Crustal anisotropy beneath the Hi-CLIMB array in Tibet from modeling receiver functions
Date Issued
2014
Date
2014
Author(s)
Huang, Yi-Xiang
Abstract
It is well known that the India-Eurasia collision beginning at about 55 Ma ago results in the world’s highest Himalayan mountain and largest Tibetan Plateau with an average elevation of 5 km and crustal thickness of 70 km. Strain-induced seismic anisotropy is commonly used as a proxy to map the distribution and characteristics of lithospheric deformation. In this study, we focus on investigation of the crustal anisotropy beneath central Tibet by means of modeling radial and transverse receiver functions (RFs) observed along a N-S striking, 800-km long linear array from the passive seismic experiment of Project Hi-CLIMB.
To quantitatively characterize the depth-varying anisotropic structure in the crust under the plateau, we combine azimuth-dependent RFs in both radial and transverse components to invert for 1-D layered anisotropic velocity models under individual stations. As the inverse problem is highly non-linear with numerous unknown model parameters, we choose a neighborhood algorithm to search for the solution with the global minimum of the optimized function, defined as the decorrelation coefficient between observed and synthetic RFs.
The resulting models show that the uppermost crust at depths less than 10 km beneath most stations reveals >10% strong anisotropy and a nearly vertical fast symmetric axis with plunge greater than 70o. For the stations near the E-W trending suture zones between terranes, the azimuth of the obtained fast axis is approximately aligned E-W, while those within the Lhasa and Qiantang terrane the fast axis is oriented more irregularly. These characteristic features suggest that the sutures and E-W striking strike-slip and N-S striking normal faults randomly distributed within the terranes may be attributed to the observed pattern of upper crust anisotropy. A strong anisotropic layer with the subhorizontal symmetric fast axis is observed in the middle crust at the depths of 20-35 km, collocated with the low shear velocity zone. The presence of a low-viscosity, ductile channel with preferred alignments of partial melt inclusions and/or anisotropic mica minerals may be attributed to the observed anisotropy in the middle crust. The anisotropy in the lower crust is generally less well constrained due to the low signal-to-noise ratios of traverse-component RFs as well as the P-to-s conversions at the velocity discontinuities in the lower crust strongly influenced by complex shallow structures.
Subjects
西藏高原
接收函數
地殼非均向性
正演方法
Type
thesis
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