Seismic anisotropy beneath the southern Himalayas-Tibet collision zone

Shear wave phases ScS, SKS, and SKKS, collected from the Project International Deep Seismic Sounding of Tibet and Himalayas II (INDEPTH-II) array in the eastern Himalayas and Tibet are analyzed for the orientation and extent of polarization seismic anisotropy beneath the southern Himalayas-Tibet collision zone. No evidence is seen for any shear wave splitting for stations in this part of the collision zone except for two northern stations located about 100 km north of the Indus-Tsangpo Suture. The lack of shear wave splitting beneath the stations in the vicinity of suture and the Tethyan Himalayas indicates that there is no appreciable large-scale mantle polarization seismic anisotropy beneath this part of the collision zone. One explanation for the lack of polarization seismic anisotropy would be a subvertical mantle shear strain field resulting from a downwelling mantle flow of a thickened Indian lower continental lithosphere. North of the Gandese belt, the fast directions of the split shear waves are oriented in a northeast-southwest direction, approximately parallel to the trend of a major left-lateral strike-slip fault zone that bounds the Gulu rift. The coincidence of fast polarization directions parallel to the surficial geologic shear zones and the direction of shear planes within the upper crustal strain rate field suggests lithospheric-wide coherent deformation.