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.

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.

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

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.

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.

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.

This document will be posted on the GA and CSIRO-Marine websites. Dr. Neville Exon was Chief Scientist and Cruise Leader for this survey.

This abstract describes the results of integrated potential firld modelling and deep-seismic interpretation from the Enderby Land and Wilkes Land margins of East Antarctica. The interpretation is based on data acquired under the Australian Antarctic and Southern Ocean Profiling Project.