Delineation of crustal structure and composition in the Northwest Himalaya and adjoining Indo-Gangetic Plain

Abstract

We utilize the H-k stacking technique on P-wave receiver functions from 38 seismic stations to estimate the crustal thickness and Vp/Vs ratio in the NW Himalaya and the adjoining Indo-Gangetic Plain (IGP). We observe varying crustal thickness from ∼29 km in the IGP to ∼50–60 km in the Higher Himalayan Crystalline Zone. An abrupt crustal thickening of ∼47–54 km observed near Jwalamukhi Thrust is due to significant crustal shortening along the fault. Inferred higher Vp/Vs values of 1.78–1.84 in the IGP may result from the underplating of mafic materials in the Precambrian. The presence of thick sediments towards the Himalayan Frontal Thrust, further, contributes to the extremely high Vp/Vs of 1.89–2.02. The intermediate Vp/Vs of ∼1.74 in the Sub Himalayan Zone indicates the reworking of the mafic crust by delamination of the lower crust or the greater influence of the overlying Himalayan wedge, comprised of felsic to intermediate rock composition. Higher Vp/Vs values in the Kishtwar (1.81–1.93) and Kashmir Valley (1.80–1.93) are possibly due to the presence of volcanic rocks and the effect of the mid-crustal low-velocity layer. Integration of our results with previous studies reveals a heterogenous crustal structure characterized by both arc-normal and arc-parallel variations which may have resulted from varying convergence rates and the influence of subsurface extension of active transverse ridges beneath the Himalaya. The varying Vp/Vs values may result from diverse crustal rock compositions and partial melt/fluid zones. These partial melts alter the mechanical properties of rocks leading to brittle failure and thus promoting seismicity.