Shear wave splitting in the southern margin of the Ordos Block, north China

The southern margin of the Ordos Block in North China marks the transition from a stable cratonic block to active transextensional deformation in the Weihe Graben, driven by the northeastward expansion of the Tibet Plateau. We deployed 29 portable digital stations across this boundary and studied the upper mantle anisotropy using SKS splitting. The anisotropy is weak (delta t < 0.5 s) at stations within the Ordos Block and in the North Weihe Graben, but strong (delta t = 1.23 +/- 0.39 s) in the South Weihe Graben and Qinling Orogen further south. The directions of the fast S-wave are 98 +/- 7 degrees, subparallel to the orientations of surface structures and the GPS site velocities relative to stable Asian continent. We suggest that this E-W oriented anisotropy is the result of both the collision between the North and South China Blocks in the Mesozoic, and the eastward upper mantle flow driven by the Indo-Asian collision.