This abstract describes the pre-breakup structures and history of the conjugate margins of southwest Australia (Naturaliste Plateau) and East Antarctica (Bruce Rise) and the subsequent formation of oceanic crust.

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.

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 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

This paper presents new interpretations of the distribution of magmatic and pre-rift rock packages in the Exmouth-Gascoyne margin, based on the integrated interpretation of two deep crustal transects with existing seismic reflection, refraction, gravity and magnetic data. Interpretations are constrained by data from sparse ODP and petroleum drilling, and dredging. There is evidence for significant accumulation of magmatic rocks and their clastic derivatives infilling extensional fault-controlled basins developed in a broad volcanic margin transition (VMT) zone between the outer Exmouth Plateau and true oceanic crust. These rocks have distinctive seismic facies in the form of Seaward Dipping Reflector Sequences (SDRS), and are dense and magnetised. Most significantly, these packages give rise to potential field anomalies that have previously been interpreted as due to seafloor spreading. Recognition of these packages in a VMT zone has implications for the recognition of the inboard edge of unequivocal oceanic crust, the Oceanic Volcanic Margin Boundary (OVMB). Notably, in the volcanic margin transition zone off the Exmouth Plateau, the main locus of igneous activity is spatially offset from a previously recognised high velocity zone, suggesting that these two phenomena may not be temporally related. Seismically imaged differences in total thinning and partitioning of thinning between upper and lower crust provide support for models of depth dependent thinning previously proposed for this margin.

This report represents an up to date analysis of the stratigraphy, structure, gelogical evolution and petroleum prospectivity of the Naturaliste Plateau based on a revised interpretation of seismic reflection data combined with drilling and gelogical sampling results and supported by potential field interpretation.

As part of the Australian Government South-east Regional Marine Plan, Geoscience Australia and the National Oceans Office collaborated through the AUSTREA marine survey to map and interpret the seafloor character over the southern Macquaire Ridge. Multibeam swath bathymetry and reflectivity, along with high-resolution seismic, reveal a complex morphology and sedimentation patterns largely influenced by plate processes and ocean-bottom current effects.

In September and October of 2011 Geoscience Australia surveyed part of the offshore northern Perth Basin in order to map potential sites of natural hydrocarbon seepage. The primary objectives of the survey were to map the spatial distribution of seepage sites and characterise the nature of the seepage at these sites (gas vs oil, macroseepage vs microseepage; palaeo vs modern day seepage) on the basis of: acoustic signatures in the water column, shallow subsurface and on the seabed; geochemical signatures in rock and sediment samples and the water column; and biological signatures on the seabed. Areas of potential natural hydrocarbon seepage that were surveyed included proven (drilled) oil and gas accumulations, a breached structure, undrilled hydrocarbon prospects, and areas with potential signatures of fluid seepage identified in seismic, satellite remote sensing and multibeam bathymetry data. Within each of these areas the survey acquired: water column measurements with the CTD; acoustic data with single- and multi-beam echosounders, sidescan sonar and sub-bottom profiler (sidescan not acquired in Area F as it was too deep in places); and sediment and biological samples with the Smith-McIntyre Grab. In addition, data were collected with a remotely operated vehicle (ROV), integrated hydrocarbon sensor array, and CO2 sensor in selected areas. Sampling with the gravity corer had limited success in many of the more shallow areas (A-E) due to the coarse sandy nature of the seabed sediments. This dataset comprises total sediment metabolism (CO2 production) and porewater pH and salinity measurments of seabed sediments.

This report contains an interpretation of the geological framework of the continental margin off the AAT based on data recorded by the Australian Antarctic and Southern Ocean Profiling Project in 2001 and 2002.

This is a compilation of all the bathymetry data that GA holds in its database for the area that covers the Diamantina Fracture Zone to the Naturaliste Plateau. This dataset consist of different 6X4 degrees tiles that are: Tiles SI48,SJ48,SK48,SL48, SI47,SJ47, SK47,SL47, SJ46,SK46,SL46, SK45 and SL45)