[1] 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 (dt < 0.5 s) at stations within the Ordos Block and in the North Weihe Graben, but strong (dt = 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°, 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.
New constraints on the pattern of crustal flow in SE Tibet are obtained from joint analysis of receiver functions and Rayleigh wave dispersion with a newly deployed seismic array. The crust in the Sichuan-Yunnan Diamond Block has an average thickness of~45 km and gradually thins toward the Indo-China Block to the west and the Yangtze Block to the east. High V P /V S ratios are detected to the west of the Xiaojiang fault, but not in the Yangtze Block to the east. The S wave velocity profile reveals that intra-crustal low-velocity zones (IC-LVZs) are strongly heterogeneous, with two LVZs in the middle and mid-lower crust, respectively, in marked contrast to previous observations of a single LVZ. Combined with other observations, the two IC-LVZs are interpreted as isolated channels of crustal flow at different depths beneath SE Tibet, resulting in the observed complex pattern of radial anisotropy and further elucidating patterns of flow and deformation.
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