Variations in mantle flow beneath the Eastern Himalaya inferred from shear wave splitting

This study attempts to understand the upper mantle deformation patterns beneath the Eastern Himalaya by performing shear wave splitting analysis of core-refracted phases. Out of the 83 broadband seismic stations used, data from 70 stations are analysed for the first time. This includes 21 stations which were newly deployed along two profiles in Arunachal Himalaya, to fill the gaps in the stations used for previous studies. In total, 172 well constrained new splitting and 215 null measurements are obtained in this study. Average delay time values of 0.64 and 0.76 s in the Bhutan and Arunachal Himalaya respectively, suggest weak anisotropy, probably due to a steep subduction of the Indian mantle lithosphere. There is a systematic variation in the orientation of fast polarization azimuths in the western (Bhutan Himalaya and western part of Arunachal Himalaya) and eastern segments (central to the eastern part of Arunachal Himalaya). In both these segments, the orientation of fast polarization azimuths varies dominantly from NE-SW or/and ENE-WSW, to E-W, from west to east. In the western and central parts of Bhutan Himalaya, the influence of absolute plate motion related strain in the asthenospheric mantle cannot be ruled out, while in its eastern part and Arunachal Himalaya, the azimuthal anisotropy can be explained by arc parallel mantle flow due to slab rollback. In addition, a few observations in the central part of Arunachal Himalaya indicate a slightly larger delay time, along NNE-SSW, which could be associated with mantle wedge flow. The eastern part of Arunachal Himalaya might be associated with a repulsive arc parallel flow from the Arunachal and Burmese arcs, resulting in null measurements. The optimal depth of anisotropy in Bhutan and Arunachal Himalaya is around $220-270$ and $200-240$ km respectively, suggesting that the source of anisotropy lies in the upper part of the asthenosphere.