Thinning and ow of Tibetan crust constrained by the seismic anisotropy

Intermediate-period Rayleigh and Love waves propagating across Tibet require marked radial anisotropy within the mid-lower crust, consistent with a thinning of it. The anisotropy is largest in the western part of the plateau where the moment tensors of earthquakes indicate active crustal thinning. A preferred orientation of mica crystals resulting from the crustal thinning can account for the observed anisotropy. The observed mid-crustal anisotropy requires a thinning of approximately 30, much more than what is estimated for the upper crust. This diierence suggests that the mid-lower crust of Tibet has thinned more than the upper crust, consistent with deformation of a mechanically weak layer in the mid-lower Tibetan crust that ows as if connned to a channel 1, 2, 3, 4. Although most of the high terrain and the thick crust of the Tibetan plateau has resulted from India's penetration into Eurasia, east-west extension and crustal thinning dominate the current active deformation in the highest parts of the plateau 5, 6, 7. Two processes may have contributed to this change in deformation: a gain in potential energy per unit area, due to removal of dense mantle material beneath the plateau 8 and a warming and weakening of middle and lower crust 1. These processes are not independent, because conductive warming of Tibetan crust to make it weak is slow, requiring tens of millions of years, unless some other process such as removal of mantle material 9 or frictional heating 10 are involved. Details of the current