【作者】 孙娅；

【导师】 柳建新； 钮凤林；

【作者基本信息】 中南大学 ， 地质资源与地质工程， 2013， 博士

【摘要】 摘要：青藏高原东南边缘大规模的地表隆起、形变和断层运动普遍认为是印度板块和欧亚板块在新生代碰撞形成的。由于缺乏浅地表、地壳和上地幔的构造信息,目前对于这种大规模地表运动的具体成因以及由碰撞引起的地壳和岩石圈形变仍未有定论,这也是当前地震学和地球动力学研究领域中的热点问题之一。本论文充分利用中国地震局在四川、云南、广西、贵州、广东、湖南等区域,自2007年7月到2010年7月共四年的宽频数字台网和其它台站所观测的地震波形资料。采用接收函数计算Moho深度和地壳Vp/Vs比值,并在接收函数的基础上引入了一种新的综合性方法计算了地壳各向异性参数。该综合性方法基于地壳各向异性介质中Moho面Ps转换波的到时,在径向分量和切向分量的接收函数所展现出的性质和谐波分析,估算横波分裂的快波方向和分裂时间。发现在青藏高原东南边缘的台站横波分裂时间变化较大,约0.24s-0.9s不等。而在相对青藏高原东南边缘较远的其他台站并没有发现地壳内部具有各向异性。同时Moho面深度和Vp/Vs自青藏高原东南边缘向东南方向逐渐变薄变弱。表明青藏高原东南部相对其周围区域含有丰富的铁镁质岩或存在部分熔融。该结果为青藏高原东南地区存在下地壳流模型提出了最新的地球物理证据。同时对比了上地幔SKS/SKKS分裂结果,发现地壳各向异性快波分裂方向与SKS/SKKS的上地幔处快波分裂方向基本一致,并且分裂时间与上地幔分裂时间几乎相同,说明青藏高原东南部地区地壳各向异性可能是SKS/SKKS横波分裂的主要贡献者。这表明上地幔构造可能是弱变形或者存在地壳与上地幔形变的解耦构造。为了进一步了解地壳与上地幔结构关系,解析上地幔岩石圈对地表抬升的作用。本文在已知地壳信息的基础上,采用有限频层析成像技术反演了该地区上地幔S波三维速度结构。由分析测量两台站所接收到穿越地球内部的地震波到时差,反演两个台站下方的构造。计算过程中选择3种不同频段的地震S波进行综合反演,分别是高频0.1-0.5hz、中频0.05-0.1hz、低频0.02-0.05hz。为了扩大研究区域,本文改进了台站对的选择方法,保证每个台站在反演矩阵中拥有合适比值。以每一个地震事件中的所有台站对首尾相连形成一个闭合圈,同时保证两个相邻台站间的地表距离大于200kmn。本文共选用3种不同地表距离分别是200km、300km、400km的走时差进行联合反演。这样既保证了射线较好地覆盖研究区域也保证了反演结果精细可靠。本文结果与前人的结果在大构造上基本一致,如四川盆地的高速异常以及在扬子克拉通在转换带处的高速异常等。除此之外,层析成像结果还显示出在青藏高原东南边缘存在一个连续的高速异常体,它沿着东经1000和1010由南往北连续分布,从北纬270约100km深处到北纬320附近约350km,其正上方覆盖着低速异常体。尽管本文结果还需要更多证据来证实,但这些新发现为岩石圈拆沉造成高原抬升提供了有力的佐证。针对以上研究,本文对青藏高原东南部地区的抬升有了一个综合认识。在印度板块与欧亚板块碰撞之后,青藏高原东南部地区由于印度板块的挤压力作用,在青藏高原地壳内出现下地壳流,造成了青藏高原的逐步抬升。另一方面来自于印度板块俯冲到欧亚板块中的密度大物质在重力作用下与岩石圈上部发生了部分拆沉现象,破坏了来自于岩石圈底部重物的牵引力作用,形成了岩石圈拆沉机制,从而使得岩石圈以上的地层被抬升。由此可知,青藏高原的地表抬升是多阶段非均匀、不等速过程,包括有下地壳流和岩石圈部分拆沉等多种动力学机制联合作用的产物。更多还原

【Abstract】 Abstract:The large amount of surface uplift, deformation and strike-slip faulting around the southeast of Tibetan plateau margin is generally believed to be associated with the Cenozoic collision between Indian and Eurasian. However, details on the linkage between the surface deformation and the collision are not well understood, largely due to the lack of observations on subsurface deformation, especially in the deep crust and upper mantle. Studying the dynamics processes involoved in the deformation is one of the highlighted fields both in seismology and geodynamic field.We analyze a large amount of receiver function data recorded by regional seismic networks of the China Earthquake Administration (CEA), specifically, the provincial networks of the Sichuan, Yunnan, Guangxi, Guizhou, Guangdong and Hunan provinces. These data come from earthquakes occurring between July of2007and July of2010with broadband stations. We develop a comprehensive analysis method that facilitates the robust extraction of azimuthally seismic anisotropy from receiver function data. The method includes an estimate of fast polarization direction and splitting time by a joint analysis of radial and transverse receiver function data, and an evaluation of measurement reliability by statistical and harmonic analysis. We find significant crustal seismic anisotropy with a splitting time of0.24-0.9s beneath the SE margin of the Tibetan plateau, and the Moho depth and Vp/Vs become weak from the SE Tibet plateau margin to southeast. These results shed new light on the deformation process of the Tibetan plateau and provide strong evidence for a lower crust flow beneath the SE margin of the Tibetan plateau. Both the splitting time and fast polarization direction are comparable to those estimated from SKS/SKKS data, suggesting that crustal anisotropy is the main cause of shear wave splitting of the SKS/SKKS wave. Mantle deformations are either weak or predominantly vertical and are obviously different from those seen in the crust. A vertically flow in the upper mantle, combined with the observation of a thin lithosphere beneath the area, leads to the inference that part of the mantle lithosphere may have delaminated and is descending into the deep mantle. Stations located in the surrounding areas, on the other hand, exhibit very little to no crustal anisotropy. The estimated Moho depth and Vp/Vs ratio also show a distinct difference between the Tibetan plateau and the surrounding regions. These observations indicate the existence of a lower crustal flow may be present beneath the SE Tibetan plateau, and that the mantle lithosphere may have been mechanically decoupled from the upper crustal motions.In order to understand lithosphere deformation process associated with the uplift of the plateau and the relationship between the lower crust deformation and the lithosphere. We apply the finite frequency tomography method to the S waves data with differential travel times between pairs of stations in the inversion to eliminate traveltime anomalies resulting from heterogeneities outside the study area. We choose three different frequencies for ajoint inversion, there are0.1-0.5hz,0.05-0.1hz,0.02-0.05hz. To ensure the above assumption to be valid for a large-scale study area, we improve in selecting proper station pairs base on the traditional method. The stations loop in different events are designed and kept the surface distance between the neighboring stations pairs in loops within the shortest is larger than200km,300km,400km, respectively. We then conduct a joint inversion with three differential time data. Our results are consistent with previous tomography in terms of large-scale seismic anomalies, such as a high velocity anomaly beneath the Sichuan basin in the uppermost mantle and a high velocity anomaly in the transition zone that may be associated with the subducted Paleo-Pacific plate beneath the Yangtze craton. In addition to these known structures, we find relatively large-scale high velocity bodies inside the upper mantle beneath the SE margin of the Tibetan Plateau, along the longitude101°from the south to north. In particular, our images show continued high velocity bodies at～100km deep beneath latitude27°N, and further to the north, the latter high velocity body around32°N located at～350km, right below a large low velocity anomaly. Although other seismic observations are required to better constrain the nature of these high velocity structure, one possible scenario is that they may be drips or delaminated pieces of the continental lithosphere, as the consequence of the progressive uplift of the plateau.With the upwards systemic studies, we can get a comprehensive understanding the Southeast of Tibetan plateau. After the Indian-Eurasian collision, the lower crust flow beneath the SE Tibetan plateau margin was caused by the extrusion of Indian plate and the deformation of Eurasian crust. With this lower crust flow working, the Tibetan plateau is uplift. On the other hand, high density of the lower lithosphere detached from the upper lithosphere under downward pulling of gravity, which destroys the negative buoyancy, and then the surface of Tibetan plateau is uplifted with the positive buoyancy. In summary, uplift of the southeast borderland of the Tibetan Plateau was thought to have intensified since the late Tertiary as the whole and local deformation associated with the Indian-Eurasian collision accelerated in the region. Therefore, the uplift of Tibetan plateau is the result of joint action with multiple mechanisms, and multi-stage process with non-uniform and different velocity.更多还原