Three-dimensional S Wave Velocity Structure And Radial Anisotropy In The Crust Beneath The Northeastern Tibetan Plateau From Ambient Noise Tomography

Due to complex topography and tectonic structure, the northeastern TibetanPlateau has become the prime site to understand the processes of continental collision,mechanisms of mountain building and interactions of tectonic blocks. Throughanalysis of Rayleigh wave and Love wave Green’s functions estimated from ambientnoise tomography, we obtained radial anisotropy and shear wave velocity structurebeneath the northeastern Tibetan Plateau. Dense station coverage from223broadbandseismic stations deployed by China Earthquake Administration, A SeismicCollaborative Experiment of Northern Tibet (ASCENT) experiment and northeasternTibet Seismic (NETS) experiment provides the unprecedented opportunity to resolvethe spatial distribution of the radial anisotropy within the crust of the northeasternTibetan Plateau.First, the study area was divided into a0.4°0.4°grid and using Occaminversion method to obtain the surface wave group velocity and phase velocitydistribution. The results of the group and phase velocity tomography show that atshort periods (<25s), Qaidam basin is clearly resolved with low velocity due to itsthick sedimentary layer, and high velocities are observed beneath the non-basinregions. With the increase of period (>30s), the velocity distribution displays thecontrary patterm compared to that of the short periods, very low velocities are imagedin the Qiangtang and Songpan-Ganze terranes, and very high velocities are imaged inthe Qaidam basin. At periods>36s, the northwestern Qilian orogen is resolved withrelative weaker low velocity, but this phenomenon seems to be died out in thesoutheastern Qilian orogen. Then based on these group velocity and phase velocitymaps for Rayleigh and Love waves, we derived the three-dimensional Vsv and Vshmodels of the crust and uppermost mantle. The results show that the Qiangtang andSongpan-Ganze terranes have a prominent LVZ in the middle crust. It is noticeablethat the northwestern Qilian orogen where the crust thickness is deeper than thesurrounding areas is also featured with a relative weak LVZ in the middle crust. Thelocal LVZ beneath the northwestern Qilian orogen may be the response to both the crustal shortening between the North China craton and Tibetan plateau and the crustalthickening of the northwestern Qilian orogen experienced from the Caledonianorogeny. Discrepancies between Love (SH) and Rayleigh (SV) wave velocities showcomplex anisotropic patterns associated with the dynamic processes of the collisionbetween the Indian and Eurasian plates:(1) In the upper crust, Vsv> Vsh anisotropyis dominant throughout the study area which probably reflects fossil microcracksinduced by the uplift, folding and erosion geodynamic processes;(2) In the middlecrust, Vsh> Vsv observed beneath the Songpan-Ganzi terrane and northwesternQilian orogen correlates well with a mid-crustal low velocity zone (LVZ);(3) Atdepths deeper than40km, Vsh> Vsv still exists in the Songpan-Ganzi terrane. Thisanisotropy could be caused by the sub-horizontal alignment of anisotropic mineralsthat has followed the collision between India and Eurasia. However, the northwesternQilian orogen is associated with Vsv> Vsh anisotropy which may be related to thevertically aligned seismic anisotropic minerals caused by the crustal thickening.