap carbonates. Recent speculation [e.g., Hoffman, 1998] has linked these low-latitude glaciations and termination cap carbonates to periods of major global cooling, triggered by massive carbon burial (and atmospheric CO^ draw-down) within the extensive subduction systems developed at the initiation of supercontinent breakup. Both of the Kundulungu Supergroups are characterised by significant differences in stratigraphic thickness and lithotypes of the marine carbonates and clastic sediments between the regional tectonic settings. The Katangan sediments were deformed and metamorphosed through to greenschist and amphibolite grade during the Lusakan (-850 Ma) and Lufilian (-650 Ma) Orogenies. REGIONAL METALLOGENY While the Central African Copperbelt is best known for its stratabound Cu-Co deposits, a number of other ore deposit types are also significant: U-(Co-Co-NiMo), Pb-Zn-(Cu-Ge-Ga), Cu-Au, massive barite, Cu-rich vermiculites and Au-PGE enrichments [Master, 1998]. In the Zambian sector of the Copperbelt, much but not all of the Cu-Co mineralisation is hosted by the 20m thick Ore Shale horizon and adjacent quartz arenites [Fleischer, 1976]. In Katanga, where there are two main stratiform Cu-Co horizons, most of the Roan is allochthonous and is bounded by breccias. The sulphide mineralisation comprises disseminated cc-bom-cpy-carrollite as dispersed grains, lenticles, fracture fillings, near-massive sulphidite lenses and replacements of pyrite, detrital oxides and silicates. Early theories on genesis of the stratiform Cu-Co ores focused on either the magmatic hydrothermal concept (relationship of the mineralisation to 'younger granites'), the syngenetic concept or to diagenesis. However, in the 1980's, investigations began to reveal the similarity in mineralogy and fluid inclusions between the stratiform Cu-Co mineralisation and the vein-type U deposits which occur in the same stratigraphic horizons, suggesting the role of metamorphic fluids as the mineralising agent. While accepting the role of primary sedimentary and diagenetic parameters in ore localisation, the relationship of the stratabound mineralisation to Lufilian-age faults and folds has been noted [e.g., Simmonds, 1980; Annels and Simmonds, 1984]. Ages of mineralising events for both stratabound Cu-Co and other ore types suggest a near-ubiquitous Lufilian to younger age for the mineralisation. These ages conform a syn-orogenic age for most of the deposits, consistent with the role of metamorphic fluids in their genesis. Given the relationship between primary sedimentary facies and active normal faulting, the potential for reactivation of early faults into later Lufilian thrusts, and the syn-orogenic ages determined for the mineralisation, examination of the thrust stacked Lufilian arc for evidence of tectonic controls on deposit localisation would appear warranted. As pointed out by Masters [1998], the region has undergone multiple mineralising events, which for different metals, happened at different times during the history of the basin. 58

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

EARTH SCIENCE RESEARCH: REGIONAL STATUS The political history of the Zambia-Zaire region has not been conducive to minerals exploration over the last 20 years. Nationalisation of the Zambian mines in the 1970's led to the closure of regional exploration programs in around 1976; on the mines, attention was focused on production at the expense of documentation and geologic research; in Zaire, a series of political upheavals triggered the dramatic reductions in production over the period; in both countries, political considerations within the parastatal operating companies, led to a decrease in capital investment on the mines and consequent progressive reductions in production capability. In consequence, the region in general and the deposits in particular have not seen the investigative attention that their importance would suggest. The situation is beginning to change but the legacy, in earth science research terms, is one of neglect. The situation has changed significantly over the last two years. Ownership changes at the operating mines have sent a breath of air to the earth scientists active in the region. Competition in regional exploration has led to regional-scale geophysical data acquisition and consequent interpretation of the regional geology and tectonics. Using the improvements in understanding of ore depositional concepts that have come in over the last two decades, old and new datasets are being analysed for their competitive edge in deposit targeting. Old deposits are being dusted off and reviewed for their economic potential under 1998 economics. During the compilation of data for this presentation, it became clear that the next phase of regional interpretation is already happening, but that we are going to have to wait for the confidentiality agreements to lapse sufficiently to allow for their presentation. There are going to be some interesting talks when the wraps do come off. REFERENCES

Annels, and Simmonds, J., 1984, Cobalt in the Zambian Copperbelt, Precambrian Research, v. 25, p. 75-98. Fleischer, V.D., 1976, Geology of the Zambian Copperbelt, m Handbook of stratabound and stratiform ore deposits, K.H. Wolf (ed.), Elsevier, Amsterdam. Hanson, R.E. et al., 1998, New geochronological constraints on the tectonic evolution of the PanAfrican Zambezi Belt, south central Africa, Abstract with Program, Gondwana-10 meeting. Cape Town. Hoffman, P. F., 1998, Personal communication, NSF final project summary. Masters, 1998, New developments in understanding the origin of the Central African Copperbelt, Abstr. Mineral Deposits Studies Group meeting, U Greenwich, 5-6 January 1998. Petters, 1991, Regional geology of Africa, Springer-Verlag, Berlin, 722 pp. Porada , 1989, Pan-African rifting and orogenesis in southern to Equatorial Africa and eastern Brazil, Precambrian Research, v. 44, p. 103-136. Simmonds, 1980, Significance of the Baluba orebodies with respect to Zambian Cu-Co mineralisation, unpublished Ph.D. thesis. University of Wales, Cardiff. Unrug, R., 1987, Geodynamic evolution of the Lufilian arc and the Kundulngu aulacogen, Angola, Zambia and Zaire, in Current research in African earth sciences, G. Matheis and H. Schandelmeier (editors). AA Balkema, Rotterdam, p. 117-120. Wendorff, 1998, Tectonostratigraphical significane of some sedimentary fragmentites in the Katangan foreland, Neoproterozoic, south Congo, Abstract with Program, Gondwana-10 meeting. Cape Town.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

EFFECTS OF SUBSIDIARY FAULTS ON THE GEOMETRIC RECONSTRUCTION OF LISTRIC NORMAL FAULT SYSTEMS Tingguang Song Tectonics Special Research Centre, School of Applied Geology Curtin University of Technology, GPO Box U1987, Perth WA 6845 Peter A. Cawood Tectonics Special Research Centre, School of Applied Geology Curtin University of Technology, GPO Box U1987, Perth WA 6845 The hangingwall rollover geometry of listric faults in extensional regimes can be used to estimate the shape and position of the fault at depth (Gibbs, 1983). This is particularly useful in petroleum-based seismic surveys where the deeper segments of the faults are either poorly imaged or unresolved. A variety of models have been proposed to reconstruct the fault geometry (Dula, 1991) with the inclined shear model considered the most reliable (White, 1992; Kerr and White, 1996). However, the effects of antithetic and synthetic faults within the hangingwall are either neglected (White, 1986) or mistreated (Davison, 1986) in these models. Such subsidiary faults disrupt the continuity of the deformed hangingwall horizons that are used to predict the main fault plane. The subsidiary faults may share extension (Williams and Vann, 1987) and hence displacement/heave with the master faults. The bed shapes and heave/displacement along the master faults are essential to calculate fault geometry at depth. Hence, the disruption of hangingwall beds and decrease of displacement/heave by subsidiary faults may significantly influence results of the fault geometry construction. We suggest that heave of individual subsidiary fault should be added to the main heave of the master fault (Fig. 1). If no faulting occurs in the hangingwall horizons, heave used for the reconstruction of the whole fault profile is constant and represented by "/i" in Figure la. Extension in the hangingwall side is normally greater than the heave and is equal to * (1+ tan0 * tana)" (Dula 1991; White 1992) where 0 is the known fault dip angle and a the simple shear angle. In situations where subsidiary faults are developed, heave needs to be determined for each individual fault bounded hangingwall block (Fig. lb). Thus, heave for segment be of the master fault is h, but for cd and the remainder of the fault is given by h+hi and h+hi+h2, respectively (Fig. lb). In the inclined shear model. White (1992) negates the effects of antithetic and synthetic faults by generating a continuous smooth bed shape across any subsidiary faults. We believe it is more reliable to leave the segment between two cut-off points blank, for example e & f or m &nm Figure lb, and to use the discontinuous bed to estimate the master fault plane at depth. In many cases the dip of the antithetic faults is the same as the inclined simple shear angle used in the inclined shear model. Thus the estimated fault shape should be a continuous plane as materials move along the inclined shear traces in these situations. However, synthetic faults may result in small gaps between each segments of the predicted fault shape. Block rotation by the small faults may also influence the bed geometry and hence the calculated master fault plane. But the influence of rotation by the subsidiary faults may be minimal because they are small in scale and blocks are mainly controlled by the master faults. Examples demonstrate this modified inclined shear method with variable-heave generates the master fault geometries at 60

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

depth which have better coincidence with the position of the fault plane delineated by seismic images (c.f., Bruce, 1973).

Regional reference

h-H

Regional reference •1h2r<-

(b)

Figure 1. Diagram shows effects of subsidiary faults on parameters for fault profile reconstruction, a). No subsidiary faults occur in the hangingwall of a Ustric fault F. b). Antithetic faults (Fi & F2) cut the hangingwall bed B. 6 is the dip angle of the upper segment of the main fault; a is the inclined simple shear angle which is assumed to equal to the dip angle of the antithetic fault.

REFERENCES Bruce, C. H., 1973, Pressured shale and related sediment deformation: Mechanism for development of regional contemporaneous faults, AAPG Bulletin, v. 57, p. 878-886. Davison, I., 1986, Listric normal fault profiles: calculation using bed-length balance and fault displacement. Journal of Structural Geology, v. 8(2), p. 209-210. Dula, J., W. F., 1991, Geometric models of listric normal faults and rollover folds, AAPG Bulletin, v. 75, p. 1609-1625. Gibbs, A. D., 1983, Balanced cross-section construction fron seismic sections in areas of extensional tectonics. Journal of Structural Geology, v. 5(2), p. 153-160. Kerr, H. G. & White, N., 1996, Application of an inverse method for calculating three-dimensional fault geometries and slip vectors. Nun River Field, Nigeria, AAPG Bulletin, v. 80(3), p. 432-444. White, N., 1992, A method for automatically determining normal fault geometry at depth. Journal of Geophysical Research , v. 97(B2), p. 1715-1733. White, N. J., Jackson, J. A., and McKenzie, D. P., 1986, The relationship between the geometry of normal faults and that of the sedimentary layers in their hanging walls. Journal of Structural Geology, v. 8(8), p. 897-909. Williams, G. & Vann, I., 1987, The geometry of listric normal faults and deformation in their hanging waUs, Journal of Structural Geology, v. 9, p. 789-795.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

OBLIQUE RIFTS: BASIN DEVELOPMENT AND TRAPS FOR HYDROCARBON ACCUMULATION IN WESTERN AUSTRALIA AND CHINA Tingguang Song Tectonics Special Research Centre, School of Applied Geology Curtin University of Technology, GPO Box U1987, Perth WA 6845

Peter A. Cawood Tectonics Special Research Centre, School of Applied Geology Curtin University of Technology, GPO Box U1987, Perth WA 6845 Oblique rifts occur when extension is not perpendicular to pre-existing faults. The influence of pre-existing structures on basin development and deformation varies depending on whether the effects of the stress field are localized along specific zones of crustal weakness, such as in the northern Perth Basin of Western Australia, or distributed across the basin, such as the Liaohe basin of northeastern China. The Perth Basin is one of the major physiotectonic features of Western Australia and is an established hydrocarbon province with 6 commercial fields. The basin formed during Permian to early Cretaceous rifting of Australia from India associated with Gondwana breakup. It is divisible into a series of fault-bounded blocks, the position and orientation of which are strongly controlled by the distribution and character of basement blocks. Obliquity between extension direction and pre-existing structures in the basin lead to the development of a transtensional environment involving both sinistral and dextral extension, and resulted in a complex and multi-cycle history of faulting and syn-sedimentary fault block movement. The eastern boundary of the basin is delineated by the Darling Fault which has a long and complex history of movement extending back until at least the Proterozoic. Reactivation and localization along specific structural zones such as the Darling Fault and environs resulted in concentrated rather than distributed zones of deformation within the basin. Hydrocarbon traps in the Perth Basin are structurally controlled, and include en echelon anticlines, inversion structures in restrained bends of the northerly striking faults, horst and tilted fault blocks, and roll-over anticlines. Extensive faulting within the cover sequence combined with localized nature of the deformation has limited the volume of traps for hydrocarbon accumulation within the basin. The Liaohe Basin is one of a series of Cenozoic rift-related basins developed along the NNE-striking Tan-Lu strike-slip fault system. This system constitutes a fundamental crustal suture in eastern China and shows evidence for a long and complex development with dextral and sinistral movement extending back until at least Early Paleozoic. The Liaohe Basin is elongated parallel to the Tan-Lu system and comprises two deep troughs separated by a central uplift. Each of these blocks is bounded by branch faults of the Tan-Lu system. Dextral movement on the TanLu fault during the Tertiary has resulted in distributed trans-tensional deformation and subsidence in the Liaohe Basin sedimentary succession. Reverse faults and inversion structures occur in the basin. En echelon anticlines, separated by transtensional faults, and fault-blocks are the main trap types within the basin which constitutes one of the major petroleum provinces in China.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup of Rodinia, Perth, September 1998

THE GEODYNAMIC MAP OF GONDWANA SUPERCONTINENT ASSEMBLY (IGCP PROJECT 288: GONDWANALAND SUTURES AND FOLD BELTS) Raphael Unrug

Wright State University, Dayton OH, USA e-mail: runmg@erinet.com

Christian Castaing

Bureau de Recherches Geologiques et Minieres, Orleans, France e-mail: castaing@brgm.fr

Jean-Louis Feybesse

Bureau de Recherches Geologiques et Minieres, Orleans, France e-mail: feybesse@brgm.fr

Pieter G. Gresse

Council for Geoscience, Bellville, South Africa e-mail: gscience@iafrica.com

C. McA. Powell

University of Western Australia, Perth WA Australia email: cpowell@geol.uwa.edu.au

Georg R. Sadowski

Universidade de Sao Paulo, Sao Paulo, Brazil e-mail: sadowski@usp.br

Luc Tack

Royal Museum for Central Africa, Tervuren, Belgium e-mail: ltack@africamuseum.be The Gondwana Supercontinent assembly resulted from reorganization of lithospheric plates and breakup of the precursor Rodinia Supercontinent. Rodinia assembled during late Mesoproterozoic (1300-1000 Ma) orogenies made up of Archaean and Paleoproterozoic rocks. Events in the 1000-725 Ma time bracket predated the final breakup of Rodinia and included widespread shearing and tectonic escape, post-tectonic magmatism, extension, rifting and intracratonic mobile belt formation. Geochronological and paleomagnetic data indicate Rodinia breakup at 725 Ma with the separation of Laurentia from east Australia-East Antarctica. The fragment of Rodinia comprising Australia, east Antarctica, India and part of Madagascar formed the stable nucleus of east Gondwanaland subject only to intracratonic deformation. The assumption that all continental crust was amalgamated into Rodinia result in the postulate that the remaining part of the Earth was covered by a global ocean of panthalassan size. The Rodinia breakup resulted in the formation of the Pacific ocean and three other oceans: the Adamastor Ocean in which the Damaran orogen was formed, the Pharusian Ocean that gave birth to the TransSaharan mobile belt exposed in the Tuareg shield of northern Africa, and the 63

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ANEKT Ocean (ANEKT is an acronym for Arabia-Nubian-Ethiopia-KenyaTanzania used for the East African Neoproterozoic ocean and orogen, to differentiate the latter from the Mozambique orogen of Mozambique proper, which is a Mesoproterozoic orogen reworked during the Neoproterozoic). In the process of West Gondwanaland assembly several lithospheric plates carrying large cratons, and a number of smaller lithotectonic terranes telescoped during closure of these ocean basins, and docked sequentially into the tectonic collage growing westward out of the east Gondwanaland nucleus. The magmatic arcs and exotic terranes assembly is well documented in the Arabian-Nubian shield of northeastern Africa and in the Tuareg shield of north-central Africa. The concept of a single collision between east and West Gondwanaland in the Neoproterozoic appears thus to be an oversimplification. Moreover the term "Pan African-Brazilian orogeny" should be applied only to events in the 725-500 Ma interval postdating the breakup of Laurentia from Rodinia and the onset of mobility of plates and terranes distinguished in West Gondwanaland, but not to the earlier Neoproterozoic events dominated by extension in the early phases of Rodinia breakup. The Pan AfricanBrazilian orogeny that shaped the agglomeration of Gondwanaland began with reorganization of crustal plates resulting from the breakup of Rodinia. Widespread late- to post-orogenic magmatism, shearing and tectonic escape, and major uplift leading to deep exhumation of the mobile belts are salient features of Gondwanaland assembly, generally attributed to a major thermal event resulting in isotopic resetting and lasting into the Cambrian-Ordovician. The Gondwanaland assembly map shows Archean-Paleoproterozoic cratons as passive blocks, without details of structure. The Mesoproterozoic mobile belts— remnants of the sutures of the Rodinia supercontinent and the Neoproterozoic mobile belts—the sutures of Gondwanaland are shown in as much detail as practicable at the map scale. The Phanerozoic cover is a convenient fig leaf covering our ignorance of basement structure of large areas. Selected important boreholes reaching the basement are shown. Only in Australia the continent-scale use of geophysical techniques allowed us to see through the cover to glean the broad-brush picture of basement blocks.

REFERENCES

R. Unrug, editor, and sixty-seven co-authors, 1996, The Geodynamic Map of Gondwana Supercontinent Assembly, scale 1:10 million, four sheets, dimensions 1,950 x 1250 mm, 19 colors. Publisher: Bureau de Recherches Geologiques et Minieres, Orleans, Price US $50, Orders: Editions BRGM, Ave Claude Guillemin, BP 6009 F45060 Orleans, Cedex 2, France Fax: 33-2-3864-36-82, or Director, Council for Geoscience, Private Bag XI12, Pretoria 0001, South Africa, attn: Information Division, Fax: 27-12-841-1221.

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SUPERCONTINENT RECONSTRUCTIONS: LESSONS LEARNED FROM THE GEODYNAMIC MAP OF GONDWANA SUPERCONTINENT ASSEMBLY APPLICABLE TO RECONSTRUCTION OF RODINIA Raphael Unrug Department of Geological Sciences Wright State University, Dayton OH 45435 Fax: +937 775 3562, E-mail: runrug@erinet.com ABSTRACT Supercontinents exist for relatively brief periods and self-destruct by heat buildup under the blanketing continental crust and/or heat advection by mantle plumes. Supercontinent breakup, dispersal and formation of new oceans overlap in time but not in space with assembly of a successor supercontinent. The assembly of Gondwanaland and dispersal of Rodinia proceeded concurrently during the interval 1000 - 650 Ma. To understand the assembly of Rodinia it will be necessary to glean the geodynamic events affecting the preRodinian Archean-Paleoproterozoic cratons. THE GEODYNAMIC MAP OF GONDWANA SUPERCONTINENT ASSEMBLY Editing the Geodynamic Map of Gondwana Supercontinents Assembly produced by researchers cooperating in IGCP Project 288 (Unrug, 1996a, b; 1997), required a judicious choice of events that could be reasonably shown at the map scale. At an early phase of the project it became apparent that the Neoproterozoic events of Gondwanaland assembly were coeval with the breakup of the end-Mesoproterozoic Rodinia. We incorporated the relevant part of Rodinian heritage into the assembly of Gondwanaland, showing Rodinian cratons and orogens in the Gondwanaland configuration. The Gondwanaland assembly map shows Archean - Paleoproterozoic cratons that rode the moving lithospheric plates during the Neoproterozoic in outline only, with no details of the older structure. The Mesoproterozoic and Neoproterozoic mobile belts are shown in as much detail as practicable. Ages of rock units at Supergroup, group, suite and complex rank are grouped in two-geon (200 Ma) intervals shown in color. Special symbols indicate the location of small igneous intrusions, dismembered ophiolitic sequences, eclogites, mafic to ultramafic metamorphic tectonites, glaciogenic sediments and regions of thermal rejuvenation, important for interpretation of Geodynamic events but too small to be shown cartographically. Sedimentary rocks were shown using symbols indicating the tectonic classification of basins. Other groups of symbols show classifications of igneous suites, metamorphic facies plate-margins and structural and tectonic details. RODINIA BREAKUP There is a broad consensus on the assembly ofRodinia in the 1.3-1.0 Ga interval. The reorganization of plate motions following the assembly of the supercontinent started shortly thereafter, as indicated by early events in Rodinia breakup. A widespread diachronous rifting event documented at 1.0 Ga in the Transamazonian (2.0-1.8 Ga) crust in the Borborema province of NE Brazil (Brito Neves et al, 65

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

1996), extended into the Archean and Ebumean (2.1-1.8 Ga) crust of Nigeria (Dada 1998), and Cameroon at 0.8 Ga (Van Schmus et al, 1991). In the ANEKT (acronym for Arabia-Nubia-Ethiopia-Kenya-Tanzania) Mobile Belt of Eastern Africa, at the eastern margin of the Tanganyika shield of the Congo craton, passive margin development, ophiolite emplacement, migmatitization, and metamorphism attributed to early collision are dated at 0.82 Ga (Shackleton 1986; Mosley, 1993). The nearest recognized rifting events are in the north in the Nubian shield at 0.870.84 Ga and in the Arabian shield at 0.88 Ga (review in: Stem, 1994). The separation of the Congo and East Sahara cratons from Rodinia could start earlier. Between the Congo and the Kalahari cratons rifting was active in the 1.1-0.95 Ga interval in a narrow deformed corridor (Unrug, 1997). The anticlockwise rotation of the Kalahari craton with regard to the Congo craton started later, in the 0.820.77Ga interval (review in Unrug, 1996). The position of the Kalahari craton shown by Weil et al. (1998), can be accepted for a period postdating their paleomagnetically analyzed time interval of 1.1 to 0.8 Ga. All these early events in Rodinia breakup (only a few examples are listed above) predate the major split of the supercontinent between East Australia, South China, East Antarctica and Laurentia at 0.745 Ga (Powell, et al, 1994), and formation of the Pacific ocean. Before the breakout of Laurentia from the Rodinia collage the Early Neoproterozoic ocean was probably panthalassan in size. Juvenile continental crust formation and arc collisions recognized in the northern ANEKT orogen (the Arabian-Nubian shield), in the Trans-Saharan orogen (the Tuareg shield), and at the margins of the South American Amazonian (Goias massif), and Rio de la Plata (Pelotas batholith) cratons dated in the 0.93-0.75 Ga interval indicate oceanic plate motion and subduction in this ocean (Unrug, 1996; 1997). Siberia linked to Laurentia prior to 1.0 Ga on geological evidence (Condie and Rosen, 1994), moved to an equatorial position by 0.75 Ga (Smethurst et al, 1988). Rifting around Siberia is reported to begin in the latest Mesoproterozoic (Khain et al, 1997). Three large oceans opened and closed during the assembly of Gondwanaland (Rogers et al, 1995). RODINIA ASSEMBLY The Rodinia reconstruction shown in Fig. 1 combines the paleomagnetically controlled model (Weil et al, 1998) with modifications based on tectonic and geochronological data and the proposed location of the South China and North China cratons (Li et al, 1996). Additional paleomagnetic and geochronological data will be necessary for refining and constraining the Rodinia assembly model. Understanding the assembly of Rodinia will require addressing the questions of Late Mesoproterozoic mobile belts continuity, opening and closing of oceans, and reorganization of Archaean Paleoproterozoic cratons. The global orogeny at 2.1-1.8 Ga included the Trans-Amazonian orogens of South America, the Ebumean orogens of Africa, the several orogens of Laurentia, the Svecofennian orogen of Baltica, and the orogens suturing the West Australian, South Australian and North Australian cratons (Myers, 1993; Myers et al, 1994). These is strong evidence of widespread juvenile crust formation and assembly of several large continents, each including a number of Archaean nuclei, (Ledm et al, 1994; Rogers, 1996). Did this global orogeny produce a pre-Rodinia supercontinent? Early Mesoproterozoic arc accretion (e.g., the Rio Negro-Juruena accretionary belt at the western margin of the Amazonian craton), and extensional intracratonic rifting, volcanism, e.g., in the Amazonian craton, (Brito Neves et al, 1996) and magmatism, e.g., the post 1.85 Ga

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

granitoid plutonism of Baltica, (Nironen, 1997) are widespread in the 1.7-1.5 Ga interval. Did the late Paleoproterozoic to earliest Mesoproterozoic extension break up a supercontinent and produce the crustal building blocks of the Rodinia puzzle? PROBLEMS NEEDING TO BE ADDRESSED Many Mesoproterozoic, Neoproterozoic and Phanerozoic mobile belts contain small terranes of older Mesoproterozoic rocks. Examples include the Andean terranes in Chile and Venezuela (Brito Neves et al, 1996), the Ribeira-Mantiqueira tectonic province of south America (review in Trompette, 1994), and the recently discovered puzzling Chewore ophiolite in the Zambezi belt of northern Zimbabwe (Oliver et al, 1988) dated at 1.39 Ga. The enormous complexity of lithotectonic terranes of the late Mesoproterozoic mobile belts suturing Rodinia can be appreciated from the study of the Grenvillian mobile belt (Rivers, 1997). The Mesoproterozoic magmatic rocks comprise petrologically peculiar types, e.g., the collision related anorthosite-mangerite-chamockite-granite complexes of the Grenville orogen (Corrigan and Hanmer, 1997), and the widespread anorogenic rapakivi and tin-bearing granites present in western Amazonia and in Baltica. Both the older inliers and the characteristic Mesoproterozoic magmatism need to be incorporated in a general model for Rodinia. Geophysical methods permitting to see the basement units through the masking younger platform covers (Shaw et aL, 1994) would contribute much to Rodinia reconstruction. Lithospheric mantle delamination and differentiation of the thermal boundary layer and mechanical boundary layer in the lithospheric mantle were proposed as controls of post tectonic uplift, extension and escape tectonics, and post-orogenic magmatism, accompanied by regional thermal rejuvenation of mineral ages (Black and Liegeois, 1993). These new ideas may be useful on constraining the Rodinia assembly model. Other aspects of Rodinia studies should include the emerging new ideas on processes controlling supercontinent breakup. Numerical models of self-consistent generation of tectonic plates (Tackley, 1998) offer interesting possibilities for testing by tectonic data. Pericratonic mobile belts are repeatedly reactivated and often contain recognizable terranes generated during several orogenic events. This was recently attributed to mechanical anisotrophy of lithospheric mantle controlled by tectonic fabric in mantle rocks and anisotropic strength of olivine (Vauchez et al, 1997). REFERENCES Black, R., and Liegeois, J.P., 1993, Cratons, mobile belts, alkaline rocks and continental .lithospheric mantle: The Pan African testimony, Journal of the Geological Society, London, v. 150, p. 89-98. Brito Neves, B.B., Winge, M., Cameiro, M.A., 1996, Orogeneses precedentes e tafrogeneses succedendo Rodinia na America do Sul, Boletim Instituto Geociencias, Universidade de Sao Paulo, Serie Cientifica, v. 27, p. 1-40. Condie K.C., and Rosen, O.M., 1994, Laurentia-Siberia connection revisited: Geology, v. 22, p. 168170. Corrigan, D., and Hanmer S., 1997, Anorthosites and related granitoids in the Grenville orogen: a product of convective thinning of the lithosphere?. Geology, v. 25, p. 61-64. Dada, S.S., 1998, Crust-forming ages and Proterozoic crustal evolution in Nigeria: a reappraisal of current interpretations. Precambrian Research v. 87, p. 65-74. Khain, V.E., Gusev, G.S., Khain, E.V., Vemikovsky V.A., and Volobuyev M.L, 1997, Circum Siberian Neoproterozoic ophiolite belt, Ofiolity, v. 22, p. 195-200. Ledru, P., Johan V., Milesi, J.P., Tegyey, M., 1994, Markers of the last stages of the Paleoproterozoic collision: evidence for a 2 Ga continent involving circum-south Atiantic provinces, Precambrian Research, v. 69, p. 169-191. Li, Z. X., Zhang, L., and Pov^ell, C.McA., 1996, Position of the East Asian cratons in Neoproterozoic

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998 supercontinent Rodinia, in Z.X. Li, I. Metcalfe, and C.Mc.A. Powell, (eds.), Breakup of Rodinia and Gondwanaland and Assembly of Asia, Australian Journal of Earth Sciences, v. 43, p. 1-20. Mosley, P., 1993, Geological evolution of the lata Proterozoic "Mozambique Belt" of Kenya, Tectonophysics, v. 221, p. 223-250. Myers, J. S., 1993, Precambrian history of the West Australian craton and adjacent orogens. Annual Review of Earth and Planetary Sciences, v. 21, p. 453-485. Myers, J. S., Shaw, R.D., and Tyler, I.M.,1994, Proterozoic tectonic evolution of Australia, Geological Society of Australia, Abstracts No. 37, p. 312. Nironen, M., 1997, The Svecofennian Orogen: a tectonic model, Precambrian Research v. 86, p. 2144. Oliver, G.J.H., Johnson, S.P., Williams, I.S., and Herd, D.A., 1988, Relict 1.4 Ga oceanic crust in the Zambezi Valley, northern Zimbabwe: evidence for Mesoproterozoic supercontinental fragmentation. Geology, v. 26, p. 571-573. Powell, C.McA., Li, Z.X., McElhinny, M.W., Meert, J.G., and Park, J.K., 1994, Paleomagnetic constraints on timing of the Neoproterozoic breakup of Rodinia and the Cambrian formation of Gondwana, Geology, v. 22, p. 889-892. Rivers, T., 1997, Lithotectonic elements of the Grenville Province: review and tectonic implications, Precambrian Research, v. 86, p. 117-154. Rogers, J.J.W., 1966, A history of continents in the past three billion years, Journal of Geology, v. 104, p. 91-107. Rogers, J.J.W., Unrug, R., and Sultan, M., 1995, Tectonic assembly of Gondwana, Journal of Geodynamics, v. 19, p. 1-34. Shackleton, R.M., 1986, Precambrian plate tectonics of eastern Gondwana, Societa Geologia Italiana, Memorie, v. 31, p. 343-350. Shaw, R.D., Wellman, P., Whitaker, A.J., Tarlowski, C., Morse M.P., 1994, Implications of the basement tectonic elements map of Australia for reconstruction of Proto-Gondwanaland, Geological Society of Australia, Abstracts No. 37, p. 401. Smethurst, M.A., Khramov, A.N., Torsvik, T.H., 1998, The Neoproterozoic and Paleozoic paleomagnetic data for the Siberian Platform: from Rodinia to Pangea, Earth Sciences Review, v. 43, p. 1-24. Stem, R.J., 1994, Arc assembly and continental collision in the Neoproterozoic East African Orogen: Implications for the consolidation of Gondwanaland, Annual Review of Earth and Planetary Sciences, v. 22, p. 310-351. Tackley, P.J., 1998, Self-consistent generation of tectonic plates in three-dimensional mantie convection. Earth and Planetary Science Letters, v. 157, p. 9-22. Trompette, R., 1994, Geology of western Gondwana, Balkema, Rotterdam, The Netherlands, 350 pp. Unrug, R., editor, 1996a, The geodynamic Map of Gondwana Supercontinent Assembly, scale 1:10 million, four sheets. Bureau de Recherches Geologiques et Minieres, Orleans. Unrug, R., 1996b, The Assembly of Gondwanaland, Episodes, v. 19, No. 1-2, p. 11-20. Unrug, R., 1997, Rodinia to Gondwana: The Geodynamic Map of Gondwana Supercontinent Assembly, Geology, v. 1, p. 1-6. Van Schmus, R.R., Toteu. S.F., Brito Neves, B.B., Hackspacher, P.C. and Dia, A., 1991, Early and Middle Proterozoic events in pre 600-Ma crust of northwestern Gondwanaland, Eos, Transactions of the American Geophysical Union, v. 72, p. 299. Vauchez, A., Barruol, G., Tommasi A., 1977, Why do continents break-up parallel to ancient orogenic belts?. Terra Nova, v. 9, p. 62-66. Weil, A.B., Van der Voo, R., Mac Niocaill, C.M, and Meert J.G., 1998, The Proterozoic supercontinent Rodinia: paleomagnetically derived reconstructions for 1100 to 800 Ma, Earth and Planetary Science Letters, v. 154, p. 13-24.

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The Assembly and Breakup of Rodinia, Perth, September 1998

End Mesoproterozoic Rodinia assembly - 1000 Ma

Figure 1. Rodinia reconstruction combining paleomagnetically controlled model (Weil et a l , 1998) with modifications based on tectonic and geochronologic data and the proposed location of the South China and North China cratons (Li et a l , 1996).

GEOLOGICAL SOCIETY OF AUSTRALL\, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

ISOTOPIC AND PALEOMAGNETIC CONSTRAINTS ON THE TIMING OF NEOPROTEROZOIC BREAKUP OF RODINIAN AUSTRALIA Michael T.D. Wingate Tectonics Special Research Centre, Deptartment of Geology and Geophysics, University of Western Australia, Nedlands WA 6907 Mafic igneous rocks in central and southern Australia record extension and rifting during the initial stages of basin formation that preceded breakup along the Neoproterozoic margin of eastern Australia (Crawford and Hilyard, 1990; Preiss, 1990; Powell et al, 1994). Ion microprobe U-Pb analyses of baddeleyite and zircon yield crystallisation ages of 827 ± 6 Ma and 824 ± 4 Ma for the Gairdner dyke swarm, and 827 ± 9 Ma for the Little Broken Hill gabbro, the latter emplaced during extension in the eastern Willyama Inlier (Glickson et al, 1996; Wingate et al, 1998). These ages confirm the synchronous nature of Willouran magmatism across most of south-central Australia, and indicate that rifting in the 'Adelaide Geosyncline' and adjacent platforms commenced at about 830 Ma. Powell et al (1994) proposed that the Sturtian glaciation in Australia preceded or accompanied continental separation. If so, an older limit to the time of breakup is provided by a U-Pb zircon age of 777 ± 7 Ma (W. Preiss, pers. comm., 1998) for the Boucaut Volcanics at the base of the Burra Group, which underlies the Sturtian rocks. Ion microprobe U-Pb dating of zircon and baddeleyite demonstrate that the Mundine Well dyke swarm (MDS) of the Pilbara Craton, Westem Australia, was emplaced at 755 ± 3 Ma and is equivalent to dykes of the Northampton Inlier. Combining new paleomagnetic data for the MDS with existing data for the Northampton dykes (Embleton and Schmidt, 1985), yields the first paleopole (136°E, 47°N, A95 = 5°) for the Australian Neoproterozoic for which the age of magnetisation is known accurately and precisely. In the Rodinia reconstruction of Powell et al (1993), however, the MDS paleopole lies 30^ away from the 780 to 740 Ma segment of the Laurentian APW path. Therefore, if the Rodinian connection between Australia and Laurentia is correct, then breakup between these two continents occurred before 755 Ma. REFERENCES

Crawford, A.J. and Hilyard, D., 1990, Geochemistry of Late Proterozoic flood basalts, Adelaide Geosyncline, South Australia, Geological Society of Australia, Special Publication 16, p. 49-67. Embleton, B.J.J., and Schmidt, P.W., 1985, Age and significance of magnetisations in dolerite dykes from the Northampton Block, Westem Australia, Australian Journal of Earth Sciences, v. 32, p. 279-286. Glikson, A.Y., Stewart, A.J., Ballhaus, C.G., Clarke, G.L., Feeken, E.H.J., Leven, J.H., Sheraton, J.W., and Sun, S.-S., 1996, Geology of the westem Musgrave Block, central Australia, with particular reference to the mafic-ultramafic Giles Complex, Australian Geological Survey Organisation, Bulletin 239, 206 pp. Powell, C. McA., Li, Z.X., McElhinny, M.W., Meert, J.G., and Park, J.K., 1993, Paleomagnetic constraints on timing of the Neoproterozoic breakup of Rodinia and the Cambrian formation of Gondwana, Geology, v. 21, p. 889-892. Powell, C. McA., Preiss, W.V., Gatehouse, C.G., Krapez, B., and Li, Z.X., 1994, South Australian record of a Rodinian epicontinental basin and its mid-Neoproterozoic breakup (-700 Ma) to form the Palaeo-Pacific Ocean, Tectonophysics, v. 237, p. 113-140. Preiss, W.V., 1990, A stratigraphic and tectonic overview of the Adelaide Geosyncline, South Australia. Geological Society of Australia, Special Publication 16: p. 1-33. 70

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998 Wingate, M.T.D., Campbell, LH., Compston, W., and Gibson, G.M., 1998, Ion microprobe U-Pb ages for Neoproterozoic basaltic magmatism in south-central Australia and implications for the breakup of Rodinia, Precambrian Research, v. 87, p. 135-159.

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NEW LATE PRECAMBRIAN PALAEOMAGNETIC RESULTS FROM NORTH CHINA AND THEIR PALAEOGEOGRAPHIC IMPLICATIONS Shihong Zhang Department of Geology and Mineral Resources China University of Geosciences, Beijing 100083, China and Tectonics Special Research Center, Department of Geology and Geophysics University of Western Australia, Nedlands WA 6907 Zheng-Xiang Li Tectonics Special Research Center, Department of Geology and Geophysics University of Western Australia, Nedlands WA 6907 INTRODUCTION In order to determine the palaeopositions of the North China Block (NCB) during the Precambrian time, in particular to answer the question of whether NCB was part of the Neoproterozoic supercontinent Rodinia, palaeomagnetic evidence is much needed. However, data for this block are scarce, and often conflict with each other. Here we firstly present new palaeomagnetic results from the southern margin of the NCB, and then make a critical selection from previous results so to establish a preliminary apparent polar wander path (APWP) for NCB for the interval of 1300500 Ma. NEW RESULTS The sampling area is located in the Lushan County, western Henan Province. Sampling was carried out over a -2000 m thick, well exposed continental shelf succession, ranging from the upper Jixianian to the lower Cambrian. Precambrian samples were mainly from purple, fine grained sandstones, muddy sandstones, siltstones and muddy dolomite. Cambrian samples were from grey sandstones and grey to purple muddy carbonates. The strata tilts gently toward the south-east, with no penetrative deformation observed at outcrops. Sandstones in different formations bear abundant glauconite which gives K-Ar ages between 1194 Ma and 650 Ma, generally following the stratigraphic order. The preservation of glauconite also indicates that there has been no major thermal event in the region after the deposition of the rocks. Samples were analysed in the Palaeomagnetic Laboratory of the University of Western Australia using the 2G-Enterprise three-axis cryogenic magnetometer. A majority of the 336 oriented core samples were subjected to stepwise thermal demagnetisation, with step sizes ranging from as large as 150°C at lower temperature, to as small as 5°C at around 570°C and above. A few grey limestone and sandstone samples were demagnetised using thermal plus AF demagnetisation treatments. Eleven different directions, defined as components A-K, have been isolated along the succession (Table 1). Components A represents the recent field overprint. It occurs in all the formations studied. Component B represents an early Mesozoic overprint, which is mainly found in carbonate rocks. Components B is probably of chemical origin. Components C to J, each from different stratigraphic units, have 72

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all been interpreted as of primary origin because (1), they are all stable components after the removal of low temperature components such as components A and B, (2), their corresponding pole positions, either in situ or tilt-corrected, are removed from known NCB pole positions from younger rocks, and (3), these components, coming from different stratigraphic levels along the more-or-less monoclinal succession, are different from each other despite their sometime similar lithology. Table 1. Palaeomagnetic results from the Lushan area (33.8°N, 112.7°E) Formation Age CP N Decs Incs ks a95 Plong Plat Dm Dp Zhushadong Fm Early Cambrian C 7 153 11.6 13.3 17.2 150.7 -42.8 17.5 8.9 XinjiFm Early Cambrian C 4 154.2 16.3 22.1 20 147.6 -41.3 20.6 10.6 DongjiaFm ca. 650Ma D 15 185.3 -14.2 7.5 14.9 101 -63 15.2 7.8 Sanjiaotang Fm ca. 900Ma E 15 223.5 -12.5 15.6 10 47 -41.3 10.2 5.2 CuizhuangFm ca. 950Ma F 19 43.3 27.8 65 4.2 217.8 46.6 4.6 2.5 Beidajian & Beicaoping ca. 1200Ma G 18 355.9 28.7 23.1 7.3 305 71.1 8 4.4 Yunmengshan U. ca. 1200Ma H 8 337.8 35.6 33.5 9.7 352.7 65.8 11.2 6.5 YunmengshanM. ca. 1200Ma I 10 97.7 46.3 15.1 12.8 175.4 9.2 16.4 10.5 ca. 1200Ma J 9 113.5 1.3 5.9 23.1 188.5 -19 23.1 11.6 Yunmengshan L. Yunmengshan L. ca. 1270Ma K 5 207.5 64 15.8 19.8 93.7 -6.7 31.5 25.1 CP: components as discussed in the text. N: number samples used for calculation. Decs, lins, Ks, a95, Plong, Plat, Dm, Dp: Parameters of directions and palaeopole positions after tilt correction.

Component C from the Cambrian strata gives a palaeopole similar to that given by Zhao et al (1992) from the east part of the craton. Among the Precambrian results, component D of the Dongjia Formation, component E of the Sanjiaotang Formation, and component F of the Cuizhuang Formation are considered most reliable because they all possess dual polarities. Component G from the Beidajian and Beicaoping Formations is also well isolated and well grouped, but is dominated by one polarity. Results from the Yunmengshan Formation are complex. This formation is over 620m thick and has been lithologically divided into four members. The top three members have been sampled for this study. By descending order, component H is from the uppermost member of the formation. Nine samples were collected and data from eight of them were used for the mean calculation. The next member down yielded component I that passes a fold test at 99% confident level. However, the age of this meters-scale fold is uncertain. The lowest member sampled yielded components J, K, and A. We prefer component J over K to be primary because (1), its direction falls closer to that of component I, and (2), it possess mixed polarities. Component K is found in five samples only, and its origin is unclear. DATA SELECTION FOR THE NCB Based on both our new results and a critical selection of the previous data, we attempt to establish a Precambrian APWP for the NCB. During our data compilation, we first rejected all the results that have Q values (Van der Voo, 1990) less than two. Among the 62 remaining data sets, 33 were further rejected because they are believed to be post-Cambrian overprints. The rest of the results have been combined according to the ages of the rock units into 15 poles. Although we believe that these 15 poles are more reliable than the ones we have discarded, there are nonetheless still discrepancies among some of them. PALAEOGEOGRAPHIC IMPLICATIONS Results from all the rock units we have studied, with ages ranging from late Mesoproterozoic to Neoproterozoic, suggest palaeolatitude of 30° or lower for the 73

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

NCB. Similar low-latitude positions were also suggested by Sinian results from the Huaibei region by Fang et al (1983), and by some Jixianian (ca. 1200-1300 Ma) results from the Jixian type section by Zhang et al (1991). The low palaeolatitude positions suggested for the Neoproterozoic times are consistent with the palaeogeographic reconstructions given by both Li et al (1996) and Wang et al. (1997). Results by Lin (1988) from the Jixian section (ca. 1100-1000 Ma), however, indicate medium to high palaeolatitude (30°-60°) for the NCB. Although a preliminary APWP for the NCB can be suggested for the 1300-500 Ma interval, due to the limited number of data sets and poor age constraints for both the stratigraphic and magnetisation ages, it is yet immature to use it to compare with that of other continents. REFERENCES

Fang, D. J., Zhu, Z. W., and Guo, Y. B., 1983, Study on the paleomagnetism of upper Precambrian in northern Jiansu and correlation between the upper Precambrian strata in south and north China, Scientia Geologica Sinica, v. 4. p. 324-337. Li, Z. X., Zhang, L., and Powell, C. McA., 1996, Position of the East Asian cratons in the Neoproterozoic supercontinent Rodinia, Australia Journal of the Earth Sciences, v. 43, p. 593-604. Lin, J. L., 1988, Middle to Late Proterozoic paleomagnetic results from Jixian, Kexue Tongbao, v. 33, p. 207-210 (in Chinese). Van der Voo, R., 1990, The reliability of paleomagnetic data, Tectonophysics, 184, 1-9. Wang, H. Z., Li, X., Mei, S., and Zhang, S., 1997, Pangaea cycles, earth's rhythms and possible earth expansion, Proc. 30th International Geological Congress, Vol. 1, p. 111-128. Zhang, H. M., Zhang, W. Z., and Elson, D. P., 1991, Paleomagnetic study on Middle and Late Proterozoic rock in Jixian, North China, Acta Geophysica Sinica, v. 34, p. 602-615 (in Chinese with English abstract). Zhao, X. X., Coe, R. S., Liu, C., and Zhou, Y., 1992, New Cambrian and Ordovician paleomagnetic poles for the North China Block and their paleogeographic implications. Journal of Geophysical Research, v. 97, p. 1767-1788.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

TECTONOTHERMAL EVOLUTION OF THE MAYUAN TECTONIC UNIT IN THE CATHAYSIA BLOCK AND ITS BEARING ON THE ASSEMBLY OF RODINIA Guochun Zhao Tectonics Special Research Centre, School of Applied Geology Curtin University of Technology, GPO Box U1987, Perth WA 6001 Peter A. Cawood Tectonics Special Research Centre, School of Applied Geology Curtin University of Technology, GPO Box U1987, Perth WA 6001 SUMMARY Four metamorphic stages (Ml to M4) are recognized from the Mayuan tectonic unit in the Cathaysia block, southeast China. Ml is represented by mineral inclusions preserved within porphyroblasts, M2 represents the formation of the porphyroblasts and growth of matrix minerals, M3 is represented by the symplectic assemblage sillimanite + biotite + quartz, and M4 is shown by the retrogressive assemblage chlorite + muscovite + epidote. These assemblages and their P-T estimates define a isothermal-decompression clockwise P-T-D path involving initial crustal thickening followed by rapid exhumation and final cooling and are related to collision of the Cathaysia and Yangtze block during final assembly of Rodinia. INTRODUCTION New geological and geophysical data indicate collision of the Cathaysia and Yangtze blocks along the Jiangshan-Shaoxing suture occurred during the Jinningian (Grenvilhan) orogeny (Shui et al, 1988; Chen et a/., 1991; Zhao et al, 1994a; Li et aL, 1995, 1996). Based on tectonostratigraphic correlations and paleomagnetic constraints, Li et al. (1995; 1996) proposed that in the Mesoproterozoic, the Yantze and Cathaysia blocks lay between southeastern Australia and northwestern America and that Grenvillian age deformation within the blocks records the final amalgamation of East Gondwana with Laurentia and the assembly of Rodinia. In this extended abstract, we present metamorphic and deformational data for the Mayuan tectonic unit (Grenville-age) from the Cathaysia Block. We use these data to estimate the P-T conditions of metamorphism and define the P-T path, which along with lithological, structural and geochronological data documents the collisional history of the Cathaysia and Yangtze blocks and provides further information to constrain their position within Rodinia. DIVISIONS OF LITHOTECTONIC UNITS The Cathaysia Block can be divided into four tectonic units: Badu, Mayuan, Wuyi-Yunkai and Changle-Nanao (Zhao et aL, 1994a). Their regional distribution is shown in Fig. 1. The Badu tectonic unit comprises a supracrustal assemblage of felsic paragneisses, pelites, quartzites, calcsilicates, carbonates and mafic volcanic rocks, represented by the Badu Group and Chencai Group, along with pre- to syn-tectonic granitoid rocks. These rocks have experienced widespread upper amphibolite facies metamorphism and polyphase deformation (Zhao et aL, 1994b) which on the basis of geochronological data occurred at round 1800 Ma (Hu, 1991). 75

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

The Mayuan tectonic unit is represented by the Mayuan Group and its equivalents. It consists of felsic paragneisses, pelitic schists, greenschists, amphibolites, marbles, calcsilicates and quartzites and underwent three episodes of deformation. Geochronological data indicate a Mesoproterozoic age for the rock units with metamorphiam at c. 1000 Ma (Li, 1989). The Wuyi-Yunkai tectonic unit is represented by the Yunkai Group and its equivalents. It comprises dominantly graywackes, siltstones, phyllites and quartzites, with minor amounts of tuffs. Most rocks within the unit underwent lowgrade metamorphism and a single phase of deformation. The available isotopic data indicate that these rocks formed between the Neoproterozoic and Early Paleozoic and underwent metamorphism at 500^50 Ma (Li, 1989).

120 km Late Mesozoic to OenozoJc cover Changle-Manao tectonic init Wuyi-Yunkai tectonte unit Mayuan tectonic uilt Badu tectonic tnit Yangtze tlock

Fig. 1 The schematic map showing distribution of the Badu, Mayuan, Wuyi-Yunkai and Changle-Nanao tectonic units in the Cathaysia block.

The Changle-Nanao tectonic unit outcrops along the southern margin of the Cathaysia block (Fig. 1) and consists of sandstones, siltstones, argillites, quartzites and carbonates. All rocks within the unit only experienced very lower grade metamorphism and a single phase of deformation. Paleontological data and regional correlations indicate a Paleozoic and Early Mesozoic age for the rocks with metamorphism and deformation at c. 190 Ma (Li, 1989).

STRUCTURAL DEFORMATION AND METAMORPHIC STAGES

Three episodes of deformation (Di to D3) are recognized in the rocks of the Mayuan tectonic unit (Zhao et al, 1994b). Di is characterized by small rootless intrafolial folds (Fi) with the Si foliation restricted to mineral inclusions in garnet, staurolite and biotite porphyroblasts from pelitic schists. D2 is the dominant deformational event recognized and is represented by ubiquitous isoclinal folds of varying scale and a penetrative gneissosity (82) with associated lineation (L2) indicated by boudins and mullion structures. The final episode of deformation (D3) produced asymmetric upright folds (F3), axial planar cleavage (S3) and a mineral lineation ( L3), indicated by orientated sillimanite and hornblende. 76

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

Based on relations between growth of mineral phases and deformational episodes, four separate metamorphic stages (Mi to M4) are associated with the three deformational phases in the Mayuan tectonic unit. Ml is represented by the mineral phases defining the Si foliation and consists of garnet (core) + chlorite + muscovite + biotite + plagioclase + quartz for pelitic schists and chlorite + epidote + albite + quartz ± actinolite in mafic rocks. All these minerals occur as inclusions within porphyroblasts. M2 represents the formation of coarse porphyroblasts of biotite, garnet, staurolite, kyanite, actinolite and hornblende and the growth of matrix plagioclase, quartz, biotite, muscovite, graphite and chlorite. The matrix phases define the regional schistocity (S2). M3 is represented by the formation of sillimanite + quartz + biotite symplectites in pelitic rocks and the formation of hornblende + plagioclase assemblage in amphibolites, associated with sillimanitebearing schists. The symplectic assemblage in pelitic rocks may be produced by the generalized reaction: garnet + muscovite = 2 sillimanite + biotite + quartz. Sillimanite from pelitic schists and hornblende from amphibolites define the D3 mineral growth lineation (L3). The last metamorphic stage M4 resulted in a retrogression assemblage of mica + epidote + chlorite, replacing garnet, staurolite, kyanite, sillimanite and hornblende. These retrogressive minerals do not show a preferred orientation, suggesting that they formed after the final deformational episode (post-D3). METAMORPHIC P-T CONDITIONS The representative mineral assemblage of Mi in pelitic rocks is garnet (core) + chlorite + muscovite + biotite + plagioclase + quartz. Temperatures and pressures for this, as well as for the following stages, were obtained with the gamet-biotite geothermometer of Ganguly & Saxena (1984), and the geobarometer of Hodges & Crowley (1985). Six samples from the garnet and the staurolite zones give P-T conditions of 5.0-5.5 kbar and 390-480* C for Mi, based on the core compositions of garnet and inclusion minerals of biotite, muscovite and plagioclase within porphyroblasts. Based on the rim compositions of garnet and matrix plagioclase, biotite and muscovite, P-T conditions of M2 are 6.5-7.5 kbar and 500-580- C. The P-T conditions of the M3 assemblages in pelitic rocks cannot be estimated by using the above geothermobarometers because of the lack of garnet in sillimanite-bearing rocks. Instead, the muscovite-biotite geothermometer of Hoisch (1989) was used and gives temperatures ranging from 570-610* C. The P-T conditions of M3 can be also estimated from the amphibolites associated with sillimanite-bearing schists by using the homblende-plagioclase thermometer of Holland and Blundy (1994) and the NaM4 in amphibole barometer of Brown (1977). These geothermobarometers produced P-T conditions of 4.0-5.0 kbar and 590-630® C for M3. The temperatures of M4 can be semi-quantitatively estimated by using the chlorite thermometer of Zang and Fyfe (1995), giving temperature between 300400* C. The pressures of M4 cannot be quantitatively estimated because of the lack of suitable barometers. P-T-D PATH AND TECTONIC IMPLICATIONS Relations between mineral growth and deformational episodes, mineral chemistry and thermobarometric calculations for the Mayuan tectonic unit define an isothermal decompression-type clockwise P-T-D path as shown in Fig. 2. This P77

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup of Rodinia, Perth, September 1998

/ T / (fi ^ m

M1/01 '

py3/D3

200 300 400 500 600

700 SOO

Tempefalyre

Fig. 2

P - T - D path of the Jinningian lithotectonic unit in the Cathaysia block

T-D path suggests that the Mayuan tectonic unit has undergone an initial phase of cmstal thickening (Mi/Di + M2/D2), followed by near-thermal exhumation (M3/D3) and final cooling and retrogression (M4). This tectonic process generally is considered to be related to continental collisional environments (England and Thompson, 1984). The collision caused the prograde metamorphism (Mi) to peak metamorphism (M2) phases. Following the peak metamorphism, the thickened crust underwent rapid exhumation (M3) driven by cmstal isostatic compensation. Finally, retrogressive cooling (M4) occurred when exhumation ceased. Thus, the tectonothermal evolution of the Mayuan tectonic unit provides evidence for the collision of the Cathaysia and Yangtze blocks along the Jiangshan-Shaoxing suture. Two ophiolite samples along the eastern part of the suture yield Sm-Nd ages of 1034 ±24 Ma and 1020 ±30 Ma (Chen et al, 1991), suggesting that the collision of the two blocks occurred in Jinningian (Grenvillian) orogeny. Recently, this collision is considered to result in the final amalgamation of East Gondwana with Laurentia and the assembly of Rodinia (Li et ai, 1995, 1996). Break-up of Rodinia occurred about ca. 700-750 Ma during which the Cathaysia and Yangtze blocks are inferred by Li et al (1995, 1996) to have rifted away from the Australia and Laurentia, and then drifted with East Gondwana to amalgamate with West Gondwana by mid-Cambrian (Li et al, 1995, 1996; Zhao et al, 1996). The early Caledonian Wuyi-Yunkai tectonic unit in the Cathaysia block may result from this tectonic event, whereas the Indosinian Changle-Nanao tectonic unit may be related to the break-up of Gondwana and the final assembly of the major China blocks in Mesozoic (Zhao et al, 1996). REFERENCES Brown, E. H., 1977, The crossite content of Ca-amphibole as a guide to pressure of metamorphism. Journal of Petrology, v. 18, p. 53-72. Chen, J., Poland, K. A., Xing, F., Xu, X., and Zhou, T., 1991, Magmatism along the southeast margin of the Yangtze block: Precambrian collision of the Yangtze and Cathaysia blocks of China, Geology, v. 19, p. 815-818. England, P. C., and Thompson, A. B., 1984, Pressure-temperature-time paths of regional metamorphism, I. Heat transfer during the evolution of regions of thickened continental crust. Journal of Petrology, v. 25, p. 894-928.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998 Ganguly, J., and Saxena, S. K., 1984, Mixing properties of aluminosilicate garnets: constraints from natural and experimental data, and applications to geothermo-barometry, American Mineralogist, V. 69, p. 88-97. Hodges, K. v., and Crowley, P. D., 1985, Error estimation in emperical geothermometry and geobarometry for pelitic system, American Mineralogist, v. 70, p. 702-709. Hoisch, T. D., 1989, A muscovite-biotite thermometry, American Mineralogist, v. 74, p. 565-572. Holland, T., and Blundy, J., 1994, Non-ideal interactions in calc amphiboles and their bearing on amphibole-plagioclase thermometer: Contributions to Mineralogy and Petrology, v. 116, p. 433447. Hu, X. J., 1991, The Precambrian geology of southwestern Zhejiang Province, Precambrian Geology, V. 5, p. 1-208 (in Chinese). Li, G. K., 1989, A discussion on the ages of the basement in Fujian Province, Fujian Geology, v. 8, p. 159-168 (in Chinese). Li, Z. X., Zhang, L., and Powell, C. McA., 1995, South China in Rodinia: part of the missing link between Australia-East Antarctica and Laurentia?, Geology, v. 23, p. 407-410. Li, Z. X., Zhang, L., and Powell, C. McA., 1996, Positions of the East Asian cratons in the Neoproterozoic Supercontinent Rodinia, Australian Journal of Earth Sciences, v. 43, p. 593-604. Shui, T., 1987, Tectonic framework of the southeastern China continental basement, Scientia Sinica, V. B30, p. 414-422 (in Chinese). Zhang, W., and Fyfe, W. S., 1995, Chloritisation of the hydrothermally altered bedrock at Igarape Bahia gold deposit, Carajas, Brazil, Mineralogita Deposita, v. 30, p. 30-38. Zhao, G. C., Sun, D. Y., and He, T. X., 1994a, Metamorphism and crustal evolution of the basement in the southeastern China, Beijing, Seismological Press, 131 p (in Chinese). Zhao, G. C., Sun, D. Y., and He, T. X., 1994b, P-T-D path of the Chencai Group, Journal of Changchun University of Earth Sciences, v. 24, p. 244-254 (in Chinese). Zhao, X., Coes, R. S., Gilder, S. A., and Frost, G. M., 1996, Paleomagnetic constraints on the paleogeography of China: implications for Gondwanaland, Australian Journal of Earth Sciences, V. 43, p. 643-672.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 50 The Assembly and Breakup ofRodinia, Perth, September 1998

AUTHOR INDEX BIRD, Robert T. BODORKOS, Simon CASTAING, Christian CAWOOD, Peter COOKE, Alasdair CORRIVEAU, Louise COSCA, Michael A. CRAWFORD, Anthony J. CROWE, Warwick A. DALGARNO, Bob DALZIEL, Ian W. D. DIREEN, Nicholas G. DYSON, Ian A. EVANS, David A. D. FEYBESSE, Jean-Louis GARTRELL, A. P. GIBSON, George M. GRESSE, Pieter G. HACKNEY, R. I. HARRIS, Lyal B. HOLLINGSWORTH, D. A. KINNY, P. D. KLEINROCK, Martin C. LEITCH, Evan C. LI, X.H. LI, Zheng-Xiang MULLER, R. Deitmar NAAR, David F. OLIVER, Nicholas H. S. PILKINGTON, Mark PIRAJNO, Franco POTTS, Graham J. POWELL, C. McA. REDDY, Steven M. RIDSDILL-SMITH, Thomas A. RIVARD, Benoit ROEST, Walter R. SADOWSKI, Georg R. SMMONDS, John SONG, Tingguang TACK, Luc TEBBENS, Sarah F. UNRUG, Raphael WANG, J. WINGATE, Michael T. D. ZHANG, Shihong ZHAO, Gouchun

1,6,7 8 63 8, 11, 40, 60, 62, 75 45 39 17 15, 24 17 19 23 15, 24 29 34 63 36 37 63 40 17, 39 40 42 6 11 42 42, 72 7 6 8 7 45 54, 56 40, 49, 63 54, 56 7 39 1 63 57 60, 62 63 6 63, 65 42 70 72 75