A seismic cross section of the Tibetan Plateau from Ambient Noise Tomography and Receiver Functions: Evidence for removal of mantle lithosphere below the Hoh-Xil Songpan-Ganzi plateau

Abstract

<p>We present results from a Sino-German cooperation project in the Tibetan plateau aiming to study fundamental questions on plateau building, processes that modify and destroy continental lithosphere and which control lithospheric dynamics beneath orogenic belts. The project focusses on the northern Tibean Plateau which features widespread Cenozoic volcanism indicating recent tectonic activity. Due to difficult accessability in this remote and high mountain terrain, previous seismological networks show still a larger gap in this region which we are aiming to fill in order to provide reliable informations from a local deployment. Data are analysed, which have been collected during several TITAN deployments from 2008 to 2018, comprising a 1000 km long north-south profile, which an average station spacing of about 10 km. <br>We performed ambient noise tomography and receiver function analysis for constructing a cross section passing the entire plateau in north-south direction along the 88.5°-Meridian.  In a first step we inverted group and phase velocities for fixed periods ranging from 5 s to 65 s and 5 s to 90 s, respectively. For depth inversion, we applied joint inversion of the surface wave model together with the receiver functions from teleseismic earthquakes.  Our 2D velocity model resolves the crust and upper mantle to a depth of about 120 km.  We interpret results from the joint inversion together with results from H-κ stacking and CCP cross sections, providing information of crustal thickness, Vp/Vs-ratio and the location of velocity contrasts, respectively. <br>The upper mantle below Hoh Xil Songpan-Ganzi terrane shows low velocities (Vs < 3.8 km/s) at depths of 60-120 km. Moreover, a strong approximately 10 km thick low velocity layer (Vs < 3.2 km/s) is resolved within the crust of the Hoh Xil Songpan-Ganzi terrane centered at 20 km depth deepening towards north to a center depth of 30 km below the Kunlun fault.  The observation of these mid-crustal low velocities coincides with the location of Late Miocene-Quartenary volcanic rocks in the northen Tibetan Plateau. Shear velocities are reduced by about 15% with respect to standard crustal models which may be explained by the presence of partial melts.  The low velocities in the upper mantle are interpreted as asthenosphere, implying removal of the mantle lithosphere and asthenospheric upwelling as putative mechanism for the uplift of the northern Tibetan plateau. </p>