Leg 5 Jean-C Sempere, Universitiy of Washington David Christie, Oregon Station Hobart to Fremantle 16 January - 17 February, 1996 The sharp boundary between the distinctive Pacific Ocean and Indian Ocean mantle isotopic provinces, presently located within the Australian- Antarctic Discordance (AAD), has been migrating westwards for at least the last 3-5 Ma, but the geometry and long-term history of this migration are not known. This leg of the R/V Melville is an ODP site survey that will focus our drilling objectives by constraining the younger (<10 Ma) history of the migration through dredge sampling and geochemical analysis. This study will also contribute to a better understanding of the physical and chemical processes underlying both the isotopic boundary and the Australian-Antarctic Discordance. This investigation will determine the geometry of the isotopic boundary over the last 10 Ma by geochemical sampling along isochrons and possibly by identification of crustal thickness variations and discordant morphotectonic traces in seafloor topography. In particular the investigators propose to determine whether the isotopic boundary has been associated with the AAD and/or its depth anomaly since rifting began, or whether it represents long-term westward migration of Pacific mantle into the widening gap between the Australian and Antarctic continents. These geophysical studies will address the spatial and temporal relationships between the isotopic boundary and the residual depth anomaly associated with the AAD, focusing on the nature and extent of crustal thickness variations and their implications for mantle flow and mantle temperature variations beneath this region. This investigation will done by performing 35 dredges, a single channel seismic survey using 80 cu. in. water guns, and a Sea Beam 2000 survey in the basic mode operation. In addtion gravity and magneticswill be collect through out this cruise.