Abstract

Mont Ross is the main volcanic feature of the Kerguelen Archipelago (terres Australes et Antarctiques françaises). 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 Ross is the subaerial part of a plutonic complex located in Galliéni 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 depth of 11 km. These are definitely higher than those encountered within surrounding basalts known as plateau basalts. These high velocities reveal, at first glance, an origin and composition of the basement of Mont Ross far distinct from those of tholeiitic or transitional lava flows generated near spreading centres. By comparison with plutonic ring complexes, it is reasonable to state that monzonite and syenite are the basic materials of the basement. Seismic velocities (6.85 to 7.30–7.35 km/s) and related Poisson ratio (σ = 0.30) within lower crust are consistent with gabbros as prominent material. The thickness of the lower crust below Mont Ross (6–7 km) is roughly the same as that below the archipelago. Gabbros are exposed around several plutonic ring complexes spread over the archipelago. The transition to mantle might be modelled by a 2 km thick transition zone, with high velocity gradient, 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 contamination of the lower crust by mantle material. Both gravity and seismic data substantiate the occurrence of high density (velocity) within the upper crust below Mont Ross. Isostatic compensation of Mont Ross is rather achieved by a flexural deflection of the lithosphere than by an Airy-type model. The structures of Mont Ross and Hawaiian volcanoes bear analogies likely related to their intraplate genesis.

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