Upper mantle deformation beneath central-southern Tibet revealed by shear wave splitting measurements

Seismic anisotropy in the upper mantle has been inferred from shear wave splitting measurements at the temporary seismic array (ANTILOPE-2) in the central-southern Tibet from September 2005 to October 2006. Teleseismic shear waves (mainly SKS and SKKS) are used to determine the splitting parameters (fast polarization direction and delay time). Weak splitting with average delay time of 0.3s is observed at stations in the southern Lhasa terrane and northernmost Tethyan Himalaya. Significant seismic polarization anisotropy with delay times from 0.8 to 1.5s and NE-oriented fast direction is obtained for stations located farther north in the northern Lhasa terrane. The transition in shear wave splitting characteristics occurs at 30.5°N and suggests distinct upper mantle deformation from south to north. The negligible anisotropy south of this transition indicates that there is no appreciable large-scale mantle seismic polarization anisotropy. The substantial anisotropy in the north may be caused by the eastward flow in a lithospheric crush zone which is squeezed and sheared between the advancing Indian plate to the south and Eurasian plate to the north.