Legacy product - no abstract available

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.

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.

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.

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.

The Naturaliste Plateau is a large marginal plateau located immediately west of the southwestern tip of the Australian mainland from which it is separated by the N-S trending Naturaliste Trough. It has an area of about 90 000 km2, extending for about 400 km E-W and 250 km N-S in water depths of 2000 to 5000 m. Results of a recent study of the Naturaliste Plateau based on new and reprocessed seismic data and RV Marion Dufresne cruise 110 (1998) dredging results indicate that large parts of the plateau are underpinned by Proterozoic metamorphic basement similar to the Leeuwin Block of southwestern Australia. The Naturaliste Plateau is a structurally complex terrain that was rifted in the Jurassic-Early Cretaceous and modified by volcanism towards the end of the Early Cretaceous. A number of variable-size rift basins are imaged on seismic profiles on the plateau. Most of these basins are half-grabens bounded by steep ENE-trending normal faults that dip S or SE. The basins extend for up to 120 km along strike, and are from 10-30 km wide. The largest basin (the western Mentelle Basin) lies beneath the Naturaliste Trough and contains more than 5 km of sediment. Basin development probably commenced with the N-S oriented Permian intracratonic rifting that characterises the western and northwestern Australian margins. These Palaeozoic structures were then reactivated by the Jurassic to Early Cretaceous rifting that preceded breakup between Greater India, and Australia-Antarctica (A-A). However, this structural fabric has been subsequently overprinted by E-W trending rifting, reflecting the Early Cretaceous stress regime developed leading to the Late Cretaceous breakup of Australia and Antarctica. A large number of smaller E-W trending rift basins formed across the southern part of the Naturaliste Plateau during this time. In the Valanginian the Naturaliste Plateau separated from Greater India along its northern and western margins. This breakup was accompanied by widespread volcanism, which partly overprinted pre-existing extensional structures. The southern margin of the plateau was formed during the Late Cretaceous A-A breakup which is interpreted to have commenced in the Santonian with a phase of very slow-spreading. A deep-seismic line extending south from the Naturaliste Plateau across the Diamantina Zone imaged a broad (250 km wide) structurally complex zone, which has been interpreted as a continent-ocean transitional zone. The inboard part of this zone appears to contain a mixture of magmatic and continental crust, while further oceanward, it is dominated by peridotite ridges alternating with basaltic intrusions, producing the characteristic rough bathymetry of the Diamantina Zone. This crust could be alternatively interpreted as an ultra-slow spreading crust or highly extended continental crust with partly unroofed mantle and it is significantly different from the slow-spreading crust described on the southern Australian margin

In January/February 2000, the Australian Geological Survey Organisation (AGSO) completed a 2S-day seabed swath-mapping and geophysical survey off south and south-east Tasmania and south of Macquarie Island for the National Oceans Office and Environment Australia. The survey, which is named AUSTREA-2 and designated as AGSO Cruise 223, used the 8S-m French oceanographic and geoscience research vessel N/O L 'Atalante, which departed Hobart on January IS and arrived in Bluff, New Zealand, on February 9. The survey covered about 10,200 km and mapped about 140,000 km2 of seabed. The initial impetus for the work was to map the foot-of-slope position in several areas to support definition of Australia's legal Continental Shelf under the United Nations Convention on the Law of the Sea. A significant additional but complementary aspect was to support marine zone planning and management, and assessment of seabed living and non-living (petroleum and mineral) resources, as an important step towards implementation of Australia's Oceans Policy and Australia's Marine Science and Technology Plan, and, in particular, the development of the South-east Regional Marine Plan by the National Oceans Office. Geophysical data collected included Simrad EM12D swath bathymetry and backscatter, 6-channel GI-gun seismic, 3.S kHz sub-bottom profiling, and gravity and magnetic profiles. This was augmented by a suite of oceanographic data, such as seawater temperature, and both current and salinity depth profiles. Weather and sea conditions were highly favourable, particularly in the more southern latitudes. Occasional periods of rough weather resulted in higher noise levels, but did not seriously affect acquisition, and data quality was generally excellent. The work conducted off Tasmania was mostly to fill in and extend previous swath coverage, and map the foot-of-slope along the eastern margin of the South Tasman Rise. It highlighted features such as the major development of slope canyons down the eastern Tasmanian margin, and the complex character of the Cascade Seamount and other seamounts adjacent to the South Tasman Rise. The work conducted over the southern Macquarie Ridge Complex highlighted features such as: a high-relief axial valley adjoining the deep Hjort Trench; the broadening to the south of the submerged Hjort Ridge, east of the Hjort Trench; the development of seafloor spreading tectonic fabric across the Hjort Ridge summit; and the presence of a linear trough/ridge feature that obliquely truncates the southern end of the Hjort Trench and adjoining axial valley. A full set of shipboard maps are held by the National Oceans Office and AGSO, and copies of the digital data are stored at AGSO. All data from the survey will be managed jointly by AGSO and the National Oceans Office.