The upper mantle beneath Ninetyeast Ridge and Broken Ridge, Indian Ocean, from surface waves

Summary The Rayleigh wave group velocities are used to determine the upper mantle structure of the aseismic ridges of the eastern Indian Ocean, i.e. the Ninetyeast Ridge and the Broken Ridge‐Naturaliste Plateau. For comparison, profiles are studied across the Bay of Bengal and to the west of the Ninetyeast Ridge. The group velocities are associated with the regions by the pure path method. Models are obtained down to 150‐200 km for the profiles on and off Ninetyeast Ridge, and down to 100km for Broken Ridge. They reveal that, except for a thick anomalous crust, the structure beneath these ridges is quasi‐oceanic. For the Ninetyeast Ridge, however, the uppermost 30 km of the mantle have lower velocities; such low velocities are also obtained for the Walvis Ridge by Chave. For the paths off ridge no such low velocities are obtained, indicating a small lateral extension of this anomaly. A thermal origin is ruled out, while a model with serpentinized peridotites as proposed by Bowin, which fits the gravimetric data, is able to explain the low velocities of the uppermost mantle beneath the Ninetyeast Ridge.