DEEP-STRUCTURE OF MOUNT-ROSS ANALYZED BY SEISMIC-REFRACTION (KERGUELEN ISLES, SOUTHERN INDIAN-OCEAN)

Mont Ross is the main volcanic feature of the Kerguelen Archipelago (t erres Australes et Antarctiques francaises). This newly formed volcano buildup over 2 Ma provides us with an outstanding model of volcanism occurring on an intraplate structure already aged 40 Ma. Mont Boss is the subaerial part of a plutonic complex located in Gallieni Peninsula . From seismic refraction studies, P-wave velocities within the upper crust range downward from 5.35 km/s at sea level to 6.60 km/s at a dep th of 11 km. These are definitely higher than those encountered within surrounding basalts known as plateau basalts. These high velocities r eveal, at first glance, an origin and composition of the basement of M ont Ross far distinct from those of tholeiitic or transitional lava fl ows generated near spreading centres. By comparison with plutonic ring complexes, it is reasonable to state that monzonite and syenite are t he basic materials of the basement. Seismic velocities (6.85 to 7.30-7 .35 km/s) and related Poisson ratio (sigma = 0.30) within lower crust are consistent with gabbros as prominent material. The thickness of th e lower crust below Mont Ross (6-7 km) is roughly the same as that bel ow the archipelago. Gabbros are exposed around several plutonic ring c omplexes spread over the archipelago. The transition to mantle might b e modelled by a 2 km thick transition zone, with high velocity gradien t, already noticed below the archipelago. Velocities of 7.30-7.35 km/s at the base of the crust below Mont Ross do not preclude contaminatio n of the lower crust by mantle material. Both gravity and seismic data substantiate the occurrence of high density (velocity) within the upp er crust below Mont Ross. Isostatic compensation of Mont Ross is rathe r achieved by a flexural deflection of the lithosphere than by an Airy -type model. The structures of Mont Ross and Hawaiian volcanoes bear a nalogies likely related to their intraplate genesis.