摘要
西太平洋俯冲带是世界上最典型、俯冲距离最远的俯冲带,太平洋板块、菲律宾海板块在此与欧亚板块相互作用,以俯冲为主兼有碰撞,影响着板块边缘及板块内部的动力过程、应力场特征及构造运动。特别是在我国东北地区的地幔过渡带(410-660km),地震层析成像结果发现了异常高速带,呈沿近水平方向,形似“舌”状结构的停滞板片,所以,这一地区成为地学家们研究地幔动力学及全球构造的重点地区。我国东北华北位于西太平洋板块消减带弧后区,属于地震和第四纪火山活动区,该区的地质构造、盆地的形成和演化以及固体矿产和能源资源分布与西太平洋俯冲带有着极为密切的联系。因此,研究东北和华北地块及其过渡带的深部电性结构,对于认识板块俯冲带弧后区东北和华北地区的构造演化、裂谷形成、岩石圈裂解和减薄等动力学问题,以及了解地震和火山活动和阴山成矿带的深部构造背景等有着重要的意义。为了查明我国东北、华北及其过渡地区的岩石圈-地幔过渡带结构特征,区内构造块体之间的深层耦合关系,在国家自然科学基金项目“西太平洋俯冲带弧后区东北和华北地块边界带岩石圈-地幔过渡带电性结构研究”支持下,设计了一条穿越东北华北地块长约1150km的内蒙古苏尼特左旗-辽宁丹东大地电磁测深剖面,开展了常规、长周期和超长周期的大地电磁探测工作。本论文主要利用常规MT与长周期MT数据,对苏尼特左旗-丹东剖面展开深部电性结构研究。为了获得研究区域可靠的地壳上地幔电性结构,作者采用了先进的大地电磁数据处理分析技术,对观测资料进行了由定性到定量全面地分析,通过二维非线性共轭梯度反演得到了沿剖面的较为详细的地壳上地幔电性结构。最后结合其它地质和地球物理资料的分析,对得到的地电模型进行了解释。论文主要研究内容和取得成果概述如下:1.大地电磁测深理论基础与应用的归纳和总结简要概况了大地电磁测深的原理,基于大地电磁测深中的两个假设,从麦克斯韦方程组出发,详细推导了视电阻率、阻抗相位的计算表达式;介绍了二维大地电磁测深的特点,对二维电性介质中电磁场的结构特征进行了讨论,详细分析了TE、TM两种极化模式。总结了大地电磁测深法在各个领域应用的最新成果,主要包括大地电磁法在中国大陆主要地块或盆地区、大陆造山带、强震活动区的深部构造探测、近代火山区或新生代裂谷区、能源、矿产资源探测等方面中的应用成果。2.大地电磁测深数据处理方法与反演方法的概述介绍了大地电磁测深法数据处理的流程,包括对时间域的电场和磁场信号进行频谱分析,求出相应频谱信息;计算测量方位所对应的阻抗张量元素;判断地下介质的电性主轴;计算主轴方位上的阻抗张量元素和视电阻率等。对数据处理过程中应用的先进方法进行了讨论,包括远参考道技术,Robust技术估算阻抗张量,Swift、Bahr、GB等阻抗张量分解技术。同时对大地电磁测深法中常用的二维反演方法(OCCAM、RRI、NLCG)进行了比较。3.苏尼特左旗-丹东剖面的大地电磁数据采集与数据分析综合分析研究地质与地球物理的资料,经过野外踏勘,制定了野外测点的布置方案,严格按照野外操作规范完成了野外数据采集工作。对沿剖面的36个常规MT点、12个长周期MT点的原始时间序列数据,利用SSMT2000、MTEDITOR软件进行处理,最终得到各测点的视电阻率与阻抗相位曲线。通过对视电阻率与阻抗、区域的二维偏离度等进行定性分析,基本了解了研究区域内电性结构的基本变化情况。分析发现绝大部分数据具有二维构造的特征,可以采用二维反演进行近似解释,同时分析了区域的主轴方位角,结合区域地质构造走向,确定区域电性构造走向基本与测线垂直。将数据直接旋转到测线方向,此时XY模式对应TE极化模式,YX模式对应TM极化模式。选择二维非性共轭梯度反演作为本剖面的反演解释方法。4.二维反演参数的选择在二维非线性共轭梯度反演中,对反演数据采用不同极化模式,正则化因子的选择、误差门槛值的选取进行了测试,本文对三个问题分别进行了讨论。通过取不同的正则化因子值进行多次反演,进行正则化因子L曲线分析,确定出最佳的正则化因子,同时选择不同的误差门槛值,进行多次反演,通过分析反演模型结果以及数据拟合情况,确定最合适的误差门槛值。采用TE、TM、TE TM三种极化模式数据分别进行反演,比较了不同极化模式数据的反演结果,发现TM模式对表层结构敏感,而TE模式对深部结构更敏感,TM模式对三维高导异常体的响应更加稳定,而TE模式对高阻异常体的响应更加稳定。最终选择对本剖面进行TM极化模式数据反演。5.苏尼特左旗-丹东剖面的带地形二维反演经过各种测试后,利用测点中心网格的自动生成技术构建带地形网格,采用TM模式进行二维反演。初始模型选用均匀半空间,正则化因子取50,TM门槛误差为2%,经过116次迭代,反演模型的总拟合均方差为4.41,最终得到了研究区域的地电模型。6.苏尼特左旗-丹东剖面深部电性结构的解释对苏尼特左旗-丹东剖面的二维电性结构进行解释,确定了主要断裂和边界带的位置以及深部延伸情况,同时确定了壳内高导层的分布位置,上地幔高导层顶界面的深度。壳内高导层的分布、上地幔高导层顶界面的起伏状况反映了研究区域内大地构造的性质和演化历史,从电性结构上,给出西太平洋俯冲带对东北、华北地块的影响的依据。
The subduction zone in the Western Pacific plate is the most typical, having the largest length of the subducted slab in the world, where intensive interaction occurs between the Pacific Plate, the Philippines sea plate and the Eurasian Plate. It poses great impact on the plate margins as well as dynamic processes, stress fields and tectonic motions in the Eurasian continent. One of the signatures of this slab is the abnormal high-velocity zone at the mantle transition zone (410-660km in depth) beneath Northeast China revealed by seismic tomography, where it looks like a horizontal“tongue”, and seems to be stagnant. Consequently, this area becomes a focused target in the research of mantle geodynamics and global tectonics.Northeast China and North China lie in the back-arc area with earthquakes and Quaternary volcanoes. Their geological structures, basin formation and evolution, and distribution of solid mineral and oil-gas resources are closely related to the West Pacific subduction zone. Therefore,the study on the deep electric conductivity structure of the Northeast in china is very important for understanding of the dynamics of these regions.In order to identify the structural features of the lithosphere-mantle transition zone and the deep coupling between the tectonic blocks at depth in Northeast and North China, a magnetotelluric (MT) profile is designed with the length about 1150km along Sunid Zuoqi to Dandong for long-period and ultralong-period MT sounding.This thesis focuses on the deep electrical structure of the profile from Sunid Zuoqi to Dandong by using normal MT data and long-period MT data. The advanced data processing and analysis technology of MT are employed and the qualitative and quantitative data from field surveys are analyzed to acquire the reliable electrical structure in crust and upper mantle. Through the data inversion using the 2D Nonlinear Conjugate Gradient (NLCG) method, a detailed electric model is yielded. At last, the interpretation of the electrical model is performed considering the geology and other geophysical data. The main contents and results are as follows:1. On foundation and application of MT theoryThe MT theory is briefly presented and the equations of apparent resistivity and phase are deduced from Maxwell equations based on the two hypothesis in MT. Also the main features of 2D electromagnetic sounding are introduced and the discussion of 2D electrical structure is conducted. The TM and TE mode are finally discussed.2. Overview of data processing and inversion of MT dataThe flow of data processing in MT is introduced including spectral analysis to the electric and magnetic fields in the time domain, yielding the corresponding spectral information. Then we compute the impedance tensor elements in survey direction, determine the electrical main-spindle of underground medium and its apparent resistivity.The advanced method of data processing has been discussed including Robust remote reference technology in estimating impedance tensors, and Swift, Bahr and GB impedance tensor decomposition technology. The comparison of 2D normal inversion methods (OCCAM, RRI and NLCG) is made at the same time.3. Acquisition and analysis of data from the Sunid Zuoqi–Dandong profileThe survey plan is carried out through field survey and consideration of other information. And acquisition of the field data follows strictly the standard codes for field measurement. The original data in the time series of 48 MT sites including 12 long-period sites and 36 broadband sites on the profile is processed by the SSMT2000 software with Robust remote reference function and MTEDITOR software to edit power spectrum. Finally we obtain the date of apparent resistivity and impedance phase. The primary lateral changes of the electrical conductivity in the target zone are analysed based on the resistivity and phase date. The dimensional feature of the structure is determined by skewness of the target with the Swift method. Meanwhile, We analyze the azimuth of the main axis, and combining the fault’s strike of the region, It is concluded that the strike of the regional electric structure is roughly perpendicular to the survey line. So the data is rotated directly to the survey orientation. In this case the XY mode corresponds to the TE mode, while the YX mode corresponds to the TM mode. The 2D NLCG method is chosen for inversion and interpretation.4. Choice of parameters in 2D inversionIn this thesis, three issues on 2D NLCG inversion are discussed separately: varied polarization modes of data inversion, choice of the regularization factors and the t value of floor errors. The analysis of regularization factor L’s curves lead to the best factor through inversions of different regularization factors, and We choose the best threshold value of errors in the same way.The 2D inversions by using of TE mode, TM mode,TE and TM joint mode are conducted, respectively. It was found that the TM mode is sensitive to the shallow structure, while the TE mode is sensitive to the deep structure. And TM mode is stable to the 3D low resistivity target, while the TE mode is stable to the 3D high-resistivity target. Finally,the TM mode to fulfill the inversion of the profile was choosed.5. Inversion of the Sunid Zuoqi -Dandong profile with topographyThe 2D initial model with topography is produced using Automatic Generating Technique for the grid of central measurement points and TM mode. The regularization factor is set to be 50, and the threshold value of errors in TM mode is 2%. The ultimate model of the target zone is obtained after 116 iteration. And the RMS value is 4.41.6. Interpretation of the Sunid Zuoqi–Dandong profileBased on the final inversion model of the target profile, I primarily determined the location of main faults and boundaries and their extension to depth. At the same time, the location of the low-resistivity layer in crust and the depth of the high-resistivity layer in upper mantle are inferred from the result. The distribution of low resistivity layers in crust and the relief of high resistivity layers in upper mantle may reflect the property and evolution history of tectonics. The deep electrical structure derived from this work may provide evidence to elucidate the influence of the subduction zone in the Western Pacific plate on Northeast and North China.