Upper mantle anisotropy structure beneath eastern Tibet and its exploration

Abstract:

Continental collision between the Indian and the Eurasian plates resulted in uplift of the Tibetan plateau and the thickening of the crust. A lot of work has been done on the crust structures beneath Tibet, and several tectonic models are proposed to explain the mechanism of the uplift and thickening. But due to the absence of the upper mantle structures, those models are still under debate. Fine upper mantle velocity and anisotropy structures can help us understand the dynamic process of the Tibetan plateau. Waveform modeling of upper mantle triplication phases can provide a good vertical resolution of upper mantle velocity structures, but present methods for calculating synthetic seismograms cannot process anisotropic media. We develop a method based on the generalized reflection and transmission method (GRTM) to calculate synthetic seismograms for wave propagating in stratified VTI media, so we can waveform model upper mantle triplications propagating in anisotropic media. In this study, we waveform model the tangential and radial seismic triplication data recorded in Chinese digital seismic stations at a epicentral distance of 10-30 degree for one events occurring in middle Tibet to constrain fine upper mantle velocity and anisotropy structures beneath eastern Tibet. The result shows that horizontal S wave velocity is larger than vertical S wave velocity in the upper mantle beneath eastern Tibet. We also build a mineral physics modeling method, which can calculate upper mantle anisotropy structures based on mantle temperatures, compositions and directions of mantle flow, and use this method to explore compositional and dynamic models that would explain the inferred seismic structures. The results suggest that in our sampling region, the water content is lower than 0.4 wt%, and there is vertical mantle flow beneath the lithospheric lid.