Abstract: Seismic anisotropy has been widely used to constrain deformation and mantle flow within the upper mantle of the Earth's interior, and is mainly affected by crystallographic preferred orientation(CPO)of anisotropic mineral in lithosphere. Anisotropy of peridotites caused by deformation is the main source of seismic anisotropy in the upper mantle. Olivine is the most abundant and easily deformed mineral to form CPO in peridotite, thus the CPO of olivine controls seismic anisotropy in the upper mantle. Based on simple shear experiments and studies of natural peridotites deformation, several CPO types of olivine have been identified, including A, B, C, D, E and AG-type. Studies on the deformation of olivine have shown that the CPO of olivine is mainly related to stress, water content, temperature, pressure, partial melting and melt/fluid percolation. Most of the seismic anisotropy has been explained by the A-type olivine CPO in the upper mantle, which is commonly found in upper-mantle peridotites and produced by the simple shear in dry conditions. Previous studies showed that anisotropy was attributed to the CPO of mica and amphibole in the middle-lower crust. The comparison between mantle anisotropy calculated from mineral CPO and regional anisotropy deduced from geophysical methods is therefore particularly useful for interpreting the deformation mechanisms and geodynamic processes which affect the upper mantle in different tectonic units such as subduction system, continental rift and continental collision zone in the world. The paper summarizes the characteristics of CPO and anisotropy of major anisotropic minerals in the upper mantle. Taking the lithosphere mantle xenoliths in the southeastern Tibetan plateau as an example, we perform detailed studies on the microstructures and seismic anisotropy to better understand the deformation mechanisms and upper mantle anisotropy in this region. Results show that the CPO of olivine in peridotite xenoliths in southeastern Tibetan plateau are A-type and AG-type. The mechanisms proposed for the formation of AG-type are different from that for the A-type. Therefore, the occurrence of AG-type olivine CPO pattern suggests that this CPO may record a change in deformation mechanism and tectonic environment of the lithosphere in southeastern Tibetan plateau. Provided that the strong SKS(shear wave splitting)observed in southeastern Tibetan plateau results from lithosphere mantle, the lithosphere mantle in this region is expected to be at least 130km thick and characterized by vertical foliation. Considering that the thickness of lithosphere in southeastern Tibetan plateau is much less than 130km and the lithosphere mantle cannot explain the anisotropy measured by SKS, other anisotropy sources should be considered, such as anisotropy in the asthenosphere and the oriented melt pockets(MPO)in the upper mantle. Therefore, detailed study of CPO of anisotropic mineral is essential for constraining geophysical measurements and analyzing the dynamic process of the lithosphere reasonably.

Key words: seismic anisotropy, crystallograpghic preferred oreintation(CPO), mantle xenoliths, southeastern Tibetan plateau

摘要： 地震波各向异性主要受岩石中矿物晶格优选方位（Crystallographic Preferred Orientation，CPO）的影响，橄榄石的CPO控制着上地幔的地震波各向异性。将岩石中矿物的CPO与全岩地震波各向异性相结合，可以解释在全球不同构造单元观测到的地震波各向异性，从而进行构造变形和动力学过程分析。文中在总结岩石圈主要各向异性矿物的CPO和各向异性特征的基础上，以青藏高原东南缘岩石圈地幔包体为例，对其显微组构和地震波各向异性进行研究，结果显示青藏高原东南缘岩石圈地幔的构造环境发生改变，岩石圈地幔无法解释观测到的剪切波分裂（SKS）地震波各向异性，需要考虑其他各向异性来源。由此可见，研究岩石圈地幔矿物的CPO对合理约束地球物理测量资料和分析岩石圈变形特征至关重要。

关键词: 地震波各向异性, 晶格优选方位, 地幔包体, 青藏高原东南缘