Crustal Structure and Deformation beneath the NE Margin of the Tibetan Plateau Revealed by Receiver Function Data

Abstract:

The large-scale surface deformation, uplifting and faulting occurring at the NE margin of the Tibetan plateau are generally believed to be caused by the continuous collision between the India and Eurasia since ~50 millions years ago. However, the style and amount of the subsurface deformation induced by the collision, especially those inside the lower crust and upper mantle, are still debated. We analyzed a large amount of receiver function data recorded by 204 broadband stations in the area between 08/2007 and 10/2013 to estimate the crustal velocity and anisotropy structure beneath the margin and its surrounding areas. Moho depth and average crustal Vp/Vs ratio were measured at each station using the H-κ stacking method, while seismic anisotropy parameterized by fast polarization direction and splitting time was computed by a joint analysis of radial and transverse receiver function data. The crust beneath the margin is around~50-70 km thick with a relatively low Vp/Vs ratio. Meanwhile, we found significant seismic anisotropy with a splitting time of 0.5-1.1s inside the crust beneath the margin. The fast polarization directions align well with surface structure, and follow the directions of the maximum horizontal tensile stress. The lower Vp/Vs ratio together with fast polarization direction suggests that whole crustal shortening might be the dominant mechanism for producing the thick crust beneath NE Tibet. We also compared the measured seismic anisotropy with those measured from SKS (PKS, SKKS), and found that crustal deformation plays a primarily role in explaining the observed SKS splitting data. Meanwhile, depth variation of seismic anisotropy may exist in some parts of the study area.