KINEMATIC AND BALANCE IMPLICATIONS OF “BightSPAN” DEEP CRUSTAL SEISMIC PROFILING OF THE CEDUNA SUBBASIN, GREAT AUSTRALIAN BIGHT MARGIN OF SOUTH AUSTRALIA

Long-offset, long-record 2D seismic reflection data have been processed to 40-km pre-stack depth-migrated images of the Ceduna Subbasin as one of the ‘SPAN’ data sets of ION/GXT’s worldwide regional reconaissance seismic programs. Twelve lines comprise the survey from the breakaway fault system at the edge of undeformed shelf to and beyond the transition to oceanic crust in 4.5 km of water. The data image to depths much greater than previously available seismic datasets, are processed in depth rather than in two-way time, and thus include the deep crust and Moho within the record length. The crystalline crust is reduced from 38 km thickness at the shelf margin to as little as 5 to 8 km in a series of fault blocks offshore. Three major tectonostratigraphic packages record subsidence and uplift during development of the margin. From bottom-up, the first package (Late J-Aptian) lies immediately above the continental crust and comprises the locus of early extension in the central part of the margin: this package is adequately imaged only landward on the margin where extension was limited, and progressively loses internal character seaward where it thickens dramatically and presumably participated in hyper-extension with the underlying continental crust. The second (Albian-Santonian) and third (Campanian-Maastrictian) packages are progradational with growth faulting inboard linked to toe-thrust systems offshore. The outboard edge of the second is uplifted and erosionally truncated beneath the third, and its basal detachment is folded. The third package lies on the angular unconformity and records the late passive sag phase of margin development. The continent-ocean transition is obscured on some lines by late volcanism on the seaward edge of the continental crust, which may have overprinted and obscured its original magnetic signature. The full extensional history includes early, locally-focused pure shear extension with a later stage of simple shear restricted to the outboard edge of the margin. Isostatic rebound on unloading and magmatic inflation of the margin edge accompanied initiation of sea floor spreading. Restoration must account for a deficiency in continental crust relative to the rift infill, possibly balanced by a departed piece of continental crust removed from beneath its sedimentary cover.