No abstract available

Orogenesis in Phanerozoic systems is rapid, diachronous, episodic, and involves the switching of tectonic modes (extension-compression). In contrast, many Archaean orogens have traditionally been viewed as having developed by relatively simple, long-lived, mono-mode deformational processes. New results, however, reveal that the late Archaean eastern Yilgarn Craton (EYC) evolved episodically and rapidly, with a diachronous series of approximately E?W coaxial switches in tectonic mode. Tectonic mode switching changed stress regimes and resulted in the development of `late basins?, the emplacement of granites, and early orogenic gold mineralisation diachronously from east to west (NE?SW). Fluids were driven from the lower crust (and below) via large-scale crustal imbricating thrust faults. These fluids promoted the passage of a compression-extension couplet along a basal detachment by successively `lubricating? faults (preparing the ground), and facilitating a propagating wave of foreland surge (D2a) and hinterland extension (D2E) followed by inversion, uplift and annealing (D2b). In this way, orogenic Au and westward orogenic surge with associated tectonic mode switches are linked. We predict that the compres-sion-extension couplets and early orogenic gold mineralisation propagated from the east to the west diachronously at a rate of ~3-5 m.y. between domains from ~2670 Ma to ~2650 Ma. Multiple mineralising episodes are also a predicted consequence of the orogenic surge model.

Keywords: Archaean; Pilbara Craton; granite-greenstone terrane (GGT); structural geology; deformation events; diapirism.

The established deformation paradigm for the Eastern Yilgarn Craton (EYC) has largely been developed from observations of overprinting relationships in the greenstone belts (Swager, 1989, 1997; Swager et al., 1992; Williams, 1993). Broadly, the recognised deformation (compressional history) involved early D1 recumbent folding and thrusting during N-S shortening, followed by E-W shortening through large-scale upright D2 folding and thrusting, then a period of strike-slip D3 faulting with associated folding, followed by continued regional D4 transpressive oblique and reverse faulting. Some authors have proposed early, intermediate, and late periods of extension throughout parts of this compressive history (Table 1). This study is aimed at providing a new deformation framework for the granites of the EYC. Granites form >60% of the surface expression of the province, so a more thorough understanding of their tectonic history is critical to our understanding of the evolution of the province. There are advantages of using deformed granites for establishing a time-space-event history. Granites have: 1. a well defined chrono-chemical framework across the entire province (pseudo-stratigraphy) (Champion & Sheraton, 1997); 2. a ductile mineralogy of quartz and feldspar that readily 'accepts' deformation; 3. multiple phases of granite, as sheets and dykes, which provide excellent markers for defining the subtleties of many of the events (especially the later ones); 4. exposure at various structural levels in the upper crust; and 5. a good U-Pb zircon geochronological database as a reference framework (Nelson, 1997; Fletcher, et al., 2001; Dunphy et al., 2003; Black et al., 2004). The scope of the study is to systematically describe the overprinting relationships of structures in granites for a range of structural levels and geographical positions (sites) for the major structural domains or terranes (Southern Cross, Kalgoorlie, Gindalbi-Kurnalpi-Laverton, and Merolia). The area chosen was the central EYC exposed in the area of the 1:250 000 scale map sheets of Leonora, Laverton, Menzies and Edjudina. This study area was chosen because new solid geology mapping (Whitaker and Blewett; 2002), new seismic reflection profiling (Goleby et al., 2002), and the available comprehensive geochronological database. The area also provided a different perspective for the regional deformation history to that determined from the better studied Kalgoorlie region (Swager, 1989). The approach was to systematically study the various structural elements for a range granite ages, with the granite age to be used as a time marker at each site. At each site, the structural elements before (cut by the dated phase) and after (overprint the dated phase) were systematically mapped and described. These structural elements form the data (available as separate data sheets) that were used to correlate events with adjacent sites in the same domain. These domain-wide event histories were also correlated to construct a new EYC deformation framework. Each stage of correlation becomes more interpretative. However, the original site data have been 'preserved' for evaluation and further testing.

The frontier deepwater Otway and Sorell basins lie offshore of south-western Victoria and western Tasmania at the eastern end of Australia's Southern Rift System. The basins developed during rifting and continental separation between Australia and Antarctica from the Cretaceous to Cenozoic. The complex structural and depositional history of the basins reflects their location in the transition from an orthogonal-obliquely rifted continental margin (western-central Otway Basin) to a transform continental margin (southern Sorell Basin). Despite good 2D seismic data coverage, these basins remain relatively untested and their prospectivity poorly understood. The deepwater (>500 m) section of the Otway Basin has been tested by two wells, of which Somerset 1 recorded minor gas shows. Three wells have been drilled in the Sorell Basin, where minor oil shows were recorded near the base of Cape Sorell 1. As part of the Federal Government funded Offshore Energy Security Program, Geoscience Australia has acquired new aeromagnetic data and utilised open file seismic datasets to undertake an integrated regional study of the deepwater Otway and Sorell basins. Structural interpretation of the new aeromagnetic data and potential field modelling provide new insights into the basement architecture and tectonic history, and highlights the role of pre-existing structural fabric in controlling the evolution of the basins. Regional scale mapping of key sequence stratigraphic surfaces across the basins, integration of the regional structural analysis, and petroleum systems modelling have resulted in a clearer understanding of the tectonostratigraphic evolution and petroleum prospectivity of this complex basin system.

The offshore Vlaming Sub-Basin comprises up to seven kilometres of Cretaceous to Cainozoic sediments, a postulated seven to ten kilometres of Jurassic sediments and an estimated thickness of two to three kilometres of Permo-Triassic sediments. As the depocentres are not coincident the maximum estimated sedimentary thickness is 16 kilometres.

Legacy product - no abstract available

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Legacy product - no abstract available