Probing lithospheric deformation beneath the Sikkim Himalaya using shear wave splitting

We investigated lithospheric deformation beneath the Sikkim Himalaya using data from core-refracted shear wave phases (SKS/SKKS) collected at 27 broadband seismic stations. This study focuses on the Dhubri-Chungthang Fault Zone (DCFZ), a significant mid crustal fault traversing Sikkim, which potentially segments the underthrusting Indian plate. Although seismic activity suggests the presence of the DCFZ, its impact on deep lithospheric structures remains unclear. Through analysis of shear wave splitting (SWS) parameters - specifically the fast polarization direction (Φ) and delay time (δt), we assessed deformation patterns across the region. Our results reveal varied deformation across Sikkim, marked by a pronounced change in δt (0.3-2.5s) and Φ across the DCFZ. In the Himalayan foreland basin, mantle flow related to absolute plate motion (APM) is predominant, with the fast-axis direction closely aligning with the APM of the Indian plate. Notably, a NE orientation of Φ is prevalent, though deviations occur, possibly due to varying driving forces associated with the plate's position. In southern Sikkim, EW pattern in Φ and lower δt values (0.3 s) suggest dominant compressional tectonics and potential multi-layer anisotropy. A sharp transition in deformation patterns across the DCFZ highlights its significant role in segmenting the Indian plate's lithosphere, with a distinct NNW pattern in Φ observed in the Higher Himalayas, indicating lithospheric segmentation facilitated by the DCFZ.