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 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.

As part of its program to define the extents of the Australian Legal Continental Shelf on the Kerguelen Plateau AGSO acquired over 5500 km of new seismic data including the first regional datasets over Elan Bank and in the Labuan Basin. This report presents results of a geological framework study carried out to underpin the Australias Law of the Sea claim on the Kerguelen Plateau. It provides an up to-date analysis of the stratigraphy, structure, geological evolution and petroleum prospectivity of the Kerguelen Plateau region taking into account recent ODP drilling, geological sampling, seismic reflection and refraction data, as well as potential field data.

The rifted margins of eastern and southern Australia formed during multiple periods of extension associated with the fragmentation and dispersal of Gondwana in the Late Jurassic to Early Eocene (Veevers & Ettreim 1988; Veevers et al. 1991). The sedimentary basins of the Southern Rift System (Stagg et al. 1990) extend from Broken Ridge in the west, to the South Tasman Rise (STR) in the east. Collectively, these depocentres cover an area in excess of 1 million square kilometres (excluding the STR), with the thickest sediments (up to 15 km) occurring in the Ceduna Sub-basin of the Bight Basin. Early phases of the extension during the late Middle Jurassic to Early Cretaceous resulted in the formation of a series of west-northwesterly trending continental rift basins along the southern margin of Australia and a series of north-northwest trending transtensional basins along the western margin of Tasmania. The amount of upper crustal extension varied between basins of the rift system. This phase of upper crustal extension preceded eventual breakup between the Australian and Antarctic plates off the Bight Basin in the latest Santonian to earliest Campanian (Sayers et al. 2001). The nature of source rocks within the rift basins reflects the eastward propagation of the rift system through time, with largely terrestrial systems dominating in the early rift stages, followed by marine inundation from the Aptian onwards (west of the Otway Basin). In the Otway Basin, the first marine influence is recorded during the early Turonian, while in the Sorell and Bass basins marine conditions prevailed from ?Maastrichtian and middle Eocene time, respectively. Terrestrial progradational systems in the Late Cretaceous are important in the maturation of potential source rocks in the Bight and Otway basins, while Neogene carbonate-dominated systems are important in the Sorell, Bass and Gippsland basins. Outside of the Gippsland Basin where exploration has reached a mature status, the southern margin basins remain frontier to moderately exploration areas, with an overall drilling density (excluding the Gippsland Basin) of approximately 1 well per 6,000 square kilometres. Key Words: Australian Southern Margin, Southern Rift System, petroleum systems References SAYERS, J., SYMONDS, P.A., DIREEN, N.G. and BERNARDEL, G., 2001. Nature of the continent-ocean transition on the non-volcanic rifted margin of the central Great Australian Bight. In, Wilson, R.C.L., Whitmarsh, R.B., Taylor, B., and Froitzheim, N., (Eds), Non-Volcanic Rifting of Continental Margins; A Comparison of Evidence from Land and Sea. Geological Society, London, Special Publications, 187, 51?77. STAGG, H.M.J., COCKSHELL, C.D., WILLCOX, J.B., HILL, A., NEEDHAM, D.J.L., THOMAS, B., O?BRIEN, G.W. and HOUGH, P., 1990. Basins of the Great Australian Bight region, geology and petroleum potential. Bureau of Mineral Resources, Australia, Continental Margins Program Folio 5. VEEVERS, J.J. and ETTREIM, S.L., 1988. Reconstruction of Australia and Antarctica at breakup (95 ? 5 Ma) from magnetic and seismic data at the continental margin. Australian Journal of Earth Sciences, 35, 355?362. VEEVERS, J.J., POWELL, C.MCA. and ROOTS, S.R., 1991. Review of seafloor spreading around Australia, I. Synthesis of the patterns of spreading. Australian Journal of Earth Sciences, 38, 373?389. WILLCOX, J.B. and STAGG, H.M.J., 1990. Australia?s southern margin, a product of oblique extension. Tectonophysics, 173, 269?281.

In 2010 and 2011, the Australian Government released exploration acreage in the Perth, Mentelle and Southern Carnarvon basins off the southwest margin of Australia. This release was underpinned by two new marine geophysical surveys (GA-310 and GA-2476) that were conducted by Geoscience Australia in late 2008 and early 2009 as part of the Australian Government's Offshore Energy Security Program. These surveys acquired a range of pre-competitive geological and geophysical data that included seismic reflection, gravity, magnetic and swath bathymetry measurements, as well as seafloor dredge samples. The new surveys provided a total of about 26 000 line km of new gravity and magnetic data that add to existing data from around 150 previous marine surveys conducted off the southwest margin since 1960. This Record describes the integration and levelling of the new gravity and magnetic data with existing data, both offshore and onshore, to produce a unified gravity and magnetic dataset for use in constraining regional tectonics, basin structure and petroleum prospectivity. Levelling is a key step in processing ship-borne gravity and magnetic data. This process minimises the mistie errors at ship-track cross-overs that arise from factors such as positioning errors, instrument drift and lack of diurnal corrections to magnetic data. Without accounting for these cross-over errors, gridded data can be rendered un-interpretable by artefacts and distortions at line cross-overs.

AUSCAN (Australian Canyons), a major research expedition that investigated Australia's southern margin from southern Tasmania in the east to off Cape Leeuwin in the west, successfully completed its mission in March 2003. The investigation included multibeam swath-mapping, geophysical profiling, geological and biological sampling, as well as oceanographic measurements. The data were collected to support marine environmental planning and management, to help model the structural and sedimentological evolution of the margin, and to assist understanding the climatic, oceanographic and environmental changes that affected the region during the late Quaternary. Important objectives were to map in detail and study the geomorphology and origins of the gigantic, but poorly known, canyon systems that exist beyond the continental shelf. The spectacular Murray Canyons south of Kangaroo Island, with their complex and extensive channel systems and 2-km high cliffs, were a special focus of the investigation. The information collected and resulting research will assist 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. The new data, integrated with the pre-existing seabed data sets, provide the basis for environmental management strategies and plans, and also provide framework information to support future biological and physical scientific field studies and research. AUSCAN was completed as two cruise legs totalling 3 weeks using the 120-m R/V Marion Dufresne of the French Polar Institute (IPEV). The survey used a range of geophysical, sampling and oceanographic equipment carried on the ship, but vital to the AUSCAN program were the ship's Thales Sea Falcon 11 multibeam sonar swath-mapper and its giant piston corer, `Calypso?. The multibeam system produces high-resolution bathymetric and backscatter images of the sea floor at 15 knots across a swath up to 20 km wide in deep water, while `Calypso' is capable of recovering deepsea sediment cores up to 60 m long. The cruise was based on excellent international scientific cooperation, and included scientists from Australia, France, Germany, other European countries, and the USA. IPEV was the main French organisation involved, while Australian institutions included Geoscience Australia (GA), the Australian National University, SARDI (South Australian Research & Development Institute) and the National Oceans Office (NOO). CSIRO Marine Research (Hobart) also provided input during the planning phase. NOO provided much of the Australian funding and support because of AUSCAN's direct and important relevance to current regional marine planning and environmental management initiatives, including development of National Bioregionalisation and the South-east Regional Marine Plan. GA managed the project for NOO. The AUSCAN program was designed to build on earlier major swath-mapping surveys involving French-Australian cooperation (GA and IPEV/IFREMER), such as TASMANTE off west Tasmania and on the South Tasman Rise, MARGAU off southwest Western Australia, and AUSTREA-1 off southeast and southern Australia. AUSCAN swath-mapped 70,000 km of seabed (about the size of Tasmania), filling many of the existing data gaps along the southern margin and now allowing detailed maps and images of almost the entire continental slope from Western Australia to Tasmania to be produced for the first time. AUSCAN also acquired 3.5 kHz sub-bottom profiler, gravity, magnetics and oceanographic data along 9,000 km of survey line.

Overall, the cruise met its objectives of studying rift and drift sedimentation, and obtaining cores for palaeo-oceanography. The east Tasmanian seismic program was completely successful. The planned sampling program was somewhat curtailed by bad weather, equipment failures and other factors. It was least successful off east Tasmania. A total of about 1300 km of 8-fold multichannel seismic data were acquired along 8 transects across the east Tasmanian margin. The quality of the seismic profiles was excellent, with good resolution and penetration, given the bad weather and the limitations of the acquisition system. The seismic source comprised 2 GI airguns (each 45/105 cu. in. capacity) giving a penetration of 2-2.5 s twt (2.5-3 km) in places. The seismic profiles indicate a structurally complex margin with rugged basement relief that includes large-scale horst/graben structures and volcanic intrusions. The sedimentary section on the continental slope is at least 1.5 s twt thick in some graben and includes Campanian-Paleocene early sag-phase deposits, which are 0.5-1.0 s twt thick. Regional compressive tectonism in the Late Paleocene-Early Eocene has produced widespread inversion (folding/faulting) in this succession. A wedge of Neogene shallow-water carbonates underlies the continental shelf. It shows seaward progradation and attains a maximum thickness of ~700 m beneath the shelf edge. Oceanic basement (?Campanian) adjacent to the margin lies at a depth of 7.0-7.5 s twt. The continental rise and Tasman Abyssal Plain in this zone are underlain by 1.5-2.0 s twt of post-breakup sedimentary section. The East Tasman Saddle is underlain by `transitional? basement and contains a sedimentary section of similar thickness. During the sampling program 58 of 86 stations were successful: 38 gravity cores (21 successful), 4 piston cores (3), 16 dredges (7) and 28 grabs (28). Total core recovery was 81.4 metres from the 16 successful cores taken in soft sediments, an average recovery of about 5 metres. The fairly low success rate with the gravity corer can be ascribed to problems with foram sand east of Tasmania, and shelly sand in Storm Bay. The low success rate with the dredge was related to the lightness of the gear. The deployment of the heavy piston corer for the first time on Franklin was successful. However, we did not attempt to piston core in deep water. East of Tasmania we recovered 8 gravity cores, and 7 dredge hauls. Deepwater dredging and coring were surprisingly unsuccessful. The upper slope stations, designed to sample older rocks, were reasonably successful. From these results and some existing information, general conclusions can be drawn about changes along the margin with increasing water depth. The shelf and upper slope wedge of Neogene, seaward-prograding sediments was sampled out to 1640 m. The sediments recovered include muddy sand, clayey sandstone with siliceous nodules, siliceous sandstone and calcarenite. The calcarenite is presumably part of the Middle Miocene shelf limestone sequence that is widespread off St Helens. Somewhat deeper on the upper slope, basement outcrops occur in steep slopes: granite, arkose, metasediments, conglomerate, quartz sandstone and gritty mudstone. The granites are probably from Devonian batholiths like those onshore up the east coast. Volcanic rocks and conglomerate form a basement block in deeper water on a ridge off northeast Tasmania at ~3750m. Deepwater outcrop ridges support manganese nodules and crusts. Nannofossil oozes cling to the slope, particularly in local basins, and are ubiquitous in deep water. The East Australian Current apparently winnows many of the oozes to form a blanket of foram sand.

Legacy product - no abstract available

Legacy product - no abstract available