Upper mantle deformation beneath the northeastern part of Indian plate from shear-wave splitting analysis

Abstract

This study investigates seismic anisotropy in the northeastern region of the Indian plate, including the Eastern Himalayan front, Eastern Himalaya Syntaxis (EHS), Indo-Burmese subduction zone, Shillong Plateau, Assam foredeep, and Bengal basin. Variations in azimuthal anisotropy are interpreted in terms of pre-existing lithospheric structures, mantle flow movement, and dynamic lithospheric stresses. Analysis of shear-wave splitting (SWS) in the waveforms recorded at 64 stations yielded 305 splittings (SKS, SKKS, and PKS phases) and 386 Null measurements. Results reveal an average delay time (δt) of 0.95 ± 0.32 s, indicating significant anisotropy. Modeling the back-azimuthal dependence of the splitting parameters indicates two-layer anisotropy along the Eastern Himalaya, Shillong Plateau, and south of the Dauki fault contiguous with the IndoBurmese arc. Application of the spatial coherency technique localizes the depth of the anisotropic layers in different tectonic subdivisions. Stresses and lithospheric strain associated with Absolute Plate Motion (APM) of India explain the deformation patterns gleaned from splitting measurements. A vertically coherent crust-mantle deformation is proposed at the Himalayan collision front, where east-west-oriented extensional shear stresses result in north-south compressive strains. APM-related stresses forge anisotropy in the Assam foredeep region that shows a coupled crust-mantle deformation. East-west-oriented fast polarization directions (FPDs) beneath the Shillong Plateau indicate localized mantle flow along the Dauki fault. The fast axes of anisotropy in the Indo-Burmese subduction zone align parallel to the arc. These findings enhance the knowledge of mantle dynamics in the subduction and continent-continent collision zones.