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| 青藏高原东南缘地震各向异性及其深部构造意义 | |
| 引用本文： | 高原, 石玉涛, 王琼. 2020. 青藏高原东南缘地震各向异性及其深部构造意义. 地球物理学报, 63(3): 802-816, doi: 10.6038/cjg2020O0003 |
| 作者姓名： | 高原 石玉涛 王琼 |
| 作者单位： | 中国地震局地震预测研究所(地震预测重点实验室), 北京 100036 |
| 基金项目： | 国家自然科学基金重点项目（41730212）和国家重点研发计划课题（2016YFC0600302）共同资助. |
| 摘 要： | 青藏东南缘是青藏高原物质东流的通道，为了更全面了解复杂的岩石圈结构和强烈的变形特征，本文介绍了青藏东南缘岩石圈各向异性的形态，综合其他研究者得到的该区域壳幔各向异性结果，增加了部分新的资料，更新了青藏东南缘岩石圈方位各向异性图像，探讨了区域深部构造意义. 基于近场小震、远震和背景噪声资料计算结果，青藏东南缘地震各向异性展现出独特的区域空间分布和垂向层次性分布形态，展现了3个主要特征.（1）青藏东南缘上地壳各向异性与地表变形测量结果相符，快剪切波偏振方向（即快波方向）呈现与地表运动特征一致的发散性，与主压应力方向一致，但受到地质构造的影响.（2）青藏东南缘下地壳方位各向异性展现了更好的方向一致性，但方位各向异性程度相对较弱，在红河断裂带西北端部和小江断裂带下方有两个下地壳低速区，其方位各向异性程度与上地壳相当.（3）青藏东南缘岩石圈方位各向异性，呈现南、北分区特征，南北分界线大致在26°20'N，快波方向在北部近似为NS方向，在南部近似为EW方向. 本文推测：（1）在26°20'N北侧的上地幔有较厚的高速体，高速体南侧边缘呈现出近EW走向的直立墙形构造，其南侧软弱的上地幔物质在EW方向上流动，导致了岩石圈方位各向异性特征在空间发生突然的变化，快波方向由北部的NS变为南部的EW方向；（2）小江断裂带是现今的华南地块的地壳西边界，但岩石圈尺度的方位各向异性展现出的趋势性表明，华南地块的上地幔物质越过了小江断裂带到达其西侧，揭示了华南地块与青藏地块接触碰撞造成的岩石圈物质变形和上地幔软流圈物质运移的深部图像.地震各向异性能揭示区域深部构造与介质变形的信息，不同观测资料的综合分析有助于获得更清晰的各向异性三维图像. |
| 关 键 词： | 青藏高原东南缘 地震各向异性 岩石圈 深部构造 方位各向异性 剪切波分裂 背景噪声资料 |
| 收稿时间： | 2020-01-31 |
| 修稿时间： | 2020-02-21 |
Seismic anisotropy in the southeastern margin of the Tibetan Plateau and its deep tectonic significances |
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| GAO Yuan, SHI YuTao, WANG Qiong. 2020. Seismic anisotropy in the southeastern margin of the Tibetan Plateau and its deep tectonic significances. Chinese Journal of Geophysics (in Chinese), 63(3): 802-816, doi: 10.6038/cjg2020O0003 | |
| Authors: | GAO Yuan SHI YuTao WANG Qiong |
| Affiliation: | Key Laboratory of Earthquake Prediction, Institute of Earthquake Forecasting, China Earthquake Administration, Beijing 100036, China |
| Abstract: | The southeastern (SE) margin of the Tibetan Plateau is a channel of the eastward escaping material flow of the Tibetan Plateau. In order to fully understand the complex lithosphere structure and strong deformation characteristics, this study introduces the anisotropic pattern in the SE margin of the Tibetan Plateau. Meanwhile, on the basis of other anisotropic studies in the crust and the upper mantle, increasing some of new data, we update the image of the lithospheric azimuthal anisotropy and discuss the significance of regional deep tectonics in the SE margin of the Tibetan Plateau. Based on the results from local seismic waveforms, teleseismic waveforms and ambient noise data, the seismic anisotropy shows the unique regional spatial distribution and the vertical hierarchical distribution, which reveals three main characteristics in the SE margin of the Tibetan Plateau. (1) The seismic anisotropy in the upper crust coincides with the measured surface deformation. The polarization direction of the fast shear-wave exhibits divergence, consistent with the crustal surface motion. It is consistent with the direction of the main compressive stress, but influenced by the geological tectonics. (2) The azimuthal anisotropy in the lower crust shows better directional consistency, but the degree of azimuthal anisotropy is relatively weak. There are two zones of low velocity beneath the northwest end of the Honghe fault belt and the Xiaojiang fault belt, where the degree of azimuthal anisotropy is as strong as that in the upper crust. (3) The lithospheric azimuthal anisotropy is characterized by two subzones, the south part and the north part. The dividing line is approximately 26°20'N between two subzones. The fast direction of S wave is approximately in NS in the north part and in EW in the south part. It is inferred that in the upper mantle on the north side of 26°20'N there is a thick high-velocity body, which southern edge presents an upright wall structure striking in nearly EW, so that in the upper mantle on the south side the material flows along EW direction. It induces the sudden spatial change of the azimuthal anisotropic characteristics in the lithosphere, the fast direction changes from NS in the north into EW in the south. This study considers that the Xiaojiang fault belt is the western boundary of the crust of the South China block. However the lithospheric azimuthal anisotropy displays that the upper mantle material of the South China block goes across the Xiaojiang fault belt to its west side and indicates that deep images of lithospheric deformation and upper mantle asthenospheric material movement are caused by collision between the South China block and the Tibetan Plateau. Seismic anisotropy reveals information on regional deep tectonics and medium deformation, and comprehensive analysis of different observations are helpful to obtain clearer 3D anisotropic images. |
| Keywords: | Southeastern margin of the Tibetan Plateau Seismic anisotropy Lithosphere Deep tectonics Azimuthal anisotropy Shear-wave splitting Ambient noise data |
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