The Indian continental lithosphere

Whilst the surface geology of the Indian continent including the Hiamalaya-Tibet collision zone is now fairly well understood, several outstanding questions remain to be resolved, relating to the deeper structures of its varied terranes: the cratonic assemblage covering most of the subcontinent and its northeastern segment including the Bengal basin and the Bay, the collision zone, and of course the upper mantle underneath. The lecture presents Rayleigh wave fundamental mode group velocity maps (10-70 secs) at a resolution of 3-5*, as well as inverted shear velocity structures, derived from recently analyzed Rayleigh wave dispersion data along 4054 source-receiver paths, gleaned from seismograms generated by IRIS and the Chinese networks, PASSCAL experiments, INDEPTH II/III, and the Indian Institute of Astrophysics/Cambridge University, NGRI and IMD stations. The group velocity maps well resolve both the shallow sedimentary basins as well as the thickened crust in the northern Bengal Bay. They also resolve the three principal cratons of the continent and their deeper seated cores embedded in a broader interconnecting keel, and show the continuance of the Indian Moho beneath southern Tibet. The latter are further imaged by shear wave structures derived from receiver functions calculated for two ~700 km long profiles across the western and eastern Himalaya. Further, Ps converted phases from mantle discontinuities have been used to image the upper mantle structure beneath the continent, particularly its Mantle Transition Zone thermometer, which point to the presence of broken off oceanic plate slabs now foundering in the mantle. Finally, the lecture presents results of SKS splittings to investigate the fast azimuth of the continental uppermost mantle and its changing configuration from the southern plate motion strained region to the collision zone with an intermediate direction in northern India.