Seismic anisotropy and implications for mantle deformation beneath the NE margin of the Tibet plateau and Ordos plateau

Abstract

The northeastern Tibet and the Ordos plateau connects east China and west China, which are dominated by horizontal extension and compression, respectively. Knowledge of seismic anisotropy beneath this transitional region can provide important constraints on deformation pattern of the crust and lithosphere mantle during an orogeny process. We measured SKS wave splitting parameters from the new installed regional networks and obtained a detailed anisotropy map for 119 broadband stations. Beneath the NE margin of Tibet, the observed WNW fast polarization direction is parallel to the surface geological features. The coherence between the observed geodetic motion of the crust and fast axis direction suggests that the vertical coherent deformation of the lithosphere is the dominate source for the observed seismic anisotropy. Further east to the Qinling orogenic belt, the fast axis direction tracks change in strike of the faults, with the fast direction aligning almost EW. We suggest that both the lithosphere mantle and east extrusion of mantle material contribute to the observed anisotropy. Small value of delay time is observed interior the thick-rooted Ordos plateau, suggesting a rigid and stable lithosphere with little deformation. The deviation of fast polarization direction beneath the adjacent rifting regions might be caused by the edge flow induced by sharp changes in the thickness of lithosphere. Our results propose that the upper crust and mantle lithosphere beneath the northeastern Tibet are at least partly coupled, or subjected to the same boundary conditions, which is inconsistent with the channelized lower crustal flow model.