Breakup of Australia and Antarctica estimated as mid-Cretaceous (95 ± 5 Ma) from magnetic and seismic data at the continental margin

Abstract

A positive magnetic anomaly marks the seaward edge of the magnetic quiet zone along the southern margin of Australia eastward between 114° and 131°E and along the conjugate Antarctic margin between 105° and 132°E. This anomaly was originally interpreted as the oldest seafloor-spreading anomaly—A22, revised by Cande and Mutter to A34—in the Southeast Indian Ocean, but is better modelled as the edge effect at the continent-ocean boundary (COB) constrained by seismic data. Continental crust abuts the oceanic sequence of normal and reversed spreading blocks, truncated within the Cretaceous normal interval at an extrapolated age of 96 Ma, rounded to 95 ± 5 Ma to take into account the uncertainty of the initial spreading rate and of the location of the COB. The occurrence of the anomaly on both margins defines this as the age of breakup. Farther east between 131° and 139°E on the Australian margin, the COB anomaly is modelled as due to the same kind of effect but with successively younger ages of truncation to 49 Ma, interpreted as indicating the most recent ridge-crest jumping to the Australian COB. The magnetic data from the conjugate sector of Antarctica, albeit scanty, are consistent with this interpretation. The 95 ± 5 Ma age of breakup coincides with that of the breakup unconformity in southern Australia, expressed by a short mid-Cretaceous lacuna in the Otway Basin between faulted Early Cretaceous rift-valley sediments of the Otway Group and the overlying Late Cretaceous Sherbrook Group, and by an unconformity of similar age in the Great Australian Bight Basin.