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 topo graphy, reflecting a weak magma supply from an unusually cold underlyi ng mantle. During the past 3-4 million years, however, a source of inc reased magma supply, coinciding with the known Indian-Pacific mantle I sotopic 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 Pa cific mantle were first erupted within the AAD. This long hiatus, comb ined with the ridge-transform geometry across the AAD boundary, constr ains the locus of mantle migration to a narrow, relatively shallow reg ion, directly beneath the spreading axis of the Southeast Indian ridge .