Investigating seismic crustal anisotropy and sub-crustal deformation beneath Northeast India through harmonic decomposition of receiver functions

Abstract

Northeast India, characterized by its active seismicity, complex tectonic structures, ongoing subduction, and diverse lithological formations, serves as an important natural laboratory for anisotropic studies. This study analyzed lithospheric anisotropy through the Harmonic Decomposition (HD) of receiver functions derived from teleseismic earthquakes recorded at ten broadband seismic stations across the region. Our findings reveal significant variations in crustal deformation across the Eastern Himalayas, Assam Valley, Shillong Plateau, and Bengal Basin, driven by distinct tectonic influences. We observed a Moho depth of approximately 50–60 km in the Indo-Burma and Eastern Himalayan regions, around 30–40 km in the Assam Valley, about 37 km in the Shillong Plateau, and between 43 and 47 km in the Bengal Basin. Further, the alignment of fast anisotropic axes generally follows the motion of the Indian Plate (N20⁰E), with localized deviations near the Kopli Fault and Indo-Burma region, indicating intricate tectonic complexities. However, the anisotropy axis direction varies from N20⁰W to E-W in the uppermost mantle. The adjacent variations in anisotropic interfaces and their tilt axes across these regions reflect extensive crustal deformation associated with subduction and collision tectonics. Notably, the Eastern Himalayas exhibit a lattice-preferred alignment in the E-W direction, while the increasing Moho depth from north to south underscores the effects of ongoing collision and crustal thickening associated with down warping of the mantle, particularly in areas like the Shillong Plateau and Bengal Basin. The simultaneous presence of shallower Moho and stress perpendicular to anisotropic tilt direction (E-W) discerned in this study validates the hypothesis of a tectonically supported uplifted Shillong Plateau. These findings highlight the role of asthenosphere flow and transpressional thickening in shaping the crustal structure in the Indo-Burma ranges due to sub-crustal geodynamical processes, with additional influences from subsidence and sediment deposition in the Bengal Basin. The observed anisotropy is consistent with past Indian plate motion, while variations in the Tripura fold belt suggest complex interactions between mantle flow and regional tectonics.