Gravity trends as an expression of lithospheric extension on the southern margin of Australia

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Abstract
Gravity anomaly trends from the southern margin of Australia and its associated ‘margin basins’ support the hypothesis that margin formation has been related to two distinct phases of lithospheric extension. Seismic data indicate that extension with a northwest to southeast orientation led to formation of the Eyre Sub‐basin in the pre‐Late Jurassic (Oxfordian): the gravity trends indicate that extension probably took place throughout the region west of the Otway Basin. Extension with a north‐northeast to south‐southwest orientation created the Gippsland Basin in the Early Cretaceous and, as indicated by the gravity, appears to have extended across the entire Otway‐west Tasmania‐South Tasman Rise portion of the margin. Structural considerations and gravity lineaments suggest that the proto‐Otway Basin, the west Tasmania margin and the northeast margin of the South Tasman Rise were probably strike‐slip features in the pre‐Late Jurassic. The almost orthogonal geometries of the two phases of extension must have resulted in ‘rift overprinting’ and/or development of a new but temporary plate boundary through either the Australian Plate or the Antarctic Plate, or both. The structural complexity of the northwestern Otway Basin and the presence of west‐northwest‐trending Cenomanian faults (of non‐basin‐forming origin) within the Great Australian Bight Basin are probably products of this complex extensional history.