Crustal anisotropy across eastern Tibet and surroundings modeled as a depth-dependent tilted hexagonally symmetric medium
Crustal anisotropy across eastern Tibet and surroundings modeled as a depth-dependent tilted hexagonally symmetric medium
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DOI：
10.1093/gji/ggx004
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发表时间：
2017-01
影响因子：
2.8
通讯作者：
Jiayi Xie;M. Ritzwoller;W. Shen;Weitao Wang
中科院分区：
文献类型：
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作者：
Jiayi Xie;M. Ritzwoller;W. Shen;Weitao Wang
地震学家经常观测到两种类型的面波各向异性，但很少将它们结合起来解释：一种是视径向各向异性，即从洛夫波和瑞利波推断出的水平偏振波和垂直偏振波之间的传播速度差异；另一种是视方位各向异性，即面波速度（通常是瑞利波）的方向依赖性。我们表明，通过将地壳建模为与深度相关的倾斜六方对称（THS）介质，可以同时解释在青藏高原东部及其周边观测到的一组新的洛夫波和瑞利波各向同性相速度以及瑞利波方位各向异性数据。我们用与深度相关的六方对称弹性张量来描述THS介质，这些张量通过倾角和走向角倾斜和旋转，并使用贝叶斯蒙特卡洛反演来估计这些量，从而在0.5°×0.5°的空间网格上生成地壳和最上部地幔的三维模型。在青藏高原东部内部和云贵高原，我们推断在上部地壳存在一个陡倾的THS介质，它覆盖在中 - 下地壳的一个缓倾的THS介质之上。这种各向异性的垂直分层可能反映了从脆性到韧性的转变，其中浅层裂缝和断层控制着上部地壳的各向异性，而各向异性（可能是云母质）矿物的晶体择优取向决定了深部地壳的各向异性。相比之下，在青藏高原周边附近，整个地壳的各向异性介质都是陡倾的，这可能是由于地壳流动在青藏高原边界附近发生旋转时，深部地壳各向异性矿物的对称轴重新定向所致。
Two types of surface wave anisotropy are observed regularly by seismologists but are only rarely interpreted jointly: apparent radial anisotropy, which is the difference in propagation speed between horizontally and vertically polarized waves inferred from Love and Rayleigh waves, and apparent azimuthal anisotropy, which is the directional dependence of surface wave speeds (usually Rayleigh waves). We show that a new data set of Love and Rayleigh wave isotropic phase speeds and Rayleigh wave azimuthal anisotropy observed within and surrounding eastern Tibet can be explained simultaneously by modeling the crust as a depth-dependent tilted hexagonally symmetric (THS) medium. We specify the THS medium with depth-dependent hexagonally symmetric elastic tensors tilted and rotated through dip and strike angles and estimate these quantities using a Bayesian Monte Carlo inversion to produce a 3-D model of the crust and uppermost mantle on a 0.5 ◦ × 0.5 ◦ spatial grid. In the interior of eastern Tibet and in the Yunnan-Guizhou plateau, we infer a steeply dipping THS upper crustal medium overlying a shallowly dipping THS medium in the middle-to-lower crust. Such vertical stratification of anisotropy may reflect a brittle to ductile transition in which shallow fractures and faults control upper crustal anisotropy and the crystal-preferred orientation of anisotropic (perhaps micaceous) minerals governs the anisotropy of the deeper crust. In contrast, near the periphery of the Tibetan Plateau the anisotropic medium is steeply dipping throughout the entire crust, which may be caused by the reorientation of the symmetry axes of deeper crustal anisotropic minerals as crustal flows are rotated near the borders of Tibet.