Regional geology of the Eastern Goldfields and component terranes; deformation history and structural architecture; previous seismic interpretations; granitoids; tectonics constraints imposed by greenstones; tectonic settings and models

Modern adakite forms in subduction zones with unusually high geotherms that result in high-pressure melting of subducted basaltic crust. Close geochemical similarities between adakite and Archaean TTG, which forms the major component of Archaean crust, have supported a view that Archaean TTG is likewise subduction related. However, recent studies have shown that conditions of adakite formation are not unique to a subducting slab and can be attained in basaltic lower crust in both subduction and non-subduction environments. Non-subduction environments may be equally relevant to the genesis of Archaean TTG. Heat flow calculations suggest that Archaean subduction (if it occurred), must have been at a low angle ? the same style of subduction that produces most modern adakites. However, if Archaean TTG was derived from a subducting slab, then like most modern adakites, it should show 1) evidence for interaction with a mantle wedge, and 2) an association with diagnostic subduction influenced magmas such as high-Mg andesite (sanukitoid), Nb-enriched basalts and boninites. Whereas this does appear to be the case for some Late Archaean terrains (e.g. Superior Province), such is not unequivocally the case for older terrains. This suggests either that Early Archaean TTG is not subduction related, or that the style of Early Archaean subduction was significantly different from modern subduction, including low-angle or flat subduction. The volume of preserved felsic crust in the Early Archaean Pilbara Craton, Western Australia, requires a complimentary volume of dense mafic material (residual after basalt melting) equaling a combined thickness of ~170 km, which is difficult to conceptualize through solely magmatic processes (e.g. underplating, mantle plume). Subduction provides a means whereby large volumes of mafic crust can be progressively cycled through a melting zone, but if applicable to the Early Archaean, features such as typically low Mg#, Cr, and Ni indicate that TTG melts of that crust must have avoided interaction with the mantle. We suggest that Early Archaean slabs were pushed or thrust beneath overriding basaltic crust, without the development of a mantle wedge. Early Archaean TTGs are melts of the underthrusted slab and their petrogenesis combines components of the subduction model for modern adakite and models for lower crustal sodic melts (e.g. Cordillera Blanca ? Peru; Ningzhen ? China).

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Introduction to a thematic issue of AJES on the Tasmanides. Most papers were originally presented at the 15th Australian geological Convention in Sydney, 3-7 july 2000.

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A deep seismic reflection profile was acquired in South Australia and Victoria in November 2009 by Geoscience Australia with project partners AuScope, Geoscience Victoria, and Primary Industries and Resources South Australia (PIRSA). Along with previously acquired deep seismic reflection data, this 145 km long line completes a continuous east-west transect across the eastern Delamerian Fold Belt into the western Lachlan Fold Belt. The project aims included determining tectonic vergence during and after amalgamation of the Gondwana Supercontinent, understanding the transition from passive margin (Rodinia breakup) to convergent margin (Tasmanides orogenesis), and locating the so-called 'Tasman Line', the extent of Proterozoic continental crust.

Thin-plate finite element models of the neotectonic deformation of the Australian plate have been calculated in order to estimate the stress and strain-rate within the plate, specifically concentrating on the Australian continent. The model includes plate-bounding faults, an anelastic brittle-ductile layered rheology and the option of laterally varying elevation and heat-flow. The results of the models are compared to: (a) the velocity of geodetic benchmarks on the Australian plate, (b) the spreading rate of the mid-oceanic ridges along the Australian plate's margins, (c) the direction of the maximum horizontal principal stress, (d) the stress regime within the plate and (e) the crustal thickness estimated from the depth to the base of Mohorovicic discontinuity's transition zone. A variety of models are tested with a wide range of input parameters. The model with the smallest misfit with observations predicts that the strain rate for most of the Australian continent is approximately 10^{-17}s^{-1}. This model has a slightly lower strain rate in the central Australia and is higher off the northern coast of Australia than for the rest of the continent. Strain rates of this magnitude would be difficult to observe from geodetic or geologic data for most parts of of Australia, but would be enough to generate much of the seismicity that has been observed over the last century.

Aspects of the tectonic history of Paleo- to Mesoproterozoic Australia are recorded by metasedimentary basins in the Mt Isa, Etheridge Provinces, and Coen Inlier in northern Australia and in the Curnamona Province of southern Australia. These deformed and metamorphosed basins are interpreted to have been deposited in a tectonically-linked system based on similarities in depositional ages and stratigraphy (Giles at al 2002). Neodymium isotope compositions of sediments and felsic volcanics, when combined with U-Pb geochronology, are independent data that are important tools for inferring tectonic setting, palaeogeography and sediment provenance in deformed and metamorphosed terrains.

This record outlines models for the tectonic evolution of Australian Proterozoic terranes, and the mineral systems that are likely to have operated in particular regions at particular times.

The rifting history of the magma-poor conjugate margins of Australia (Great Australian Bight) and Antarctica (Terre Adélie) is still a controversial issue. In this paper, we present a model for lithosphere-scale rifting and deformation history from initial rifting to breakup, based on the interpretation of two regional conjugate seismic profiles of the margins, and the construction of a lithosphere-scale, balanced cross section, sequentially restored through time. The model scenario highlights the symmetric pattern of initial stretching resulting to pure shear at lithospheric-scale accompanied by the development of four conjugate detachments and crustal half-graben systems. This system progressively evolves to completely asymmetric shearing along a single south-dipping detachment at the scale of the lithosphere. The detachment accounts for the exhumation of the mantle part of the Australian lithosphere, and the isolation of a crustal klippe separated from the margin by a peridotite ridge. Antarctica plays the role of the upper plate with the formation of an external crustal high separated from the unstretched continental crust by a highly extended zone still active during the Australian exhumation phase. The total elongation amount of the Australian-Antarctic conjugate system reaches ~413km (61%). Elongation was partitioned through time: ~189km and ~224km during symmetric and asymmetric stages, respectively. During symmetric stage, both margins suffered relatively the same elongation accommodated by crustal stretching (~105km (45%) and ~84km (38%) for Australia and Antarctica, respectively). Again, both margins accommodated relatively the same elongation during the asymmetric stage: the Antarctic upper plate records an elongation amount of ~225km (40%) as crustal tectonic stretching, above the inferred low-angle south dipping detachment zone, whereas the Australian lower plate suffered ~206km (61%) of elongation through mantle exhumation.