Study on Pn velocity and anisotropy in the uppermost mantle of the Eastern Himalayan Syntaxis and surrounding regions

The Eastern Himalayan Syntaxis (EHS) is a singular point of continental collision between the Indian and Eurasian plates, and has been focused by many geoscientists. We added the ISC data of 1964 similar to 2006 to previous data set, Chinese national and provincial events data used by Pei et al, and 349475 Pn arrivals were selected in all. We have obtained seismic velocities and anisotropy in the uppermost mantle around EHS and surrounding regions by performing tomographic inversion of Pn travel times. Comparing Pn velocity variation with geological results, we find they have distinct correlation: particularly high velocities always exist in old stable regions such as Sichuan Basin and Indian Plate. Low Pn velocities lie in active tectonic regions, volcanic areas and magmatic rock regions, for example, eastern Tibetan Plateau and North-South Seismic Zone, southern Yunnan province and northern Indochina. Pn anisotropy results show that the fast directions of Pn are clockwise around the EHS from NE in southeastern Tibetan Plateau, then SE to NS in North-South Seismic Zone. But the direction sharply changes to EW in the northern Indochina. The clockwise Pn fast direction maybe results from movement of mass extruded from Tibet under simple shear, which is caused by mass flow in southeastern Tibetan Plateau and North-South Seismic Zone relative to stable EHS and Sichuan Basin. The EW Pn direction probably results from pure shear, which is caused by compression in NS and extension in EW in Burma back-arc extensional area. The polarization directions of SKS have similar rotation to Pn, but in northern Indochina the oriented EW fast direction area of Pn has a southward offset of about 3 degrees than SKS, which maybe results from the faster movement of upper lithosphere in this area.