Victoria's Proterozoic basement controls the distribution of its southern margin petroleum basins

Abstract

There are three petroleum basins of differing character off the Victorian coast: the Otway, Bass and Gippsland basins. These formed during continental rifting between Australia, Antarctica and New Zealand, associated with the break up of Gondwana Marked variation in the development of these basins appears to have been largely controlled by the distribution of Proterozoic basement—the Selwyn Block—under central Victoria. Lying deep under central Victoria, this block surfaces towards the coast and continues southward as the Proterozoic crust of western Tasmania. The boundaries of this block are coincident with the boundaries separating the three basins. The Otway Basin in western Victoria represents a clean break between Australia and Antarctica. The Otway Basin has thick fill upon thinned continental crust with an outboard break to a continent-ocean boundary. The overall geometry here is a classic lower plate margin. This clean continental break-up failed to propagate eastward across the Proterozoic Selwyn Block. Instead, localised continental stretching resulted in some grabens and the overlying steers head sag of the Bass Basin. True continental separation was transferred southward to the margin of the Tasmania/Selwyn Block. The Gippsland Basin lies east of the Selwyn Block. Its development reflects initial southern margin rifting, but this was overtaken by orthogonal-oriented Tasman rifting. This left the Gippsland Basin with a complex interplay of north-south and east-west structures controlling the platforms, terraces and deeps.