A case for Australia-Antarctica separation in the Neocomian (ca. 125 Ma)

Abstract

The age of separation of Australia and Antarctica (that is, the onset of seafloor spreading) has usually been determined by the identification of seafloor-spreading magnetic anomalies adjacent to the margin, or by extrapolation of the spreading rate/time span between the oldest identified anomaly and the continent-ocean boundary (COB) as interpreted from magnetic and single-channel seismic data. The most recent estimate of the age of breakup using these methods is 95 ± 5 Ma, in the Cenomanian. However, identification of the oldest magnetic anomalies, formed during the early phase of slow drift between Australia and Antarctica, is tenuous, particularly between southwest Australia and the central Great Australian Bight (GAB). An alternative approach for establishing the age of breakup is to determine the relationship of the continental margin sequences and the oceanic crust, using seismic data. Application of this technique to the western Great Australian Bight demonstrates that it is likely that the oldest interpreted oceanic crust is overlain by several hundred metres of sediment that is no younger than Valanginian (ca. 125 Ma); that is, separation of Australia and Antarctica must have occurred prior to this time, perhaps contemporaneously with the separation of Australia and “Greater India” off Western Australia. The similarity of structural and magnetic anomaly trends between the southern and western margins of Australia further suggests that breakup and spreading on those two margins are closely linked. However, there may be further complicating factors in the Australian-Antarctic extension and breakup history. High-quality migrated seismic data from the central Great Australian Bight, across what has been previously interpreted as oceanic crust, show the presence of listric faulting and apparent syn-rift sedimentation within “basement”. The gross structure is similar in appearance to published models of a metamorphic core complex. Other possibilities are that the observed structuring is the product of the re-rifting of oceanic crust of undetermined (pre-Valanginian) age, or that the zone oceanwards of the “COB” consists of highly thinned lower plate, probably injected by ribbons of oceanic basalt towards its southern edge. If such structures can also be identified in the western GAB, then it is clear that the age of the emplacement of the first oceanic crust between Australia and Antarctica is still undetermined.