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Geological Society of Australia

ABSTRACTS Number 113

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Riding the Wave Caloundra, Queensland, Australia 22 - 27 November 2015 Specialist Group in Tectonics and Structural Geology

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Riding the Wave A conference to showcase current research and ideas in Structural Geology and Tectonics

Caloundra, Queensland, Australia, November 2015

Specialist Group in Tectonics and Structural Geology (GSA)

Abstract Volume 113 Edited by C. Siegel, C. Verdel & G. Rosenbaum

ISSN: 0729 Oil X National Library of Australia Cataloguing-in-Publication Data © Geological Society of Australia Incorporated, Specialist Group in Tectonics and Structural Geology, 2015 Copies of this abstract volume may be obtained from Geological Society of Australia, Email: info@gsa.org.au Web: www.gsa.org.au Example citation for papers in this volume: Abdullah R., Rosenbaum G., Dunstan S. and Babaahmadi A., 2015. Curvilinear east-west structures at the southern Thomson Orogen and their geodynamic significance, in: Siegel C., Verdel C. and Rosenbaum G. (eds.), Riding the Wave: GSA Specialist Group in Tectonics and Structural Geology Conference, November 2015. Geological Society of Australia Abstract No. 113, pp 1.

Foreword This volume includes the proceedings of the Specialist Group in Tectonics and Structural Geology conference held at the Events Centre in Caloundra from 22 to 27 November 2015. The conference is focused on seven themes: (1) Deformation mechanisms, microstructure, and rheology; (2) Orogens and continental deformation; (3) The coupling of fluid flow, deformation, and resources; (4) Structure and reactivation of basin margins; (5) Pressure oscillations in orogens; (6) Supercontinent cycles and global geodynamics and (7) Challenging paradigms. This volume is organised by alphabetical order of the presenting author (underlined).

The organising committee members are as follow: Gideon Rosenbaum (UQ) Rod Holcombe (HCO/UQ) Charles Verdel (UQ) Derek Hoy (UQ) Scott Bryan (QUT) Christoph Schrank (QUT) Charlotte Allen (QUT) Paul Donchak (GSQ) Ian Withnall (GSQ) Thomas Flottmann (Origin) Coralie Siegel (QUT)

Abstract List Presenting author is underlined 1

R. Abdullah, G. Rosenbaum, S. Dunstan & A. Babaahmadi Curvilinear east-west structures at the southern Thomson Orogen and their geodynamic significance

F. Arboit, K. Amrouch, A.S. Collins, R. King & C. Morley Determination of the tectonic evolution from fractures, faults and calcite twins on the southwestern margin of the Indochina Block

F. Arboit, A.S. Collins, C. Morley, R. King & K. Amrouch In-situ detrital zircon geochronology and Hf isotopic analyses within the south-western margin of the Indochina terrane, central Thailand: New insights on the Indosinian orogeny

D.B. Archibald, A.S. Collins, J.D. Foden, J.L. Payne, P. Holden & T. Razakamanana Stenian-Cambrian tectonic evolution of central Madagascar: Insights from U-Pb, O, and Hf isotopes from zircon in magmatic rocks

S.E. Armistead, P.G. Betts, L. Ailleres & R.J. Armit Structural evolution of the North Portia Cu-Au-Mo deposit, Curnamona Province, South Australia

A. Babaahmadi & G. Rosenbaum Fault systems in Queensland and New South Wales and their implications to the tectonic evolution of eastern Australia since the Permian

P. Betts, R. Armit, R. Cayley, L. Moresi & D. Moore Australia, Nuna and beyond - Ribbon tectonics and the growth of the Australian continent

M.L. Blades, A.S. Collins, J. Foden, J. Payne, S. Al-Khirbash, B. Thorpe, G. Murray & B. Alessio Using isotopic techniques to constrain the tectonic evolution of the Oman basement

S. Brooke-Barnett & G. Rosenbaum Structure of the Texas Orocline below the sedimentary cover: insights from geophysical data

J. Brownlow The paradox of Permian-Trias sic subduction in the Southern New England Orogen

J. Brownlow How complex was the last lOOMy? - geomorphic evidence from the Mesozoic Clarence-Moreton Basin in northeastern New South Wales

J. Duque-Trujillo, L. Ferrari, S. Bryan, T. Orozco-Esquivel & M. Lopez-Martinez The Gulf of California (Mexico): Constraining the timing and evolution of rifting from the plutonic record

D. Burke-Shyne, D. Wiemer, C. Schrank & D. Murphy Carbonate alteration is the dominant weakening mechanism in the Doolena Gap Greenstone Belt preserving the keel rock in a dome-and-keel terrane

R. Cayley A new geodynamic model for Cambrian Tasmania/Australian East Gondwanaland - continentcontinent collision above doubly divergent Dundas Trough subduction zones.

T. Chapman, G.L. Clarke, S. Piazolo & N.R. Daczko Arc root buoyancy: metastable igneous assemblages in deformed lower crust, Fiordland, New Zealand

J. Chen, D.R. Cooke, D. Selley & J. Piquer Structural geology, veins and breccias at the Zijinshan high sulfidation Cu-Au deposit, Fujian Province, China

G.L. Clarke, S.K. Bhowmik, T.R. Ireland, J.C. Aitchison & S.L. Chapman Inter-zone dependences in an inverted Oligo-Miocene metamorphic succession, Arunachal Pradesh, India

A.S. Collins, D. Archibald, M. Blades, C. Clark, D. Plavsa, J. Foden, S. Glorie, T. Razakamanana, T. Alemu, G. Woldetinsae, S. Al-Khirbash, P. Siegfried, S. Pisarevsky & J.L. Payne Plate Topologies and Palaeogeography of the Neoproterozoic-Cambrian Amalgamation of Eastern Gondwana

W.J. Collins, N. Priyatkina & E. Martin Application of zircon Hf isotope arrays to tectonics and geodynamics

S.F. Cox, A. Flatten & D. Beck The dynamics of injection-driven failure and fluid pathways in high fluid flux regimes: insights from ore deposits, seismicity and numerical modelling

J. Cunneen, F. Pym, J. Hammelswang & C. Elders Basement influences on structural styles in the Bight Basin, southern Australia

K. Czarnota & N. White Pure Shear Extension of Thick Lithosphere - Rift but no Sag: NE Canning Basin, Western Australia

N.R. Daczko, J.A. Halpin, J.M. Whittaker & I.C.W. Fitzsimons The Neoproterozoic East Gondwana suture: Reconciling geological and geophysical evidence

J.A. Halpin, N.R. Daczko, J.M. Whittaker, S.E. Williams, R.L. Gardner, M.E. Kobler & P.G. Quilty Microcontinents offshore Western Australia: insights into the make-up and break-up of East Gondwana

N.R. Daczko Undergraduate teaching with a Virtual Petrographic Microscope

P.J.T. Donchak & D. Clark Structural architecture of deformed subsurface basins flanking the Mount Isa Inlier interpreted from recent deep crustal seismic transects

M.P. Doublier, K. Czarnota, D.C. Champion & R.J. Korsh Lithospheric architecture of the Yilgarn Craton: an integrative approach

P.G. Lennox, H. De Wall & D.W. Durnev Relationship of AMS to solid-state strain and biotite microstructure in the foliated Wyangala Granite, Eastern Lachlan Orogen

D.W. Durnev What drives mass transfer during deformation? An insight from angular distributions of thermodynamic variables around a spherical inclusion.

C. Elders What moved in the Fitzroy Movement?

C. Fergusson & B. Henderson Oroclines, superfans, the Thomson-Lachlan connection and Early Paleozoic development of the Tasmanides

M.A. Finch & R.F. Weinberg The role of water in the formation of a thick ultramylonitic shear zone

C. Gaina Microcontinents: Stories about complexities in the Wilson tectonic cycles

49 50

R. Gardner, S. Piazolo, L. Evans & N. Daczko Does microstructure matter? Investigating the effect of spatial distribution of viscosity on bulk strength and strain localisation G.M. Gibson, T. Meixner, L. Hutton & J. Holzschuh Basin inversion as a driver for sedimentation, fluid flow and Pb-Zn mineralisation on the Lawn Hill Platform, north Queensland: evidence from deep seismic reflection profiling and gravity modelling

J. Gillespie, S. Glorie, W. Xiao, Z. Zhang, A.S. Collins, N. Evans, B. Mclnnes & J. De Grave Mesozoic reactivation of the Beishan, southern Central Asian Orogenic Belt: Insights from low temperature thermochronology

S. Glorie, J. De Grave, W. Xiao, A.S. Collins & N. Evans Deciphering the Meso-Cenozoic tectonic history of southern Eurasia using thermochronology on Central Asian faults

D. Green, M. Cracknell & G. Cumming Structural interpretation of high resolution Digital Elevation Models derived from LiDAR remote sensing

L. Grose, L. Ailleres & G. Laurent Characterising fold geometries from structural data: application to implicit modelling

57 59 60 61 62

B. Williams, A.C. Hack & G. Phillips High-temperature low-pressure metamorphism & accretionary orogen geodynamics: Insights from the Wagga-Omeo Metamorphic Belt. A.D. Haisley A Palinspastic Reconstruction: Basement and lithological controls on the development of the Jellinbah Thrust Belt. J.W. Hall, S. Glorie, A.S. Collins, K. Agostino, M. Reddy, C. Trenough, N. Evans, A. Reid & R. Dutch Unlocking and linking the low-temperature thermochronological histories of inland South Australia K.S. Havward, S.F. Cox & J.G. Fitzgerald Rarely made or rarely preserved? Experimental insights into the formation and preservation of frictional melt K.S. Havward & S.F. Cox Faulting, fluids and fusion: frictional melting and cohesive strengthening on low displacement, misoriented fault interfaces

K.A. Heilbronn & R.J. Holm Revisiting the plate dynamics of the northwest Coral Sea: Modelling the Gulf of Papua and the break-up of northeast Australia

B. Henderson & C. Fergusson Northern Tasmanides: shared orogenic threads but unique properties

65 66

B. Hobbs The Influence of Deformation on Mineral Phase Equilibrium. R. Holcombe, C. Fielding, R. Sliwa, G. Rosenbaum & D. Hoy Time-Space discontinuum in the NEO: partitioning of Hunter-Bo wen events

R.J. Holm, C. Spandler & S. Richards Late Cenozoic convergent tectonics in Papua New Guinea

R.J. Holm, G. Rosenbaum & S.W. Richard Late Neogene and Quaternary reconstruction of Papua New Guinea and the Solomon Islands

D. Hoy & G. Rosenbaum The end of the Hunter-Bowen Orogeny and younger deformation in eastern Australia: insight from the Gympie Terrane

N.J.R. Hunter, C.J.L. Wilson & R.F. Weinberg Evaluating quartz crystallographic texture strengths using classic eigenvalue methods

G.M.M. Jepson, S. Glorie, D. Konopelko, A.S. Collins & N. Evans Exhumation history of the Western Tien Shan (Uzbekistan and Tajikistan): Preliminary thermochronological results

G.M.M. Jepson, A.H.E. Bailey, R.C. King, S.P. Holford & M. Hand In-situ stress and natural fracture networks in the Carnarvon Basin, North West Shelf, Australia

K. Jessop Steep metamorphic field gradients in HTLP regional aureoles: structurally controlled advection of heat by water versus magma

K. Jessop A new map of the Wongwibinda Metamorphic Complex: its relationship to other HTLP complexes of the New England Orogen

X. Jiang & W. Gong Tectonics of the reactivated Kuqa Foreland Basin - Tien Shan Orogen

I. Jones & C. Verdel An animated reconstruction of the Cenozoic motion of Australia

M. Keep The Concertina Coast: a history of repeated inversion during basin formation on Australia's northern margin

A. Khudoley, R. Rainbird & N. Priyatkina Laurentia - Siberia connection: An overview and new data

M. Lee & C. Verdel Ar/39Ar geo- and thermo-chronology of the northern Thomson Orogen

J. Yan, P.G. Lennox, R. Offler & B. Kelly The enigmatic Hastings Block - history of emplacement and subsequent deformation

Z-X Li Supercontinent-superplume coupling and a new global geodynamic working model

S. Liu, K. Czarnota & A. Stewart Continental chronostratigraphic solid geology mapping of Australia

L. Mahoney, S. McLaren & K. Hill Regional scale structural modelling along a geological transect: insights from the NW Fold and Thrust Belt, PNG

N. Mancktelow Pressure oscillations in orogens: lithostatic versus overpressure

S. Wex, N. Mancktelow, F. Hawemann, A. Camacho & G. Pennacchioni Inverted localization of deformation in the "dry" middle crust across the Woodroffe Thrust, Central Australia

F. Hawemann, N. Mancktelow, S. Wex, G. Pennacchioni & A. Camacho Pseudotachylytes as evidence for lower crustal earthquakes (Musgrave Ranges, Central Australia)

R,J. Manton, S. Buckman & A.P. Nutman Application of Rb-Sr geochronology to date HP/LT metamorphic events in the southern New England Orogen, NSW: implications for tectonic models, eastern Gondwana.

S. Marshak Insights into the kinematics of deformation in fold-thrust belts, from field and modeling studies

S.R.B. McAlpine & J.A. Goodwin Uncovering the mineral potential of the Stavely Arc using 3D geological modelling

100

N.E. Molnar & A.R. Cruden The role of boundary conditions and rheological heterogeneities during the rifting to drifting process: 3D laboratory experiments with application to the Red Sea

102

D.H. Moore, P.G. Betts & M. Hall The subdivisions of western Tasmania and their effects across Bass Strait

104

M.K. Mukheriee Basement-cover relations in the intracratonic Kaladgi basin, southwestern India: Deformtional evidence of a Mesoproterozoic gravity gliding of the cover over the basement.

106

J.A. Mulder, J.A. Halpin, N.R. Daczko Mesoproterozoic Tasmania: Witness to the East Antarctica-Laurentia connection within Nuna

108

J.A. Mulder, R.F. Berry, S. Meffre, J.A. Halpin The metamorphic sole of the western Tasmanian ophiolite: New insights into the Cambrian tectonic setting of the Gondwana Pacific margin

109

R.J. Musgrave What can geophysics tell us about the mobile phase of the Lachlan Orogen?

111

A. Ord Representing the kinematics of deforming rocks

112

X. Wang, M. Munro & A. Ord Understanding vein-hosted mineralization at the Sunrise Dam gold deposit through coupled mechanical-fluid flow modelling

113

C.W. Passchier Problems of overprinting foliations - an example from NW-Namibia

114

M.A. Pearce & A.J.R. White Deformation and K-metasomatism in a Grantic Shear Zone, Capricorn Orogen, WA

115

G. Phillips, R. Offler, D. Rubatto & D. Phillips A continuous subduction model for the Tasmanides: evidence of long-lived subduction preserved by high-pressure rocks in the Peel-Manning Fault System

117

S. Piazolo, L. Spruzeniece, M.A. Mamtani, D. Czaplinska & L. Evans The influence of local neighbourhood on the dominance of deformation mechanisms and bulk rheology of polymineralic rocks: Examples from garnet, quartz and magnetite

118

V. Faucheux, S. Picazo, G. Manatschal & S. Piazolo Carbonatisation processes in tecto-sedimentary breccias drilled at ODP Sites 1068 and 1070 along the Zone of Exhumed sub-Continental Mantle (ZECM) offshore Iberia

120

J.M. Pownall, G.S. Lister & R. Hall Rollback and extension in Eastern Indonesia

122

N. Privatkina, W.J. Collins, A.K. Khudoley & D. Zastroshnov Itinerant Siberia during the Neoproterozoic

124

R. Quentin de Gromard, H.M. Howard, R.H. Smithies, F. Jourdan, C.L. Kirkland & M.T.D. Wingate Coupling deep seismic with 40 Ar- 39 Ar thermochronology: reinterpretation of the structural evolution of the Musgrave Province, Central Australia

126

K. Regenauer-Lieb, G. Rosenbaum, R.F. Weinberg & G. Manatschal Tectonic overpressure in continental collision and the origin of ultra-high-pressure rocks

128

M. Roach, S. Cox, P. King, C. Verdel, K. Welsh, A. George & S. McLaren AusGeol.org - A Virtual Library of Australia's Geology

130

T. Roache Ore shoot emplacement: differentiating architectural from syn-deformational controls

132

J.A. Robinson & G. Phillips 3D structural architecture of the western Tamworth Belt, southern New England Orogen: regional scale controls on hydrothermal mineral systems

134

L. Royden & O. Jagoutz The geodynamics of convergence between India and Eurasia (and the roles of Afro-Arabia, Indonesia and Australia)

136

I.V. Sanislav, P.H.G.M. Dirks, T. Blenkinsop & Y.A. Cook Vertical vs horizontal tectonics - insights from the Tanzania Craton

137

C.E. Schrank, A. Karrech, D.A. Boutelier & K. Regenauer-Lieb Simple shear of single inclusions with a hyperelastoviscoplastic rheology at finite strain

138

D. Boutelier, M. Henriquet, C. Schrank, A. Karrech & K. Regenauer-Lieb Ductile elasto-plastic shear zone at large strain: insight into elastic energy storage and dissipation from analogue experiments

140

U. Shaanan & G. Rosenbaum Provenance and palaeogeography of Early Permian back-arc basins in eastern Australia, and implications for the role of trench retreat in accretionary orogens

141

U. Shaanan, G. Rosenbaum, S. Pisarevsky & F. Speranza Paleomagnetic data from the New England Orogen (eastern Australia) and implications for oroclinal bending

142

C. Siegel, S.E. Bryan, C.M. Allen, D.J. Purdy, A.J. Cross & D.A. Gust Unravelling the nature of crustal basement beneath large tracts of the Thomson Orogen

144

R. Sliwa, A. Babaahmadi & J. Esterle The Jellinbah Fold-Thrust Belt in the Bowen Basin: a late phase of the Hunter-Bowen contraction in Queensland.

146

L. Sonnette & J-C. Lee The NE Taiwan salient: a bent progressive mountain belt

148

N.E. Timms, C. Tucker, D. Pearce, P. Wilkes, T. Johnson & I.C.W. Fitzsimons The anatomy of the Leeuwin Complex: Insight into the assembly of the southwest corner of Australia

150

R.F. Weinberg, M.A. Finch, M.G. Fuentes & R. Becchio The Pichao-Ovejeria shear zone in Argentina: an introduction to very thick ultramylonites

151

A. Weisheit, B.L. Reno, E.E. Beyer & J.A. Whelan Pulses of progressive shearing and reactivated fault structures: The 1.5 b.y. Palaeoproterozoic to Palaeozoic structural and metamorphic evolution of eastern Arunta Region, central Australia

153

D. Wiemer, C.E. Schrank, D.T. Murphy, A.H. Hickman Structural development of the early Archaean Doolena Gap greenstone belt, East Pilbara Terrane (Western Australia)

155

C.J.L. Wilson, N. Hunter, V. Lutzin, M. Peternell & S. Piazolo Investigation of fabrics in quartz and ice: Comparison and applications of different analytical methods

157

N.M. Wright, M. Seton, S.E. Williams, R.D. Miiller The Late Cretaceous to recent tectonic history of the Pacific Ocean basin

SGTSG 2015: Riding the Waves

Curvilinear east-west structures at the southern Thomson Orogen and their geodynamic significance RASHED ABDULLAH 1 , GIDEON ROSENBAUM 1 , SAM DUNSTAN 1 AND ABBAS BABAAHMADI 1 1

School of Earth Sciences, The University of Queensland, Australia.

The dominant ~N-S trend of the Tasmanides is generally attributed to the orientation of the paleoPacific plate boundary during the Paleozoic and early Mesozoic. However, a number of major structures deviate from this general orientation, including the curvilinear ~E-W structures at the southern Thomson Orogen and the oroclines in the Lachlan and New England orogens. The presence of such discordant structural features raises fundamental questions on the geodynamic evolution of eastern Australia during the Paleozoic. We have investigated the ~E-W structures in the area of the Thomson-Lachlan boundary (northwestern NSW). Geophysical observations show evidence for dextral separation in a broad zone (~ 150 km wide) that includes the Olepoloko Fault, Louth-Eumarra Shear Zone and the Culgoa Fault. In cross sections, reinterpretation of 2D regional seismic reflection profiles shows an additional component of reverse faulting, particularly along the ~E-W-trending Olepoloko Fault, which has penetrated through the whole crust and shows offset of the Moho. A reverse component is also recognised along the Culgoa Fault in the seismic section DMR 98-02. The widespread evidence for dextral-reverse kinematics in the southern Thomson, and the fact that some of these structures have penetrated through the whole crust, indicate that a major zone of dextral transpression is preserved in the area of the Olepoloko Fault, Louth-Eumarra Shear Zone and the Culgoa Fault. The geodynamic signficance of this broad shear zone is yet to be resolved, but we speculate that it may have played a role in linking the Delamerian Orogen with an equivalent "Delamerian" belt that may currently resides in the Anakie Inlier (northeast Queensland) and the Nebine Ridge. The probable "Delamerian" basement in the Nebine Ridge is recognised in seismic line BMR 84-14. Two crustal-scale structures (Foyleview and Westgate geosutures) are clearly recognised in the seismic section, bounding a zone of thicker lower crust and deeper Moho. We hypothesise that this zone is the continuation of the Delamerian belt, thus providing the missing link between the Anakie inlier, the ~EW structure of the southern Thomson Orogen, and the Delamerian belt sensu stricto.

SGTSG 2015: Riding the Waves

Determination of the tectonic evolution from fractures, faults and calcite twins on the south-western margin of the Indochina Block

FRANCESCO ARBOIT **, KHALID AMROUCH , ALAN S. COLLINS \ ROSALIND KING AND CHRISTOPHER MORLEY 2

Centre for Tectonics Resources and Exploration (TRaX), Department of Earth Sciences, The University of Adelaide, SA 5005, Australia. The Australian School of Petroleum, The University of Adelaide, SA 5005, Australia. Chiang Mai University, 239 Huaykaew Road, Tumbol Suthep Amphur Muang, Chiang Mai, Thailand. 1

South East Asia is formed from a number of continental fragments and volcanic arcs, separated by oceanic suture zones, which accreted to the growing Asian continent during the latest Paleozoic-early Mesozoic Indosinian Orogeny. In Thailand, this tectonic event, developed mainly during the Triassic and is the result of the collision between the Sibumasu, Sukhothai and Indochina Terranes. Models for the tectonic evolution of Thailand from the Indosinian Orogeny to the present have been defined using biostratigraphical, petrological, structural and geochronological data and involve broadly N-S trending tectonic domains converging in E-W directions (with respect to the present-day orientation). However, these are based on the present day orientations of the tectonic domains and little constraints on the original orientations or paleo-stress determinations have been previously attempted. Central Thailand, and more specifically the Khao Khwang Fold-Thrust Belt (KKFTB), provides a natural laboratory to obtain the necessary information, with many well exposed quarries along Highway 21. The complex structural architecture in the KKFTB evolved from the Permian, mainly with the growth of E-W to ENE-WSW striking reverse faults and associated drag-folds that developed in this thin-skinned fold and thrust belt. Associated with this deformation, there are a number of N-S to E-W trending sets of fractures that developed before, during, and after, the main Layer-Parallel Shortening (LPS) event of the Indosinian Orogeny. In order to develop a permissive model for the dynamic evolution of the region, we took advantage of the rocks of the Khao Khad Formation to compare fracture and fault sets with a paleostress analysis carried out using calcite twins. In combination, these data provide an accurate estimate on the evolving stress orientation and resulting deformation before, during, and after, the Indosinian Orogeny. In polyphase tectonic zones such as the KKFTB, integrating a study of fault and fracture with calcite twin analysis can determine the evolving paleo-stress magnitudes and principle stress directions that affected the area. Here we presents the results of the analyses of fractures, striated faults and calcite twins collected within the KKFTB in central Thailand, by the integration of these data we attempt to reconstruct the orientation of the principal stresses that developed during the tectonic evolution of this highly deformed, polyphase orogen. Tectonic data were collected in the Permian carbonates of the Khao Khad Formation of the Saraburi Group, and five successive tectonic stages are determined that are interpreted to have developed before, during, and after, the Triassic Indosinian Orogeny. The first three stages pre-date the main deformation event: the first stage is interpreted as a pre-Indosinian N-S extensional stage, the second stage described a N-S strike-slip and compressional regime, largely perpendicular to the fold axes of the main structures, while the third stage is associated with an E-W compressional strike-slip phase. A further two stages took place after, or during, the main folding event and correspond to N-S compression and to an E-W composite strike-slip/contractional stage, the latter which is interpreted to represent Cenozoic deformation related to the India-Asia collision.

SGTSG 2015: Riding the Waves

In-situ detrital zircon geochronology and Hf isotopic analyses within the south-western margin of the Indochina terrane, central Thailand: New insights on the Indosinian orogeny FRANCESCO ARBOIT **, ALAN S. COLLINS \ CHRISTOPHER MORLEY 2 , ROSALIND KING 1 AND KHALID AMROUCH 3 1 Centre for Tectonics Resources and Exploration (TRaX), Department of Earth Sciences, The University of Adelaide, SA 5005, Australia. 2 Chiang Mai University, 239 Huaykaew Road, Tumbol Suthep Amphur Muang, Chiang Mai, Thailand. 3 The Australian School of Petroleum, The University of Adelaide, SA 5005, Australia.

The Khao Khwang Fold-Thrust Belt (KKFTB), central Thailand developed within a basin that formed on the south-western margin of the Indochina Block. Limited geochronological and provenance constraints mean that the time of deposition, sediment source location, and tectonic significance of the basin have been uncertain. Here, we present 837 U-Pb detrital zircon ages and 271 Hf isotope in-situ analyses from Permian-Triassic clastic units within the KKFTB in order to constrain the provenance, maximum depositional ages and depositional environment of the south-western margin of the Indochina terrane through the Late Palaeozoic to Early Mesozoic. The key lithological units: the Sap Bon, Pang Asok and Nong Pong Formations are part of the Saraburi Group and have detrital age spectra spanning from Upper Triassic to Palaeoarchean. The entire dataset have a common age peak at ca. 450 Ma, and all samples contain zircons with ages between 0.2-0.3, 0.4-0.6, 1.0-1.3, 1.7-1.8, 2.22.7 Ga. A few zircons predate 3.0 Ga. Multidimensional-scaling analysis of detrital zircons from throughout SE Asia demonstrate that the detrital zircon age spectra of the siliciclastic units of the Saraburi Group resemble that of Permian-Triassic detritus found elsewhere in the Khorat Plateau and throughout Vietnam and southeast China, implying that these areas share similar sources. These sources may be the, now largely covered, Indochina basement, and/or contiguous continental crust in terranes already amalgamated to Indochina at that time. Detrital zircons as young as 205 ± 6 Ma show that some formations of the Saraburi Group, previously considered being of Middle-Late Permian age, are no older than Late Triassic. Therefore, we propose a depositional model, for the region, of a Permian rift or passive margin setting that evolved into piggy back and foredeep basins during an extended period of folding and thrusting in the Triassic.

SGTSG 2015: Riding the Waves

Stenian-Cambrian tectonic evolution of central Madagascar: Insights from U-Pb, O, and Hf isotopes from zircon in magmatic rocks

DONNELLY B. ARCHIBALD , ALAN S. COLLINS , JOHN D. FODEN , JUSTIN L. PAYNE , PETER HOLDEN AND THEODORE RAZAKAMANANA 1

Tectonics, Resources and Exploration (TRaX), Department of Earth Sciences, The University of Adelaide, Adelaide, Australia Centre for Tectonics, Resources and Exploration (TRaX), School of Built and Natural Environments, University of South Australia, Mawson Lakes, SA, Australia Research School of Earth Sciences, The Australian National University, Canberra, Australia Departement des Sciences de la Terre, Universite de Toliara, Toliara, Madagascar 1

Madagascar occupies an important location within the East African Orogen, which involves a collection of Neoproterozoic microcontinents and arc terranes lodged between older cratonic units during the final assembly of the supercontinent Gondwana. The Malagasy basement records evidence of tectonic disturbances throughout most of the middle to late Proterozoic. The oldest basement units are the Antongil and Masora Domains. These rocks are primarily Mesoarchaean ortho- and paragneiss intruded by Neoarchaean granitic rocks. These units are inferred fragments of India left behind in the Mesozoic during the break-up of Gondwana. The largest unit, the Antananarivo Domain consists of orthogneiss, paragneiss and Neoarchaean granitoids (Betsiboka Suite) interlayered with Tonian to Cambrian granite, syenite, and gabbro. The Itremo, Ambatolampy and Maha Groups are suggested Mesoproterozoic supracrustal packages overlying the Antananarivo Domain. The Ikalamavony Group is a Meso-to Neoproterozoic volcano-metasedimentary package deposited in association with the Dabolava Suite. The youngest Proterozoic unit (Manampotsy Group) represents a supracrustal package deposited in a narrow Neoproterozoic basin in east central Madagascar near the location of the proposed Neoproterozoic Betsimisaraka suture zone. The tectonic evolution of central Madagascar during Meso- to Neoproterozoic time by studying the consumption Mozambique Ocean is the focus of this paper. Magmatic arc rocks emplaced along plate margins during the subduction of oceanic crust are used to investigate the extent and destruction of this Proterozoic ocean. Central Madagascar preserves three magmatic episodes between -1080-960 Ma (Dabolava Suite), -850-750 Ma (Imorona-Itsindro Suite) and -650-540 Ma (Kiangara and Ambalavao Suites). Gabbroic and granitoid rocks of the Stenian-Tonian Dabolava suite combined with the probable coeval Ikalamavony Group represent a magmatic arc and marginal volcano-sedimentary sequence deposited within a continental back-arc tectonic setting based on geochemical characteristics. The Tonian Imorona-Itsindro suite represents contemporaneous emplacement of magmas with varied geochemical characteristics with their collective genesis most reliably explained by plate margin processes. Tonian arc magma generation is believed to coincide with oceanic plate subduction during closure of the Mozambique Ocean along the Betsimisaraka Suture in eastern Madagascar. Recently, some authors have questioned the presence of an active continental margin and consequent suture zone in favour of intraplate magma generation based on geochemical characteristics. The Ediacaran to Cambrian Kiangara-Ambalavao suite is a post-collisional, mainly granitoid suite emplaced during the final assembly of Gondwana. In this U-Pb, hafnium and oxygen isotope study in zircon we attempt to: (1) resolve the conundrum of the presence or absence of possible suture zones in east central Madagascar and (2) reconstruct the tectonic geography of central Madagascar from the Stenian to Cambrian. Preliminary inferences question published palaeogeographic reconstructions. Interpretations also imply the possibility of a previously unrecognised and long-lived (-500 Ma) active subduction margin similar to the present-day western Pacific Ocean. This large isotopic dataset is essential for understanding Madagascar's tectonic evolution from the breakup of Rodina, the closure of the Mozambique Ocean, and the final amalgamation of Gondwana.

SGTSG 2015: Riding the Waves

Structural evolution of the North Portia Cu-Au-Mo deposit, Curnamona Province, South Australia SHEREE E. ARMISTEAD1,2, PETER G. BETTS 2 , LAURENT AILLERES 2 AND ROBIN J. ARMIT 2 Centre for Tectonics, Resources and Exploration (TRaX), Department of Earth Sciences, The University of Adelaide, SA 5005 2 School of Earth, Atmosphere and Environment, Faculty of Science, Monash University, VIC 3800

The Curnamona Province that straddles the South Australia and New South Wales border is an emerging region of IOCG exploration interest, with several IOCG deposits already recognised. Within the Curnamona Province is the Benagerie Ridge Magnetic Complex (BRMC) which contains the North Portia Cu-Au-Mo deposit. We integrate detailed drillcore logging, petrophysics, petrology, 3D modelling, with high resolution aeromagnetic data to produce a detailed structural interpretation of the BRMC and North Portia deposit. A major control on Cu mineralisation in the Curnamona Province is the extensive regional redox boundary which separates stratigraphically lower oxidised magnetic units from upper reduced non-magnetic units. The redox boundary is identifiable in aeromagnetic images as a sharp boundary between magnetic and non-magnetic units. The boundary is well represented at North Portia with the most significant Cu-Au-Mo mineralisation occurring at or proximal to this redox boundary. Significant sulphide mineralisation also occurs along bedding parallel shear structures, which, coupled with the redox boundary mineralisation supports a strong sedimentary control on Cu mineralisation at all scales. The North Portia deposit contains pervasive iron oxide alteration, albite alteration and brecciation which are characteristics consistent with IOCG mineralisation. Leapfrog 3D geological modelling was undertaken to quantitatively confirm the observations from drillcore that sulphide mineralisation is controlled by the redox boundary - in this case, the boundary between lower magnetic psammopelites and upper, altered metasedimentary rocks. Leapfrog is an implicit modelling tool that allows rapid construction of geological models. Interpolation shells for Cu were formed based on drillcore assay data. The results from this confirm that the highest values of Cu occur directly above the redox boundary. The BRMC is entirely undercover, so the development of a 3D structural interpretation was largely dependent on high resolution aeromagnetic data (figure 1). The TMI RTP image of the BRMC shows a refolded structure similar to the simplified type 2 interference pattern which is representative of recumbent folding being orthogonally overprinted by upright folding (Grasemann et al., 2004). This large refolded structure has then been overprinted by later shearing and faulting.

RIP first vertical derivative

RTP total magnetic intensity

mk mm -

Figure 1: RTP first vertical derivative (top); and RTP total magnetic intensity (lower) of the BRMC. Location of North Portia

The following three deformation events are interpreted to be responsible d e P o s i t indicated by hatched for the current configuration of the BRMC and are consistent with J™*-Map extent-12 km x 22 current interpretations for the Olarian Orogeny:

SGTSG 2015: Riding the Waves

D1 structures occur as bedding-parallel foliations (SI) which are observed in drillcore. Locally, the regional redox boundary occurs parallel to this foliation and therefore the orientation of this magnetic boundary in aeromagnetic images is representative of D1 structures. The D1 event is interpreted to be analogous to the earliest layerparallel foliation of the Olarian Orogeny. The strong sedimentary control on Cu-Au-Mo mineralisation, with no apparent effect from later folding indicates that the relative timing of mineralisation occurred syn- to post-Dl, but prior to D2. The SI foliation has then been refolded around F2 folds. Based on the current configuration of the type 2 interference pattern, the second deformation event was a north to south directed shortening event which produced very shallow to horizontal axial surfaces, or approximately recumbent style folds. These folds are locally developed as cm-scale isoclinal recumbent folds in drillcore. The folding of D2 structures during D3 has produced the type 2 interference pattern observable in aeromagnetic images. This D3 event is interpreted as a north-west to south-east compression which produced large upright folds. In the BRMC this is represented as the large Benagerie Ridge antiform (figure 1). This large F3 fold plunges moderately to the north. The north-easterly trend of this anticline is consistent with other trends recorded in the Olary Domain and Mulyungarie Domain (Conor and Preiss, 2008). The structural evolution described, and visualised in figure 2 is consistent with the c. 1620-1590 Ma Olarian Orogeny (eg. Conor and Preiss, 2008). We propose that the Cu-Au-Mo mineralisation occurred syn- to post-Dl, before the onset of D2 recumbent folding. This suggests that the Cu-Au-Mo mineralisation in the North Portia deposit is associated with the earliest phase of the Olarian Orogeny, and is controlled predominantly by the redox boundary and layer-parallel structures.

Figure 2: Noddy 3D block models of D1 to D3 showing the development of the type 2 interference pattern observed in aeromagnetic images

References Conor C. H. & Preiss W. V. 2008. Understanding the 1720-1640 Ma Palaeoproterozoic Willyama Supergroup, Curnamona Province, Southeastern Australia: Implications for tectonics, basin evolution and ore genesis. Precambrian Research 166, 297-317. Grasemann B., Wiesmayr G., Draganits E. & Fusseis F. 2004. Classification of refold structures. The Journal of Geology 112, 119-125.

SGTSG 2015: Riding the Waves

Fault systems in Queensland and New South Wales and their implications to the tectonic evolution of eastern Australia since the Permian ABBAS BABAAHMADI 1 AND GIDEON ROSENBAUM 1 1

School of Earth Sciences, The University of Queensland, Brisbane, 4072, Australia

Eastern Australia has been affected by numerous fault systems, but their roles in the tectonic evolution of eastern Australia have hitherto remained relatively understudied. We addressed this issue by investigating fault systems and their reactivation history since the Permian. We studied the role of fault systems in the formation of the Texas and Coffs-Harbour oroclines in the southern New England Orogen. The results show that layer-parallel faults with a major strike-slip component occur parallel to the curved structure of the oroclines. This may indicate that a flexural slip mechanism operated during oroclinal bending in the Early-Middle Permian, although younger reactivation with a similar sense of kinematics is clearly recognised. Reactivated likely occurred during the Middle Permian to early Late Triassic contractional phases of the Hunter-Bowen orogeny. These contractional phases are expressed by thrust faulting in the Bowen, Gunnedah, and Sydney basins (Korsch et al., 2009), and thrusting in the Devonian-Carboniferous subduction-related units (Holcombe et al., 1997). Structural analysis of faults in some of Triassic-Jurassic sedimentary basins in eastern Australia shows that intermittent phases of rifting events occurred during the Triassic (Babaahmadi et al., 2015). An early stage syn-sedimentary normal faulting in the Nymboida Coal Measures suggests that a rifting phase occurred in the Early-Middle Triassic. This phase of rifting was followed by a contractional event that resulted in tilting, folding, and thrust faulting. Moreover, evidence of syn-sedimentary normal faults and bimodal volcanism during the early Late Triassic is indicative of another rifting phase, resulting in the development of the Ipswich Basin. The alternating episodes of rifting and contraction during the Triassic were possibly controlled by plate boundary migration and switches between trench retreat and advance. The N-striking dextral strike-slip Demon Fault is interpreted to be a post-Late Triassic fault because it displaced some Triassic NW-striking faults and Early to Late Triassic magmatic rocks (Babaahmadi and Rosenbaum 2013). The major activity of the Demon Fault occurred contemporaneously with the development of the Clarence-Moreton Basin in the Jurassic. A more recent reverse dextral activity of the Demon Fault is possibly related to mid-Cretaceous contractional deformation or Cenozoic deformation associated with collisional processes at the northern boundary of the Australian plate. The NNW-striking North Pine Fault System (NPFS) in southeast Queensland has undergone sinistral reverse strike-slip movement with offsets ranging from -3.4 to -8.2 km (Babaahmadi and Rosenbaum 2014a). Based on a Triassic granophyre dyke parallel to the southeastern NPFS, and the contribution of parallel NNW-striking strike-slip and normal faults in the development of the Late Triassic-Early Cretaceous Maryborough Basin, the NPFS has likely been active during the Mesozoic. The NPFS is interpreted to have been reactivated with oblique sinistral-normal kinematics during the Late Cretaceous-early Eocene in response to regional oblique extension associated with the opening of the Tasman and Coral seas. The recent strike-slip reverse movement was likely due to far-field contractional stresses from collisional tectonics at the eastern and northern boundaries of the Australian plate in the Cenozoic. New evidence shows that numerous strike-slip faults with a reverse component have displaced Cenozoic volcanic rocks, ranging in ages from - 31 to - 2 1 Ma, in southeast Queensland (Babaahmadi and Rosenbaum 2014b). These ages point out that faulting must have occurred after the late Oligocene. The reactivation of major faults resulted in the occurrence of brittle subsidiary faults in Cenozoic volcanic rocks. Intraplate transpressional deformation resulted from far-field stresses transmitted from the collisional zones at the northeast and southeast boundaries of the Australian plate since the late Oligocene. 7

SGTSG 2015: Riding the Waves References Babaahmadi A. & Rosenbaum G. 2013 Kinematics of the Demon Fault: Implications for Mesozoic strike-slip faulting in eastern Australia, Australian Journal of Earth Sciences, vol. 60, 255-269. Babaahmadi A. & Rosenbaum G. 2014a Late Mesozoic and Cenozoic wrench tectonics in eastern Australia: Insights from the North Pine Fault System (southeast Queensland), Journal of Geodynamics, vol. 73, pp. 83-99. Babaahmadi A. & Rosenbaum G. 2014b Late Cenozoic intraplate faulting in eastern Australia, Journal of Structural Geology, vol. 69, pp. 59-74. Babaahmadi A., Rosenbaum G. & Esterle J. 2015 Alternating episodes of extension and contraction during the Triassic: Evidence from Mesozoic sedimentary basins in eastern Australia, Australian Journal of Earth Sciences, vol. 64/5. Holcombe R. J., Stephens C. J., Fielding C. R., Gust D., Little T. A., Sliwa R., McPhie J. & Ewart A. 1997 Tectonic evolution of the northern New England Fold Belt: The Permian-Triassic Hunter-Bowen event, in: Ashley, P.M., Flood, P.G. (Eds.), Tectonics and metallogenesis of the New England Orogen., Geological Society of Australia Special Publication, pp. pp. 52-65. Korsh R. J., Totterdell J. M., Fomin T. & Nicoll M. G. 2009b Contractional structures and deformational events in the Bowen, Gunnedah and Surat Basins, eastern Australia, Australian Journal of Earth Sciences, vol. 56, no. 3, pp. 477-499.

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Australia, Nuna and beyond - Ribbon tectonics and the growth of the Australian continent

PETER BETTS , ROBIN ARMIT , ROSS CAYLEY , LOUIS MORESI AND DAVID MOORE 1

Monash University, Clayton, Victoria Geological Survey of Victoria,Melbourne, Victoria University of Melbourne, Parkville, Victoria

;

2 3

Micro-continental ribbons can develop in several tectonic settings. Some form during continental break-up in response to type 1 passive margn development, in which micro-continental ribbons are trapped by oceanic crust caused by mid-ocean-ridge jumping. The Mascarene Plateau is a modern example of such a micro-continent. The second mechanism that develops large ribbons is subduction roll-back. Elongate ribbons oriented parallel with convergent margin are 'ripped' from the over-riding plate during back arc basin development. The Lord Howe Rise represents a modern analogue of this type of ribbon. The two episodes of significant continental growth of the Australian continent have profoundly influenced by ribbon tectonics by modifying the plate geometry as well as increasing the aerial extent of the continent. Much of the Australia's Archaean and Palaeoproterozoic geology, which comprises two thirds of the continent, formed during rapid accretion of geological terranes between ca 1860 Ma and 1800 Ma (Betts et al., in press). Our reconstruction, based on mapping major suture zones and identifying terranes of similar geochemical footprint, suggest that continental growth occurred as a series of accretions of micro-continents and microcontinental ribbons positioned along at the edges of the protoNorth Australian Craton (NAC). Continental growth (ca 1860-1850 Ma) along the southern margin of the proto-NAC is recorded by the accretion of a ribbon micro-continent comprising the Aileron Terrane (northern Arunta Inlier) and its lateral continuation in the Gawler Craton. Eastward growth of the proto-NAC occurred during the accretion of the buried Numil Terrane (seismic province) and the Abingdon Seismic Province, which forms part of a broader zone of collision with the northwestern margin of Laurentia. The Tickalara Arc initially accreted with the Kimberley Craton at c. 1850 Ma and together these collided with the proto-NAC at ca 1820 Ma. Collision between the West Australian Craton and the proto-NAC at ca 1790-1760 Ma terminated the rapid growth of the Australian continent. Subsequent tectonism along the southern margin of the continent involved cycles of continental reworking. Several continental ribbons are interpreted to have ripped off the plate edge and re-accreted along the margin between ca 1800 Ma and 1640 Ma. The Gawler Orocline defined by folding of ribbon-terranes of the Gawler Craton was caused by the arrival of a mantle plume and an associated buoyant plateau forming the present day geometry of the craton. Subsequent ribbons are accreted onto the southern margin of the proto-NAC. These include the ribbons of the Gawler Craton and the Warumpi terrane which have been rifted off and re-accreted onto the margin and are likely to have been derived from the proto-NAC (Betts et al., 2011). Phanerozoic Australia experienced a second protracted period of accretionary tectonism and continental growth along the eastern margin of Gondwana. This growth occurred via back arc inversion (Collins, 2002), accretion of microcontinental ribbons and juvenile arc terranes. Several ribbons of east Gondwana are likely to have formed during the Neoproterozoic break-up of Rodinia associated with the development of type 1 passive margins (Huisman and Beaumont, 2011) as proposed by Moore et al., (2015) for the western Tasmanian terranes. These ribbons comprised continental crust separated by oceanic back arc or transitional crust accreted duirng the Cambrian Tyennan Orogeny to form the VanDieland micro-continent (Moore et al., 2015). The internal ribbons of VanDieland (Moore et al., 2015) are likely to represent the remnants of a microcontinents formed as a passive margin developed along the Antarctic margin of Gondwana. The final accretion of VanDieland during the Devonian had a profound effect on the Siluro-Devonian evolution of the Tasmanides (Cayley and Musgrave, 2013; Moresi et al., 2014), resulting in the development of the large Lachlan Orocline and wholesale lateral translations of Ordovician continental arc (Macquarie arc) and back arc terranes behind VanDieland, essentially embedding the micro-continent into the interior of the continent. In North Queensland, two micro-continental ribbons comprising rocks

SGTSG 2015: Riding the Waves deformed and metamorphosed during the Late Cambrian Delamerian Orogeny were rifted from the Gondwanan margin, resulting in the development of several oceanic back arc basins during the retreat of a west-dipping subduction zone. Late Ordovician to Early Silurian accretion of these microcontinental ribbons (basement to the Hodgkinson Province and Barnard Province) inverted oceanic back-arc basins and the Lucky Spring Arc, which fomed in response to east-dipping subduction beneath the Hodgkinson Province. Ribbons of North Queensland are likely to represent fragments of crust stranded in continental back arc regions following the Delamerian Orogeny. There are similarities in the tectonic system for the Palaeoproterozoic and Phanerozoic episodes of accretion and continental growth in the Australian Plate. Both accretionary orogens faced large external oceans at the edges of supercontinents characterised by protracted episodes of roll-back driven extension interrupted by transient episodes of accretion.

References

Betts, P.G., Giles, D., Aitken, A.RA 2011. Paleoproterozoic Accretion Processes of Australia and comparisons with Laurentia. International Geology Reviews 53, 1357-1376. Betts, P.G., Armit, R.J., Stewart, J., Aitken, A.R.A., L. Ailleres, Donchak, P., Hutton, L., Withnall., I., Giles, D. in press. Australia and Nuna. In: Li, X and Evans, D.A.D., Supercontinents through time. Geological Society of London Special Publication. Cayley, R.A., Musgrave, R.J., 2013. A paradigm change - the giant Lachlan Orocline: consequence of microcontinental ingestion, stalled subduction and southeast-directed Siluro-Devonian subduction rollback superimposed ona single accreted Ordovician arc assemblage: the Lachlan Fold Belt of eastern Australia. AGU Fall meeting abstracts 1, 07. Collins W.J. 2002. Hot orogens, tectonic switching, and creation of continental crust. Geology 30, 535-8 Huismans, R., Beaumont, C 2011, 'Depth-dependent extension, two-stage breakup and cratonic underplating at rifted margins', Nature 473, 74-78. Moore, D.H., Betts, P.G., Hall, M., 2015. Fragmented Tasmania: the transition from Rodinia to Gondwana. Australian Journal of Earth Sciences 62, 1-35. doi: 10.1080/08120099.2014.966757. Moresi, L.M., Betts, P.G., Miller, M.S., Cayley, R. 2014. Dynamics of continental accretion. Nature 508, 245248.

Acknowledgements We would like to acknowledge insightful discussion with Laurie Hutton, Paul Donchak, Ian Withnall, Bob Musgrave, Bill Collins, David Giles, Laurent Ailleres, Alan Aitken, John Stewart, Megan Miller, and David Willis who have all contributed to ideas related to this research.

SGTSG 2015: Riding the Waves

Using isotopic techniques to constrain the tectonic evolution of the Oman basement MORGAN L. BLADES 1 , ALAN S. COLLINS 1 , JOHN FODEN 1 , JUSTIN PAYNE 2 , SALAH ALKHIRBASH3, BENJAMIN THORPE 1 , GEORGE MURRAY 1 AND BRANDON ALESSIO 1 tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Sciences, The University of Adelaide, Adelaide, SA 5005, Australia. 2 The University of South Australia, Mawson Lakes, South Australia, Australia 3 Sultan Qaboos University, Muscat, Sultanate of Oman

Neoproterozoic rocks form in the periphery of a number of mega sutures that stitch together the supercontinent of Gondwana and Oman holds an important position at the eastern margin of the East African Orogen, approaching its boundary with Neoproterozoic India. However, the lack of exposure east of the Arabian Nubian Shield raises an essential question regarding the association between the isolated exposures of basement in Oman and the ANS; are the basement exposures in Oman an extension of the ANS or do they represent a completely different tectonic regime? The timing and nature of the accretion of Oman into the Arabian - Nubian Shield has continued to be problematic and the significance of the Neoproterozoic basement provides an important laboratory to unravel the Neoproterozoic tectonic geography of the region. There are three main outcrop areas in Oman where crystalline basement and overlying Neoproterozoic sedimentary rocks are exposed. The basement Huqf Supergroup of Oman crops out in northern Oman (Jebel Akhdar), east - central Oman (Huqf area, A1 Jobah and Jebel Ja'alan) and southern Oman, in the Mirbat area. Here we use U/Pb, Hf and other isotopic techniques to characterise the nature of these basement rocks and to further constrain 1) the timing and location of oceanic-crust subduction, 2) whether the subduction zones involved the consumption of old continental crust or the formation of new continental crust. Jebel Akhdar basement diamictites yield U/Pb ages between 794 - 767 Ma and are juvenile with epsilon hafnium values ranging between + 2.12 and + 9.01. This suggests that these rocks are Mantlederived with little crustal input. Further south we have basement windows in central Oman (A1 Jobah, Jebel Ja' Alan). Samples collected from A1 Joba yield U/Pb ages between 855 - 839 Ma. Similarly, plutonic rocks of the Jebel Ja'alan yield U/Pb ages between ca. 887 - 831 Ma and are juvenile (eHf(t) = 4.3 - 13.3) with any involvement of continental crust being minimal. Geochemistry conducted on a series cross cutting dolerite dykes of differing bulk chemistry and age give eNd (t) values between 6.78 - 0.59 (varying between each phase), suggesting different pulses of mantle derived magmatism. Metasedimentary sequences from Jebal Ja'alan have detrital zircon U/Pb ages ranging between ca. 2109 - 780 Ma. These have a wide sHf (t) range from +14.0 to -9.0, suggesting that these metasedimentary sequences represent not only juvenile Mantle-derived rocks, but also reworked continental crust. A dominant Tonian-aged population exists in all samples, ranging from ca. 876 Ma to 928 Ma and the youngest zircon population at 876 ± 1 2 Ma provide a maximum depositional age. In the south we have the largest basement outcrop in the Mirbat region. Plutonic rocks from this region yield a U/Pb igneous crystallisation ages between 764 - 810 Ma. These ages are broadly contemporaneous with other basement outcrops further north. Collectively from these data we suggest that Oman, saw a period of crustal growth from ca. 880 - 750 Ma via arc accretion. Evidence of deformation and metamorphism can be seen in central Oman with amphibolite grade metamorphism occurring at ca. 830 Ma - 845 Ma, which we suggest dates the amalgamation of these juvenile arcs with the western margin of Neoproterozoic India. The relatively early cessation of accretion here compared to the Arabian Nubian shield suggests that that this active Tonian margin of Neoproterozoic India transformed to a passive margin by Cryogenian times with consumption of the Mozambique Ocean switching to the African margin.

SGTSG 2015: Riding the Waves

Structure of the Texas Orocline below the sedimentary cover: insights from geophysical data SAMUEL BROOKE-BARNETT 1 AND GIDEON ROSENBAUM 2 Origin Energy, 135 Coronation Drive, Milton 4064, Australia School of Earth Sciences, The University of Queensland, Brisbane 4072, Australia

The New England Orogen in eastern Australia contains multiple orogenic-scale curvatures (oroclines). The largest of these, the Texas Orocline, is mostly obscured by post-oroclinal sedimentary basins. These basins also cover the boundary between the northern and southern New England Orogen, and consequently inhibit understanding of how these two components of the New England Orogen are linked. Here we present an interpretation of multiple geophysical datasets that elucidate the structure of the Texas Orocline below the sedimentary cover. 2D seismic, aeromagnetic total magnetic intenity and Bouguer gravity data are used in conjunction with outcrop and well data to determine the depth to the New England "basement", and to identify major faults intersecting both basement and the sedimentary cover. These data were also used to trace the subsurface continuation of the Peel-Yarrol Fault System, which is identifiable from high gravity and magnetic anomalies owing to the presence of serpentinite along the fault system. Using the seismic transects, we have indentified a fault bounded sub-trough of the Bowen Basin filled with Early Permian sedimentary rocks. However, this subtrough deviates from the general north-south trend of the Bowen Basin and is oriented roughly parallel to the western limb of the Texas Orocline. The results therefore suggest that the development of the Bowen Basin was contemporaneous with initial stages of oroclinal bending. Subsequent phases of strike-slip and contractional deformation have further tightened the pre-existing curvatures.

SGTSG 2015: Riding the Waves

The paradox of Permian-Triassic subduction in the Southern New England Orogen JEFF BROWNLOW 1 7

9 Belinda Place Armidale, NSW 2350. (Retired, formerly GSNSW, Armidale)

Permian-Triassic geological development of the Southern New England Orogen (sNEO) presents a paradox: available evidence implies subduction and denies alternatives but only poorly matches standard subduction models. This paradox arises despite: (a) an active continental margin setting; (b) widespread calc-alkaline magmatism including Cordilleran granitoids, andesites, and basalts with arc signatures; (c) other orogenic attributes such as multiple episodes of deformation, metamorphism, serpentinite emplacement, basin formation, and mineralisation; and (d) a complementary record of a shared history preserved in fringing basins. Resolving this paradox required answers to three key questions: (a) which geological model; (b) which subduction mode (free or flat); and (c) how were any dipping slabs removed - rollback or breakoff? Volcanism is considered key to selecting an appropriate geological model because it: (a) links the mantle and the surface; (b) provides the clearest basinal record of orogenic activity; (c) occurs as discrete episodes reflecting a distinctly episodic geological history and (d) associated activity exhibits temporal and spatial recurrence (Brownlow, 2010). Recurrence typically involves deformation before volcanism (with an intervening gap where the stratigraphic record is adequate) plus granitoids with or following volcanism, initially outboard (mainly mesogranitoids) and later inboard (commonly large, Sn-F mineralised leucogranitoids). The Permian-Triassic stratigraphic record encompasses seven volcanic episodes of which focus herein is on the first six (all of likely continental margin setting) and principally on the fourth (Wandsworth - the most complex, problematic and potentially informative). The literature broadly distinguishes two standard subduction modes - free and flat. Free subduction 90% globally) is a simple continuum process driven internally by negative buoyancy and exhibits dips > 30°, simpler profiles, hot-based upper plates due to in-place asthenosphere, slab descent free of upper plate interference, and outboard arc volcanism. Rollback is common. In contrast, flat subduction is atypical (~ 10% globally), involves continental advance (± oceanic plateau subduction) and exhibits dips < 30°, complex profiles, cold-based upper plates, displaced asthenosphere which leads to upper plate deformation and inhibits slab descent, plus volcanism that is displaced inboard (e.g., central Mexico), anomalous (e.g., adakites), or suppressed (potentially leading to fluid transfer to the overriding plate as inferred for the Laramide Orogeny). Rollback is atypical (incompatible? without mode change). Testing these subduction modes against geological recurrence raises problems for free subduction: 1. Distinctly episodic, rapidly changing sNEO geology seems incompatible with the sedate continuum of free subduction implied by negative buoyancy-controlled slab descent. 2. The distribution and NE alignment of the Wandsworth system contrasts with that of the two preceding and the two following systems. Those systems collectively exhibit a crude NNW alignment and a crude outward age progression suggestive of rollback (free subduction). However, the Wandsworth system is out of place, off trend, overlaps other systems and requires a double change (from the norm and back) difficult for free subduction. 3. Temporal overlap of the third, fourth (Wandsworth) and fifth systems severely constrains feasible subduction. For example the largely Permian Wandsworth system likely continued into the Early Triassic to produce inboard Sn-F leucogranitoids (correct alignment and too far inboard for other systems), whereas basal Triassic deformation in the Gunnedah Basin likely marks the start of the succeeding system. The Wandsworth system also overlaps the preceding system. Free subduction provides no obvious mechanism for such system overlaps. 4. Two or more subducting slabs cannot occupy the same 3-D mantle space at the same time, yet subduction of the third, fourth (Wandsworth) and fifth slabs implies such a configuration unless the slabs were flat and stacked (hence flat rather than free subduction).

SGTSG 2015: Riding the Waves 5.

Geological recurrence, specifically initial deformation and late inboard granitoid younging, is unexplained by free subduction, particularly if rollback is involved.

Thus, these tests largely deny free subduction as a cause of the Wandsworth system. Flat subduction is less constrained by these tests, in part because external control by continental advance could potentially initiate two or more subduction systems that overlap in time and place. Flat Wandsworth subduction also helps explain the contrast with the penecontemporaneous, subparallel Gerringong system on the South Coast. Contemporary Sydney Basin records suggest coeval onsets but contrasting evolutions: narrow, fast, and brief for the Gerringong system, but wide, slow and protracted for the Wandsworth system. As the latitic Gerringong system is conventionally attributed to (rapidly evolving) free subduction, the Wandsworth requires a different mode - likely slowly evolving, flat subduction. Four stages (up until and including widespread volcanism) are envisaged: 1.

Compression leading to deformation and uplift until the plate breaks;

Slab descent lubricated by dehydration, plausibly down-dragging the overlying plate;

Buoyancy leading to slab flattening and contact with the overlying plate, uplifting it (potentially augmented later by thermal uplift and uplift due to hydration); and

Loss of slab buoyancy leading to down-bending and asthenosphere inflow to produce calc-alkaline volcanism from reaction with the hydrated lithosphere of the overriding plate.

Finally, the key question arises — what happened to once subducting slabs? Rollback is unconvincing due to: (a) lack of supporting evidence of significant crustal extension or rotation; (b) contrasting spatial trends between various systems; (c) difficulty in explaining early deformations and late inboard younging of granitoids, and (d) likely 3-D plate interference. Thus, breakoff is favoured by default. This suggests two additional stages: 5.

Further slab subsidence leading to slab breakoff and outboard granitoids.

Slow inboard sliding, sinking and/or slab rotation leading to asthenospheric upwelling behind the trailing edge and intrusion of inboard, Sn-F mineralised leucogranitoids.

Thus in summary, Permian-Triassic geological development of the sNEO presents a paradox: available evidence implies subduction and denies obvious alternatives but only poorly matches standard models. Classic subduction is too continuous for multiple systems, too sedate for the tempo of sNEO geology, provides no repeatable mechanism for early deformation, and appears unable to accommodate inboard granitoid younging, system overlap and trend contrasts. In contrast, flat subduction is better able to accommodate the width, diversity and overlap (through slab stacking) of the Wandsworth and other systems and provides a mechanism (continental advance) for deformation. However, it needs to incorporate slab breakoff as a means of slab removal and to facilitate completion of one regime even after another has begun. A specific issue arises for the earliest Triassic deformation (fifth system): longitudinal offset of outboard Wandsworth granitoids to the SW relative to inboard Wandsworth granitoids suggesting overall WSW movement of sNEO crust rather than the opposite sense of movement required if continental advance were the driving force. Hence flat subduction is a better starting point than free subduction for modelling sNEO development, but paradoxically, standard models appear to require modification (addition of slab breakoff and optionally compression from the oceanic side) to sensibly model the Wandsworth and other sNEO systems. Likely applicable systems are those that involve inboard Sn-F mineralised leucogranitoids, including the first, fifth and sixth and possibly the third. References Brownlow, J., 2010, Mid Permian to mid Triassic development of the southern New England Orogen. In: Buckman S. & Blevin P.L. (eds) New England Orogen 2010: Proceedings of a conference held at the University of New England, N.S.W., November 2010, pp. 62-68. University of New England, Armidale.

SGTSG 2015: Riding the Waves

How complex was the last lOOMy? - geomorphic evidence from the Mesozoic Clarence-Moreton Basin in northeastern New South Wales

JEFF BROWNLOW ;

9 Belinda Place Armidale, NSW 2350. (Retired, formerly GSNSW, Armidale)

Modern interpretations of SE Australian geomorphology emphasise landscape stability ± antiquity with increasingly sophisticated analysis and numerical modelling seeking to link a single, protracted (delayed?) continuum response to Tasman Sea opening (ca 80 Ma-52 Ma) to the development of the Great Divide and the Great Escarpment (e.g., review by Bishop, 2007). These interpretations differ from traditional interpretations of a complex, multi-system Tertiary geological and geomorphic history. Observations in and around the Mesozoic Clarence-Moreton Basin (CMB) in NE NSW suggest that the basin was repeatedly deformed after the end of the Jurassic and probably after the inferred end of basinal sedimentation in the mid Cretaceous (i.e., during the last 100 My), causing geomorphic expression that was potentially widespread across SE Australia. These observations seem to fit far better with traditional rather than modern geomorphic interpretations. Widespread folding in the CMB is manifest geomorphically by pronounced relief on the latest Triassic Laytons Range Conglomerate (LRC), Early Jurassic Gatton Sandstone (GS) and Late Jurassic Kangaroo Creek Sandstone (KCS) compared to subdued relief on most other rock units. LRC dips NE along the NW-trending Laytons Range near the SW basin margin at Nymboida and dipping GS and KCS form parallel ridges nearby to the north. GS dips moderately westerly along the NNE-striking Coast Range and Shark Creek Ridge on the eastern CMB limb and dipping KCS forms a partly buried, parallel ridge further north (south of Lismore). Parallel thrust faulting along the eastern side of Shark Creek Ridge is part of a major fault repetition of the eastern basin sequence. Westerly dipping GS forms prominent coastal headlands in the Angourie-Yamba-Woody Head area. KCS dips gently inwardly on either side of the N-S trending CMB basin axis, forming prominent, axis-parallel ridges. KCS also forms irregular areas of pronounced relief along the E-W trending Kungala Anticline in the basin south, and along a N-E line of domes (Clifden-Banyabba-Tullymorgan) north of Grafton. The CMB environs also exhibits ordered drainage. Laytons Range at Nymboida separates two NWflowing Clarence tributaries: the lower Nymboida-lower Mann and the parallel lower Orara Rivers. Parallel to these is a low, asymmetric ridge and adjacent NE-facing scarp near South Grafton which separates the lower Orara Valley from the extensive lower Clarence Valley to the NE. The lower Clarence flows to the NE, approximately perpendicular to the Layton Range trend, in a likely fault- or fracture-controlled corridor that extends across the easterly, N-S-trending, axis-parallel KCS ridge near Maclean. Quartzose "pea"-gravels unconformably overlie the CMB south of Casino (close to the basin axis) and north of Maclean (locally capping the eastern, N-S-trending, axis-parallel KCS ridge). The gravel composition indicates derivation from a deeply weathered terrain. The gravels resemble and potentially correlate with Eocene Armidale beds and other similar occurrences in the surrounding region. If so, the drainage basin might have been quite different from the modern Clarence River. The unconformity beneath these Eocene? gravels limits the age of NS folding of the KCS to Early Eocene or older. Likely E-W shortening is incompatible with both the early transpressional phase and later extensional phase of Tasman Sea opening (ca 80 Ma-52 Ma), so probably developed earlier (potentially 100 Ma-80 Ma). If widespread ferricrete development on KCS formed during regional Palaeocene? weathering, then ferricrete occurrence on KCS along the Kungala Anticline and on domes north of Grafton also suggests formation of those folds prior to Tasman Sea opening. The coincident NW strike of Laytons Range, the lower Nymboida-lower Mann and parallel lower Orara Rivers plus the NE-facing scarp at South Grafton suggests common origin involving NE-SW shortening leading to folding around Nymboida, downwarping along the lower Orara and upwarping plus erosion and scarp retreat around South Grafton. Sinuous aeromagnetic anomalies adjacent to the 15

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NE-facing scarp at South Grafton are interpreted as maghemite-bearing gravels along scarp-derived gullies. Their inferred composition and configuration suggest erosion of an upwarped old (Palaeocene?) land surface onto which the lower Orara is weakly incised along a parallel downwarp. Plausibly, Eocene scarp initiation occurred along the side of a NW-aligned trunk drainage stream that deposited the Eocene? "pea"-gravels near Casino and Maclean. The perpendicular, NE-SW trend of the lower Clarence is consistent with formation later in the episode of NE-SW shortening, but after strain accommodation began to change from folding to fracturing. The deeply leached (K-poor radiometric signature), polylithic composition of high-level gravels along the Clarence River near Grafton contrasts with unleached modern fluvial sediments (K-rich where draining certain Miocene volcanics), suggesting a pre-Miocene, Oligocene? age. Their clear association with the Clarence River and compositional contrast with Eocene? gravels suggest deposition following drainage basin rearrangement for which the most likely cause was the earth movements that formed the NNE-striking fold limbs and ridges in the eastern basin and the perpendicular Powapar Fault in the western basin. Miocene basalts close to Lismore unconformably overlie the northern end of the N-S and NNE trending KCS ridges cited above, establishing a minimum age for these folding events and for any associated drainage rearrangements. Contrasting beach ridge shapes (McAuleys lead v present day beach north of Woody Head) suggest the possibility of Late Pleistocene/ Early Holocene uplift of coastal sandstones in the Yamba-Woody Head area. More broadly across SE Australia, the lower Clarence parallels the Darling River and other streams, and those streams plus the perpendicular Laytons Range potentially formed through Eocene? (postTasman Sea opening age), NE-SW shortening. Similarly, NNE trends are evident in the strike of the Illawarra Escarpment on the South Coast and the long axis of the Murray-Darling, and these plus parallel structures in the CMB and offset of the Tasmantid Seamount chain in the Tasman Sea are consistent with WNW-ESE shortening during the Oligocene. Possible late coastal uplift may correlate with Pleistocene fault block uplift and related offset of the Murray River in SW NSW. Thus, deformation affecting the CMB since Tasman Sea opening (including faulting and folding not cited above) and its geomorphic expression likely extends broadly across SE Australia. The strength and complexity of that inferred, multi-stage geological and geomorphic history appears difficult to reconcile with modern geomorphological interpretations that seek to explain the development of the Great Escarpment and the Great Divide by a protracted, single-stage process initiated at Tasman Sea opening. Whether deformation affecting the CMB early in, or prior to, Tasman Sea opening left a stillrecognisable geomorphic expression in SE Australia is less clear but broad geological impact is consistent with the syn- and post- depositional records of other basins in the broader region. Finally, the linkage inferred herein between geology and geomorphology potentially better fits a Hortonian model of drainage limited upstream by erosional thresholds (leading to a zone of no erosion adjacent to a quasi-stable watershed which mduringight potentially survive as a long-lived record of uplift) than a Davisian model of headward erosion constrained downstream by base levels. Ironically, that Davisian model, first introduced regionally a century ago by Griffith Taylor, remains integral to modern geomorphic interpretations1.

References Bishop, P., 2007, Long- term landscape evolution: linking tectonics and surface processes. Earth Surface Processes and Landforms, 32(3), 329-365.

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The Gulf of California (Mexico): Constraining the timing and evolution of rifting from the plutonic record

JOSE DUQUE-TRUJILLO , LUCA FERRARI , SCOTT BRYAN , TERESA OROZCO-ESQUIVEL AND MARGARITA LOPEZ-MARTINEZ Centra de Geociencias, Universidad Nacional Autonoma de Mexico, Campus Juriquilla, Queretaro, 76230 Queretaro, Mexico School of Earth, Environmental and Biological Sciences, Queensland University of Technology (QUT), Brisbane, Queensland 4001, Australia Departamento de Geologia, Centro de Investigacion Cientifi ca y de Educacion Superior de Ensenada (CICESE), Carretera Ensenada-Tijuana No. 3918, 22860 Ensenada, Baja California, Mexico 1

Insights on the timing of rifting can be obtained by studying plutonic rocks emplaced across the zone of crustal rupture and their exhumation driven by extensional tectonics. The Gulf of California is one of the youngest examples of crustal rupture that has produced a new and developing oceanic basin at a formerly convergent margin. Previous studies principally focused along the outboard rifted margin in Baja California indicated rifting was initiated after subduction and related magmatism ceased at ca. 14-12.5 Ma (e.g., Umhoefer, 2011). However, the geologic record along the inboard and Mexican mainland margin provides a much longer record and earlier inception of crustal stretching that began as early as late Oligocene. As early as -30 Ma, crustal extension had already affected a 200 km wide region spanning from the eastern Sierra Madre Occidental to easternmost Baja California (Ferrari et al., 2013). A novel approach we have taken to resolve the timing, pace and evolution of rifting in the Gulf of California has been to focus on plutonic rocks exposed both along the conjugate Baja California and western Mexico rift margins, as well as recently recovered plutonic rocks now exposed on submerged rifted blocks inside the Gulf. This is because the timing of cooling and exhumation of pre- and synrift plutonic rocks can provide constraints on the timing and rate of rifting. Forty-one samples were dated via U/Pb zircon and Ar/ Ar mineral ages, yielding emplacement age and thermochronological constraints on timing and rate of cooling (Duque-Trujillo et al., 2015). U/Pb zircon dating identified three general age suites of plutonic rocks across the Gulf of California: Cretaceous (-100-65 Ma); Early Miocene (-25-18 Ma): and Middle Miocene (-16-13 Ma). The early and middle Miocene plutons form an extensive suite emplaced at shallow depths within the basement Cretaceous-Paleocene batholiths. Early Miocene granitoids define an elongated WNW-ESE belt crossing the entire southern gulf. Most have an intermediate composition (<67 Si0 wt%), but a distinctive group of high-silica granites (>75 Si0 wt%) was emplaced -20-18 Ma, near the end of the early Miocene. Age span and chemical composition of the early Miocene silicic plutons essentially overlap rhyolite ignimbrites and domes exposed in the southern Sierra Madre Occidental of mainland Mexico and in southern Baja California, suggesting that eruptive sources for the early Miocene ignimbrite flare-up (Bryan et al., 2008; 2014; Ferrari et al., 2013) may also have resided within the Gulf. Importantly, Early Miocene plutons cooled below the Ar/ Ar biotite closure temperature (350400 °C) in less than 2.5 m.y., while middle Miocene plutons show an even more restricted cooling window of <1.5 m.y. from emplacement defined by the U/Pb zircon age to the Ar/ Ar biotite age. The middle Miocene granitoids are less widely distributed, and their age and intermediate-composition overlap with lower-volume volcanism recorded by the middle and upper members of the onshore Comondu Group in Baja California. These middle and upper members of the Comondu Group have persistently been interpreted as the final expression of supra-subduction zone arc volcanism along the margin. However, our widely spaced sampling of the generally sediment-covered igneous crust suggests that middle Miocene primary volcanic rocks are much less abundant than implied by previous models. Thermobarometry data for the middle Miocene plutonic rocks also indicate a very shallow depth (<5 km) of emplacement with some rocks showing a distinctive inequigranular texture of feldpsar phenocrysts set in a fine grained holocrystalline groundmass. The rapid cooling of Early and Middle Miocene plutons to <350 °C is attributed to their shallow emplacement and cooling enhanced by exhumation soon after intrusion. Importantly, the timing and rate of cooling, and the extensive present-day exposure of Miocene plutonic rock on the submerged rift margins contrasts markedly with previous interpretations for a high-standing andesitic volcanic arc to have existed in this region and for 40

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rifting (and thus any tectonic exhumation) to have occurred extremely rapidly between - 1 2 and 6 Ma when the first oceanic crust was generated in the Gulf. W e therefore find no evidence for a volcanic arc to have existed during the Middle Miocene along western Mexico-Baja California. Instead, these new data are consistent with a model whereby a switch in rift modes occurred - 1 8 Ma, where extension became focused along the site of the future Gulf of California (Bryan et al., 2014). This switch to a narrow rift mode was associated with a change to more effusive and intermediate composition volcanism and coincided with rapid cooling of young plutons. The culmination of wide and narrow rifting, from - 3 0 - 1 2 Ma, had thinned the crust to half of its original thickness ( - 2 0 km) and transtensional deformation related to the dragging of Baja California by the Pacific Plate aided in completing the rifting process to form the modern Gulf. References Bryan, S.E., Ferrari, L., Reiners, P.W., Allen, C.M., Petrone, C.M., Ramos-Rosique, A., and Campbell, I.H., 2008, New insights into crustal contributions to large volume rhyolite generation at the mid-Tertiary Sierra Madre Occidental Province, Mexico, revealed by U/Pb geochronology: Journal of Petrology, v. 49, no. l , p . 47-77. Bryan, S.E., Orozco-Esquivel, T., Ferrari, L., and Lopez-Martinez, M., 2014, Pulling Apart the Mid to Late Cenozoic Magmatic Record of the Gulf of California: Is there a Comondu Arc?, in Gomez-Tuena, A., Straub, S.M., Zellmer, G.F., eds. Orogenic andesites and crustal growth: Geological Society, London, Special Publication 385, 389-407. Duque-Trujillo, J., Ferrari, L., Orozco-Esquivel, T., Lopez-Martinez, M., Lonsdale, P., Bryan, S.E., Kluesner, J., Pinero-Lajas, D., Solari, L., 2015 Timing of rifting in the southern Gulf of California and its conjugate margins: Insights from the plutonic record. Geological Society of America Bulletin, v. 127 no. 5-6 p. 702-736. Ferrari, L., Lopez-Martinez, M., Orozco-Esquivel, T., Bryan, S.E., Duque-Trujillo, J., Lonsdale, P., and Solari, L., 2013, Late Oligocene to middle Miocene rifting and syn-extensional magmatism in the southwestern Sierra Madre Occidental, Mexico: The beginning of the Gulf of California rift. Geosphere, v. 9, p. 1161-1200, doi: 10 .1130/GES00925 .1. Umhoefer, P.J., 2011, Why did the southern Gulf of California rupture so rapidly?—Oblique divergence across hot, weak lithosphere along a technically active margin. GSA Today, v. 21, no. 11, p. 1-10.

SGTSG 2015: Riding the Waves

Carbonate alteration is the dominant weakening mechanism in the Doolena Gap Greenstone Belt preserving the keel rock in a dome-and-keel terrane DUNCAN BURKE-SHYNE 1 , DANIEL WIEMER 1 , CHRISTOPH SCHRANK 1 AND DAVID MURPHY 1 Queensland

University of Technology, 2 George St, Brisbane, QLD 4000, Australia

We hypothesise that strain localisation facilitates large vertical displacements in dome-and-keel structures by a coupled thermo-hydro-mechanical process controlled by carbonate alteration of the mafic keel rocks. The partial crustal overturn of these dome-and-keel structures is often ascribed to a Rayleigh Taylor (RT) instability [1], Numerical models suggest that RT instabilities can only progress in dome-and-keel terranes if the effective viscosity of the mafic rock decreases such that localised shear zones form, allowing for protracted periods of deformation accommodating large strain [2, 3]. We examined anastomosing shear zones and preserved lozenges of dense keel rock exposed in the Doolena Gap Greenstone Belt (DGGB), East Pilbara Terrane (EPT, Western Australia), to test our hypothesis. The DGGB provides an excellent study area because it represents the synclinal limb of a greenstone keel, containing dominantly mafic schists and minor felsic volcanics and meta-sediments, in which a brittle to ductile gradient of deformation is observed towards the domal Muccan Granitoid Complex. The increasing deformational intensity within the mafic keel rocks and associated occurence of localised anastomosing carbonate shear zones record large vertical displacement between dome-andkeel while preserving the early structures associated with dome-and-keel formation due to not being completely overprinted by subsequent deformation events. We present microstructural and geochemical analyses of three detailed transects across anastomosing shear zones within the most deformed part of DGGB greenstones, the Central Fold Belt, which abuts the mylonitic shear zone separating the Muccan dome in the North from the keel rocks [4]. Carbonate alteration is spatially associated with shear zones at all scales and with all deformation phases, as indicated by overprinting relationships. Here, we focus on the deformation event (D2 after [4]), which accommodated the largest amount of greenstones displacement relative to the Muccan dome. D2 displays highly transposed, rootless isoclinal folds in an anastomosing shear zone network, which locally preserves ultramafic to mafic intrusives and extrusives in metric to decametric lozenges. The studied transects feature three 10-m wide shear zones with carbonate alteration bounded by relatively less deformed lenses of mafic rock exhibiting remnant igneous textures. We investigate the spatial correlation of relative strain intensity, microfabrics, deformation mechanisms, and trace- and major-element geochemistry to unravel the interplay of carbonate alteration and strain localisation. The mafic low-strain lenses comprise mafic schists that have been altered by carbonate-rich fluids under greenschist-facies conditions. Carbonate rocks within the shear zones represent alteration products of the mafic protolith having experienced almost complete replacement by carbonate minerals gradationally transitioning to mylonitic carbonates at the centre. The shear zones exhibit a gradual increase of foliation intensity towards their centres, which is interpreted as an increase in finite strain. The mafic schists in the low strain domain are composed of dominantly chlorite and calcite ± dolomite with minor quartz and opaques. Within the shear zones the composition is almost entirely calcite ± dolomite with minor quartz, opaques, goethite and accessory micas. The mineralogical transition displays a sharp disappearance of chlorite at the mafic schist to carbonate contact replaced by calcite, with gradually decreasing quartz towards the centre of the shear zones. Some relict magmatic textures are observed within the mafic schists. These remnant igneous textures gradually disappear in the carbonate towards the centre of the shear zones. The S2 foliation is defined by cleavage domains of chlorite and carbonate minerals, anastomosing around microlithons of microcrystalline quartz. Deformation mechanisms on the micro-scale include the shape preferred orientation of chlorite and carbonate minerals and significant grain size reduction in quartz by bulging and subgrain rotation and similar mechanisms in carbonate minerals. From major element geochemical analysis there is a clear trend of decreasing Si0 2 and associated relative increase in CaO towards the centre of the shear zones. Trace element analysis shows that the carbonate material in the shear zones is derived from a mafic protolith. 19

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The observed spatial assocation of increasing foliation density with an increase in dynamic recrystallisation and the proportion of carbonate minerals suggests that carbonate replacement played a major role during strain localisation. Ongoing work focuses on identifying the source and composition of fluids associated with the multiple carbonate replacement events.

References 1.

O'Neill, C. and V. Debaille, The evolution of Hadean-Eoarchaean geodynamics. Earth and Planetary Science Letters, 2014. 406(0): p. 49-58.

Thebaud, N. and P.F. Rey, Archean gravity-driven tectonics on hot and flooded continents: Controls on long-lived mineralised hydrothermal systems away from continental margins. Precambrian Research, 2013.229: p. 93-104. de Bremond d'Ars, J., C. Lecuyer, and B. Reynard, Hydrothermalism and diapirism in the Archean: gravitational instability constraints. Tectonophysics, 1999. 304(1-2): p. 29-39. Wiemer, D., C. Schrank, and D. Murphy, Lithostratigraphy and structure of the early Archaean Doolena Gap greenstone belt, East Pilbara Terrane (Western Australia). 2014: AGU Fall Meeting.

3. 4.

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A new geodynamic model for Cambrian Tasmania/Australian East Gondwanaland - continent-continent collision above doubly divergent Dundas Trough subduction zones. Ross CAYLEY 3

Geological Survey of Victoria, 10/121 Exhibition Street, Melbourne, Victoria, Australia.

A new geodynamic model developed for the wide and complex Lachlan Fold Belt (LFB) of eastern Australia advocates for clockwise oroclinal (mega-) folding and fault fragmentation related to asymmetric south-east-directed slab-rollback during the Silurian (435-390Ma), superimposed onto an simple continent-dipping accretionary system - the Ordovician Macquarie Arc (Cayley & Musgrave, in review). The trigger for slab rollback and consequent Macquarie Arc system fragmentation was accretion of the microcontinent Vandieland, drawn obliquely into the southern end of the Macquarie Arc subduction zone during the Ordovician. Microcontinent collision at ~440Ma congested the southern end of the subduction zone, precipitating a dramatic tectonic mode switch to dextral transtension (Moresi et al., 2014). The Lachlan Orocline developed to completion as a giant Z-shaped, upper-plate megafold in rollback through NSW, eastern Victoria and north-eastern Tasmania. Beginning around 405 Ma Vandieland transferred abruptly to the upper plate of the system via a 'slab window' opening within the congested subduction-zone limb wrapping its eastern flank. Southeasterly asthenospheric flows through this window left a distinctive legacy in Vandieland microcontinent lithosphere - magmatism related to lithospheric tearing and rafting; a second-order orocline superimposed over Precambrian-Cambrian Tasmanian geology (the Dundas-Fossey Orocline); and long-term lithospheric thinning - Bass Strait. The Lachlan Orocline model constrains retrodeformation of the Dundas-Fossey Orocline to produce a startlingly different - simpler and symmetrical - pre-Silurian Tasmania comprising two linear, elongate strips of Meso-Proterozoic continental crust - the Rocky Cape Block to the west and the Tyennan Block to the east - separated by a linear strip of complexly-deformed Cambrian rocks - the Dundas Trough. All three elements trend north-south and continue into central Victoria as Vandieland. In the Cambrian, Vandieland lay directly south along-strike from the Mt Stavely in western Victoria. New structural scenarios can now be tested for Cambrian Tasmania. The Rocky Cape Block and the Tyennan Block both contain Cambrian metamorphic complexes (Meffre et al, 2000). Retro-deformed, the metamorphic complexes are linear, disposed symmetrically either side of the Dundas Trough. They share P-T-t characters, a -510-511 Ma age, and cooling histories. Their metamorphic peaks match Tyennan Orogeny onset and timing of Cambrian fore-arc ophiolite emplacement onto the Rocky Cape Block. Their ages overlap Mt Read Volcanics in the intervening Dundas Trough, which have distinct continental magmatic arc geochemical character (Corbett et al., 2014). The metamorphic complexes, subduction signature in volcanics, and fore-arc ultramafics all suggest the Proterozoic crustal blocks and Dundas Trough occupied upper plate settings during the Tyennan Orogeny. Symmetrical disposition of the metamorphic complexes on either side of the Dundas Trough is matched by reversal of Cambrian transport vectors and vergence directions - top-to-west in the Rocky Cape Block, and topto-east in the Tyennan Block, explained as the Rocky Cape and Tyennan Blocks occupying separate, but mirrored, upper plates of a doubly-divergent subduction system (eg. Soesoo et al., 1998) active in the Cambrian west Paleopacific. The Dundas Trough represents the suture formed as these two systems drew together in a 'soft' continent-continent collision in the middle Cambrian. Prior to Tyennan Orogeny collision, the two Tasmanian systems were separated. The western subduction zone beneath the Rocky Cape Block eastern margin was part of the continent-dipping subduction zone active along East Gondwanaland. The eastern subduction zone beneath the Tyennan Block continental ribbon western margin was part of a larger, mostly intra-oceanic, east-dipping subduction system farther outboard in the western Palaeopacific. Both continental margins were fringed by Cambrian forearc successions and active continental magmatic arcs. Throughout the Cambrian, overall oblique-sinistral convergence of the Gondwana and paleoPacific plates drew the Tyennan Block continental ribbon towards the Gondwana margin, the intervening lower-plate progressively subducting along both sides. Convergence accelerated at -515 Ma, evolving into roll-back to form and partially exhume back-arc 21

SGTSG 2015: Riding the Waves metamorphic complexes in both continental blocks, also seen in the coeval Glenelg River Metamorphic Complex in western Victoria, which lay along strike from the Arthur Lineament in the Cambrian. Around 508 Ma, the divergent subduction systems began to collide, prelude to final upperplate continent-continent collision at ~500Ma. Rocky Cape arc- and fore-arc material was backthrust west at this time, most probably eroded. The Arthur Lineament was inverted. Simultaneously, Tyennan Block arc and forearc material was backthrust east over, with east-vergent inversion of Tyennan Block metamorphic complexes. The Tyennan and Rocky Cape forearcs and arcs collided and intermixed into the 'Dundas Trough' suture. North of Vandieland, evidence for double-divergent Cambrian subduction zones in the Delamerian and proto-western Lachlan Fold belts of mainland Australia persists. Long-lived west-dipping subduction beneath the east Gondwanaland margin at Mt Stavely matches the Rocky Cape system in Tasmania. Outboard, east-dipping intraoceanic subduction explains the upper plate geochemistry of intra-oceanic Cambrian igneous rocks of the proto-western Lachlan Fold Belt (Crawford et al., 1984). In both cases, mirrored upper-plates were brought together at the end of the Cambrian by foundering of the intervening doubly-divergent lower plate, leading to self-extinguishing convergence. On mainland Australia, the 'soft' collision was between a continental margin arc and a low-relief, extended intraoceanic arc with very limited crustal thickening. In Tasmania, between the Rocky Cape Block and the Tyennan Block microcontinent, with the Dundas Trough preserved as the suture - a collage of continental margin boninitic fore-arc and calc-alkaline arc materials sourced from both sides. Self-extinguishment of paired divergent subduction zones explains how the continent-dipping subduction zone active along the Australian margin of East Gondwana could become extinct at the time of Vandieland accretion and form an oceanic 'trapped plate segment' the proto-Bendigo Zone - without proceeding into the rollback seen in subsequent Lachlan Orogenesis (eg. Moresi et al., 2014). References Corbett, K.D., Berry, R.F., Everard, J.L., Calver, C.R., Crzwford, A.JU., Vicary, M.J., Bottrill, R.S. McNeill, A.W., 2014. Cambrian Tasmania. In Corbett, D.D., Quilty, P.G., & Calver, C.R. (eds.) Geological Evolution of Tasmania. 95-240 Geological Society of Australia Special Publication 24. Geological Society of Australia (Tasmania Division). Crawford. A.J., Cameron, W.E. & Keays, R.R., 1984. The association bonininte low-Ti andesite-tholeiite in the heathcote greenstone belt, Victoria; ensimatic setting for the early Lachlan Fold Belt. Australian Journal of Earth Sciences, 31. 161-174. Meffre, S., Berry, R.F., & Hall, M., 2000. Cambrian metamorphic complexes in Tasmania: tectonic implications. Australian Journal of Earth Sciences 47, 971-985. Moresi, L., Betts, P.G., Miller, M.S. & Cayley, R.A., 2014. Dynamics of continental accretion. Nature. doi:10.1038/naturel3033 Soesoo, A., Bons, P.D., Gray, D.A. & Foster, D.A., 1997. Divergent double subduction: tectonic and petrologic consequences. Geology. 25, 755-758.

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Arc root buoyancy: metastable igneous assemblages in deformed lower crust, Fiordland, New Zealand TIMOTHY CHAPMAN 1 , GEOFFREY L. CLARKE \ SANDRA PIAZOLO 2 AND NATHAN R . DACZKO 2 School of Geosciences, F09, The University of Sydney, Sydney, NSW, 2006, Australia ARC Centre of Excellence for Core to Crust Fluid Systems and GEMOC, Department of Earth and Planetry Sciences, Macquarie University, NSW, 2109, Australia 2

The Breaksea Orthogneiss in Fiordland, New Zealand comprises dioritic omphacite granulite interlayered with basic eclogite, both formed in the root of the Cretaceous Pacific Gondwana margin at 850°C and P ~ 1.8 GPa. Deformed metadioritic components of the orthogneiss preserve: (1) relict igneous grains and microstructures in low-strain domains; and (2) microstructures dominated by recrystallised and metamorphic assemblages in high-strain domains. Recrystallization of the diorite and its influence on metamorphism are characterised along a strain profile in the orthogneiss. Detailed microstructural and physiochemical relations indicate that up to 35% of the host orthogneiss comprises relict igneous minerals; the remainder is formed of recrystallised (metamorphic) grains. Such extensive persistence of igneous minerals results in lower densities for dry, metastable lower crustal rocks, even if at eclogite facies conditions. This reduces the chances of lower crustal foundering and delamination and therefore increases the likelihood for the preservation of an arc root, including dense and cumulate components embedded in the metastable orthogneiss.

SGTSG 2015: Riding the Waves

Structural geology, veins and breccias at the Zijinshan high sulfidation CuAu deposit, Fujian Province, China JING CHEN, DAVID R . COOKE, DAVID SELLEY AND JOSE PIQUER ARC Centre of Excellence in Ore Deposits (CODES), School of Physical Sciences, University of Tasmania, Private Bag 79, Hobart, Tasmania, Australia 7001

The Jurassic to early Cretaceous Yanshanian period (180-90 Ma) was characterised by major magmatic and mineralization events in southern China. Subduction of the Pacific plate beneath the Eurasian plate produced large volumes of volcanic and intrusive rocks. The Zijinshan ore field in southwest Fujian province, southeast China, formed at that time. It is currently one of the largest Cu and Au producers in southern China. The Zijinshan high-sulfidation deposit is located in the middle of the Zijinshan ore field, and contains 326 t Au and 2 Mt Cu. The deposit is hosted in the Zijinshan lithocap, which developed primarily within the Zijinshan intrusive complex, a Yanshanian granite complex (168 ± 4 Ma; Zhao et al., 2008). High sulfidation Cu-Au mineralization is Cretaceous (102.5 ± 1.5 Ma; Zhang et al., 2003) and associated with dacite dykes (110 Ma ± 1 Ma; Jiang et al., 2013). The mineralization is hosted within tectonic-hydrothermal breccias that formed NW-trending faults. Systematic Anaconda-style mapping of the Zijinshan open pit was conducted in order to obtain detailed information about lithotypes, structures, breccias, veins and alteration mineral assemblages. Mapping has shown that the dacite porphyry intruded the Zijinshan granite as dykes, mostly in the southern part of the deposit. Dykes are north-west and east-trending and steeply dipping. Tectonichydrothermal breccias, coupled of clasts of dacite and dickite-alunite-altered granite and dacite, set in alunite-covellite-cement and altered matrix, are elongated in NW-trending syn-mineralization faults. The syn-mineralization faults that controlled the emplacement of dacite porphyry and hydrothermal breccias are mostly northwest-trending normal faults that dip moderately to the northeast (Fig.l). Sinistral strike-slip faults that dip steeply to the southwest were also active during mineralization, although to a much lesser extent than the normal faults. The normal and sinistral strike-slip faults typically have dickite and alunite fault fibres, which preserve evidence of normal and strike-slip senses of displacement. Sinistral strike-slip movement post-dated normal fault movements, based on the horizontal mineral fibres overprinting the oblique fibres on individual fault surfaces. The kinematic and dynamic analysis shows a NNE direction of extension (a 3 ) and very high angle plunges of c h

Figure. 1: Kinematic axis plots for the syn-mineralization faults at Zijinshan. All stereoplots are lowerhemisphere, equal-area projections. The left stereoplot consists of the fault planes (great circles) with slip vectors (points). The middle and right stereoplots show the calculated orientations of Gi and o 3 for subgroups of fault-slip data using the Multiple Inverse Method (Yamaji, A., 2000).

Post-mineralization northeast-trending dextral strike slip faults dip steeply to the northwest (Fig. 2). They truncated and disrupted mineralized veins and breccias. Strike-slip senses of displacement have been recorded by hematite, jarosite and minor goethite mineral fibres. This fault activity relates to WNW-oriented <j\ and NE- oriented a 3 . 24

SGTSG 2015: Riding the Waves

(a2-o3)/(oi-a3) Figure 2: Kinematics axis plots for the post-mineralization faults at the Zijinshan deposit. Same plots and legend as in Fig. 1. Previous studies at the structural evolution of the Zijinshan district concluded that the northeaststriking reverse faults formed prior to mineralization and controlled the geometry of the early Jurassic granite intrusions, e.g. Zijinshan granite complex (Tao and Xu, 1992; Zhang et al., 2003). However, little record of northeast-trending reverse movement was found during our study and was restricted spatially northeast part of the ore field. Either reactivation of the northeast-trending faults elsewhere in the Zijinshan district obscured evidence for the previous reverse movements, or there was no reverse movement on these faults prior to mineralization. Several stages of veins have been recognized within the Zijinshan lithocap. The earliest veins are premineralization quartz-pyrite veins with or without quartz alteration halos. Syn-mineralization veins are infilled with alunite-pyrite-covellite-digenite veins, and dickite-alunite-covellite-digenite veins. There are also syn- and post- mineralization dickite and alunite veins. The pre-mineralization quartzpyrite veins do not have any preferred orientations, whereas the syn-mineralization veins are preferentially aligned to the northwest, and dip moderately NE and SW. Post-mineralization veins are northwest-striking and dip steeply northeast. Three different types of tectonic-hydrothermal breccias have been identified at Zijinshan. Premineralization matrix-rich polymict breccias occur as elongated dykes that are on average lm wide, which have little or no hydrothermal cement and consist predominantly of granite and dacite porphyry clasts in a rock flour matrix. Syn-mineralization hydrothermally cemented breccias occur as subparallel breccia veins with widths that average 20 to 40 cm. These mineralized breccias strike northwest and dip less than 45° to the northeast. The hydrothermal cements include quartz and pyrite, locally with abundant alunite, dickite, digenite, covellite and minor enargite. The syn-mineralization hydrothermal cemented breccias are indicated to be dilational jogs on the normal-striking faults, based on the geometry of the breccia dykes and the deformed clasts in the breccia veins. Dacite cemented breccias are the third category of breccias at Zijinshan. They are spatially associated with dacite porphyry dykes. The polymict breccias contain clasts of dacite, granite and quartz veins. North-west trending normal faults controlled the emplacement of the dacite dykes, tectonichydrothermal breccias and mineralized veins at Zijinshan during the Yanshanian period. The NWtrending Shanghang-Yunxiao fault is located in the south-west part of the Zijinshan district and was active as a major normal fault during the late Yanshanian period (Tao and Xu, 1992), NNE-directed extension is consistent with the regional fault movements during mineralization. Slab rollback is suspected to have triggered extension in the late Yanshanian and is considered to be the cause for the extension faulting and epithermal mineralization at Zijinshan (Li et al., 2012). The post-mineralization northeast-trending dextral strike slip faults have been identified throughout the ore field, which dip steeply to the northeast. The principal compression stress direction is WNW and the minimum principal stress is NE-trending. This indicates that the fault activity transferred from an extensional regime to a strike slip regime soon after mineralization. 25

SGTSG 2015: Riding the Waves

Inter-zone dependences in an inverted Oligo-Miocene metamorphic succession, Arunachal Pradesh, India GEOFFREY L. CLARKE 1 , S . K . BHOWMIK 2 , T . R . IRELAND3, J.C. AITCHISON4 AND S.L. CHAPMAN1 1

School of Geo sciences F09, University of Sydney, NSW 2006, Australia. Department of Geology & Geophysics, Indian Institute of Technology, Kharagpur-721 302, India Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia 4 School of Geography, Planning & Environmental Management, University of Queensland, QLD 4072, Australia 2

Exposures of the Lesser Himalaya sequence in the Siyom Valley, eastern Arunachal Pradesh, expose a telescoped and inverted greenschist-granulite sequence over c. 20 km. Pelitic and psammitic samples all preserve similar detrital zircon age spectra, characterised by c. 2500, 1750-1500, 1200 and 1000 Ma Gondwanan populations. Metamorphic isograds broadly parallel a penetrative NW-dipping S2 foliation. P-T conditions and c. 19-27 Ma U-Th-Pb monazite metamorphic ages increase with structural height. Garnet growth in garnet, staurolite and kyanite zone schists commenced at P>8 kbar and 7^550°C, before the syn- to post-S2 heating of staurolite and kyanite zone rocks to 7^640°C at P~8.5 kbar at c. 19 Ma. Peak conditions of P^ 14 kbar and 7^850°C in kyanite-rutile-garnet-bearing psammitic migmatite at c. 21 Ma preceded its syn-D2 thrusting onto, and heating of, lower grade schists; monazite ages of 16-19 Ma in rocks either side of the thrust date their juxtaposition. A SEpropagating structural discontinuity was contemporary with early Miocene metamorphism; it accreted lower grade footwall rocks and controlled their exhumation to at least mid-crustal levels between 22 and 19 Ma.

SGTSG 2015: Riding the Waves

Plate Topologies and Palaeogeography of the Neoproterozoic-Cambrian Amalgamation of Eastern Gondwana

ALAN S. COLLINS , DONNELLY ARCHIBALD , MORGAN BLADES , CHRIS CLARK , DIANA PLAVSA , JOHN FODEN , STIJN GLORDE , THEODORE RAZAKAMANANA , TADESSE ALEMU , GIRMA WOLDETINSAE , SALAH AL-KHIRBASH , PETE SIEGFRIED , SERGEI PISAREVSKY AND JUSTIN L. PAYNE 1

Centre for Tectonics Resources and Exploration (TRaX), Department of Earth Sciences, School of Physical Sciences, University of Adelaide, Adelaide 5005, South Australia, Australia ^Department of Applied Geology, Curtin University, Perth, GPO Box U1987, Perth, Western Australia Departement de Sciences des terre, Universite de Toliara, Toliara, Madagascar Geological Survey of Ethiopia, Addis Ababa, Ethiopia Ministry of Mines, Addis Abba, Ethiopia Department of Earth Sciences, Sultan Qaboos University, Muscat, Oman GeoAfrica, Windhoek, Namibia Centre for Tectonics, Resources and Exploration (TRaX), School of Built and Natural Environments, The University of South Australia, Adelaide, South Australia 1

* Alan. collins@adelaide. edu. au

Australian Gondwana formed by the Ediacaran collision of Neoproterozoic Australia (consisting of cratonic Australia and its continuation into East Antarctica) with Neoproterozoic India (also consisting of the Indian subcontinent with a large extension into East Antarctica), the Congo-TanzaniaBangweulu Block of central Africa and Azania (a Neoproterozoic continent consisting of southern peninsular India, central Madagascar and parts of East Africa and southern Arabia). Outline palaeogeographies of the interactions between these continents can be constructed by looking at the available high-quality palaeomagnetic solutions for these continents. However, these are sparse for the continents in question in the Neoproterozoic. Interrogating the timing and nature of orogenesis in the collision zones between these continents (the East African Orogen and the Kuunga Orogen) provides more precision when integrated with the palaeomagnetic data. However, these only really provide palaeogeographic information for the final stages of the formation of Gondwana. A third major paradigm is to investigate the ocean plate tectonic history. This is only possible by investigating the timing and nature of the remnant volcanic arc rocks now preserved metamorphosed and deformed in the orogens—and by looking at the detrital products of these arcs. Here we present the first steps to an oceanic-plate integrated tectonic geography of the Neoproterozoic of what was to become central and eastern Gondwana. This is the first time that a tectonic geography has attempted to integrate the evolution of oceanic-plate margins with continental interactions to better constrain whole plate evolutions in the Proterozoic—at a time when the first voluminous, productive source rocks were being deposited.

SGTSG 2015: Riding the Waves

Application of zircon Hf isotope arrays to tectonics and geodynamics WILLIAM J. COLLINS1, NADEZHDA PRIYATKINA1'2 AND ERIN MARTIN 1 ]

We demonstrate how to use zircon age clusters and Hf isotope arrays (sHf) to evaluate tectonic setting of silicic magmatism and geodynamic evolution of orogenic belts. Because of their refractory nature and resistance to chemical and physical breakdown, detrital zircons survive partial melting episodes and often yield an age limit greater than that of the host orogen. The resultant zircon sHf arrays inform on the varying contributions of mantle (+sHf) and crust (-eHf) through time, from inception of magmatism to orogen termination, and the transition from one to another is a response to geodynamic evolution. When plotted against time, large vertical sHf arrays almost uniquely indicate continental arc magmatism, because the extended crystallisation history of hydrous arc magmas facilitates extended melting and mixing of mantle and pre-existing arc crust; ideal conditions for zircon growth. By contrast, bimodal magmatism in continental rifts rarely records the mafic end-member; only an evolved crustal signature is evident, providing information in the timing of rifting and the nature of the rifted basement. Backarc-related magmatism is also usually bimodal, with zircons from ophiolite complexes providing the mantle end-member, if present. Occasionally, the backarc sHf array extends from crust-dominated to mantle-like values, but this usually occurs in proximal backarcs, not distal backarcs. Oceanic arcs almost exclusively produce arrays with +sHf values, but they rarely lie on the Depleted Mantle (DM) evolution line, indicating that the zircons are derived from remelted juvenile crust. Zircon Hf isotope arrays also record the evolution from oceanic to continental arc, and vice versa. Accretionary orogens can either advance or retreat, expressed in the rate of change from crustal (-sHf) to mantle (+ sHf) values, as exemplified by the Andean and Tasmanides systems of the circumPacific orogenic system. Long-term accretionary orogenic systems like the circumPacific record a systemic progression to positive Hf values over a 500 Ma period. Slowly retreating arc-backarc systems, such as the Phanerozoic orogenic systems of Europe (Variscides, Alpides), produce dominantly reworking arrays, reflecting meagre mantle input during protracted orogenic activity. In collisional orogens, "precollisional" accretionary events are recorded by distinct excursions to -sHf values, as seen through the Mesozoic evolution of the Himalayan system. However, Wilson cycles may not be recorded by sHf arrays, as with Pangean amalgamation in Europe. Rather, the array simply records reworking of earlier-formed, but opposed orogenic belts (Appalachians/Caledonides vs Variscides). On the other hand, supercontinental cycles are usually, but again not always, recorded by sHf arrays. Gondwana and Nuna amalgamation produced similar long-term fanning sHf arrays, culminating in major excursions to highly - sHf values, whereas Rodinian amalgamation is surprising mute on the long-term sHf evolutionary array. The difference in sHf arrays between different supercontinental cycles reflects global cyclicity on a billion-year scale, relegating the supercontinental cycle to a second order phenomena on Earth.

SGTSG 2015: Riding the Waves

The dynamics of injection-driven failure and fluid pathways in high fluid flux regimes: insights from ore deposits, seismicity and numerical modelling STEPHEN F C o x 1 , ARND FLATTEN 2 AND DAVID BECK 3 1

Research School of Earth Sciences, The Australian National University, Canberra, ACT 2601, Australia Beck Engineering Pty Ltd, Applied Mechanics Group, Niedstrasse 20, 12159 Berlin, Germany Beck Engineering Pty Ltd, 9 Reid Drive, Chatswood West, NSW 2067, Australia

2 3

Fluid pathways associated with formation of hydrothermal ore deposits in intrinsically low permeability host rocks are controlled by permeability enhancement associated with episodic growth of extension fractures and repeated reactivation of faults in high fluid flux regimes. Opening of extension fractures is driven by fluid injection and pressurization of connected networks of hydraulic fractures at low differential stresses and severely overpressured fluid conditions. Vein orientations are controlled by near-field stress regimes. Injection-driven failure of faults occurs mostly at higher differential stresses. The internal structure of faults and veins in high fluid flux systems indicate that genesis of hydrothermal ore deposits in these regimes involves hundreds to thousands of episodes of fluid migration that accompany bursts of injection-driven failure. Seismicity styles during both deep fluid injection experiments, and in contemporary natural, overpressured hydrothermal systems, are dominated by injection-driven swarm (IDS) sequences. Bursts of seismicity, associated with injection-driven rupture propagation, involve up to thousands of slip events in the range -1 < M w < 3 over days to weeks; co-seismic slips range from 0.1mm to 10 mm, over rupture lengths of several meters to several hundred meters. For example, a typical swarm at Hakone caldera (Japan) in 2009 exhibited intense seismicity over 8 days, and migrated through several small faults with strike lengths less than 2000m. Time migration of seismicity and the associated fluid pressure front occurred at rates up to several hundred meters per day. Total moment release during IDS sequences indicates injection of approximately 104 - 105m3 of fluid over the duration of typical swarms. Episodic swarm seismicity commonly occupies volumes greater than 10km3 over decadal time-scales. Fluid fluxes and associated slip rates indicate that major, fault-controlled ore deposits can accumulate on timescales of 104 to 105 years. The dynamics of fluid-driven failure and propagation of fluid pressure fronts associated with injection of overpressured, magmatic-hydrothermal fluids into low permeability host-rocks is illustrated using fully-coupled hydro-mechanical simulations with continuum/discontinuum constitutive relations that incorporate the regional stress field, rock mechanical properties, mechanical heterogeneity associated with pre-existing faults, and depth-dependent fluid properties. The models track dynamic changes in fluid pressure and stress states during fluid injection, along with permeability enhancement coupled with failure. The 3D simulations also track evolution of fluid pathways, intensity of fracture damage, and fluid flux. The modelling closely simulates the formation of heavily fractured domains that host vein systems around cupolas in porphyry and some Sn-W systems, and allows exploration of how the dimensions of these domains and ore distribution are influenced by fluid production rates, initial permeability distributions and far-field stress regimes. A major result is that nucleation of new faults and re-activation of suitably oriented, pre-existing faults can generate fluid pathways that facilitate fast fluid transport up to several kilometres away from the injection source. Propagation of fracture networks through low permeability domains into high permeability, near-hydrostatic fluid regimes at shallow depths leads to rapid fluid depressurization, depletion of driving pressures in the fluid reservoir and cessation of individual flow episodes. The results provide new insights about the dynamics of magmatic-hydrothermal systems and have implications for exploration for the distal footprints of these systems.

SGTSG 2015: Riding the Waves

Basement influences on structural styles in the Bight Basin, southern Australia

JANE CUNNEEN, FLETCHER PYM, JAKE HAMMELSWANG AND CHRIS ELDERS Department of Applied Geology, Curtin University, Western Australia

The Bight Basin on the southern margin of Australia is nearly 2000 km wide from west to east and overlies a number of different basement terranes (Figure 1). Major basement terrane divisions occur between the basement of the Ceduna delta and the Eyre and Bremer sub-basins, resulting in a variety of structural styles in the basin sequences. Overlying the Albany Fraser Orogen's northern foreland, the Bremer Sub-Basin is dominated by WSW-ESE trending half graben structures and large rollover anticlines associated with JurassicCretaceous rifting. To the east in the Eyre Sub-Basin, half graben basement structures trend E-W to WSE-ENE but the overlying basin sediments are less deformed. Further along strike to the east, the Polda Trough is an E-W trending Neoproterozoic structure also reactivated in the Jurassic- Cretaceous rift event, although its relationship with the Eyre Sub-Basin is unclear. The Eyre Sub-Basin overlies the boundaries of the Proterozoic Madura and Coompana basement provinces, which are separated by the Mundrabilla shear zone. The shear zone is a N-S trending, continent-wide structure visible in magnetic data which appears to extend offshore in the Eyre SubBasin and is also visible as a north-trending present-day fault scarp in the onshore Eucla Basin. Seismic data interpretation suggests that the shear zone steps to the east in the region of the Jerboa-1 well, resulting in a broad zone of uplift in the Madura Shelf. The Ceduna Sub-Basin overlies the Archaean-Proterozoic Gawler Craton which is subdivided onshore by regional-scale shear zones. Offshore, terrane boundaries are interpreted to trend N-S, effectively dividing the Ceduna Sub-Basin. Gravity data suggests a decrease in basement thickness in the southeast portion of the sub-basin, supported by a change in structural style and increase in Eocene magmatism visible in seismic data.

References Totterdell, J.M. & Bradshaw, B.E. 2004. The structural framework and tectonic evolution of the Bight Basin. In: Boult, P.J., Johns, D.R. & Lang, S.C. (eds) Eastern Australasian Basins Symposium II. Petroleum Exploration Society of Australia, Special Publication, 41-61.

SGTSG 2015: Riding the Waves

Pure Shear Extension of Thick Lithosphere - Rift but no Sag: NE Canning Basin, Western Australia KAROL CZARNOTA 1 AND NICKY WHITE 2 1 2

Geoscience Australia, GPO Box 378, Cnaberra ACT, Australia Bullard Laboratories, Madingley Rise, Madingley Road, Cambridge, CB30EZ,

Whether rift basins form as a consequence of pure shear or simple shear stretching of the lithosphere or a hybrid of these two end members has long been the focus of debate (McKenzie, 1978; Wernicke, 1985; Rosenbaum et al., 2008). It is generally accepted that under low strain pure shear dominates yet the debate rages with respect to highly extended continental margins. The key dataset to resolve this debate is the spatial distribution of syn-rift and post-rift basin subsidence resulting from mechanical thinning of the lithosphere and subsequent thermal re-thickening of the lithospheric mantle to its prerift thickness. An often-overlooked element of this debate is what lithospheric template is being stretched (Crosby et al., 2010). Most geodynamic models simply assume a standard lithospheric thickness of 100-120 km, yet in the last decade teleseismic tomography has revealed that much of the Earth's continental land mass is underlain by lithosphere over double this thickness (Priestley and McKenzie, 2013). Here, we kinematically model the subsidence history of the Canning basin following Crosby et al. (2010). This intracratonic rift basin putatively overlies lithosphere > 180 km thick, imaged using shear wave tomography (Kennett et al., 2013). The entire subsidence history of the, < 300 km wide and < 6 km thick, western Canning Basin is adequately explained by Ordovician rifting of pre-existing 100120 km thick lithosphere followed by post-rift thermal subsidence as described by the established pure shear model. In contrast, the < 150 km wide and 15 km thick Fitzroy Trough of the eastern Canning Basin reveals an almost continuous phase of normal faulting between Ordovician and Carboniferous Periods followed by negligible post-rift thermal subsidence. This pattern cannot be accounted for by a simple shear model (c.f. Drummond et al., 1991), as there is no record of excess post-rift subsidence in the basin, nor does the data fit the standard pure shear model. We attribute this difference in subsidence to a sharp change in mantle lithospheric thickness between the west and eastern Canning Basin. The presence of - 2 0 Ma diamond bearing lamproites intruded into the basin depocentre indicate that the present lithospheric thickness exceeds -180 km (Evans et al., 2012). In order to account for the observed subsidence, at standard crustal densities, the lithospheric mantle is required to be depleted by 50-70 kg m . The actual depletion of the lowermost lithospheric mantle was assessed by modeling REE concentrations of the - 2 0 Ma lamproites along with other ultrapotassic rocks from the Kimberley, Yilgarn and Pilbara blocks following the method of Tainton and McKenzie (1994) which reveal a depletion of 40-70 kg m . This result suggests that thermal re-thickening of the lithospheric mantle did not occur following rifting, as it is unlikely that such a strongly depleted mantle source was available in the Phanerozoic to be frozen into the lowermost lithospheric mantle. Therefore, we conclude that thinning of thick lithosphere to thicknesses > 120 km is thermally stable and is not accompanied by post-rift thermal subsidence driven by thermal re-thickening of the lithospheric mantle. The discrepancy between estimates of lithospheric thickness derived from subsidence data in the Western Canning and that derived from shear wave tomography suggests that the latter technique cannot resolve lithospheric thickness variations on < 300 km half wavelengths.

References Crosby, A. G., S. Fishwick, and N. White (2010), Structure and evolution of the intracratonic Congo Basin, Geochem Geophy Geosy, 77, 1-20. Drummond, B. J., M. J. Sexton, T. J. Barton, and R. D. Shaw (1991), The Nature of Faulting Along the Margins of the Fitzroy Trough, Canning Basin, and Implications for the Tectonic Development of the Trough, Exploration Geophysics, 22, 111-116. Evans, N., B. A. Mclnnes, B. McDonald, M. Danisik, F. Jourdan, C. Mayers, E. Thern, and D. Corbett (2012), Emplacement age and thermal footprint of the diamondiferous Ellendale E9 lamproite pipe, Western Australia, Mineral Deposita, 1-9.

SGTSG 2015: Riding the Waves Kennett, B. L. N., A. Fichtner, S. Fishwick, and K. Yoshizawa (2013), Australian Seismological Reference Model (AuSREM): mantle component, Geophysical Journal International, 192(2), 871-887. McKenzie, D. (1978), Some Remarks on Development of Sedimentary Basins, Earth and Planetary Science Letters, 40(1), 25-32. Priestley, K., and D. McKenzie (2013), The relationship between shear wave velocity, temperature, attenuation and viscosity in the shallow part of the mantle, Earth and Planetary Science Letters, 381(0), 78-91. Rosenbaum, G., R. F. Weinberg, and K. Regenauer-Lieb (2008), The geodynamics of lithospheric extension, Tectonophysics, 458(\-A), 1-8. Tainton, K. M., and D. McKenzie (1994), The generation of kimberlites, lamproites, and their source rocks, Journal of Petrology, 35(3), 787-817. Wernicke, B. (1985), Uniform-sense normal simple shear of the continental lithosphere, Canadian Journal of Earth Sciences, 22(1), 108-125.

SGTSG 2015: Riding the Waves

The Neoproterozoic East Gondwana suture: Reconciling geological and geophysical evidence

NATHAN R. DACZKO , JACQUELINE A. HALPIN , JOANNE M. WHITTAKER AND IAN C.W. FITZSIMONS 1

2,3

department of Earth and Planetary Sciences, Macquarie University, Sydney NSW, Australia ARC CODES, School of Physical Sciences, University of Tasmania, Hobart TAS, Australia Institute for Marine and Antarctic Studies, University of Tasmania, Hobart TAS, Australia Department of Applied Geology, Curtin University, Perth WA, Australia

2 3

Several fundamental tectonic boundaries, inferred in the amalgamation of Gondwana, are largely concealed under ice in Antarctica. The hidden geology has caused considerable speculation of rock types and their age, resulting in widely spaced interpretations of the position and type of tectonic boundaries central to the Neoproterozoic unification of the East Gondwanan elements Indo-Antarctica and Australo-Antarctica. Geological-based interpretations rely heavily on (1) incomplete coastal outcrops and (2) inland rock samples and sediment brought to the coast by ancient and modern processes. Increasingly sophisticated and detailed geophysical studies provide new opportunities to constrain the sub-glacial geology, though non-unique solutions are possible and high-resolution data is spatially limited, leading to multiple additional hypothesized locations of key boundaries. We explore the controversy using time-continuous plate tectonic reconstructions in GPlates based on sparse geological, geophysical and plaeomagnetic constraints to investigate the Neoproterozoic plate motions. We propose that the amalgamation of Indo-Antarctica and Australo-Antarctica involved two key plate boundaries: (1) a subduction boundary that consumed a >1000 km wide ocean basin, linked and approximately perpendicular to (2) a sinistral transform boundary that accommodated >1000 km of strike-slip motion. In our reconstructions, subduction and arc activity along the first boundary persisted from c. 720 Ma to c. 620 Ma concurrent with activity along the strike-slip boundary. The plate motion led to continent-continent collision at c. 600 Ma. We use an extensive compilation of zircon isotopic data for grains derived from the Gamburtsev Subglacial Mountains to evaluate recent geophysical interpretations of the Neoproterozoic East Gondwana suture being located directly north of the Gamburtsev Subglacial Mountains. Our analysis demonstrates a marked change in the zircon isotopic data compilation at c. 620-600 Ma, modeled in our reconstruction as the onset of continent-continent collision between India and Antarctica. During the inferred period of subduction (720-620 Ma), zircon with positive epsilon Hf isotopic composition dominates the compilation, whereas significantly negative compositions predominate following the inferred continent collision. A lack of significant topography along the inferred strike-slip boundary and limited c. 720-620 Ma zircon from the region along this boundary support the interpretation of a dominantly transform system for the second boundary. This study reconciles multiple geophysical interpretations of the ice-covered Neoproterozoic plate boundaries for the first time. The location of the proposed "Gamburtsev Suture" is reinforced by zircon isotopic data that supports a subduction boundary preceding continent-continent collision.

SGTSG 2015: Riding the Waves

Microcontinents offshore Western Australia: insights into the make-up and break-up of East Gondwana JACQUELINE A. HALPIN1'2 , NATHAN R. DACZKO3, JOANNE M. WHITTAKER1, SIMON E. WILLIAMS4, ROBYN L. GARDNER3, MADELLINE E. KOBLER3 AND PATRICK G. QUILTY2 department of Earth and Planetary Sciences, Macquarie University, Sydney NSW, Australia ARC CODES, School of Physical Sciences, University of Tasmania, Hobart TAS, Australia 1 Institute for Marine and Antarctic Studies, University of Tasmania, Hobart TAS, Australia 4 Earthbyte Group, School of Geo sciences, University of Sydney, Sydney NSW, Australia 2

The seafloor offshore Western Australia records the spreading history between Greater India and Australia-Antarctica during the Cretaceous breakup of East Gondwana. During 2011, we retrieved the first dredge samples from prominent bathymetric highs, Gulden Draak and Batavia knolls - two submarine plateaux that lie > 1000 km west of the Perth Basin, at the boundary between the Perth Abyssal Plain and Wharton Basin, Indian Ocean. We recovered continental rocks from the steep western slopes that rise more than 3000m above the surrounding seafloor including granite, gneiss, schist and sandstone. Isotopic analysis of zircon and paleontology has constrained the age and affinity of basement and sedimentary rocks, confirming that these knolls are microcontinents rifted from the Indian plate during the Mesozoic breakup of East Gondwana. Gulden Draak Knoll comprises a high-grade basement complex, including pelitic paragneiss (deposited < 1.1 Ga) and mafic orthogneiss (emplaced > 600 Ma) intruded by Cambrian granite (-540 Ma). Boulders and cobbles of felsic orthogneiss with Archean (-2.85 Ga), Mesoproterozoic (~ 1.3-1.2 Ga) and Cambrian (-530-510 Ma) zircon ages were likely locally sourced. Sampled basement from Batavia Knoll comprises granite and felsic orthogneiss emplaced at -540-530 Ma. Dating of metamorphic zircon constrains the timing of granulite facies metamorphism to -530-510 Ma. Fossil assemblages and detrital zircon ages constrain sampled sandstones to the late Early Cretaceous (Albian). The newly discovered microcontinents represent the Indian conjugate to both the Antarctic Wilkes-Queen Mary Land margin, and the Australian Naturaliste Plateau. Our new data provide important constraints on the pre-rift configuration of East Gondwana. Incorporation of the newly discovered microcontinents in a 'Leeuwin' full-fit model satisfies key geological and geophysical constraints. Onshore in East Antarctica, rare geological outcrops recording latest Neoproterozoic to Cambrian-aged magmatism and metamorphism in this sector have been interpreted to represent part of a Pan-African-aged orogenic belt. The path of this belt into the Antarctic interior as well as the tectonic context is controversial, but the prevailing model is that a Gondwana-forming collision zone between blocks of broadly Indo-Antarctic and Australo-Antarctic affinity. There is strong evidence for Cambrian metamorphism and magmatism in the dredge samples from the knolls. However, our isotopic data do not easily fit within a convergent plate boundary model, as clear evidence of Cambrian subduction and arc magmatism during ocean closure is lacking. Instead we explore alternative models whereby the Gulden Draak and Batavia basement rocks were located at a transpressive plate margin or major shear zone at -540-510 Ma. Exhumation some 350-400 Myrs later occurred near the nexus of East Gondwanan Mesozoic breakup, with final rifting from India at -102 Ma.

SGTSG 2015: Riding the Waves

Undergraduate teaching with a Virtual Petrographic Microscope N . R. DACZKO

ARC Centre of Excellence for Core to Crust Fluid Systems and GEMOC, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia

An overview of the unrestricted standalone desktop software tool, Virtual Petrographic Microscope (VPM; available for download at http://eps.mq.edu.au/vpm/) is presented. The program is intended for rock thin section analysis in the absence of a petrographic microscope. Virtual Petrographic Microscope enables the analysis of prepared high-resolution images of rock thin sections incorporating features familiar to users of microscopes such as stage rotation, objective zoom, and switching between plane-polarised light (PPL) and crossed-polarised light (XPL). Additionally, 'virtual' features include auto-scaling grid overlays and thin section image annotation, with the ability to save, export, and import annotation files for undergraduate teamwork and instruction. Our second year class has used the program since 2013 in addition to learning at a physical petrographic microscope. The extra practice involved in using the program improved the student's skill at recognising common rock-forming minerals. The final examination results have improved. Prior to using VPM, students correctly identified three-quarters of minerals in an examination question on average, with 11% of the class correctly identifying all eight minerals. These results improved for students that used VPM with -80% of minerals correctly identified on average and 24% of the class correctly identifying all eight minerals. The benefits of the software include that all students see the same example, samples can be focussed on smaller areas of complex thin sections, and the scale includes an extra large field of view not common on physical microscopes.

SGTSG 2015: Riding the Waves

Structural architecture of deformed subsurface basins flanking the Mount Isa Inlier interpreted from recent deep crustal seismic transects P A U L J . T . DONCHAK 1 AND D.CLARK 2 1 Geological Survey of Queensland, Department of Natural Resources and Mines, Level 12, 61 Mary Street, Brisbane, QLD, 4000 Australia 2 Onshore Energy and Minerals Division, Geoscience Australia, GPO Box 378, Canberra, ACT, 2601, Australia

A recent deep seismic transect east of the Mount Isa inlier (14GA-CF1) was commissioned by the Queensland Geological Survey in collaboration with Geoscience Australia to open up greenfields exploration opportunities in northwest Queensland. The transect extends in a north-northwest direction along public highways for over 670km from Longreach in the south to the small township of Four Ways in the north. The survey crosses the currently defined southern margin of the Precambrian North Australian Craton (NAC), which is well defined in regional magnetic and gravity data. The boundary lies beneath a relatively thin cover-basin veneer comprising strata of the late Carboniferous-Triassic Galilee Basin and overlying Eromanga and Carpentaria Basins. The geophysically-defined southern NAC margin is marked by the Diamantina Suture (locally expressed at the surface as the Cork Fault), a complex zone which is well imaged in the seismic data. The crustal section to the south is generally assigned to the Cambrian to Ordovician Thomson Orogen. The NAC crustal section to the north of the Diamantina Suture comprises multiple layers (from top to bottom): (1) an upper thin layer of Palaeozoic cover basins (Eromanga and Carpentaria Basins underlain by late Carboniferous to Triassic Galilee Basin sediments in the south and Triassic subbasins in the north, (2) underlying strata of the Mesoproterozoic? Millungera Basin (first identified in the 2007 GA-GSQ Isa-Georgetown deep seismic program), overlying (3) weakly to moderately reflective deformed strata of the Soldiers Cap Group (the remnants of a deep basin up to 18km thick informally referred to here as the Julia Creek Basin), and (4) a moderately to strongly reflective mid to lower crustal layer up to 30km thick (equivalent to the Numil Seismic province of Korsch et al (2012)) bounded by a well-defined reflective MOHO surface at its base. An ENE trending, mid-crustal magnetic and gravity high formed over a 60km wide wedge of highly reflective strata occurs at CDP 45800 and may represent an extension of the Leichhardt Superbasin volcanic sequences inferred to be at deeper crustal levels below the Soldiers Cap Group within the Mount Isa Inlier. To the west of the transect, in the Cloncurry region, the mineralised Soldiers Cap Group and equivalent Kuridala Group metamorphics are exposed at the surface and commonly display strong north-south trending shortening fabrics which characterise the late (1570-1500Ma) Isan Orogeny throughout most of the Mount Isa Inlier. These trends swing eastwards to a more ENE orientation at the location of the seismic transect and are defined by tightly folded mafic sills and lavas evident in the regional magnetic imagery. These roughly northeast trending shortening fabrics in the Julia Creek Basin are expressed in the seismic transect as moderate to tight folding, thrusting and duplex development sporadically developed over a distance of around 200kms. Early normal faulting is also evident in the lower crust beneath the Julia Creek Basin. Coincident gravity and magnetic trends indicate ENE trending basement highs, from which we in infer an early NNW-SSE extension direction consistent with that of the Calvert Superbasin, which evolved synchronously in the western part of the Mount Isa Inlier. The strong shortening fabric at right angles to the seismic transect is consistent with inversion of the ENE trending basement architecture during the Isan D1 deformation event. West of the seismic transect, zones of strongly partitioned Isan D2 deformation overprint and re-orient D1 fabrics within the outcropping Mount Isa Inlier. Further east within the NAC, metamorphics of the Etheridge Group are characterised by similar strong D1 folding of mafic sills and lavas equivalent in age and affinity to those of the Soldiers Cap Group. Strong E-W trending D1 fold fabrics in these rocks are interpreted to correlate with those evident in the CF1 seismic transect. The extensive shortening deformation evident along 800km of the southern margin of the currently exposed NAC between Julia Creek and Georgetown is interpreted to result 36

SGTSG 2015: Riding the Waves from subduction-related convergence to the south during the interval 1620-1570Ma, expressed in the Northern Territory as the Chewings Orogeny and also perhaps as the Olarian Orogeny within the Broken Hill region of NSW. The Diamantina Suture is expressed in the seismic data as a complex region of rifting flanked by a broader basin to the south. In detail, the suture lies at the northern margin of a narrow rift (here termed the Diamantina Rift) about 60kms wide and 30kms deep, floored by a strongly reflective seismic package interpreted to be dominated by mafic volcanics. This package markedly thickens towards the northern side of the rift, probably indicating growth faulting and greater associated subsidence rates along the northern rift margin. The reflective mafic package is overlain by thicker meta-sedimentary packages, which are interpreted to be juxtaposed against packages of similar character forming a wide basin to the south by long-lived growth faults. The uppermost sediments of the Diamantina Rift recorded from basement-penetrating wells show strong evidence of being sourced from the Musgrave Province in the Northern Territory (Brown et al, 2014; Carr et al , 2014) probably during the Petermann Orogeny in the earliest Cambrian. A continuous connection is inferred at this time between the Diamantina Rift and the Larapinta Rift along strike in the Northern Territory. A major change in lower crustal basement character takes place south of the rift where a transition occurs from poorly-moderately reflective extended continental crust (with a well-defined MOHO) to extended highly reflective (oceanic?) crust (with a less well-defined MOHO). This reflective lower crustal basement typifies the central Queensland interior of the Thomson Orogen as revealed in profiles from BMR deep seismic programs in the early 1980's. This change in character of the lower crustal basement occurs above a sub-horizontal upper mantle reflector (at - 50km depth) which becomes listric to the north, either transitioning into the MOHO or linking with south-dipping probable extensional faults that characterize the NAC margin along the Diamantina Suture. The Diamantina Rift margin exhibits several periods of inversion, with the most intense evident along a series of reactivated listric normal faults inferred to have produced more than 3 kilometres of uplift along the adjacent Proterozoic margin. Partial exhumation of the rift sediments also occurred at this time, pre-dating late Carboniferous development of the overlying Galilee Basin.

References

Brown, D., Purdy, D., Carr, P., Cross, A., Kositcin, N., 2014. New isotopic data from the Thomson Orogen basement cores: a possible link with the Centralian Superbasin. Geological Society of Australia Abstracts 110, 243-244. Carr, P., Purdy, D., Brown, D., 2014. Peeking under the covers: undercover geology of the Thomson Orogen. Geological Society of Australia Abstracts 110, 244-245. Korsch R.J., Huston D. L., Henderson R.A., Blewett R. S., Withnall I.W., Fergusson C.L., Collins W. J., Saygin E., Kositcin N., Meixner A.J., Chopping R., Henson PA., Champion D. C., Hutton L. J.,Wormald R., Holzschuh J. & Costelloe R.D., 2012. Crustal architecture and geodynamics of North Queensland, Australia: Insights from deep seismic reflection profiling. Tectonophysics 572-573, 76-99.

SGTSG 2015: Riding the Waves

Lithospheric architecture of the Yilgarn Craton: an integrative approach M . P . POUBLIER 1 , K. CZARNOTA 1 , D . C . CHAMPION1 AND R.J. KORSCH1 1

Geoscience Australia, Cnr Jerrabomberra Avenue and Hindmarsh Drive, Symonston, ACT

One of the holy grails of geoscience is to understand the evolution of lithospheric architecture through time. Of particular importance is how the architecture of cratons has evolved. Are cratons a result of ancient processes or has the lithosphere been reshaped more recently (e.g. Kaban et al., 2015)? This problem necessitates studies at the scale of a craton, often hampered by insufficient data coverage and the non-uniqueness of geological and geophysical investigations. Through the Australian Government's Onshore Energy Security Program (OESP) and the AuScope initiatives, supported by state and federal government funding initiatives such as the Royalties for Regions Exploration Incentive Scheme (EIS, Western Australia), various geophysical and geological datasets in the Yilgarn Craton have matured over the last decade to the point that such craton-scale investigations are now possible. Here, we present a study combining disparate geological and geophysical datasets along a west to east transect in the northern Yilgarn Craton. We assess the architecture along five deep seismic reflection profiles which transect the craton largely subperpendicular to the main structural trend, and which, when combined, define the northern Yilgarn Craton geotransect (Kennett and Blewett, 2013; Korsch and Doublier, 2015; and references therein). The cross-section starts in the Narryer Terrane in the northwest of the Yilgarn Craton, transects the Youanmi Terrane and Eastern Goldfields Superterrane (EGF), ending in the Yamarna Terrane in the east. We integrate reflection seismic interpretations along these profiles with P and S-wave velocity data derived from teleseismic and ambient noise tomography studies (Salmon et al., 2013; Kennett et al., 2013), receiver function studies (Yuan, 2015), broad band magnetotelluric imaging (Milligan et al., 2013; Duan et al., 2013), potential field data (Bacchin et al., 2008; Milligan et al., 2010), electrical conductivity derived from the Australia-wide array of geomagnetic stations (Wang et al., 2014), and Sm-Nd model ages (Champion, 2013). These techniques vary in the physical properties they image, resolution both vertically and horizontally, and also in their ability to image different parts of the lithosphere. We focus on the lithospheric architecture of the craton, and (a) map/assess the architecture using different geophysical methods which vary with respect to the physical properties they target and/or the measurement technique used; (b) compare results from different techniques, and explore if and how they may complement each other; (c) explore to what degree the geodynamic evolution of the craton through time is preserved in these geophysical datasets. For the latter, we focus on zones where the geological record implies major lithopsheric thinning at some stage during craton evolution, which are important from an economic perspective. In order to synthesise the crustal architecture at this scale, the depth in two way travel times of the major crustal layers/boundaries (i.e. Moho, top lower crust, top middle crust, etc) has been mapped at constant intervals (every 500 CDP = 10 km) along the seismic profiles. Results are consistent with, and complement, more detailed interpretations (Korsch et al., 2013). They include: (a) the main geological entities traversed (Narryer Terrane, Youanmi Terrane, EGF) are all characterised by distinct crustal layering; (b) lateral thickness changes in the lower crust are only moderate, but the thickness of the middle and upper crust can vary significantly; (c) these changes are primarily not caused by first order, but lower order structures; and (d) the crust is thinnest under the Youanmi Terrane, with little topography of the Moho, but thickens towards the east. The architecture of the underlying lithospheric mantle is best imaged by seismic wavespeed distributions (AuSREM), where high wavespeeds, likely related to depleted mantle lithosphere, define a keel-shaped geometry underneath the Youanmi Terrane, and a layer of constant thickness under the

SGTSG 2015: Riding the Waves EGF. This layer deepens towards the east, hence somewhat resembling the eastward thickening of the overlying crust. Although the geodynamic evolution of the Yilgarn Craton remains controversial in detail (e.g. Korsch et al., 2013), Sm-Nd isotope mapping robustly delineates zones/corridors of more juvenile crust (with respect to the surrounding crust), suggesting lithospheric thinning occurred at various times across the craton (Champion and Cassidy, 2007). Another dataset worth considering is the space-time distribution of volcanic hosted massive sulfide deposits (VHMS) and related volcanic rocks, which form during extension over thinned lithosphere (e.g. Huston et al., 2015). Integration of such geological and geophysical data sets indicates that while first order structural boundaries are expressed in both data sets shorter wavelength features such as the Murchison juvenile corridor may not be resolvable in the deep geophysical datasets. References Champion DC, and Cassidy KF (2007), Geoscience Australia Record 2007/14, 8-13. Bacchin M et al. (2008), Gravity anomaly map of the Australian region (Third Edition), Geoscience Australia. Blewett RS, and Kennet BLN (2014), Preview, 172, 47-50. Duan J et al. (2013), Geoscience Australia Record 2013/28, 9-23. Huston et al. (2015), Ore Geology Reviews; DOI: 10.1016/j.oregeorev.2015.05.010. Kaban et al. (2015), Nature Geoscience; DOI: 10.1038/ngeo2525. Kennett BLN et al. (2013), Geophys. J. Int., 192, 871-887. Salmon M et al. (2013), Geophys. J. Int., 192, 190-206. Korsch RJ, and Doublier MP (2015), Ore Geology Reviews; DOI: 10.1016/j.oregeorev.2015.09.005. Korsch RJ et al. (2013), GSWA Record 2013/6, 147-166. Milligan PR et al. (2010), Magnetic anomaly map of Australia (Fifth Edition), Geoscience Australia. Milligan PR et al. (2013), GSWA Record 2013/6, 13-25. Wang L et al. (2014), Geophysical Journal International, 198, 1143-1158. Yuan H (2015), Nature Geoscience; DOI: 10.1038/NGE02521.

SGTSG 2015: Riding the Waves

Relationship of AMS to solid-state strain and biotite microstructure in the foliated Wyangala Granite, Eastern Lachlan Orogen

P.G. LENNOX \ H. DE WALL AND D.W. DURNEY 2

School of BEES, The University of New South Wales, Sydney 2052, Australia GeoZentrum Nordbayern, D-19054, Erlangen, Germany The Wyangala Granite is a foliated, porphyritic Silurian granite from the Eastern Lachlan Orogen (ELO) near Cowra in central New South Wales. It is an ilmenite-bearing, S/marginal I type mainly biotite-granite but also a primary two-mica granite with local chlorite-clinozoisite alteration. Previous studies have shown that granites (sensu lato) of this age in the ELO were emplaced in a back-arc extensional setting prior to convergent regional deformation in Middle Devonian (Tabberabberan event) to Early Carboniferous (Kanimblan event) time. They are therefore pre-tectonic with respect to that deformation rather than syn-tectonic as once thought and as suggested in some recent overseas studies of foliated granites. Here we describe how this deformation is expressed in the magnetic fabric, foliations, strain of quartz aggregates and microstructure of biotite in the rock. New data on the mineralogy of K-feldspar (Ab _6), plagioclase (An _ ) and biotite (low Ti) in synkinematic extensionveinlets show that metamorphism was (as expected) well below solidus conditions but (in contrast to previous suggestions) above greenschist facies during the deformation. Anisotropy of magnetic susceptibility (AMS) in the granite is due mainly to biotite +/- chlorite. In this study, AMS is used to differentiate possible magmatic, sample heterogeneity and tectonic fabric components with the help of the Benn (1994) theoretical models of superposed strains on an initial biotite fabric. Within-sample variations of AMS can be attributed to heterogeneity of this medium to coarse grained rock. Site-average AMS data follow a tight trend of increasing oblateness with increasing magnetic anisotropy, mostly sub-parallel to the dominant S-foliation and lineation, attributed to strain. Initial magmatic fabric of biotite, if present, appears to be masked by tectonic strain in both the AMS and biotite microstructure in our lowest strain site (quartz XJZ aspect ratio 1.4:1). 1

Site-average degree of magnetic anisotropy CP'AMS 1.03-1.14) correlates well with hand-specimen and outcrop quartz-aggregate strain [finite P (e) 1.4-19] over a transition from premylonitic S-foliations to mainly protomylonitic S-C foliations. The main difference is between the 'flattening' shapes recorded by the AMS (shape factor T^MS 0.0-0.6) and a near neutral piano-linear contraction style of deformation in this area evidenced by 3D shapes of quartz aggregates and deformed enclaves A suggested cause may be waviness of the biotite solid-state deformation microstructure, related to localised shearing, which gives rise to a bimodal orientation of (001) traces in XZ section, especially in samples with S-C structure. In contrast, the magnetic fabric due to epidote in samples of an ultramylonitised quartz-epidote vein from the same area has a near neutral shape ( JAMS -0.2- +0.1), as expected for a strongly sheared rock. Qtz

References Benn, K. 1994 Overprinting of magnetic fabrics in granites by small strains: numerical modelling. Tectonophysics 233, 153-162.

SGTSG 2015: Riding the Waves

What drives mass transfer during deformation? An insight from angular distributions of thermodynamic variables around a spherical inclusion.

D.W. DURNEY

Department of Earth & Planetary Sciences, Macquarie University, New South Wales 2109. The rock record displays many examples of localised mass transfer related to deformation: a phenomenon that has occupied numerous geologists and physicists for well over a century (see, for example, Durney 1978 for a historical review). Here, I examine the type of effect where inferred soluble components migrate from one part of a polymineralic rock matrix to another, giving rise to changes of bulk composition known as 'differentiation' or 'segregation'. Among the possible thermodynamic drivers that have been suggested for mass transfer at this scale are pressure, a normal component of stress, strain energy and rate of strain energy production/dissipation (see Hobbs & Ord 2011 for a recent discussion). Unfortunately, not all of these variables are readily distinguished by observation of the natural structures, either because of complexity or because particular structures have been explained by different variables. In the case of inclusions, however, where light and dark differentiated zones form in relation to mechanical disturbances around hard objects such as porphyroblasts (Fig. 1), the variables show different angular distributions. In the classical theory of creeping viscous flow around a rigid spherical inclusion, pressure, velocity and components of stress have angular dependences (0, (p) governed by so-called spherical surface harmonic functions S = S(0, (p), expressible as Vi n • E*, • n times a constant (E^ far-field strain-rate tensor, n unit vector normal direction around the sphere), which are identical to those of the far strain-rate tensor. That is, their values or radial and tangential components are repeated exactly twice in every circuit around the object. Previously unreported algebraic values for the magnitude of deviatoric stress, deviatoric strain rate, viscous rate of strain energy production and (in a Maxwell fluid) quasi-static elastic strain energy also vary from high to low symmetrically about the axes of far strain rate but are repeated four times around the inclusion. Although the classical theories are conservative in that they do not allow internal changes of mass, it is possible to view the thermodynamic variables as potential drivers of mass transfer if they were allowed to relax by internal reaction and diffusion. In particular, it is suggested that the angular frequencies and positions of their highs and lows may be used and compared with those of natural losses and gains around hard inclusions, such as in Fig. 1. Such comparison then provides a geometrical test for identifying which of the two kinds of variables could have been responsible for the mass transfer. Fig. 1. Differentiated zones of angular frequency 2 around retrogressed cordierite porphyroblasts. Lachlan Fold Belt, New South Wales.

References Durney, D.W., 1978. Early theories and hypotheses on pressure-solution-redeposition. Geology, 6, 369-372. Hobbs, B.E., Ord, A., 2011. Microstructures in deforming-reactive systems. Geological Society, London, Special Publications, 360, 273-299.

SGTSG 2015: Riding the Waves

What moved in the Fitzroy Movement? CHRIS ELDERS Department of Applied Geology, Curtin University, GPO Box 1987, Perth, Western Australia, 6845

The Fitzroy Movement is a well defined compressional deformational event in the Fitzroy Trough of the Canning Basin, although the precise timing is uncertain because of the large time gap separating deformed Middle Triassic sediments from unconformably overlying, undeformed Middle Jurassic sediments. Various structures in the offshore basins of the North West Shelf, recognised mainly on the basis of regional 2D seismic sections, have also been interpreted as evidence of more widespread compressional and transpressional deformation associated with this event. Although the initial interpretation of these compressional structures in the offshore basins was based on relatively few published observations, the event is widely referenced in subsequent literature, with little supporting evidence or additional observations. It is problematic, however, in that the Upper Triassic and Lower Jurassic has also been established as a period of significant extension on the North West Shelf.

Post-CaHoviars sag

Early-Middle Jurassic Synrift

Middle-Late Triassic Inversion

Perrao-Carboniterous Synrift

Permo-Triassic Post-rift sag

Figure 1. Original interpretation of inversion structures in the Browse Basin. From Struckmeyer et al. (1998). Note that the supposed syn-inversion package thickens towards the supposed inversion anticline, rather than away from it.

Since these original interpretations were published much more seismic data has become available. A re-evaluation of the original interpretations suggests that most of the structures can in fact be interpreted as extensional in nature, consistent with regional tectonics. It is therefore suggested that the Fitzroy Movement was largely confined to the Canning Basin, and was probably restricted in time to the earlier part of the period represented by the regional unconformity in that basin, predating extension on the northwest shelf, and perhaps co-incident with the onset of deposition of thick sequences of Mungaroo Formation sediments in the Northern Carnarvon Basin. 42

* SGTSG 2015: Riding the Waves

Figure 2: Re-interpretation of the structure illustrated by Struckmeyer et al. (1998). The package originally interpreted as syn-inversion onlaps onto the rotated footwall of the fault to the south - a result of extensional reactivation of this fault during the main Triassic rift event.

Reference Struckmeyer, H. I. M., Blevin, J. E., Sayers, J., Totterdell, J. M., Baxter, K., and Cathro, D. 1998. Structural evolution of the Browse Basin, North West Shelf: New concepts from deep-seismic data. In: Purcell, P.G., and Purcell, R.R. (eds), The Sedimentary Basins of Western Australia 2: Proceedings of the Petroleum Exploration Society of Australia Symposium, Perth, 345-367

SGTSG 2015: Riding the Waves

Oroclines, superfans, the Thomson-Lachlan connection and Early Paleozoic development of the Tasmanides CHRIS FERGUSSON 1 AND BOB HENDERSON 2 School of Earth & Environmental Sciences, University ofWollongong, New South Wales 2522, Australia Department of Earth and Ocean Sciences, James Cook University, Townsville, Queensland 4811, Australia

Rifting in the Late Neoproterozoic in the Tasmanides was followed by island arc and exotic microcontinent collisions in the Delamerian Orogeny culminating in an active continental margin in western Victoria and northwestern New South Wales and the cryptic formation of the Thomson Orogen in Queensland. The extent to which the lower crust of the Thomson Orogen involves extended Precambrian basement or Neoproterozoic paleo-Pacific crust remains unresolved. Two main rock assemblages are known in the Thomson Orogen from outcrop in its northeastern extremity and from scattered basement cores beneath younger cover over most of its extent. The older assemblage consists of a siliciclastic succession containing an almost unimodal zircon population of Grenville-age indicating derivation from the Musgrave Province and/or an eastern continuation of it. This is consistent with either an AUSMEX or AUSWUS reconstruction of Rodinia. The younger assemblage of siliciclastics is characterised by the Pacific-Gondwana detrital zircon ages and has been widely found in basement cores (Geoscience Australia/Geological Survey of Queensland data). This assemblage formed by widespread turbidite deposition (Barcoo superfan) in the Late Neoproterozoic(?) to Late Cambrian (ca 495 Ma) and was rapidly accreted during the last part of the Delamerian Orogeny. The Delamerian Orogeny in the Thomson Orogen involved medium-P metamorphism and major uplift in the northeast and formation of a wide orogenic belt bounded to the northwest by the Diamantina Structure. To the southwest the Thomson Orogen is gradational into the Warburton Basin, which was unaffected by the Delamerian Orogeny. In the southeastern Anakie Province, and further north in the Greenvale Province of north Queensland, the Delamerian component of the Thomson Orogen is bordered to the east by assemblages strongly shortened in the Late Ordovician to Early Silurian Benambran Orogeny consistent with eastward growth of the orogen. The Olepoloko Fault in northwestern New South Wales has been taken as the contact between Ordovician turbidites of the so-called southern "Thomson Orogen" and the Girilambone Group of the Lachlan Orogen to the south, although the significance of this boundary is disputed with a suggestion that Lachlan Orogen rocks continue northwards into southern Queensland. Development of the curving structural patterns of the apparent "southern Thomson orocline" was by accretion of the Lachlan superfan in the Late Ordovician to Early Silurian Benambran Orogeny. Just how relationships are accommodated across the poorly resolved junction between the Koonenberry Belt, Warburton Basin and southwestern Thomson Orogen remains unresolved. In the southern Tasmanides, accretion of the Selwyn Block and formation of wide zones of deformed Ordovician turbidites of the Lachlan superfan that envelops and occurs within the Macquarie Arc are hallmarks of the Benambran Orogeny. In western Victorian, the Stawell and Bendigo zones have accreted to the Delamerian Orogen showing eastward continental growth as evident around the eastern and southern margins of the Thomson Orogen. The role the Tabberabbera Zone has played in this event is poorly understood with arguments for orocline development as opposed to a minimal rotation model involving double divergent subduction either side of the Melbourne Zone. The structural pattern in the Tabberabbera Zone reflects multiple deformation with prominent northwest-southeast structural trends and early east-west trending structures especially in its southeastern part. The orocline hypothesis of Ross Cayley infers that the southern Tabberabbera Zone is linked to the Bega and Narooma zones to the east with the northwestern Tabberabbera Zone connected to the northern extension of the Bendigo Zone. This Z-shaped megafold, reminiscent of the Texas-Coffs Harbour Megafold of the New England Orogen, is presumed to account for the "anomalous" width of the Lachlan Orogen thereby inferring that the Lachlan superfan developed along rather than across the active margin. Alternatively, the Bega and Narooma zones continue southwards linking with the Mathinna Group in northeast Tasmania, which generally has been lumped with the rest of Tasmania and the Selwyn Block despite its distinct surface and subsurface content. 44

SGTSG 2015: Riding the Waves

The role of water in the formation of a thick ultramylonitic shear zone

MELANIE A. FINCH AND ROBERTO F. WEINBERG 1

School of Earth, Atmosphere and Environment, Monash University, Clayton, VIC, 3800

Shear-zone widening is a key process for the generation of thick ultramylonites and is promoted by processes that weaken the host rock or harden the shear zone. Water weakens rocks and minerals and its mobility is determined by pressure gradients, which can be dynamic during simple shear, causing movement of water in and around the shear zone and affecting rock strength. We examine the role of water in an unusually thick shear zone: the 1 km-thick ultramylonitic layer of the El Pichao shear zone. We used Fourier Transform Infrared spectroscopy to measure water content in quartz and feldspar comparing ultramylonitic rocks deformed by diffusion creep to mylonites and weakly-deformed rocks deformed by dislocation creep. We found that quartz and feldspar in ultramylonites contained half the amount of water of weakly-deformed rocks (Fig. 1), a result contrary to previous studies of water in shear zones. We propose that the thick ultramylonite formed in three stages (Fig. 2): (1) localized deformation and recrystallisation caused release of intracrystalline water to grain boundaries and, once a critical grain size was reached, promoted dissolution-precipitation, which gave rise to an ultramylonite, (2) high pressure in the shear zone compared to the surroundings continuously expelled intercrystalline water (Mancktelow, 2002), gradually drying the grain boundaries and leading to strain hardening (Fig. 2b), (3) water migrated to neighbouring, less-deformed rocks causing hydrolytic weakening, initiating diffusion creep, and repeating the cycle, widening the ultramylonite (Fig. 2c; Oliot et al., 2014). 9000 A • A •

8000 CO

7000

Quartz sample mean Feldspar sample mean Quartz overall mean Feldspar overall mean

T—

& 6000

o 2 4000 - H t

Weakly deformed

Mylonites

Ultramylonites

Figure 1: Mean molar water concentration in quartz and feldspar for weakly-deformed rocks, mylonites, and ultramylonites. Error bars are one standard deviation from the mean.

SGTSG 2015: Riding the Waves (a) Protolith: granitic diatexite

Figure 2: Model for shear zone widening: the water expulsion cycle, (a) Granitic diatexite protolith begins shearing, (b) A mylonitic band forms and shear heat diffuses out to shear zone margins. Recrystallisation causes liberation of intracrystalline water to grain boundaries (inset) and towards the margins of the shear zone, in response to pressure gradients, (c) Water on grain boundaries accelerates diffusion, GBS and dissolution-precipitation which causes ultramylonitisation and mixing of phases. During steps (b) and (c) the outward flux of water may be aided by pumping due to dynamic porosity created during GBS. At this stage, the balance between water liberation by recrystallisation and water fluxing outwards is lost and the ultramylonites begin to dry and harden, while the shear zones shoulders weaken. This process causes the locus of maximum strain rate to move outwards, where the cycle begins again.

References Mancktelow, N. S., 2002. Finite-element modelling of shear zone development in viscoelastic materials and its implications for localisation of partial melting. Journal of Structural Geology 24(6-7), 1045-1053. Oliot, E., Goncalves, P., Schulmann, K., Marquer, D., and Lexa, O., 2014, Mid-crustal shear zone formation in granitic rocks: Constraints from quantitative textural and crystallographic preferred orientations analyses: Tectonophysics, v. 612-613, p. 63-80.

SGTSG 2015: Riding the Waves

Microcontinents: Stories about complexities in the Wilson tectonic cycles CARMEN GAIN A 1 1 Center for Earth Evolution and Dynamics, CoE, Department of Geosciences, University of Oslo, Norway email: carmen, gaina @ geo. uio. no

Microcontinents are small continental slivers surrounded by oceanic crust, which are detached from large continental masses by rifting and subsequent seafloor spreading. They are common in the accreted continental geological record, and also described in modern tectonic settings. Many of today's microcontinents are found scattered in large oceanic basins like the North Atlantic and Indian oceans, but also in smaller basins like in the Tasman and Coral seas or in the High Arctic. In this contribution I will describe the present day crustal architecture of several microcontinents found in the North Atlantic, Indian Ocean, the Arctic realm and east and northeast of Australia. Based on geophysical data and sometimes rock samples recovered from these small continental slivers, one can establish the crustal thickness, sedimentary cover and the amount of volcanic material. For example, in the case of the Jan Mayen microcontinent, situated in the NE Atlantic midway between the Eurasian and Greenland margins, it has been established that the crustal thickness is decreasing from north to south, volcanic extrusive layers (sea dipping reflectors) are observed only in the northeastern part, but the southern part was affected by later volcanic flows due to the proximity to the Iceland plume (e.g. Gudlaugsson et al., 1988; Peron-Pinvidic et al, 2013a, b). In other cases, the original continental crust was completely covered by the magmatic material supposedly generated by a mantle plume, but geochemical signature of surface rocks can reveal the continental signature of deeper layers (as it has been suggested for the Mauritius Island from the Indian Ocean by Torsvik et al., 2013, 2015). In the Arctic Ocean, there are many microcontinents dispersed within a confined space; some of them have undisputed continental nature (like in the case of the Lomosov Ridge), in other areas -like the Alpha-Mendeleev ridges - their nature is controversial due to thick volcanic layers which may hinder evidences of continental origin. However, new seismic data and dredged samples may hint to the presence of small continental fragments within these igneous plateaus (D0ssing et al.y 2013), a situation observed in other Large Igneous Provinces (LIPs) - like the Kerguelan Plateau, south Indian Ocean, e.g. Gaina et al., 2007). Several studies aimed to build conceptual models for the formation of microcontinents based on present day examples, especially the ones in the Indian Ocean, North Atlantic and around Australia (e.g. Mueller et a., 2001; Gaina et al, 2003 and Gaina et al., 2009). The succession of tectonic events leading to the isolation of microcontinents include: (1) rifting of a passive margin, (2) possible volcanism (sometimes LIP formation) due to the presence of a mantle plume, and (3) ridge jumps which will relocate rifting and seafloor spreading few hundred or thousands kilometres away from the previous plate boundary and complete the formation of the microcontinent. So far there have been only very few attempts to model (numerically or analogue) the formation of microcontinents, a scenario which is more complex than the formation of a microplate with homogeneous rheology within the continental or oceanic domains. Moreover, most of the settings tested in these models are mainly from extensional tectonics linked to rifting and seafloor spreading in large oceanic basins. Here we will outline plans for a new project which aims to test other possible mechanisms that may lead to continental sliver detachments in subduction environments. Possible implications for understanding the formation of numerous microcontinents east and northeast of Australia (Gaina et al., 1998, 1999) and elsewhere will also be discussed. References D0ssing, A., Jackson, H.R., Matzka, J., Einarsson, I., Rasmussen, T.M., Olesen, A., and J.M. Brozena, 2013, On the origin of the Amerasia Basin and the High Arctic Large Igneous Province-Results of new aeromagnetic data. Earth and Planetary Science Letters, Vol. 363, 2013, p. 219-230.

SGTSG 2015: Riding the Waves Gaina, C., R.D. Miiller, J.-Y. Royer, and P. Symonds, 1999, The evolution of the Louisiade Triple junction, Journal of Geophysical Research, 104, 12927-12939. Gaina, C., R.D. Miiller, J.-Y. Royer, J. Stock, J. Hardebeck, and P. Symonds, 1998, The tectonic history of the Tasman Sea: A puzzle with thirteen pieces, Journal of Geophysical Research, 103, 12413-12433. Gaina, C., Miiller, R.D. Brown, B., and Ishihara, T., 2003, Microcontinent formation around Australia, in The Evolution and Dynamics of the Australian Plate, Geological Society of Australia Special Paper 22 and Geological Society of America Special Paper 372, ed. by Hillis R. and Miiller, R.D., 405-417. Gaina, C., Miiller, R.D, Brown, B., Ishihara, T. and S. Ivanov, 2007, Breakup and early seafloor spreading between India and Antarctica, 2007, Geophysical Journal International, 170, 151-169. 9 Gaina, C., Gernigon, L., and P. Ball, 2009, Paleocene-Recent Plate Boundaries in the NE Atlantic and the formation of Jan Mayen microcontinent, Journal of Geological Society, 166, 601-616, doi: 10.1144/0016-76492008-112. Gudlaugsson, S.T., Gunnarsson, K., Sand, M. &and J. Skogseid, 1988. Tectonic and volcanic events at the Jan Mayen Ridge microcontinent. In: Morton, A.C. & Parson, L.M. (eds), Early Tertiary Volcanism and the Opening of the NE Atlantic. Geological Society, London, Special Publications, 39, 85-93. Miiller, R.D., Gaina, C., W.Roest, and D. L. Hansen, 2001. A recipe for microcontinent formation, Geology, 29, pp. 203-206. Peron-Pinvidic, G., Gernigon, L., Gaina, C. and P. Ball, Insights from the Jan Mayen system in the NorwegianGreenland Sea: Mapping of a microcontinent, 2012, Geophysical Journal International, DOI: 10.111 l/j.l365-246X.2012.05639.x. Peron-Pinvidic, G., Gernigon, L., Gaina, C. and P. Ball, Insights from the Jan Mayen system in the NorwegianGreenland Sea: Architecture of a microcontinent, 2012, Geophysical Journal International, DOI: 10.1111/j. 1365-246X.2012.05623.x. Torsvik, T. H., Amundsen, H., Hartz, E. H., Corfu, F., Kusznir, N., Gaina, C., Doubrovine, P.V., Steinberger, B., Ashwal, L., Jamtveit, B., 2013. A Precambrian microcontinent in the Indian Ocean. Nature Geo science, doi: 10.1038/ngeo 1736. Torsvik, T.H., H. E.F. Amundsen, F. Corfu, R. G. Tr0nnes, P. V. Doubrovine, C. Gaina, N. Kusznir, B. Steinberger, L. D. Ashwal, W. L. Griffin, S. C. Werner, B. Jamtveit, 2015, Continental crust beneath Iceland, Proceeding of National Academy of Sciences, doi/10.1073/pnas. 1423099112.

SGTSG 2015: Riding the Waves

Does microstructure matter? Investigating the effect of spatial distribution of viscosity on bulk strength and strain localisation ROBYN GARDNER 1 , SANDRA PIAZOLO 1 , LYNN EVANS 1 , 2 AND NATHAN DACZKO 1 1 Australian Research Council Centre of Excellence for Core to Crust Fluid Systems/GEMOC, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia 2 School of Earth, Atmosphere and Environmental Sciences, Monash University, Clayton, Vic 3800, Australia

It is well established that viscosity and the stress exponent of the flow law in rocks is highly dependent on grain size. Here we use the microdynamic modelling platform Elle to investigate the impact of grain-size dependent stress exponent and viscosity on the bulk strength and strain localising behaviour of a material deformed in simple shear. We are particularly interested in the effect of the spatial distribution (i.e. the pre-deformation microstructure) and stress related evolution of grain size on the bulk properties and strain localization behaviour during deformation. For example, we specify that an initial 20% of the material area is small grained and thus has a relatively low viscosity. We vary the geometry of the spatial distribution of these grains within the material. We test different microstructures with small grains exhibiting: (I) a random distribution, concentration into (II) horizontal and (III) vertical bands, (IV) clustered distribution, and (V) clusters with a "hard" large "prophyroclast" in the cluster centre. Measurements of stress, strain and effective viscosity of the different geometries are compared with calculated values from end-member scenarios where all the material has either relatively low or high viscosity. The initial experiments are completed with all grains undergoing Newtonian (stress exponent = 1) flow, then a combination of non-Newtonian flow (stress exponent = 3) for the larger, more viscous grains and Newtonian flow for the smaller weaker grains. The results indicate that the geometry of the small grain-size, low viscosity domains significantly impacts the material strength through time, and the ability of the material to successfully localise strain into shear bands. Horizontal bands readily concentrate strain, while vertical bands, which rotate as the experiment proceeds, cause the material strength to vary depending on the angle of the bands to the direction of simple shear. The cluster scenarios readily form sigmoidal-shaped microstructures, and "stable" shear bands form if these were originally close together. Our work emphasises that the pre-deformation histories (i.e. variable microstructures) in the deforming rock packages have a significant effect on the strength of a material and the localization of strain.

SGTSG 2015: Riding the Waves

Basin inversion as a driver for sedimentation, fluid flow and Pb-Zn mineralisation on the Lawn Hill Platform, north Queensland: evidence from deep seismic reflection profiling and gravity modelling

GEORGE M GIBSON , TONY MEIXNER , LAURIE HUTTON AND JOSEF HOLZSCHUH 1

Research School of Earth Sciences, Australian National University, Canberra, Australia Geoscience Australia, Canberra, Australia Geological Survey of Queensland, Brisbane, Australia

2 3

Deep seismic reflection profiling, combined with forward modelling of gravity data, lend strong support to the idea that the Paleo- to Mesoproterozoic Mount Isa mineral province comprises three vertically stacked and partially inverted sedimentary basins preserving a record of intracontinental rifting followed by passive margin formation. Passive margin conditions were established no later than 1655 Ma before being interrupted by plate convergence, crustal shortening and basin-wide inversion across the Lawn Hill Platform at 1640 Ma in the older Calvert (1730-1640 Ma) and Leichhardt (17901740 Ma) superbasins. Crustal extension and thinning recommenced after 1640 Ma with formation of the 1635-1595 Ma Isa Superbasin and continued up to ca. 1615 Ma when extensional faulting ceased and a further episode of basin inversion commenced, driven by onset of the Isan Orogeny. This younger episode of syn-orogenic basin inversion was accompanied by deposition of carbonaceous sediments hosting the 1575 Ma Century Pb-Zn deposit and was largely accommodated on east- or northeast-dipping reactivated intrabasinal extensional faults and footwall shortcut thrusts. These structures extend to considerable depths and served as fluid conduits during basin inversion, tapping thick syn-rift sequences of immature siliciclastic sediments floored by bimodal volcanic sequences in the two older basinal sequences from which the bulk of metals and mineralizing fluids are thought to have been sourced. Basin inversion and fluid expulsion at this stage were entirely submarine and are consistent with a syn-sedimentary to early diagenetic origin for Pb-Zn mineralization at, or close to, the seafloor. Migration of the ore-forming fluids to such shallow crustal depths was probably assisted by an increase in rock permeability induced by fabric development and metamorphism at deeper structural levels during the course of orogenesis. Basin inversion and synorogenic processes are also implicated in the formation of several other SEDEX-type Pb-Zn deposits in northern Australia, including the 1640 Ma McArthur River, as evidenced by their age relative to prominent bends in the polar wander path for this part of the continent. A comparable depositional and tectonic environment (foreland basin) has been proposed for carbonate-hosted Mississippi-type Pb-Zn deposits elsewhere in the world, highlighting the importance of basin inversion and orogenesis as drivers of fluid flow and mineralization more generally, if not globally. An analogous passive margin setting has been proposed for SEDEX-style Pb-Zn deposits in the equally richly-endowed but considerably younger (Paleozoic) Selwyn Basin of western Canada although it remains to be seen whether mineralization in this basin is similarly of post-extensional origin.

SGTSG 2015: Riding the Waves

Mesozoic reactivation of the Beishan, southern Central Asian Orogenic Belt: Insights from low temperature thermochronology JACK GILLESPIE \ STUN GLORIE \ WENJIAO XIAO 2,3> ZHIYONG ZHANG 2 , ALAN S. COLLINS \ NOREEN EVANS 4 , BRENT MCINNES 4 AND JOHAN DE GRAVE 5 1 Centre for Tectonics, Resources and Exploration (TRaX), Department of Earth Sciences, School of Physical Sciences, The University of Adelaide, Adelaide SA-5005, Australia 2 State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China 3 Xinjiang Research Centre for Mineral Resources, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China 4 John de Laeter Centre for Isotope Research, Department of Applied Geology/Applied Physics, Curtin University, Perth WA-6945, Australia 5 Department of Geology and Soil Sciences, Mineralogy and Petrology Research Unit, Ghent University, Ghent 9000, Belgium.

The Beishan Orogenic Collage (BOC) is located in the southeast of the Central Asian Orogenic Belt (CAOB) and formed during the late Palaeozoic with the final consumption of the Palaeoasian Ocean. In order to constrain the Meso-Cenozoic history of the BOC we applied low temperature thermochronology. Apatite fission track and apatite U-Th-Sm/He data obtained for granitoid samples along a north-south transect through the BOC indicate three distinct phases of exhumation during (1) the late Triassic early Jurassic (-225 - 180 Ma), (2) early Cretaceous (-130 - 95 Ma) and (3) late Cretaceous - early Palaeogene (-75 - 60 Ma). Samples from the northern BOC reveal a more profound early Cretaceous signal and a weaker late Triassic - early Jurassic signal than those in the southern BOC. Crustal-scale fault zones in the northern BOC that are interpreted to have undergone repeated reactivation throughout the Mesozoic may explain the differences between the north and south. These faults may therefore have acted as a major control on exhumation in the region. This pattern is consistent with results from elsewhere in the CAOB, such as in the Tianshan and the Altai, where regional widespread exhumation occurred since the early Cretaceous while major fault zones record localised exhumation during the late Cretaceous and Cenozoic. Late Cretaceous - early Palaeogene cooling ages were found only in the south of the BOC, suggesting that exhumation at that time was relatively localised in contrast to the more regional earlier events. The Meso-Cenozoic cooling events described here are thought to be related to the progressive closure of the (Palaeo-)Tethys ocean to the south. Associated collision and accretion of microcontinental blocks and island-arcs at the southern Eurasian margin are interpreted to have induced more widespread reactivation and exhumation in Central Asia than previously anticipated, extending to the northern margin of the Tarim Craton. Further studies applying low-temperature thermochronology to the easternmost Tianshan and the West Junggar Mountains are currently being conducted in order to further refine the existing tectonic history models for Central Asia.

SGTSG 2015: Riding the Waves

Deciphering the Meso-Cenozoic tectonic history of southern Eurasia using thermochronology on Central Asian faults

STUN GLORIED JOHAN DE GRAVE , WENJIAO XIAO , ALAN S. COLLINS AND NOREEN EVANS 2

Centre for Tectonics, Resources and Exploration (TRaX), Department of Earth Sciences, School of Physical Sciences, The University of Adelaide, Adelaide SA 5005, Australia Dept. Geology & Soil Science, MINPET Group, Ghent University, 281-S8 Krijgslaan, Ghent 9000, Belgium Division ofTethys Research Center, Institute of Geology and Geophysics, Chinese Academy of Sciences (CAS) John de Laeter Centre for Isotope Research, Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia Corresponding Author: Stijn.Glorie@adelaide.edu.au 1

The mountainous Central Asian landscape (to the north of the Himalayas and Tibet) predominantly formed as a response to recurrent tectonic deformation (e.g. Hendrix et al., 1992; De Grave et al., 2007). The cause for these episodes of deformation is not yet fully understood. It has been suggested widely that the most recent pulse of intracontinental deformation that affected Central Asia, is related to distant forces that originate at the India-Eurasia collision zone (e.g. Molnar and Tapponnier, 1975; Knapp et al., 1996). Subduction of the Tethys Ocean led to convergence and collision of Gondwanaderived India into Eurasia, which generated stresses that caused shortening and uplift in the Himalayas and Tibet (e.g. Harrison and Copeland, 1992). Continuous convergence between India and Eurasia and the growth of the Tibetan plateau induced convergence-driven (e.g. Abdrakhmatov et al. 1996) and/or flexure related (Aitken, 2011) stresses that propagated into the Eurasian interior where it deformed the weaker crust of Central Asia (e.g. Knapp et al., 1996). This deformation is preferentially accommodated by strength heterogeneities such as pre-existing fault zones within the crust of Tibet and Central Asia (e.g. England and Houseman, 1985), resulting in fault reactivation and associated rapid exhumation (Walker et al., 2007; Clark et al. 2010; Jolivet et al. 2010; Glorie et al., 2011; 2012; Glorie and De Grave, 2015). The reactivation history of these fault systems, hence, not only archives the associated exhumation history of the Central Asian mountain ranges, but also maps out the strain distribution in this region, which provides unique insights into the dynamics of the Meso-Cenozoic collisions at the Eurasian margin such as the India-Eurasia collision (e.g. Peltzer and Saucier, 1996; England and Molnar, 1997; Glorie and De Grave, 2015). This study documents that besides the occurrence of preserved Early Mesozoic geomorphic features (such as internally drained plateaus or old erosion surfaces), most of the current Central Asian relief is related to an important phase of Late Jurassic - Early Cretaceous exhumation. This Mesozoic exhumation pulse is thought to be related with the progressive consumption of the Palaeo-Tethys Ocean and associated collisions of Gondwana-derived terranes to Eurasia in the south and to the closure of the Mongol-Okhotsk Ocean to the northeast. Major fault systems within southern Central Asia (Tian Shan) record Cenozoic episodes of fault-induced rapid exhumation during the Early Palaeogene (-55-45 Ma) and Oligocene (-33-22 Ma) to Miocene (-10-8 Ma), related with the consumption of the Neo-Tethys and subsequent India-Eurasia collision (Glorie and De Grave, 2015). Our findings indicate a major episode of fault reactivation since ~33Ma, which correlates well with a -35 Ma age for the India-Eurasia collision (Aitchison et al., 2007; Jiang et al., 2015). Although many of the hypothesised links between intracontinental exhumation and the prevailing plate-margin tectonic processes need to be tested further, they allow a first-order insight into stress propagation pathways from the Eurasian margin to the continental interior. Ongoing research focusses on fault systems in the verges of the Tian Shan (Uzbekistan and China) to further constrain the dynamic response of the collisions at the southern margin to Central Asia.

References

Abdrakhmatov, K.Ye., Aldazhanov, S.A., et al., 1996. Relatively recent construction of the Tien Shan inferred from GPS measurements of present-day crustal deformation rates. Nature 384, 450-453. Aitchison, J.C., Ali, J.R., Davis, A.M., 2007. When and where did India and Asia collide? Jour. Geophys. Res. 112, B05423.

SGTSG 2015: Riding the Waves Aitken, A.R.A., 2011. Did the growth of Tibetan topography control the locus and evolution of Tien Shan mountain building? Geology 39, 459-462. Clark, M.K., Farley, K.A., et al. 2010. Early Cenozoic faulting of the northern Tibetan Plateau margin from apatite (U-Th)/He ages. Earth Plan. Sci. Lett. 296, 78-88. De Grave, J., Buslov, M.M., Van den haute, P., 2007. Distant effects of India-Eurasia convergence & Mesozoic intracontinental deformation in Central Asia: Constraints from apatite fission-track thermochronology. Jour. Asian Earth Sciences 29, 188-204. England, P., Houseman, G., 1985. Role of lithospheric strength heterogeneities in the tectonics of Tibet and neighbouring regions. Nature 315, 297-301. England, P., Molnar, P., 1997. Active deformation of Asia: from kinematics to dynamics. Science 278, 647-650. Glorie, S., De Grave, J., et al., 2011. Tectonic history of the Kyrgyz South Tien Shan (Atbashi-Inylchek) suture zone: the role of inherited structures during deformation-propagation. Tectonics 30, TC6016. Glorie, S., De Grave, J., et al. 2012a. Structural control on Meso-Cenozoic tectonic reactivation and denudation in the Siberian Altai: Insights from multi-method thermochronometry. Tectonophysics 544-545, 75-92. Glorie, S., De Grave, J., 2015. Exhuming the Meso-Cenozoic Kyrgyz Tianshan and Siberian Altai-Sayan: A review based on low-temperature thermochronology. Geoscience Frontiers, in press. Harrison, T.M., Copeland, P., et al., 1992. Raising Tibet. Science 255, 1663-1670. Hendrix, M.S., Graham, S.A., et al. 1992. Sedimentary record and climatic implications of recurrent deformation in the Tian Shan: Evidence from Mesozoic strata of the north Tarim, south Junggar, and Turpan basins, northwest China. GSA Bulletin 104, 53-79. Jiang, T., Aitchison, J.C., Wan, X., 2015. The youngest marine deposits preserved in southern Tibet and disappearance of the Tethyan Ocean. Gondwana Research, in press. Jolivet, M., Dominguez, S., et al., 2010. Mesozoic and Cenozoic tectonic history of the central Chinese Tian Shan: Reactivated tectonic structures and active deformation. Tectonics 29, TC6019. Knapp, J.H., 1996. How Earth created heaven. Nature 384, 406-409. Molnar, P., Tapponnier, P., 1975. Cenozoic Tectonics of Asia: Effects of a Continental Collision. Science 189, 419-426. Peltzer, G., Saucier, F., 1996. Present-day kinematics of Asia derived from geologic fault rates. J. Geophys. Res. 101,27943- 27956.

SGTSG 2015: Riding the Waves

Structural interpretation of high resolution Digital Elevation Models derived from LiDAR remote sensing

DAVID. GREEN , MATHEW CRACKNELL AND GRACE CUMMING 1

Mineral Resources Tasmania, PO Box 56, Rosny, TAS 7018 School of Earth Sciences, University of Tasmania

Gaps in the geological knowledge of Tasmania are in large part due to the inaccessibility and forested nature of large areas. Remote sensing methods have provided datasets useful in producing interpretive maps for field checking but are less useful in areas that lack rocks with contrasting geophysical properties. LiDAR (light detection and ranging) has the capability of providing a digital elavation model (DEM) in remote and forested areas that is rich in geological information, revealing bedding traces and folds. LiDAR surveys in the lightly forested north east of Tasmania proved to be successful in revealing enough mappable micro-topographic bedding features to produce geological maps. However similar surveys in more heavily forested areas produced relatively poor DEMs, characterised by poor ground point density, high noise and numerous classification errors. Recent advances in LiDAR instrumentation, including increased power, increased pulse frequency, decreased echo separation and faster detection electronics provide the potential to improve the resulting DEMs in rain forested areas. We conducted experimental surveys in western Tasmania to find the LiDAR survey specifications that maximise returns from the ground through thick forest. The most efficient survey specification was that which maximises the number of LiDAR pulses emitted, but sufficient ground returns from beneath tall trees could only be achieved by increasing the spot diameter. A variety of interpolation methods were tested to produce a 2m DEM that maximised geologically related micro-topography. The minimum curvature (i.e. thin plate spline) method with small tension (lambda) was found to be optimal. A 5m 2 derivative (curvature) image was most useful in identifying bedding traces. Interactive GIS tools have been developed to mark bedding traces traversing topography and calculate strike and dip using the moment of inertia method. In the Mathinna area in northeast Tasmania 1150 structural measurements were interpreted from 40 km , improving mapping efficiency by 40% and providing important constraints on structural interpretation. At Waratah in western Tasmania, LiDAR is being used to interpret the structural geology of remote areas covered by rain forest. Stratigraphic units assumed to be continuous are actually highly segmented, with blocks entrained in structurally complex zones. LiDAR remote sensing using appropriate survey specifications and effective GIS tools to aid interpretation has delivered the ability to efficiently build reconnaissance geological maps rich in structural information. The technique promises to deliver a step change in the geological mapping and structural interpretation of forested terrain. nd

SGTSG 2015: Riding the Waves

Synclines are revealed in a hill-shaded LiDAR DEM over almost entirely forested terrain south of Corinna, in western Tasmania.

The 4 LiDAR survey specifications conducted in western Tasmania are indicated on a diagram illustrating the available range of survey specifications in grey shade, bounded by regions inaccessible to the instrumentation. The LiDAR specification space is contoured by the cost of obtaining 1000 ground points in areas with tall trees. The cost is strongly dependent on spot diameter.

Geological information in an image of 2nd vertical derivative topography (left) is interpreted to provide strikes and dips used to interpret a cross section across the 250m wide scene (above).

SGTSG 2015: Riding the Waves

Characterising fold geometries from structural data: application to implicit modelling LACHLAN GROSE 1 , LAURENT AILLERES 1 AND GAUTIER LAURENT 1,2 1 2

Monash University, School of Earth Atmosphere and Environment U. de Lorraine (Nancy, France), Georessources, Research for Integrative Numerical Geology.

Folds present a challenge for implicit interpolation because the information associated with the geometry of the structure is not explicit in the orientation of the folded foliation. Traditional implicit approaches involve interpolating between observations from orientation measurements and stratigraphic contacts. To constrain the geometry of folds, the user is generally required to draw fold profiles on cross sections or level maps using bedding structural elements. A new method allows for more structural elements (e.g. foliations, intersection lineations) and overprinting relationships to be incorporated into the modelling process. Each generation of folding is successively modelled, starting with the most recent folding event. At each stage, several scalar and vector fields are interpolated to represent the axial surfaces, the fold axis and vergence of each fold. The geometry of each fold generations are defined by structural parameters. These parameters need to be derived from available data and specified to the interpolator. We present an adaptation of geostatistical tools to structural geology for quantifying fold geometry. In this framework, each fold is described with a fold frame based on structural elements (fold axial surface, stretching lineation and intersection lineation). We use the angle rotating the direction of the folded foliation towards the surface of the structural element defining the fold axial surface (fold rotation angle). The fold rotation angle can be calculated in two ways: (1) angle between younger foliation, Sn and older foliation Sn_1 or; (2) angle between Sn and Sn_1 form line around the fold axis Ln. We present adaptations of geostatistical tools where cartesian space is substituted for the foliation scalar field of the younger foliation (S n ): a cross plot of fold rotation angle and the scalar field (S-Plot); and a variogram of fold rotation angle (S-Vario). Using these plots it is possible to characterise the geometry of folds such as the fold axial trace, wavelength, tightness, asymmetry and periodicity, for a single fold or a fold series. This approach is applicable to complex poly-deformed terranes because we use the foliation scalar field representing the state prior to sequential stages of deformation. Our method provides a robust method for identifying these parameters from observations and provides the framework for automated fold modelling. We expect a number of different applications of this approach and present an application to parametric fold modelling.

SGTSG 2015: Riding the Waves

High-temperature low-pressure metamorphism & accretionary orogen geodynamics: Insights from the Wagga-Omeo Metamorphic Belt. BRADLEY WILLIAMS1, ALISTAIR C. HACK1 AND GLEN PHILLIPS2 1 Earth Sciences, School of Environmental and life Sciences, Faculty of Science and IT, The University of Newcastle (UoN), NSW, Australia 2 Regional Mapping & Exploration Geoscience, Geological Survey of New South Wales \ Division of Resources & Energy | Department of Industry, 516 High St Maitland NSW 2320 \ GPO Box 344 Hunter Regional Mail Centre NSW 2310

This study focuses on quantifying the thermal and structural history of the Wagga-Omeo Metamorphic Belt (WOMB) and its implications for (i) models of high-temperature low-pressure (HTLP) metamorphism and (ii) contrasting geodynamic models of Tasmanides evolution. The results are based on new field observations (Fig. 1), metamorphic-structural analysis and analytical geochronological methods. w e m • • • • • • • • • •

Figure 1: Metamorphic zonation in the Holbrook area mapped in this study integrated with the Victorian Omeo Metamorphic Complex. Note the high-grade migmatite zone is inferred to remain west of the Tallangatta Creek Fault Zone. This implies the west side is uplifted exposing deeper crustal levels (inset), supported by the kinematic indicators observed along this fault in this study.

I PH

I S-type granite f-type granite

£T1

A-type granite

i Gramte (N.S.W.)

[ Saurian and Devonian volcanic and sedimentary rocks Early Devonian catdera volcamcs

Triassic intrusions and trachyte

CNorrte and btobte nones r.-TJ I

Cordierite zone Andaiusite-K-letdspar zone

W1BBP SiBimanite-muscovrte zone •H

Stttimanite-K-feidspar and migmatite zones 147* E I

The oldest fabric identified in metamorphic rocks (SI, Fig. 2a) is a continuous foliation associated with high-temperature mineral growth, e.g. cordierite (crd), sub-parallel to bedding. No folding was identified in association with the development SI. U-Th-Pb chemical dating of individual monazite grains grown within SI yields a tanh age of 443 ± 3 Ma, which is interpreted as the age of peak metamorphism. The first contractional deformation (D2) is associated with tight upright folding, 57

SGTSG 2015: Riding the Waves development of a differentiated crenulation cleavage (S2), and coincides with the termination of crd growth (Fig. 2b). A weak S3 fabric is locally developed in the vicinity of ca. 430 Ma S-type granites, though its significance is uncertain due to lack of exposure.

Figure 2: SEM BSE images showing the textural context of monazite grains dated by the U-Th-Pb chemical method from samples within the cordierite zone in the Holbrook area. a. Crd porphyroblast-hosted equant monazite in contact with biotite inclusion aligned with SI. Mnz from both textural settings are statistically identical with a tanh age of 443 ± 3 Ma. b. Matrix-hosted equant monazite proximal to S2 crenulation hinge. 6

5 15

•4 3

10.

5 1 450 500

550

600

Figure 3: (left) P-T phase diagram showing minimum geothermal gradients with associated surface heat flows for cordierite-assemblages in metamorphic rocks of the WOMB. Note in the case of the minimum geothermal gradient for crd-biotite (bi) is not stable at subsolidus conditions due to the wet granite solidus (WGS) and bidehydration melting reaction (BDS), higher geothermal gradient are required to stabilise subsolidus crd-and. (Fig after White et al., 2014). Extensional tectonometamorphic crd+bi fabrics of rocks around Holbrook formed at less than 10 km depth (sillimaniteabsent) in very hot thinning crust.

650 700 750 800 850 900

Temperature (°C) Analysis of calculated phase equilibria suggest crd-assemblages in the study area require geothermal gradients in excess of > 80 °C km"1 with corresponding surface heat flow in excess of 200 mW m"2 (Fig. 3). Such conditions are capable of inducing biotite dehydration melting at depths < 15 km (e.g. Omeo-zone). Examination of global surface heat flow data shows only young, hot, thin oceanic crust associated with back-arc environments are capable of generating HTLP conditions on a regional scale. Significantly the results of this study indicate that HTLP metamorphism (i) shares no temporal relation to granite emplacement and (ii) was terminated by crustal thickening during Dl. This thermal and structural history supports a retreating accretionary geodynamic model in which a submarine sedimentary pile undergoes HTLP metamorphism during an early phase of extensional back-arc spreading (Dl). Peak HTLP metamorphism was followed by a relatively short-lived crustal thickening event (D2) and subsequent S-type granite emplacement. References White, R. W., Powell, R., Holland, T. J. B., Johnson, T. E. & Green, E. C. R. 2014. New mineral activitycomposition relations for thermodynamic calculations in metapelitic systems. Journal of Metamorphic Geology, 32, 261-286.

SGTSG 2015: Riding the Waves

A Palinspastic Reconstruction: Basement and lithological controls on the development of the Jellinbah Thrust Belt. ALB AN DAVID HAISLEY School of Earth Sciences The University of Queensland

The complex geological structure known as the Jellinbah Thrust Belt lies between the Jellinbah Fault and Yarrabee Fault on the eastern flank of the Permian Bowen Basin. The detail of the Jellinbah Thrust Belt is the subject of much discussion, partly because it interacts with coal-bearing sediments of economic importance, but also because it provides evidence of structural trends developed during subduction episodes along the entire eastern margin of Australia in the later stages of the HunterBowen Orogeny. This project interprets the complex structural architecture within the system in this part of the Bowen Basin, using structural restoration of the stratigraphy to its Pre-Permian state. By estimating the major thrust fault movements, one can understand how the basement and stratigraphy controlled the mechanics of the fault system. Seismic data from the 2007 Dingonose acquisition (07-DNG-01, 07-DNG-02 and 07-DNG-08) was depth converted using velocity picks in SKUA ™ software. Well data was used to confirm the accuracy of the depth conversion and to estimate stratigraphic boundaries that were traced as horizon reflectors in the seismic data. Finally a kinematic reconstruction was performed in the software package Move ™. The basement high features identified in section 07-DNG-01 act as obstructions to compression focussing larger reverse displacements in this part of the fault system. Displacement is interpreted to occur most freely along decollement surfaces associated with clay and other ductile beds, which coincide with the base of the Bowen Basin and at various intervals within the Permian coal formations. Ductile units glide atop more resistant units easily dissipating the strain energy. Smaller related faulting between decollements occurs within the more competent stratigraphic units. Flattening of the basement high to the south-east resulted in less confinement of stresses on specific faults and a more even distribution of displacement within the fault system. The depth of the Bowen Basin as interpreted from seismic conversion varies between approximately ~2.5km to 4.5km within the project area, with the stratigraphic sequence thickening toward the Taroom Trough. With increased section depth the number of ductile stratigraphic surfaces increases. This creates a more uniform displacement distribution across the fault system along a thicker stratigraphic section. The palinspastic interpretation concurs with the broad evolutionary framework of the basin during the Permian period. Basement morphology plays a significant role in the transfer of strain between faults. The study highlights that multiple deformation mechanisms occur during the compressional phases, generating both relatively low angle thrust faults and higher angle kink bands. Keywords: Bowen Basin, Jellinbah Thrust Belt, Rangal Coal Measures, Seismic Interpretation, Hunter Bowen Orogeny, Structural Geology, Reverse Faulting, Thrust Faulting, Palinspastic Restoration.

SGTSG 2015: Riding the Waves

Unlocking and linking the low-temperature thermochronological histories of inland South Australia JAMES W . HALL 1 , STIJN GLORIE1, ALAN. S. COLLINS1, KATE AGOSTINO1, MAX REDDY1, CHRIS TRENOUTH1, NOREEN EVANS 2 , ANTHONY REID 3 AND RIAN DUTCH 3 1 Centre for Tectonics Resources and Exploration (TRaX'), Department of Earth Sciences, School of Physical Sciences, University of Adelaide, Adelaide 5005, South Australia, Australia John de Laeter Centre for Mass Spectrometry, Curtin University, Perth, GPO Box U1987, Perth, Western Australia 3 Geological Survey of South Australia, Department of State Development, 101 Grenfell St, Adelaide 5001 2

The thermal evolution of a region can be revealed using a combination of low-temperature thermochronological techniques which can then potentially illuminate the palaeo-stresses, namely exhumation, of an intra-continental setting. This can be used to infer the timing and extent of deformation events such as orogenies. Apatite fission track, and zircon and apatite U-Th-Sm/He analyses were conducted on granitiod samples collected within four regions within inland South Australia to deduce the thermal histories of these regions. These regions, the Peake and Denison Ranges, central Gawler Craton, eastern Musgrave Province, and buried Stuart Shelf, all revealed complex and longlived thermal histories. However, all regions presented here contain evidence for thermal events during the late Cambrian - Ordovician, mid to late Carboniferous, and Early Jurassic, while select samples also suggest localised Cretaceous cooling within parts of South Australia. These recorded periods of thermal activity can be linked to denudation resulting from orogenies such as the Delamerian and Alice Springs orogenies, or far-field stresses caused by the break up of Australia and Antarctica. Within the Musgraves and the buried basement of the Stuart Shelf, older thermal events were identified related with the late Ediacaran Peterman Orogeny and a Grenvillian thermal event, respectively. Very little data was previously available from this region of inland South Australia, despite this region being the catchment of the transcontinental Late Cretaceous Ceduna River (MacDonald et al., 2013; Lloyd et al., 2015) and a site of considerable controversy about the cause of the rejuvenation of the river system to deposit the upper lobe in the Bight Basin (MacDonald et al., 2013). The data presented here also suggest that despite the Gawler Craton having experienced its last major tectonothermal event in the early Mesoproterozoic (Hand et al., 2007), it appears to have far from technically inactive through the Neoproterozoic and Phanerozoic. We explore the possibility that the data are revealing phases of reactivation of the huge shear zone networks in the region and/or the incision of major canyon systems (Reddy et al., 2015). Recent developments in the applied methodology allow fast access to larger and more accurate datesets. Therefore, the extent of the above descrbied thermal events within South Austalia can now accurately be mapped out and linked with the prevailing tectonic regimes at the plate margins. It is furthermore anticipated that this thermal map may aid future mineral exploration within South Australia. References Hand, M., Reid, A., Jagodzinski, L., 2007. Tectonic Framework and Evolution of the Gawler Craton, Southern Australia. Economic Geology 102, 1377-1395. Lloyd, J., Collins, A.S., Payne, J.L., Glorie, S., Holford, S., Reid, A.J., 2015. Tracking the Cretaceous transcontinental Ceduna River through Australia: The hafnium isotope record of detrital zircons from offshore southern Australia. Geoscience Frontiers. MacDonald, J., D., Holford, S.P., Green, P.F., Duddy, I.R., King, R.C., Backe, G., 2013. Detrital zircon data reveal the origin of Australia's largest delta system. Journal of the Geological Society, London 170, 3-6. Reddy, M., Glorie, S., Collins, A.S., 2015. Phanerozoic cooling history of the central Gawler Craton: implications of new low-temperature thermochronological data. Mesajournal 75, 36-40.

SGTSG 2015: Riding the Waves

Rarely made or rarely preserved? Experimental insights into the formation and preservation of frictional melt K.S. HAYWARD1, S.F. COX 1 AND J.G. FITZ GERALD, 1

Research School of Earth Sciences, Australian National University, Canberra

The propagation of seismic slip is generally attributed to the activation of fault weakening mechanisms at high slip velocities, with one such mechanism being the formation of frictionally induced melting. Indeed, the presence of fault related pseudotachylyte is generally conceded to be the only unequivocal evidence of seismogenic slip within the geological record, placing important constraints on the interpretation of rupture properties. However, field evidence indicates that pseudotachylytes are rare, implying either: melting does not occur within all seismogenically active fault zones (possibly due to the activation of different weakening mechanisms); or, that many pseudotachylytes are not preserved over geological timescales. During this talk, new insights into the behaviour and the microstructural development of fault surfaces during the early stages of seismic instability will be presented. This work is based on the findings of a series of high temperature experiments undertaken at high normal stress and over small slip displacements using a triaxial deformation apparatus and an essentially pure-quartz sandstone starting material. Results reveal complex transitions in fault behaviour with changing temperature and pressure conditions, including marked differences in slip velocities and acceleration between the different slip regimes. The mechanical results are coupled with microstructural analysis using high resolution SEM and FIBTEM techniques that provide insights into mechanisms controlling fault behaviour over a range of scales. Significant findings include the recognition of textures that capture the transition from flash heating to the early stages of frictional melting. Microstructures range from the formation of a partially amorphous gouge layer during aseismic sliding through to the generation of frictional melt during stick-slip. Quenched melt is localised, forming distinctive drawn-out glass filaments and fractured patches up to 2^im thick that cover between 10-60% of the fault surface. These microstructural changes occur over slip distances that are several orders of magnitude less than have been previously identified during high-velocity experiments, with melt forming within 50|im of the commencement of slip. The rapid onset of micromechanical and microstructural changes has implications for understanding the dynamics of fault rupture including weakening distances and co-seismic fault strength. It also suggests that pseudotachylytes may, in fact, be far more common than field observations suggest. Further experimental treatment of the melt-covered fault surfaces at hydrothermal conditions shows that the pseudotachylyte has a short lifespan (<1 hour) in the presence of high temperature, reactive fluids. Consequently, the scarcity of natural pseudotachylytes may arise from both a failure to recognise micron-thick melt layers and also an inability to preserve such features on geological timescales.

SGTSG 2015: Riding the Waves

Faulting, fluids and fusion: frictional melting and cohesive strengthening on low displacement, misoriented fault interfaces K . S . HAYWARD 1 AND S . F . COX 1 1

Research School of Earth Sciences, Australian National University, Canberra

Many faults, in various tectonic regimes remain active despite being unfavourably oriented for reactivation relative to the prevailing regional stress field. This implies that even under non-ideal stress conditions a fault zone can remain a comparatively weak structure within the host rock that may accommodate continued slip. Once reactivation occurs, fault behaviour is controlled by dynamic weakening and strengthening mechanisms that either enhance or impede the propagation of slip. One such mechanism is the formation of frictional melt (pseudotachylyte) along the sliding interface. However, observations from natural faults suggest that pseudotachylyte generation is largely limited to single slip events within low-porosity, dry crystalline host rocks, with such processes as thermal pressurization of fault fluids potentially inhibiting melt generation in fluid saturated environments. Until now, apparatus limitations have prevented exploration of fault properties during high velocity slip under saturated conditions and on macroscopically porous slip surfaces. This poster presents the results of a number of room-temperature, low- to high-velocity, small displacement triaxial experiments that have been undertaken using an essentially pure-quartz sandstone starting material. The samples are arranged with pre-ground fault surfaces inclined between 25° and 70° to the maximum shortening direction, representing faults that vary from optimally-oriented to severely-misoriented for failure. Faults are reactivated in both dry and water-saturated conditions, with the former experiments involving the increase of axial load at constant rate until failure (stress-driven failure), whereas in saturated conditions fault reactivation is achieved by maintaining a constant axial load and increasing pore fluid pressure until slip occurs (fluid-driven failure). A significant result from these experiments is the generation of a semi-continuous melt layer during both the stress- and fluid driven reactivation, with the extent of melt formation being positively correlated with an increasing angle of misorientation and hence increasing normal stress. The generation of pseudotachylyte on the fault surface is observed to have significant effects on the strength and behaviour of the fault via a process termed 'melt-welding'. The use of sophisticated imaging techniques, including high resolution SEM and microcomputed X-ray tomography, has provided a 3-dimensional constraint on the spatial extent of melt-welding, with approximately 50% of the fault surfaces of highly misoriented faults being fused. This process significantly contributes to the increase in damage observed along the slip zone; on the most unfavourably oriented faults the increase in cohesive strength due to melt-welding can drive fault lock-up and generation of a new optimallyoriented fault. These results raise the possibility that frictional melt formation may be prevalent under conditions and in materials thought not to generate pseudotachylytes. Further, these results suggest that thin layers of melt may play a previously unrecognised role in the development of off-fault damage.

SGTSG 2015: Riding the Waves

Revisiting the plate dynamics of the northwest Coral Sea: Modelling the Gulf of Papua and the break-up of northeast Australia KELLY A. HEILBRONN1,2 AND ROBERT J. HOLM 1 ' 2 department of Earth and Oceans, College of Science, Technology and Engineering, James Cook University, Townsville, Queensland 4811, Australia Economic Geology Research Centre (EGRU), College of Science, Technology and Engineering, James Cook University, Townsville, Queensland 4811, Australia

The timing and nature of the Late Mesozoic-Cenozoic breakup of eastern Gondwana is generally well constrained by previous tectonic modelling, however, our understanding of the break-up of northeastern Australia and Papua New Guinea has been brought into question by recent studies. Research into the Maramuni Arc and the Louisiade Archipelago of Papua New Guinea interpret the existence of northward subduction beneath Papua New Guinea at the Pocklington Trough, ceasing as recently as 12 Ma (Webb et al., 2014; Holm et al., 2015). Previous reconstructions have assumed that closure and suturing of the Pocklington Trough preceded the opening of the Coral Sea, however, this new evidence demands the existence of another oceanic basin north of the Coral Sea, the Pocklington Sea. The Pocklington Sea has been completely removed from the geological record by subduction, but by using modern convergence rates across the region of up to 10 cm year over an inferred 14 million years of subduction, we estimate there was likely in excess of 1000 km of oceanic lithosphere that existed north of the Coral Sea. Is is not possible to directly examine the Pocklington Sea lithosphere at the present day due to subduction. In lieu of direct evidence as to the nature of the Pocklington Sea, we investigate the regional tectonic framework through a review of datasets and tectonic models for the northwest Coral Sea and northeast Australia. We take two different approaches in an effort to shed light on the tectonic history of the region. Firstly, we created a 3D model for the Pandora Trough within the Gulf of Papua using Leapfrog Geo software. This model provides an oppurtunity to examine the structure of the western Coral Sea and permits the comparison between structures in the Pandora Trough and the Queensland Trough, which may assist in the tectonic interpretation of basin formation in the Coral Sea region. In conjunction, tectonic reconstructions were created using GPlates software that incorparate the major spreading events involved in break-up of eastern Gondwana and the isolation of the Australian continent between 160 Ma to 40 Ma. The reconstructions highlight a significant gap in our understanding of the extension and sea floor spreading history to the north of Australia. We test multiple scenarios for sea floor spreading in the Pocklington Sea assumming this was spatialy and temporally related to either Jurassic sea-floor spreading off Australia'a northwest shelf, or Cretaceous-Paleogene spreading in the Tasman and Coral Seas. The development of such tectonic models for this region have the potential to provide an improved understanding of the plate dynamics that formed the Coral Sea, and offers insights into the nature of the Pocklington Sea and tectonic history of the northern Australian plate. References Holm, R. J., Spandler, C., and Richards, S. W., 2015, Continental collision, orogenesis and arc magmatism of the Miocene Maramuni arc, Papua New Guinea: Gondwana Research, v. 28, no. 3, p. 1117-1136. Webb, L. E., Baldwin, S. L., and Fitzgerald, P. G., 2014, The Early-Middle Miocene subduction complex of the Louisiade Archipelago, southern margin of the Woodlark Rift: Geochemistry, Geophysics, Geosystems, v. 15, no. 10, p. 4024-4046.

SGTSG 2015: Riding the Waves

Northern Tasmanides: shared orogenic threads but unique properties BOB HENDERSON1 AND CHRIS FERGUSSON2 department of Earth and Oceans and Economic Geology Research Centre, College of Science, Technology and Engineering, James Cook University, Townsville, Queensland 4811, Australia 2 School of Earth & Environmental Sciences, University ofWollongong, New South Wales 2522, Australia

The Mossman Orogen occupies the northern part of the Tasman Orogenic Zone where it is separated from the North Australian Craton by a thin selvage of rocks assigned to the Thomson Orogen. These include metavolcanics, granites and sedimentary rocks, including quartz-rich turbidites that are variously overprinted by metamorphism. Units of Thomson association are generally assigned as Cambro-Ordovician, based on isotopic dating and a solitary graptolite, overlapping in age with the later stage development of the Thomson system to the south. Early Silurian Benambran orogenesis 430 Ma) strongly affected these rocks and was coincident with the accretion of a proximal late Ordovician island arc within the northern Tasmanides. A pre-Delamerian history for the Thomson Orogen in its northernmost development is suggested by a -500 Ma Ar-Ar metamorphic age for an amphibolite from the Gray Creek Complex, a mafic-ultramafic assemblage locally developed on its outboard side. The Mossman Orogen represents a classic active margin of Siluro-Devonian age, with coeval magmatic arc, forearc basin and subduction complex elements. It was initiated in response to easterly directed subduction following closure of the Thomson orogenic system by Benambran orogenesis. Its development was terminated by Tabberabberan orogenesis in the Frasnian, in which its palinspastic architecture was substantially reorganised by thrusting from the east. Subduction complex rocks dominate surface exposure. They are dominantly quartz intermediate turbidites with subordinate chert and mafic volcanics, with ubiquitous development of melange fabrics. Their established age range, based on biostratigraphic evidence and detrital zircon age spectra is late Silurian - late Devonian with a younging trend generally to the east. Their provenance was local, with derivation largely from recycled rocks of the adjoining craton, Thomson Orogen and the contemporary magmatic arc with the relative proportions of these sources changing through time. Tabberabberan thrusting appears to have intercalated Thomson quartz-rich turbidites into the southwestern part of the subduction complex. In its northern part the subduction complex was overprinted by Permian HunterBowen orogenesis in addition to that of the Tabberabberan. Stratigraphic and facies architecture of the forearc basin assemblage suggests a horst and graben extensional setting with juxtaposition of deep water siliciclastics against shelf limestone and with basalt eruption in its northern part. Biostratigraphic age control shows it developed between the Telychian and early Frasnian (438-380 Ma). Tabbereabberan thrusting extensively disrupted the northern part of the forearc which contains a substantial belt of overturned rocks. Thrusting also carried subduction complex rocks over the forearc, interrupting its along-strike continuity. During the late Silurian - early Devonian, the magmatic arc developed in older rocks adjoining to the west but in the late Devonian it jumped to the east and is represented by syntectonic granites emplaced in the subduction complex. Exposure of the Mossman Orogen terminates to the south on the Clarke River Fault, a crustal scale sinistral dislocation induced in Tabberabberan orogenesis. The orogen is bounded on its eastern side by the Barnard Metamorphics, an assemblage of older, early Paleozoic rocks distributed along the coast north of Innisfail towards Cairns. These rocks were once part of the Thomson Orogen. Their present location is due to either strike-slip translation from the Thomson Orogen terrain to the south, or to upthrusting from older crust beneath the Mossman subduction complex. The latter is favoured by deep crustal seismic interpretation, with the implication of extensive development of the Thomson Orogen within the crust but now covered by westward Tabberabberan thrust transport of the subduction complex across the continental margin at grand scale.

SGTSG 2015: Riding the Waves

The Influence of Deformation on Mineral Phase Equilibrium.

BRUCE HOBBS 1

Centre for Exploration Targeting, The University of Western Australia.

The aim of this presentation is to discuss a unified approach to the influence of deformation (and hence, non-hydrostatic stress) on the conditions and nature of mineral equilibrium. By conditions is meant the position and (Clapeyron) slope of the equilibrium phase boundary in P-T space and by nature is meant the number of mineral phases and their microstructural arrangements. A number of entrenched concepts are questioned including the influence of normal stress in controlling dissolution and precipitation, the role of hydrostatic and mean pressure, whether equilibrium can exist in deformed rocks, and how deformation influences the mineral assemblage and the position of the phase boundary. The basis for any self-consistent approach has been in existence for at least 60 years and was laid out by Gibbs (1878), Landau (1937) and Born and Huang (1954). The pressure relevant to influencing the equilibrium is always the thermodynamic pressure which is conceptually distinct from the mechanical pressure (the mean stress) and from the hydrostatic pressure equal in magnitude to the overburden pressure. For a material undergoing elastic deformations the thermodynamic pressure is always equal to the mean stress. Thermodynamic equilibrium is controlled solely by elastic distortions of crystalline lattices together with the thermodynamic pressure and the temperature. This is true for both elastic deformations and for crystal lattices with high concentrations of lattice defects (point and planar defects, dislocations and disclinations). The normal stress on interfaces is not a thermodynamic state variable and is irrelevant. The Clapeyron slope is always influenced by deformation. For fluid present reactions the effect is small. For fluid absent reactions the effect on the Clapeyron slope depends on how elastic moduli soften at the phase transition. In many instances the elastic moduli soften to zero at a phase transition and so the effect of non-hydrostatic stress is negligible. However if there is strong coupling between diffusive effects and/or plastic deformation on the softening behaviour of elastic moduli then the effect can be very large. This seems to be true for the sillimanite-andalusite transition but is true for many other phase transitions as well. The phase rule always involves extra degrees of freedom under non-hydrostatic conditions, for both fluid present and fluid absent reactions, so that extra "phases" are always present at equilibrium compared to what one would expect under hydrostatic conditions. These "phases" can be of a chemical or microstructural nature. The theory outlined here provides a new tool box for interpreting the microstructure and chemistry of deformed metamorphic rocks and for interpreting the processes involved in their development.

References

Born M., and Huang, K.l954.Dynamical Theory of Crystal Lattices. Oxford University Press. Gibbs, J.W. 1878. On the equilibrium of heterogeneous substances. Trans.Connecticut Academy of Arts and Science. 3, 108-248 (1775-1876), 343-524 (1877-1878). Landau, L. D. 1937. On the theory of phase transitions. Phys. Zs. Sowjet. 11, 26, 545.

SGTSG 2015: Riding the Waves

Time-Space discontinuum in the NEO: partitioning of Hunter-Bowen events ROD HOLCOMBE1, CHRIS FIELDING2, RENATE SLIWA3, GIDEON ROSENBAUM1 AND DEREK HOY 1 School of Earth Sciences, The University of Queensland, St Lucia Qld 4072 School of Earth & Atmospheric Sciences, University of Nebraska, Lincoln, Nebraska, USA integrated Geoscience Pty Ltd, P.O.Box 462, Yandina QLD 4561

The New England Orogen (NEO) is a broad term for a collage of superposed terranes related to the active Gondwanan subduction margin from Late Devonian to Late Triassic. During this 130+ million year interval the location of the arc oscillated across the continental margin and the orogen underwent an oscillating series of crustal-scale extensional and contractional events, inferred to result from variations in the rate of convergence, the geometry of the down-going slab, and the degree of coupling with the overriding plate. In addition, major oroclinal flexures that developed within this time span require some degree of accommodating wrench (e.g. Murray et al., 1987; Cawood et al., 2011; BrookeBarnett & Rosenbaum, 2015). It has long been recognised that evolutionary models for any terrane within the orogen need to be strongly tied to the appropriate time slice. Analysis of a Time-Space plot for the NEO shows not only events that are common orogen-wide, but also shows along-orogen and across-orogen event discontinuities that still require explanation. The elements in common include: the Carboniferous active accretionary margin elements; the transition into orogen-wide, Early Permian extension; the broadly defined 30my interval of the Permo-Triassic Hunter-Bowen Orogeny (H-BO); the linear belt of Late Permian arc magmatism; the development of a major Triassic foreland basin system (Bowen-Gunnedah-Sydney Basins); and a rapid transition into widespread extension at the beginning of the Late Triassic (~230Ma). All of these have logical explanations in terms of slab geometry, location, or degree of coupling. The most prominent discrepancies within the time-space plot are spatial-temporal details within the fold-thrust belt formed by the Hunter-Bowen Orogeny (New England Fold Belt [NEFB]). Two have important implications for NEO tectonic models for SEQld: 1. The initial contractional pulse of the Hunter-Bowen Orogeny affects all elements of the NEFB at -260 ma - except the Gympie province, which then requires some explanation as to how that province was partitioned from that event. The earliest orogenic pulse of the H-BO is tightly constrained within the northern NEO where it has a strong sedimentological signature in the Moah creek Beds and Barfield Formations (-260 Ma) that were subsequently deformed in a prominent fold-thrust belt prior to 253 Ma. (At which time the early thrust belt was overridden by the Marlborough nappe sheet as described below). In southeast Queensland (SEQld), structures associated with the initial phase are more difficult to separate from the later deformation, but the evidence of an early phase of deformation is quite strong. An abundance of -260 Ma Ar-Ar and K/Ar ages in older rocks in the D'Aguilar Range marks exhumation of basement rocks that already contain metamorphic fabrics attributed to H-BO deformation, and a strong angular unconformity at the base of the units in the Esk Basin overlies steeply dipping Early Permian rocks. In stark contrast, the Gympie province remains unaffected by this initial H-B pulse, even though, in its present location, it is only 30 km across-strike from rocks in the D'Aguilar-Conondale ranges with early H-BO fabrics. The stratigraphic sequence at Gympie is more or less concordant and continuous throughout much of the Permian. Recent chronostratigraphy (Li et al., 2015) suggests that the largely Kungurian South Curra Limestone may have persisted up to 258 Ma, and was concordantly succeeded by marine clastics of the Tamaree Formation up to at least 255 Ma, indicating little, if any, effect of the H-BO during this interval. It appears that the Gympie Province was not closely linked to the remainder of the NEFB in the Late Permian phase of the H-BO (and may have remained so until the last pulse of the H-BO at ~235Ma). The boundary between the Gympie Province and the NEFB rocks of the D'Aguilar-Conondale Ranges is orogen-parallel, but identifying it precisely is complicated by both Late Triassic rifting and the Cenozoic strike-slip faulting. It may correspond to a set of NNW trending faults that occur sporadically along the eastern margin of the metamorphic rocks D'Aguilar and Conondale Ranges (e.g. Bracalba Fault; Normanby Fault?). 66

SGTSG 2015: Riding the Waves 2. A major mid-H-BO deformation event at -253 Ma in the northern NEFB has no counterpart at all in the SEQld segment of the NEFB, requiring a crustal-scale partitioning boundary. The mid H-BO event in the northern segment includes a major nappe (including basement ophiolitic rocks) emplaced over the early fold-thrust belt (Holcombe et al., 1997) at about 253Ma (Harbort et al., 2001). This event precedes a profound change in the rate and type of sedimentation of the foreland basin (Bowen Basin). A similar change in sedimentation patterns occurred simultaneously in the southernmost NEO to form thick terrestrial sequences in the Sydney Basin suggesting that this southern sector of the NEFB underwent a similar orogenic pulse at the same time as the northernmost sector. In contrast, the entire central section of the NEFB from about Bundaberg to perhaps as far south as Coffs Harbour (and including the Gympie Province), shows no contractional deformation at this interval at all. Instead, sediment accumulation continues throughout the Early and Middle Triassic. Thus the entire central segment of the NEFB has been partitioned from the -253 Ma contractional pulse, before itself undergoing very strong contraction at ~235Ma. The boundaries between these segments of vastly contrasting structural styles at -253 Ma must have been crustal in scale in order to partition the -253 Ma orogenic pulses. The southern boundary of this central NEFB segment is poorly constrained, and complicated by the Texas-Coffs Harbour orocline. Likewise, the northern boundary is unconstrained in location or orientation, t is possible that a structure extending SW from about Childers on the coast could have formed a crustal-scale tear fault during this phase of the H-BO, but it is difficult to define such a feature on geophysical imagery. Alternatively, it is possible, and perhaps more likely, that the partitioning structure lies parallel to the grain of the orogen and may correspond in part to the Yarrol Fault as initially proposed by Murray etal. (1987). Perhaps a more likely solution to both of the Time-Space discrepancies listed above is the possibility that each partitioning structure is strike-slip and broadly orogen-parallel, and perhaps related in some way to the late evolution of the Texas-Coffs Harbour Oroclines. The recent models of Cawood et al. (2011) and Brooke-Barnett & Rosenbaum (2015) both produce the oroclines with local strike-slip accommodation of variable slip sense. The problem with invoking a relationship with orocline formation is that most models require the oroclines to form in the Early Permian, with some some difference of opinion as to whether the orocline accompanied, or post-dated, formation of the extensional terranes. Cawood et al. (2011) invoked a final stage of orocline formation at 270-260 Ma, which does correspond to the initial pulse of the H-BO (except at Gympie). But none of the models, as currently formulated, consider any oroclinal activity as young as 253Ma, the age of the Permo-Triassic pulse in the northern NEFB. However it is not inconceivable, that late readjustments of the oroclinal system may have been responsible for the apparent lack of coupling between events in the northern NEFB and those in southern NEFB at this time.

References

Cawood P.A., Pisaveresky S. & Leitch E. 2011. Unravelling the New England Orocline, east Gondwana accretionary margin. Tectonics 30, TC5002. Brooke-Barnett S, & Rosenbaum, G. (2015). Structure of the Texas Orocline beneath the sedimentary cover (southeast Queensland, Australia), Australian Journal of Earth Sciences, 62, 425-445. Holcombe, R.J., Stephens, C.J., Fielding, C.R., Gust, D. Little, T.A., Sliwa, R., McPhie, J., Ewart, A. 1997. Tectonic evolution of the northern New England Fold Belt: The Permian-Triassic Hunter-Bowen event. Geol. Soc., Australia Spec. Publ., 19, 52-65. Harbort, T. Holcombe R.J., Vasconcelos P. & Fielding C.R. 2001. Latest Permian emplacement of the Marlborough Block duplex: the major mountain-building phase of the Hunter-Bowen Orogeny in the northern NEFB. Geological Society of Australia Abstracts Series 64, 77-78. Li, P., Rosenbaum, G., Yang, J.-H., and Hoy, D. 2015. Australian-derived detrital terrane (eastern Australia): Evidence for an autochthonous origin, Tectonics, 34, 1-17. Murray, C.G., Fergusson, C.L., Flood, P.G., Whitaker, W.G., and Korsch, R.J. 1987. Plate tectonic model for the Carboniferous evolution of the New England Fold Belt, Aust. J. Earth Sci., 34,213-236.

SGTSG 2015: Riding the Waves

Late Cenozoic convergent tectonics in Papua New Guinea ROBERT J. HOLM 1 , CARL SPANDLER1 AND SIMON RICHARDS 1 College of Science, Technology & Engineering, James Cook University, Townsville, Queensland

The geodynamic history of the northern Australian plate boundary throughout the Late Cenozoic is one of the most contentious and poorly understood aspects of Australian and southwest Pacific tectonics. The relict history of the northern Australian plate boundary throughout this time is expressed in the composite terranes that form Papua New Guinea, and are the result of multiple collision events and episodes of crustal growth. This region, however, is also characterised by a multitude of different tectonic models that reflect the limited pool of available data and limitations of our knowledge. One of the most important elements of this yet unresolved geological history is the convergent dynamics of Miocene subduction and associated magmatism of the Maramuni arc of Papua New Guinea. The Maramuni arc represents the only continuous record of the tectonic evolution of Papua New Guinea during the Miocene, and hence provides an opportunity to gain insight into subduction dynamics, orogenesis and crustal processes that operated throughout this dynamic period. We present an integrated U-Pb geochronology, Hf isotope and geochemical investigation of the Maramuni arc utilizing a suite of intrusive rocks from the Kainantu region of the eastern Papuan Highlands that span the Late Miocene from ca. 12 Ma to 6 Ma. The magmatic rocks formed from ca. 12-9 Ma have compositional affinities of subduction-zone magmas, but record increasing incompatible trace element contents and decreasing sHf with time, which we interpret to reflect a progressive increase in the crustal component of the magmas. Porphyry suites emplaced at 7.5-6 Ma are distinct from the older magmatic rocks by their marked HREE-depletion, which reflects a dramatic shift in arc-mantle dynamics. Based on these results we propose a revised geodynamic model for the tectonic evolution of Papua New Guinea involving the arrival of the Australian continent at a north-dipping Pocklington trough from ca. 12 Ma. Continent collision then led to growth of the New Guinea Orogen from 12 Ma aided by the underthrusting of the leading continental margin, which contributed crustal material to magma-genesis at ca. 9 Ma. Recent studies of the Louisiade Archipelago subduction complex substantiate north-dipping subduction at the Pocklington trough and closure of the trench at approximately 12 Ma (Webb et al., 2014). From ca. 7 Ma slab break-off and lithospheric delamination are reflected in a second phase of orogenesis that produced the HREE-depleted geochemical signatures of the contemporaneous magmatic rocks. References Webb, L.E., Baldwin, S.L., Fitzgerald, P.G., 2014. The Early-Middle Miocene subduction complex of the Louisiade Archipelago, southern margin of the Woodlark Rift. Geochemistry, Geophysics, Geosystems, 15, 4024-4046.

SGTSG 2015: Riding the Waves

Late Neogene and Quaternary reconstruction of Papua New Guinea and the Solomon Islands ROBERT J. H O L M 1 , GIDEON ROSENBAUM 2 AND SIMON W . RICHARDS 1 1 2

College of Science, Technology & Engineering, James Cook University, Townsville, Queensland School of Earth Sciences, The University of Queensland, Brisbane, Queensland

Papua New Guinea and the Solomon Islands lie in a complex tectonic setting trapped between the converging Ontong Java Plateau of the Pacific plate and the Australian continent. We present a Late Neogene to Quaternary plate tectonic reconstruction for this region. The reconstruction was performed using GPlates software and is based on data derived from a range of geological datasets, such as magnetic isochrons associated with seafloor spreading, subducted slab models, palaeomagnetic data, deformation patterns, and spatio-temporal distribution of arc magmatism. The reconstruction shows that since ca. 6 Ma, crustal elements began interacting in advance of the impending collision between the Ontong Java Plateau and Australian continent, leading to the inception of regional microplate tectonics and escalation in tectonic complexity. The Bismarck Sea region developed initially as a back arc basin, but was subsequently modified in response to arc-continent collision. Accordingly, the west Bismarck Sea became a transpressional zone associated with subduction of the Caroline plate and North Bismarck microplate at the New Guinea trench. Farther east, sea floor spreading has taken place in response to the clockwise rotation of the South Bismarck microplate. The Solomon Sea was subjected to anticlockwise rotation relative to Papua New Guinea in response to west-dipping subduction at the New Britain trench, which resulted in initiation of subduction at the Trobriand Trough and extension in the Woodlark Basin. Subduction resistance of the spreading Woodlark Basin at the San Cristobal trench drove crustal shortening in the Solomon Islands and convergence with the Ontong Java Plateau, which led to differential plate kinematics between the Solomon Islands and North Bismarck microplate with opening of the Feni Deep. Our reconstruction shows that the style of tectonics at convergent plate boundaries could involve intricate relationships between microplate rotations, sea floor spreading and subduction segmentation over timescales considerably less than 10 million years. Through unraveling the tectonic evolution of this complex region we can gain insight into the development of microplate tectonics at convergent margins, and the plate kinematics that might be preserved in the geological record of ancient orogenic systems.

SGTSG 2015: Riding the Waves

The end of the Hunter-Bowen Orogeny and younger deformation in eastern Australia: insight from the Gympie Terrane

DEREK HOY AND GIDEON ROSENBAUM 1

School of Earth Science, The University of Queensland, Brisbane, QLD 4072, Australia.

The Gympie Terrane of eastern Australia contains a near complete record of volcanism and sedimentation that spans the transition from Early Permian rifting into the establishment of a Permo Triassic magmatic arc during the Hunter Bowen Orogeny (-265-230 Ma). The stratigraphy of the Gympie Terrane has recently been updated, and the succession now comprises Upper Carboniferous (?) to Lower Permian volcanic, volcanoclastic, and associated sedimentary rocks of the Highbury and Dawn formations, which are overlain by the Middle Permian Rammutt Formation. The latter comprises intermediate to felsic volcanic, volcanoclastic, and coarse clastic rocks. The Middle to Upper Permian South Curra Limestone is succeeded by rhythmically interbedded siltstone and sandstone of the Upper Permian Tamaree Formation. Lower Triassic coarse sandstone and conglomerate of the Keefton Formation are overlain by shale, slate, and phyllite of the Lower to Middle Triassic Kin Kin beds. Despite being the site of a major mesothermal gold deposit, many aspects of the geology of the Gympie Terrane remain enigmatic. The structure of the terrane is complex and strain is strongly partitioned as a function of both the proximity to major faults and the lithological response to deformation. The Kin Kin beds, which is the youngest and most easterly unit, exhibits a pervasive slaty cleavage that is only moderately developed in the older rock units of the Gympie Terrane farther west. The intensity of cleavage in these older rocks generally increases towards the contact with the Kin Kin beds, suggesting that proximity to a major thrust has possibly influenced deformation in the eastern part of the terrane. The geological evidence for an east-dipping thrust is compelling, although the tectonic transport of young rocks over older units suggests that it may be an out-of-sequence-thrust. While the slaty cleavage is most likely related to thrusting during the final contractional phase of the Hunter Bowen Orogeny at around -230 Ma, subsequent dyke intrusion and several phases of younger deformation clearly postdate this orogenic event. Dykes crosscutting the slaty cleavage may be coeval with regional extension in the Late Triassic. Small scale thrust faults and associated kink folds overprint the slaty cleavage and dykes and may therefore be Late Triassic or younger. Normal faults and extensional reactivation of earlier thrusts overprint the folded slaty cleavage. The youngest deformation in the Gympie Terrane is related to a series of major predominantly strike slip faults that are well developed in the western part of the terrane. One such fault juxtaposes steeply dipping and overturned beds of the South Curra Limestone from shallowly dipping South Curra Limestone and Rammutt Formation. The fault zone contains sheared coal with tuffaceous partings which is characteristic of the Lower Jurassic Tiaro Coal Measures, constraining the age of deformation to the middle Mesozoic or younger. The scale, intensity, and style of this subsequent deformation indicates that the Gympie Terrane has been subjected to locally intense deformation that has hitherto not been fully recognised. These results indicate that parts of eastern Australia continued to experience intense deformation after the end of the Hunter Bowen Orogeny, well into the Mesozoic and possibly the Cenozoic.

SGTSG 2015: Riding the Waves

Evaluating quartz crystallographic texture strengths using classic eigenvalue methods

N.J.R. HUNTER , C J . L . WILSON AND R.F. WEINBERG 1

School of Earth, Atmosphere and Environment, Monash University, 3800, Australia

During crystal-plastic deformation, crustal rocks develop crystallographic preferred orientations or 'textures'. When plotted on a hemispheric pole figure, the individual orientations of crystallographic axes from such rocks typically define a non-random distribution where the 'strength' of maxima can provide approximations of strain intensity (Lister and Hobbs, 1980). While it is practical for textures to be assessed qualitatively, quantitative texture strength measurements are useful for understanding the evolution of microstructures with increasing strain, and may be essential to quantifying rheological weakening in the lithosphere (Austin, 2011). Several methods of texture strength measurement have been proposed in the literature (Bunge, 1982; Skemer et al., 2005), but their major restriction is that they require knowledge of the orientation density function (ODF) of the sample. Because ODF computation requires the full crystallographic spectrum of crystals in Euler space, strength measurements cannot be made using textural data from universal stages and polarizing microscopes (Fabric Analyser and CIP), where only the 0001 (c-axis) plane is attainable. Thus, a strength estimate that can be expressed across all texture analysis instruments is preferable. A classical approach to strength estimation can be made by computing eigenvalues from the 'orientation tensor' of a sample (Scheidegger, 1965; Woodcock, 1977). Several eigenvalue-based analyses have been developed to quantify both the shape and strength of axial data in (i) structural research, to describe the orientations of faults and fractures (Vollmer, 1990); and (ii) sedimentology research, in order to relate clast shapes to sedimentary facies and deposition settings (Benn, 1994). By comparison, eigenvalue-based strength measurements in crystal-based analysis, particularly for quartz textures, are underused.

79*2900"

79*3000"

79*31 00"

79"3200*

79*33W

79*34 00"

Figure 1: (A) Geological map and location of samples around the Main Central Thrust in the Alaknanda region of the Garhwal Himalaya (NW India); (B) Texture strength variation along the strain gradient in the MCT, measured using the E test. The critical value at 99% confidence is 15.09 (refer text).

We review classic eigenvalue-based analysis and present a revised method for quantitatively analysing the strength of quartz textures. The method is a modification of the uniformity test statistic originally 71

SGTSG 2015: Riding the Waves proposed by Mardia (1972), and suggested for geological purposes by Lisle (1985). The 'strength' of the orientation tensor, defined here as E, measures the deviation of the eigenvalues in a highly deformed rock (SiD) from their original distribution in an undeformed protolith (SiP): 3

i=l Where N is the number of measured orientations. In this form, E acts as a simple/ 2 test where SiP is the null hypothesis and the values 11.07 and 15.09 are the respective 95% and 99% critical values. The E test has additional benefits: (i) texture strength is directly related to the undeformed host rock, and thus is similar to classic strain analysis tests (Lisle, 1977); (ii) any previous deformation textures recorded in the protolith are taken into account during the test. We demonstrate that relatively small sample sizes are required for the test (monomineralic rocks: n = -175 points, polymineralic rocks: n = -350 points). The mean orientation tensor of samples can also be constrained over multiple trials using bootstrap analysis. We have applied the E test to four quartzite mylonite rocks of the Main Central Thrust in the Alaknanda region of the Garhwal Himalaya (NW India; Fig. la). Textures for these rocks were acquired using neutron diffraction, from which 500 random c-axis orientations were collected and compared against an undeformed protolith (AK-39; Fig la). The E test demonstrates a systematic increase in texture strength towards the NE, which peaks in the core of the shear zone (Fig. lb). Our example shows that strength analysis of crystal textures using eigenvalue-based methods is a viable alternative to the ODF-restricted texture strength measurements described in the literature, and has the potential for wider use in texture analysis. References Austin, N.J., 2011. The microstructural and rheological evolution of shear zones. Geological Society, London, Special Publications 360, 193-209. Benn, D.I., 1994. Fabric shape and the interpretation of sedimentary fabric data. Journal of Sedimentary Research A: Sedimentary Petrology & Processes, 4. Bunge, H.J., 1982. Texture Analysis in Materials Science: Mathematical Models. Butterworth-Heinemann, London. Lisle, R.J., 1977. Estimation of the tectonic strain ratio from the mean shape of deformed elliptical markers. Geologie en Mijnbouw 56, 140-144. Lisle, R.J., 1985. The use of the orientation tensor for the description and statistical testing of fabrics. Journal of Structural Geology 7, 115-117. Lister, G.S., Hobbs, B.E., 1980. The simulation of fabric development during plastic deformation and its application to quartzite: the influence of deformation history. Journal of Structural Geology 2, 355-370. Mardia, K.V., 1972. Statistics of Directional Data. Academic Press, London. Scheidegger, A.E., 1965. On the Statistics of the Orientation of Bedding Planes, Grain Axes, and Similar Sedimentological Data. U.S. Geologic Survey. Skemer, P., Katayama, I., Jiang, Z., Karato, S.I., 2005. The misorientation index: Development of a new method for calculating the strength of lattice-preferred orientation. Tectonophysics 411, 157-167. Vollmer, F.W., 1990. An application of eigenvalue methods to structural domain analysis. Geological Society of America Bulletin 102, 786-791. Woodcock, N.H., 1977. Specification of fabric shapes using an eigenvalue method. Geological Society of America Bulletin 88, 1231-1236.

SGTSG 2015: Riding the Waves

Exhumation history of the Western Tien Shan (Uzbekistan and Tajikistan): Preliminary thermochronological results G.M.M. JEPSON1, S. GLORIE1, D. KONOPELKO2, A.S. COLLINS1 AND NOREEN EVANS3 1 Centre for Tectonics, Resources, and Exploration (TRaX), School of Physical Sciences, Department of Earth Sciences, The University of Adelaide, Australia, 5005. 2 Geological Faculty, Saint Petersburg State University, 7/9 University Embankment, SPb 199034, Russia. 3 John De Laeter Centre, Dept. Applied Geology, Curtin University, Perth, Australia, 6845.

The theory of plate tectonics predicts that mountain belts are generated at plate boundaries. However, Central Asia is dominated by vast mountain ranges, with peaks up to 7000m, while being thousands of kilometers away from the nearest current plate margin. The timing and mechanisms of deformation within Central Asia are still largely unclear and there is particularly little known on the deformation history of the southwestern verges of Central Asia. This study is focussed on the thermal history of major suture - fault zones in the western Tien Shan (Uzbekistan and Tajikistan), in order to document the timing of deformation and exhumation in southwestern Central Asia. The Tien Shan is the southernmost and highest expression of the Central Asian Orogenic Belt, which stretches 5000km from Uzbekistan in the West to Xinjiang, China in the East. The mountainous Tien Shan relief was built by punctuated reactivations during the Meso-Cenozoic due to the incremental accretion of continental fragments onto southern Eurasia during closure of the Tethys Ocean. Its current topography is still growing as a result of ongoing India-Eurasia convergence, making the Tien Shan the world's most active intracontinental mountain range. A series of -120 samples have been collected along and across the main suture zones within the Uzbek and Tajik Tien Shan. These samples are currenlty being analysed using multi-method (apatite fission track, apatite and zircon U-Th-Sm/He) thermochronology, aiming to determine the reactivation history of the western Tien Shan. Preliminary results indicate preserved fast cooling events during the late Triassic - early Jurassic (-230-185 Ma) and mid Cretaceous (-125-100 Ma) in the western Tien Shan verges and Oligocene (-30-25 Ma) and late Miocene cooling ages along the Tien Shan - Pamir suture zone. Further analyses will allow to decipher the thermal and exhumation history of the Western Tien Shan, aiming to shed more light on the timing and extent of strain propagation and fault reactivation within southwestern Central Asia.

SGTSG 2015: Riding the Waves

In-situ stress and natural fracture networks in the Carnarvon Basin, North West Shelf, Australia

G.M.M. JEPSON , A.H.E. BAILEY, R.C. KING , S.P. HOLFORD AND M. HAND 1

Centre for Tectonics, Resources, and Exploration (TRaX), School of Physical Sciences, Department of Earth Sciences, The University of Adelaide, Australia, 5005. The Australian School of Petroleum, The University of Adelaide, South Australia 5005. 1

Hydrocarbon exploration in the Carnarvon Basin on the North West Shelf of Australia has proven significant reserves, making it Australia's pre-eminent hydrocarbon province. However, there is little understanding of the naturally occurring fracture networks and their impact on the permeability of the basin, which is crucial to resource recovery and future basin development. This study has analysed resistivity image logs from twelve petroleum wells in the offshore Carnarvon Basin in order to map the fracture networks to determine the structural permeability. The in-situ stress field is a major control on the ability for fractures to transmit fluid. One hundred and twenty-three drilling-induced tensile fractures and 175 borehole breakouts present in 12 image logs, determined a mean maximum horizontal stress orientation of 110. Leak-off tests and density logs were used to calculate the in-situ stress magnitudes with a vertical stress (5 ) of 21.7 MPa/km, a minimum horizontal stress ( S ) of 16.8 MPa/km and a maximum horizontal stress of 23.4 MPa/km ( S ) , indicative of a strike-slip fault stress regime ( S > S > S ) in the Carnarvon Basin. Using these stress orientations and magnitudes, we are able to predict the fracture sets that are open to fluid flow at present day. Using fracture susceptibility plots and Mohr circles constrained by the in-situ stress values, we show that the majority of E-W striking conductive fractures are optimally oriented within the in-situ stress field, demonstrating a high likelihood for fluid transmission. Additionally, several of these fractures demonstrate significant losses of drilling fluids at corresponding depths. It is likely that the identified conductive fractures are indeed open to fluid flow; demonstrating that these fracture networks provide secondary permeability the Carnarvon Basin subsurface. A total of 517 naturally occurring fractures are identified on 12 resistivity image logs. A range of fracture orientations were present, with the mean fractures strike approximately E-W. The fractures can be divided into two sets: 1) Electrically resistive and conductive fractures orientated NE-SW, and; 2) electrically resistive and conductive fractures orientated E-W. Electrically resistive fractures that are considered to be cemented with electrically resistive cements are dominantly orientated NE-SW. While conductive fractures that are considered to be uncemented and filled with drilling mud, thus, considered to be open for fluid flow are dominantly orientated E-W. Indicating that fractures that are orientated with the in-situ stress regime are more likely to be conductive and open for fluid flow. Four different fracture sets indicate that four different stress regimes were experienced by the Carnarvon Basin. The two NE-SW fracture sets can be linked to two periods of extension during the Jurassic. While the two E-W fracture sets correlate to a period of Miocene compression and the in-situ stress regime. Demonstrating that both the paleo and in-situ stress regime have a direct control of fracture formation and thus permeability in the Carnarvon Basin. V

h m i n

H m a x

hmin

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Steep metamorphic field gradients in HTLP regional aureoles: structurally controlled advection of heat by water versus magma KIM JESSOP123 department of Earth and Planetary Sciences ARC Centre for Core to Crust Fluid Systems Macquarie University, North Ryde, NSW

2 s

A recently mapped steep metamorphic field gradient (>70 0Ckm_1) is well exposed along the Aberfoyle River in the Wongwibinda Metamorphic Complex, northeast of Armidale, NSW, in the southern New England Orogen. The exposed metasedimentary rocks progressively increase in metamorphic grade from biotite- to garnet-cordierite-bearing rocks over a traverse of approximately 2 km. The Aberfoyle transect lacks intrusive rocks that are common elsewhere in the complex. Quartz veins are common. Results of whole rock XRF and isotopic analyses of samples from the Aberfoyle transect provide strong evidence for metasomatism resulting from fluid-rock interaction and show a positive correlation between metamorphic grade and potassium content, 87Sr/86Sr and DO18. In contrast, there is a negative correlation between grade and sodium content, calcium content and 143Nd/144Nd. On the basis of the spatial relationships and geochemical changes, it is proposed that metamorphism was driven by an influx of hot hydrous fluids along fault channels. Higher-grade rocks are exposed approximately 14 km to the east of the Aberfoyle transect in a second steep metamorphic field gradient. This second field transect builds to metasedimentary migmatite that is spatially associated with a broad shear zone. Garnet and/or cordierite surrounded by leucosome in metasedimentary rocks and abundant dykes and small bodies of two-mica granite suggest that anatectic melt formed in situ and was also injected into the migmatite. Muscovite-bearing granite is consistent with high water contents and links to the influx of hot hydrous fluids inferred in the Aberfoyle transect. Abundant collapse structures are inferred to trace magma pathways through the migmatite. The magmatic advection of heat through the broad shear zone is thought to heat these rocks to above their solidus. The mechanisms of heat advection at Aberfoyle and in the migmatites of Wongwibinda are interpreted to be part of a continuum. The source of fluid is interpreted to derive from the first metamorphic cycle of the accretionary complex of the New England Orogen. Early metamorphic fluid produced in the deeper accretionary complex is channelled up fault zones, advecting heat to shallower crustal levels. As metamorphism in the deep crust progresses, partial melts produce S-type magma that migrates up the same channels, further advecting heat. The decreasing availability of water in the deep crust as metamorphism progresses self arrests the system, resulting in short-lived (<10 m.y.) metamorphism in the shallow crust of narrow extent.

SGTSG 2015: Riding the Waves

A new map of the Wongwibinda Metamorphic Complex: its relationship to other HTLP complexes of the New England Orogen

KIM JESSOP

123

department of Earth and Planetary Sciences ARC Centre for Core to Crust Fluid Systems Macquarie University, North Ryde, NSW

2 s

In comparison to common low-grade metasedimentary rocks in the Carboniferous-Triassic New England Orogen (NEO), pockets of high-pressure, contact, or regional high-temperature/low-pressure (HTLP) metamorphic rocks are conspicuous. Exhumed blocks of Cambrian and Ordovician meta-igneous and serpentinite melange along the Yarrol-Peel-Manning fault system include blueschist and eclogite, related to subduction metamorphism. Devonian serpentinite and blueschist are exposed adjacent to the Mt Mia Thrust in Queensland; the exhumation of these rocks involved the development of a metamorphic core complex prior to thrust faulting. Remnants of Carboniferous blueschist are exposed in the Tia Metamorphic Complex. Contact aureoles exist around the numerous granitoid intrusions along the length of the NEO, however many are insignificant in terms of metamorphic grade and/or aureole width. Extensive (~4km wide) amphibolite facies aureoles have been reported from the Triassic Carrai Suite of I-type granites that intrude the Nambucca Block in NSW. Occurrences of HTLP regional aureoles involving amphibolite facies rocks are dotted along the orogen, particularly within the Carboniferous accretionary complex. Ages of c. 300 Ma have been obtained from several of these complexes and many, such as the Wongwibinda Metamorphic Complex, are associated with S-type granites. HTLP amphibolite facies rocks include the: • Key Creek Gneiss and Dead Horse Gneiss - Connors Subprovince, Connors-Auburn Province, near Bowen, Queensland • Yerilla Metamorphics - Auburn Subprovince, Connors-Auburn Province, near • Broome Head Metamorphics - Wandilla Province, north of Rockhampton, Queensland • Marlborough Metamorphics, metasedimentary thrust slices - Marlborough Province, north of Rockhampton, Queensland • Shoalwater Formation, Facing & Hummocky Islands - Coastal Subprovince, Wandilla Province, south of Rockhampton, Queensland • Glen Eva Metamorphics - Wandilla Province, south of Gladstone, Queensland • Metamorphics of the Yarraman Subprovince including the Chahpingah Complex - Wandilla Province, near Kingaroy, Queensland • Wide Bay Creek Gneiss & Manumbar Metamorphics - North D'Aguilar Subprovince, Wandilla Province, west of Gympie, Queensland • Wongwibinda Complex - Woolomin/Tablelands Province, north-east of Armidale, NSW • Tia Complex - Woolomin/Tablelands Province, south of Walcha NSW Although some have been described as contact aureoles, recent mapping and analysis of the Wongwibinda Complex has highlighted the potential role of fluids in their formation 76

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Tectonics of the reactivated Kuqa Foreland Basin - Tien Shan Orogen XIAODIAN JIANG1 AND WEI GONG 1 1

Department of Marine Geology, Ocean university of China, Qingdao, China, 266100,

xdjiang@ouc.edu.cn

The Tien Shan is bounded by the Tarim Basin to the south, the Junggar Basin to the northwest and the Kazakh Plate to the northwest. It is formed in late Paleozoic and reactive in Eocene responding to uplifting of Tibet. The Kuqa foreland basin is developed at southern Tien Shan front. As the response of the shortening and uplift of the orogenic belt, the lithosphere and the deep structure change significantly, supported by the seismic tomography and the deep seismic reflection, and the moho depth increases along the structural zone. The thickness of the lithosphere increases from 130km inside the basin to 150km at the edge, and reduced suddenly to less than 100km beneath the southern Tien Shan, because of the extrusion and bend by the thickening crust and the upwelling of the asthenosphere (Fig.l). Additional, the high velocity body distributes within the depth of 200-300km, as a result of slab detachment caused by the migration of the deep mantle material. It is accepted that the wide distribution of low velocity body implies the existence of the small-scale clockwise mantle convection which contributes to the subduction of the Tarim plate and accelerates the deformation and uplift of the Tien Shan without a clear mountain root, and as for the mechanism between the convection and Tibet plateau uplift, it needs further research. Center Tien Shan

-3%

0% AVp

+3%

Figure 1: P wave velocity perturbation along Kuqa-Kuytun profile (revised from Guo et al., 2006). The small white arrow represents the mantle convection, and the big one shows the subduction of the lithosphere. The black solid line illustrates the depth of the moho, and the grey one indicates the location of the fault.

According to the yield strength envelope (Liu et al., 2003), the lithosphere shows the stratification and heterogeneities of the rheological strength along the structural zone. The effective elastic thickness of the lithosphere decreases northwards from 50-60km to 20-30km near South Tien Shan. The above characteristics imply that the lithosphere deforms from coupling to decoupling northwards. The upper crustal layers are detached and overthrust, with the intensity decreasing southwards, and the ductile middle-lower crust and upper-mantle subduct beneath South Tien Shan. The detachment phenomenon is also supported by the low velocity zone at depth of 20km whose morphology implies that it would probably be initiated by the rock anisotropy enhanced by the low-angle shear activity of the crust. The structural belt should be devided into vertical uplift deformation zone, oblique compression deformation zone and horizontal stratified contraction deformation zone from the orogenic belt to the basin. In the vertical uplift deformation zone, the thrust extends through the entire upper crust, with the action of nearly vertical shearing induced by the uplift of the Tien Shan. However, the activity shows the "stratification" characteristics obviously in the other zones, and that means the tectonic model transforms from thick-skinned uniform to thin-skinned stratified structures southwards. Bounded by the Palaeogene Kumugeliemu gypsiferous rock Group, the structural layer can be classified into three 77

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layers. The pre-salt layer is characterized by basement-involved compressional structures and the complicated tectonic pattern shifts from imbricated thrust faults to detachment-thrust faults and paleohigh controlled thrust faults in the south. s

Figure 2: The geological structure of the Mountain front across from Kuqa basin to the margin of the southern Tien Shan (revised from Qi et al., 2009; Neng et al., 2012)

Motivated by the India-Eurasian collision and small-scale mantle convection, the tectonic stress field activates in the tectonic belt of the Kuqa foredeep basin-Tien Shan mountains in Cenozoic. The deformation process involves the whole lithosphere. It suggests the tectonic activity shows obviously the spatial heterogeneity, which horizontal zonality and vertical stratification.

SGTSG 2015: Riding the Waves

An animated reconstruction of the Cenozoic motion of Australia ISABELLE JONES1 AND CHARLES VERDEL1 1

School of Earth Sciences, University of Queensland

Many previous studies have addressed the Cenozoic motion of Australia, including recent investigations that utilised paleomagnetic data. Models arising from these studies fall into two groups: those showing largely linear northward motion[2,4], and those that, conversely, show significant variations in the longitudinal position of Australia, including a mid-Miocene westward excursion[1,6]. Interestingly, this westward excursion seems to correlate with the initial soft-docking of the Ontong Java Plateau (OJP) and the concurrent eastward offset in the Tasmantid and Lord Howe seamount hotspot tracks[3]. Our study utilised animated reconstructions to highlight relationships between the Cenozoic motion of Australia, development of the eastern Australia Cenozoic volcanic provinces, and the collision of the OJP with the Solomon Islands. Two reconstructions were created: a "standard rotation" model that uses the linear path of Seten et al. (2012), and an "adjusted" model that utilises modified reconstruction poles based on paleomagnetic data evaluated by Embleton and McElhinny (1982). A key difference between the models was the timing of OJP collision: initial docking occurs much later (at approximately 16 Ma) in the standard rotation model than in the adjusted model (docking at -22 Ma; Fig. 1). The earliest Miocene period of docking from the adjusted reconstruction agrees with some prior estimates for initial collision between the OJP and the Solomon Islands[7]. Perhaps more importantly, in only the adjusted reconstruction is there a correlation between OJP collision and the eastward offset in the hotspot tracks. Our reconstructions thus illustrate that the adjusted model agrees more closely with geologic observations.

Figure 1: Paleogeographic reconstructions at 22 and 16 Ma illustrating approximate age of docking of OJP for a) the adjusted model and b) the standard rotation model.

References [1]

[2]

[3]

[4]

EMBLETON B. J. J. & MCELHINNY M. W. 1982. Marine magnetic anomalies, palaeomagnetism and the drift history of Gondwanaland. Earth and Planetary Science Letters 58, 141-150. IDNURM M. 1985. Late Mesozoic and Cenozoic palaeomagnetism of Australia-I. A redetermined apparent polar wander path. Geophysical Journal of the Royal Astronomical Society 83, 399-418.

KNESEL K. M., COHEN B. E., VASCONCELOS P. M. & THIEDE D. S. 2008. Rapid change in drift of the Australian plate records collision with Ontong Java plateau. Nature 454, 754-757. MUSGRAVE R. J. 1989. A weighted least-squares fit of the Australian apparent polar wander path for the last 100 Myr. Geophysical Journal International 96, 231-243.

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[6]

[7]

SETON, M., MULLER, R. D„ ZAHIROVIC, S„ GAINA, C„ TORSVIK, T„ SHEPHARD, G., & CHANDLER, M. (2012). Global continental and ocean basin reconstructions since 200Ma. Earth-Science Reviews, 113(3), 212-270. WELLMAN, P. (1975). Palaeomagnetism of two mid-Tertiary basaltic volcanoes in Queensland, Australia. In Proceedings of the Royal Society of Queensland (Vol. 86, pp. 147-153). YAN, C. Y„ & KROENKE, L. W. (1993). A plate tectonic reconstruction of the Southwest Pacific, 0-100 Ma. In Proceedings of the Ocean Drilling Program, Scientific Results (Vol. 130, pp. 697-709). Texas: College Station.

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The Concertina Coast: a history of repeated inversion during basin formation on Australia's northern margin MYRA KEEP School of Earth and Environment, The University of Western Australia, 35 Stirling Hwy, Crawley, WA, 6009. Email myra.keep@uwa.edu.au

The present day configuration of Australia's northern margin includes a series of Phanerozoic sedimentary basins forming the North West Shelf. Their polyphase history, dominantly extensional, and closely associated with the breakup of Eastern Gondwana, includes the early formation of intracratonic basins (from the mid-Devonian), overprinted by Permo-Carboniferous rifting that generated the dominant NE-trending structural trends that persist to the present-day. Subsequent Mesozoic extension, associated with the formation of abyssal plains, further refined the margin, creating additional depocentres. During this polyphase rift history, a number of periods of inversion have punctuated the margin. These include a Carboniferous event, the Meda Transpression, late Permian to Early Triassic event, sometimes referred to as the Bedout Movement (possibly transtensional), and two events, one in the Middle to Late Triassic, followed by another in the Late Triassic to Early Jurassic, often referred to as the Fitzroy events. These various events, recorded locally in specific basins, caused inversion, folding, uplift and erosion where documented, with the Fitzroy events described as transpressional, resulting from right-lateral oblique inversion. Subsequent inversion during the Cretaceous, also attributed to dextral transpression, caused long wavelength folding and fault inversion in some basins. Whereas the effects of earlier inversions are somewhat sporadic across the North West Shelf, the effects of Neogene inversion have been documented across both the active and passive segments of the present day North West Shelf, and also appear to be strongly controlled by right-lateral oblique reactivation mechanisms, with associated seismicity and focal mechanism solutions. The history of the North West Shelf therefore includes 6 discrete episodes of reactivation and inversion, apparently strongly dominated by oblique mechanisms, which punctuate the long, multiphase extensional history. Whereas Neogene to Recent inversions can be attributed at least in part to plate collision (locally) and far-field stress (generally), the cause(s), distribution(s) and intensity of these previous events remains unclear. This presentation will examine the locations and manifestation of Late Permian to Recent events, and examine possible causes and consequences of repeated inversion of this rift/passive margin region.

SGTSG 2015: Riding the Waves

Laurentia - Siberia connection: An overview and new data ANDREI KHUDOLEY1, ROBERT RAINBIRD2, NADEZHDA PRIYATKINA3 1 Institute of the Earth Sciences, St. Petersburg State University, 7/9 University Nab., St. Petersburg 199034, Russia 2 Geological Survey of Canada, Ottawa, ON, Canada 3 New South Wales Institute of Frontiers Geoscience, University of Newcastle, Newcastle, NSW 2308, Australia

Continental reconstructions that display a connection between Siberia and Laurentia since the amalgamation of Nuna ca. 1.9-1.8 Ga are well supported. Their separation is considered to have initiated during the break-up of Rodinia, mainly between ca. 780 Ma and 720 Ma. Paleomagnetic data support a north Laurentia - south Siberia connection, but allow for several options, which can be tested by comparing their basement structures, magmatic events and provenance evolution. The youngest orogenic belts in the Siberian Craton are ca. 1.9-1.8 Ga. In contrast, eastern Laurentia is defined by the 1.8-1.6 Ga Yavapai and Mazatzal provinces and adjacent ca. 1.45-1.0 Ga Grenville Province. Proterozoic inliers of the Wernecke and Ogilvie mountains lying on the northern margin of Laurentia, host mafic magmatic rock with U-Pb ages ca. 1.38, 1.27, 0.78 and 0.72 Ga. Further south (cratonward) are the Great Bear Lake mafic magmatic events at ca. 1.74, 1.67 and 1.59 Ga. Mafic magmatic events at 1.75-1.7, 1.67, and 0.78-0.72 are recognized in the southern Siberia, and ca.l340±55 Ma intrusions documented on the southeast margin of Siberia may correlate with ca. 1.38 Ga event from northwestern Laurentia. However, southeast Siberia contains ca. 1.0-0.95 Ga mafic intrusions not recognized in the northern Laurentia and evidence for the extensive ca. 1.27 Ma Mackenzie event has yet to be recorded in Siberia. In northern Siberia, mafic magmatic events at ca. 1.75, 1.5 and 1.38 Ga are documented, but there is no evidence for younger events. Pre-780 Ma quartzarenite units in northwestern Canada contain distinctive 1.4-1.0 Ga (Grenvillian) detrital zircon grains supporting a paleogeographic model whereby extensive river systems transported detritus across Laurentia to its northern margin. In Siberia broad distribution of Grenvillian detrital zircon grains is known only in upper Mesoproterozoic - lower Neoproterozoic successions on the southeast margin of the Siberian Craton. Recent studies of the southwest margin of Siberia show that Mesoproterozoic - lower Neoproterozoic clastic rocks typically contain a subordinate population of Grenvillian detrital zircon grains with predominance of Paleoproterozoic and Archean grains. No Grenvillian detrital zircon grains are recorded in comparable successions located on the northern margin of Siberia. Provenance and magmatic event studies as well as paleomagnetic data show that north Laurentia south Siberia connection remains the most viable option. The southern and northern margins of the Siberian Craton show evidence for synchronous late Neoproterozoic extension. However, terranes located south of the Siberian Craton were likely accreted during Ordovician-Devonian Caledonian orogeny and should not be considered for any Proterozoic reconstructions of Siberia-Laurentia connection. Of concern is the location of Pearya terrane in the early Neoproterozoic. If it was a part of Laurentia, it has no counterpart on the south Siberian margin and a separate landmass between Siberia and Laurentia is required.

SGTSG 2015: Riding the Waves 40

Ar/ Ar geo- and thermo-chronology of the northern Thomson Orogen 39

MELANIE LEE AND CHARLES VERDEL 1

School of Earth Sciences, University of Queensland, Australia

The Thomson Orogen of NE Australia was deformed during a number of Paleozoic and Mesozoic orogenic phases related to convergent margin tectonism in eastern Australia. The timing of these events is recorded by mineral thermochronometers, including several minerals that can be dated with Ar/ Ar step-heating. In this study we compile previous K-Ar and Ar/ Ar results and add new Ar/ Ar data from key locations to better constrain the temporal and spatial pattern of metamorphism and exhumation of the northern Thomson Orogen. Much of the region is characterised by early- to mid-Paleozoic K-Ar and Ar/ Ar ages that are variably attributable to prograde metamorphism accompanying contractile deformation during the Delamerian and Benambran Orogenies, or to Ordovician (Wood & Lister, 2004) and Late Devonian periods of extensional exhumation. In contrast, new biotite, muscovite, and plagioclase Ar/ Ar data from some early Paleozoic granitic rocks in the eastern part of the northern Thomson Orogen have medium-T step-heating steps that converge on -270 Ma. Similar Ar/ Ar ages have been widely reported from the New England Orogen (e.g., Shaanan et al., 2014) and have been attributed there to metamorphism during onset of the Hunter-Bowen Orogeny, a period of shortening that affected much of eastern Australia and other parts of eastern Gondwana. Our compilation and new data may therefore be helpful in delineating a broad-scale division between eastern Thomson Orogen-New England Orogen rocks that were strongly affected by the HunterBowen Orogeny and western Thomson Orogen rocks that preserve Ar/ Ar records of older periods of deformation. References 40

Shaanan, U., Rosenhaum, G., Li, P., and Vasconcelos, P. 2014. Structural evolution of the early Permian Nambucca Block (New England Orogen, eastern Australia) and implications for oroclinal bending. Tectonics 33, 1425-1443. Wood, D.G., and Lister, G.S. 2013. Dating deformation in the Anakie Metamorphic Group and Gem Park Granite. In Queensland Geology, P. Jell, ed. (Brisbane: Geological Survey of Queensland), pp. 133-135.

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The enigmatic Hastings Block - history of emplacement and subsequent deformation

JIE YAN , PAUL G. LENNOX , ROBIN OFFLER AND BRYCE KELLY 1

School of BEES, The University of New South Wales, Sydney 2052 New South Wales Institute of Frontier Geosciences, University of Newcastle, NSW 2308

The Hastings Block is a ~ 3000 km fault-bounded block consisting of mainly Devonian to Carboniferous arc-derived sedimentary and volcanic rocks and an Early Permian sedimentary cover sequence (Figure 1). It is located, out of place, on the outboard edge of the Tablelands Complex, a subduction complex and northern margin of the Tamworth Belt of the Southern New England Orogen (Roberts et al. 1995). 2

Eastern Australia

LEGEND F z\ • Triassic volcanics k-rLtl + granites

HMMJCCA

nillll Triassic sediments Permian cover sequence NHB Carboniferous

BLOCK

p T ^ SHB Carboniferous ^ SHB Devonian Subduction complex rocks Undifferentiated rocks

Bowen

Volcanic arc Forearc basin Subduction complex Gunnedah, Sydney Basin

NNEO

YARROWITCH BLOCK

31* 30'

SNEO / - k

10 km

SOUTHERN

W F

PORT MACQUARIE BLOCK

60 120

Tamworth ; I l l l i

Peel Manning Fault ~ System Lachlan Orogen

Geological boundary, observed, inferred

^Tablelands Complex Hastings Block

Jg Newcastle

153.20

Faults, observed, inferred Serpentinite exposures Boundary (?) between Northern and Southern Hastings Block

TAMW(

3 2 93

Figure 1: Location and tectonic setting of the Hastings Block, (a) within eastern Australia, (b) within the

southern New England Orogen (NEO); NNEO = Northern NEO, SNEO = Southern NEO. (c) Major tectonic units and faults within and adjacent to the Hastings Block (simplified from Roberts et al. 1995). The three dashed black lines show possible positions for the boundary between the Northern Hastings Block and Southern Hastings Block. Abbreviations: CWF = Cowarra Fault; PD = Parrabel Dome & YFS = Yarras Fault System.

The Hastings Block was translated with rotation between the Yarras and Parrabel fault systems, from a position along strike of the Tamworth Belt, in the Late Carboniferous (Schmidt et al. 1994). 84

SGTSG 2015: Riding the Waves Subsequently, the Nambucca and Hastings Blocks, and southern Tamworth Belt, were deformed by the Hunter Orogeny. However, they have distinctly different deformation histories reflecting the near field (Nambucca Block), mid field (Hastings Block) and far field (Tamworth Belt) accommodation of the southward movement of the Coffs Harbour Orocline. The Northern Hastings Block (NHB) consists of Carboniferous to Early Permian sequences. It is dominated by the open, gently plunging NW- trending Parrabel Dome (Figure lc). Four episodes of cleavage and fold development and extensive post-dome faulting occurred during the Hunter Orogeny (Yan 2015). The earliest E-W trending cleavage Si of the Permian sequences of the southern Nambucca Block were subsequently re-orientated as a result of the NHB acting as a massif during shortening related to southward translation of the Coffs Harbour Orocline. Worm analysis of gravity/magnetic data reveals a possible boundary between the NHB and SHB (heavy dashed line; Figure lc). It is defined by a series of faults some of which underwent movement over extended periods and were in part active up into the Late Triassic with late sinistral-strike-slip movement. The Southern Hastings Block (SHB) consists of generally east-dipping and facing, Devonian to Late Carboniferous sequences which have been affected by two episodes of folding (Fi N-S, F NW-SE and faulting (N-S, NE-SW, NW-SE) south of the southern dashed line (Figure lc). After dome formation in the NHB there was extensive faulting in the region between the two thin dashed lines shown on the Figure lc. It represents an accommodation zone between the domal, rigid NHB and the east-dipping panel of rocks in the northern SHB. This led to northwest translation of the northern block with localised rotation, translation and re-arrangement of some smaller fault blocks in this zone (Yan 2015). Limited Triassic movement on some faults in the Hastings Block took place reflecting continued accommodation of the block during the Hunter-Bowen Orogeny prior to Triassic granite intrusion that stitched the block to the subduction rocks to the west. 2

References Roberts J.R., Leitch E.C., Lennox P.G. & Offler R. 1995. Devonian - Carboniferous Stratigraphy of the Southern Hastings Block, eastern Australia. Australian Journal of Earth Sciences 42, 609-634. Schmidt P.W., Aubourg C., Lennox P.G. & Roberts J. 1994. Palaeomagnetism and Tectonic Rotation of the Hastings Terrane. Australian Journal of Earth Science 41, 547-560. Yan, J. 2015. Unravelling the deformation history of the Northern Hastings Block, southern New England Orogen. PhD (unpubl.), University of New South Wales, Sydney.

SGTSG 2015: Riding the Waves

Supercontinent-superplume coupling and a new global geodynamic working model

ZHENG-XIANG LI

ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS) and The Institute for Geoscience Research (TIGeR), Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845, Australia

The question of what drives plate tectonics remains a major challenge to geoscientists. Major breakthroughs over the past 30 years include (1) the recognition of a likely cyclic nature of supercontinent evolution, including the Pangea (320-170 Ma), Rodinia (900-700 Ma), and Nuna/Columbia (16007-1300? Ma) cycles; (2) seismic tomographic evidence for subducting slabs reaching the lower mantle; (3) the discovery of the two equatorial, antipodal, large low shear-wave velocity provinces (LLSVPs, or superplumes) in the lower mantle that underlie (and fed?) almost all the know mantle plumes since the Permian; and (4) the increasing recognition of TPW events (True Polar Wander - rotation of the entire mantle and lithosphere relative to the planetary rotation axis) in Earth history that reflect changes in mantle structure and convection patterns. Although many believe that subducting slabs can influence lower mantle structure and dynamics, some believe that the two LLSVPs/superplumes are long-lived in Earth history and are thus independent of the plate system. Here I present a self-consistent working hypothesis (see Li and Zhong, 2009) that challenges this belief and features the foliowings: (1) The Earth's history has been dominated by cycles of supercontinent assembly and breakup coupled by antipodal superplume events (Fig. 1); 900 Ma Rodinia assembled over a single downwelling

320 Ma Pangea assembled over a mantle downwelling

<«-825-750 Ma Rodinia superplume and breakup

550-530 Ma

Gondwanaland

Superplume(?) and breakup continues

>200-80 Ma • Pangean s u p e r - . plume and breakup, Paleo-Pacific superplume

^ 320 Ma

assembled (-half of all continents)

Present

Residuals of the Pagean W ' (African. A) and Paleo- " Pacific (Pacific, P) Superplumes superplumes, and an continue Eurasia downwelling

Figure 3: Proposed supercontinent-superplume coupling (Li and Zhong, 2009)

_oc

Pangea

_„ K . _ a s s e m D i e a

. 280(?)Myinto the future

(2) Circum-supercontinent subduction leads to the formation of two antipodal superplumes (and individual plumes above them) corresponding to the positions of the supercontinent and the superocean, respectively, but with a time delay of a few tens of million years from the final assembly of the supercontinent (Li et al., 2008) (Fig. 2c); (3) Earth's spinning can bring the antipodal superplumes and the coupled supercontinent to equatorial positions through TPW events (Evans, 2003; Li et al., 2004; Zhong et al., 2007), and eventually lead to the breakup of the supercontinent. Equatorial antipodal superplumes also allow for inertial interchange true polar wander; (4) The breakup of the supercontinent gradually changes the Earth from a dominantly circular subduction system to one with scattered subduction systems, thus weakening the antipodal

SGTSG 2015: Riding the Waves

superplumes until after the formation of the next supercontinent. It is unclear whether one or both superplumes can become nearly diminished during the transition between supercontinents. This working model, along with other hypotheses, can be further tested with improved knowledge of (1) supercontinent cycles (Fig. 2a) and TPW events, (2) of the temporal and spatial variation of plume activity in both the continental and oceanic realms, and their relationships to supercontinent cycles (Fig. 2a-b), (3) of the Earth's internal structure and properties (Fig. 2c), and through integrated global syntheses and geodynamic modeling. IGCP project #648 "Supercontinent Cycles and Global Geodynamics" (2015-2019) was established to coordinate a global campaign to address this fundamental geoscience question and its wide implications (see http://geodynamics.curtin.edu.au). The participation of, and contributions from, Australian researchers will be crucial for the success of the project. (a) Supercontinent cycles

in gijui

Pangea

Rodinia

Nuna/Columbia

i i i i r™ f i i i i1500 i ii 500 i i T i 1000

Continental LIP

Kenorland?

T T

2000

• I 1 1 1 1 I

2500

3000

LIPs record (relative intensity)

3500 (Ma)

Continental LIPs

Oceanic LIPs Record mostly missing Oceanic

Figure 2: Data suggesting cyclic, and possibly coupled (a) supercontinent and (b) global plume (LIP - large igneous province) events (after Prokoph et al., 2004), and (c) a geodynamic working model (Li et al., 2008; Li and Zhong, 2009).

References Evans, D. A. D., True polar wander and supercontinents. Tectonophysics, v. 362, p. 303-320, 2003. Li, Z. X., Bogdanova, S. V., Collins, A. S., Davidson, A., De Waele, B., Ernst, R. E., Fitzsimons, I. C. W., Fuck, R. A., Gladkochub, D. P., Jacobs, J., Karlstrom, K. E., Lu, S., Natapov, L. M., Pease, V., Pisarevsky, S. A., Thrane, K., and Vernikovsky, V., Assembly, configuration, and break-up history of Rodinia: A synthesis. Precambrian Research, v. 160, no. 1-2, p. 179-210, 2008. Li, Z. X., Evans, D. A. D., and Zhang, S., 2004, A 90° spin on Rodinia: possible causal links between the Neoproterozoic supercontinent, superplurne, true polar wander and low-latitude glaciation. Earth and Planetary Science Letters, v. 220, no. 3-4, p. 409-421, 2004. Li, Z. X., and Zhong, S., Supercontinent-superplume coupling, true polar wander and plume mobility: plate dominance in whole-mantle tectonics. Physics of Earth and Planetary Interiors 176, 143-156, 2009. Prokoph, A., Ernst, R. E., and Buchan, K. L., Time-series analysis of large igneous provinces: 3500 Ma to present. Journal of Geology, v. 112, p. 1-22, 2004. Zhong, S., Zhang, N., Li, Z. X., and Roberts, J. H., Supercontinent cycles, true polar wander, and very longwavelength mantle convection. Earth and Planetary Science Letters, v. 261, p. 551-564, 2007.

SGTSG 2015: Riding the Waves

Continental chronostratigraphic solid geology mapping of Australia

SONGFA Liu, KAROL CZARNOTA AND ALASTAIR STEWART

Geoscience Australia, GPO Box 378, Canberra, ACT, 2601, Australia

Our understanding of the geological history and resource potential of Australia has been underpinned by over half a century of surface geological mapping. A synthesis of this effort is captured in the 1:1 000 000 surface geology map of Australia (Raymond et al., 2012), which shows that -80% of the bedrock geology of Australia is covered by a veneer of sediment. Now, the challenge is to continue to unravel the geological history and resource potential of Australia beneath this cover. With this goal in mind, Geoscience Australia (GA) is embarking on compiling national chronostratigraphic solid geology maps starting with the pre-Cenozoic in South Australia, New South Wales, and Victoria. This mapping effort is complemented by a program to construct chronostratigraphic isopach maps of the cover starting with the Cenozoic Murray Basin. These maps will be compiled at an optimal scale of 1:1,000,000 exploiting potential field datasets, radiometric coverages, seismic profiles, borehole data and regional solid geology compilations. In the interest of efficiency, solid geology compilations at scales between 1:500 000-1:2 500 000 will be incorporated with minimum modification. The end product will be a national geology database in chronostratigraphic order starting with the 1:1 000 000 surface geology of Australia and the preCenozoic solid geology.

References Raymond, O.L., Liu, S.F., Gallagher, R, Highet, L.M., Zhang, W., 2012. Surface Geology of Australia, 1:1 000 000 scale, 2012 edition [Digital Dataset]. Geoscience Australia, Commonwealth of Australia, Canberra.

SGTSG 2015: Riding the Waves

Regional scale structural modelling along a geological transect: insights from the NW Fold and Thrust Belt, PNG

LUKE MAHONEY , SANDRA MCLAREN AND KEVIN HILL 1

School of Earth Sciences, The University of Melbourne Oil Search Limited

Papua New Guinea is arguably one of the most technically complex regions of the world. As such, the Papuan Fold Belt is characterised by significant geological complexity. Despite being host to a wealth of hydrocarbon and mineral resources, the Papuan Fold Belt is under-explored compared to similar settings worldwide. This is in large part due to the significant logistic, and therefore financial, commitment required to safely and effectively conduct geological research within this challenging environment. In particular, the Papuan Highlands are remote, experience unfavourable weather patterns and complex community relationships are present in inhabited areas. Together, these factors have hampered basic geological research over long periods. A number of tectonostratigraphic domains have been recognised within the Papuan Fold Belt (Fig. 1), but are poorly understood. Most geological knowledge of the Papuan Fold Belt has come from over 20 years of hydrocarbon exploration and production in the Kutubu Fold Belt (Fig. 1). However, it is unclear how the spatial and temporal evolution of the Kutubu Fold and Thrust Belt (KFTB) relates to surrounding regions such as the neighbouring North West Fold and Thrust Belt (NWFTB). Folding wavelength in the KFTB is around 7 km, while the much more elevated NWFTB contains structures, such as the Muller Ranges, with wavelengths of over 30 km. The KFTB and NWFTB regions are separated by the Bosavi Lineament, a zone of structural disruption and abundant Plio-Pleistocene volcanoes. The underlying mechanisms responsible for such significant changes in fold belt structure are largely unknown. The jungle-covered NWFTB ranges from the platform near sea level in the SW, to mountains over 3000 m high in the NE, with none of it accessible by road, hence the geological structure and evolution are poorly constrained. This study involves field mapping and analysis of remote sensing data tied to wells and available seismic data to facilitate improved local and regional structural models for the NWFTB. A 150 km long regional cross-section has been constructed across the NWFTB from the platform, across the Juha gasfield, the giant Muller Anticline, NW Thrust Belt, the highly elevated Yangi Fold Belt and into the Om Metamorphic terrane (Fig. 1). A transition from platform to slope to basin is reflected in the stratigraphy. Poor quantity and/or quality of data at depth make it difficult to identify the primary controls on structural style. Field and seismic data across the NWFTB reveals major detachments are widespread, particularly within the Cretaceous Ieru Formation. The massive nature of the Muller Anticline and Om Terrane suggest crustal scale structures. Seismic data across Juha indicate a thick sedimentary section compared to the platform and probable inversion. Geological maps and the Korka well suggest thrusting along multiple detachments in the NW Thrust Belt and Yangi Fold Belt. Iterative forward kinematic modelling approaches are used to reproduce the complex fault-by-fault evolution from pre-compression to present day and test the kinematic and geometric validity of the section. Models reveal that structural style, and its variability, relates to the relative influence of basement-connected and detachment faulting. The former is crucial in forming regionally significant structures such as the Muller Ranges and Om Thrust. Reproducing the present day elevation from the Muller Ranges to Om Terrane requires relatively steep NE dipping basement decollements extending to crustal depths of greater than 30 km. Basement structures may be Early Mesozoic rifted passive margin features reactivated during Late Cenozoic compression. Smaller wavelength structures in the NW Thrust Belt and forelandward of the Juha Anticline are best reproduced by thrusting from multiple detachment-levels, with minimal basement involvement. Forward kinematic models indicate that orogenic shortening across the elevated NWFTB (20-30%) is significantly less than across the KFTB 89

SGTSG 2015: Riding the Waves (-40%; Hill et al, 2010), suggesting a relative increase in the abundance of deep basement-connected structures throughout the NWFTB. Future work will involve forward mechanical modelling to address potential shortfalls related to the use kinematic methodology. Additionally, radiometric dating methods will be used on field samples to improve temporal constraints on structural sections.

Figure 1: Tectono-stratigraphic domains and Plio-Pleistocene volcanoes of the Papuan Fold Belt, modified from Hill et al 2000. NWFTB = Northwest Fold and Thrust Belt, KFTB = Kutubu Fold and Thrust Belt. Digital elevation model from Ryan et al. (2009).

References

Hill, K, Norvick, M, Keetley, J & Adams, A 2000, 'Structural and stratigraphic shelf-edge hydrocarbon plays in the Papuan Fold Belt', in P Buchanan, A Grainge & R Thornton (eds), Petroleum exploration, development, and production in Papua New Guinea: proceedings of the fourth PNG Petroleum Convention, Port Moresby, pp. 67-85. Hill, K, Lucas, K & Bradey, K 2010, 'Structural styles in the Papuan Fold Belt, Papua New Guninea: constraints from analogue modelling', in G Goffey (ed.), Hydrocarbons in Contractional Belts, Geological Society, London, Special Publications, vol. 348, no. 1, pp. 33-56. Ryan, W, Carbotte, J, Coplan, S, O'Hara, A, Melkonian, R, Arko, R, Weissel, V, Ferrini, A, Goodwillie, F, Nitsche, J, Bonczkowski & Zemsky, R 2009, 'Global Multi-Resolution Topography synthesis', Geochemistry, Geophysics, Geosystems, vol. 10, no. 3.

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Pressure oscillations in orogens: lithostatic versus overpressure

NEIL MANCKTELOW department of Earth Sciences, ETH Zurich, CH-8092 Zurich, Switzerland email:mancktelow @erdw.ethz.ch 1

In continuum mechanics, pressure is defined as the negative of the "hydrostatic" or mean stress (corresponding, in 3D, to one third of the first invariant of the stress tensor). In an elastically compressible material, this pressure is linearly related to the relative change in volume (the dilatation, again corresponding to the first invariant of the infinitesimal strain tensor) through a scalar material property, the compressibility. Dense, low-porosity rocks are almost incompressible, so that the dilatation associated with even large changes in pressure is small. Such small (and elastic) volume changes are obviously not preserved in the geological record, so that the record of pressure variations in rocks can only be indirect, either through the effect on rock rheology or on variably preserved metamorphic mineral assemblages or variation in mineral composition. The P-t history can sometimes be estimated, but direct constraints on the depth of burial of rocks through time are generally lacking. Although the vertical load of overlying rock generally provides the major component of the total pressure, elastically loaded or viscously flowing natural rocks at depth in the earth will always have a tectonic or dynamic component of pressure. The question is not whether it occurs but rather whether significant effects of tectonic over- or underpressure are preserved in the geological record. For a Maxwell material, as commonly used to model the viscoelastic behaviour of rocks, the relaxation time (the time for the stress to drop to 1/e of the initial value produced by an instantaneously imposed strain) is given by the effective viscosity divided by the elastic shear modulus. It follows that even for large deviatoric stress magnitudes on the order of 1 GPa, relaxation times are still only on the order of 1 Ma, so that stress and pressure can change rapidly by geological time standards. For an (incompressible) viscous material, as commonly employed in many numerical models, the relaxation time is zero and the tectonic pressure component can change instantaneously. It follows that changes in tectonic pressure (or even oscillation) can occur rapidly if the boundary conditions or geometry changes, for example through fluctuations in subduction or roll-back rates, or due to slab break-off. For a viscous rheology, tectonic and background (lithostatic) pressure are independent of each other and the total pressure is a simple addition. For pressure dependent rheology (e.g. Mohr-Coulomb plasticity as a model for fracture), there is a feedback effect and the total pressure has to be included in any numerical model. The potential magnitude of tectonic pressure variation scales with the stress sustained by the stronger material(s) in the system, through a geometric scaling factor. Higher stress implies higher effective viscosity or faster strain rates. Higher effective viscosity in turn implies a more elastic response, with longer stress relaxation times and therefore an increased tendency for damping of stress (and pressure) oscillations. In many natural deformation structures (folds, boudins, isolated inclusions), this geometric scaling factor is on the order of 1-2 relative to the deviatoric stress supported by the stronger material. However, strong confinement (e.g. converging channel flow, corner flow, extrusion) can generate much higher values, which could oscillate with only minor cyclical changes in the geometry. Changes in stress and pressure can be the result of changes in geometry (e.g. in a developing fold, narrowing or broadening of a subduction channel, seamounts, ridges, or irregular fault topography on a subducting plate), rheology (e.g. variation due to metamorphic reaction /hydration /dehydration, shear heating, heat conduction), or boundary conditions (e.g. rate of plate convergence). Regardless of the magnitudes involved, the direction of fluid flow in deforming rocks will reflect local pressure gradients which can be profoundly influenced by tectonic pressures. These fluids are important for minerals reaction and, by their influence on the effective stress, on the rheological response of the rock. In particular, the localization of earthquakes in the subducting plate may be determined by fluid flow, localization and entrapment. The geometrically necessary bending and unbending of downgoing plates is directly comparable to folding and will generate tectonic pressure gradients perpendicular to plate boundaries, so that fluid may flow both upwards and downwards relative to the plate boundaries. Local tectonic pressure variation can also influence the location of 91

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partial melting, providing a potential feedback mechanism between melting and shearing and an explanation for the common observation of leucosomes localized in shear zones. In compositionally and rheologically layered rocks, stress refraction and tectonic pressure differences are linked effects. Any natural example of stress refraction (e.g. cleavage refraction) implies a corresponding difference in tectonic pressure. For example, gradual cleavage refraction due to graded bedding implies a corresponding gradient in pressure that can drive fluid flow perpendicular to bedding. Wedge-shaped fractures developed in the stronger material perpendicular to the layer interface are clear examples of both stress refraction and pressure differences, as for example veins on the outer arc of folds or pseudotachylyte injection veins approximately perpendicular to the generation surface. Brittle fracture always involves lower tectonic pressure in the fracture core. This is implicit in rheological models for fracture, such as Mohr-Coulomb plastic yield, where strong localization follows from the feedback between pressure-dependent yield and reduced pressure in the zone of plastic flow. In contrast, viscous localization in planar shear zones should occur initially at 45° to the shortening direction (i.e. on planes of maximum shear stress and shear strain rate) and continued deformation should result in their rotation into orientations where they are transpressive "stretching" shear zones. To continue localizing, the effective viscosity within the approximately planar shear zone layer must be less than in the matrix and such a stretching weak layer should have a higher tectonic pressure than the surrounding matrix. The common observation of enhanced fluid-rock interaction in (especially retrograde) shear zones stands in contradiction to this prediction and implies that heterogeneous "ductile" shear zones cannot be modelled with a simple viscous rheology. One possibility is that fluid influx may reflect an initial brittle precursor, with the associated fluid-rock interaction changing the rheology (e.g. increased amounts of biotite or white mica) and localizing subsequent ductile shear. Localized, heterogeneous "ductile" shear zones may also not be strictly viscous but involve an interplay between crystal-plastic flow and localized porosity development and fracture (cavitation), with the interplay varying in time and space during progressive shearing. Shearing on a network of localized zones can only continue if the intervening blocks also change their shape. Tectonic underpressure in these stronger intervening blocks can promote new fracture and inflow of water-rich fluids, with subsequent localization of shear on these new fracture surfaces providing a mechanism for progressive spreading of strain through a heterogeneously deforming rock volume. In summary, considering the realistic variation of tectonic pressure on a time and length scale that is much shorter than can be achieved by depth alone provides an explanation for many observations in natural rocks that would otherwise remain enigmatic.

SGTSG 2015: Riding the Waves

Inverted localization of deformation in the "dry" middle crust across the Woodroffe Thrust, Central Australia SEBASTIAN WEX 1 , NEIL MANCKTELOW1, FRIEDRICH HAWEMANN1, ALFREDO CAMACHO2 AND GIORGIO PENNACCHIONI3 department of Earth Sciences, ETH Zurich, Sonneggstrasse 5, CH-8092 Zurich Department of Geological Sciences, University of Manitoba, 125 Dysart Rd, Winnipeg, Manitoba, R3T2N2 Canada 3 Department of Geosciences, University of Padua, Via Gradenigo 6, 35131 Padua

The Musgrave Ranges region in Central Australia is a very well exposed, semi-desert area, in which numerous large-scale shear zones developed during the intracratonic Petermann Orogeny around 550 Ma. The most prominent structure is the -600 km long E-W trending Woodroffe Thrust, which placed -1.2 Ga granulites onto similarly-aged granitoids and amphibolite-granulite facies gneisses along a south-dipping thrust plane, with a top-to-north shear sense. Ductile deformation related to this thrusting was mainly accommodated in a continuous sequence of protomylonites, mylonites, ultramylonites, and sheared pseudotachylytes, which preferentially localized in the footwall rather than in the hanging wall. On a regional scale, conditions are generally "dry", as indicated by: 1) lack of syn-tectonic quartz veins; 2) the fine dynamically recrystallized grain size of quartz (av. 30 |im) and feldspar (<10 \im)\ 3) dominantly porosity-free quartz grain boundaries; 4) metastable plagioclase in the presence of Kfeldspar, which rarely shows any significant sericitization; and 5) breakdown of plagioclase to kyanite + garnet, rather than to kyanite + clinozoisite. Thorium concentrations estimated from airborne surveys are markedly higher in the footwall compared to the hanging wall. This is an inherited feature due to 1) partial melting and associated depletion in incompatible thorium during the earlier -1.2 Ga granulite facies metamorphism and 2) the predominance of granitoids rather than granulites in the footwall. However, the spatial resolution is insufficient to precisely determine the original boundary between the two compositionally similar protoliths within the thrust zone. Measuring the thorium concentration in felsic assemblages across the Woodroffe Thrust, using gamma spectrometry, allowed a precise quantification of the extent to which the hanging wall was reworked into the Woodroffe Thrust mylonites. The preferential concentration of mylonites in the footwall of a thrust is unusual and unexpected from the geothermobarometric results, which give consistent P-T estimates of ~650°C and 1.2 GPa in the hanging wall and lower values of ~600°C and 0.8 GPa for the immediately underlying footwall. Pseudotachylytes, which have been identified as the preferred nucleation sites for shearing under "dry" mid-crustal conditions, are present on both sides. The most likely explanation for the inverted localization of deformation across the Woodroffe Thrust is either 1) the observed greater abundance of hydrous minerals in the footwall and potentially also in the pseudotachylytes, or 2) externally derived fluids, introduced on a very local scale. Field and microscopic observations suggest that externally introduced fluids are scarce and, where present, possibly rich in C0 2 , as indicated by the crystallization of calcite in otherwise completely non-carbonaceous rocks. On the other hand, the higher abundance of hydrous minerals in the footwall could more easily localize deformation and recrystallization. We favor a combination of model 1) and 2) based on the close correlation between the variable extent of hanging wall reworking and the mylonite thickness with regional trends in: 1) the variation of plagioclase breakdown reactions; 2) the abundance of fine-grained calcite; and 3) the variation in the size and abundance of micro-scale etch pits on recrystallized quartz grains ("fossil" porosity acting as a proxy for free fluid activity during deformation).

SGTSG 2015: Riding the Waves

Pseudotachylytes as evidence for lower crustal earthquakes (Musgrave Ranges, Central Australia)

F. HAWEMANN , N. MANCKTELOW , S. WEX , G. PENNACCHIONI AND A. CAMACHO 1

department of Earth Sciences, ETH Zurich, Zurich, Switzerland Department of Geosciences, University of Padua, Padua, Italy Department of Geological Sciences, University of Manitoba, Winnipeg, Canada

Current models and extrapolated laboratory data generally predict viscous flow in the lower continental crust and any localized brittle deformation at these depths has been proposed to reflect downward propagation of the frictional-viscous transition zone during short-term seismic events and related high strain rates. Better natural constraints on this proposed rheological behaviour can be obtained directly from currently exposed lower crust that has not been strongly overprinted during its exhumation. The Musgrave Ranges in Central Australia expose a series of amphibolite to sub-eclogitic facies shear zones developed in generally felsic granulites and dolerite dykes during the intracratonic Petermann Orogeny (-550 Ma). Large scale shear zones show mutually overprinting mylonites and pseudotachylytes and are interpreted as fossil seismogenic shear zones, with pseudotachylytes representing seismic rupture and subsequent mylonitization the post-seismic creep. Ductile shearing in the Davenport Shear Zone took place under sub-eclogitic conditions of 650 °C and 1.2 GPa, generally typical of the lower continental crust. Sheared and recrystallized pseudotachylytes typically show the assemblage Grt+Cpx+Fsp±Ky±Bt (Fig 1A), whereas unsheared injections of the same vein may show cauliflower-shaped overgrowths of garnet (Fig IB). The chemistry of this fine grained (1-2 ^im) assemblage can be accessed using 'XMapTools' (developed by Pierre Lanari), which allows quantification based on x-ray maps. The P-T conditions derived from the pseudotachylytes using this technique are similar to those from the ductile shear zones (600-680 °C, 1.0-1.2 GPa). The development of pseudotachylytes under high-grade conditions is also evident from fractured garnets with diffusion rims. Diffusion in garnet is generally only efficient above 500 °C. Old granulite facies garnets show a 30 \im diffusion rim of high-Ca, associated with the high-P Petermann event. This diffusion pattern is offset by fractures, but diffusion also occurs along these fractures, demonstrating that fracturing is developed under high temperatures. Fracturing of garnet is often proposed as an indicator of seismic faulting, since a high differential stress is required. In the study area, fractured garnets are only found in close proximity to pseudotachylyte veins.

Figure 4: A: Eclogitic dolerite (left) separated from sheared pseudotachylyte (right) by a preserved chilled margin, lightest grey: grt, intermediate grey: cpx, black: fsp.. B: dendritic grt overgrowing unsheared pseudotachylyte.

SGTSG 2015: Riding the Waves

Application of Rb-Sr geochronology to date HP/LT metamorphic events in the southern New England Orogen, NSW: implications for tectonic models, eastern Gondwana.

RYANJ.MANTON \ SOLOMON BUCKMAN AND ALLEN P. NUTMAN 1

GeoQuEST Research Centre, School of Earth and Environmental Sciences, University of Wollongong, Wollongong, NSW 2522, Australia

Eclogites and blueschists record direct evidence of subduction events in ancient convergent margins such as the New England Orogen (NEO) in eastern Australia. Dating these rare HP/LT (high pressure/low temperature) metamorphic rocks is critical to piecing together accurate tectonic reconstructions but geochronological studies of these rocks are not without its problems. Zircon dating is robust but not always an option if the rocks do not contain metamorphic zircon, which generally requires temperatures of > 550 - 500°C. White micas can be readily dated using K-Ar (Ar-Ar) techniques but in low temperature high pressure environments they can have issues with excess radiogenic argon trapped from older detrital components results in inaccurate, older ages (Li et al., 2009). Sm-Nd can be applied to high pressure garnets but minor amounts of garnets retrogression to chlorite yields spurious ages. Recently published detrital zircon ages of ~ 250 Ma from the Port Macquarie eclogite contrast starkly with previous K-Ar Ordovician 470 Ma) suggesting either an issue with contamination in the zircon study or excess Ar issues associated with the excess K-Ar date. Here, as an alternative approach to determining the timing of metamorphic events, we use Rb" Sr dating techniques on white micas from the same samples used for the zircon study. The Rb-Sr methods does not suffer the same issues of excess radiogenic Ar gas associated with older detrital components e.g, Sherlock and Kelley (2002), but does require separation of Rb and Sr from the whole rock and mineral matrix using resin extraction methods in a clean laboratory, prior to isotopic analysis using either thermal ionisation mass spectrometry (TIMS) or MC-ICMPS. 87

Three samples were collected for white mica separation towards the northern end of Rocky Beach, Port Macquarie (Nutman et al., 2013; Och et al., 2003). Sample Rb-2 is a blueschist which is intercalated with eclogite where the difference in mineralogy, is controlled by a compositional change in the layered sedimentary protolith (Oh et al., 1991). Its mineral assemblage is glaucophane + garnet + phengite ± lawsonite ± albite + titanite + pyrite. Rb-3 is a retrogressed block in close proximity to Rb2, which also illustrates compositional layering RB1402 is a small cobble found in a small cove, ~ 100 m north of the Rb-2, 3 site, with the main non-foliated mineral assemblages consisting of glaucophane + phengite + albite + rutile, with retrogressive actinolite + calcite + chlorite. Based on whole rock geochemical analysis published previously, the samples protolith is a mafic to intermediate volcaniclastic unit. White mica compositions from these samples with Si per formula unit (p.f.u.) compositions of 3.32 - 3.52, based on 110 with total iron assumed to be Fe . All white micas group closely to the phengite end-member component [K (MgAl3)(AlSi 02o)(OH)4] on the A1 - A1 - Mg + Fe ot) ternary composition plot. Na-amphiboles are dominated by a glaucophane component, with (Na)M4 values of 1.76 - 1.99 and 7.92 - 8.04 p.f.u. Phengite growth is often associated with higher grade greenschist facies. In the samples small laths of glaucophane occurs coeval to this period of growth, or shortly after followed by retrogression assemblages. Both of these are pre-dated by veins of calcite. PT estimates of <1 GPa, < 450 °C are by previous authors are based on the low A1 0 contents of omphacites, the coexistence of lawsonite with garnet + omphacite. Fe-Mg cation exchange thermometry has failed to replicate garnet-omphacite temperatures if > 560°C, reported by Och et al. (2003). Rb-Sr ages from phengites at Port Macquarie record the growth of metamorphic white micas from upper greenschist to blueschist facies during subduction. Our results of Rb-Sr geochronology of the white micas will help to establish an accurate age of peak HP/LT metamorphism at Port Macquarie, which will build more accurate tectonic reconstruction of this enigmatic serpentinite melange. 2+

SGTSG 2015: Riding the Waves References Li, S., Jagoutz, E., Lo, C.H., Chen, Y., Li, Q., Xiao, Y., 1999. Sm/Nd, Rb/Sr, and 40Ar/39Ar isotopic systematics of ultrahigh-pressure metamorphic rocks in the Dabie-Sulu Belt, Central China: a retrospective view. International Geology Review 41, 1114-1124. Nutman, Allen P., Buckman, Solomon, Hidaka, Hiroshi, Kamiichi, Tomoyuki, Belousova, Elena, Aitchison, Jonathan, 2013. Middle Carboniferous-Early Triassic eclogite- blueschist blocks within a serpentinite melange at Port Macquarie, eastern Australia: Implications for the evolution of Gondwana's eastern margin. Gondwana Research 24, 1038-1050. Och, D.J., Leitch, E.C., Caprarelli, G., Watanabe, T., 2003. Blueschist and eclogite in tectonic melange, Port Macquarie, New South Wales, Australia. Mineralogical Magazine 67, 609-624 Oh, C.H., Liou, J.G., Maruyama, S., 1991. Low-temperature eclogites and eclogite schists in Mn-rich metabasites in Ward Creek, California; Mn and Fe effects on the transition between blueschist and eclogite. Journal of Petrology 32, 275-301. Sherlock, S., Kelley, S., 2002. Excess argon evolution in HP-LT rocks: a UVLAMP study of phengite and K-free minerals, NW Turkey. Chemical Geology 182, 619-636.

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Insights into the kinematics of deformation in fold-thrust belts, from field and modeling studies STEPHEN MARSHAK 1 ] Dept. of Geology and School of Earth, Society, & Environment, University of Illinois at 605 E. Springfield Ave., Champaign IL 61820 smarshak@illinois. edu

Urbana-Champaign,

Fold-thrust belts occur in the forelands of collisional and convergent margin orogens and result, overall, in the, horizontal shortening of the upper crust. Mapping and seismic-reflection studies have provided basic constraints on the architecture of these belts. But since development of fold-thrust belts takes millions of years, visualizing how rocks move during the formation of fold-thrust belts (i.e., how the belts evolve kinematically) has proven to be difficult. Insight into this issue can come from sandbox (analog) modeling in the lab, and from field analysis of mesoscopic structures exposed in outcrop. This talk will present three examples. Specifically, sandbox modeling emphasizes that the development of map-view curves in thin-skinned fold-thrust belts does not necessarily require oroclinal bending (rotation of segments of the belt around a vertical axis) after thrusting. Curves can be a consequence of shortening of a sedimentary wedge that changes thickness along its length (perpendicular to the regional shortening direction). In fact, the map-view shape of structural trendlines in a fold-thrust belt directly correlate with along-strike thickness change in the deforming basin. In places where mapview curves are due to a second, non-coaxial phase of deformation, mesoscopic structural analysis reveals overprinted slip lineations, and cross-cutting cleavages. Sandbox modeling using particle image velocimetry (PIV) can provide insight into the progressive evolution of ramp faults. Traditional cross-sectional models of the development of ramps commonly imply that such faults initiate at the tip of a detachment and then cut up-section through overlying strata, first producing a fault-propagation fold and then a fault-bend fold. PIV analysis demonstrates, however, that the detachment propagates far into the foreland of the site of ramp initiation. The ramp initiates in the interior of the layer above the detachment and grows up-dip and down-dip until it intersects the detachment well to the hinterland of the detachment tip line. At the moment of intersection, the detachment to the foreland of the intersection becomes temporarily inactive, while displacement takes place entirely on the ramp. This evolutionary model explains the field observation that layer-parallel shortening fabrics occur well to the foreland of the leading ramp in a thrust belt. Progressive deformation and fabric development in fold-thrust belts can lead to an accommodation of vertical shortening during that later stages of deformation. Specifically, in cases where pelitic units in thrust belts develop phyllitic foliation, progressive deformation produces antithetic kink bands and an associated asymmetric crenulation cleavage. This fabric rotates into a horizontal orientation, resulting in sub-simple shear that not only allows continued foreland displacement, but also yields vertical thinning.

SGTSG 2015: Riding the Waves

Uncovering the mineral potential of the Stavely Arc using 3D geological modelling SARLAE R. B. MCALPINE 1 AND JAMES A. GOODWIN1 1

Geo science Australia, Cnr Jerrabomberra Avenue & Hindmarsh Drive, Symonston, Canberra, ACT.

3D geological models have been produced for two major geological units in the Grampians-Stavely structural geological zone in western Victoria. The Grampians-Stavely Zone is located on the eastern limit of the Cambrian-aged Delamerian Orogen in Victoria (VandenBerg et al., 2000; Crawford et al., 2003; Miller et al., 2005) and several belts of Cambrian igneous rocks with arc affinities have been recognised within this zone (Crawford and Keays, 1978; Buckland, 1987; VandenBerg et al., 2000; Crawford et al., 2003); including the exposed Mount Stavely Volcanic Complex (Buckland, 1987). The Mount Stavely Volcanic Complex, together with other belts of Cambrian igneous rocks, have been interpreted as fault slices of a now mostly buried magmatic arc system referred to as the Stavely Arc (Schofield et al., 2015; Cayley et al., in prep). In order to address the outstanding geological questions and challenges to exploration in the Grampians-Stavely Zone, Geoscience Australia and the Geological Survey of Victoria established the collaborative Stavely Project in 2013. The Stavely Project forms part of the broader UNCOVER initiative (Australian Academy of Science, 2012) and aims to provide the fundamental framework for discovery in the Grampians-Stavely Zone. This is done using a mineral systems-based approach (Wyborn et al., 1994) through the provision of pre-competitive geoscientific data. This approach involves characterising the subsurface geology, recognising favourable geological environments for the formation of major mineral systems, identifying important elements that demonstrate mineral systems potential, and understanding the depth and nature of cover across the region. This study will focus on understanding the depth and nature of specific cover units across the region. Cover, in this study, is defined as anything that overlys the prospective rocks of the Stavely Arc and includes: regolith, the Murray Basin and various sedimentary and volcanic rock units such as the Grampians Group, Rocklands Volcanic Group and the Newer Volcanic Group. Of these cover units, 3D geological models have been produced for the Newer Volcanic Group volcanics and the Grampians Group sediments. In order to provide a framework for discovery in the region it is essential that the depth and spatial extent of these units are identified. GeoModeller 2014 software was used to create the two 3D geological models being presented. GeoModeller utilises an interpolator method for creating 3D geology that is based on potential field theory (Chiles et al., 2004; Mclnerny et al., 2005). The 3D geological model provides a space where interpretations from multiple datasets can be represented together. Information included in these models includes surface geology, stratigraphic drill-holes, and interpretations from seismic reflection, gravity and magnetic data. The primary goal of the Stavely Project was the acquisition of pre-competitive geoscientific data. This included the completion of fourteen stratigraphic drill holes which tested regional geological interpretations and recovered material for detailed lithological analysis (Schofield et al., 2015). The new information derived from these stratigraphic drill holes has been incorporated into the Newer Volcanic Group and Grampians Group 3D models which cover areas of 122 x 155 km and 62 x 94 km respectively in the Grampians-Stavely Zone. Initial results for the Newer Volcanics Model show an average thickness for the volcanics of ~30m across the model area. This value is derived from surface geology, geophysics and >600 drillholes that intersect the unit. The Grampians Group Model tests the validity and depth of a low angle decollement that lies between the Grampians Allocthon and the Cambrian basement below. 3D modelling provides an estimate of the depth to prospective basement and provides an insight into the geometry and geodynamic evolution of the Stavely area. This project will assist in further developing ideas for the

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exploration of arc and back-arc-related mineral systems, utilising and presenting the most recent understanding of the geological structure and evolution of the Grampians-Stavely Zone.

References Australian Academy of Science 2012. Searching the deep earth. Accessed at http://www.science.org.au/sites/default/files/user-content/searchingthedeepearth.pdf Buckland, G.L. 1987. Geology and mineral potential of the Mount Stavely Volcanic Complex. Geological Survey of Victoria Report 80. Geological Survey of Victoria. Cayley, R.A., Korsch, R.J., Kennett, B., Skladzien, P.B., Jones, L., Morand, V.J., Gibson, G.M., Rawling, T.J. and Betts, P.G. in prep. Results of deep seismic reflection imaging of the eastern Delamerian Orogen, South Australia and western Victoria, Australia. Chiles, J.P., Aug, C., Guillen, A. and Lees, T. 2004. Modelling the Geometry of Geological Units and its Uncertainty in 3D From Structural Data: The Potential-Field Method: Workshop Proceedings: Orebody Modelling and Strategic Mine Planning, Perth, WA, 22-24 November. Crawford, A., Cayley, R., Taylor, D., Morand, V., Gray, C., Kemp, A., Wohlt, K., VandenBerg, A., Moore, D., Maher, S., Direen, N., J, E., Donaghy, A., Anderson, J. and Black, L. 2003. Neoproterozoic and Cambrian: continental rifting, continent-arc collision and post-collisional magmatism. In: Birch, W. ed., Geology of Victoria (3rd edition). Geological Society of Australia Special Publication 23. Geological Society of Australia, Victoria Division, 73-93. Crawford, A.J. and Keays, R.R. 1978. Cambrian greenstone belts in Victoria: marginal sea-crust slices in the Lachlan Fold Belt of southeastern Australia. Earth and Planetary Science Letters, 41, 197-208. Mclnerney, P.M., Guillen, A., Courrioux, G., Calcagno, Ph. and Lees, T. 2005. Building 3D Geological Models Directly from the Data? A new approach applied to Broken Hill, Australia., in Soller, D.R., ed., Digital Mapping Techniques '05 - Workshop Proceedings: U.S. Geological Survey Open-file Report. Miller, J.M., Phillips, D., Wilson, C.J.L. and Dugdale, L.J. 2005. Evolution of a reworked orogenic zone: the boundary between the Delamerian and Lachlan fold belts, southeastern Australia. Australian Journal of Earth Sciences, 52, 921-940. Schofield, A., Cayley, R.A., Barton, T., Taylor, D.H., Nicoll, M. and Cairns, C.P. 2015. Regional geology and mineral systems of the Stavely region, western Victoria: data release 1 - stratigraphic drilling field data. Record 2015/13. Geoscience Australia, Canberra. VandenBerg, A.H.M., Willman, C.E., Maher, S., Simons, B.A., Cayley, R.A., Taylor, D.H., Morand, V.J., Moore, D.H. and Radojkovic, A. 2000. The Tasman Fold Belt system in Victoria: geology and mineralisation of Proterozoic to Carboniferous rocks. Geological Survey of Victoria Special Publication. Department of Natural Resources and Environment, Melbourne. Wyborn, L. A. I., C. A. Heinrich, and A. L. Jaques. 1994. Australian Proterozoic mineral systems: essential ingredients and mappable criteria. The AusIMM Annual Conference, 109-115.

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The role of boundary conditions and rheological heterogeneities during the rifting to drifting process: 3D laboratory experiments with application to the Red Sea

NICOLAS E. MOLNAR AND ALEXANDER R. CRUDEN 1

School of Earth, Atmosphere and Environment, Monash University, Melbourne, VIC 3800, Australia

The transition from continental break up to sea-floor spreading is a poorly resolved aspect in plate tectonics theory. The evolution and resulting structural patterns are controlled by plate fragmentation, pre-existing weaknesses, lithospheric decoupling, complex crust-mantle thermal interactions, or a combination of these factors. Here, we investigate the role of inherited rheological heterogeneities and lithospheric decoupling during rifting by means of analogue modelling. In this study we focus on the Red Sea, which provides an ideal setting to study the role of boundary conditions during rifting to drifting processes, as it represents the best example of a modern juvenile rift-related oceanic basin on Earth. The general geodynamic evolution of the Red Sea-Gulf of Aden rifting system is well constrained. Numerous structural, geophysical and geodetic datasets have constrained the understanding of its complex, polyphase evolution. However, there is evidence that lithospheric heterogeneities associated with Neoproterozoic sutures are associated with strain localisation during rifting and subsequent evolution. These major morphotectonic features indicate a strong structural control on continental rifting, but their role as stress guides, barriers to axial propagation or precursors to oceanic transforms is poorly understood. To address these topics we have designed a multipurpose experimental apparatus that can impose pure, oblique and rotational rifting boundary condition on analogue lithospheres over a range of strain rates. In the experiments we reproduce strong and weak lithosphere scenarios to simulate rheological coupling/decoupling and also include variably oriented strength heterogeneities. We quantify surface strain and dynamic topography using a high-resolution particle imaging velocimetry and digital photogrammetry monitoring system. Analysis of the resulting fault patterns, dynamic topography, velocity fields and strain localization will allow us to compare the experimental outcomes with published numerical models and ongoing structural analysis of geophyisical data from the Red Sea. Preliminary results of the first series of experiments will be presented.

100

SGTSG 2015: Riding the Waves EXPERIMENTAL SET UP 3D stereoscopic setup PIV camera 1 PIV camera 2

Data acquisition + velocity controller

PLAN VIEW ,

65 cm

44 cm

Heterogeneity

SIDE VIEW

LEGEND l: : : ::: l Upper crust E ~ J Lower crust Lithospbertc mantle H I Weak lithospheric mantte Asthenospheric mantle

Fixed wall

Moving wall

40.5 cm

Figure 1: Experimental set up. Plan view figure includes a reference map (modified from Bos worth et al., 2005) with the approximated area represented in the experiments. Bottom is a side view of how the models are constructed.

101

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The subdivisions of western Tasmania and their effects across Bass Strait

DAVID H. MOORE , PETER G. BEITS AND MIKE HALL 1

Monash University, Clayton, Victoria

VanDieland continues north from western Tasmania across Bass Strait to become the Selwyn Block in central Victoria. Moore et al. (2015) subdivided western Tasmania and the submarine plateaux further south into 7 zones, of which three, the King Island, Rocky Cape and Burnie zones can be traced north into Victoria (Figure 1). The King Island Zone includes the Mesoproterozoic and Neoproterozoic metasedimentary rocks exposed on King Island, including the 580 Ma rift tholeiite to MORB mafic volcanic rocks on southeastern King Island. Magnetic and gravity data indicate this mafic package continues north and can be correlated with the basalt present on the south coast of Phillip Island. Further north, outcrops of the Ceres Gabbro may have formed during the extension associated with the breakup of Rodinia, implying that they are also Neoproterozoic. The most northerly effects of the King Island Zone may be seen in the Mt Disappointment Granodiorite, where there is evidence of a mafic precursor in rare enclaves that contain cummingtonite and green biotite pseudomorphs after orthopyroxene (Clemens and Benn, 2010). Upper Devonian granites that intrude the northern extension of the King Island Zone have £ values as high as +2 compared to those elsewhere in central Victoria, where values are as low as -6 are obtained (Maas and Nicholls, 2012), suggesting a more juvenile source in the King Island Zone, and consistent with derivation from the 580 Ma mafic rocks. In Tasmania, the outcropping Rocky Cape Zone is comprised of Mesoproterozoic marginal marine sedimentary rocks that are overlain by the Neoproterozoic Smithton Basin that includes both carbonates and mafic volcanic rocks. While no Rocky Cape Zone outcrops are present in Victoria, unexplained calc-silicate enclaves are present in some granites, and clasts of marginal marine quartzite are present in Early Devonian marine slope deposits in the southeastern Melbourne Zone (VandenBerg et al., 2006). The broad magnetic highs that originate from bodies under the Strathbogie and Tynong batholiths and the Cerberean Cauldera in central Victoria may result from metamorphism of an underlying mafic rock package. Finally, results from the 2006 seismic survey in Victoria (Cayley et al., 2011) can be interpreted as indicating the same layering as inferred to be present in the Rocky Cape Zone in Tasmania. The Burnie Zone in Tasmania is comprised of Neoproterozoic rift-related metaturbidites that are commonly overlain by Cambrian mafic-ultramafic rocks and the Mount Read Volcanics. In Victoria, Mount Read Volcanic equivalents are known from the eastern Melbourne Zone (Morey et al., 2002) and clasts of mafic-ultramafic rocks are also known from the slope deposits referred to above. The mafic rocks at Cape Liptrap may also be part of the Neoproterozoic and Cambrian packages. The northern end of line 4 of the 2006 seismic survey has a different character to other parts and can be interpreted as indicating the presence of Burnie Zone rocks. Nd

References Cayley, R.A., Korsch, R.J., Moore, D.H., Costelloe, R.D., Nakamura, A., Willman, C.E., Rawling, T.J., Morand, V.J., Skladzien, P.B., O'Shea, P.J., 2011. Crustal architecture of Central Victoria: results from the 2006 deep crustal reflection seismic survey. Australian Journal of Earth Sciences 58, 113-156. doi: 10.1080/08120099.2011.543151. Clemens, J.D., Benn, K., 2010. Anatomy, emplacement and evolution of a shallow-level, post-tectonic laccolith: the Mt Disappointment pluton, SE Australia. Journal of the Geological Society, London 167, 915-941. doi:http://dx.doi.org/10J 144/0016-76492009-120. Maas, R., Nicholls, I., 2012. A Proterozoic basement block in SE Australia: Sr-Nd-0 isotope tracing of the Selwyn Block, Victoria, 34th International Geological Congress. Australian Geoscience Council, Brisbane, Qld. Moore, D.H., Betts, P.G., Hall, M., 2015. Fragmented Tasmania: the transition from Rodinia to Gondwana. Australian Journal of Earth Sciences 62, 1-35. doi:10.1080/08120099.2014.966757.

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SGTSG 2015: Riding the Waves Morey, A.A., Bierlein, F.P., Cherry, D.P., Turner, G., 2002. Genesis of greenstone-hosted Cu-Au mineralisation at Hill 800, Mt Useful Slate Belt, eastern Victoria. Australian Journal of Earth Sciences 49, 787-799. doi:10.1046/j.l440-0952.2002.00954.x. VandenBerg, A.H.M., Cayley, R.A., Willman, C.E., Morand, V.J., Seymon, A.R., Osborne, C.R., Taylor, D.H., Haydon, S.J., McLean, M., Quinn, C., Jackson, P., Sandford, A.C., 2006. Walhalla-Woods PointTallangallook special area geological report, Report, 127. Geological Survey of Victoria, Melbourne, p. 448.

144°E

148°E

Basement Geology of Bass Strait & Surrounding Areas 37°S Upper Devonian granite Eastern Tasmania & Tabberabbera Zone Bendigo & Stawell zones

Sorell - Badger Head Zone

Tyennan Zone

Pedder Zone

Burnie Zone 40°S Rocky Cape Zone

King Island Zone

Zone boundary

Fault Kilometres

Figure 1: Interpreted basement geology of Bass Strait & surrounds overlaid on an image of the magnetic of the region. C marks the location of the Cerberean Caldera, D, Mount Disappointment Granodiorite, G, the Ceres Gabbro, L, Cape Liptrap, P, Phillip Island, S, the Strathbogie Batholith and T, the Tynong Batholith.

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Basement-cover relations in the intracratonic Kaladgi basin, southwestern India: Deformtional evidence of a Mesoproterozoic gravity gliding of the cover over the basement. MRINAL KANTI MUKHERJEE1

department of Applied Geology, Indian School of Mines, Dhanbad-826004, mrinal_km67@yahoo. co. in

Jharkhand, INDIA. Email:

The structural relationship between the basement and the Mesoproterozoic sedimentary cover in the intracratonic Kaladgi basin, south western India, has been studied with reference to association, mode of occurrence, distribution, interrelationship, variation and chronology of development of structural elements and deformation microstructures. The Mesoproterozoic sedimentary cover of the Kaladgi basin, unconformably overlie a basement that forms a part of the Dharwar Craton. The basement is an assemblage of the Archaean Peninsular Gneissic Complex (PGC), Late Archaean Hungund Schist Belt (HSB) and Granites (Closepet Granite). The HSB, composed of Banded Iron Formations, metapelites and mafic metavolcanics have undergone multistage deformation (Dl, D2, and D3 respectively). The D1 stage involved a combination of layer parallel shortening along NE-SW (with reference to present day geographic coordinates) and distributed layer parallel shear leading to the development of tight-to-isoclinal recumbent folds that were refolded in the D2 stage to develop plane non-cylindrical D2 folds with transposed compositional banding, schistocity and D2 crenulation lineations. The D3 stage involved a mild shortening of D2 structures along NW-SE to develop broad open D3 folds and D3 crenulation lineations. The orientation of the overall structural grain of the HSB is 310°. The PGC consisting of granitoid gneisses are deformed in phase with the HSB through development of foliations in the Dl stage of deformation and folding of the foliations in the D2 stage. The general trend of D2 folds in the PGC is 310° and are parallel to the D2 structures of the HSB. The Closepet granite (c. 2.5 Ga) is intrusive within the PGC and the HSB and is technically undeformed, in general. The Mesoproterozoic sedimentary cover of the Kaladgi basin are composed of repetitive cycles of coarse siliciclastics that include granular arkose, quartz arenites and lensoidal bands of polymictic conglomerates along the basin margin that grades gradually through argillites to carbonates (limestones and dolomites) towards the basin interior. Based on type, geometry, distribution and association of structural elements, the deformation structures of the Mesoproterozoic sedimentary cover rocks of the basin can be grouped to define an extensional domain in the northern sectors and a contractional domain in the south-central sectors of the basin. In the northern sectors, the cover rocks are gently dipping (10°-15°) due south and define a homocline that is affected at places by normal faults with associated strata drags, tensile and hybrid joints, and torn - apart segments of the cover. Together, these define an extensional deformation zone. In the south-central sectors of the basin, an association of WNW-ESE trending, both northerly and southerly verging, asymmetric-to-overturned, plane noncylindrical, gently plunging folds with axial planar cleavages, E-W striking thrusts, and N-S trending strike-slip faults, together define the contractional deformation zone. Distribution, mode of occurrence, variation and interrelationship of structural elements in the basin reveal that the extensional and contractional domains can be spatially linked as a continuous deformation sequence that are related to a single deformation event of the cover.

104

Jamkhandi

kilometres

Legend Deccan Trap (Upper Cretaceous to Eocene) Badami Group (Neoproterozoic) Bagalkot Group (Mesoproterozoic) Closepet Granite (2.5 Ga) HSB

}

Archaean

PGC

Plunging Antrform ^ ^ ^ Plunging Synform —r—r Fault

fo Oip of Bed

•

^r-r 20 to

rrTTs"

Saundatti

Strike of Structure Form Surface

Figure 1: Simplified geological map of the deformed sectors of the Kaladgi basin showing the deformation patterns of the Mesoproterozoic sedimentary cover (Bagalkot Group). The extensional and contractional domains of deformation are also indicated.

The structural anatomy of the basement cratonic assemblage and the Mesoproterozoic cover sediments are markedly contrasting in terms of geometry, association, inter-relationship, deformation history and deformation microstructures. The deformation of the cover rocks is younger and its structural geometry of contractional domain require a NNW-SSE to N-S directed shortening (with reference to present day geographical coordinates) and the effect of such shortening would expectedly reorient the NW-SE oriented structures of the HSB and align them along WNW-ESE similar to that of the cover. However, orientation of fold axis and the general trend of structures in the HSB, show a NW-SE trend that continues beyond the basin in the SE, where it translates into Kushtagi Schist belt with the same orientation. In addition, the Granite which is also a constituent of the basement rocks of the basin, show no visible penetrative deformation fabric related to N-S shortening of the cover and is in general undeformed throughout. Furthermore, the microstructures in the cover rocks corresponding to low-temperature deformation mechanisms, have not been overprinted on the microstructures of the basement rocks corresponding to relatively high-temperature deformation mechanisms. It is therefore evident that the basement rocks have escaped the effects of the deformation of the cover. The non-involvement of the basement during the deformation of the Mesoproterozoic sedimentary cover and the linked extensional and contractional domains of deformation of the cover rocks from north to south, in the Kaladgi Basin, together indicate that the deformation of the Mesoproterozoic sedimentary cover originated by a southerly directed gravity gliding of the cover over the basement.

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Mesoproterozoic Tasmania: Witness to the East Antarctica-Laurentia connection within Nuna

JACOB A. MULDER , JACQUELINE A. HALPIN AND NATHAN R. DACZKO 1

ARC Centre of Excellence in Ore Deposits (CODES), School of Physical Sciences, University of Tasmania, Private Bag 79, TAS 7001, Australia ARC Centre of Excellence for Core to Crust Fluid Systems and GEMOC, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia 1

Most recent paleogeographic reconstructions of the supercontinent Nuna juxtapose the North Australian craton, Mawson continent (South Australia-East Antarctica), and Laurentia between 1.6 Ga and 1.3 Ga but differ in their relative positioning. The >10-km-thick siliciclastic Rocky Cape Group of Tasmania was deposited in an opening marine basin on the margin of East Antarctica during Nuna breakup. Based on a similar detrital zircon signature and depositional age, the Rocky Cape Group has been correlated with the upper Belt-Purcell Supergroup in Laurentia, thus representing a key tie point within Nuna. Here the detrital zircon age signature of Mesoproterozoic Rocky Cape Group quartzites is investigated by comparing new detrital zircon U-Pb-Hf isotopic data to an extensive compilation of zircon isotopic data from Australia, East Antarctica, and Laurentia. Our analysis demonstrates that the Rocky Cape Group is unlikely to have been sourced from any geological terrane exposed in presentday Australia. Instead, zircon U-Pb-Hf isotopic data from basement terranes in Laurentia and East Antarctica show striking similarities to the Rocky Cape Group detrital signature. Paleocurrent data indicate that the majority of sediment in the Rocky Cape Group was sourced from Laurentia, which was to the southeast (present-day coordinates) of Tasmania, supporting a SWEAT-like (southwest United States-East Antarctica) configuration for Nuna. We suggest that rifting left a thinned continental connection between East Antarctica and Laurentia onto which the lower-middle Rocky Cape Group was deposited between 1.45 and 1.30 Ga.

Thinned \ •ntinentaf , crusty

Laurentia Belt-

Purcell Basin

Tasmania

C D 1.45-130 Ga basins 1.76-1.45 Ga basins

] Mesoproterozoic Paleoproterozoic— Mesoproterozoic F I Archean

106

Figure 1: Proto-SWEAT paleogeographic reconstruction of Nuna at -1.45 Ga showing basement terranes and Paleoproterozoic— Mesoproterozoic sedimentary basins relevant to this study following (e.g.) Goodge et al. (2008) and Medig et al. (2014). The relative positioning of the continents follows Pisarevsky et al. (2014) but are rotated -180° to reflect present day orientations. Inset shows inferred paleoshoreline and dominant paleocurrent modes derived from cross-bedding in lower-middle RCG (present day coordinates). The paleoshoreline of the Tasmanian Mesoproterozoic basin has been rotated to parallel the orientation of the inferred Mawson Continent margin. The exact position of Tasmania along this paleomargin is poorly constrained (dashed outline shows possible alternative position), but our detrital zircon data suggest a close link to 1.45 Ga crust in the southern Mawson Continent. NAC; North Australian Craton, SAC; South Australian Craton, nMC/sMC: northern/southern Mawson Continent, TA; Terre Adelie Land, Y/MP; Yavapai/Mazatzal Province, MoP; Mojave Province, GR; Grenville Province, LBP; Lower-Belt Purcell Correlates, MF; Marquenes Formation, BG= Byrd Glacier, MCB = boundary between the northern and southern Mawson Continent. Paleocratonic margins are unknown and approximated by light gray fill.

SGTSG 2015: Riding the Waves

References Goodge, J.W., Vervoort, J.D., Fanning, C.M., Brecke, D.M., Farmer, G.L., Williams, I.S., Myrow, P.M., and DePaolo, D.J., 2008, A positive test of East Antarctica-Laurentia juxtrapositoin within the Rodinia supercontinent: Science, v. 321, no. 5886, p. 235-240. Medig, K.P.R., Thorkelson, D.J., Davis, W.J., Rainbird, R.H., Gibson, H.D., Turner, E.C., and Marshall, D.D., 2014, Pinning northeastern Australia to northwestern Laurentia in the Mesoproterozoic: Precambrian Research, v. 249, p. 88-89. Pisarevsky, S.A., Elming, S.A., Pesonen, L.J., and Li, Z. X., 2014, Mesoproterozoic paleogeography: Supercontinent and beyond: Precambrian Research, v. 244, p. 207-225.

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The metamorphic sole of the western Tasmanian ophiolite: New insights into the Cambrian tectonic setting of the Gondwana Pacific margin JACOB A . MULDER, RON F. BERRY, SEBASTIEN MEFFRE AND JACQUELINE A . HALPIN 1 1 ARC Centre of Excellence in Ore Deposits (CODES), School of Physical Sciences, University of Tasmania, Private Bag 79, TAS 7001, Australia

The Cambrian Ross-Delamerian Orogeny records the first phase of accretional tectonics along the eastern margin of Gondwana following breakup of the supercontinent Rodinia. Western Tasmania represents a key area for understanding the Cambrian tectonic setting of the eastern margin of Gondwana as it is one of the few places where a Tethyan-type ophiolite is preserved and contains the only known exposures of a sub-ophiolitic metamorphic sole associated with the Ross-Delamerian Orogen. This paper presents an integrated study of the field, petrographic, geochemical, and metamorphic characteristics of the metamorphic sole to the western Tasmanian ophiolite. The structurally highest levels of the metamorphic sole consist of granulite-upper amphibolite facies metacumulates and metagabbros. A transition to amphibolite and epidote-amphibolite facies conditions is recorded by metadolerites and metabasalts towards the base of the metamorphic sole. Kinematic indicators in mylonitic amphibolites suggest the metamorphic sole formed in an east-dipping subduction zone located to the east of the Proterozoic continental crust of Tasmania. Major and trace element whole rock and relict igneous spinel geochemistry indicates that the protoliths to the metamorphic sole formed at a back arc basin spreading centre. Our new data supports a model in which east-dipping subduction in Tasmania was driven by collapse of a back arc basin developed above an earlier west-dipping subduction zone outboard of the eastern margin of Gondwana. The proposed model may help to resolve a controversy related to apparent along-strike variations in subduction zone polarity during the Ross-Delamerian Orogeny and suggests a complex geodynamic setting had developed along the eastern margin of Gondwana by the Middle Cambrian. (a)]

>515 M a

m Tasmania

Back Arc Basin

Arc?

505-495 M a Isolated ophiolite remnants

"tyennan Re9,on

Figure 1: Geodynamic scenario proposed for Cambrian evolution of Tasmania and the eastern margin of Gondwana. (a) Protoliths to the metamorphic sole form in a back arc basin above a west-dipping subduction zone developed along the eastern margin of Gondwana. Tasmania is a microcontinent outboard of the eastern margin of Gondwana. (b) Collapse of the back arc basin to form a new east-dipping subduction zone east of Tasmania. The metamorphic sole to the western Tasmanian ophiolite forms in the new east-dipping subduction zone as hot, young back arc basin crust is accreted to the hanging wall of the upper plate, (c) The Tasmanian microcontinent is drawn into the east-dipping subduction zone initiating the main phase of the Tyennan Orogeny. Parts of the microcontinent are subducted and undergo high-pressure metamorphism coincident with obduction of the western Tasmanian ophiolite. Blocking of the east-dipping subduction zone drives convergence to be taken up west of Tasmania causing the eventual accretion of the Tasmanian microcontinent onto the eastern margin of Gondwana during the late Cambrian, (d) Deeply subducted crust is rapidly exhumed following slab break-off, erosion of the ophiolite during uplift contributes to the isolation of the mafic-ultramafic complexes throughout Tasmania.

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What can geophysics tell us about the mobile phase of the Lachlan Orogen? ROBERT J. MUSGRAVE 1 Geological Survey of NSW, NSW Department of Industry, PO Box 344 Hunter Region Mail Centre, NSW 2210 e-mail: robert. musgrave @ industry.nsw.gov. au

Competing models for the Lachlan Orogen prior to the cratonising Tabberabberan orogeny differ in their implications for the character and motion history of the package of Ordovician calc-alkaline rocks comprising the Macquarie Arc. The Lachlan Orocline model (Cayley et al., 2012; Moresi et al., 2014) suggests a highly mobile Macquarie Arc, migrating rapidly behind a retreating trench pinned at its southern end by the Selwyn continental block. Southward migration of the trench as the upper plate wrapped around the indentor drove clockwise oroclinal rotation of the arc. Fergusson (2009) also proposed a rotation phase for the Macquarie Arc, as a mechanism to explain the enigmatic distribution of Ordovician quartz turbidites on both sides of the arc, but differed in the timing (Ordovician) and sense of rotation (anticlockwise). Recent reinterpretation of the Macquarie Arc as a feature of back-arc extension (Quinn et al., 2014; Bruce & Percival, 2014) suggest not only a different motion history, but a fundamentally different crustal composition and structure. Palaeomagnetism provides the most direct evidence for local vertical-axis rotation. Existing preDevonian palaeomagnetic data from the Lachlan Orogen (Luck, 1973; Goleby, 1980) have been considered unreliable, owing to their apparent inconsistency with likely Gondwana apparent polar wander paths and because of challenges to their legitimacy (Schmidt et al., 1990). McElhiny et al. (2003) recognised that the apparent scatter of these poles could be explained by local rotations. Reexamination of the palaeomagnetic data as a test of the Lachlan orocline hypthesis (Figure 1) reveals evidence not only for a latest Silurian - early Devonian phase of clockwise rotation, in general agreement with the orocline model but implying a larger (55°-75°) rotation angle, but a possible additional late Ordovician - early Silurian phase of anticlockwise rotation (-90°). Recently acquired palaeomagnetic data from Cambrian rocks around the western hinge of the Lachlan orocline likewise confirm oroclinal rotaiton, but also suggest an earlier phase of anticlockwise rotation relative to Gondwana. Long-wavelength components of aeromagnetic anomalies provide additional constraints on elements of the Lachlan Orogen. Although the Macquarie Arc is usually regarded as intra-oceanic, the longwavelength magnetic low that characgterises the arc (and which contrasts with the long-wavelength magnetic high over the western Lachlan Orogen) indicates a middle to lower crust with intermediate, continental-like geoochemistry. This could represent a continental basement sliver, or may instead reflect a highly mature oceanic arc with a very substantial, evolved substrate. Such a basement would be incompatible with the back-arc model for the Macquarie Arc, but matches the interpreted basement of the Izu Arc (Suyehiro et al., 1996), especially the Izu rear-arc, which shares features of localised extension with the Macquarie Arc, and a similar high-K geochemistry. Together, the palaeomagnetic and aeromagnetic data suggest a highly mobile early Palaeozoic history for the Lachlan Orogen, with a setting and tectonic style sharing features of modern western and southwestern Pacific arcs. References Bruce, M.C., Percival, I.G., 2014. Geochemical evidence for provenance of Ordovician cherts in southeastern Australia. Australian Journal of Earth Sciences 61, 927-950, doi: 10.1080/08120099.2014.956792. Cayley, R., Musgrave, R., Preiss, W., 2012. South-directed oroclinal folding in the Lachlan Fold Belt: a solution to apparent Ordovician-Early Silurian complexity. Program and Abstracts, SGTSG biennial conference, Waratah Bay, Australia, Geological Society of Australia Abstracts 102, 19-21. Fergusson, C.L., 2009. Tectonic evolution of the Ordovician Macquarie Arc, central New South Wales: arguments for subduction polarity and anticlockwise rotation. Australian Journal of Earth Sciences 56, 179-193, doi: 10.1080/08120090802547017. Goleby, B.R., 1980. Early Palaeozoic palaeomagnetism in South East Australia. Journal of Geomagnetism and Geoelectricity 32, Supplement III, pp. SIII 11 - SIII21.

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SGTSG 2015: Riding the Waves Luck, G.R., 1973. Palaeomagnetic resyults from Palaeozoic rocks of southeast Australia. Geophysical Journal of the Royal Astronomical Society 32, 35-52. McElhinny, M.W., Powell, C.McA., Pisarevsky, S.A., 2003. Paleozoic terranes of eastern Australia and the drift history of Gondwana. Tectonophysics 362, 41-65, doi: 10.1016/S0040-1951(02)00630-3. Moresi, L., Betts, P.G., Miller, M.S., Cayley, R.A., 2014. Dynamics of continental accretion. Nature 508, 245248, doi:10.1038/naturel3033. Quinn, C.D., Percival, I.G., Glen, R.A., Xiao, W.-J., 2014. Ordovician marginal basin evolution near the palaeoPacific east Gondwana margin, Australia. Journal of the Geological Society, London 171, 723-736, doi: 10.1144/jgs2012-034. Schmidt, P.W., Powell, C.McA., Li, Z.X., Thrupp, G.A., 1990. Reliability of Palaeozoic palaeomagnetic poles and APWP of Gondwanaland. Tectonophysics 184, 87-100, doi: 10.1016/0040-1951(90)90122-0. Suyehiro, K., Takahashi, N., Ariie, Y., Yokoi, Y., Hino, R., Shinohara, M., Kanazawa, T., Hirata, N., Tokuyama, H. & Taira, A. (1996). Continental crust, crustal underplating, and low-Q upper mantle beneath and oceanic island arc. Science 272, 390-392.

Age (Ma)

Figure 1. Palaeomagnetic declination data from the Molong-Monaro terrane (Macquarie Arc)

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SGTSG 2015: Riding the Waves ALISON ORD

Representing the kinematics of deforming rocks

Centre for Exploration Targeting, The Universityof Western Australia

An important advance was made by Passchier (1991) in identifying that dilatant flow can be represented by eigenvectors of the flow. He erected a classification of flow fields based on these eigenvectors. An eigenvector of the flow is one that does not change direction during deformation. He pointed out that some fabric elements would be attracted to these eigenvectors so that the geometry of the deformation and the kinematics are nicely represented. Passchier addressed only plane strain involving simple shearing whereas Iacopini et al. (2010) extended the argument to 3D general homogeneous deformations. The analysis is extended here to include 3 dimensional general deformations including both homogeneous and inhomogeneous flow. For any general deformation in 2 dimensions there are always 2 eigenvectors. It is proposed that SC fabrics and oblique foliations are represented by these 2 eigenvectors. In 3 dimensions there are always 3 eigenvectors and a classification of deformation fields is presented for both homogeneous and inhomogeneous 3 dimensional deformation fields. The origins of foliations, crenulation cleavages, folds, and boudinage are discussed in terms of this classification. Analysing the deformation fields in terms of eigenvectors is a powerful means of understanding the kinematics of deformed rocks.

References

Iacopini, D., Carosi, R., Xypolias, P. 2010. Implications of complex eigenvalues in homogeneous flow: a threedimensional kinematic analysis. Journal of Structural Geology 32, 93-106. Passchier, C.W. 1991. The classification of dilatant flow types. Journal of Structural Geology 13, 101-104.

Ill

SGTSG 2015: Riding the Waves

Understanding vein-hosted mineralization at the Sunrise Dam gold deposit through coupled mechanical-fluid flow modelling

XIAOHAN WANG, MARK MUNRO AND ALISON ORD

Centre for Exploration Targeting, The University of Western Australia

We consider hydrothermal mineralization, a complex process that combines basic physical and chemical processes in mineral deposition with deformation and fluid flow. Structural interpretation is integrated with numerical modeling to improve our understanding of the interactions between fluid migration and brittle-ductile shear zones at the Sunrise Dam gold deposit. The emphasis is on the development of and interactions between rock fragmentation and veining. In addition, wavelet analysis is used to explore multifractal distributions of field and modelled structures. Model results are compared with and validated against laboratory and field observations.

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Problems of overprinting foliations - an example from NW-Namibia CEES W. PASSCHffiR University of Mainz, Germany Foliations are one of the most important tools in structural geology, used to determine aspects of flow kinematics and strain, relative timing of metamorphic cycles and deformation, and overprinting relations of deformation events. The basis of many structural analyses is the overprinting of one foliation by another, leading to the formation of crenulation cleavages or similar interference patterns. Although the principle of foliation overprinting is simple, there are many pitfalls and unusual situations which may lead to errors, both in the field and in microstructural studies. The Ugab Terrain in NW Namibia is used here to illustrate some of the problems, and to show how foliations can be used to reconstruct the structural and metamorphic history of mobile belts. Neoproterozoic meta-turbidites of the Ugab Terrain were deposited on the SW edge of the Angola craton, and were affected by Cambrian deformation and metamorphism at the triple junction of the Kaoko and Damara Belts. These mobile belts formed in collision of the Angola Craton with the Rio de la Plata and Kalahari Cratons respectively during the amalgamation of Gondwanaland. The Ugab terrain shows low-grade deformation in meta-pelites with locally up to five overprinting foliations of different type in a single outcrop. Most interesting are "sector foliations", which do not overprint but fill adjacent space, and "flame foliations", crenulation cleavages that derive their spacing from nucleation on vein-like structures. The foliations have a complex regional distribution pattern and have been used to reconstruct the development of the Kaoko and Damara Belt triple junction.

Figure. 1: Overprinting foliations in meta-turbidites, Twijfelfontein, NW Namibia

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Deformation and K-metasomatism in a Grantic Shear Zone, Capricorn Orogen, WA

MARK A. PEARCE AND ALISTAIR J.R. WHITE 1,2

CSIRO Mineral Resources, Australian Resources Research Centre, 26 Dick Perry Avenue, Kensington, WA 6151, Australia Department of Applied Geology, Curtin University, GPO U1987, Perth, WA (5845, Australia !

The Capricorn Orogen is a broad zone of crustal deformation and reworking between the Pilbara and Yilgarn Cratons in Western Australia. Multiple deformation, crustal melting and metasomatic events have affected the rocks that now make up the Capricorn Orogen. In spite of being located between cratons that host some of the largest mineral deposits in Australia, the Capricorn has remained large unprospective for either gold or iron ore, and contains only a few economic base metal occurrences. The complexity of the basement, thick sedimentary basins and weathered cover material all present exploration challenges. The "Capricorn Distal Footprints Project", of which this study forms part, aims to increase exploration succes in the region by providing an exploration toolkit for the Capricorn to aid understanding of the geological, geochemical and geophysical signatures of ore-deposits. A key part of this toolkit is understanding the background geological signatures of deformation and fluid-flow within the orogen. This study aims to provide insights into fluid-rock interactions within a shear zone localised in the granitic basement rocks of the Minnie Creek Batholith. The Minnie Creek bathlith forms part of the 1820-1775 Ma Moorarie Supersuite that was intruded during the intracontinental Capricorn Orogeny. The batholith is broadly granodioritic in composition but encompasses a range of rock types including tonalites and granites. In the area around the studied shear zone, the rocks are also intruded by mafic dykes and coarse-grained alkali-feldspar granites. Strain is localised in a metre to 10m scale high strain zone with a sub-vertical, NW-SE striking shape fabric. The granodioritic protolith containing two feldspars, quartz and biotite is altered to a chlorite, epidote, albite, quartz mylonite with quartz and K-feldpar-after-plagioclase porphyroclasts. Outside of this main mylonite zone, the rocks are cut by discrete brittle faults trending parallel to the mylonitic fabric. Truncation of lithologies across the river that exposes the shear zone rocks suggests that there is a second set of WNW trending faults that offset many of the structures and lithological boundaries. Whole rock geochemistry, microchemical XX-ray fluorescence mapping and microstructural analysis of deformed and altered samples are used to show how the variations in mineralogy observed in this deformation zone are variably a function of original compositional heterogeneity, intensity of metasomatism and finite strain. Whilst deformation is accompanied by K-metasomatism at the 100m scale, in detail the locus of the alteration and the most intensly strained rocks are not co-located. We suggest that the late-syn to post deformational intrusion of an acidic, alkali feldspar granite, facilitated by brittle deformation, provided a fluid source to cause alteration of surrounding rocks. The exploitation of existing fault zones by later fluid-rich intrusives can provide a source mineralising fluids that are associated with base metal deposits in the overlying sedimentary basins.

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A continuous subduction model for the Tasmanides: evidence of long-lived subduction preserved by high-pressure rocks in the Peel-Manning Fault System

GLEN PHILLIPS , ROBIN OFFLER , DANIELA RUBATTO AND DAVID PHILLIPS 1,2

4,5

Geological Survey of New South Wales, Division of Resources and Energy, NSW Department of Industry, Maitland, NSW, 2320, Australia. Discipline of Earth Sciences, School of Environmental and Life Sciences, University of Newcastle, Callaghan, NSW 2308, Australia. New South Wales Institute of Frontiers Geoscience, School of Environmental and Life Sciences, University of Newcastle, NSW 2308, Australia. Research school of Earth Sciences, Australian National University, Canberra ACT 2601, Australia. Institute of Geological Sciences, University of Bern, 3012 Switzerland. School of Earth Sciences, University of Melbourne, Parkville, Vic. 3010, Australia. 1

The Tasmanides of eastern Australia record 320 million years (520-200 Ma) of convergent margin orogenesis. The assembly of the southern Tasmanides took place during three main stages, which from west to east are represented by the Delamerian, Lachlan and New England orogens. While subduction is acknowledged as the primary driver of orogenesis, the number, location and mode of subduction remains controversial. Central to this debate is the mechanism responsible for the transition from one orogenic stage to the next. There are two models currently invoked to account for this phenomenon, which involve: (i) long-lived, continuous subduction zone where hinge retreat results in the outboard translation of the convergent margin and the initiation of the next, orogenic stage (Collins, 2002), and; (ii) punctuated subduction, where collision and accretion events result in subduction reversal, or outboard transference of subduction to a new, or existing subduction zone (Aitchison & Buckman 2012). In an attempt to test the applicability of the two models, we present new geochemical, metamorphic and geochronological data from high-pressure rock samples (eclogite-blueschist facies) from the PeelManning Fault System of the southern New England Orogen. The Peel-Manning Fault System is host to fault-bounded sequences of serpentinite-matrix melange, norite, gabbro, pyroxenite, dolerite, basalt, rodingite, plagiogranite and chert, which are interpreted to represent a heavily tectonized and dismembered ophiolite (Aitchison et al., 1994). Relict blocks of harzburgite, along with exotic blocks of eclogite, blueschist, amphibolite, plagiogranite, basalt and chert are located in the serpentinitematrix melange; which arguably represents a tectonized harzburgite layer from the original ophiolite sequence. Age control on ophiolite genesis is provided by U-Pb dating of plagiogranite blocks (c. 530 Ma) in the serpentinite-matrix melange, which are interpreted to represent felsic differentiates formed during the crystallisation of the mafic rocks (Aitchison & Ireland, 1995). Whereas the samples dominantly have geochemical compositions that are comparable to MORB, one sample shows a considerable depletion in Nb and Ti and enrichment in Th and the light rare earth elements, which is consistent with genesis in a continental arc-type setting. The samples preserve two main metamorphic mineral assemblages, summarised as: (i) eclogitic garnet containing inclusions of omphacite, lawsonite and glaucophane that is surrounded by sodic augite, hornblende and actinilte in a groundmass of chlorite and albite, and; (ii) glaucophane, phengite and lawsonite in a groundmass of albite. Glaucophane grains within the latter assemblage commonly have cores of ferroan-hornblende, which are interpreted to have formed during an earlier, higher-temperature metamorphic stage. Pressure-temperature (P-T) constraints on the metamorphic evolution of the samples are calculated using optimal (THERMOCALC AvP-T) and conventional thermobarometry, and provide the following results: (i) eclogitic garnet and omphacite formed at conditions of 22.4±1.9 kbar and 556±23°C, which was followed by the growth of sodic augite at 7.5-10.0 kbar and 650-700°C, and; (ii) ferroanhornblende grew at 12.5-17.5 kbar and 620-700°C, which was followed by the growth of glaucophane, phengite and lawsonite at c. 7.5 kbar and 350-300°C. U-Pb (SHRIMP) and geochemical analysis of zircon from the sample containing the eclogite-facies mineral assemblage reveals a two-stage thermal history, defined by zircon growth and/or resetting at 115

SGTSG 2015: Riding the Waves 514±6 and 480±5 Ma. Trace element data from these zircons dominantly show flat heavy rare earth signatures, consistent with zircon growth in the presence of garnet (Rubatto et al., 2002). We therefore suggest that these two age populations correspond to the development of eclogite (514±6) and amphibolite (480±5) facies assemblages in the sample. 40Ar/39Ar dating of phengite from two samples preserving a lawsonite-blueschist assemblage yield well-defined single grain fusion ages of 482±1 Ma and 482±2 Ma. Given our temperature estimate calculated for the two samples recording the lawsoniteblueschist facies assemblage, we suggest that these ages approximate the timing of metamorphic recrystallization. Using these data, we test the applicability of the continuous vs. punctuated subduction models for the Tasmanides. Most notable are clear overlaps between the pressure-temperature-time histories and important evolutionary stages of the Tasmanides. Firstly, the timing of eclogite facies metamorphism (514±6 Ma) overlaps with the incipient stages of the Delamerian Orogen. Secondly, the recrystallization of high-pressure rocks at amphibolite and then blueschist facies conditions (485-480 Ma) is broadly coeval with the transition from the Delamerian to Lachlan orogens. Given these correlations, we suggest that the high-pressure rocks contained by the Peel-Manning Fault System were recrystallised during the Delamerian and-/-or Lachlan orogenic stages, and therefore define a link between all three orogenic stages. Consequently, we support a continuous subduction model for the Tasmanides. By contrast, the punctuated subduction model predicts that subduction complexes should be exhumed and accreted inboard of accreted magmatic arcs and terranes. Subduction complexes should therefore get progressively younger from west to east and be younger then the timing of arc accretion and subduction reversal. This model can only account for the high-pressure rocks in the PeelManning Fault System if these rocks formed in a Cambrian-Ordovician subduction zone that was unrelated and isolated from that responsible for the Delamerian and early Lachlan orogenic stages. Given the overlapping nature of the P-T-t histories recorded by the high-pressure rocks in the PeelManning Fault System and key evolutionary stages of the Tasmanides, we argue that this is not the case. References Aitchison, J.C., and T.R. Ireland (1995), Age profile of ophiolitic rocks across the Late Palaeozoic New England Orogen, New South Wales: Implications for tectonic models, Australian Journal of Earth Sciences, 42, 11-23. Aitchison, J.C., and S. Buckman (2012), Accordion vs. quantum tectonics: insights into continental growth processes from the Paleozoic of eastern Gondwana, Gondwana Research, 22, 674-680. Aitchison, J.C., M.C. Blake, P.G. Flood, and A.S. Jayko (1994), Paleozoic ophiolitic assemblages within the southern New England orogen of eastern Australia: Implications for growth of the Gondwana margin, Tectonics, 13, 1135-1149. Collins, W.J. (2002), Hot orogens, tectonic switching, and creation of continental crust, Geology, 30, 535-538. Rubatto, D. (2002) Zircon trace element geochemistry: distribution coefficients and the link between U-Pb ages and metamorphism. Chemical Geology 184:123-138.

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The influence of local neighbourhood on the dominance of deformation mechanisms and bulk rheology of polymineralic rocks: Examples from garnet, quartz and magnetite

SANDRA PIAZOLO , LIENE SPRUZENIECE , MANISH A MAMTANI , DARIA CZAPLINSKA AND LYNN EVANS 1

1,3

Australian Research Council Centre of Excellence for Core to Crust Fluid Systems/GEMOC, Department of Earth and Planetary Sciences, Macquarie University, NSW, Australia. Department of Geology & Geophysics, Indian Institute of Technology, Kharagpur-721302, West Bengal, INDIA, ijiamtan i @ je.g- iitkgp. ernet. in School of Earth, Atmosphere and Environmental Sciences, Monash University, Clayton, Australia 2

In order to derive the rheological behaviour of rocks and deformation conditions, geologists commonly use 'deformation mechanism maps". Such maps are based on experimental data of mainly monophase materials, where the dominance of a specific deformation mechanism is directly related to grain size, temperature, stress and strain rate. Using such deformation mechanism maps allows the geologist to estimate the strain rates and temperatures of deformed rock once the dominant deformation mechanism and grain size has been established. The dominant deformation mechanism is determined using microstructural "indicators". While the use of deformation mechanism maps is a powerful tool in nearmonophase materials such as quartzite and dunite, many natural rocks are polyphase rather than monophase. In this work we show several examples where the approach outlined above does not always hold true in the case of a polyphase rock. We show that the deformation mechanism by which a specific phase deforms is determined not by temperature, grain size and strain rate alone but, importantly, by the local neighbourhood. The neighbourhood, that is the phases surrounding a specific grain, influences local stress and strain heterogeneities and hence affects the deformation behaviour of the material. Furthermore, in a polyphase rock the difference in strength between different phases can directly aid the influx of fluids and/or melt through either "detachment" of a soft matrix from a central, Theologically strong grain or brittle fracturing of the strong phase. Fluid/melt influx in such "cavities" may have important consequences for the rheological behaviour during subsequent deformation. Here, we present data from magnetite bearing deformed granite, quartz-muscovite bearing shear zones developed in granodiorites and deformed garnet-omphacite eclogites. Microdynamic numerical simulations are used to assess the variations in differential stress magnitude in different geometric phase-neighbourhood configurations.

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Carbonatisation processes in tecto-sedimentary breccias drilled at ODP Sites 1068 and 1070 along the Zone of Exhumed sub-Continental Mantle (ZECM) offshore Iberia V . FAUCHEUX1' S. PICAZO 2 , G . MANATSCHAL1 AND S. PIAZOLO3 1

Ecole et observatoire des Sciences de la Terre, Institut de Physique du Globe de Strasbourg - CNRS UMR7516, Universite de Strasbourg, 1 rue Blessig, F-67084 Strasbourg Cedex, France University of Lausanne, Institut des Sciences de la Terre, Batiment Geopolis CH-1015 Lausanne, Switzerland 3 Department of Earth and Planetary Sciences - Macquarie University, NSW 2109, Australia 2

Extensional detachment faults have been drilled during ODP Leg 173 in the Zone of Exhumed subContinental Mantle (ZECM) offshore Iberia. The drilled fault zones are formed by serpentinized mantle rocks showing a strong cataclastic overprint that ranges upsection into fault gouges. It is overlain by tectono-sedimentary breccias consisting of reworked footwall rocks. The fault rocks and breccias undergo a partial carbonatisation with a fluid-assisted process forming so called ophicalcites or ophicarbonates (OC). These ophicarbonates, commonly observed in Alpine type ophiolites, have been interpreted to form during or after mantle exhumation at the seafloor. Two types of OC have been defined in the Alps: a first type (OC1) related to brittle deformation and carbonatisation, and a second type (OC2) due to reworking and sedimentation of OC1 (Bernoulli et Weissert, 1985; Lemoine et al., 1987). ODP drilling along the Iberian margin showed that OC occur at the top of exhumed mantle and may have formed during a late stage or after its exhumation at the sea-floor (Evans et al, 1988; Milliken et al., 1996; Agrinier et al., 1996). The aim of this study was to understand the link between the late-evolution of mantle exhumation, the formation of tectono-sedimentary breccias, fluid flow and the carbonatisation processes. In contrast to the Alpine examples, the rocks drilled from the Iberia margin were not affected by a subsequent orogenic event. For our study, we focused on two ODP Sites drilled across the Iberia rifted margin. ODP Site 1068 is located in the most proximal part of the ZECM near exhumed crustal rocks; whereas ODP Site 1070 is located at the most distal part of the ZECM near first oceanic crust (Whitmarsh et al, 1998). Our study provides a detailed description of the tectono-sedimentary breccia and the underlying fault rocks, with a particular focus on the OC and their relation with the related rocks and breccias. We made macro- and microscopic observations of drilled samples and performed geochemical analysis to constrain the conditions under which carbonatisation occurred. Cold Cathodoluminescence (CL) enabled us to characterize the Mn/Fe ratio in calcite families to constrain variations of oxidation-reduction fronts within the carbonates. We also used Electron Micro Probe (EMP) to analyse the compositional distribution of elements associated to carbonatisation. Our observations point to an evolution from an initial tectonic breccia (i.e. cataclastic and gouge) recorded at the base of the succession to a predominantly sedimentary breccia toward the top of the succession. In some intermediary samples, clasts are polymictic and the serpentine/chlorite/argileous cement is foliated, suggesting a tecto-sedimentary origin. The highly coherent sub-vertical orientation of fractures shows that the principal stresses were sub-vertical, which is in agreement with an extensional tectonic context. These observations are not compatible with a purely sedimentary or tectonic origin of the breccias but suggest that most breccias are the result to tectono-sedimentary processes. Thus, these breccias cannot be characterized as either OC1 or OC2 but seem to be more complex and hybrid. It is also important to note that at ODP Site 1068, no mantle derived clasts have been observed, while veins filled by serpentine are observed to cross-cut all calcite generations. Chemical analyses (EMP) were carried out on successive calcite families defined by CL. Carbonation succession can therefore be defined as (1) being associated to replacement of breccia with thin growing patches, (2) precipitations in multiple generations of thin and sub-vertical jig-saw fractures, and (3) resulting from the development of larger fractures in the same orientation associated to coarser calcite precipitation. Although at ODP Site 1068 the last stage shows an increase in the oxidation conditions (i.e. a bigger Mn/Fe ratio and high presence of oxides), the CL study at samples from ODP Site 1070 shows a global decrease of this oxidizing state from the first to the last stage. Just the last precipitation event shows an increase like the one described from ODP Site 1068. We analysed the last stage of the carbonatisation history recorded at ODP Site 1070. In this site, sedimentary injections are followed by 118

SGTSG 2015: Riding the Waves successive and most complete calcite-precipitation events that are recorded in multiple veins. A decrease in the MgO content in carbonate phases toward the centre of veins marks a possible decrease in temperature through time (Burton E.A. et al, 1987 ; Morse J. W. et al., 1987 ; Lopez O. et al., 2009). The last precipitation event shows a high increase in the concentration of MnO in the carbonate phases and therefore of the Mn/Fe ratio confirming more oxidizing conditions. The overall results are coherent with the idea that carbonatisation occurred near or at the seafloor, but was enough rapid to interact with the extensional detachment faults and/or hydrothermal zones. We can make the hypothesis that the decrease in temperature and the more oxidizing last stage observed at ODP Sites 1068 and 1070 shows the evolution from an exhumation system to a more open depositional environment. In this study, we followed the carbonatisation processes within a system that was related to mantle exhumation within the ZECM. The results show that there is a clear link between exhumation, sedimentation and fluids most likely related to hydrothermal systems. The results confirm the use of OC as markers of complex tectono-sedimentary processes under the presence of fluids during mantle exhumation. This study also shows the highly variable environments related to calcite precipitation and its potential implication for the C0 2 storage in these yet little constrained environments. References Agrinier P., Cornen G., Beslier M.-O., 1996. Mineralogical and oxygen isotopic features of serpentinites recovered from the Ocean/Continent transition in the Iberia Abyssal Plain. Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 149, 541-552. Bernoulli D., Weissert H., 1985. Sedimentary fabrics in Alpine ophicalcites, South Pennine Arosa zone, Switzerland. Geology, Vol.13, 755-758, doi: 10.1130/0091-7613(1985) 13<755:SFIAC)S>2.0. CO;2 Burton E. A., Walter L. M., 1987. Relative precipitation rates of aragonite and Mg calcite from seawater: Temperature or carbonate ion control? Geology, Vol. 15-2, 111-114. doi: 10.1130/0091-7613(1987)15 <111:RPROAA>2.0.CO;2 Evans C. A., Baltuck M., 1988. Low-temperature alteration of peridotite, Hole 637A. Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 103, 235-240. Lemoine M., Tricart P., Boillot G., 1987. Ultramafic and gabbroic ocean floor of the Ligurian Tethys (Alps, Corsica, Apennines): In search of a genetic model. Geology, Vol. 15, 622-625, doi: 10.1130/00917613(1987) 15<622:UAGOFO>2.0.CO;2 Lopez O., Zuddas P., Faivre D., 2009. The influence of temperature and seawater composition on calcite crystal growth mechanisms and kinetics: Implications for Mg incorporation in calcite lattice. Geochimica et Cosmochimica Acta, Vol. 73, 337-347. doi:10.1016/j.gca. 2008.10.02 Milliken K.L., Morgan J.K., 1996. Chemical evidence for near-seafloor precipitation of calcite in serpentinites (site 897) and serpentinite breccias (site 899), Iberia Abyssal Plain. Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 149 Morse J. W., Wang Q., Tsio M. Y., 1987. Influences of temperature and Mg:Ca ratio on CaC03 precipitates from seawater. Geology, Vol. 25-1, 85-87. doi: 10.1130/0091-7613(1997)025<0085: IOTAMC>2.3.CO;2 Whitmarsh R.B., Beslier M.-O., Wallace P.J., et al., 1998. Proceedings of the Ocean Drilling Program, Initial Reports, leg 173, chapter 7-9, p.l63-218/347-433;p.265-294/463-493.

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Rollback and extension in Eastern Indonesia

JONATHAN M . POWNALL , GORDON S. LISTER AND ROBERT HALL 1

Research School of Earth Sciences, Australian National University, Canberra, Australia

SEAsia Research Group, Department of Earth Sciences, Royal Holloway University of London, UK Eastern Indonesia (Fig. 1) presents a rare snapshot in time of a complex tectonic system responding to the ongoing collision of two continents. It is an ideal region to investigate the mechanics of active subduction rollback and the tectonic controls of crustal metamorphism. However, many aspects of the geology of Eastern Indonesia, including some that are fundamental to deciphering its tectonic evolution, either remain unknown or are the subject of conflicting interpretations. The 180° orocline of the Banda Arc, from Timor round to Seram (Fig. 1), exposes upper-mantle rocks and granulite-facies migmatites in its northern portion (Pownall et al., 2013, 2014; Pownall, 2015), and encloses the 7.4 km deep forarc basin of the Weber Deep (Bowin et al., 1980; Pownall et al., in review). The formation of both these features required considerable extension. One interpretation is that extension in both instances may have been driven by the rolling back Banda Slab (Spakman & Hall, 2010; Hall, 2011; Hall & Spakman, 2015), which exhumed the upper mantle beneath the northern part of the Arc, and 'rolled open' the Weber Deep (Pownall et al., in review). The Weber Deep—a 7.4 km-deep depression within the tightly curved Banda Arc of eastern Indonesia (Fig. 2)—is Earth's deepest forearc basin, and also the deepest point of the Earth's oceans not within a trench. Several models have been proposed to explain the tectonic evolution of the Banda Arc in the context of the on-going (c. 23 Ma-present) Australia-SE Asia collision, but no model explicitly accounts for how the Weber Deep achieved its anomalous depth. Here we propose that the Weber Deep opened due to forearc extension driven by eastward subduction rollback of the Banda Arc. Considerable lithospheric extension in the upper plate was accommodated by a major, previously unidentified low-angle normal fault we name the 'Banda Detachment', which bounds the eastern extent of the basin (Fig. 3). High-resolution (15 m) multibeam bathymetry data (Fig. 2) reveal that the Banda Detachment is exposed for over 40 km down-dip and over 600 km laterally, thereby having produced the largest bathymetric expression of any fault discernable in the world's oceans. The Banda Arc serves as a potential modern analogue for highly extended terranes preserved in the older geological record that may also have 'rolled open' behind migrating subduction zones. 2

Figure 1: Tectonic map of Eastern Indonesia (from Pownall et al., in review). The Banda Detachment bounds the eastern edge of the 7.4 kmdepth Weber Deep. Red triangles represent active volcanoes. Black polygon indicates the location of Fig. 2A. The digital elevation model uses topographic data (30 m resolution) acquired by the ASTER satellite (http: //asterweb.j pi .nasa. gov) and bathymetry data (900 m resolution) from the GEBCO project (http://gebco.net). FZ— Fault Zone; UHT—Ultrahightemperature metamorphic rocks.

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SGTSG 2015: Riding the Waves Figure 2: Digital elevation model of the Weber Deep and Aru Trough (from Pownall et al., in review). Perspective view, looking from south, of the Weber Deep (vertically exaggerated bathymetry) showing the topographic expression of the Banda Detachment. The pinnacle reef structures (marked) could alternatively be submarine volcanoes. Bathymetric dataset (15 m resolution) used for both parts of the figure courtesy of TGS and GeoData Ventures.

Volcanic Arc

Banda Detachment

ROLLBACK

Sula Spur (Australian crust) Kai Kai Kecil Besar

Seram-Timor fold-and-thrust belt 100 km L

Figure 3: Geological crosssection (true scale) through the Weber Deep and Aru Trough, from Pownall et al. (in review). Crustal thicknesses are approximate. L—base of lithosphere.

References Bowin, C., Purdy, G.M., Johnston, C. et al., 1980. Arc-continent collision in Banda Sea region. AAPG Bulletin, 64, 868-915. Hall, R., 2011. Australia-SE Asia collision: plate tectonics and crustal flow. In: The SE Asian Gateway: History and Tectonics of the Australia-Asia Collision (eds Hall, R., Cottam, M.A. & Wilson, M.E.J.), Geological Society of London, Special Publication, 355,75-109. Hall, R. & Spakman, W., 2015. Mantle structure and tectonic history of SE Asia. Tectonophysics. doi: 10.1016/j.tecto.2015.07.003 Pownall, J.M., 2015. UHT metamorphism on Seram, eastern Indonesia: reaction microstructures and P-T evolution of spinel-bearing garnet-sillimanite granulites from the Kobipoto Complex. Journal of Metamorphic Geology 33, doi: 10.1111/jmg. 12153. Pownall, J.M., Hall, R. & Watkinson, I.M., 2013. Extreme extension across Seram and Ambon, eastern Indonesia: evidence for Banda slab rollback. Solid Earth 4, 277-314. Pownall, J.M., Hall, R., Armstrong, R.A. & Forster, M.A., 2014. Earth's youngest known ultrahigh-temperature granulites discovered on Seram, eastern Indonesia. Geology 42, 279-282. Pownall, J.M., Hall, R. & Lister, G.S., (still) in review. Rolling open Earth's deepest forearc basin. Submitted to Geology. Spakman, W. & Hall, R., 2010. Surface deformation and slab-mantle interaction during Banda arc subduction rollback. Nature Geoscience, 3, 562-566.

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Itinerant Siberia during the Neoproterozoic NADEZHDA PRIYATKINA 1,2 , WILLIAM J. COLLINS1, ANDREI K. KHUDOLEY2 AND DMITRY ZASTROZHNOV 3 ' 4 J

New South Wales Institute of Frontiers Geo science, University of Newcastle, Newcastle, NSW 2308, Australia Institute of the Earth Sciences, St. Petersburg State University, 7/9 University Nab., St. Petersburg 199034, Russia 3 All Russian Geological Research Institute (VSEGEI), Sredny Prospect 74, St. Petersburg, 199106, Russia 4 University of Oslo, The Centre for Earth Evolution and Dynamics (CEED), Sem Scelands vei 2A, 0371, Oslo, Norway 2

Siberia's position in Neoproterozoic continental reconstructions is problematic, largely because of a lack of geological data establishing its affinity to other continents. The margins of NE Laurentia, eastern Baltica and northern and western Siberia that probably were located along the periphery of Rodinia are surrounded by Neoproterozoic to Early Cambrian orogenic belts, but how they related geodynamically remains enigmatic. To establish cratonic affinities of Neoproterozoic arc terranes currently located near the northern (Taymyr belt) and western margins (Yenisey Ridge, East Sayan) of the Siberian Craton, we compared U-Pb-Hf detrital zircon signatures from pre-accretionary Mesoproterozoic to post-accretionary Neoproterozoic and Cambrian sediments. The detrital signature of Siberia is significantly different from that of Laurentia and Baltica, because the Siberian cratonic basement incorporates Archean and Paleoproterozoic but not Mesoproterozoic crust (e.g. Rosen, 2002). Neoproterozoic to Early Cambrian sediments of both western and northern Siberian margins reveal ca. 980-800 Ma and ca. 750-600 Ma zircon populations, broadly consistent with ages of the magmatic belts recognized within Taymyr, Yenisey Ridge (Vernikovsky et al., 2004) and East Sayan (e.g. Turkina et al., 2007). Both margins were surrounded by continental arcs at ca. 900-800 Ma, as evidenced by large vertical Hf isotopic arrays extending sHf (T) values from 12 to -10. A peri-Siberian origin for the Early Neoproterozoic Taymyr arc can be inferred from inheritance of 2.15-2.0 Ga juvenile zircon population, which is derived from Siberian basement. The 1.86 Ga peak in detrital age spectra of post-accretionary sediments from near Yenisey Ridge provides a clear link to granitoids from the Angara belt along the western cratonic edge. However, another accretionary event that ocurred at ca. 600 Ma introduced a 750-600 Ma arc-related zircon population and an exotic Mesoproterozoic detrital signature to the platform cover of northern Siberia. This U-Pb-Hf detrital signature is similar to the coeval detrital record of eastern Baltica, supporting juxtaposition of the two margins during the Timanian orogeny at the Precambrian/Paleozoic transition. By contrast, all post-accretionary sediments near the Yenisey Ridge and East Sayan inherit detritus derived solely from the Siberian cratonic basement. Integration of U-Pb-Hf detrital zircon data and available geological records suggest that the northern and western margins of Siberia became active at ca. 970 Ma, likely as an extension of coeval Valhalla orogen that formed on the margin of NE Laurentia (Cawood et al., 2010). Possibly, at ca. 800-750 Ma, subduction rollback along this margin of Rodinia promoted the rift and drift of Siberia from northern Laurentia and ultimately caused its collision with Baltica at -600 Ma. In this model, Neoproterozoic crust that formed along this subduction zone currently occupies the northernmost (Arctic) parts of Siberia, Baltica and Laurentia, commonly referred to as Arctida. By contrast, the western margin of Siberia remained a part of a peri-Siberian accretionary orogen until the Early Paleozoic. References Cawood, P.A., Strachan, R., Cutts, K., Kinny, P.D., Hand, M., Pisarevsky, S., 2010. Neoproterozoic orogeny along the margin of Rodinia: Valhalla orogen, North Atlantic. Geology 38, 99-102. Li, Z.-X., Evans, D.A.D., Halverson, G.P., 2013. Neoproterozoic glaciations in a revised global palaeogeography from the breakup of Rodinia to the assembly of Gondwanaland. Sedimentary Geology 294, 219-232. Metelkin, D.V., Vernikovsky, V.A., Matushkin, N.Y., 2015. Arctida between Rodinia and Pangea. Precambrian Research 259, 114-129.

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Pisarevsky, S.A., Gladkochub, D.P., Konstantinov, K.M., Mazukabzov, A.M., Stanevich, A.M., Murphy, J.B., Tait, J.A., Donskaya, T.V., Konstantinov, I.K., 2013. Paleomagnetism of Cryogenian Kitoi mafic dykes in South Siberia: Implications for Neoproterozoic paleogeography. Precambrian Research 231, 372-382. Rosen, O., 2002. Siberian craton—a fragment of a Paleoproterozoic supercontinent. Russian Journal of Earth Sciences 4, 103-119. Shatsillo, A., Pavlov, V., Didenko, A., 2006. Paleomagnetism of Vendian rocks in the southwest of the Siberian Platform. Russ. J. Earth Sci 8, 1-30. Turkina, O.M., Nozhkin, A.D., Bayanova, T.B., Dmitrieva, N.V., Travin, A.V., 2007. Precambrian terranes in the southwestern framing of the Siberian craton: isotopic provinces, stages of crustal evolution and accretion-collision events. Russian Geology and Geophysics 48, 61-70. Vernikovsky, V.A., Vernikovskaya, A.E., Pease, V.L., Gee, D.G., 2004. Neoproterozoic Orogeny along the margins of Siberia. Geological Society, London, Memoirs 30, 233-248.

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Coupling deep seismic with 40Ar-39Ar thermochronology: reinterpretation of the structural evolution of the Musgrave Province, Central Australia RAPHAEL QUENTIN DE GROMARD1, HEATHER M . HOWARD1, ROBERT H. SMITHIES1, FRED JOURDAN2, CHRISTOPHER L. KIRKLAND2 AND MICHAEL T . D . WINGATE 1 Geological Survey of Western Australia, 100 Plain Street, East Perth, WA, 6004, Australia Department of Applied Geology and JdL Centre, Curtin University, Perth, WA, 6845, Australia

Coupling deep seismic imaging with targeted fabric geochronology is a powerful tool to reconstruct the evolution of ancient deformation belts. Such coupled examination using the Yilgarn Craton-Officer Basin-Musgrave Province (YOM) deep seismic reflection line (11GA-Y01) and 40 Ar- 39 Ar geochronology, prompts radical reinterpretation of the structural evolution of central Australia. The Mitika Fault is interpreted as a steeply south-dipping reverse fault that offsets the Moho whereas the Woodroffe Thrust is reinterpreted as a shallow south-dipping thrust that soles onto the Mitika Fault. Further north, a strongly reflective multiply duplexed upper crust is the crustal manifestation of the Petermann Nappe Complex. West-directed thrusts and west-verging overturned folds of the Mitika area, south of the Mitika Fault, are revealed in seismic images as sub-horizontal reflectors as the YOM line runs sub-parallel to these structures. Further south, the Talbot sub-basin is dominated by a regional scale open fold and south-directed reverse faults. Ten muscovite and hornblende 40 Ar- 39 Ar thermochronology samples constrain the timing of movement along the aforementioned key structures of the west Musgrave Province (Fig. 1). Kyanite-bearing metarhyolites of the Talbot sub-basin yielded a c. 715 Ma plateau age interpreted to date the time of cooling below 400 ± 50 °C (retention temperature of argon in muscovite) during regional-scale folding, south-directed reverse movement, and uplift of the southern end of the west Musgrave Region in a N-S compression setting. Alternatively, this result could reflect incomplete isotopic resetting during the Petermann Orogeny. North-trending structures and metamorphism in the Mitika area are interpreted to have occurred at c. 630-615 Ma, as dated through U-Pb analyses of metamorphic zircon overgrowths in upperamphibolite facies rocks and by the age of muscovite crystallization in the surrounding greenschist facies rocks (Fig. 1). These ages may represent a distinct and previously unrecognised E-W shortening event prior to dextral transpressive deformation related to the Petermann Orogeny. Ar-Ar dates ranging from 590 to 565 Ma indicate progressive exhumation related to the Petermann Orogeny along the Mitika Fault and the Woodroffe Thrust. Granulite facies rocks of the Wanarn area, cooled below 400 ± 50 °C at c. 590 Ma during exhumation along a northeast-directed thrust splay of the Mitika Fault synchronously with the exhumation of upper-amphibolite facies rocks in the Mitika area (Fig. 1). At the same time, the northern end of the Wanarn area passed through the 550 ± 50 °C isotherm (temperature of argon retention in hornblende) during exhumation along the Woodroffe Thrust revealing an in-sequence northward migration of the Petermann thrust system. The Wanarn area was then cooled below 400 ± 50 °C at c. 565 Ma during ongoing exhumation along the Woodroffe Thrust.

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Ar-Ar ages (Ma) •

Cooling < 550 ± 50 °C (Hbl)

•

Cooling < 400 ± 50 °C (Ms)

Crystallization (Ms)

U-Pb ages (Ma) O

Metamorphic zircon

MITIKA AREA-

Talbot sub-basin

20 km

NORTH AUSTRALIAN CRATON

Paleozoic Basins

Musgrave Orogeny I Mt West r Orogeny |_|

WEST AUSTRALIAN CRATON

Talbot r Warakurna sub-basin I Supersuite Bentley I Basin Pitjantjatiara Supersuite (1220-1150 Ma)

1090-1040 Ma Giles Event

Rudall Province Hamersley

Ramarama Basin I Wankanki Supersuite | (1345-1293 Ma) Warlawurru Supersuite (1575 Ma)

ALBANY-FRASER AUSTRALIAN

OROGEN

CRATON

3 1 Proterozoic basin | Proterozoic orogen | Archean craton Proterozoic craton margin

Figure 1: Interpreted bedrock geology map of the western end of the Musgrave Province showing the location of the YOM seismic line (11GA-Y01) and the location of samples used for Ar-Ar thermochronology. Inferred blocking temperatures of the argon system for hornblende (Hbl) and muscovite (Ms) are indicated in the inset. The location of the study area is shown on the tectonic map of Australia.

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Tectonic overpressure in continental collision and the origin of ultra-highpressure rocks KLAUS REGENAUER-LIEB 1 , GIDEON ROSENBAUM 2 , ROBERTO F. WEINBERG 3 AND GIANRETO MANATSCHAL 4 1

School of Petroleum Engineering, UNSW Australia, Sydney, 2052, Australia School of Earth Sciences, The University of Queensland, Brisbane, Queensland 4072, Australia School of Geosciences, Monash University, Clayton, Victoria 3800, Australia 4 Institut de Physique du Globe de Strasbourg, IPGS—UMR 7516, CNRS—Universite de Strasbourg, 1 rue Blessig 67084 Strasbourg cedex, France 2 3

Ultra-high-pressure (UHP) rocks in collisional orogens are typically interpreted to indicate burial of rocks to great depths (>100 km) assisted by subduction. The assumption behind this explanation is that pressures recorded in metamorphic assemblages represent lithostatic pressures. Over the last three decades a large number of occurrences of UHP mineral assemblage (e.g. coesite, microdiamonds) have been reported from metasedimentary continental rocks in collisional orogens. These indicate pressures of 4-5 GPa, and the accepted paradigm for their formation is that they were physically taken to depths where such lithostatic pressures existed. In other words, the metamorphic pressure (p) is translated directly into depth through the assumption that it records lithostatic pressure (pLith = pgh). Under the current paradigm, UHP rocks require a mechanism that can take them to depths beyond 100 km and bring them back to the surface. Tectonic overpressure is commonly dismissed on the grounds that ductile metamorphic rocks cannot possibly support stresses larger than their yield strength. There are certain rheological and geometrical conditions, however, that allow an increase in pressure beyond the strength limit of rocks, for example if these rocks are confined and "protected" by stronger walls. Therefore, the debate on tectonic overpressure revolves around questions on how strong the confinement is, what happens when it breaks, and how it can maintain higher pressures during metamorphism in non-lithostatic conditions. We bring a fresh element to this discussion by illustrating classical concepts of contact mechanics (Johnson, 1985) through numerical models that explore the implications of overpressure during continental collision. Using well-established scale-invariant concepts from this important discipline in applied mechanics, we derive upper and lower bounds of tectonic pressures, ptecU during continental collision and show that substantial overpressures can be generated and possibly explain the origin of UHP rocks. The presented models also account for 3-D and plate boundary effects, and demonstrate that overpressures associated with vertical thickening and strike-slip movements are much larger than expected from simple Andersonian criteria. This finding has extensive implications to how we interpret pressure determinations from mineral assemblages. Most significantly, overpressures in continental collision (e.g. Alps, Himalayas) could account for UHP metamorphism without the need to bury and exhume rocks from extreme depths. Contact mechanics is concerned with the stresses and deformation that arise when two solid bodies are brought into contact (Johnson, 1985). Unlike the Andersonian approach, contact mechanics takes into account the full 3D geometry of the problem, including thickness variations and the distance and nature of open boundaries. The contact between these surfaces, i.e. contacts between plates, blocks, or grains, can be conforming or non-conforming, depending on whether or not they fit exactly before deformation. Geometric complexity of the contact surface leads to situations of confined plastic flow where areas of plastic deformation are trapped in an elastic stress cage, which explains the large overpressures that can be achieved. The scale invariance of the theory also suggests a possible additive effect of contact pressures through locally raising the pressures from large scale continental collision, through regional interactions of embedded strong blocks to the well known indentation of individual minerals as can be seen in undulose extinctions in thin sections. All of these scales are additive and can substantially boost the theoretical pressure from a single value of tectonic overpressure of 5 times the yield stress in shear of the material to up to 15 times the yield stress in shear if a constructive addition of multiscale geometric 126

SGTSG 2015: Riding the Waves interaction occurs around e.g. the corner point of an indenter. Numerical calculations show that overpressure peaks in the proximity of such singularity (corner) points of the indenter, giving rise to local occurrences of UHP metamorphism, such as in the Dora Maira Massif in the European Alps. Elsewhere, overpressure is less extreme and may have contributed to increased metamorphic pressures but has not necessarily resulted in UHP metamorphism. Overpressure during indentation is scaled by the yield stress, which peaks at the brittle-ductile transitions. This stress can be expected to be particularly high in cases such as the Dora Maira Massif due to the existence of a cool, thermally equilibrated, exhumed subcontinental mantle lithosphere. Another interesting outcome is that our models predict a relatively fast flow of material through the corner point (slightly faster than the convergence velocity of the indenter). This flow is characterized by rapid pressurization as material enters the high-pressure zone, followed by rapid decompression as material leaves it, compatible with the pressure drop inferred in nature. This prediction removes the requirement of intervening rapid phases of exhumation in subduction-driven model for UHP rocks, and is consistent with an overall convergent setting in a spatially restricted ocean. Our models also predict that downward from the brittle-ductile transition, into the hotter crust in overpressured zones, there should be a steep negative pressure gradient. References Johnson, K. L., 1985, Contact Mechanics, Cambridge, UK, Cambridge University Press, 457 p.:

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AusGeol.org - A Virtual Library of Australia's Geology MICHAEL ROACH 1 , STEPHEN COX 2 , PENNY KING 2 , CHARLES VERDEL 3 , KEVIN WELSH 3 , ANNETTE GEORGE 4 AND SANDRA MCLAREN 5

School of Physical Sciences, University of Tasmania Research School of Earth Sciences, Australian National University School of Earth Sciences, The University of Queensland 4 School of Earth and Environment, The University of Western Australia 3 School of Earth Sciences, The University of Melbourne 2

Recent advances in digital imaging techniques now facilitate simple, cost-effective, generation of digital representations of geological features. These geological visualisations can be utilised to augment conventional geological educational programs and also provide resources for professional education and research. A major collaborative project to generate a virtual library of Australia's geology is currently underway. The project is funded by the Australian Government Office for Learning and Teaching, and supported by a consortium of universities, state and federal geoscience agencies. The visualisations generated in this project are freely available as Open Educational Resources and are distributed through the AugGeol.org website. This presentation will briefly describe visualisation generation methods, showcase selected visualisations, outline the current status of the digital library and describe plans for future enhancement of this educational resource. Digital photogrammetry, based on 'structure from motion' algorithms, can be easily undertaken using standard digital photographic equipment to generate photo-realistic texture-rendered digital models of geological outcrops, digital elevation models and orthoimagery. If appropriate control points are established in the field during photography, then photogrammetric models can be fully coordinated in three dimensional space and hence capture all the geometric and textural features of the real outcrop. Photgrammetry can be undertaken at any scale. Imagery for large-scale features (>50m) are best acquired using UAVs. Medium-scale (0.5-50m) visualisations can be generated from UAV, pole and terrestrial photography. Small scale features (0.05 to 0.5m) are acquired using macro photographic methods. Multi-resolutional (gigapixel) photography enables depiction of outcrop features at both small and large scales within a single image. These enormous images can be generated by stitching together hundreds of conventional photographs acquired using a high power zoom lense and usually a specialised robotic panorama camera head. Multi-resolutional images can also be generated as an additional product of the photogrammetric process. These huge images are stored and visualised using a multi-resolution tiling technique, similar to that employed by Google Earth, or alternatively in a single file using wavelet compression techniques (ECW format). High resolution, full spherical panoramas are easily generated using a tripod, panorama head and a good DSLR camera with a fisheye lense. These images, when viewed in 3D viewing software, enable a seamless, zoomable, 360 degree view of a locality from a single viewpoint. Multiple full spherical panoramas can be linked together, and with other visualisations and additional data, to generate walkthrough tours of geological sites. All visualisations we produce have coordinates (latitude and longitude for the site at minimum) and have associated geological attributes that encode stratigraphic, lithological, age and mineralogical characteristics. The AusGeol.org site delivers visualisations via a web mapping platform that currently provides a spatial view of the distribution of visualisations with features displayed on the basis of attribute searches (Figure 1). Tabular display of data based on search criteria will also be implemented. All visualisations are available as downloadable files for display using local hardware and software (Figure 2). We are currently developing open-access software that will enable annotation and geometric analysis of 3D models. This software will allow measurement of the orientation of planar and linear features on fully coordinated models and hence structural analysis of the virtual models. We will use the AusGeol visualisations and these new tools to develop educational resources for Tertiary Earth science education that will augment, but not replace, conventional field based geoscience 128

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education. Field and structural geology will be a particular focus of educational resource development but resources in other areas such as sedimentology and volcanology will also be developed. The ultimate objective of the AusGeol project is to generate, and freely disseminate, visualisations and educational resources that document the important and iconic aspects of Australia's geology. This will be accomplished, in-part, by targeted field data acquisition by collaborating organisations. However, the task of recording geological features across the whole continent really requires a collaborative effort by the entire Earth Science community to generate a truly representative and comprehensive resource. Contributions of imagery and visualisations to the database are welcome from all geoscientists.

Figure 1: Extracts from the AusGeol.org web map interface. The image on the left shows the distribution of visualisations is SE Australia (September 2015) with the number in each circle showing the number of visualisations at each locality. The Image on the right shows a thumbnail of a visualisation at Cape Conran in Victoria

Figure 2. The 3D texture-rendered model Conranl7 downloaded from the Ausgeol.org site as a 3D PDF and visualised in Adobe Acrobat.

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Ore shoot emplacement: differentiating architectural from syndeformational controls

TONY ROACHE * 1

AngloGold Ashanti Australia Ltd, Lvl 13, 44 St Georges Tee, Perth, WA, 6000, Australia.

* troache@anglogoldashanti.com

Structural controls on orogenic-style mineralisation is integral to the understanding of their formation, but depending on interpretation of the structural, hydrothermal and stratigraphic framework can lead to significantly contrasting geological - and exploration - models. Pre-existing structural architecture is an important element in the creation of giant ore bodies, but in many cases requires further discussion as to how mineralised fluids are localised within particular architectures. The classical source-conduittrap mechanism in whatever variation requires significant stimulation - through structural and chemical means - at the depositional site to allow fluids to precipitate metals, which is why the role of active deformation has to be re-addressed in many examples ore deposit formation. This paper will present two examples - already discussed in this context within the literature - that aim to show active deformation including the intersection of structure/s within an existing architecture is in most cases the dominant factor in the establishment of ore shoot geometry. The New Holland gold deposit in the Eastern Goldfields Superterrane of the Archaean Yilgarn Craton, Western Australia, is hosted within a turbiditic sedimentary succession. Mineralisation is hosted by quartz-rich tension vein arrays predominantly contained within a single N-S trending, subverticallydipping sandstone unit that is unconformably in contact with siltstone on its eastern margin. Given the arrangement of gold lodes periodically distributed along the length of the sandstone unit, in two dimensions it may appear that the coarse sandstone fraction acted as the conduit for focussing mineralised fluid from depth (Fig. 1). However, as mineralisation is restricted to a short strike length of the sandstone, a previously undocumented set of westerly-dipping shear zones that bound the northern and southern mineralised limits provide the essential architectural control in the third dimension. Another potential architectural control on the position of mineralisation at the Wallaby gold deposit, also in the Eastern Goldfields Superterrane, involves a pipe-like distribution of syenite dykes and amphibole-magnetite-epidote-calcite (AMEC) alteration that was overprinted by gold-stage alteration (e.g. Salier et al., 2004). Long-lived N-S trending structures including Thet's and Slaughteryard Faults that bound the E-W limits of the syenite dykes, in conjunction with a significant E-W magnetite component of the AMEC alteration that is open to the west, combine to suggest at least two structural sets resulted in the linear dimension of magmatic and hydrothermal alteration bodies (Roache et al. 2010). Furthermore, the AMEC is not a simple, single-phase alteration assemblage, but a series of overprinting, composite assemblages with intermediate isotopic signatures at the juncture of chemically distinct, structurally-controlled end-member alteration types (Roache et al. 2010).

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w EMU SHEAR ZONE SSF/SSL SSF/SSL

Magmatic / metamorphic fluid source at depth

Ultramafic Volcanic Rocks

Fluid pressure < lithostatic pressure .-.no brittle fracture of fine grained sandstone (SSF) to siltstone (SSL) and medium to very coarse grained sandstone (SSVC)

Not to Scale

Figure 1: Schematic geological cross section across the New Holland deposit showing travel direction of interpreted mineralised fluid up the sandstone units from basal shear zone (modified from Ackroyd 2001). References

Ackroyd, B., 2001, Hydrothermal alteration and gold mineralisation at the New Holland gold deposit, Leinster Gold Camp, Western Australia: University of Western Australia, BSc Honours thesis (unpublished). Roache, T.J., Walshe J.L. and Huntington, J.F., 2010, On-Site Validation and Implementation of New Hylogging Technologies - Technology Transfer and Re-Skilling, M400 final report, Minerals and Energy Research Institute of Western Australia, pp. 81. Salier, B. P., Groves, D. I., McNaughton, N. J. and Fletcher, I. R., 2004, The world-class Wallaby gold deposit, Laverton, Western Australia: An orogenic overprint on a magmatic-hydrothermal magnetite-calcite alteration pipe?: Mineralium Deposita, v. 39, p. 473 - 494.

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3D structural architecture of the western Tamworth Belt, southern New England Orogen: regional scale controls on hydrothermal mineral systems JAMIE A ROBINSON AND GLEN PHILLIPS Geological Survey of New South Wales, PO Box 344, Hunter Region MC, NSW 2310

The spatial distribution of mineral systems in the western Tamworth Belt of the southern New England Orogen is controlled by proximity to faults that extend down to an east- or northeast-dipping basal thrust (the Hunter-Mooki Fault System). In the Rocky Creek Block, most hydrothermal mineral systems are localised along, or immediately surrounding, the Peel-Manning Fault System. In constrast, within the Rouchel and Gresford blocks, most hydrothermal systems occur in linear clusters up to 60 km south of the Peel-Manning Fault System. Mapped geology shows a significant change in structural character from the Rocky Creek Block to the Rouchel and Gresford blocks. Here, we examine the distribution of the hydrothermal systems in relation to the recently completed 3D model of the western Tamworth Belt, and attempt to recognise structural relationships that shed light on how changes in structural character influence regional fluid flow and mineralisation. As part of the Geological Survey of New South Wales (GSNSW) 3D mapping program, a regional structural-stratigraphic model has been developed to a depth of 30 km for the western Tamworth Belt. The belt is bound by the crustal scale Hunter-Mooki and Peel-Manning Fault systems (Figure 1), which together form a wedge that hosts deformed Devonian to Permian rocks. The model consists of broad lithological volumes representing the Devonian, Devonian-Carboniferous, Carboniferous and Permian rocks that are folded and offset by numerous second and third order fault systems. The model is based on a series of 2D cross-sections developed through integration of the surface mapping, 16 reflection seismic profiles, as well as magnetic and gravity data. One of the key interpretations in this work is that the Peel Fault dips west and terminates against the Hunter-Mooki Fault at depths of around 20 km. The deep crustal seismic profile across the Tamworth Belt (BMR91_G01) that has been used constrain this relationship is ambiguous at depth. However, use of seismic trace attributes has indicated that the Mooki Fault extends down-dip further than the projected intersection with the Peel Fault, and therefore it is not backthrust to the Peel Fault. The distribution of hydrothermal mineral systems in the Tamworth Belt, and the general lack of systems west of the Peel Fault, is also considered to indicate that the Peel Fault is a backthrust to the Mooki Fault because if the reverse sense were true, mineralisation would be expected around subordinate structures in its hangingwall. 3D modelling highlights the contrast in structural style in the Rocky Creek Block relative to the Rouchel and Gresford blocks (Figure 1). Much of the central and western Rocky Creek Block is characterised by open folding with minimal faulting. Of the few strike-extensive faults in the area that are interpreted to intersect the Mooki Fault, most dip to the east. Towards the Peel Fault, higher strain is manifest in tighter folding and numerous steep west-dipping reverse faults with shallower eastdipping backthrusts. Based upon mapped relationships, most of the east-dipping faults are interpreted to terminate against the west-dipping faults. East-dipping faults also appear to be subordinate to steep west-dipping faults in the Gresford and Rochel blocks. The Rouchel Block is highly disrupted by NEand NNE-trending fault zones. The Karakurra Fault, which bounds the eastern margin of the Rouchel Block (Figure 1) is considered to have originated as a syn-sedimentary structure (Roberts & Engel 1987), which along with mapped cross-cutting relationships, has been used to infer that the fault cuts all units in the Tamworth Belt and extends down to the Hunter-Mooki Fault System. Southeastdipping faults in the eastern Rouchel Block terminate against the Karakurra Fault at depth or against the Hunter-Mooki Fault System further west. Most faults in the western and central Gresford Block are interpreted to dip to the east, have listric geometries and terminate against major west-dipping faults along the eastern margin of the block. Strike extensive, west-dipping faults along the eastern margin of the Gresford Block, such as the Williams River Fault, are also inferred to cut all units and extend down to the Hunter-Mooki Fault System.

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Sydney

Karakurra Williams River Fault loucester

Creek Block Williams River Fault

Gresford Block

50km

Figure 1: 3D Fault architecture of the western Tamworth Belt with small to large hydrothermal and magmatic mineral systems. A) Isometric view of the model with an east-west slice in the foreground showing a crosssection through the Rouchel Block and Gresford Block. B) Plan view of the model.

Known hydrothermal and magmatic mineral systems in the western Tamworth Belt are focussed along west-dipping fault zones that intersect the Hunter-Mooki Fault System at depth (Figure IB). The clusters of systems within the belt away from the Peel-Manning Fault System in the Rouchel and Gresford blocks are linked to the more widespread major west-dipping faults that intersect the basal thrust. Structures of this type occur only in the immediate hangingwall to the Peel Fault in the Rocky Creek Block, so the distribution of hydrothermal systems is strongly localised. The strike of the faults, along with subtle changes in strike, also appears to influence clustering of hydrothermal mineral occurrences. Most occurrences occur around faults that strike north-south at a regional scale. The northwest-trending part of the Peel-Manning Fault System is devoid of mineral systems with the exception of a segment north of the Williams River Fault, where the strike changes to a north-northwest orientation. The clustering of systems along north-south-trending faults occurs where subtle strike variations to north-northwest occur. The relationship between fault architecture and mineral system distribution highlights the importance of interconnectivity with a basal thrust that is likely to be sourcing and focussing mineralising fluid from the deep crust. The apparent steeper geometry of the west-dipping faults, along with their deep penetration, is likely to provide a steeper hydrostatic gradient and thus most of the mineralising fluid is focussed around these faults rather than the large east-dipping thrusts.

References Roberts J. & Engel B.A. 1987. Deposition and tectonic history of the southern New England Orogen. Australian Journal of Earth Sciences. 34, 1-20.

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The geodynamics of convergence between India and Eurasia (and the roles of Afro-Arabia, Indonesia and Australia) LEIGH ROYDEN AND OLIVER JAGOUTZ Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge MA 02139

The northward movement of India from Late Cretaceous to Recent time presents a number of puzzling features related to the high rates of convergence that obtained both before and after its collision with Eurasia. In particular, the pre-collisional convergence rate between India and Eurasia exceeded -140 mm/yr a period of more than 20 m.y., nearly twice as fast as maximum sustained rates of major plate convergence today. In addition, post-collisional convergence between India and Eurasia has continued at rates of -40-50 mm/yr since -40 Ma despite the lack of an obvious driving mechanism. An understanding of subduction dynamics, coupled with reconstruction of the broader tectonic boundaries of the central, western and eastern NeoTethys, and their evolution through time, can provide important insights into both phenomena. Quantitative modeling of "coupled" double subduction shows that two parallel subduction systems with the same sense of dip can drive overall convergence at rates up to, and sometimes exceeding, that of a single subduction system. The overall rate of convergence depends on the density of the subducting slab, the trench length of each subduction systems, and the separation between systems. Because asthenosphere must escape laterally from between the two converging slabs, asthenospheric pressure is increased between the slabs and can begin to inhibit the overall rates of subduction and convergence when this pressure becomes sufficiently high. In general, increasing trench length and decreasing slab separation lead to greater intervening pressure in the asthenosphere and slower rates of convergence. Geologic reconstructions show that at least two north dipping subduction systems, one Andean-style and the other intra-oceanic, were present between the southern continents and Eurasia by Early Cretaceous time. Overall convergence rates remained slow prior to - 8 0 Ma due to the extreme length (>10,000 km) and relatively narrow separation (<3000 km) of the two systems. West of the current location of the Himalaya, the intra-oceanic subduction boundary collided with the Afro-Arabia from 90 to 70 Ma, while east of the Himalaya this system collided with continental crust attached to Eurasia beginning at -90-80 Ma. This left a small oceanic plate (Kshiroda plate) bordering Eurasia to the south and bounded by a relatively short (3000 km) intra-oceanic boundary. Quantitative modeling shows that the timing of onset and the anomalously rapid convergence rate of India and Eurasia can be explained by "double" slab pull along this short intra-oceanic subduction system and the longer Andean-style margin that bounded Eurasia. The anomalously rapid convergence ends when Indian continental lithosphere enters the intra-oceanic subduction zone at 50 Ma - not when India collides with Eurasia at 40 Ma. During Cretaceous and Early Cenozoic time a north-dipping subduction boundary extended from the eastern Mediterranean to Indonesia. East and west of the Himalaya this subduction boundary collided with continental areas at -80-95 Ma and the domain between India and Eurasia included a small oceanic plate bounded to the east and west by transform boundaries. Quantitative modeling shows that the anomalously rapid convergence of India and Eurasia, which reached rates of 130-180 mm/a at 7050 Ma, can be explained by slab pull along two coupled subduction zones (one intra-oceanic and one Andean) and that rapid convergence ends when Indian continental lithosphere enters the intra-oceanic subduction zone at 50 Ma - not when India collides with Eurasia at 40 Ma. Rapid and protracted post-collisional convergence between India and Eurasia can also be understood by recognizing that, after -40-50 Ma, the Indian and Australian plates were effectively united into a single larger "plate", albeit containing zones of rather significant deformation. Importantly, the boundary between former India and former Australia is a region where oceanic lithosphere is under strong compression and subject to shortening (without subduction). Our quantitative modeling shows that when continental collision occurs across a subduction boundary that also includes segments where 134

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dense oceanic lithosphere continues to be subducted, continent-continent convergence may continue at nearly the same rate as pre-collisional convergence. (Our modeling also shows that ridge-push is ineffective in changing rates of subduction and convergence as slight changes in subduction rate, without changing slab buoyancy, trench length, etc., can generate much larger compressional stresses in the surrounding lithosphere than can be generated by ridge push.) In the geometry presented by the Indo-Australian plate, our model results indicate that northward subduction of the Australian portion of the Indo-Australian "plate" beneath the Sunda-arc drives northward motion of the Australian portion of the plate. Because the Australian portion of the plate is strongly coupled to the Indian portion, this also drives northward movement of India with forces that are much greater than can be generated through ridge push. The large magnitude of these forces are reflected in the compression of oceanic lithosphere beneath the northern Indian ocean and in the compression and shortening of continental lithosphere beneath Tibet. In this conceptual framework, Australia is responsible for the continued shortening beneath the Himalaya and the long-term uplift of the Tibetan plateau.

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Vertical vs horizontal tectonics - insights from the Tanzania Craton 10AN V. SANISLAV1, PAUL H.G.M. DIRKS1, THOMAS . BLENKINSOP2 AND YVONNE A. COOK1 1

Economic Geology Research Centre (EGRU) and Department of Earth and Oceans, James Cook University, Townsville, 4011, QLD, Australia; phone: (+61)07 4781 3293; fax: (+61)074781 5581 School of Earth & Ocean Sciences, Cardiff University, Cardiff CF10 3AT, United Kingdom

Tanzania Craton is a large, middle to late Archean craton located in East Africa. It is surrounded by a series of Proterozoic mobile belts and the East African Rift system. The stratigraphy of the Tanzania Craton is subdivided into three distinct supergroups: the Dodoman Supergroup, the Nyanzian Supergroup and the Kavirondian Supergroup. The Dodoman Supergroup is interpreted to represent the basement unit and consists mainly of high grade felsic and mafic gneisses, granulites, migmatites and schists. It occurs mainly in the central and southern part of the craton. The Nyanzian Supergroup is subdivided into the Lower Nyanzian and the Upper Nyanzian. The Lower Nyanzian consists mainly of mafic volcanics with subordinate felsic volcanics and volcanoclastics, deformed and metamorphosed under lower amphibolite facies conditions. The Upper Nyanzian is dominated by felsic volcanoclastics, ironstones and turbiditic sediments deformed and metamorphosed under greenschist facies conditions. The Kavirondian Supergroup unconformably overlies the Nyanzian Supergroup and consists of isolated outcrops of quartzitic conglomerates, arkosic sandstones and grits. The craton can be subdivided into two distinct terranes with specific age and tectonic evolution: the Central Tanzania Terrane and the Northern Tanzania Terrane. The Central Tanzania Terrane is dominated by the > 3 Ga Dodoman Supergroup which contains slivers of 2.6 Ga to 2.8 Ga greenstone belts. In contrast, the Northern Tanzania Terrane is dominated by the 2.6 Ga to 2.8 Ga greenstone belts of the Nyanzian Supergroup which have been disrupted and fragmented by granitoid intrusion and shear zones. The oldest rocks in the Northern Tanzania Terrane are the mafic volcanics of the Lower Nyanzian Supergroup which were erupted at -2.82 Ga. They have geochemical and isotopic composition similar to modern day oceanic plateaus. These include flat chondrite normalized REE patterns and trace element ratios similar to the primitive mantle ratios. The next period of tectonism and crustal growth in the Northern Tanzania Terrane is characterized by the deposition of the Upper Nyanzian sediments and the associated magmatism. This period is constrained by detrital and igneous zircons between -2.75 Ga and 2.7 Ga. The main period of deformation occurred between 2.7 Ga and 2.65 Ga with eventual stabilization by 2.62 Ga. The overall magmatic history of the Northern Tanzania Craton shows a progressive evolution from juvenile mafic melts that progressively mature into felsic melts: its starts with mafic magmatism pre-2.8 Ga, followed by diorite and TTG magmatism between 2.75 Ga and 2.7 Ga and transitions into more felsic magmatism so that between 2.65 and 2.62 Ga the magmatism in Northern Tanzania Craton is dominated by the high-K granites. This magmatic history is mirrored by the detrital record of Kavirondian sediments which contain a detrital zircon population spreading between 2.75 Ga and 2.64 Ga. The Hf isotopic composition of the detrital zircons is dominated by positive epsilon Hf between 2.75 Ga and 2.7 Ga, reflecting juvenile dioritic and TTG magmatism, and becomes dominated by negative epsilon Hf values post 2.7 Ga, reflecting the transition into more evolved crustal sourced magmatism. This overall transition from primitive mafic magmatism (pre-2.8 Ga) to juvenile diorite and TTG magmatism (2.75 to 2.7 Ga) and into evolved crustal melts (post-2.7 Ga) reflects a change in the tectonic processes. Between 2.75 Ga and 2.7 Ga the tectonism was dominated by vertical magma segregation and the extraction of juvenile melts by melting the base of an oceanic plateau. At ca. 2.7 Ga a transition into more evolved melts occurred which was synchronous with the onset of horizontal accretion. This transition at 2.7 Ga from juvenile to more evolved magmatism may mark the accretion of the Northern Tanzania Terrane to the Central Tanzania Terrane.

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Simple shear of single inclusions with a hyperelastoviscoplastic rheology at finite strain CHRISTOPH ECKART SCHRANK 1,2 , ALI KARRECH 3 , DAVID ALEXANDRE BOUTELIER 4 AND KLAUS REGENAUER-LIEB 5 Queensland University of Technology, School of Earth, Environmental and Biological Sciences, 2 George St, Brisbane, 4001, QLD, Australia 2 The University of Western Australia, School of Earth and Environment, 35 Stirling Highway, Crawley, 6009, WA, Australia 3 The University of Western Australia, School of Civil, Environmental and Mining Engineering, 35 Stirling Highway, Crawley, 6009, WA, Australia 4 The University of Newcastle, School of Environmental and Life Sciences, University Drive, Callaghan, NSW 2308, Australia 5 The University of New South Wales, School of Petroleum Engineering, Tyree Energy Technologies Building, H6, Anzac Parade, Sydney, NSW, 2052, Australia

We investigate isothermal 2D simple shear deformation of round, matrix-bonded, deformable single inclusions with a hyperelastoviscoplastic rheology. The large parameter space of loading conditions and material properties encountered in the ductile lithosphere is explored systematically for inclusions with weak and strong viscosities. The shape evolution of inclusions in our models differs significantly from that of linear-viscous inclusions in a linear-viscous matrix over most of the relevant parameter space. Inclusion orientation history is more strongly affected by elastic and plastic contributions to rheology for strong inclusions. Moreover, at high Weissenberg numbers (> 10"2), strong inclusions deform in the transient viscoelastic stress regime up to y > 3. At smaller Weissenberg numbers, inclusion deformation is mainly controlled by the ratio of yield stress over steady-state viscous stress. Deviations from purely viscous solutions become more pronounced with increasing yield stress. Our results imply that studies on deformed objects for finite-strain analysis or viscosity-ratio computation should establish appropriate rheology and loading conditions carefully. It seems likely that fairly strong deformable clasts in shear zones retain stored energy up to high shear strains. Therefore, viscous models of sheared inclusions may underestimate the energy budget available for dissipation within and around natural inclusions.

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Ductile elasto-plastic shear zone at large strain: insight into elastic energy storage and dissipation from analogue experiments

DAVID BOUTELIER , MAXIME HENRIQUET , CHRISTOPH SCHRANK , ALI KARRECH AND KLAUS REGENAUER-LIEB 1

1,2

3,4

The University of Newcastle, School of Environmental and Life Sciences, University Drive, Callaghan, NSW 2308, Australia Laboratoire de Geologie de Lyon, Ecole Normale Superieure de Lyon, Universite Lyon-1, Universite de Lyon, CNRS UMR5276, Lyon, F-69364 France Queensland University of Technology, School of Earth, Environmental and Biological Sciences, 2 George St, Brisbane, 4001, QLD, Australia The University of Western Australia, School of Earth and Environment, 35 Stirling Highway, Crawley, 6009, WA, Australia The University of Western Australia, School of Civil, Environmental and Mining Engineering, 35 Stirling Highway, Crawley, 6009, WA, Australia The University of New South Wales, School of Petroleum Engineering, Tyree Energy Technologies Building, H6, Anzac Parade, Sydney, NSW, 2052, Australia 1

We present preliminary physical experiments designed to test the new finite-strain theory of Karrech et al. (2011). This theory employs a logarithmic frame-indifferent co-rotational stress rate and the logarithmic Hencky strain tensor to obtain a proper measure for large transformations. This difference in strain measures predicts a difference in the elastic energy budget of non-coaxial deformation. The challenge therefore is to design physical experiments, in which the energy budget of deformation can measured and decomposed accurately. We present a series of simple-shear analogue experiments monitored with Particle Imaging Velocimetry (PIV), which provide quantitative tests for the new large-strain theory. Elastic energy storage and dissipation as well as their association with strain localization during ductile shearing are investigated using three-dimensional thermo-mechanical physical experiments with temperature-dependent elasto-plastic and strain-softening analogue materials. The model consist of a thin plate (23 x 13 x 1 cm in length, width and thickness) floating on water, which is sheared in between two rigid blocks moving horizontally parallel to each other at a constant velocity. Displacement and deformation are calculated by correlation of successive time-lapse images through PIV. The temperature in the model is constant and homogeneous during each experiment, but has been varied between experiments to simultaneously vary the elastic modulus, peak plastic yield stress and rate of softening following failure. All experiments initiate with homogeneous elastic shearing across the entire weak plate. Once elastic limit is reached, strain localization may or not initiate, depending on imposed temperature. Strain localization appears in the form of a narrow shear zone (width of ~l-5 mm) close to one of the interfaces between the weak plate and the rigid vices imposing simple shear. Rheological tests indicate that the rate of softening in our thermo-mechanical analogue materials after plastic failure decreases with increasing temperature. Therefore, we observe a more efficient strain softening and strain localization process at lower temperatures. At low temperatures (T<40C) with efficient strain localization, a weak through-going narrow plastic zone forms parallel to the displacement direction. After localization of this narrow shear zone, the weak plate unloads elastically, reducing its stored energy by up to 10%. Then a steady-state regime is obtained in which the narrow ductile plastic shear zone accommodates the imposed displacement without any further loading or unloading in the intact part of the weak plate. When the imposed deformation is stopped, deformation continues only within the weak domain; it unloads further, dissipating another 1% of the stored elastic energy. At higher temperature (T>40), corresponding to little/no strain softening, plastic failure leads to the formation of a network of interconnected anastomosing shear zones instead of one through-going feature. During plastic deformation, the velocity profiles across the weak domain remain linear 138

SGTSG 2015: Riding the Waves suggesting that further elastic deformation is being stored in the weak domain, albeit at a lower rate compared to the initial purely elastic initiation of the experiment. Our results show that both the energy budget in the model and the deformation localization are very sensitive to the rheology. A significant quantity of stored energy is conserved in the sheared domain, even after several dissipation events, and elastic energy might be added to the system during large strain plastic deformation when strain softening is not significant. By varying the rheological properties via the imposed temperature (in the range 37-41C) and varying the initial and boundary conditions (e.g. length, width, deformation rate, presence of inclusion), we can construct a series of observations that ought to be matched by numerical simulations using the new finite theory (Schrank et al., 2015), thereby providing a way to test the new theory quantitatively. Then the impact of each rheological parameter on the storage/dissipation of energy and formation of narrow shear zones will be investigated. References Karrech, A., K. Regenauer-Lieb, and T. Poulet (2011), Frame indifferent elastoplasticity of frictional materials at finite strain, Int. J. Solids Struct., 48(3-4), 397^07, doi:10.1016/j.ijsolstr.2010.09.026. Schrank, C. E., A. Karrech, D. Boutelier, and K. Regenauer-Lieb (2015), Ductile deformation of single inclusions in simple shear with a finite-strain hyperelastoviscoplastic rheology, in Ductile Shear Zones: From Micro- to Macro-scales, edited by S. Mukherjee and K. F. Mulchrone, pp. 46-58, John Wiley & Sons. 1 Time: 1 1 0 s I 8 x 10 2 m m / s — 1

Y x 10 8

- o- 6

37°C

8 x 10 2 mm/s - * J

i i J!

4 2 o •ro- 0 -

-2 -4 Ci -vO • (N -6 -8

f x 10 3

37°C

2 1

-1

-2 -3

Figure 1: Successive images of the model surface with PIV velocity vectors and shear rate showing elastic deformation (t=110), shear localization (t=190), dissipation of stored elastic energy (rebound, t=215) and steadystate ductile plastic shear (t=340s).

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Provenance and palaeogeography of Early Permian back-arc basins in eastern Australia, and implications for the role of trench retreat in accretionary orogens

URI SHAANAN AND GIDEON ROSENBAUM 1

School of Earth Sciences, University of Queensland, Brisbane, Queensland 4072, Australia

The New England Orogen of eastern Australia is characterised by tight orogenic curvatures (oroclines). Oroclinal bending commenced in the Early Permian during a period of crustal extension that involved extensional faulting and development of rift basins, crustal melting and widespread emplacement of Stype granitoids, exhumation of metamorphic complexes, and local high-temperature metamorphism. At the 'core' of the oroclinal structure in the southern New England Orogen, the Dyamberin and Nambucca blocks are Early Permian sedimentary successions that supposedly represent rift-related basins. However, the origin of the sediments and the exact time of deposition have hitherto been poorly constrained. Here we present new U-Pb ages and morphological analysis of detrital zircons from the Dyamberin and Nambucca blocks. Results show equivalent age populations, which are as young as 285 Ma, thus confirming the Early Permian deposition of the successions. Furthermore, we use the abrasion of crystals as a proxy for transportation distances and we separately plot ages for periods that correspond with plausible source regions. The collective dataset provides a powerful correlation tool between samples and terranes, and an important insight into the transportation systems that brought detritus into the basin. Our results indicate that: (1) the successions of the Dyamberin and Nambucca blocks are correlative and were deposited in a single basin; (2) a local transportation system mobilised Carboniferous detritus within the New England Orogen; (3) a regional fluvial system transported pre-Devonian detritus from continental Gondwana across the landscape of the simultaneously developing Sydney-GunnedahBowen Basin System and the former (Devonian-Carboniferous) magmatic arc; (4) the successions of the Dyamberin and Nambucca blocks were deposited in a back-arc setting, this receiving detritus both from the continent and the arc. Based on these findings, we present a paleogeographic reconstruction for the Early Permian. The simultaneous formation of the Dyamberin-Nambucca back-arc basin and the New England oroclines suggest that oroclinal bending was controlled by trench retreat. The recognition of a genetic link between oroclinal bending and back-arc extension may explain how accretionary orogens, such as the eastern Australian Tasmanides, were able to obtain a substantial width without much contribution from accreted exotic terranes. A similar mode of tectonism may have played an important role in other accretionary orogens.

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Paleomagnetic data from the New England Orogen (eastern Australia) and implications for oroclinal bending URI SHAANAN1, GIDEON ROSENBAUM1 SERGEI PISAREVSKY 2 , 3 AND FABIO SPERANZA 4 1

School of Earth Sciences, University of Queensland, Brisbane, Queensland 4072, Australia. School of Earth and Environment, University of Western Australia, Crawley, Western Australia, Australia. The Institute for Geoscience Research (TIGeR), Department of Applied Geology, Curtin University of Technology, Perth, Western Australia, Australia. 4 Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy.

2 3

Two competing hypotheses are commonly invoked to explain the origin of curved orogenic belts (oroclines). The first one explains the formation of oroclines by lithospheric buckling associated mainly with orogen-parallel contraction, whereas the second hypothesis assumes that many oroclines have formed in response to trench retreat and contemporaneously with back-arc extension. Here we focus on the New England oroclines of eastern Australia, the formation of which had been broadly constrained to the Early to Middle Permian. This time interval encompasses periods of both back-arc extension (at -300-280 Ma) and subsequent contractional deformation (Hunter-Bowen Orogeny) that commenced at -270 Ma along the paleo-Pacific and Gondwanan subduction plate boundary. We present new paleomagnetic data from volcanic rocks that were extruded during the transition from extension to contraction (at -272 Ma), and we show that the oroclinal structure must have formed prior to the emplacement of the volcanic rocks. Our results thus indicate that oroclinal bending in the southernmost New England Orogen has been completed prior to the onset of Middle Permian contractional deformation. It is therefore concluded that the oroclines have likely formed during backarc extension, and that a major contribution to the orogenic curvature was driven by trench retreat.

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Unravelling the nature of crustal basement beneath large tracts of the Thomson Orogen CORALIE SLEGEL1, SCOTT E. BRYAN1, CHARLOTTE M. ALLEN1'2, DAVID J. PURDY3, ANDREW J. CROSS4 AND DAVID A. GUST1 1

School of Earth, Environmental and Biological Sciences, Science and Engineering Faculty, Queensland University of Technology, Brisbane, Australia Central Analytical Facility, Institute for Future Environments, Queensland University of Technology, Brisbane, Australia 3 Geological Survey of Queensland, Brisbane, Australia 4 Geoscience Australia, Canberra, Australia The Thomson Orogen is a poorly understood tectonic element in eastern Australia that separates Precambrian cratonic regions of central Australia from Phanerozoic fold belts developed along the eastern margin (see recent reviews by Fergusson & Henderson, 2013 and Purdy et al., 2013). Unravelling the nature of the crustal basement beneath the Thomson Orogen is crucial for providing a better understanding of the origin of elevated geothermal gradients in southwest Queensland and prospectivity for hot dry rock geothermal energy, and to the tectonic history and Paleozoic transitions of eastern Australia. Currently, debate exists as to whether crustal basement is oceanic in origin (Harrington, 1974; Glen et al., 2013) or continental, and if continental, whether it represents a possible extension of the Musgrave Province-type crust to the east (Fergusson et al 2007) or other Precambrian continental crust (Henderson, 1980; Finlayson, 1990). However, the Thomson Orogen is dominantly concealed by thick sedimentary cover making investigations of its geologic history and basement difficult. An alternative approach to shed light on the nature of crustal basement was to undertake ID stochastic thermal modelling (Siegel et al., 2014) across the region, which demonstrated that elevated geothermal gradients in southwest Queensland must result from the presence of a moderately heatproducing silicic crust between 5 and 40 km depth, therefore supporting the continental crust hypothesis. Siegel et al. (2014) also proposed such crust contains portions of Musgrave-type crust beneath the Thomson Orogen. We have since tested this hypothesis using mineralogy, whole-rock chemistry, and zircon U-Pb geochronology associated with Hf and O isotope in zircons from subsurface granitic rocks where the granite are probes of their deeper crustal sources. U-Pb zircon geochronology on the granitic rocks established emplacement ages that are Devonian, Silurian and Ordovician within the Thomson Orogen, and Late Devonian-Early Carboniferous for the Roma Shelf granites along the eastern margin of the Thomson Orogen. Zircons from the Devonian Itype tonalite of AOP Balfour 1 have mantle-like O isotopes (~5 to 6.5 per mil) with two-stage Hf model ages ranging between 1200 and 1330 Ma, consistent with reworking of Musgrave-type crust during the Devonian. The pattern of zircon inheritance for the Late Devonian-Early Carboniferous, Silurian and Ordovician granitic rocks is similar to the detrital zircon age signature observed in the exposed northern Thomson Orogen with both Pan-African and Grenvillian age peaks. Such similarity indicates that both Pan-African and Grenvillian age zircons were present in crustal materials at the site of anatexis. A possibility is that this signature derives from the reworking of similar sedimentary materials. This is in agreement with crustal-like O isotope in zircons (>6.5 per mil) and the presence of muscovite +/- cordierite for the Late Devonian-Early Carboniferous, Silurian and Ordovician granitic rocks, which suggest that a portion of the source was sedimentary or metasedimentary. Most granitoids studied here are highly silicic (>70 wt%) and relatively non-fractionated and must therefore have acquired their high-silica character by partial crustal melting excluding large material contributions from mantle-derived magmas in their genesis. Each sample shows a spread of Hf model ages (3 to 6 £Hf units), which upper and lower limits can be used in association with known igneous events in Australia, to help establish the nature of those potential crustal sources. Hf isotopic signatures from all analysed granites reveal the reworking of Paleoproterozoic or/and Mesoproterozoic igneous sources during the Ordovician, Silurian, Devonian and Early Carboniferous. The crustal sources involved are interpreted to include mantle-derived material added into the crust at -2400-2700 Ma as evident in the Mount Isa region, -2100-2300 Ma in the Mount Painter region, -1900 Ma in the Musgrave Province, during Mount Isa and Mount Painter magmatism at 1700-1800 and 1500-1600 Ma, respectively, and 142

SGTSG 2015: Riding the Waves during the Mount West Orogeny and Musgrave Orogeny. The occurence of inherited zircons and crustal Hf and O isotopic signatures in most rocks indicate the crust beneath the Thomson Orogen is continental. The presence of Precambrian inherited zircons and two-stage Hf model ages reveal that portion of this continental crust are Precambrian in age. Zircon inheritance, Hf and O isotopic signatures, along with results from a ID stochastic thermal modelling (Siegel et al 2014), argue against an oceanic type of crust as basement for the Thomson Orogen as suggested by Harrington (1974) and Glen et al (2013). In particular, subsurface granitic rocks reveal that the underlying crust beneath the Thomson Orogen contains Precambrian igneous material with portions of Musgrave-type crust. Consequently, Precambrian material must lie well east of the Tasman line of Veevers and Powell (1984). References Fergusson CL, Henderson RA, Fanning CM & Withnall IW (2007) Detrital zircon ages in Neoproterozoic to Ordovician siliciclastic rocks, northeastern Australia: implications for the tectonic history of the East Gondwana continental margin. Journal of the Geological Society, 164 (1): 215-225. Fergusson CL & Henderson RA (2013) Thomson Orogen. In: Jell P (ed.) Geology of Queensland. Brisbane, Queensland: Geological Survey of Queensland pp. 113-224. Glen RA (2013) Refining accretionary orogen models for the Tasmanides of eastern Australia. Australian Journal of Earth Sciences, 60 (3): 315-370. Harrington HJ (1974) The Tasman Geosyncline in Australia. In: Denmead AK, Tweedale GW & Wilson AF, eds. The Tasman geosyncline—a symposium in honour of Professor Dorothy Hill. Geological Society of Australia, Queensland Division, pp. 383-407. Henderson RA (1980) Structural outline and summary geological history for northeastern Australia. The geology and geophysics of northeastern Australia: l-26.Finlayson D (1990) Basin and crustal evolution along the Eromanga-Brisbane Geoscience Transect: precis and analogues. In: Finlayson D (ed.) The Eromanga-Brisbane Geoscience Transect: a guide to basin development across Phanerozoic Australia in southern Queensland. Bureau of Mineral Resources Bulletin 232 pp. 253-261. Purdy DJ, Carr PA & Brown DD (2013) Review of the geology, mineralisation and geothermal potential of the Thomson Orogen. Brisbane. Geological Survey of Queensland. 2013/01, 212 p. Siegel C, Schrank CE, Bryan SE, Beardsmore GR & Purdy DJ (2014) Heat-producing crust regulation of subsurface temperatures: A stochastic model re-evaluation of the geothermal potential in southwestern Queensland, Australia. Geothermics, 51 (0): 182-200. Veevers JJ & Powell CM (1984) Epi-Adelaidean: regional shear. Phanerozoic Earth History of Australia: 278284.

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The Jellinbah Fold-Thrust Belt in the Bo wen Basin: a late phase of the Hunter-Bowen contraction in Queensland. RENATE SLIWA 1,2 , ABBAS BABAAHMADI 1 AND JOAN ESTERLE 1 1 2

School of Earth Sciences, The University of Queensland, St Lucia Qld4072 Integrated Geoscience Pty Ltd, P. O. Box 462, Yandina QLD

The Jellinbah fold-thrust belt (JFTB) is a 10-50km wide zone of compressional deformation that reaches for >500km from Collinsville in the far northern Bowen Basin to Baralaba in the east where it merges with the Gogango Overfolded Zone to the east. Within the JFTB strain decreases from east to west as deformation transitions from upright map-scale folding to a network of connected thin-skinned thrust faults that are associated with fault propagation folds and numerous mesoscopic accommodation structures, to a western low-strain zone of disconnected minor thrust faults and fractures in the west. Extensive folding and regional-scale thrust faults in the poorly exposed northern Bowen Basin have been known since the 1960's when the region was extensively explored for coal (Maione & Olgers, 1969; Devey & Price, 1985). A deep seismic profile near Blackwater showed that the larger thrust faults are linked to a regional east-dipping detachment surface at depth (Korsch et al., 2009). The recent mining boom has now generated numerous 2D & 3D seismic surveys as well as extensive mining exposures that now allow a more detailed characterisation of the faults and folds across the JFTB. The JFTB is best described as three strike-parallel domains with distinct structural styles. The easternmost domain which extends from the exposed Connors Arch to the easternmost significant thrust fault is dominated by upright symmetrical folds with l-3km wavelength that are cause bedding dips up to 80°. The folds affect the whole Bowen Basin succession including the mid-Triassic Rewan Group and are continuous beneath the Early Tertiary Duaringa Basin. The central zone in the JFTB is characterised by three to eight parallel thrust faults with a combined vertical separation of 800-1000m. Individual thrust faults generally trend north-south or northnorthwest and dip -60° to the east near the surface, but shallow out to <20° at depth. West-dipping structures are uncommon. The locations of individual thrust faults are closely linked to regional-scale folds, typically cutting out the eastern limb of asymmetric synclines. Mining has exposed abundant accommodation structures along the thrust faults, including normal faults, drag folds, bedding plane shears and thrust stacks at various scales. Coal in particular is commonly contorted into complex geometries to accommodate space problems along the faults. Most of the accommodation structures are brittle, but some of the deepest exposed parts of the larger thrust faults appear brittle-ductile with weaker rocks appearing to flow into the core of tight folds. The westernmost zone of the JFTB is the least deformed. Thrust faults are much shorter (<6km) and have vertical separations of less than 20m. Bedding dips gently at 3-5° to the east. The main difference between the smaller thrust faults in this zone and the regional structures to the east is the strong relationship between the thrust geometry and the distribution of competent rock types in the coal measures. Thrust faults commonly exploit weak coal seams and ramp up section along the contacts between strong sandstone bodies and weaker siltstone units (Esterle and Sliwa, 2003). The folds and faults deform all the Permian and Triassic units of the Bowen Basin, suggesting a Late Triassic maximum age for the deformation (~235Ma). The only definitive minimum age constraint for the JFTB is provided by several Early Cretaceous intrusions into the northern Bowen Basin (~129Ma). However, there are large thrust movements along the Burunga and Moonie-Goondiwindi Fault systems that predate deposition in the Surat Basin (~200Ma). By correlation with these faults, the JFTB most likely formed during a late phase of the Hunter Bowen contraction between 235-200Ma.

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References Devey D. & Price I., 1985. Some structural features along the western margin of the Bowen Basin. Geological Society of Australia Abstracts 17. Esterle J.S. & Sliwa R., 2002. Bowen Basin Supermodel 2000. ACARP Project Report Korsch R.J., Totterdell J.M., Fomin T. & Nicholl M.G., 2009. Contractional structures and deformational events in the Bowen, Gunnedah and Surat Basins, eastern Australia. Australian Journal of Eargth Sciences, 56, 477-499. Malone E.J. & Olgers F., 1969. The geology of the Duaringa and Saint Lawrence 1:250,000 Sheet Areas, Queenland. BMR Report 121.

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The NE Taiwan salient: a bent progressive mountain belt

LIONEL SONNETTE AND JIAN-CHENG LEE

institute of Earth Sciences, Academia Sinica, 128 Academia Road, Section 2, Taipei 115, Taiwan.

The Taiwan orogen is the result of the collision of the Luzon volcanic arc pushed northwestward by the Philippine Sea plate against the Eurasian plate since the Late Miocene, that seemed to be significantly influenced by the opening of the Okinawa Trough in its NE end since 2 Ma. The fold-and-thrust belt in NE Taiwan shows a characteristic convex shape with a horizontal bend of more than 60 degrees for its foreland part and more than 90 degrees for its hinterland part. How is the curvature of the NE Taiwan saliend associated with horizontal rotations? What is the part of plate indentation and back-opening in the curvature of the NE Taiwan salient? In this study, we carried out a paleomagnetic study with 62 sampling sites for 1333 specimens. By intergrating all the previous studies, we are able to highlight both a complex pattern of local block rotations together with pre- and post-folding magnetizations; and two magnetic lineation patterns (evidence by the anisotropy of magnetic susceptibility), with one remains parralel to the local fold axis trend and the other is counterclockwise deviated by about 30 degrees to the fold axis trend. In addition the regional fold axis trend and the main thrust fault strikes are quite consistent with the yielded magnetic lineation from one tip of the salient to the other one. Such consistency favors the interpretation of a succession of large-scale regional clockwise then anticlockwise rotations in NE Taiwan.

Counterlockwise Rotations itation amount of rotatioi angular uncertainty

Clockwise Rotations

Figure 5: Paleomagnetic rotations along the NE Taiwan salient. Average rotations are indicated for each sections. The formation of the foreland part of the salient can be correlated with to main events associated with magnetic fabric development. The first event is marked by tilting/folding during the propagation of thrust sheets, and the record of magnetic lineation both affected by a 15 to 70 degrees clockwise rotation. The youngest formation, the kuechulin formation, affected by this rotation is 4.1 Ma old. This event is proper to a progressive arc definition. The second event clearly post-tilting/folding, is marked

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SGTSG 2015: Riding the Waves by the record of a second magnetic lineation pattern and the occurrence of a 5 to 40 degrees counterclockwise rotation. And youngest and volcanic formations, younger than 1.5 Ma, have recorded no rotation. The second rotation occured while the fold-and-thrust belt was mainly already formed; this event can be associated to an oroclinal bending. We propose to associate: 1) the clockwise bending of the NE Taiwan salient to the indentation of the Luzon volcanic arc against the Eurasian plate and; 2) the anticlockwise rotation or "un-bending" to the opening of the Okinawa Trough.

1.5 Ma

? Ma

4.1 Ma

Figure 6: Paleomagnetic restoration of the leading edge of the NE Taiwan salient. The main geometry of the salient (i.e., leading edge, black thick line) have been restored considering the different amount and sense of rotations and a succession of clockwise rotations followed by counterclockwise rotations (grey and light grey thick lines). Ages inferred from the age of the sampled formations are indicated. One can notice the remarkable similarity in shape between the Central Range (composed exclusively by metamorphic rocks, grey dashed line) and one step of the restoration of the leading edge (the grey thick line).

Keywords: Taiwan, Fold-and-thrust belt, AMS, Paleomagnetism and Salient. 147

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The anatomy of the Leeuwin Complex: Insight into the assembly of the southwest corner of Australia NICHOLAS E. TIMMS 1 , CHRIS TUCKER 1 , DAVID PEARCE1, PAUL WILKES 2 , TIM JOHNSON1 AND IAN C. W . FITZSIMONS1 1

The Institute for Geoscience Research (TIGeR), Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845, Australia CSIRO, Perth, Western Australia

The Leeuwin Complex in the southwestern corner of Australia is one of several Proterozoic inliers that collectively define the Pinjarra Orogen, an important yet poorly understood stage in the assembly of eastern Gondwana. The rocks of the Leeuwin Complex comprise gneisses of variable composition that record a complex history of igneous intrusion, folding and fabric formation, partial melting and faulting. However, outcrops are restricted to exposures along the coast, which has hampered resolution of the structural architecture. This resulting lack of context has led to difficulties in interpreting published zircon U-Pb age data from the Leeuwin Complex, which range from -1090 Ma to -500 Ma (discounting some inherited grains as old as 1200 Ma). Consequently, details of the tectonic history of the Leeuwin Complex remain poorly constrained. This study utilises high-resolution aeromagnetic and gravity imagery in order to examine the structural architecture of the Leeuwin Complex of southwestern Australia in much greater detail than was previously possible. These data, when combined with new detailed geological mapping, structural field measurements, in situ magnetic susceptibility measurements and existing mineralogical, geochemical and geochronological constraints, permit a detailed characterisation of regional-scale brittle and ductile deformational features that are preserved in the Leeuwin Complex. The results are integrated into a new 1:50,000 geological/geophysical interpretation map of the Leeuwin Complex. Four 'domains' have been identified within the Leeuwin Complex based on aeromagnetic and structural characteristics. The boundaries between these domains are inferred to be ductile, eastdipping high strain zones that variably truncate west-verging regional folds. The regional scale N N W SSE-trending folds in the eastern domains are tight to isoclinal, and become more asymmetrical with overturned western limbs towards the west. The asymmetry of the large-scale folds indicate an east over west sense of vergence. However, shear sense from small-scale kinematic indicators is less clear. Generally, folds in the northern Leeuwin Complex plunge at gentle to moderate angles towards the north, whereas folds in the southern part of the Complex are sub-horizontal or plunge gently towards the south. Regional Type 3 'hook' refolded folds are also evident from aeromagnetic images. The dip of regional fold axial planes and thrust zones becomes progressively more shallow towards the east. This major phase(s?) of folding / high strain ductile deformation dominates the pattern on aeromagnetic images, and is consistent with evidence of upper amphibolite to granulite facies peak metamorphic conditions, during which most lithologies partially melted. The -1090 Ma zircon ages are exclusively from the westernmost domain. Orthogneisses in the central and eastern domains preserve both inferred protolith ages of 780-680 Ma, and inferred metamorphic ages of 550-530 Ma. This is interpreted to record variable reworking within the central and eastern domains of the Leeuwin Complex during the main phase of predominantly west-verging folding and thrusting. Hypotheses for the tectonic development of the Leeuwin Complex that are compatible with its structure and whole-rock geochemistry include: (1) rift-related emplacement of orthogneiss protoliths followed by west-vergent accretion; (2) strike-slip emplacement of the Leeuwin Complex followed by west-vergent accretion. However, further work is required to test these hypotheses. Finally, sets of NE-SW, NNW-SSE and minor E-W faults correspond with aeromagnetic lows, truncation of aeromagnetic units, and rivers and beaches on the coast. These structures correlate with faults in the Perth Basin, and most are interpreted to have a normal sense of displacement consistent with the breakup of Gondwana in the Phanerozoic. These faults control the elevation of the

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SGTSG 2015: Riding the Waves unconformity between the basement gneisses of the Leeuwin Complex and overlying Quaternary Tamala Limestone.

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The Pichao-Ovejeria shear zone in Argentina: an introduction to very thick ultramylonites

ROBERTO F. WEINBERG , MELANIE A. FINCH , M. G. FUENTES AND R. BECCHIO 1

School of Earth, Atmosphere and Environment, Monash University, Clayton, VIC, 3800 Instituto Geonorte, National University of Salta, INENCO-CONICET. Av. Bolivia 5150. 4400. Salta, Argentina.

The Pichao-Ovejeria shear zone is a several kilometre thick band of ultramylonites and mylonites that thrusted granulite facies migmatites onto amphibolite facies rocks during the 470 Ma Famatinian orogeny in NW Argentina (Finch, et ah, 2015). At the base of the shear zone there is a 1 km thick band of ultramylonites that grades upwards to weakly sheared migmatites over 3 to 4 km. Mylonitic rocks define a geochemical field narrower than the protolith, suggesting they underwent mixing and homogenization through folding and stretching during shearing. Ultramylonites this thick are uncommon. We explore the origin of such thick ultramylonites. The width of a shear zone, in the absence of significant compositional rheological contrasts controlling strain localization, is controlled by the balance between shear heat generation and diffusion. Under typical crustal conditions, a strain rate of 10" s" is required to form a 1 km-thick ultramylonite, and this is achieved when large movement velocities are imposed across the shear zone. This shear zone was later cut by younger shearing that caused a 7km sinistral displacement, and later still it was overprinted by pseudotachylites. We postulate that the Pichao-Ovejeria shear zone and its thick ultramylonite could have accommodated a significant fraction of convergence velocities driving the orogeny, and that the wide mylonitic shear zones characteristic of the Cambrian-Ordovician deformation of the Sierras Pampeanas result from the convergent movement being taken up by only a few active major shear zones. Another alternative will be explored by Finch and Weinberg in a twin talk. 12

References Finch, M. A., Weinberg, R. F., Fuentes, M. G., Hasalova, P. & Becchio, R., 2015. One kilometre-thick ultramylonite, Sierra de Quilmes, Sierras Pampeanas, NW Argentina. Journal of Structural Geology, 72, 33-54.

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Pulses of progressive shearing and reactivated fault structures: The 1.5 b.y. Palaeoproterozoic to Palaeozoic structural and metamorphic evolution of eastern Arunta Region, central Australia ANETT WEISHEIT 1 , BARRY L. RENO 2 , ELOISE E. BEYER 1 AND JO A . WHELAN 2 1 2

Northern Territory Geological Survey, AZRI, South Stuart Highway, Alice Springs NT 0870 Northern Territory Geological Survey, 3rd Fir Centrepoint Building, Smith Street, Darwin NT 0800

Palaeoproterozoic metasedimentary and metaigneous rocks of the eastern Arunta Region preserve a complex polymetamorphic and polydeformational history with pulses of activity from the late Palaeoproterozoic through to the Palaeozoic. The eastern Arunta Region is subdivided into two provinces: the Palaeoproterozoic Aileron and Neoproterozoic to Palaeozoic Irindina, each with distinct protolith ages and stratigraphic and tectonic evolution. Major shear zones and fault structures several km wide and over 100 km long separate Aileron Province basement from unmetamorphosed Neoproterozoic to Palaeozoic flat-lying Georgina Basin sediments to the north and its high-grade equivalent, the Irindina Province to the south. New structural and geochronological analysis of the steeply south dipping, E-W striking Delny Shear Zone in Jervois Range (-250 km NE of Alice Springs) and Mopunga Range area (-200 km) indicate a prolonged history. Compressional regional gneissic structures and km-scale folding in basement rocks north of the Delny Shear Zone developed progressively until a shift in the regional stress field refocussed deformation along this Shear Zone and possibly along km-scale oblique NNE and NNW trending shear structures. Development of early gneissic to mylonitic and ultramylonitic structures with predominant dip-slip stretching lineation in the Delny Shear Zone indicates a steeply oblique dextral, south-side down movement during granulite-facies metamorphism. This deformation coincides with near-peak-P zircon growth during low-pressure, high-temperature metamorphism at ca. 1760-1750 Ma in Jervois Range and migmatite crystallisation during cooling at ca. 1730 Ma in Mopunga Range (Scrimgeour and Raith, 2001). The change in grade and timing of metamorphism in the Aileron Province basement from east (Jervois Range) to west (Mopunga Range) is interpreted to be a result of progressive exhumation along the Delny Shear Zone in a stable compressional setting during that time. By the early Neoproterozoic the Aileron Province basement was at the surface. Extension reactivated the NNW trending shear zones and caused the formation of half graben structures in the lower Georgina Basin stratigraphy that were successively buried by wide-spread Cambrian sediments. Part of the basin, the Irindina Province, experienced granulite facies metamorphism at around 4 8 0 ^ 6 0 Ma (Buick et al, 2001) accompanied by progressive simple shear folding and stretching, likely related to the re-activation of the E, NNE and NNW trending regional shear structures. A change in the stress field to compression resulted in a regional sinistral, transpressional setting that caused north-vergent exhumation along the Delny Shear Zone and sub-horizontal nappe structures in the Irindina Province. Anastomosing, progressive mylonite and ultramylonite shear zones with moderately plunging fabric attractor to the SW form locally in amphibolite and greenschist facies rocks in the Delny Shear Zone during the initiation of the Alice Springs Orogeny at ca. 450 Ma (Scrimgeour et al, 2001). The Irindina Province cooled progressively during the following 150 million years indicating local shearing and later faulting activity that not only juxtaposed the Irindina and Aileron provinces, but also Aileron Province basement with Georgina Basin sediments. All major Palaeoproterozoic E, NNE and NNW trending compressional structures in the eastern Arunta Region have been re-activated during Neoproterozoic and Ordovician extension, and again during compression in the Alice Springs Orogeny. This indicates a general stable intra-cratonic setting with repeated strain localisation at pre-existing zones of weakness during this long-lasting, ca. 1.5 billion year evolution of the central Australian crust.

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SGTSG 2015: Riding the Waves References Buick I.S., Miller J.A., Williams I.S., Cartwright I., 2001, Ordovician high-grade metamorphism of a newly recognised late Neoproterozoic terrane in the northern Harts Range, central Australia, Journal of Metamorphic Geology, 19, 373-394 Scrimgeour I., Raith J.G., 2001, High-grade reworking of Proterozoic granulites during Ordovician intraplate transpression, eastern Arunta Inlier, central Australia, In: Miller J.A., Holdsworth R.E., Buick I.S., Hand M. (eds), Continental Reactivation and Reworking, Geological Society, London, Special Publication, 184, 261-287 Scrimgeour I., Smith J.B., Raith J.G., 2001, Palaeoproterozoic high-T, low-P metamorphism and dehydration melting in metapelites from the Mopunga Range, Arunta Inlier, central Australia, Journal of Metamorphic Geology, 19, 739-757

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Structural development of the early Archaean Doolena Gap greenstone belt, East Pilbara Terrane (Western Australia) DANIEL WIEMER 1 *, CHRISTOPH E. SCHRANK 1 ' 2 , DAVID T. MURPHY 1 AND ARTHUR H . HICKMAN 3 Queensland University of Technology, 2 George St, Brisbane, QLD 4000, Australia University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia Geological Survey of Western Australia, Department of Mines and Petroleum, 100 Plain St, East Perth, WA 6004, Australia *correspondance: d.wiemer@qut.edu.au

2 3

Regional crustal-scale dome-and-keel structures in the Archaean East Pilbara Terrane (EPT, Western Australia) are explained through gravitational overturn of buoyant felsic substrate overlain by dense mafic greenstones [1]. In the southeastern EPT, dome-and-keel development is thought to postdate deposition of the -3.33 Ga Euro Basalt. Thermal softening of buried radiogenic 3.47-3.43 Ga granitic mid-crust triggered vertical re-organization through sagduction of greenstones and buoyant rise and expansion of domes [2]. Older (>3.43 Ga) deformational structures have been reported locally, but their development remains largely unexplained [3]. Here, we present a detailed lithostratigraphic and structural analysis of a granite-greenstone traverse in the western Doolena Gap greenstone belt (DGGB) of the northern EPT. The traverse reveals four structural domains: i) orthogneisses of the polyphase Muccan Granitoid Complex (MGC), ii) the mylonitic South Muccan Shear Zone (SMSZ), iii) a Central Fold Belt (CFB) of dominantly mafic greenschists, and iv) a well-preserved southern Low Strain Belt (LSB) featuring pillow basalts and intercalated metasediments. From the keel to the dome margin, metamorphic facies and deformational intensity increase across domain boundaries. Domain boundaries and anastomosing intra-domain shear zone networks are marked by significant carbonate ± quartz alteration and high-strain non-coaxial shear deformation with dome-up kinematics. The studied greenstone successions have been ascribed to the 3.47 Ga Mount Ada Basalt (MAB) and the Duffer Formation (DF), overlain by the <3.43 Ga Strelley Pool Formation (SPF) across an angular unconformity [4] that suggests -45° tilting of the MAB and DF prior to -3.43 Ga, provided that the stratigraphic correlation with the SPF is correct. We interpret this tilting as a consequence of doming of the MGC to the North. Geometry and shape of synanatectic tight folds within the MGC resemble those of isoclinal folds (F2) within the CFB. We therefore propose that partial melting within the MGC triggered its buoyant rise and expansion, resulting in D2 passive shear folding and heterogeneous shear deformation accompanied by influx of carbonaceous fluids along discrete deformation zones within the CFB, and development of the mylonitic SMSZ at the dome-keel interface. Within the CFB, prolate fabrics indicated by D2 Ltectonites become increasingly common towards the South. In conjunction with mineral lineations (L2) this may indicate constrictional flow (sagduction) of the keel rocks to the SW, likely coupled with the early doming. F2 folds deform an earlier localized shear foliation (Si) and associated Di carbonate veins, which possibly record the initiation of the main D2 event along the flanks of the rising MGC. Structures post-dating -3.43 Ga SPF deposition include asymmetric, open upright folds (F3) superimposed on earlier structures within the CFB, and shear zone networks penetrating the LSB including the SPF. D3 indicates tightening of the D2 greenstone keel associated with protracted domeup dextral flow towards a dome triple junction to the SW. The entire belt is cut by late NE-SW-striking faults (D4) that exhibit dominantly brittle deformation in the LSB but ductile drag folding (F4) in the CFB. Therefore, the LSB may have overlain the CFB during this D4 event. The two domains were juxtaposed during late-stage oblique strike slip (D5) along local reactivated S2 foliation and carbonate zones. In summary, we propose a structural history of the DGGB in which successive deformation events can be related to the episodic emplacement of the MGC and associated sagduction of the keel rocks. Our study suggests that dome-and-keel initiation commenced at least locally prior to -3.43 Ga. Polyphase localized thermo-hydro-chemical weakening of mafic greenstones accommodated strain during the deformational episodes within the keel.

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SGTSG 2015: Riding the Waves References [1]

Collins et al., 1998. Journal of Structural Geology, 20, 9/10, pp. 1405-1424

[2]

Sandiford et al., 2004. Tectonics, 23, TC1009, doi:10.1029/2002TC001452

[3]

Van Kranendonk et al., 2002. Economic Geology, 97, pp. 695-732

[4]

Van Kranendonk, 2010. Geology of the Coongan, Geological Survey of Western Australia Report

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Investigation of fabrics in quartz and ice: Comparison and applications of different analytical methods C.J.L. WILSON 1 , N . HUNTER 1 , V . LUTZIN 2 , M . PETERNELL3 AND S. PIAZOLO 4 1

School of Earth, Atmosphere and Environment, Monash University, 3800, Australia Australian Nuclear Science and Technology Organisation (ANSTO), Lucas Heights, 2234, Australia Institute of Geo sciences, University of Mainz, 55099, Mainz, Germany 4 Department of Earth Sciences, Macquarie University, 2109, Australia 2 3

Our understanding of fabric evolution in rocks and ice masses has advanced from simply measuring the c-axis. So too have the methods available to extract crystallographic orientation data. Methods such as neutron diffraction (ND), electron backscattering diffraction (EBSD) can potentially provide a higher density of information and data on additional crystallographic axes. In addition, techniques utilizing EBSD and automated optical fabric analysers (FA) have the advantage of producing spatially referenced data with the ability to generate Achsenverteilungsanalyse (AVA) or axial distribution diagrams (Sander, 1950). Such a diagram, essentially a map of crystallographic orientation within a 2D slice from a sample, can help identify spatially distinct grains or groups of grains. In this contribution we will illustrate and draw comparisons between the outputs between the three main techniques currently being used in the structural geology community.

FABRIC ANALYSER

EBSD

Propcted using Onent, gnd resolution 50, 10 contour levels. Kamb method (sigma • 4)

Protected using Onent, gnd resolution 50, 20 contour levels. Kamb method (sigma • S)

Fig. 1: Comparison of FA, EBSD and ND data from AK-6-1. FA data analysed using Investigator software (Wilson et al., 2007). EBSD data collected using Channel5 software. ND data collected by exporting 8000 individual orientations randomly selected from the total dataset. Two stereonet contouring types were used to identify parts of the maxima that are slightly 'masked' by higher density areas.

In the case of quartz we have collected quartz c-axis data for a sample (AK-6-1), from the Main Central Thrust in NW India, using each technique. Rather than working with 'one data point per grain', we collected individual orientation points from a transect of the sample: FA data was taken from 8000 points in a 2.5 x 2.5cm transect of a thin section; EBSD data was taken from -6000 points in a 2.15 x 2.15 mm transect; ND data was taken from 6000 full orientations in a 2.5 x 2.5cm cube. All FA data points with either a geometric quality or retardation quality <75 was omitted from the data (Peternell et al., 2010); these data points and are typically associated with non-quartz phases or grain boundaries. After omission, the total points for FA data was 5688, giving a similar sample size (n) across the data sets. All datasets were projected as pole figures and a combined PGR diagram, which, describes the data based on eigenvectors and defines the degree to which the data define a point (P), girdle (G) or random (R) distribution (Fig. 1). Each quartz pole figure defines an asymmetric single girdle. In the FA data there is a broad area around the Y-axis where there is very little data. This is a common issue in FA data acquisition and is related 155

SGTSG 2015: Riding the Waves to issues with the FA to capture such high angle c-axis orientations. As a result, the data trends more towards a P-distribution than a G-distribution. The EBSD data is very dispersed and is probably related to the small transect area where the 5 major spots probably represent 5 very large quartz grains. This is the major problem with the EBSD technique as collecting 'maps' with decent beam precision is time consuming. Whereas, the ND data is, by far, the most accurate. The activity of prism-<a> slip is clear in the sample, and the faint cross-girdle maxima (second column) also demonstrates rhomb-<a> and basal-<a> slip. An obvious advantage of ND is that you can obtain fully quantitative microfabric results from a 3D sample. This we have employed in undertaking 3D in situ experiments to collect data from polycrystalline heavy water (D 2 0) ice deformed in a dynamic regime (Piazolo et al., 2013). The ice and temperature (-7 °C) chosen for this study is used as a direct analogue for deforming natural-water ice (and quartz) as it offers a unique opportunity to link grain size and texture evolution in natural ice at -10 °C. Despite some limitations, the FA is also an invaluable technique to understand the processes operating on a grain scale during 2D in situ deformation experiments. Where deformation maps of grain microstructure can be compared with the temporal evolution of crystallographic fabrics. To illustrate this, we will use experiments conducted on deuterated ice. In constant strain rate experiments fabric stabilizes between a single-point maximum and a girdle distribution. If there is long strain-rate cycling then the principal eigenvector El is rotated approximately parallel to the shortening direction, and the fabric becomes an intermediate girdle-cluster-type fabric pattern. For short-interval strain-rate experiments the fabric remains unchanged and near isotropic. All experiments illustrate that steadystate rheology is not necessarily coupled to microstructural and fabric stability. References Sander, B., 1950. Einfuehrung in die Gefuegekunde der Geologischen Koerper; Zweiter Teil, Die Korngefuege. Springer-Verlag, Wien-Innsbruck. Peternell, M., Hasalova, P., Wilson, C.J.L. Piazolo, S. and K. Schulmann. K., 2010. Evaluating quartz crystallographic preferred orientations and the role of deformation partitioning using EBSD and Fabric Analyser techniques. J. Struct. Geol., 32, 803-817. Piazolo, S., Wilson, C.J.L., Luzin, V., Brouzet, C., Peternell, M., 2013. Dynamics of ice mass deformation: Linking processes to rheology, texture and microstructure. G-Cubed, 14, 4185-4194. Wilson, C. J. L., Russell-Head, D. S., Kunze and Viola, G., 2007, The analysis of quartz c-axis fabrics using a modified optical microscope, J. Microscopy, 227, 30-41.

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The Late Cretaceous to recent tectonic history of the Pacific Ocean basin NICKY M . WRIGHT1, MARIA SETON1, SIMON E. WILLIAMS1 AND R. DIETMAR MULLER1 ]

EarthByte Group, School of Geosciences, The University of Sydney, Sydney, NSW 2006, Australia

A vast ocean basin has spanned the region between the Americas, Asia and Australasia for well over 100 Myr, represented today by the Pacific Ocean. However, the longevity of the Pacific basin belies the complex history of the oceanic crust that lies beneath, involving an ever-changing arrangement of tectonic plates and plate boundary configurations. These changes have been dominated by a number of plate fragmentation and plate capture events, such as the formation of the Vancouver, Nazca, and Cocos plates from the break-up of the Farallon plate, and the incorporation of the Bellingshausen, Kula, and Aluk (Phoenix) plates. Previous regional tectonic models of the Pacific typically restrict their scope to either the North or South Pacific, and global kinematic models fail to incorporate many of the complexities in the Pacific plate evolution (e.g. Bellingshausen and Aluk independent motion), thereby limiting their usefulness for understanding events and processes occurring in the Pacific realm. We derive relative plate motions (with 95% uncertainties) for the Pacific-Farallon/Vancouver, KulaPacific, Bellingshausen-Pacific, and early Pacific-Antarctic spreading systems, based on recent data including marine gravity anomalies, well-constrained fracture zone traces and a large compilation of magnetic anomaly identifications. We find our well-constrained relative plate motions result in a good match to the fracture zone traces and magnetic anomaly identifications in both the North and South Pacific. In conjunction with recently published and well-constrained relative plate motions for other Pacific spreading systems (e.g. Aluk-Antarctic, Cocos-Pacific, recent Pacific-Antarctic spreading), we explore variations in the age of the oceanic crust, seafloor spreading rates and crustal accretion and find considerable refinements have been made in the central and southern Pacific. Asymmetries in crustal accretion within the overall Pacific basin (where both flanks of the spreading system are preserved) have typically deviated less than 5% from symmetry, and large variations in crustal accretion along the southern East Pacific Rise (i.e. Pacific-Nazca/Farallon spreading) appear to be unique to this spreading corridor. Through a relative plate motion circuit, we explore the implied convergence history along the North and South Americas, where we find that the inclusion of small tectonic plate fragments such as the Aluk plate along South America are critical for reconciling the history of convergence with onshore geological evidence.

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Author Index Abdullah R. Agostino K. Ailleres L. Aitchison J.C. Alemu T. Alessio B. Al-Khirbash S. Allen C.M. Amrouch K. Arboit F. Archibald D.B. Armistead S.E. Armit R.J. Babaahmadi A. Bailey A.H.E. Becchio R. Beck D. Berry R.F. Betts P.G. Beyer E.E. Bhowmik S.K. Blades M.L. Blenkinsop T. Boutelier D.A. Brooke-Barnett S. Brownlow J. Bryan S.E. Buckman S. Burke-Shyne D. Camacho A. Cayley R. Champion D.C. Chapman S.L. Chapman T. Chen J. Clark C. Clark D. Clarke G.L. Collins A.S. Collins W.J. Cook Y.A. Cooke D.R. Cox S.F. Cracknell M. Cross A.J. Cruden A.R. Cumming G. Cunneen J. Czaplinska D. Czarnota K. Daczko N.R. De Grave J. De Wall H. Dirks P.H.G.M. Donchak P. Doublier M.P. Dunstan S. Duque-Trujillo J.

1 60 5,56 26 27 11 11,27 142 2,3 2,3 4, 27 5 5,9 1,7, 144 74 150 29 108 5, 9, 102 151 26 11,27 136 137,138 12 13, 15 17, 142 95 19 93, 94 13, 25 38 26 23 24 27 36 23, 26 2,3,4, 11,27, 51,52, 60, 73 28, 122 136 24 29,61,62, 128 54 142 100 54 30 117 31,38, 88 23, 33, 34, 35, 49, 106 51,52 40 136 36 38 1 17

Durney D.W. Dutch R. Elders C. Esterle J. Evans L. Evans N. Faucheux V. Fergusson C. Ferrari L. Fielding C. Finch M.A. Fitzgerald J.G. Fitzsimons I.C.W. Flatten A. Foden J.D. Fuentes M.G. Gaina C. Gardner R.L. George A. Gibson G.M. Gillespie J. Glorie S. Gong W. Goodwin J. A. Green D. Grose L. Gust D.A. Hack A.C. Haisley A.D. Hall J.W. Hall M. Hall R. Halpin J. A. Hammelswang J. Hand M. Hawemann F. Hay ward K.S. Heilbronn K.A. Henderson B. Henriquet M. Hickman A.H. Hill K. Hobbs B. Holcombe R. Holden P. Holford S.P. Holm R.J. Holzschuh J. Howard H.M. Hoy D. Hunter N.J.R. Hutton L. Ireland T.R. Jagoutz O. Jepson G.M.M. Jessop K. Jiang X. Johnson T. Jones I. Jourdan F.

40,41 60 30, 42 144 49, 117 51,52, 60, 73 118 44, 64 17 66 45, 150 61 33, 148 29 4, 11,27 150 47 34, 49 128 50 51 27,51,52, 60, 73 77 98 54 56 142 57 59 60 102 120 33, 34, 106, 108 30 74 93, 94 61,62 63 44, 64 138 153 89 65 66 4 74 63, 68, 69 50 124 66, 70 71, 155 50 26 134 73, 74 75,76 77 148 79 124

Karrech A. Keep M. Kelly B. Khudoley A.K. King P. King R.C. Kirkland C.L. Kobler M.E. Konopelko D. Korsh R.J. Laurent G. Lee M. Lennox P.G. LiZ-X Lister G.S. Liu S. Lopez-Martinez M. Lutzin V. Mahoney L. Mamtani M.A. Manatschal G. Mancktelow N. Manton R.J. Marshak S. Martin E. McAlpine S.R.B. Mclnnes B. McLaren S. Meffre S. Meixner T. Molnar N.E. Moore D.H. Moresi L. Morley C. Mukherjee M.K. Mulder J.A. Miiller R.D. Munro M. Murphy D.T. Murray G. Musgrave R.J. Nutman A.P. Offler R. Ord A. Orozco-Esquivel T. Passchier C.W. Payne J.L. Pearce D. Pearce M.A. Pennacchioni G. Peternell M. Phillips D. Phillips G. Piazolo S. Picazo S. Piquer J. Pisarevsky S. Plavsa D. Pownall J.M.

137,138 81 84 82,122 128 2, 3, 74 124 34 73 38 56 83,146 40, 84 86 120 88 17 155 89 117 118, 126 91,93, 94 95 97 28 98 51 89, 128 108 50 100 9, 102 9 2,3 104 106, 108 157 112 19, 153 11 109 95 84, 115 111, 112 17 113 4, 11,27 148 114 93, 94 155 115 57,115, 132 23,49, 117, 118, 155 118 24 27, 141 27 120

Priyatkina N. Purdy D.J. Pym F. Quentin de Gromard R. Quilty P.G. Rainbird R. Razakamanana T. Reddy M. Regenauer-Lieb K. Reid A. Reno B.L. Richard S.W. Roach M. Roache T. Robinson J. A. Rosenbaum G. Royden L. Rubatto D. Sanislav I.V. Schrank C.E. Selley D. Seton M. Shaanan U. Siegel C. Siegfried P. Sliwa R. Smithies R.H. Sonnette L. Spandler C. Speranza F. Spruzeniece L. Stewart A. Thorpe B. Timms N.E. Trenough C. Tucker C. Verdel C. Wang X. Weinberg R.F. Weisheit A. Welsh K. WexS. Whelan J.A. White A.J.R. White N. Whittaker J.M. Wiemer D. Wilkes P. Williams B. Williams S.E. Wilson C.J.L. Wingate M.T.D. Woldetinsae G. Wright N.M. Xiao W. YanJ. Zastroshnov D. Zhang Z.

28, 82, 122 142 30 124 34 82 4, 27 60 126, 137, 138 60 151 68, 69 128 130 132 1,7, 12, 66, 69, 70, 126, 140, 141 134 115 136 19,137, 138, 153 24 157 140, 141 142 27 66, 144 124 146 68 141 117 88 11 148 60 148 79, 83, 128 112 45,71, 126, 150 151 128 93, 94 151 114 31 33, 34 19, 153 148 57 34, 157 71, 155 124 27 157 51,52 84 122 51