Lithospheric Velocity Structure beneath Eastern Part of the Chhotanagpur Plateau, India

The Chhotanagpur Plateau in eastern India was formed due to severe upheaval of tectonic forces deep inside the earth. The deep geological processes associated within its subsurface along with geodynamical rationales were never understood properly due to the dearth of good quality geophysical data. Also, the regional studies that can penetrate beyond the crust are limited to magnetotelluric and resistively surveys only. Hence, we use earthquake data to jointly model discrete but complementary datasets such as Ps and Sp receiver functions and Rayleigh wave group velocity dispersion curves. These datasets were computed from teleseismic events (Δ > 30⁰) of magnitude five or greater for the years between 2008 to 2015 of the seismic stations located in Dhanbad and Bokaro. An iterative time domain deconvolution technique was employed to compute the Ps and Sp receiver functions.

Using an incorrect starting seismic velocity model during joint modeling may produce unreliable image of subsurface, especially in the presence of heterogeneous lateral velocity variations. So, we decide to use the H-ĸ stacking technique for deducing the reliable values of average crustal thickness and bulk Vp/Vs ratios. During the process of joint modeling, we varied this starting model iteratively using nonlinear optimization process of Very Fast Simulated Annealing (VFSA). We use Posterior Probability Density function (PPD) for estimation of uncertainties. The PPD function was used to distinguish the portions of search space where acceptable models are more or less probable to occur. Our results show that, on an average the eastern part of Chhotanagpur plateau which is mostly composed of Granite Gneiss Complex is characterized by thick crust of nearly 43 km. This thick crust may be associated with the removal of lower lithosphere in the deep mantle. We also found the appreciable amount of variation in Vp/Vs ratio ranging from 1.65 to 1.89, respectively. These higher values of Vp/Vs ratios are higher than the global average and may be indicative of partial melt present in the lower crust.