The Structure of the Continental Margin off Wilkes Land and Terre Adélie Coast, East Antarctica

In 2001 and 2002, the Australian Government acquired approximately 9000 km of high-quality geophysical data over the margin of East Antarctica between 110-142 degrees E that provide a sound framework for understanding the geology of the region. The data comprise 36-fold deep-seismic, gravity and magnetic data and non-reversed refraction/wide-angle reflection sonobuoys recorded along transects that extend from the lower continental slope out to oceanic crust at a spacing along the margin of approximately 90 km. The continental slope is underlain by a major rift basin beneath which the crust thins oceanwards through extensive faulting of the rift and pre-rift sedimentary section and by mainly ductile deformation of the crystalline crust. Outboard of the margin rift basin, the 90 to 180 km wide continent-ocean transition zone is interpreted to consist primarily of continental crust with magmatic components that can account for the lineated magnetic anomalies that have been interpreted in this zone. The thick sedimentary section in the COT zone is floored by dense lower crustal or mantle rocks indicating massive (>10 km) thinning of the lower and middle crust in this zone. The boundary between the margin rift basin and the COT is marked by a basement ridge which potential field modelling indicates is probably composed of altered/serpentinised peridotite. This ridge is similar in form and interpreted composition to a basement ridge located in a similar structural position at the inboard edge of the COT on the conjugate margin of the Great Australian Bight. On both margins, the ridge is probably the product of mantle up-welling and partial melting focussed at the point of maximum change/necking of crustal thickness. Integrated deep-seismic and potential field interpretations point very strongly to the boundary between unequivocal oceanic crust and largely continental crust of the continent-ocean transition as lying in very deep water, and considerably seaward of most previous interpretations (often based on inadequate seismic data or magnetic data only). We consider the continent-ocean boundary to be well-constrained from 124-131 degrees E and unequivocal from 131-140 degrees E, but open to debate in the sector from 110-124 degrees E. There is a strong degree of pre-breakup symmetry between the conjugate margins of southern Australia and East Antarctica east of about 120 degrees E. In addition to the crustal symmetry,there is also a strong correlation in seismic character between the margins, which allows us to date the major unconformities as probably of base Turonian, Maastrichtian and early Middle Eocene age.