P-wave data from the Indian subcontinent

The Indian subcontinent has a complex geological and geophysical history that must be better understood. To accomplish this, we have recently obtained P-wave data from the Indian subcontinent. Twenty DSS profiles have been conducted since 1972 totaling more than 6000 km. Long range refraction and wide angle reflection techniques with a dense detector spacing of 80-200 m and a shot point interval of 10-40 km were used to acquire these data. Measurements of crustal thickness fall into the range of 35-40 km, with the biggest exception being the Himalayas which are known to have thicknesses of up to 80 km (Mahadevan, 1994). Here we show three velocity models that were developed as a result of these experiments. The Moho depth is seen to be around 38 km, and we note several deep lenses of 7.3 km/s velocity across the subcontinent. Also, the transition depth from the lithospheric to the asthenospheric mantle fluctuates by about 50 km across the profile.

The velocity model of Kaila et al. (1990) for the SW Marwar Terrain shows a shallow midcrust with 6.6 km/s velocity and a lower crustal velocity of 7.3 km/s. The high-velocity lower crustal layer in this region is believed to be due to the underplating of the crust by mantle upwelling, crustal extension and continental rifting related to the Reunion hotspot. A thick, high-velocity lower crustal layer (7.0-7.5 km/s) is often observed between the lower crust and upper mantle in areas of continental rifts, where extension has been the last deformation process to occur (Mooney et al., 1983; Catchings and Mooney, 1988).

This extensional process is attributed to igneous accretion of the upper mantle material at the base of the crust (White and McKenzie, 1989; Furlong and Fountain, 1986) and is observed at several rift zones of the world.  In this case, this plume activity coincides with the breaking up of the Rodinian supercontinent, of which the Indian continent was a part, during the Mid-Neoproterozoic (750 Ma). During periods of supercontinent break-up, rifting generally takes place at old suture zones, as these are relatively weak and therefore prone to rifting (Vink et al., 1984). The location of the rifting event (750 Ma) in the Marwar Terrain is in close proximity to the earlier Delhi Suture (1100 Ma), and strikes in the same direction (NE-SW) as the earlier Aravalli Delhi Fold Belts. It seems then that there is much to support the idea that the Indian shield has evolved through processes that are known to be active on a global scale.