The spatial and temporal evolution of strain during the separation of Australia and Antarctica

Abstract

A re‐evaluation of existing onshore and offshore gravity, magnetic, seismic reflection, and well data from the Australo‐Antarctic margins suggests that magmatism and along‐strike lithospheric heterogeneities have influenced the localization of initial rifting. The 3‐D crustal architecture of the Australian and Antarctic margins, which formed during multiple rifting episodes spanning ∼80 Myr, reveal local asymmetries along strike. Rift structures from the broad, late Jurassic (165–145 Ma) rift zone are partially overprinted by a narrower, mid‐to‐late Cretaceous rift zone (∼100 Ma), which evolved in highly extended crust. This late‐stage rift zone is located within a region of heterogeneous crust with faults that cut late syn‐rift strata, interpreted as a continent ocean transition zone. This late stage transitional rift is populated by seismically identified rift‐parallel basement highs and intracrustal bodies with corresponding positive Bouguer gravity and magnetic anomalies. These undrilled features can be interpreted as exposures of exhumed mantle rocks, lower crustal rocks and/or as discrete magmatic bodies. Our results suggest that strain across an initially broad Australo‐Antarctic rift system (165–145 Ma) migrated to a narrow rift zone with some magmatism at 100–83 Ma. Breakup did not occur until ∼53 Ma within the eastern Bight‐Wilkes and Otway‐Adélie margin sectors, suggesting a west to east propagation of seafloor spreading. The prolonged eastward propagation of seafloor spreading processes and the increased asymmetry of the Australian‐Antarctic margins coincides with a change from rift‐perpendicular to oblique rifting processes, which in turn coincide with along‐strike variations in cratonic to Palaeozoic lithosphere.