Crustal Transects of the Great Australian Bight : A Cruise Proposal in Support of the Ocean Drilling Program and the Law of the Sea

The southern margin of Australia has long been considered an example of a classic passive rifted continental margin. In recent times, it has been cited as an example of a 'lower plate' margin in the terminology describing detachment models of passive margin formation.However, despite extensive study by both industry and government since the early 1970s, some fundamental aspects of the structure and geological history of the margin remain speculative. It is proposed here to use the AGS0 research vessel Rig Seismic to acquire 4087 km of deep-seismic data (16 s record length) in the central Great Australian Bight (GAB) and across the continental margin south of Western Australia. The survey has three principal objectives: 1. To enhance understanding of the tectonic evolution / event history of the southern margin in support of DPIE' s Acreage Release Program and thus to encourage successfulpetroleum exploration.

To provide the necessary framework data in support of a proposal submitted to the Ocean Drilling Program (ODP) for drilling in the GAB; and

To provide data in the GAB that support Australia's claim to a Legal Continental Shelf beyond the 200 n.m. Exclusive Economic Zone (EEZ) as defmed under the 1982 UNConvention on the Law of the Sea (UNCLOS).

The specific scientific aims of the work include: - Definition of the deep crustal structure of the region and the mode of margin formationacross a number of key transects. In particular, to image the key detachment surfaces which are believed to have controlled the extensional processes. Interpretation of the deepstructures will be enhanced by seismic refraction data recorded by Lamont-Doherty Geological Observatory in 1976 (Talwani et al., 1979).

Determination of the location and structural setting of the continent-ocean boundary. In places, stratified tilt-blocks that lie oceanwards of the magnetically determined COBsuggest that either the COB identification is incorrect, old oceanic crust has been re-rifted, or the seismic data are imaging an amalgam of oceanic crust and continental slivers.

Determination of the structure and origin of the enigmatic Diamantina Zone, west of the GAB, and its relationship, if any, to the change in seafloor spreading rate at 44 Ma. Is the crust to the north oceanic, or is it highly extended continental?