Mapping lithospheric seismic structure beneath the Shillong plateau (India) and adjoining regions by jointly fitting receiver functions and surface wave dispersion

Abstract

SUMMARY The northeastern Indian region is characterized by complex lithospheric structure that developed due to collision between the Indian and Eurasian tectonic plates, in the north, and to subduction beneath the Burmese arc, in the east. We report results from joint modelling of Ps and Sp receiver functions and Rayleigh wave group velocity dispersion curves in which a broad search for acceptable models is performed via simulated annealing. We identify three tectonic domains, the Shillong plateau, Brahmaputra valley and Indo-Burma convergence zone (IBCZ), sampled by teleseismic earthquake data recorded by nine broad-band seismic stations. Our results reveal that the region's thinnest crust lies beneath the Shillong plateau, where it increases slightly from the plateau's eastern edge to its centre and reaches a maximum at the western edge of the plateau. Crustal Vp/Vs ratios range between 1.69 and 1.75 for the Shillong plateau, which is consistent with a felsic composition. Deeper Moho depths beneath the Brahmaputra valley, adjacent to the northern front of the Shillong plateau, may be due to the flexure of Indian lithosphere subducting beneath Asia. Low velocity zones are indicated at ∼5–10 km depth beneath the Brahmaputra valley, which may have been developed by NE–SW trending compressional stresses from the collision at the Himalayan arc and subduction at the Burmese arc. The crust is thickest in Kohima, beneath the Naga thrust in the IBCZ, where a high velocity zone is observed for both Vp and Vs at a depth of 25–40 km. This anomaly may be associated with a high velocity slab, trending N–NE to S–SW, that comprises the subducting Indian lithosphere in the IBCZ.