No abstract available

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.

About 80% of the Archaean Corunna Downs Granitoid Complex in the East Pilbara Granite Greenstone Terrane is of c. 3315 Ma monzogranites. They are typically highly fractionated, K-rich and Al-poor, and have trace element compositions consistent with remelting of an older tonalitic-trondhjemitic-granodioritic (TTG) crust at a mid-crustal level. The remaining 20% of the complex comprises tonalites, trondhjemites, and granodiorites. Some of these granitoids are as old as c. 3400 Ma, however, the majority are thought to be similar in age to the monzogranites. The tonalites, trondhjemites, and granodiorites have high Y and Yb, and low Sr and Al2O3 concentrations compared to classical Archaean TTGs, which are thought to form through high-pressure melting of hydrated mafic crust. In contrast to TTGs, the tonalitic, trondhjemitic and granodioritic rocks of the complex have a low-pressure, mid- to lower-crustal, amphibolite-source that was garnet-free and probably also had residual plagioclase. It is proposed that, at the same time as mid-crustal melting of TTG formed the monzogranites, melting of a mafic intraplate formed the tonalites, trondhjemites, and granodiorites.

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.

This record is a compilation of the abstracts of oral and poster papers presented at a symposium held at the Bureau of Mineral Resources, Canberra 13-16 February 1989. The symposium was entitled "Seismicity and Earthquake Studies in the Australian Plate and its Margins", and was co-sponsored by the Specialist Group on Solid Earth Geophysics of the Geological Society of Australia and the Bureau of Mineral Resources. The abstracts in this paper are in the same order as in the symposium program at the beginning of the paper.

Models for the crustal evolution of the Yilgarn Craton have changed in the last 25 years from generally autochthonous greenstone development on sialic crust (Gee et al. 1981, Groves & Batt 1984) to alloch-thonous models that highlight the importance of accretionary tectonics (Myers 1995). Recent models highlight the importance of mantle plumes and long-lived convergent margins for both Au and Ni (Barley et al. 1998). The role of sialic crust in the development of the abundant mineral systems in the Yilgarn, and Archaean cratons in general, however, remains problematic. Felsic rocks from across the Yilgarn Craton are used as crustal probes, with their geochronology, zircon inheritance and Nd isotopic character used to constrain the age and extent of basement terranes. The studies reveal a collage of crustal fragments and implicate both autochthonous and allochthonous crustal development, with increasing importance of accretionary tectonics, particularly after 2.8 Ga. The crustal evolution places significant constraints on the development of metallogenic associations.

No abstract available

The origins of high heat production (HHP) granites - with high concentrations of the heat-producing elements Th, U and K (HPE) - is controversial, particularly large areas of such rocks. To constrain possible controls on HHP granites, we have investigated temporal changes in Th, U and K contents of Paleoarchean to Mesozoic granites in Australia, and how these relate to peri-ods of HHP magmatism. Australian HHP granites range in age from Mesoarchean to Triassic, but are most abundant in the Neoarchean, the Paleoproterozoic - early Mesoproterozoic, and the Carboniferous. HHP magmatism ranges from relatively short lived (<30 Ma) geographically-restricted events in the Neoarchean and Carboniferous, to geographically widespread, (possibly unrelated) repetitive events over an extended time period (ca. 1800 to 1500 Ma) for the Proterozoic.

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.

Formed throughout some 40% of the earth's history (>2500 Ma), Archaean cratons now comprise <10% of the continents, but contribute disproportionately to the world's mineral wealth. Remnant Archaean terrains vary in age from fragments as old as 3.6 to 4.0 Ga in age (e.g., Isua - Greenland, Acasta's Slave Province), to more common younger cratons (3.6 to 2.5 Ga) of various sizes, the largest being the Superior Province (1,572,000 km2), which alone constitutes greater than 20% of the total exposed Archaean (Thurston, 1991). Better known Australian examples include the small but well exposed (3.6 Ga and younger) Pilbara Craton (45,000 km2), and the significantly larger, but poorly outcropping Yilgarn Craton (>600,000 km2), both in Western Australia. Granitic rocks form the main component of most Archaean Cratons (e.g., ~70% of the Yilgarn). They occur as syn-volcanic and younger intrusive units within volcano-sedimentary assemblages (greenstone belts), as intrusive components of batholiths, and as components of high-grade gneiss terrains. Their compositional range is extensive and reflects both short-lived or local tectonic processes as well as longer-term process that relate to regional or global evolution.