Delineation of Average 1-D Shear Velocity Structure below North India by Surface Wave Dispersion Study

During 2014–16, a semi-permanent network of four 3-component broadband seismographs was operational in the Rajasthan craton and Aravalli mobile belt in the NW Indian shield. The reliable and accurate broadband data from 16 selected regional Indian earthquakes of Mw 5.5–7.8 from this seismic network enabled to estimate the group velocity dispersion characteristics and one-dimensional regional shear velocity structure of northern India, covering the region below north India between the entire Himalaya (from the Pakistan Himalaya in west to the Burmese arc in the east) and Rajasthan (including Aravalli mobile belt). First, Rayleigh- (at 7–87s) and Love- (at 7–82s) wave group velocity dispersion curves were measured and then these curves were inverted to estimate the crustal and upper mantle structure below north India. It is observed that group velocities are of variable nature within the region. This could be attributed to the complex crust-mantle structure in the study region resulted from the magmatism episodes associated with the Proterozoic collision, 65 Ma Deccan volcanism and the Himalayan collision. The best model in the study region reveals a two-layered crust, with a 15-km thick upper-crust (UC) of average shear velocity (Vs) of 3.12 km/s and a 25-km thick lower-crust(LC) of average Vs of 3.44 km/sec. The modeling detects a drop in Vs (∼1-2%) at 79–120 km depths, underlying north India, representing the probable seismic lithosphere-asthenosphere boundary (LAB) at 79 km depth. A geothermal gradient extrapolated from the surface heat flow (∼74 mW/m2) shows that such a gradient would intercept CO2-bearing mantle peridotite solidus at 100 km depth, and thus could signal the presence of small amounts of partially melted magma below 100 km depth. Therefore, this 1–2% drop in Vs could be attributed to the presence of carbonatite melts in the upper mantle related to magmatic episode of 65 Ma Deccan plume activity as also suggested by existing geological and seismological evidence.