The initiation and evolution of compressional intracontinental orogens are favored by rheologically weak lithosphere underneath; however, how this weakened lithosphere responds to the regional stress regime remains vigorously debated. The Tien Shan mountains in central Asia provide the best example to illustrate the deep deformational responses to intracontinental orogenesis. We present new constraints on the nature of seismic anisotropy in the crust and upper mantle of the central Tien Shan through shear-wave splitting analyses. Our results reveal a sharp change in the orientations of crustal anisotropic fabrics on two sides of the mountains. The convergence-parallel fast orientations in the northern segment are closely related to the lower-crustal simple-shear deformation caused by the underthrusting of the Kazakh Shield, whereas the depth-independent orogen-parallel fast orientations in the southern segment suggest vertically coherent pure-shear thickening of the Tien Shan lithosphere in response to the northward indentation of the Tarim Basin. The thickened lithosphere has partly foundered into the deep mantle, contributing to the accelerated shortening deformation in the late Cenozoic. Our observations demonstrate the complex tectonic processes in the Tien Shan and suggest that the rheological properties of bounding blocks can play a significant role in shaping the lithospheric structures of intracontinental orogens.

中文翻译：

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天山中部的地震各向异性揭示了陆内造山过程中流变控制的变形

挤压陆内造山带的萌生和演化有利于下方流变较弱的岩石圈；然而，这个被削弱的岩石圈如何响应区域压力机制仍然存在激烈的争论。中亚的天山山脉为说明陆内造山运动的深部变形响应提供了最好的例子。我们通过剪切波分裂分析提出了对天山中部地壳和上地幔地震各向异性性质的新约束。我们的研究结果揭示了山脉两侧地壳各向异性织物的方向发生了急剧变化。北段辐合平行快速方位与哈萨克地盾下冲引起的下地壳简剪变形密切相关，而南段与深度无关的造山带平行快速方向表明天山岩石圈的垂直连贯纯剪切加厚响应塔里木盆地向北的压陷。加厚的岩石圈部分沉入地幔深处，导致晚新生代加速缩短变形。我们的观察表明天山复杂的构造过程，并表明边界块的流变特性可以在塑造陆内造山带的岩石圈结构中发挥重要作用。加厚的岩石圈部分沉入地幔深处，导致晚新生代加速缩短变形。我们的观察表明天山复杂的构造过程，并表明边界块的流变特性可以在塑造陆内造山带的岩石圈结构中发挥重要作用。加厚的岩石圈部分沉入地幔深处，导致晚新生代加速缩短变形。我们的观察表明天山复杂的构造过程，并表明边界块的流变特性可以在塑造陆内造山带的岩石圈结构中发挥重要作用。