The oldest magnetic anomalies in the Australian‐Antarctic Basin: Are they isochrons?

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Abstract
We present a revised tectonic interpretation of Australia and Antarctica incorporating new magnetic data off of Wilkes Land, Antarctica, for the earliest period of seafloor spreading on the Southeast Indian Ridge, from the Late Cretaceous to Early Tertiary. Reconstructions based on our revised anomaly identifications are characterized by a surprisingly large amount of continental overlap, involving the South Tasman Rise, Tasmania, and the Victoria Land region of Antarctica. The overlap is due to continental extension and/or deformation of oceanic crust in the Australian‐Antarctic Basin. The second hypothesis would imply that the magnetic anomalies older than chron 31 in the Australian‐Antarctic Basin are not isochrons. We also find evidence for a previously unrecognized period of extremely slow spreading on the Southeast Indian Ridge from chron 31 (68.7 Ma) to chron 24 (53.3 Ma) in the Australian‐Antarctic Basin. In the western end of the basin we have tentatively identified a hiatus in the generation of crustal accretion associated with a minimum sustainable threshold half spreading rate of 1.5 mm/yr. In addition, we recognize a new fracture zone on the Antarctic plate, the Vincennes fracture zone, as the conjugate to the Perth fracture zone on the Australian plate, in the western Australian‐Antarctic Basin. These fracture zones record the initial NW‐SE opening direction between Australia and Antarctica.