The Jurassic-Cretaceous Bight Basin is situated along the western and central parts of the southern Australian continental margin. The largely offshore basin extends from the southern tip of Western Australia in the west, to just south of Kangaroo Island in the east, where it adjoins the Otway Basin. The thickest depocentre in the basin is the Ceduna Sub-basin, which contains a sedimentary section in excess of 15 km thick. The deepwater Recherche Sub-basin adjoins the Ceduna Sub-basin and extends west along the southern margin as far as the Leeuwin Fracture Zone. Perched half-graben systems of the Denmark, Bremer and Eyre sub-basins lie to the north of the Recherche Sub-basin. The Duntroon Sub-basin adjoins the Ceduna Sub-basin to the east, and consists of a series of oblique extensional depocentres. The Bight Basin evolved through repeated episodes of extension and thermal subsidence leading up to, and following, the commencement of sea-floor spreading between Australia and Antarctica. The basin was initiated during a period of Middle-Late Jurassic to Early Cretaceous upper crustal extension. A northwest-southeast to north-south extension direction, superimposed on east-west and northwest-southeast-oriented basement structures, resulted in oblique to strongly oblique extension and the formation of en echelon half graben in the Denmark, Bremer, Eyre, inner Recherche, eastern Ceduna and Duntroon sub-basins. The areal extent of the early extensional structures beneath the thick Ceduna Sub-basin cannot be determined at present. The anomalously thick nature of the Ceduna Sub-basin may indicate, however, that Jurassic-Early Cretaceous rifts are present at depth. Post-rift thermal subsidence was followed by a phase of accelerated subsidence, which commenced in the Late Albian and continued until continental break-up in the Late Santonian-Early Campanian. During this phase of enhanced subsidence, the dominant structural feature was a system of gravity-driven, detached extensional and contractional structures, which developed in the Ceduna Sub-basin during the Cenomanian as a result of deltaic progradation. Evidence for upper crustal extension during this basin phase is limited to Turonian-Santonian extensional faulting, and the reactivation and propagation of Cenomanian growth faults. The commencement of sea-floor spreading at ~83 Ma was followed by a further period of thermal subsidence and establishment of a passive margin

In early 2008 Geoscience Australia and Mineral Resources Tasmania acquired 141,234 km of high resolution (800m line spacing) aeromagnetic data over Bass Strait and the offshore marginal basins of western Tasmania. The data fill a gap in the existing aeromagnetic coverage between Tasmania and mainland Australia and provide fresh insights into basement structure and its control on basin architecture and sedimentation patterns during Gondwanan continental break-up and the separation of Australia from Antarctica. Prominent in the new data are several northwest-trending basement faults that extend from the mainland into westernmost Tasmania and the South Tasman Rise; they appear to represent an offshore extension of previously mapped structures in western Victoria (Hummocks and Yarramyljup Faults). These structures postdate, truncate and offset in a sinistral sense many older north- and northeast-trending basement structures, including the late Neoproterozoic Arthur lineament in Tasmania, the Bambra fault in central Victoria and the boundary between the Lachlan and Delamerian Orogens (Moyston Thrust) in western Victoria. The Hummocks Fault coincides with a narrow belt of ultramafic rocks and possibly continues offshore as a series of prominent magnetic anomalies whereas the Yarramyljup Fault may form the western limit of Proterozoic (Tyennan) basement in Tasmania. The distribution and geometry of Mesozoic-Tertiary offshore sedimentary basins in western Tasmania and the South Tasman Rise is consistent with reactivation of the older basement structures in a north-south-directed transtensional tectonic regime. Magmatic rocks intruded into the Bass, Otway and Sorell Basins and Torquay Sub-Basin are clearly delineated in the new aeromagnetic data.

The Australian Southern Margin SEEBASE® Compilation represents many years of work by SRK in southern Australia in the petroleum, mineral and coal sectors. During this time SRK has undertaken numerous projects in southern Australia with both the private and government sectors. These projects have resulted in the development of a model of the geological evolution of southern Australia from Archean to Recent that is summarised in this GIS and report. The model is consistent with a wide range of datasets including airborne and satellite remote sensing, seismic, well and outcrop observations. The basins of Australia's Southern margin formed by the repeated reactivation of long-lived basement structures. By understanding the genesis and geometry of the old basement structures, we have produced a model for the evolution of the Southern Margin basins that explains their structural framework and architecture. This SEEBASE model and structural interpretation can now be used as the basis for a new understanding of the sequence stratigraphy and petroleum systems of the margin.

Australian passive margins display a large variety of styles, including narrow, wide, volcanic and non-volcanic margins. Their tectonic history has been complicated by tectonic reactivation and anomalous subsidence/uplift, widespread at various times during the post-rift phase. Defining the exact location of the boundary between continental and oceanic crust (COB) is of key importance to understanding the structure and evolution of continental passive margins. Here, we review the history of Australian passive margins and the location of the COB, and we assess rift-related magmatism and anomalous post-rift subsidence based on recently acquired seismic reflection data, available industry data, and current tectonic models for margin evolution.

Integration of conventional interpretation of deep-seismic data with potential field modelling is a powerful tool for elucidating the geology of continental margins and particularly the continent-ocean transition zone (COT). Recent work carried out on the Wilkes Land margin of East Antarctica using new seismic and potential field data shows the power of combining these techniques. In this study, the initial deep-seismic interpretation was combined with sonobuoy- and stacking-derived velocity information to provide a starting model for the potential field modelling; the potential field model and aspects of the seismic interpretation were iterated until a consistent interpretation was reached. The most important observations from the COT zone on the Wilkes Land margin include: ? The outer edge of the COT, that is the point at which the crustal type becomes 100% oceanic, is much further offshore than previously interpreted from seismic data and seafloor spreading anomalies, and lies beneath the deep ocean basin. ? The COT is dominated by a basement ridge complex which may represent serpentinised, unroofed mantle peridotites and associated intrusions and extrusions related to decompression melting, similar to features inferred on the conjugate southern Australian margin. ? The lower crust is inhomogeneous, probably due to massive localised intrusion landward of the COT; however, pre-existing inhomogeneities cannot be ruled out. ? The base of the crust has considerable relief landward of the COT, increasing from ~10 km to 16 km depth over a distance of about 40 km.

New deep-seismic data acquired from offshore East Antarctica by Australia provide more than 40 crossings of the continent-ocean transition (COT) along the 5500 km length of the continental margin that was formerly adjacent to the east coast of India and the southern margin of Australia prior to East Gondwana breakup. As volcanic activity is relatively subdued on this margin, except in the vicinity of the southern Kerguelen Plateau, the data provide a window into the late-rift and post-rift stages of formation of a non-volcanic rifted margin. Three characteristic margin segments are interpreted in the data. From west to east, these are: Offshore Enderby Land to Prydz Bay (38-80oE): The COT is not clearly defined in this zone, probably due in part to the effect of the Kerguelen hot spot in the Late Cretaceous. Delineation of the COT here will rely heavily on potential field modelling. Queen Mary Land 90-105oE): southeast of the Kerguelen Plateau, the margin is dominated by Bruce Rise, a continental marginal plateau that was formerly conjugate to the Naturaliste Plateau off southwest Australia. The transition from continental to oceanic crust is sharp, in apparent contrast to the conjugate margin off southwest Australia. Wilkes Land (105-140oE): in the area formerly conjugate to southern Australia, the margin is characterised by a broad zone of deeply-subsided continental and transitional crust beneath the inner edge of the deep ocean basin in an area previously considered to have been oceanic crust formed by seafloor spreading.

The southern Australian margin is unique as it is the only known passive margin that formed over and orthogonal to a Mesozoic subducted slab in the mantle. The tectonic subsidence pattern observed along the southern Australian margin primarily reflects the extensional processes that were associated with the development of the divergent continental margins of Australia and Antarctica, coupled with Cretaceous mantle dynamics and the influence of intra-plate stress on the Australian plate in the Late Tertiary.

Deep-seismic data acquired by Geoscience Australia from the Naturaliste Plateau off southwest Australia in 1997, and from Bruce Rise on the margin of East Antarctica in 2001, allow direct comparison of these conjugate margins for the first time. Sampling has shown that the Naturaliste Plateau is at least partly of continental origin and composed of Proterozoic to Cambrian metamorphic rocks. A rift phase on the Naturaliste Plateau resulting in a series of predominantly E-W oriented grabens is estimated to have occurred in the Late Jurassic to earliest Cretaceous. Similar grabens are also present beneath the Bruce Rise. The Diamantina Zone, south of the Naturaliste Plateau, has been interpreted as a continent-ocean transitional zone with sampling indicating that the southernmost part is comprised of peridotite ridges. In contrast, to the north and northwest of Bruce Rise, the basement seismic character, and headwave velocities interpreted from sonobuoys, suggest that it is likely to be of oceanic origin. This crust lies about 2000 m deeper than the fast-spreading Eocene crust of the Australian-Antarctic Basin. Correlation of magnetic anomalies and the seismic character of the deep crust suggest that it formed either during an episode of Early Cretaceous seafloor spreading coincident with the opening of the Perth and Enderby Basins, rather than during the very slow spreading between Australia and Antarctica that started in the early Campanian. In either case, the breakup of the Naturaliste Plateau and the Bruce Rise appears to have been highly asymmetric with most of the extended continental or transitional crust being attached to the Australian margin.

This abstract contains a summary of the broad scientific results coming out of the interpretation of data acquired under the Australian Antarctic & Southern Ocean Profiling Project.

This abstract provides an interpretation of the margin structures and breakup processes in the separation of Elan Bank (Kerguelen Plateau) from Enderby Land, east Antarctica.