The Crustal And Upper Mantle Structure And Azimuthal Anisotropy Of Surface Wave Beneath The Tibetan Plateau And Adjacent Areas

Tibetan Plateau is the dynamic consequence of the collision between India and Eurasian plates, and the crust and upper mantle structure in this region is extremely complex.The research of deep structure of the Tibetan Plateau is the basis and key for understanding the dynamic mechanism of the Tibetan Plateau uplift.In the recent studies on the geodynamics of Tibetan Plateau, the deformation and coupling of crust and mantle is one of the current foci of research.This thesis attempts to use the broadband digital records from the Tibetan Plateau and surrounding areas to perform a Rayleigh surface wave tomography to obtain group volocity and azimuthal anisotropy images and3-D S wave velocity images from crust to lithospheric mantle beneath the Tibetan Plateau, to understands the subduction-collision model and probes into the tectonic significance of the crust-mantle deformation coupling. To ensure a better path coverage, the long period surface wave data are collected from broadband seismic stations in a large region (12°-5°N,70°-145°E) including those of CDSN, IRIS, PASSCAL in1991-1992, INDEPTH in1994and1999, and the China Earthquake network established in2002,especially broadband temporary networks(total35stations)deployed in Yunnan during2000-2001, and from eastern Tibet to western Sichuan during2004-2006. More than6000mixed path dispersions of Rayleigh wave group velocity were measured by means of time-frequency analysis based on multiple-filter.China and adjacent regions(12°S-55°N,70°-145°E) was divided into a1°×1°grid, and group velocity and azimuthal anisotropy distributions of fundamental Rayleigh wave between7-184s were determined by Occam’s inversion.From the pure-path dispersion curves, the three-dimensional shear velocity structures in0-350km depths are inverted at each node in the Tibetan Plateau and surrounding areas(18°-45°N,70°-115°E). Seven average velocity distributions of shear wave in different depth rages, eleven profiles along parallels and seven profiles along meridians of shear wave velocity were displayed to analyze and explain the velocity structure and azimuthal anisotropy of each terrane of the Tibetan Plateau and surrounding areas, and then compare the velocity structure and azimuthal anisotropy images with those of the phase velocity of surface waves and SKS results.The inversion results indicate that the Tibetan plateau and surrounding areas show obvious lateral variation of crust and upper mantle velocity structure and azimuthal anisotropy images in different terranes. At first, the Tibetan plateau has a thick crust (60-70km) and very thick lithosphere (exceeding200km), there also exists a low velocity layer (Vs<3.2km/s) in the middle crust (about in25-45km depth range).The Qiangtang and Songpan-Ganze-Hoh Xil terranes in north Tibetan plateau show high velocity of upper mantle lithosphere, but Himalayan and Lhasa terranes located in south Tibetan plateau depict low velocity of upper mantle lithosphere. Second, the crust anisotropy images show clockwise rotation surrounding the eastern Himalayan syntaxis. The fast wave directions for azimuthal anisotropy in west of88°E are NS,while in east part the fast wave directions change from NE-EW-NW which are consistent with fast wave directions of phase velocity and SKS. Third, the huge low velocity body exists in Yunnan-Burma-Thai block and Indochina block of southeast Tibetan plateau and upwells to crust which may be associated with the Indian plate eastward subducted under the Burma Arc, causing an uplift of the asthenosphere. Fourth, the fast wave directions of surface wave anisotropy are small changes from the crust to upper mantle in most regions inside Tibetan plateau which may belong to the vertical coherent deformation of the crust and mantle deformation model; while in Yunnan outside the plateau, the fast directions from the crust to upper mantle are large changes which may belong to the model of the crust-mantle decoupling.Fiveth. in the areas of good path coverage (excluding Indian plate on the southwest of the Tibetan Plateau) of the Tibetan Plateau and adjacent areas((18°-45°N,70°-115°E) the group velocity and anisotropic features of scope greater than400km and strength greater than2%are reliable.At last, the inversion result constrains the collision model between the India and Eurasian plates. In Himalayan and Lhasa terranes of south Tibetan plateau, there is a north- dipping lithosphere lid of high velocity similar to that of India plate, mimicking a subducting plate. In north Tibetan plateau, upper mantle shows a sub-Moho low velocity zone, which could be explained by lithosphere delaminating and sinking and asthenosphere uplifting to the top of upper mantle.