TECTONOPHYSICS ELSEVIER Tectonophysics 277 (1997) 259-270 Shear velocity structure of the laterally heterogeneous uppermost mantle beneath the Indian region G. Mohan a,*, S.S. Rai b, G.F. Panzac crust and ’ Department of Earth Sciences, Indian Institute of Technology, Powai, Mumbai 400076, India h National Geophysical Research Institute, Uppal Road, Hyderabad, 500 007, India ’ Institute di Geodesia e Geojisica, Via de1 Universita 7, 34124, Trieste. Italy Received 21 July 1995; accepted 23 January 1997 Abstract The shear velocity structure of the Indian lithosphere is mapped by inverting 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 with depth in each unit. The Hedgehog Monte Carlo technique is used to obtain the shear wave velocity structure for each geologic unit, revealing distinct velocity variations in the lower crust and uppermost mantle. 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 central Indian platform comprised of Proterozoic basins and cratons is marked by a distinct low-velocity (4.G4.2 km/s) upper mantle. Lower crustal 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 and eastern convergence fronts of the Indian plate. The South Burma region along the eastern subduction front of the Indian oceanic lithosphere shows significant velocity enhancement in the lower crust and upper mantle. High velocities (~4.8 km/s) are also observed in the upper mantle beneath the Ninetyeast ridge in the northeastern Indian Ocean. Keywords: lithosphere; Monte Carlo technique; Rayleigh wave; regionalization; shear velocity 1. Introduction The Indian plate is a collage of numerous con- tinental and oceanic fragments that carries imprints from early Archaean crustal evolution to present- day tectonics. The geological vitality of the re- gion is seen in the presence of exposed/covered Archaean provinces, Proterozoic sedimentary basins, zones of ongoing continental convergence, exposed and covered basaltic provinces, super-thick and old * Corresponding author. Fax: Fax: 091-22-578-3480; e-mail: gmohan@zircon.geos.iitb.ernet.in ocean basins, aseismic ridges, trails of hotspots etc. (Fig. 1). Such remarkable space-time variability of geotectonic history in the Indian plate region is a consequence of past and ongoing geodynamic pro- cesses, the unravelling of which requires detailed exploration of the lithospheric architecture under- neath it. The geological and geophysical frameworks of the region are discussed in Valdiya (1984), Naqvi and Rogers (1987) and Mahadevan (1994). Seismic surface waves have been used extensively to infer lateral and vertical shear wave velocity vari- ation beneath continents and oceans by modelling 0040- 195 l/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOO40- I95 1(97)00029-2