| Summary: | 博士 === 國立中央大學 === 地球物理研究所 === 97 === In this study, we attempt to understand the seismic anisotropy beneath the orogeny from the observations of fundamental surface waves. The array analyses were adopted for average phase velocities of Rayleigh and Love waves, respectively. For the observations in the Tibet plateau, the inverted shear-wave structures show the noticeable contrasts between the Lhasa and Qiangtang terranes. The crustal-velocity decreases about 0.3 km/s from south to north with a significant change of Poisson’s ratio from normal value in the south to relative high value in the north. Besides, the mantle is cold and high-velocity of Lhasa but is hot with unusual low velocity of Qiangtang. In north Tibet, the model also shows no obvious lid which indicates the thinning or melting lithospheric mantle. On the whole, the opposite geophysical properties indicate the mantle lithosphere is not identical from south to north of the Tibet plateau. The Love-Rayleigh discrepancies reveal the radial anisotropies varying with depth to infer the feasible origin of anisotropy. The radial anisotropy is derived within the mid-to-lower crust beneath southern Tibet and is related the fluid-filled cracks or the horizontal partial-melting layer. The anisotropy is inferred within the upper mantle beneath northern Tibet and is coincident with the shear-wave splitting, which is considered as the results of alignments of olivine.
For the phase velocities revealed by the modified two-station method, the azimuthal anisotropy parameters of the Central Range show significant variations with period. The upper layer is displayed by the anisotropic terms of short -period signals, characterized with fast polarization in east-west direction, and is related to the crustal cracks. In contrast, the lower layer is retrieved by the extensive long-period signals and is characterized with fast polarization in approximately northeast direction. The mantle anisotropy is commonly regarded as the olivine alignments accommodated to the compression or transcurrent motion during plate collision. The mantle anisotropy extending to 150 km deep demonstrates the anisotropic properties beneath the Central Range and corresponds with the fast direction and large time-delay revealed from the shear-wave splitting.
|
|---|
