Shear Wave Structure In The Northeastern Tibetan Plateau From Two-plane-wave Method

The northeastern Tibetan Plateau has undergone extensive tectonic activities andhas been an ideal laboratory for studying the formation mechanism of the TibetanPlateau because of continuous collision and subduction between the Indian plate andEurasian plate. In this paper, we collected vertical component teleseismic datarecorded by35temporary seismic stations crossing the Songpan-Ganzi Terrane,Qilian-Qinling Orogen and Gobi-AlaShan from south to north deployed by ChinaUniversity of Geoscience (Beijing), China Academy of Geological Sciences,University of Rhode Island, University of Houston and University of Missouribetween July2008and July2010. Then we apply the two-plane-wave tomographymethod which represents the incoming wavefield with the interference of two planewaves and therefore can account for the non-planar energy in the incoming wavescaused by scattering and multipathing. We first calculate the fundamental modeRayleigh wave phase velocity distributions at14periods between20s and100s andthen extract pure dispersion curves for each grid. Based on those dispersionmeasurments, we conduct one-dimensional shear wave velocity inversion for eachgrid, then all these1-D profiles are combined into3-D shear wave velocity models.Rayleigh phase velocity distribution maps show that the south of the Kunlun fault andnearby areas are characterized by significant low velocities at periods20～35s, whichcould be affected by the low velocity layer in the middle crust. At periods40-50s,phase velocity is mainly sensitive to shear wave velocity in the depth between40and80km and is primarily influenced by crustal thickness and the structure of the lowercrust and uppermost mantle, we notice that at this period, the East Kunlun fault stillshows a prominent low-velocity. At long periods (76～100s), velocities in theQilian-Qinling and Gobi-AlaShan block are still higher than the Songpan-Ganzi block.Based on these dispersion measurements, shear wave velocities are inverted. Our3-Dshear wave velocity results reveal that upper mantle in the study area generally showsa relatively low velocity, especially in the Songpan-Ganzi terrane. Compared highvelocity anomalies in the north and northeast of Qilian and Qinling Orogens, low velocity anomaly in the mid-to-lower crust to the south of the Kunlun fault is clearlyobserved that is primarily caused by a low velocity layer in the mid-to-lower and athicker Moho beneath Songpan-Ganzi terrane. Especially, at depth between90～120km, the low velocity anomaly became more pronounce beneath theSongpan-Ganzi terrane, we interpret it as the local feature due to the restraining bendof the Kunlun fault, and heat anomaly related to the upwelling asthenosphere andpartial melting.