Seismic anisotropy beneath the Chinese mainland

We investigated the upper mantle anisotropy beneath China by applying teleseismic shear wave splitting measurements at 119 seismic stations from CDSN and GSN/IRIS networks in China. The splitting observations are characterized by apparent diversity of anisotropy pattern in adjacent tectonic domains, including the Tianshan orogenic belt, Tibetan plateau, the Yangtze craton, the North China craton and northeastern region. In western China (Tianshan orogenic belt and Tibetan plateau), fast polarization directions of split SKS waves coincide strikingly well with the dominating trend of deformational crustal features and delay times range from 0.5 s to 1.6 s. While in eastern China, seismic anisotropy deduced from shear wave splitting reveals a homogeneous NW-SE trending structure, almost perpendicular to the strike of large-scale surface structures. The observed delay times of 1.5 s to more than 2 s favor consistent mantle flow over large mantle thicknesses. Based upon the straightforward relationships between seismic anisotropy and the development of lattice preferred orientation of mineral in upper mantle rocks, we interpret the splitting results in terms of tectonic fabric within the upper mantle. Since the lithosphere is less than 100 km thick beneath eastern China and the observed fast directions are subparallel to the trend of the absolute plate motion (APM) of Eurasian plate, we propose that the asthenosphere may mainly contribute to the anisotropic effects beneath eastern China. However, the upper mantle anisotropy beneath western China may have developed more recently in the subcrustal lithosphere because of rather small delay times and thick lithosphere. We also use the opportunity of the dense geodetic measurements available in China to discuss the coupling between the crust and mantle. In the Eurasia-fixed reference frame, GPS and shear wave splitting both depict a similar trend beneath eastern China, suggesting a lithospheric block "escaping" toward the east that could orient olivine 001 axis in the upper mantle. There is a strong coupling between the crust and the mantle in eastern China. A different behavior is observed in western China. The GPS vectors trend NS-NE in Tibet and NW in Tianshan, close to the regional compression direction, whereas the fast directions trend EW in Tibet and NE in Tianshan, suggesting a tectonic regime close to a mode of axial shortening, generating the development of EW-trending foliation in Tibet and NE-trending foliation in Tianshan at depth. The crust and mantle deform independently in western China.