Palaeomagnetism argue against a stable supercontinent during the Archaean-Proterozoic transition

Liu Yebo1, Mitchell Ross N.2,1, Li Zheng-Xiang1, Kirscher Uwe1,3, Pisarevsky Sergei A1,4, and Wang Chong1,2,5

1Earth Dynamics Research Group, The Institute for Geoscience Research (TIGeR), School of Earth and Planetary Sciences, Curtin University, Bentley, Western Australia, 6102, Australia; 2State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; 3Department of Geosciences, University of Tübingen, Sigwartstr. 10, 72076 Tübingen, Germany; 4Institute of the Earth’s Crust, Siberian Branch of the Russian Academy of Sciences, Irkutsk 664033, Russia; 5Department of Geosciences and Geography, University of Helsinki, Helsinki, Finland

Many Archaean cratons exhibit Proterozoic rifted margins, implying they were pieces of some ancestral landmass(es). The idea that such an ancient continental assembly represents an Archaean supercontinent has been proposed, but remains to be justified. Starkly contrasting geological records between different clans of cratons have inspired an alternative hypothesis where cratons were clustered in multiple, separate “supercratons”. A new palaeomagnetic pole from the Yilgarn Craton of Australia, when compared with available coeval poles globally, is compatible with either two successive but ephemeral supercontinents, or two stable supercratons across the Archaean-Proterozoic transition. Neither interpretation supports the existence of a single, long-lived supercontinent, suggesting that Archaean geodynamics were fundamentally different from subsequent times (Proterozoic-present), which were influenced largely by supercontinent cycles.


Biography

Yebo Liu is currently a member of the Earth Dynamic Research Group at Curtin University. His main research interests are palaeogeography reconstructions using geological and palaeomagnetic investigations. Studying the secular variation of Earth magnetic field and the deep Earth process using paleointensity.

Proterozoic crustal evolution of NE Australia during Nuna assembly: Insights from geophysical and radiogenic isotope data

Jiangyu Li1, H. K. H. Olierook2, 3, Zheng-Xiang Li1, Adam R. Nordsvan1, 4, Amaury Pourteau1, Silvia Volante1,5, Chris Elders2, William J. Collins1, Luc S. Doucet1

1Earth Dynamics Research Group, The Institute for Geoscience Research (TIGeR), School of Earth and Planetary Sciences, Curtin University, Perth, Australia, 2Faculty of Science and Engineering, The Institute for Geoscience Research (TIGeR), School of Earth and Planetary Sciences, Curtin University, Perth, Australia, 3Timescales of Mineral Systems, Centre for Exploration Targeting – Curtin Node, and John de Laeter Centre, Curtin University, GPO Box U1987, Perth, Australia, 4Department of Earth Sciences, University of Hong Kong, Pokfulam, China, 5Institute of Geology, Mineralogy and Geophysics, Ruhr-Universität Bochum, Bochum, Germany

The final assembly of the Proterozoic supercontinent Nuna occurred via a collisional event between Australia and Laurentia in NE Australia at ca. 1.60 Ga[1]. However, detailed collisional processes and the resulting orogenic architecture in NE Australia remain elusive. Here, we combine aeromagnetic[2] and gravity data[3] with surface geological data and a reinterpretation of seismic profiles[4-5] to depict the deep crustal structure that developed during this collisional event. Neodymium and hafnium isotopic data from Proterozoic mafic and felsic intrusions[6] were also compiled to investigate the crustal evolutionary processes. A N–S trending, distinctive terrane boundary is recognized on the eastern margin of the Mount Isa Inlier from the filtered aeromagnetic and gravity grid, coinciding with the Gidyea Suture Zone previously recognised from the seismic reflection data. Between the Mount Isa and Georgetown inliers, a west-dipping, crustal dissecting fault is interpreted as another suture zone with additional smaller-scale thrusts that are antithetic to the main suture. The duplexed crustal architectures between the Mount Isa and Georgetown inliers are interpreted to have formed during a crustal thickening event associated with the docking of the Georgetown Inlier along a west-dipping subduction zone. The Georgetown Inlier is isotopically distinguishable from the Mount Isa Inlier at ca. 1.68 Ga, but shares the same isotopic history after ca. 1.60 Ga. Subduction may have initiated at ca. 1.64 Ga but ceased at 1.60 Ga, with the Georgetown Inlier accreted to the Mount Isa Inlier, possibly along the Empress Suture Zone during the final Nuna assembly. 

[1] A Pourteau et al., Geology 46 (11), 959 (2018).

[2] M Greenwood, (2018).

[3] C Roger, (2014).

[4] JL Maher, (2008).

[5] JL Maher, (2009).

[6] D Champion, (2013).


Biography

Jiangyu Li is a Ph.D student joint the Earth Dynamic Group at Curtin University in 2017. His Ph.D. project involves understanding the Proterozoic crustal evolutionary process in NE Australia during the 1.6 Ga supercontinent Nuna assembly. His tools are Argon thermochronology, aeromagnetic and gravity data processing and seismic profile interpretation.

Assembly of proto-Australia prior to the formation of the Nuna supercontinent in the Paleoproterozoic

Kirscher, Uwe1,2, Mitchell, Ross N3,1, Liu, Yebo1, Nordsvan, Adam R4,1, Wu, Lei5,1, Pisarevsky, Sergei1, Li, Zheng-Xiang1

1Earth Dynamics Research Group, School of Earth and Planetary Sciences, The Institute for Geoscience Research (TIGeR), Australian Research Council Centre of Excellence for Core to Crust Fluid Systems (CCFS), Curtin University, Perth, Australia, 2Department of Geosciences, Eberhard Karls University of Tuebingen, Tuebingen, Germany, 3State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China, 4Department of Earth Sciences, University of Hong Kong, Pokfulam, Hong Kong, 5Department of Earth & Planetary Sciences, McGill University, 3450 Rue University, Montréal, Canada

The paleogeography, chronology and importance of the Paleoproterozoic assembly of the supercontinent Nuna are still debated. To further test the paleogeographic evolution of the Australian cratons in the leadup to Nuna formation, we present new paleomagnetic results from two Paleoproterozoic rock formations in North Australia. First, we obtained paleomagnetic directions from the 1825±4 Ma, bimodal Plum Tree Creek Volcanics sequence of the North Australian Craton (NAC). Second, we studied the 1855±2 Ma layered mafic-ultramafic ‘Toby’ intrusion from the Kimberley Craton (KC). Samples from both study areas reveal high quality, stable, magnetite related characteristic remanent magnetization directions. Combining within-site clustered mean directions, we obtained two paleopoles, which plot proximal to each other in the present day central Pacific Ocean, off the east coast of Australia. These results agree with previous interpretation that the Kimberly Craton was amalgamated with the rest of the NAC prior to ca. 1.85 Ga. Comparing these new results with slightly younger poles from the NAC and slightly older, rotated poles form the West Australian Craton (WAC) reveal a high degree of similarity suggesting minimal absolute plate motion between ca. 1.9 and 1.65 Ga. All available paleomagnetic poles agree with an assembly, or close juxtaposition, of the two major Australian cratons (NAC and WAC) before 1.8 Ga. Furthermore, the individual virtual geomagnetic poles from the potentially slow cooled Toby intrusion show a non-fisherian distribution along a great circle. This spread might be related to previously interpreted major true polar wander events based on data from Laurentian cratons, which would be global if such an interpretation is correct. The assembly of proto-Australia prior to ca. 1.85 Ga roughly 250 to 300 Myr before the final stage of supercontinent Nuna’s amalgamation ca. 1.6 Ga suggests that assembling of major building blocks, such as Australia and Laurentia for the supercontinent Nuna and Gondwana for the supercontinent Pangea, is an important step in the formation of supercontinents.


Biography

UK did his undergraduate and PhD at Ludwig-Maximilians University in Munich, Germany. He then moved to Perth for a three-years postdoc at Curtin University, before he moved back to Germany for another postdoc at Tuebingen University.

Unravelling the Palaeoproterozoic tectonic evolution of the Tanami Region and northwest Aileron Province

McFarlane, Helen1, Blaikie, Teagan1

1CSIRO Mineral Resources, Perth, Australia

Stradling the Northern Territory-Western Australia border approximately 600 km to the northwest of Alice Springs, the Tanami Region comprises regionally expansive, polydeformed metasedimentary and volcanic rocks (Tanami and Ware groups: ca. 1885–1816 Ma). Recording a protracted and complex tectonic history, the sequences are extensively intruded by 1825–1790 Ma granites and dolerite dykes. To the southeast, the adjoining Aileron Province is comprised largely of metasedimentary and magmatic rocks of a comparable age. The region is highly prospective for gold, and preserves several deposits including the world class Callie deposit. Geological studies of the region are made challenging by extensive Mesoproterozoic to Cenozoic sedimentary cover and Early Cambrian basalt flows, with most geological information derived from sporadic outcrop and drill core. Geophysical data is therefore critical in this region, and is relied upon to understand the structural architecture and extent of potential gold bearing metasedimentary and volcanic rocks under cover.

Newly acquired and legacy government and industry aeromagnetic data available across the Tanami Region and northwest Aileron Province were reprocessed and interpreted for this work to produce new seamless solid geological and structural maps. This new interpretation attempts to resolve inconsistent geological correlations in the Northern Territory and develop a new cohesive structural framework for the entire region.

Deformation is typically best resolved in the magnetic stratigraphy of the Tanami Group which preserve several styles of fold interreference patterns. The earliest deformation event (D1) is characterised by isoclinal folding and low angle thrust faulting. D1 structures are extensively overprinted by subsequent deformation events; however, evidence suggests it is associated with an episode of SW-directed tectonic vergence. D2 deformation is characterised by the refolding of D1 structures by tight to isoclinal NNE- to NE-striking F2 folds, associated with WNW-ESE to NW-SE shortening. Both D1 and D2 are attributed to the ca. 1830 Ma Tanami Event which also involved regional greenschist to amphibolite facies metamorphism and early gold mineralisation. This was followed by an episode of sedimentation and volcanism, preserved as the Ware Group. The oldest recognised deformational structures in the Ware Group are attributed to NE-SW shortening during D3 as evidenced by NW- to NNW-striking chevron folds. The overprinting relationship of D3 on D1 structures generated a Type-2 fold interference pattern in the magnetic stratigraphy of the Tanami Group. The localised development of tight, E-W striking chevron folds is attributed to D4 N-S shortening and is associated with comparatively weaker deformation compared to earlier events. The final folding event (D5) generated long wavelength, open, NE-striking F5 folds during NW-SE shortening. Deformation events D3–D5 are interpreted as polyphase deformation during the 1810-1790 Ma Stafford Event and was coeval with widespread felsic magmatism. The transition to brittle-ductile deformation (D6) is associated with the development of the regionally prominent WNW-ESE to NW-SE striking dextral and sinistral shear zones, associated with the major period of gold mineralisation in the region.


Biography

Helen McFarlane is a structural geologist at CSIRO, with a diverse background, spanning Palaeoproterozoic tectonics in West Africa to MVT mineralisation in the Andes. Her current role examines structural controls in Au-, Cu- Mn- and multi-commodity mineral systems around Australia.

New interpretations of high-resolution aeromagnetic data and implications for stratigraphic correlations in the Tanami Region and northwest Aileron Province

Blaikie, Teagan1, McFarlane, Helen1,

1CSIRO Mineral Resources, Perth, Australia

The Tanami Region, located 600 km to the northwest of Alice Springs, preserves an important record of basin development, deformation, magmatism, and the assembly of the North Australian Craton during the Paleoproterozoic. The region comprises extensive polydeformed metasedimentary and volcanic rocks, preserved as the ca. 1885-1840 Ma Tanami Group and the ca. 1824–1816 Ma Ware Group. The Lander Rock Formation of the adjoining Aileron Province represents a comparably aged metasedimentary package that is considered to be laterally equivalent to the Killi Killi formation in the upper part of the Tanami Group. These metasedimentary packages were extensively intruded by 1825–1790 Ma granites and dolerite dykes, and deformed during the ca. 1840 Tanami Event, and the 1825–1790 Ma Stafford Event.

Extensive cover makes geological studies in this region challenging, but newly acquired 200 m line spaced aeromagnetic data and previously acquired gravity and seismic data offers valuable information on the underlying geology. This data were interpreted to produce a solid geological map, and develop a new cohesive structural framework. This aimed to test the different stratigraphic, structural and tectonic models that presently exist for the region. The interpretation initially focussed on mapping the fault architecture and lithological units under cover, and was constrained by the petrophysical characteristics of each unit and correlation of outcropping geology with distinct geophysical features.

Results of the interpretation led to revision of the extent of previously mapped or interpreted units, and recognition of potential correlative units across the two terranes. In the northwest Aileron Province,  the extent of the Archean inlier known as the Billabong Complex was revised based on correlation of outcrop with a distinct geophysical response characterised by a variable moderate to high amplitude magnetic intensity and featuring strong magnetic lineaments. The complex is now defined as a fault bounded arcuate belt flanked by granitic units. A continuous geophysical signal, defined by a moderate to low gravity response, and low magnetic intensity with a smooth texture was also recognised between the Killi Killi and Lander Rock formations. Magnetic units with a moderate magnetic intensity and strong linear fabric, which are very similar in character to the Dead Bullock Formation were also noted within the north-western Aileron province, and may represent a previously unrecognised lateral equivalent of the Dead Bullock Formation in this area. These observations support previous interpretations of the deep crustal seismic data which suggest stratigraphy is continuous and drapes the crustal boundary between Tanami Region and Aileron Province. This implies the two terrains were joined together prior to deposition of the Tanami Group and Lander Rock Formation and the onset of deformation and magmatism related to the Stafford Event.


Biography

Dr Teagan Blaikie is a research scientist in geophysics based at the CSIRO in Perth.  Her work focuses on regional scale interpretation and modelling of geophysical data to understand the architecture and tectonic evolution of sedimentary basins and polydeformed terranes

Self-consistent geodynamic models through the supercontinent cycle — testing the introversion and extroversion supercontinent assembly and the stability of LLSVPs

Chuan Huang1, Zheng-Xiang Li1, Nan Zhang2

1Earth Dynamics Research Group, ARC Centre of Excellence for Core to Crust Fluid Systems and The Institute for Geoscience Research (TIGeR), Department of Applied Geology, Curtin University, Perth, Western Australia, Australia, 2Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing, China

We developed a series of new dynamic models aiming to more realistic model the full supercontinent cycle, testing parameters critical for introversion vs. extroversion assembly, and mantle response to the alternative supercontinent evolution paths. The numeric simulations allow for self-generated subduction and supercontinent break-up during the supercontinent cycle. Subduction within the oceanic realm is implemented by considering plastic yielding in the oceanic lithosphere, through which rapid viscous weakening occurs when convection stress is larger than the yield strength. For subduction along continental margins, weak zones are introduced in oceans near the continental edge when the age of oceanic lithosphere is greater than a certain value (e.g., 180 Ma). Under such a model setup, our models are able to naturally generate Earth-like ocean-ocean and ocean-continent subductions.

By simulating the mantle evolution from the breakup of a supercontinent to the formation of the next one, we found that heat distribution (monitored by mantle temperature) between the mantle domains under either the supercontinent or the surrounding superocean, divided by the subduction girdle, provides an important control on how the next supercontinent forms. During the breakup stage, the average mantle temperature beneath the supercontinent (here denoted by Tc) is higher than that under the superocean (To) partially due to thermal isolation by the supercontinent. After the breakup, Tc decreases with the vanish of the thermal isolation effect, but To maintains at a similar level. It causes To shifts to a value slightly larger than Tc in the time soon after the breakup. Despite the limited higher energy level in the superocean-side, subduction/girdle retreat maintains the continuous drift of continents. After that, dispersing continents will reach their maximum distance from each other. The relative value of Tc and To after the time that greatest distance is reached determines whether the next supercontinent assembles through introversion or extroversion. Generally, models with a dense chemical layer above the core-mantle boundary tend to have introversion cycle, due to the much higher heat level (~50 K) in the superocean-side reserved by its larger chemical layer volume than the continent side. Two LLSVPs formed in our models during the full cycle locating in the ocean and continent sides, respectively. However, migrations up to several thousand kilometers for the two structures can be also observed.


Biography

Chuan Huang is a research fellow at Curtin University. He is working on geodynamic modelings and is currently focusing on the coupled continent-mantle process during a supercontinent cycle.

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