Shear wave velocity structure of the upper mantle under the NW Indian Ocean

Shear wave velocity structure of the NW Indian ocean is analysed by using fundamental mode Rayleigh wave dispersion data of 67 events occurred during 1990-98 at the central Indian Ridge and Carlsberg Ridge and recorded at Hyderabad Geoscope station (HYB). These events provide a dense coverage of the NW Indian ocean and Chagos-Laccadive Ridge (CLR) in the back-azimuthal range of 192-253° with respect to HYB. The dispersion curves, corrected for continental and young ocean paths, indicate large variations in the shear wave velocity structure of the region. The group velocities along the CLR path support a typical aseismic ridge-type structure. However, the central region bounded between the Central Indian Ridge and India in the back-azimuth of 206-234° indicates a decrease in the group velocity by 0.1 km/s. Inversion of these data sets indicates presence of aseismic-ridge type lithospheric structure for CLR, a thin lithosphere and high velocity block in the depth range of 125-200 km for the central region, and a continental-type lithospheric structure for the northern-most part of the Indian ocean. It is inferred that the dynamic state of the upper mantle in this region has been significantly perturbed during the recent geological past.