Despite long history of studies the Wallaby Plateau offshore Western Australia remains a controversial feature. Analysis of interval seismic velocities from Geoscience Australia's 2008/09 seismic survey 310 in conjunction with seismic reflection interpretation provides new insights into the geology of the Plateau. Seismically distinctive divergent dipping reflector (DDR) packages have been identified. The seismic character of the DDR packages is similar to seaward dipping reflector (SDR) packages of inferred volcanic composition. Initial analysis of seismic velocity profiles indicated affinities between the DDR packages and known sedimentary strata in the Houtman Sub-basin. Effect of water loading on seismic velocities is commonly ignored in offshore studies. However, direct comparative analysis of interval velocity patterns between areas of significantly different water depth requires various water pressure related changes in velocity to be accounted for. There are controversies in methodology and application of water depth adjustment to seismic velocities, and presentation of velocity models as function of pressure rather than two-way time, or depth emerges as a possible solution. Water depth adjustment of seismic velocities analysed in our study reduces distinction between SDRs, DDRs and sedimentary strata such that discrimination between volcanic and sedimentary strata in DDR or SDR packages is equivocal. A major uncertainty of this interpretation is due to a lack of the reference velocity model of SDRs and DDRs investigated globally.

This report (Record 2009/38) contains the description and preliminary analysis of datasets acquired during Geoscience Australia marine reconnaissance survey GA2476 to the west Australian margin. The survey, completed as part of the Federal Government's Offshore Energy Program, was undertaken between 25 October 2008 and 19 January 2009 using the German research vessel RV Sonne. The survey acquired geological, geophysical, oceanographic and biological data over poorly known areas of Australia's western continental margin. Data from the marine reconnaissance survey (GA2476) and the concordant regional seismic survey (GA0310) will improve knowledge of frontier sedimentary basins and marginal plateaus and allow assessment of their petroleum prospectivity and environmental significance. These data will be used to improve resource management and underpin decisions regarding future acreage release in offshore Western Australia and marine zone management. Four key areas were targeted: the Zeewyck and Houtman sub-basins (Perth Basin), the Cuvier margin (northwest of the Southern Carnarvon Basin), and the Cuvier Plateau (a sub-feature of the Wallaby Plateau). Over the duration of the survey a total of 229,000 km2 (26,500 line-km) of seabed was mapped with the multi-beam sonar, 25,000 line-km of digital shallow seismic reflection data and 25,000 line-km of gravity and magnetic data. A variety of sampling equipment was deployed over the duration of the survey, including ocean floor observation systems (OFOS), deep-sea TV controlled grab (BODO), boxcores, rock dredges, conductivity-temperature-depth profilers (CTD) and epibenthic sleds. A total of 62 stations were examined throughout the survey, including 16 over the Houtman Sub-basin, 16 over the Zeewyck Sub-basin, 13 in the Cuvier margin, 12 over the Cuvier Plateau and four in the Indian Ocean. This report is intended to provide a comprehensive overview of the survey activities, equipment used and preliminary results form survey GA2476.

No abstract available

The continental margin of East Antarctica between Dronning Maud and George V Lands shows no evidence of widespread breakup-related volcanism, other than adjacent to the southern Kerguelen Plateau, and it therefore constitutes one of the largest tracts of non-volcanic rifted margin on the planet. The integrated interpretation of deep-seismic, velocity and potential field data acquired in 2001/02 shows that there are first-order structural variations between the two main sectors of this margin. In the west, the Enderby Land margin formed by the separation of Greater India from Antarctica, commencing in the Valanginian (ca. 131 Ma). Seawards of a major basement fault zone underlying the continental slope, deep-seated continental crust is characterised by high-angle brittle faulting; seismic data provide little information on the style of the deeper crustal deformation. The other prominent structural features of this margin are a sharp continent-ocean boundary (COB), characterised by an oceanwards step-up in the basement level of up to 1 km, and an earliest phase of ocean crust that is distinctive for its rich internal reflection fabric and strong Moho reflection. Potential field modelling indicates that the gross margin structure is relatively simple, and that both the continental and oceanic crusts have behaved as a semi-rigid plate that has been depressed landwards by the thick (4-9 km) post-rift sediment loading. In the east, the Wilkes Land margin formed during the separation of Australia and Antarctica, commencing with very slow seafloor spreading in the early Campanian (ca. 83 Ma). This margin is dominated by the broad and highly structured transition from the attenuated continental crust inboard on the margin, to the mechanically extended and largely amagmatic oceanic crust that formed during the initial seafloor spreading. From inboard to outboard across this zone, the structuring is characterised by: ? plastic and brittle deformation of the lower continental crust and upper mantle; ? a ridge of interpreted serpentinised peridotite that can be traced along strike on the margin for a distance of at least 800 km; ? a sedimentary basin outboard of the ridge that appears to be underlain by fragments of crystalline continental crust; and ? a COB between mechanically extended continental and oceanic crust that is complex and not readily delineated. The structures of the Wilkes Land margin are very similar to those on the conjugate southern Australian margin, indicating that at least the final stage of rifting between Antarctica and Australia was inherently symmetrical. The wide differences between the structural styles documented on the Enderby and Wilkes Land margins indicate that tectonic processes forming non-volcanic rifted margins may differ significantly depending on a range of factors that may include pre-existing heterogeneities in the continental crust, the thermal regime and the pre-breakup intra-plate stress regime.

Geoscience Australia is custodian of ship-track magnetic and gravity data from close to 700 marine surveys conducted between 1960 and 2009. These data were last combined and levelled in the late 1990s. New levelling has been motivated by specific requirements within projects being conducted as part of the Australian Government's Energy Security Program (2006-2011). These projects rely on marine potential-field datasets to help constrain sediment thickness and basement architecture in remote offshore basins. Recently-levelled datasets cover: 1) the Capel and Faust basins, deepwater basins about 800 km off the east coast, and 2) the southwest margin of the Australian continent. The levelling involved the following steps: consistent computation of gravity anomalies; splitting lines into straight-line segments to facilitate cross-over computations; low-pass line filtering where necessary; editing to remove problematic ship-tracks; and levelling to minimise cross-over misties. Using methods developed by Intrepid Geophysics for the late-1990s Australia-wide work, magnetic data were levelled by minimising misclosures around loops and then gridded and merged with aeromagnetic data in onshore and near-shore areas. Gravity data were levelled using a polynomial technique and satellite-altimeter-derived gravity data as a reference surface. The resulting levelled datasets provide information at a higher-resolution than is available from satellite-derived gravity measurements or from global compilations of magnetic data. Despite this, the levelled datasets are limited to areas of specific scientific or exploration interest, which highlights the need for levelled datasets that cover the whole of Australia's marine jurisdiction.

Legacy product - no abstract available

In January 2000, the Australian Geological Survey Organisation (AGSO) completed a major, 25-day seabed swath-mapping and geophysical survey off southeast Australia for the National Oceans Office (NOO) and Environment Australia (EA). The survey, named AUSTREA-l and designated as AGSO Cruise 222, used the 85-m French oceanographic and geoscience research vessel L 'Ata/ante, departing Noumea on 18 December 1999 and ending in Hobart on 11 January 2000. The survey covered 11,000 km and mapped about 120,000 km2 of seabed - an area about 1.5 times the size of Tasmania. The work was done for marine zone planning and management, for assessment of seabed living and non-living (petroleum and mineral) resources, and geological and biological research, as a major step towards implementation of Australia's Oceans Policy and Australia's Marine Science and Technology Plan, and in particular, the development of the Southeast Regional Marine Plan by the National Oceans Office. Data collected included Simrad EM 12D swath-bathymetry and backscatter imagery, 6- channel GI-gun seismic, digital 3.5 kHz sub-bottom profiles, gravity and total field magnetics. Also collected was oceanographic information - XBTs to 1800 m depth and underway ADCP (current), sea surface temperature and salinity measurements. Weather and sea conditions were generally favourable, though stormy conditions with 30-35 knot winds and associated rough seas were encountered at times. Data quality was mostly excellent. The survey mapped the volcanic slopes of Lord Howe Island and Ball's Pyramid to the 12 nautical mile outer limits of a proposed Marine Protected Area, revealing a rugged terrain of volcanic cones, flows and canyons likely to harbour diverse benthic communities. The steep and narrow rifted continental margin off the NSW South Coast was shown to be deeply dissected by canyons and to contain gigantic continental fauit blocks fuld ?syw-ift volcanic seamounts and ridges. The survey completed mapping of the huge Bass Canyon complex off southeast Victoria, revealing detailed morphology of tributary canyons up to 1000 m deep adjacent to the Gippsland oil fields. Important fishing grounds of the Southeast Trawl Fishery were mapped off Tasmania, including volcanic and carbonate pinnacle terrain off St Helens, volcanic seamounts of the Southern Hills, and the heads of canyon systems incised into the sedimented upper slope off west Tasmania. Mapping of the Tasmanian Seamounts Marine Protected Area, south of Hobart, was completed, with thirty additional volcanic seamounts found just east and north of the MP A. The seismic profiles confirmed the existence of potential frontier petroleum basins off the east, southern and west coasts of Tasmania. Parts of the deeply-canyoned upper and mid slope of the Otway Basin were mapped off northwest Tasmania, Victoria and South Australia. The Great Australian Bight Benthic Protected Area of the GAB Marine Park was fully surveyed below the 500 m isobath and was shown to be generally a uniform slope, with the gigantic Nullarbor Canyon crossing its southeastern comer, gouged into deformed Late Cretaceous sediments. A full set of shipboard maps was provided to the National Oceans Office; copies of the digital swath-data are held for NOO at AGSO. All data from the cruise will be jointly managed by AGSO, NOO and EA.

Legacy product - no abstract available

Flexural backstripping applied to 5 seismic reflection lines has been used to constrain the distribution of lithosphere thinning, crustal structure and the location of the ocean-continent transition (OCT) of the southern Australian Bight Basin rifted margin. The amplitude of the anomalous subsidence in the southern margin of Australia was determined for each seismic line by means of residual depth anomalies (RDA) and included in all models. Sensitivity tests of predicted syn-breakup melt generated during lithosphere thinning and breakup were investigated assuming a magma-poor and a normal non-volcanic margin. The Bight Basin shows an apparent crustal segmentation between the Eyre-Recherche and the Ceduna-Recherche sub-basins. Whilst the lateral variation of lithosphere thinning in the Ceduna-Recherche sub-basin is gradual, in the Eyre-Recherche it is abrupt, predicting a highly extended continental crust underneath the Recherche sub-basin. Predicted ocean-continent transition (OCT) widths for the Bight Basin range from approximately 100 km to a maximum of 200 km, although the structure of the OCT itself is unclear. The predicted OCT determined from flexural backstripping compared with independent estimates of basement thickness derived from gravity inversion are in general agreement. Linear marine magnetic anomalies appear to be lying within the predicted OCT of the Bight Basin. If the magnetic anomalies are due to the presence of highly serpentinized peridotites and mafic bodies, the magnetic lineations cannot be used to constrain conventional seafloor spreading.

Legacy product - no abstract available