Chaotic topography, mantle flow and mantle migration in the Australian-Antarctic discordance

Oceanic crust formed over the past 30 million years at the Australian-Antarctic discordance (AAD) is characterized by chaotic sea-floor topography, reflecting a weak magma supply from an unusually cold underlying mantle. During the past 3-4 million years, however, a source of increased magma supply, coinciding with the known Indian-Pacific mantle Isotopic boundary, has propagated into the eastern AAD, displacing the chaotic terrain and replacing it with normal sea floor, Pacific mantle reached the eastern boundary of the AAD at least 7 million years ago, but it was not until 3-4 million years ago that lavas derived from Pacific mantle were first erupted within the AAD. This long hiatus, combined with the ridge-transform geometry across the AAD boundary, constrains the locus of mantle migration to a narrow, relatively shallow region, directly beneath the spreading axis of the Southeast Indian ridge.