DEEP-STRUCTURE OF THE SOUTHERN KERGUELEN PLATEAU (SOUTHERN INDIAN-OCEAN) FROM OCEAN-BOTTOM SEISMOMETER WIDE-ANGLE SEISMIC DATA

Wide-angle seismic data collected during the Kerguelen ocean bottom se ismometer experiment provide the first images of the deep structure of the southern Kerguelen Plateau and support a new interpretation of th e origin of the plateau. Velocity models based on travel time inversio ns and reflectivity synthetic seismograms show a 22-km-thick crust com posed of similar to 1.6 km of sedimentary cover, similar to 5.3 km of upper crust, similar to 11.0 km of lower crust, and a 4- to 6-km-thick reflective zone immediately above Moho. Velocities in the upper crust (from 3.8-4.5 km/s at top to 6.0-6.5 km/s at bottom) are consistent w ith the basaltic nature of this layer, the top of which was sampled du ring the Ocean Drilling Program. Velocities ill the lower crust increa se continuously from 6.60 km/s at the top to 6.90 km/s at 19.5 km dept h. The reflective zone at the base of the crust identified by wide-ang le reflections is observed only along the NNW-SSE direction. It consis ts of alternating high- and low-velocity layers with an average veloci ty of 6.70 km/s in the NNW-SSE direction and greater than or equal to 6.90 km/s in the perpendicular direction. Strong azimuthal anisotropy is also observed in the upper mantle with velocities of 8.60 and 8.00 km/s, in the NNW-SSE and E-W directions, respectively. The absence of high velocities at the base of the crust that characterizes many large -volume mafic provinces, the reflective lower crust, and anisotropy in upper mantle suggest that the southern Kerguelen Plateau represents a stretched continental fragment overlain by basaltic flows isolated fr om the Antarctic margin during the early opening of the Indian Ocean.