Abstract:

Based on the SKSwave splitting analysis on the seismic records at 143 broadband permanent and temporary stations deployed in the eastern Tibetan Plateau and its vicinity, we have determined the fast polarization direction and delay time between fast and slow waves at individual stations; the pattern of SKSsplitting fast polarization direction exhibits the flow direction of upper mantles material in the eastern Tibetan Plateau, i.e., the clockwise rotation around the Eastern Himalayan Syntaxis. The style of orogenic deformation can be quantitatively evaluated by the joint analysis of the surface deformation field, as inferred from GPS and Quaternary fault slip, and the mantle deformation field from seismic anisotropy. The greatly expanded data set of surface deformation (GPS) and seismic anisotropy (SKS splitting) from eastern Tibet, Yunnan and Sichuan regions provides a stringent test for the manner in which the Tibetan lithosphere actually deforms. These new data not only strengthens the evidence for the coupled deformation onplateau, but also argues this same property offplateau. The vertically coherent deformation of a mechanicallycoupled lithosphere for Tibet and its vicinity has two implications for continental dynamics. First, the lithospheric vertical strength profile is constrained by the requirement that GPErelated stresses are being efficiently transmitted from the crust into the mantle. Second, the observed spatial variations in anisotropy reflect the largescale pattern of lithospheric deformation, as well as a transition in deformational style from simple shear on the Tibetan Plateau to pure shear offplateau. The model of mechanicallycoupled deformation in lithosphere presented in this paper has different implications from the previously presented models of orogenic deformation. Therefore, the mantle deformation plays a major role in the uplift process in the Tibetan plateau.

Key words:

Tibetan Plateau; lithospheric deformation; mechanical coupling; seismic anisotropy; vertically coherent deformation