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.

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.

Detrital zircon record of Proterozoic strata in the Priest River region of western Laurentia: Evaluating “SWEAT” relationships for supercontinents Nuna and Rodinia

Brennan, Daniel T.1, Li, Zheng-Xiang1, Link, Paul K.2, Johnson, Tim3

1Earth Dynamics Research Group, School of Earth and Planetary Sciences, Curtin University, , , Australia, 2Idaho State University, Pocatello , USA, 3School of Earth and Planetary Sciences, The Institute for Geoscience Research (TIGeR), Curtin University, , Australia

Correlation of rocks across purportedly paired margins, such as Proterozoic strata (notably the Belt-Purcell and Windermere Supergroups) of western Laurentia with coeval rocks and/or magmatic sources in and around the Gawler Craton, have long been used as a key piercing point for SWEAT-like reconstructions of supercontinents Nuna and Rodinia. Here we evaluate the nature and timing of the proposed correlations through U-Pb and Lu-Hf analysis of detrital zircon (DZ) from the Proterozoic Gold Cup Quartzite, Belt-Purcell Supergroup, Deer Trail Group, and Buffalo Hump Formation of the Priest River region, northwestern USA.

The <1.7 Ga Gold Cup quartzite contains mostly ca. 2.6 and 1.8 Ga DZ grains, indicating it is likely a western equivalent of the Neihart Formation. Lu-Hf values from these grains suggest that the younger ca. 1.8 Ga population (εHft = -9 to -3) resulted from a reworking event on the ca. 2.6 Ga crust involving juvenile mantle input (εHft = -2 to 4). This is consistent with the sediments being sourced from proximal Neoarchean Laurentian terranes such as the Clearwater/Medicine Hat block, that were intruded by Paleoproterozoic magmatism associated with the collision of the Wyoming and Medicine Hat blocks. Thus, these units do not require a SWEAT configuration (or the existence of Nuna) at ca. 1.7 Ga. In the overlying western (ca. 1.48–1.37 Ga) Belt Supergroup units, significant juvenile (εHft =2 to 8) ca. 1.6 Ga DZ grains are present. These grains fall within the North American Magmatic Gap and likely indicate provenance from the Gawler Craton, supporting a proto-SWEAT configuration for Nuna during ca. 1.5–1.4 Ga as in most Nuna reconstructions. The overlying <1.3 Ga, fine-grained and carbonate Deer Trail Group is interpreted as a passive margin succession and contains mostly ca. 1.9–1.65 Ga DZ grains with a wide range of Lu-Hf values (εHft = -6 to 9), notably ca. 1.6 Ga DZ grains are absent. This provenance shift could be indicative of Nuna breakup, removal of the Gawler Craton from its Nuna position along western Laurentia, and a south-western Laurentia provenance or recycling from underlying rocks of the Lemhi group of the Belt-Purcell Supergroup.

Coarse, locally conglomeratic, rocks of the Buffalo Hump Formation unconformably overly Deer Trail group strata. Prior small-n DZ study of the Buffalo Hump Formation identified a ca. 1.1 Ga youngest DZ population, which was suggested to record deposition at ca. 1.0 Ga during Rodinia amalgamation. However, our large-n study of the Buffalo Hump Formation identified for the first time a minor (~1%) yet significant ca. 760 Ma DZ population, which constrains the maximum age of deposition. These geochronology results redefine the onset of Rodinia rift-related sedimentation to after ca. 760 Ma in this region. Additionally, the Buffalo Hump Formation lacks any ca. 900–790 Ma DZ grains. Such a DZ age-spectrum, and inferred rift history, is difficult to reconcile with an immediate neighbourhood between Laurentia and Australia in Rodinia as the latter had an earlier start of continental rifting (with ca. 830–750 Ma rifting and syn-rift magmatism).


Biography

Daniel Brennan is a PhD student with the Earth Dynamics Group at Curtin University.

When did Australia’s Cratons come together?

Gorczyk, Weronika1, Tyler, Ian1, Aitken, Alan1, Kohanpour, Fariba1

1Centre of Exploration Targeting, School of Earth Science, University of Western Australia, Perth, Australia

Assembly of Australian Cratons as part of Nuna assembly has been a subject of debates for decades. Especially within Australian geoscience community the timing and style of Western Australia Craton (WAC) with Norther Australian Craton (NAC) collision causes a lot of controversy. The dispute arise mostly due to sparsity of data available. As the Proterozoic knowledge of Australian cratons, as well as others grew new models for Nunan assembly were proposed, and the literature become overcrowded with variable models, based on localised and limited data.

Here, without presenting any new data, an attempt is made to analyse existing models in a unbiased style, questioning and correlating all the tectonic and sedimentation events across WAC, NAC and SAC (Southern Australia Craton). The position and interactions with Laurentia and North China – which are believed to be proximal to Australia at the time of paleo-meso Proterozoic, as also considered.

To achieve this task, plate reconstruction software (GPlates) is used. Publicly available geological data that describe tectonic and sedimentary events affecting WAC, NAC and SAC, as well as paleomagnetic data to are taken into account to support or contradict conceptual models. The immense advantage of this approach is continuous space and time visual representation of the plates interactions and occurrence of events.

Three (with variations) time models of WAC and NAC collision are shownwith different subduction polarty: (1) 1800- 1765 Ma, (2) 1590-1550Ma, (3) ~1300 Ma. Essentially, in the first model one can corelate all the tectonic events across WAC an NAC and SAC with one another post-collision, but spatial problem arises between the cratons and events that follow. In the second model the collision of WAC and NAC can be corelated with metamorphic and magmatic events in Arunta region, as well as in Mt Isa, but does not allow for correlation of prevous events across WAC and NAC. The third model with subduction under WAC combines the tectonic evolution of Paterson region, Wankanki Arc and Stage I of Albany Fraser in a very elegant way, but again keeps WAC on a separate palate prior 1300 Ma, and does not allow for correlation of events across the cratons of with similar styles and timings. Pros and cons for all models will be presented, and the verdict will be left to you.


Biography

Weronika Gorczyk is a research fellow at Center of Exploration Targeting at University of Western Australia. She is a geodynamicist with interest in tectonic process from the edge of the plate to its interior.

The emergence of eclogites linked to global arc chemistry change at 2 Ga

Tamblyn,Renée1, Hasterok, Derrick1, Hand, Martin1, Gard,Matthew1

1Department of Earth Sciences, the University of Adelaide, South Australia, Australia

The thermal state of the solid Earth determines the interactions between the mantle and the crust. The only way to probe the thermal conditions of the ancient Earth is from the mineralogical and geochemical record of thermally-driven processes, i.e. metamorphism and magmatism. The generally accepted model for the thermal budget of the Earth balances heat accumulated from accretion and the decay of heat producing elements, and indicates an overall cooling trend from ca. 3 Ga to present, encompassing the emergence of modern plate tectonics. The geological record however indicates this simple cooling model may not hold true. Thermally sensitive metamorphic mineral assemblages, such as eclogites, emerge in the rock record transiently from 2.2–1.8 Ga, and disappear again until ca. 0.8 Ga. Coincident with this transient emergence of eclogite, the global record of arc granite chemistry also shows significant step changes, most notably decreased Sr and Eu and increased Y and rare earth element concentrations, from 2.0–1.8 Ga, both of which point to a global increase in thermal gradients that intersected granite genesis. We suggest these changes occurred as the secular cooling of the mantle and crust was reversed by a net increase in the spatial extent of continental crust between 2–1.8 Ga, resulting in thermal insulation of the mantle. The following 1.2 billion years on Earth was dominated by a warm, insulated mantle and crust, maintained by stable continental volumes, which eventually cooled to allow the second emergence and widespread preservation of eclogites from ca. 0.8 Ga until present. While novel, this idea combines unrelated global petrological and geochemical datasets to explore the sensitivity of switches in the thermal evolution of the solid Earth.


Biography

Renee is finishing her PhD at the University of Adelaide. Her early PhD focussed on the pressure-temperature-time evolution of high-pressure rocks formed in the subduction channel. More recently, she has looked into the emergence of these subduction-related rocks on Earth.

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