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 Indianplate, including the Eastern Himalayan front, Eastern Himalaya Syntaxis (EHS),Indo-Burmese subduction zone, Shillong plateau, Assam foredeep and Bengalbasin. Variations in azimuthal anisotropy are interpreted in terms of pre-existinglithospheric structures, mantle flow movement and dynamic lithospheric stresses.Analysis of shear-wave splitting (SWS) in the waveforms recorded at 64 stationsyielded 305 splitting (SKS, SKKS, and PKS phases) and 386 Null measurements.Results reveal an average delay time (δt) of 0.95 ± 0.32 s, indicating significantanisotropy. Modelling the backazimuthal dependence of the splitting parametersindicates two-layer anisotropy along the Eastern Himalaya, Shillong plateau, andsouth of the Dauki fault contiguous with the Indo-Burmese arc. Application ofthe spatial coherency technique localizes the depth of the anisotropic layers indifferent tectonic subdivisions. Stresses and lithospheric strain associated withAbsolute Plate Motion (APM) of India explain the deformation patterns gleanedfrom splitting measurements. A vertically coherent crust-mantle deformation isproposed at the Himalayan collision front, where east-west-oriented extensionalshear stresses result in north-south compressive strains. APM-related stressesforge anisotropy in the Assam foredeep region that shows a coupled crust-mantledeformation. East-west-oriented fast polarization directions (FPDs) beneath theShillong Plateau indicate localized mantle flow along the Dauki fault. The fastaxes of anisotropy in the Indo-Burmese subduction zone align parallel to the arc.These findings enhance the knowledge of mantle dynamics in the subduction andcontinent-continent collision zones.