Basement and crustal results from the Bremer Sub-basin, SW Australia and its Antarctic counterpart drive Australia-Russia cooperation

Brief description

As part of the Australian Government's "New Oil" initiative, Geoscience Australia undertook a geophysical survey (the Southwest Frontiers Survey) of the south-western Australian continental margin in late 2004. The survey acquired 2700 km of industry-standard, 106-fold seismic data recorded to 12 seconds two-way time using a 6-8 km digital streamer and 4900 cui gun array. Marine reflection seismic acquisition was supplemented by recording of refraction seismic data by sonobuoys at sea and by land stations in the onshore/offshore observation scheme. Marine reflection survey shots were used as sources of seismic energy for both add-ons. The main scientific objectives of refraction work were: 1. Provide accurate seismic velocity information to improve depth conversion of reflection seismic data and to define type of basement and crust below it underneath the sedimentary basins. 2. Provide estimates of crustal thickness underneath major sediment deposition centres in the area to better constrain interpretation of tectonic evolution of the region. The new refraction seismic data have substantially supplemented coverage of the area resulting from old sonobuoy work and from very few onshore stations. When applied to a basement travel time pick interpreted in the multi-channel reflection data set, the new velocity data indicate a maximum sediment thickness in excess of 9 km. A comparison of stacking velocities and modelled sonobuoy data suggests that, unlike in other areas, stacking velocities from the Bremer Sub-basin survey 280 can be used as a reliable substitute for acoustic velocities down to 3 s two-way time into the sediments, for the purpose of calculating sediment thickness. One of the key findings of the refraction seismic study is that velocities in the basement are generally in the 5.2-5.6 km/s range, indicating that, contrary to priory expectations, basement in the area is mostly not granitic in composition. Results from the Bremer area conjugate counterpart in Antarctica obtained by the 50th Russian Antarctic Expedition (December 2004 - April 2005) also show low velocities in the basement on the inner side of Antarctic continent-ocean boundary, and therefore are consistent with our results from the Australian margin. If we combine all results available to us, it appears that a ~400km wide zone in Gondwana prior to break-up had basement velocities significantly lower than normal continental values of 6.0 - 6.2 km/s typical for granites and gneisses. The presence of low grade metasediments of the Albany-Fraser Province and its Antarctic equivalent is our preferred interpretation of this observation. Metasediments produce substantially less heat than granites and this leads to a different scenario, than granitic basement, for hydrocarbon maturation in the Bremer sub-basin, which is one of the targets of Geoscience Australia's Big New Oil program. Advanced burial and thermal geo-history modelling in this area was carried out for the first time in Australia without relying on default values (such as Heatflow or geothermal gradient) in modelling packages. Results of this work will be presented at the APPEA 2006 Conference. The similarities in seismic properties of the crust between the Bremer/Recherche area on the SW Australian continental margin and its conjugate on the Antarctic margin, has generated interest in Russia. As a result, planning of Russian Antarctic Expedition 51 (to start in December 2005) will take into consideration the need to record additional data at specific locations to answer questions of interest to both Australia and Russia.

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