【作者】 王琼；

【导师】 高原；

【作者基本信息】 中国地震局地震预测研究所 ， 固体地球物理学， 2012， 硕士

【摘要】 青藏高原南部受印度板块碰撞和挤压，北部受远及俄罗斯、西伯利亚地台的阻挡，在这种构造格局下，青藏高原一方面通过自身的地壳缩短、增厚来吸收南北向的强烈挤压作用，形成了构造复杂的青藏高原东北缘；另一方面在其东西方向寻求物质流逸的通道。不论物质以何种方式逃逸，位于青藏高原东南缘的川滇地区都与高原物质东流有直接关系。因此，通过研究青藏高原东部边缘地壳上地幔形变机制，可以探查块体内深部构造、形变场特征，深化对应力应变场和构造过程的认识，对于解释板块运动、软流圈流动、地震成因和强震深部背景力源等问题具有重要的意义，尤其对青藏高原隆起的动力学过程有特殊意义。本文基于青藏高原东北缘甘肃区域台网41个宽频带地震台站的远震记录资料，通过PKS、SKS和SKKS震相（文中统称为XKS）的剪切波分裂分析，研究获取了每个台站下方介质的各向异性分裂参数，得到青藏高原东北缘地区上地幔各向异性分布图像，并结合GPS速度场和地壳剪切波各向异性分析青藏高原东北缘各向异性形成机制及壳幔各向异性特征。研究结果表明，阿尔金断裂带西侧，各向异性快波偏振呈WNW-ESE方向，与断裂带走向有一定夹角；在祁连山-河西走廊构造区，XKS快波偏振呈NW-SE方向，一致性较好，与区域断层走向方向相同；在海原断裂东部陇中盆地及其周围，XKS快波虽然总体上呈WNW-ESE方向，但在快波方向图像上也显示出较其他地区更复杂的分布，这可能与该地区处于青藏块体与鄂尔多斯地块之间的过渡区域有关。由整个区域慢波时间延迟可以得出，青藏高原东北缘各向异性层厚度呈两端厚中间薄的趋势。将本文的结果与地壳剪切波分裂结果综合对比分析发现，在祁连山-河西走廊构造区地壳剪切波快波偏振在该区域呈NE-SW方向，与相对于稳定欧亚大陆GPS运动速率一致，与XKS的快波偏振方向不同，地壳和上地幔快波偏振方向的差异说明了壳幔变形可能有着不同的机制；而在陇中盆地及其周缘，地壳快剪切波偏振和上地幔XKS快波偏振总体上呈WNW-ESE方向，暗示了可能相同的壳幔变形机制。在青藏高原东南缘，利用云南区域台网56个宽频带地震台站的连续波形数据，根据噪声互相关方法，反演得到研究区域0.5°×0.5°周期5-40s相速度分布图像和方位各向异性分布图像，并结合近震S波分裂、Pms转换波分裂和远震XKS分裂，对青藏高原东南缘地区壳幔各向异性进行综合分析。相速度分布结果显示，短周期(5-12s)的相速度与地壳浅部结构有关，低速异常区位于澜沧江断裂以东和程海-红河断裂以西之间、普渡河断裂以西和弥勒-师宗断裂以北一带。易门断裂和普渡河断裂之间则存在高速异常。周期16-26s相速度分布图像主要反映中下地壳范围内速度的变化情况。由红河断裂、小江断裂和丽江-剑川断裂围成的“川滇菱形块体”内呈现大区域的低速异常。高速异常主要在滇东南地区。周期30-40s主要反映下地壳至上地幔顶部附近的速度结构，其中川滇菱形块体内由低速异常又逐渐变为高速异常。纵观整个范围川滇菱形块体内相速度的变化情况，暗示中下地壳处的低速异常带很可能就是青藏高原下地壳流的通道。方位各向异性分布结果显示，短周期Rayleigh面波快波偏振方向与区域断裂走向有很好的一致性，快波方向随着断裂走向的变化而变化；周期16-26s相速度偏振方向与上地壳基本相同，其中，滇中块体中易门断裂和普渡河断裂附近，各向异性快波方向从NS向WN方向旋转；易门断裂以西呈NW向。一方面反映了青藏高原物质东流，一方面又说明了川滇块体受到青藏块体的南东向挤压。周期30-40s范围的各向异性，滇缅泰块体和印支块体，快波偏振方向为NS和NNW向；而在滇中块体内部，各向异性快波方向呈顺时针旋转变化，可能与青藏高原物质向东逃逸有关。通过与近震S波分裂、Pms转换波分裂和远震XKS分裂的对比研究，发现随着周期的增大，得到的快波偏振方向与XKS剪切波快波偏振方向接近一致，与地壳剪切波方向呈一定夹角，整体上来说，壳幔各向异性可能有着不同的形成机制。更多还原

【Abstract】 Driven by crashing, extrusion and continuous drifting of Indian plate in the south,and synchronously prevented by Russian-Siberia platform in the north, theQinghai-Tibet plateau, on the one hand, absorbs the north-south strong compressionthrough the crust thickening and shortening, so that it forms complicated structure inthe northeast of Qinghai-Tibet plateau. On the other hand, the plateau materialsescape to east and west respectively. However, no matter what the escape way theyare, the Sichuan-Yunnan regions in the south-eastern Tibetan plateau are the domainswhere the tectonic escape and lower crust flow pass through. Investigating the crustand mantle anisotropy and deformation mechanisms, will be helpful to know the deepstructure, the characteristic of deformation field, the stress-strain field and tectonicprocess and so on. Furthermore, it is very important to make acquaintance of themechanism of the plateau uplift and the escaping of the deep materials.Based on teleseismic PKS，SKS and SKKS phases (XKS) recorded by41broadband stations in Gansu province in northeastern margin of Qinghai-Tibet plateau，this study obtains the splitting parameters of fast polarization direction and delay timebetween the fast and slow shear waves at each station using the minimum transverseenergy method，and then，plots the distribution map of upper mantle anisotropyaround this area. Furthermore， combined with GPS velocity field and crustalanisotropy from near-field shear-wave splitting using SAM method，we discuss thecharacteristics and formation mechanism of the crust-mantle anisotropy in thenortheastern margin of Qinghai-Tibet plateau.The main results demonstrate that the fast wave polarization trends WNW-ESEin the west part of Altun fault，siting at a certain degree angle to the strike of regionalstructures. In Qilian-Hexi Corridor tectonic region，the XKS fast direction is NW-SE，consistent with strike direction of the main fault in this area. In Longzhong Basin andadjacent regions， which located in the transition zone between the active Qinghai-Tibet block and comparatively stable Ordos block，with more complextectonic background, the fast direction trends WNW-ESE, but owing to local feature’sinfluence，the results vary obviously with these stations.By comparing the crustal anisotropy of our study with that from othershear-wave splitting analysis of direct S-wave, we find that the fast direction obtainedfrom near-field shear-wave splitting is NE-SW，consistent with GPS velocity relativeto stable Eurasian Continent, and different with XKS fast direction, which suggeststhat the deformation mechanism of them may be different. In Longzhong Basin andadjacent regions, the fast directions in the crust and in the upper mantle are consistentby and large，which means that the crust and the upper mantle possibly have the samedeformation.Based on continuous ambient noise recorded by56broadband stations in Yunnanprovince in southeastern margin of Qinghai-Tibet plateau，this study obtains thedistribution of Rayleigh wave phase velocity and the splitting parameters of fastpolarization direction and delay time between the fast and slow waves in5-40s periodusing ambient noise correlation. Furthermore，combined with anisotropy separatelyfrom shear-wave splitting using S-wave, Pms and XKS, we discuss the characteristicsand formation mechanism of the crust-mantle anisotropy in the southeastern marginof Qinghai-Tibet plateau.The results of phase velocity distribution shows that, at5-12s period, the phasevelocity is related to shallow crust structure. Low velocities exist between theLancangjiang fault and Chenghai-Honghe fault, west of Puduhe fault and north ofMile-Shizong fault. High velocities exist between Yimen fault and Puduhe fault. At16-26s period, phase velocity mainly reveals the speed variation in the middle-lowercrust. The Sichuan-Yunnan rhombic block, which enclosed by Honghe fault, Xijiangfault and Lijing-Jianchuan fault, scatters low velocity in large regions. And highvelocities mainly exist in northeast of Yunnan. At30-40s period, phase velocitymainly reveals the speed variation from the lower crust to the upper mantle.Remarkably, the velocity in Sichuan-Yunnan rhombic block gradually turns to highvelocity anomaly. According to the variation of Sichuan-Yunnan rhombic block, it is possible that the middle-lower crust is channel flow of Qinghai-Tibet Plateau.The results of azimuthal anisotropy distribution show that, at5-12s period, thefast wave polarization is influenced by the strike of the regional structures. At16-26speriod，fast wave polarization direction coincides with upper crust. In nearby Yimenfault and Puduhe fault in middle Yunnan block, the fast wave polarization rotates fromNS to WN, and in the west of Yimen fault, the direction trends NW, which suggestthat plateau materials flow east, and Sichuan-Yunnan block is squeezed by theQinghai-Tibet plateau. At30-40s period, in Yunnan-Burma block and Indosinianblock, the fast wave direction trends NS and NNW. And in middle Yunnan block,there is clockwise rotation about direction.According to comparison with S-wave splitting, Pms splitting and XKS shearwave splitting, we find that along with the increase of the period, the fast wavepolarization is similar to the XKS shear wave splitting, and sites at a certain degreeangle to the direction of fast wave in the crust. In short, crust-mantle formationmechanism may be different in the southeast of Qinghai-Tibet plateau.更多还原