Laboratoire de Geodynamique sous Marine, France

Digital grids, with a spatial resolution of .5 degrees , were compiled using bathymetry, sediment thickness, and oceanic crustal ages for the southern hemisphere to calculate sediment-free palaeobathymetric charts for late Cretaceous (110 Ma) to the present. These reconstructions allow the definition and quantification of the bathymetric evolution of the seaways and gateways within this region. Pre-60 Ma the palaeobathymetry and the interconnection of the deeper ocean basin was dominated by the formation of hotspot-related ridges and plateaux. Between 40 Ma and the present the palaeobathymetry is characterized by the opening of numerous gateways as the Tasman Rise separated from Antarctica, the Mascarene Plateau split from the Chagos-Laccadive Ridge, Broken Ridge rifted from the northern Kerguelen Plateau, the Crozet Plateau separated from the Madagascar Ridge, and the Scotia Sea opened. The validity of thse palaeobathymetric reconstructions is demonstrated by comparing the digital gridded data sets, from which they were calculated, with data derived from 39 Deep Sea Drilling Project and Ocean Drilling Program drill sites. Comparison of drill site data with the gridded data revealed positive correlations between crustal ages and bathymetry, whereas the sediment thickness data showed no correlation. The lack of any correlation for the sediment thickness data, however, did not preclude a positive correlation between sediment-free bathymetries. The difference between expected 'true' palaeobathymetry, that is with sediments older than the time of reconstruction decompacted and reloaded back onto oceanic crust, and the sediment-free palaeobathymetry was shown to be less than 600 m for 87.5% of the data. The greatest differences occur where sediment thicknesses are greatest, i.e. within the deep ocean basins which are in close proximity to the continental margin.