Crustal velocity structure beneath Saurashtra, NW India, through waveform modeling: Implications for magmatic underplating

Abstract

The Saurashtra peninsula in the northwestern segment of the Deccan volcanic province of India, is characterized by several high gravity and magnetic anomalies, which correspond to mafic crustal intrusions. This study attempts to quantify the alterations to the crust caused by the Deccan volcanism, by estimating the crustal Poisson’s ratio, shear wave velocity (Vs) structure and the shear velocity contrast across the Moho, through teleseismic waveform modeling. The P receiver functions (RFs) for six broadband seismic stations were constructed using about 575 high quality (S/N≥2.5) teleseismic waveforms of earthquakes (M≥5.5) recorded during the period 2004–2010. The moveout corrected RF summation stacks were inverted using the Neighborhood algorithm (NA) to estimate the shear velocity structure beneath each station. The crustal thickness is estimated to range from 38km in western Saurashtra to 33km close to the southern extension of the Cambay rift. A low velocity zone possibly corresponding to sub-basaltic sediments is detected beneath all the stations. The average crustal Vs and Poisson’s ratio are estimated to be 3.68km/s and 0.276 respectively. The crustal Poisson’s ratio indicates an intermediate to mafic composition for the crust. Modeling reveals a relatively high velocity lower crust with an average Vs≈3.88±0.1km/s which is consistent with the high Vp ≈ 7.1km/s reported for this region through seismic refraction and wide angle reflection studies, overlying a relatively low velocity (Vs ≈ 4.4±0.1km/s) upper mantle. The shear velocity contrasts across the Moho, derived from the amplitudes of the P-to-s (Pms) conversions from the Moho range from 0.08 to 0.17 which are much smaller than those (>0.20) observed across the Indian shield, implying a gradational Moho due to underplating. This study reveals that the crust beneath Saurashtra is distinctly different from that of the Archean Indian shield largely due to significant alterations by the Deccan volcanism.