Summary of forward gravity and flexure modelling of the New Caledonia Trough to highlight temporal variations in lithospheric rigidity during its evolution.

Compressional deformation is a common phase in the post-rift evolution of passive margins and rift systems. The central west Western Australian margin, between Geraldton and Karratha, provides an excellent example of strain partitioning between inverting passive margin crust and adjacent oceanic and continental crust. The distribution of contemporary seismicity in the region indicates a concentration of strain release within the basins diminishing eastward into the cratons. Very few data exist to quantify uplift or slip rates, however this pattern can be qualitatively demonstrated by tectonic landforms which indicate that the last century or so of seismicity is representative of patterns of Neogene and younger deformation. Pleistocene marine terraces on the western side of Cape Range indicate uplift rates of several tens of metres per million years, with similar deformation resulting in sub-aerial emergence of Miocene strata on Barrow Island and elsewhere. In the southern Carnarvon Basin, marine strandlines of unknown age are displaced by a few tens of metres, indicating uplift rates an order of magnitude lower than further west. Relief production rates in the western Yilgarn Craton are lower still - numerous scarps (e.g. Mt Narryer) appear to relate individually to <10 m of displacement across Neogene strata. The en echelon arrangement of such features distinguish them from those representing strain concentration in the craton proper, where scarps are isolated and typically <5 m high. Quantitative analysis of time-averaged deformation preserved in the aforementioned landforms, including study of scarp length as a proxy for earthquake magnitude, has the potential to provide useful constraint on seismic hazard assessments in a region which contains major population centres and nationally significant infrastructure.

Many aspects of the evolution and overall architecture of the Australian southern rifted margin are consistent with current models for the development of non-volcanic rifted margins. However, when examined in detail, several key features of the southern margin provide useful points of comparison with the Atlantic and Alpine Tethyan margins from which these models derive. Extensive petroleum industry and government seismic and geophysical data sets have enabled detailed mapping of the basins of the southern margin and an improved understanding of its tectonostratigraphic evolution. Australia's southern rifted continental margin extends for over 4000 km, from the structurally complex margin south of the Naturaliste Plateau in the west, to the transform plate boundary adjacent to the South Tasman Rise in the east. The margin contains a series of Middle Jurassic to Cenozoic basins-the Bight, Otway, Sorell, Gippsland and Bass basins, and smaller depocentres on the South Tasman Rise (STR). These basins, and the architecture of the margin, evolved through repeated episodes of extension and thermal subsidence leading up to, and following, the commencement of sea-floor spreading between Australia and Antarctica. Break-up took place diachronously along the margin, commencing in the west at ~83 Ma and concluding in the east at ~ 34 Ma. In general, break-up was not accompanied by significant magmatism and the margin is classified as 'non-volcanic' (or magma-poor). Initial NW-SE ultra-slow to slow seafloor spreading (latest Santonian-Early Eocene), followed by N-S directed fast spreading (Middle Eocene-present), resulted in: (1) an E-W oriented obliquely- to normally-rifted marginal segment extending from the westernmost Bight Basin to the central Otway Basin; (2) an approximately N-S oriented transform continental margin in the east (western Tasmania-STR), and (3) a transitional zone between those end-members (southern Otway-Sorell basins).

Legacy product - no abstract available

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Three hundred and sixty-five surface and near-surface seabed samples provide the basis for an assessment of regional lithofacies variations on the Tasmanian shelf and in eastern and western Bass Strait. Quartz-rich sands with variable amounts of shell debris occur on the innermost shelf and on the rises flanking the central Bass Strait basin. They are essentially modern deposits derived in the main from Pleistocene near shore sand bodies reworked and transported landwards during the Holocene marine transgression. Muddy sediments of the middle shelf off eastern Tasmania and in central Bass Strait are sites of present-day sedimentation, but they are likely to form only a thin veneer, and include coarse material probably reworked from the Pleistocene and early Holocene substrate. Extensive areas of the middle and outer shelf, particularly off southern and western Tasmania, are floored by dominantly relict bryozoan sands and gravels. Fine-grained and shelly, slightly quartzose sands in areas of the middle shelf consist of relict sediment, and sediment from the late Holocene transgressive marine sand sheet, in about equal proportions. Four main suites of heavy minerals are present in the surface sediments. Provenance relationships with sources in the adjacent hinterland suggest that little offshore sediment transport parallel to the coastline has taken place. Rare grains of cassiterite were identified in marine sediments lying off the tin-producing areas of northeastern Tasmania, but 10 ppm Sn was the maximum value recorded in the geochemical analyses. Some phosphatisation of relict limestone gravels on the middle and outer shelf off northwestern Tasmania has taken place, but the highest recorded whole-rock analysis was 3.6 percent Pi>0. Density of sample stations in this part of the shelf is low.

Under the Australian Government's Energy Security Program, Geoscience Australia conducted a seismic survey and a marine reconnaissance survey to acquire new geophysical data and obtain geological samples in frontier basins along the southwest Australian continental margin. Specific areas of interest include the Mentelle Basin, northern Perth Basin, Wallaby Plateau and southern Carnarvon Basin. The regional seismic survey acquired 7300 km of industry-standard 2D reflection seismic data using an 8 km solid streamer and 12 second record length, together with gravity and magnetic data. These new geophysical datasets, together with over 7000 km of re-processed open-file seismic data, will facilitate more detailed mapping of the regional geology, determination of total sediment thickness, interpretation of the nature and thickness of crust beneath the major depocentres, modelling of the tectonic evolution, and an assessment of the petroleum prospectivity of frontier basins along the southwest margin. The scientific aim of the marine reconnaissance survey was to collect swath bathymetry, potential field data, geological samples and biophysical data. Together with the new seismic data, samples recovered from frontier basins will assist in understanding the geological setting and petroleum prospectivity of these underexplored areas.

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This record is a compilation of the abstracts of oral and poster papers presented at a symposium held at the Bureau of Mineral Resources, Canberra 13-16 February 1989. The symposium was entitled "Seismicity and Earthquake Studies in the Australian Plate and its Margins", and was co-sponsored by the Specialist Group on Solid Earth Geophysics of the Geological Society of Australia and the Bureau of Mineral Resources. The abstracts in this paper are in the same order as in the symposium program at the beginning of the paper.

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