Upper mantle anisotropy of the eastern Himalayan syntaxis and surrounding regions from shear wave splitting analysis

Abstract

Polarization analysis of teleseismic data has been used to determine the XKS (SKS, SKKS, and PKS) fast polarization directions and delay times between fast and slow shear waves for 59 seismic stations of both temporary and permanent broadband seismograph networks deployed in the eastern Himalayan syntaxis (EHS) and surrounding regions. The analysis employed both the grid searching method of the minimum tangential energy and stacking analysis methods to develop an image of upper mantle anisotropy in the EHS and surrounding regions using the newly obtained shear wave splitting parameters and previously published results. The fast polarization directions are oriented along a NE-SW azimuth in the EHS. However, within the surrounding regions, the fast directions show a clockwise rotation pattern around the EHS from NE-SW, to E-W, to NW-SE, and then to N-S. In the EHS and surrounding regions, the fast directions of seismic anisotropy determined using shear wave splitting analysis correlate with surficial geological features including major sutures and faults and with the surface deformation fields derived from global positioning system (GPS) data. The coincidence between structural features in the crust, surface deformation fields and mantle anisotropy suggests that the deformation in the crust and lithospheric mantle is mechanically coupled. In the EHS, the coherence between the fast directions and the NE direction of the subduction of the Indian Plate beneath the Tibetan Plateau suggests that the lithospheric deformation is caused mainly by subduction. In the regions surrounding the EHS, we speculate that a westward retreat of the Burma slab could contribute to the curved anisotropy pattern. The Tibetan Plateau is acted upon by a NE-trending force due to the subduction of the Indian Plate, and also affected by a westward drag force due to the westward retreat produced by the eastward subduction of the Burma slab. The two forces contribute to a curved lithospheric deformation that results in the alignment of the upper mantle peridotite lattice parallel to the deformation direction, and thus generates a curved pattern of fast directions around the EHS.