SEISMIC VELOCITY, RADIAL ANISOTROPY IN THE CRUST AND UPPER MANTLE OF NORTHEASTERN TIBETAN PLATEAU FROM SURFACE WAVE TOMOGRAPHY
Li, Lun 1984-
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The northeastern (NE) Tibetan Plateau with complex interaction between mountain building and strike-slip faulting is a prime site to study the growth of the plateau and to understand the dynamic processes at its boundaries. This dissertation research investigates the crust and upper mantle structure in the NE Tibetan Plateau using surface wave tomography from teleseismic data recorded at the 36 seismic stations of NorthEast Tibet Seismic (NETS) array. The first part of this dissertation is to construct a 3-D SV velocity (VSV) model using Rayleigh wave data in the NE Tibetan Plateau. The average and 2-D variations of phase velocities were obtained at 14 periods from 20 to 100 s and then used to constrain 1-D and 3-D VSV models. The entire region is characterized by relatively low absolute velocity, indicating a weak upper mantle. One pronounced slow anomaly centered at ~120 km depth is imaged right beneath a restraining bend of the Kunlun fault and is probably caused by anomalously high temperature and partial melting resulted from localized asthenosphere upwelling after the delamination of a thick lithosphere root beneath the bend. In the second part of the dissertation, a 3-D SH velocity (VSH) model was obtained from the Love wave phase velocities at 13 periods from 20 to 91 s, and then used to construct a 3-D radial anisotropic model combined with the VSV model. The lower crust is characterized with positive anisotropy (VSH>VSV). The large positive anisotropy can be explained by horizontal alignment of anisotropic minerals such as mica at the formation of lower crust. The mantle lithosphere above 90 km is largely isotropic while weak positive anisotropy appears beneath 90 km, which probably marks the lithosphere-asthenosphere boundary. The Kunlun restraining bend and its surrounding region are distinguished by a relatively negative radial anisotropy in the entire lithosphere, providing evidence for coherent lithosphere deformation in this area where local compression stress is high. Such coherent lithosphere deformation is not observed in the eastern portion of the Kunlun neither in the Qinling and Qilian orogens, suggesting that the lithosphere shortening in most of the NE Tibet is not as strong as in the interiors of the plateau.