The Role of Isostasy in the Evolution and Structural Styles of Fold and Thrust Belts

Ibrahim, Youseph1, Rey, A/ Prof. Patrice1

1University Of Sydney, Sydney, Australia

Fold and thrust belts (FTB) are highly deformed regions that form as the crust accommodates shortening. The evolution of FTB’s records the dynamic interplay between crustal and surface
processes, in conjunction with the rocks’ intrinsic properties. The stacking of thrust sheets and mass transfer of sediment during orogenesis imposes a load on the lower crust and the mantle underneath, inducing isostatic adjustment and a flexural response, which may also contribute to the overall architecture of FTB’s. The tempo at which a fold and thrust belt forms is a consequence of plate kinematics. The tempo of the isostatic response, however, is reliant on the rheology of the mantle and the elastic thickness of the crust. Here, we focus on the role isostasy plays in controlling structural style in FTB’s. We run two-dimensional, coupled thermal and mechanical, numerical experiments using the Underworld framework to explore the interplay between the rate of compression and the rate of isostasy on the structural evolution of FTB’s.

The numerical model runs in a cartesian domain by solving the conservation of energy, mass, and momentum equations. The numerical domain is 42 km wide and 16 km tall, with a grid resolution of 80 m. From top to bottom, the model consists of ‘sticky air’, 4 km of sediment that alternates in competence at 500 m intervals, a 3 km thick basement, and a virtual basal layer, which allows us to implement a local ‘psuedo-isostasy’ boundary condition. Models are run with varying compressional velocities and isostatic rates.

Our suite of models demonstrates the relationship between tectonic and isostatic rates. When the tectonic rate is greater than the isostatic rate, subsidence or flexure is post-tectonic mainly, and
therefore isostasy is unlikely to play a role in the development of the FTB, however, it may modify its architecture post-loading. Alternatively, when the tectonic rate is slower than or equal to the isostatic rate, subsidence will keep pace with tectonic loading. In this scenario, isostasy plays an important role in the development of FTB’s, influencing the topographic elevation generated, the outward extent of the FTB, and thrust fault angles.


Biography

Youseph is a first year Ph.D. student at the University of Sydney studying the evolution and structural styles of fold and thrust belts in Central Australia and Papua New Guinea.

Distinct formation history for deep mantle domains reflected in geochemical differences

Doucet, Dr. Luc1, Li, Pr. Zheng-Xian1, El Dien, Hamed Gamal1,2, Pourteau, Dr. Amaury1, Murphy, Pr. Brendan1,3, Collins, Pr. William1, Mattielli,Pr. Nadine4, Olierook,Dr. Hugo5,6, Spencer,Asso. Pr. Christopher 1,7, Mitchell, Asso. Pr. Ross8

1Earth Dynamics Research Group, TIGeR, School of Earth and Planetary Sciences, Curtin University, Perth WA 6845, Australia, Bentley, Australia, 2Geology Department, Faculty of Science, Tanta University, Tanta, Egypt, 3Department of Earth Sciences, St. Francis Xavier University, Antigonish, , Canada, 4Laboratoire G-Time, Université Libre de Bruxelles, Brussels, Belgium, 5Timescales of Mineral Systems, Centre for Exploration Targeting – Curtin Node, Curtin University, Bentley, Australia, 6John de Laeter Centre, Curtin University,, Bentley, Australia, 7Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Canada, 8State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China

The Earth’s mantle is currently divided into the African and Pacific domains, separated by the circum-Pacific subduction girdle and each domain features a large low shear-wave velocity provinces (LLSVPs) in the lower mantle. However, it remains controversial as to whether the LLSVPs have been stationary through time or dynamic, changing in response to changes in global subduction geometry. Here we compile radiogenic isotope data on plume-induced basalts from ocean islands and oceanic plateaus above the two LLSVPs which show distinct Pb, Nd and Sr isotopic compositions for the two mantle domains. The African domain shows enrichment by subducted continental material during the assembly and breakup of the supercontinent Pangaea, whereas no such feature is found in the Pacific domain. This deep-mantle geochemical dichotomy reflects the different evolutionary histories of the two domains during the Rodinia and Pangaea supercontinent cycles and thus supports a dynamic relationship between plate tectonics and deep mantle structures.


Biography

After a PhD in St Etienne, France (2012), Luc moved to Brussels to apply the “non-traditional” stable-isotope systematics on Archean igneous. After an academic career break, he joined Professor Li and the Earth Dynamics Research Group (Curtin University) to work on the present-day and past connections between Earth’s mantle, supercycles.

Paleogeography of the western margin of Rodinia: New findings from Madagascar and Seychelles

Zhou, Dr Jiu-long1, Li, Prof. Xian-Hua1,2, Li,Prof. Zheng-Xiang3

1State Key Laboratory Of Lithospheric Evolution, Institute Of Geology And Geophysics, Chinese Academy Of Sciences, Beijing, China, 2College of Earth Sciences, University of Chinese Academy of Sciences, Beijing, China, 3Earth Dynamics Research Group, ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS) and The Institute for Geoscience Research (TIGeR), School of Earth and Planetary Sciences, Curtin University, Perth, Australia

* Email: zhoujl_geo@126.com

We present new and existing geological and geochemical observations from Madagascar and Seychelles, both are believed to have faced the Mirovoi superocean during Rodinia time, in order to shed new light on the evolution of Rodinia’s western margin. (1) 870-750 Ma plutonic rocks in central Madagascar (i.e., the Imorona-Itsindro Suite) have a general bi-modal nature. The felsic components have a ferroan A-type composition, were likely derived from ancient continental crust, and show a positive zircon εHf(t) excursion during a 790 Ma high-flux magmatic pulse. The gabbroic components host layered Fe-Ti-V oxides and chromitites, and were likely derived from an enriched lithospheric mantle with no clear asthenospheric input. (2) Clastic sedimentary rocks in central Madagascar coeval with the Imorona-Itsindro Suite contain detrital zircons of dominantly Archean to Paleoproterozoic ages. (3) Neoproterozoic granitoids in Seychelles were emplaced during two discrete magmatic pulses: an older and weaker one at 810 Ma and a younger and more intense one at 760–745 Ma. Primary low-δ18O signals are absent from the 810 Ma granitoids but characterize the 750 Ma granitoids. The 750 Ma Mahé-type (Nd-isotopically juvenile) granitoids appear to display a stronger 18O-depletion than the 750 Ma Praslin-type (Nd-isotopically enriched) granitoids. Some of the 750 Ma 18O-depleted rocks even document intra-zircon δ18O decreases from core to rim. Thus, the Seychelles granitoids likely acquired their low-δ18O signatures through the dynamic magma process of crustal cannibalization, instead of through source inheritance from a pre-existing basement. (4) Neoproterozoic magmatism in the Bemarivo Terrane (northern Madagascar) also has a two-episodes age-distribution: an older 760–745 Ma one represented by the Antsirabe Nord Suite, and a younger 740–700 Ma one represented by the Manambato Suite and the Daraina-Milanoa Group. The Antsirabe Nord granitoids are ferroan A-type in chemistry, have negative ɛHf(t) values and low-δ18O signatures, and were likely generated by remelting of an ancient basement source. In contrast, the Manambato granitoids and Daraina-Milanoa rhyolites are chemically calc-alkaline and Hf-isotopically juvenile, with progressively disappearing low-δ18O signatures. Generation of these rocks involves a significant input of asthenospheric materials.

The pre-745 Ma Neoproterozoic igneous rocks in central Madagascar, the Bemarivo Terrane, and Seychelles bear strong resemblance to each other in geochemistry and petrogenesis, and likely have been produced within continental rift(s). However, the post-745 Ma igneous rocks, only found in the Bemarivo Terrane, likely signified the onset of arc magmatism. We thus conclude that Rodinia’s western margin experienced a transition from a protracted passive margin setting at 870–745 Ma to an arc setting at 740–700 Ma at this segment of the eastern edge of the Mirovoi superocean. The 740 Ma  subduction initiation age here is significantly younger than previously proposed.


Biography

Jiu-Long Zhou obtained his Ph.D. degree from the China University of Geosciences (Beijing) in 2016, and then received postdoctoral training at the Institute of Geology and Geophysics, Chinese Academy of Sciences. His research interests focus on utilizing geochemical methods to understand supercontinent cycles and associated igneous processes.

Linking supercontinents to a convective mantle framework

Martin, Erin L.1,3, Cawood, Peter A.1, Murphy, J. Brendan2,3

1School of Earth, Atmosphere and Environment Science, Monash University, Clayton, Australia, 2Department of Earth Sciences, St Francis Xavier University, Antigonish, Canada, 3Earth Dynamics Research Group, The Institute for Geoscience Research (TIGeR), School of Earth and Planetary Sciences, Curtin University, Perth, Australia

The amalgamation of continental fragments into supercontinents can occur by processes of introversion, involving the closure of interior oceans, extroversion, in which the exterior ocean closes, or orthoversion, entailing formation 90° from the centroid of the previous supercontinent. However, individual supercontinents are often defined as forming by contradictory mechanisms; for example, Pangea has been argued as forming by introversion and by extroversion. Conflicting interpretations arise, in part, from attempting to define an ocean as interior or exterior based on paleogeography, or the age of oceanic crust relative to the time of supercontinent breakup. We argue that interior and exterior oceans should be defined relative to the peripheral subduction ring and its associated accretionary orogens that surround the amalgamated supercontinent. The subduction ring broadly divides the Earth into two cells, which conform to the spherical harmonic degree-2 structure of the mantle: one associated with supercontinent assembly, and therefore dominated by continental crust with only minor oceanic crust, and the other containing almost exclusively oceanic crust, which is subducted to create peripheral accretionary orogens at the margin of the supercontinent. All oceans within the cell that contains continental blocks are interior oceans, as they are interior to the continental cell of the degree-2 planform. By contrast, the exterior ocean is the oceanic cell antipodal to the continental cell, separated by the subduction ring. Interior oceans close following asymmetrical subduction and collisional orogenesis. However, for the exterior ocean to close, the subduction ring must collapse upon itself, leading to the juxtaposition of long-lived accretionary orogens within the core of the supercontinent. Employing this geodynamic definition for interior and exterior oceans, Rodinia formed by extroversion, but all other supercontinents formed by introversion which cannot occur without orthoversion.


Biography

Erin Martin is a research associate working with as part of the Pulse of the Earth ARC Laureate Fellowship team at Monash University. Erin completed her PhD at Curtin University with the Earth Dynamics Research Group.

Development of William’s Ridge, Kerguelen Plateau and Broken Ridge: tectonics, hotspot magmatism, microcontinents, and Australia’s Extended Continental Shelf

Coffin, Millard F1.; Whittaker, Joanne1; Daczko, Nathan2; Halpin, Jacqueline1; Bernardel, George3; Picard, Kim3; Gardner, Robyn2; Gürer, Derya4; Brune, Sascha5; Gibson, Sally6; Hoernle, Kaj7; Koppers, Antonius8; Storey, Michael9; Uenzelmann-Neben, Gabriele10; Magri, Luca1; Neuharth, Derek5; Christiansen, Sascha Høegh9; and Easton, Laura3

1Institute For Marine & Antarctic Studies, University Of Tasmania, Hobart, Australia, 2Macquarie University, Sydney, Australia, 3Geoscience Australia, Canberra, Australia, 4University of Queensland, Brisbane, Australia, 5GFZ German Research Centre for Geosciences, Potsdam, Germany, 6University of Cambridge, Cambridge, United Kingdom, 7GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany, 8Oregon State University, Corvallis, United States of America, 9Natural History Museum of Denmark, Copenhagen, Denmark, 10Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany

William’s Ridge, a ~300-km-long salient extending southeast from the Central Kerguelen Plateau, and Broken Ridge are conjugate divergent margins in the southern Indian Ocean that separated at ~43 Ma. In early 2020, scientists aboard Australia’s Marine National Facility, RV Investigator, acquired multichannel seismic reflection (MCS), sub-bottom profiling, multibeam bathymetry, and gravity data on these margins, as well as dredged rock samples, on a 57-day voyage. The research project constitutes the first-ever case study of conjugate oceanic plateau end-member tectonic plates, with the goal of advancing knowledge of lithospheric rifting, breakup, and initial plate separation processes. The first-ever dedicated multibeam mapping of William’s and Broken ridges encompassed ~52,000 km2 and ~43,000 km2, respectively. Four new RV Investigator MCS profiles (500 line-km) across William’s Ridge complement one legacy RV Rig Seismic and three new RV Sonne MCS profiles; five new RV Investigator MCS profiles (603 line-km) across the conjugate portion of Broken Ridge are the first to be acquired on that feature. Multibeam bathymetry and MCS transects of William’s Ridge show multiple linear ridges and troughs interpreted as horst and graben. In contrast, multibeam bathymetry and MCS transects of Broken Ridge show a prominent E-W scarp (Diamantina Escarpment) with a complex morphology of emanating en echelon crustal blocks and depressions at the base of the scarp. Prominent angular unconformities (middle Eocene hiatus?) characterize the sedimentary section on some ridges, and dipping reflection sequences within interpreted igneous basement suggest subaerial basalt flows. Rock dredges on the facing conjugate margin fault scarps targeted all stratigraphic levels exposing basement rocks. Nine on William’s Ridge yielded both oceanic and (in situ?) continental rocks; eight on Broken Ridge yielded solely oceanic rocks. The new geophysical data and geological samples may justify a new or revised submission to the United Nations Commission on the Limits of the Continental Shelf to extend Australia’s marine jurisdiction on and around William’s Ridge under the United Nations Convention on the Law of the Sea.


Biography

Marine geophysicist Mike Coffin investigates interactions between the solid Earth and the oceanic environment. Educated at Dartmouth College (A.B.) and Columbia University (M.A., M.Phil., Ph.D.), he has pursued an international career in Australia, France, Japan, Norway, the UK, and the USA. His 35 research voyages span the global ocean.

The first continuous global full-plate animation back to 2 Ga

Li, Zheng-Xiang1, Wu, Lei1,2, Liu, Yebo1, Pisarevsky, Sergei1,

1Earth Dynamics Research Group, TIGeR, School of Earth and Planetary Sciences, Curtin University, Perth WA 6845, Australia, 2Department of Earth & Planetary Sciences, McGill University, 3450 Rue University, Montréal, Québec, H3A 0E8, Canada

Creditable global plate reconstructions back to early Earth are crucial for understanding the evolution and driving mechanisms of the Earth system. Here we present the first continuous global full-plate reconstruction model back to 2 Ga, featuring the assembly and break-up of three successive supercontinents: Nuna (tenure ca. 1600–1300 Ga), Rodinia (900–700 Ma) and Pangaea (320–170 Ma). We revised the configuration of both Nuna and Rodinia based on the updated global palaeomagnetic database with correction applied to inclination shallowing, and calibrated our reconstruction using geological and tectonic databases and prior knowledge. We generally adopt an orthoversion longitudinal rule for supercontinent assembly (i.e., each supercontinent is longitudinally 90° away from the previous one), but provide an alternative for longitudinal evolution between Nuna and Rodinia. We also adopt an alternating introversion and extroversion mechanism for supercontinent assembly, with Rodinia being assembled through introversion, and Pangaea extroversion. For Rodinia reconstruction, we recognise the possible repeated true polar wander events during the tenure of the supercontinent and interpret the phenomenon as inertia interchange true polar wander (IITPW) due to an inherited degree-2 mantle structure from the Nuna cycle. We used the palaeomagnetic reference frame (i.e., a reference frame fixed to the spin axis) for our reconstruction instead of plume or LLSVP reference frames as increasing evidences suggest a dynamic nature of deep mantle structures that are linked to the supercontinent and superocean cycles and plate tectonics.


Biography

Zheng-Xiang Li is an ARC Laureate Fellow and a Co-Leader of IGCP 648. After PhD at Macquarie, he worked for UWA for >17 years before joining Curtin in 2007. The Earth Dynamics Research Group that he leads aims to explore Earth’s evolution since the Proterozoic and the dynamic driving mechanisms.

Structural evolution of a 1.6 Ga orogeny related to the final assembly of the supercontinent Nuna: coupling of episodic and progressive deformation

Volante, Dr Silvia1,2, Collins, Prof William J.1, Pourteau, Dr Amaury1, Li,Prof Zheng-Xiang1, Li, Jiangyu1, Nordsvan, Dr Adam1,3

1Earth Dynamics Research Group, Australian Research Council Centre of Excellence for Core to Crust Fluid Systems (CCFS) and The Institute for Geoscience Research (TIGeR), School of Earth and Planetary Sciences, Curtin University, Perth, Australia, 2Institute of Geology, Mineralogy and Geophysics, Ruhr-Universität Bochum, Bochum, Germany 3Department of Earth Sciences, University of Hong Kong, Pokfulam, Hong Kong

The poly-deformed Georgetown Inlier (GTI) in NE Australia has recently been suggested to record a 1.60 Ga orogenic event related to final Nuna assembly. However, the structural evolution of the inlier has remained poorly constrained at the regional-scale, and major tectono-thermal events occurred at c. 1.55 Ga. The GTI is the type-region for conceptualisation of crenulation cleavage development and where the foliation intersection axes (FIAs) approach has been applied. We re-evaluated both concepts by combining a multiscale petrostructural analysis with recent petrological and geochronological data. Three main deformation events (D1, D2, D3) and associated composite fabrics (S1, S2, S3) are identified in the GTI. The original NE-orientation of 1.60 Ga D1 compressional structures is preserved in the low-grade western domain, and the associated composite S1 fabric is retained as microstructural relicts within c. 1.55 Ga D2 low-strain domains to the east. Extensional D2 structures, characterised by a pervasive, high-grade, composite S2 foliation throughout the central and eastern domains, are interpreted as the footwall of a regional N-S-trending, W-dipping crustal-scale detachment zone. Syn-D2 S-type granites formed at 1.55 Ga as the detachment evolved. D1 stage was associated with Nuna assembly, whereas D2 represents post-collisional extension. Progressive foliation development occurred twice in the GTI, at 1.60 Ga (D1) and 1.55 Ga (D2), but the previous FIA analysis only records the 1.60 Ga event and cannot be easily reconciled with the regional structural analysis. This study highlights that a multiscale and multi-disciplinary approach is required to unravel the structural history of orogenic belts.


Biography

Our interest lies in reconstructing the structural, magmatic and metamorphic history of Proterozoic inliers to unravel the evolution of NE Australia during the Mesoproterozoic final assembly of the supercontinent Nuna by applying a multi-disciplinary and multi-scale approach which combines structural analysis with geochronology, metamorphic and igneous petrology and geochemistry.

Mantle refertilization from 3.2 billion years ago points to an early start of plate tectonics

Gamal EL Dien, Hamed1*, Doucet, Luc-Serge1, Murphy,J. Brendan1, 2, Li,Zheng-Xiang1

1 Earth Dynamics Research Group, The Institute for Geoscience Research (TIGeR), School of Earth and Planetary Sciences, Curtin University, GPO Box U1987, Perth, WA 6845, Australia, 2 Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia, Canada

* Corresponding author: E-mail address: hamed.gamaleldien@curtin.edu.au

Progressive mantle melting during the Earth’s earliest evolution led to the formation of a depleted mantle and a continental crust enriched in highly incompatible elements. Re-enrichment of Earth’s mantle can occur when continental crustal materials begin to founder into the mantle by either subduction or, to a lesser degree, by delamination processes, profoundly affecting the mantle’s trace element and volatile compositions. Deciphering when mantle re-enrichment/refertilization became a global-scale process would reveal the onset of efficient mass transfer of crust to the mantle and potentially when plate tectonic processes became operative on a global-scale. Here we document the onset of mantle re-enrichment/refertilization by comparing the abundances of petrogenetically significant isotopic values and key ratios of highly incompatible elements compared to lithophile elements in Archean to Early-Proterozoic mantle-derived melts (i.e., basalts and komatiites). Basalts and komatiites both record a rapid-change in mantle chemistry around 3.2 billion years ago (Ga) signifying a fundamental change in Earth geodynamics. This rapid-change is recorded in Nd isotopes and in key trace element ratios that reflect a fundamental shift in the balance between fluid-mobile and incompatible elements (i.e., Ba/La, Ba/Nb, U/Nb, Pb/Nd and Pb/Ce) in basaltic and komatiitic rocks. These geochemical proxies display a significant increase in magnitude and variability after ~3.2 Ga. We hypothesize that rapid increases in mantle heterogeneity indicate the recycling of supracrustal materials back into Earth’s mantle via subduction. Our new observations thus point to a ≥3.2 Ga onset of global subduction processes via plate tectonics.


Biography

I am a Ph.D. student working at Earth Dynamics Research Group, Curtin University in three major projects: (1) Geochemical records linking mantle plumes, supercontinent cycles and plate tectonics, (2) Crustal growth of the Arabian-Nubian Shield and Neoproterozoic mantle dynamics, and (3) Subduction zone geochemical cycle

Partial melting, granulites, retrogression and their control on late orogenic exhumation processes

Bénédicte Cenki-Tok 1, 2, Patrice F. Rey 2, Diane Arcay 1

1 Géosciences Montpellier, Université de Montpellier, CNRS, 34095 Montpellier cedex 5, France, 2 Earthbyte Research Group, School of Geosciences, University of Sydney, NSW 2006, Sydney, Australia

Orogenesis drives the differentiation of the continental crust through metamorphic and magmatic processes, the exhumation of deep metamorphic terranes and the concomitant formation of sedimentary basins. A major consequence of prograde metamorphism following a typical orogenic thermal gradient is the dehydration and partial melting of buried rocks leading to the formation of migmatites and granulites. Partial melting and granulitisation are often intertwined and primarily linked to the availability of fluids. Here, we consider the thermal and mechanical consequences of coupled partial melting, granulitisation and strain-rate dependent retrogression during the orogenic cycle, in particular during the recovery phase when the crust’s thickness and geotherm re-equilibrate. We explore through 2D thermo-mechanical modelling how the interplay between mechanical weakening due to partial melting and mechanical strengthening due to granulitisation impacts the formation and preservation of crustal roots, the exhumation of the partially molten crust in gneiss domes, the formation of HT/UHT terranes and the partitioning of deformation through the crust.

Our results show that the survival of granulites, which strengthen the lower crust and decrease its capacity to flow under gravitational stresses, impedes the formation of migmatite-cored gneiss domes, and controls the formation and preservation of thick and strong granulitic roots. These are strong enough to stay immune to gravitational stresses and persist over hundreds of million years. These can be actually compared with stable intracontinental regions where the presence of localized crustal roots explains the remarkable variability – from 25 to 65 km – of crustal thickness. Finally, our results highlight the importance of an elevated radiogenic heat production in the upper crust in order to form the long-lived HT/UHT terranes often resulting from supercontinents amalgamation. Our experimental results explain as well why some ancient orogenic domains expose at the Earth’s surface dominantly granulitic terranes (e.g., South India, Sri Lanka, Madagascar, Antarctica, Baltica), whereas others (Variscides) expose dominantly migmatitic and granitic crust.


Biography

Bio to come

Review of SHRIMP zircon ages for the Eastern Succession of the Mount Isa Province and its provenances and comparison with the Etheridge Province

Withnall, Ian1

1Geological Survey Of Qld, Brisbane, Australia

The migration of zircon geochronology data collected by Geoscience Australia (GA) and Geological Survey of Queensland(GSQ)  from the Mount Isa Province into the Online Geochron Delivery System, an important repository maintained by GA, provided an opportunity to review the data and replot it using a consistent approach. This included data for which only preliminary plots of had been available to GSQ and never published.

The review highlighted that the main magmatic events that would have contributed zircon to the Eastern Succession sedimentary rocks occurred at 1850–1870 Ma, 1790–1800 Ma, 1780 Ma, 1760 Ma, 1735–1745 Ma, 1725 Ma, 1705–1715 Ma and 1670–1680 Ma and volumetrically smaller events at 1770 Ma, 1755 Ma, 1655–1660 Ma and 1650 Ma.

The Soldiers Cap Group in the easternmost part of the Mount Isa Province and extending under cover to the east is younger than most of the eastern succession. It consists of Llewellyn Formation, Mount Norna Quartzite and Toole Creek Volcanics in ascending stratigraphic order. The Kuridala Group comprises the Starcross Formation and Hampden Slate.

Samples of the two lowermost units of the Soldiers Cap Group and Starcross Formation have similar maximum depositional ages. A closer comparison has been made of their respective provenances by pooling analyses for units in each group. These provenances are similar, indicating a minor, very old source around the Archean–Proterozoic boundary and then almost none up to ~1900 Ma (the Barramundi Orogeny). Except for minor components from the Kalkadoon–Leichhardt basement (1850–1870 Ma ) and Argylla Formation (1780 Ma), by far the major sources appear to be the Wonga–Burstall–Gin Creek plutonic suites at ~1740 Ma and Fiery Creek Volcanics or Weberra Granite at ~1710 Ma. They also both have a significant younger component (slightly older in the Soldiers Cap Group at ~1685 Ma, and ~1675 Ma in the Starcross Formation). Pooling analyses from the Hampden Slate indicates that apart from the youngest component being ~1655 Ma, other components are almost identical to those in the Starcross Formation.

By contrast the provenance of the Toole Creek Volcanics is dissimilar to the other units. It shows an isolated, almost unimodal population at ~1658 Ma, with small populations at ~1795Ma, ~1850Ma, and ~ 2680Ma.

Comparing the provenance spectra of the lower part of the Soldiers Cap and Kuridala Groups with those of the lower part of the Etheridge Group in the Etheridge Province (Georgetown region) suggests that they were probably deposited at about the same time, but the provenance patterns are strikingly different. The Etheridge Group shows a large Archean component as well as almost continuous spread of data points throughout the Paleoproterozoic including peaks around 1900–2000 Ma. This dissimilarity has been cited as evidence that the Georgetown rocks were not distal to Mount Isa and were part of Laurentia until welded to the Australian craton during the assembly of Nuna. The provenance of the upper part of the Etheridge Group, however, is like that of the Toole Creek Volcanics.


Biography

Ian Withnall spent 42 years with GSQ in regional studies. He was principal compiler of the Queensland Geology 1:2M-scale map and a major contributor to the Geology of Queensland volume, before retiring in 2014. He continues at GSQ voluntarily, working on NW Queensland geology and assisting with map publishing.

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About the GSA

The Geological Society of Australia was established as a non-profit organisation in 1952 to promote, advance and support Earth sciences in Australia.

As a broadly based professional society that aims to represent all Earth Science disciplines, the GSA attracts a wide diversity of members working in a similarly broad range of industries.