SHEAR VELOCITY STRUCTURE OF THE LATERALLY HETEROGENEOUS CRUST AND UPPERMOST MANTLE BENEATH THE INDIAN REGION

The shear velocity structure of the Indian lithosphere is mapped by in verting regionalized Rayleigh wave group velocities in time periods of 15-60 s. The regionalized maps are used to subdivide the Indian plate into several geologic units and determine the variation of velocity w ith depth in each unit. The Hedgehog Monte Carlo technique is used to obtain the shear wave velocity structure for each geologic unit, revea ling distinct velocity variations in the lower crust and uppermost man tle. The Indian shield has a high-velocity (4.4-4.6 km/s) upper mantle which, however, is slower than other shields in the world. The centra l Indian platform comprised of Proterozoic basins and cratons is marke d by a distinct low-velocity (4.0-4.2 km/s) upper mantle. Lower crusta l velocities in the Indian lithosphere generally range between 3.8 and 4.0 km/s with the oceanic segments and the sedimentary basins marked by marginally higher and lower velocities, respectively. A remarkable contrast is observed in upper mantle velocities between the northern a nd eastern convergence fronts of the Indian plate. The South Burma reg ion along the eastern subduction front of the Indian oceanic lithosphe re shows significant velocity enhancement in the lower crust and upper mantle. High velocities (approximate to 4.8 km/s) are also observed i n the upper mantle beneath the Ninetyeast ridge in the northeastern In dian Ocean.