【作者】 刘峡；

【导师】 傅容珊；

【作者基本信息】 中国科学技术大学 ， 固体地球物理学， 2007， 博士

【摘要】 华北地区位于我国北部，阴山、燕山以南，秦岭、大别山以北、鄂尔多斯高原以东延伸至沿海一带的广袤区域，地理坐标范围为东经112°～124°，北纬31°～42°，是我国政治、经济、文化中心。同时，该区域囿于太平洋板块、菲律宾板块和印度板块之间，新生代以来构造运动活跃，为我国大陆强地震频发区域之一。因此，研究该区域现今地壳运动及形变、构造应力场特征，继而探讨其动力学机理，具有重要理论和现实意义。本文采用有限元方法在大型有限元软件平台ANSYS基础上，综合现有地质、地球物理资料构建华北地区岩石层（地壳和上地幔顶部）框架。以区域GPS观测、地形变观测以及震源机制解结果作为约束条件及检验标准，通过数值模拟的方式，研究该区域现今地壳运动和形变、构造应力场特征，并将表层运动与深部运动相联系，以区域动力学环境和驱动力为中心，探讨其动力学机制。根据华北地区新生代构造发育特征和深部地球物理观测数据，本文选取了108°E～127°E，32°N～42°N矩形区域构建有限单元数值模拟的几何框架。与此同时，还将现今活动断层及断裂带分布、地形起伏以及地壳—上地幔速度结构等纳入模型中。本文构建和测试了三个基本模型：即三维模型3DCM_Ⅰ，3DCM_Ⅱ利二维模型2DDM_Ⅰ。模型3DCM_Ⅰ不考虑地壳—上地幔的实际分层，将模型沿纵向分为七个厚度均匀的水平层，分别代表上、中、下地壳和上地幔（四层）。模型3DCM_Ⅱ则依据该区域深反射地震研究结果进行水平分层。在模型3DCM_Ⅰ、3DCM_Ⅱ中，区域内主要活动断裂带处理为宽度5—6Km，深度不等（10～15km）的软弱带，模型含14736个节点，24829个三维实体单元。在模型2DDM_Ⅰ中断层及断裂带则处理为非连续接触边界，模型含1843个节点，3547个二维实体单元。论文采用有限元弹性静力分析方法，以模型区周边GPS观测作为边界约束，计算华北岩石层形变的水平速度场和应力、应变增量场。并以模型区内部GPS观测作为标准，通过调整模型中断层的物理参数方式，寻求与GPS观测的最佳吻合的有限元解。计算显示，水平运动场的最佳数值解和GPS观测值的平均离散度为10171mm²/year（三维模型）、1.176mm²/year（二维模型）。与此同时，数值模拟预测的区内主要活动断层的错动方式、水平错动速率（三维模型结果为～0.1mm/year，二维模型结果为～0.3mm/year）与地质及跨断层形变观测结果基本一致。三维模型还显示，在10～15Km深度上，最大、最小主应力轴接近水平，主张应力为主压应力的2～8倍，主张应力方向为NNW，主压应力方向为NEE。这一结果与震源机制解、地应力观测结果比较吻合。三维模型得到的全区应变率较低约10^-9/year。并且，大致沿汾渭地堑断裂系、唐山—河间—磁县断裂带和郯庐断裂带存在三个北东走向的剪应变梯度带。不过二维模型预测的张家口—渤海断裂带水平运动速度为1.35～1.45mm/year，远高于其他断层以及三维模型结果。数值模拟结果可以推论：华北地区岩石层水平形变运动主要受控于周边大型构造块体的相对运动，其次才是岩石横向、纵向非均匀性的影响。同时，在周边块体运动的控制下，区内主要断裂带现今的错动方式基本是继承性的，且错动速率较低。只有张家口—渤海断裂带错动速度较高，值得关注。论文考虑岩石应力—应变遵从幂指数本构关系，基于三维模型3DCM_Ⅰ，3DCM_Ⅱ的基本框架，采用弹性—蠕变静力学有限元方法对华北地区晚近时期（～4Ma）构造运动进行数值模拟，探讨华北地区现今地壳运动演化的动力学环境和驱动力因素。模拟时以模型周边GPS观测数据作为表层的边界约束，计算时顾及岩石的流变性、重力作用以及大变形导致的位移—应变非线性。本文通过选择侧面、底面边界约束方式，调整岩石的物性参以及分析板块运动、周边块体运动、断层活动、岩石层流变分层等因素的影响，寻求与区内GPS观测、构造应力场观测数据最佳符合的预测模型。论文计算了22个模型。结果表明，当侧面以深部稍快、浅部稍慢的方式运动，并考虑岩石层下部的拖曳运动时，则模拟预测果更接近实际观测。在该约束方式下，相应于模型3DCM_Ⅰ、3DCM_Ⅱ框架的预测与GPS观测的最低平均离散度分别为1.1529mm²/year、1.1451mm²/year，低于弹性模型，而且断层错动与地质研究结果相一致。在相同边界条件下，3DCM_Ⅱ结果优于3DCM_Ⅰ结果，考虑断层运动结果优于不考虑断层运动结果。模型预测应力场显示，深度10Km处的最小主压应力轴均接近水平，方向为NNW向，最大主压应力轴在南部区域接近水平，在北部区域则垂直于水平面，方向以NEE为主。这与其它研究显示的最大、最小应力主轴的方向基本吻合，但与现有的最大、最小主应力轴均接近水平的基本认识存在一定差别。预测应力场随深度的变化明显受控于边界加载方式（应力环境），而断层的影响仅局限于断层内部以及断层附近较小区域内。为研究局部特别是隐伏构造对地表形变的影响，提取形变异常信息，论文发展了GPS差异形变分析方法。并用此方法，对华北地区和中国大陆GPS观测结果进行了GPS差异形变分析。华北地区GPS差异形变分析显示，张家口—渤海断裂带、唐山—河间—磁县断裂带为该地区现今构造活动较强烈区域，在未来若干年内需密切关注及防范强震发生。对中国大陆而言，论文分别使用三种有限元模型（均一、分块和多驱动力模型）而进行了GPS差异形变分析。结果表明，中国大陆现今构造运动是多种驱动力共同作用的结果。其中，印度板块向北的强烈推挤以及地幔对流对岩石层底部的拖曳作用占据着非常重要的地位。前者主导了以青藏高原为中心的大陆西部地区构造变形运动，而后者则对华北、华南地区的影响重大。同时，现今构造运动十分活跃大型断裂系（如阿尔金、张家口—渤海等）对大陆岩石层构造变形的影响也是不可忽视的。综合数值模拟结果可以得到如下认识：华北地区岩石层形变的区域动力学环境和驱动力十分复杂。总体而言，一方面在太平洋板块的俯冲和印度—欧亚板块的碰撞挤压作用下，鄂尔多斯活动地块、华南活动地块以及东北亚活动地块的运动状态决定了华北地区表层运动的基本格局。另一方面，地幔对流对岩石层底部的拖曳作用将直接影响该区域岩石层形变运动。与此同时，以张家口—渤海断裂为代表的现今构造运动十分活跃大型断裂系以及岩石层内流变性非均匀分布（特别是中地壳软弱层的存在），对区域构造变形的影响也是不可忽视的。更多还原

【Abstract】 North China Region is situated in the north of the China main land, bounded by the Ordos Plateau in the west, the East Coast in the east, Yin Shan Mountain and Yan Shan Mouthtain in the north, and Qin Ling Mountain and Dabie Shan Mountain in the south. Its longitude and latitude range is 112° -124° E, 31°～42° N. This region is the economical, cultural and political center of China. In addition, located among Pacific Plate, Philippine Plate and Indian Plate, the tectonic activity of this region is remarkable during Cenozoic. It’s one. of the areas where strong earthquakes frequently occur in China. Studying the crustal movement and deformation pattern, the feature of the constructional stress field, and its dynamic mechanics of this region is of great theoretical and practical importance.Based on Finite element method and ANSYS software, the thesis establishes several lithosphere modeling frames （crust-up mental） of North China region by incorporating existing geological and geophysical information of this region. Using digital simulation, and taking GPS observation, crustal deformation observation and focal mechanism results as basic boundary condition and testing standard, the thesis studies the present-day crustal movement, the strain and stress field of this region. And by connecting the surface movement with its deep tectonic movement, the thesis discusses the dynamic mechanics of the region, focusing on its dynamic setting and the driving force.According to the Cenozoic tectonic feature and deep geophysical observation, the author choose a rectangular region of 112° -124° E and 31° -42° N to establish the geometrical frame of the finite element models. In addition, present-day active faults distributing, topography and crust-up mantle structure of this region is included. The author creates and tests three finite element models, 3D model, 3DCM_I,3DCM_II, and 2D model, 2DDM_I. Ignoring the real crust-up mantle stratum structure of this region, Model 3DCM_I contains 7 layers with each layer being of equal thickness, including upper crust, mid-crust, lower crust, and up mantle（4 layers）. The layering of Model 3DCM_II is consistent with the deep seismic sounding results. In 3DCM_I / 3DCM_II , faults are treated as weak zones of 5-6 km wide and 10-15km deep. Each model contains 14736 nodes and 24829 3D solid elements. In Model 2DDM_I faults are treated as discontinuous contact boundary. And it contains 1843 nodes and 3547 2D solid elements.Using static structural finite element analysis method, taking GPS observations around the model area as boundary condition, the horizontal velocity field of lithosphere deformation, stress increment field, and strain increment field are calculated. Using GPS observation within the model area as standard, the optimum solution which fits the GPS observation best is searched by adjusting the parameter of fault elements in those models. The results show that the average discreteness between the optimum calculations and GPS observations is 1.171mm²/year （3D model） and 1.176 mm²/year （2D model）. In addition, the predicted slip features and slip velocities of main faults（ ～0.1mm/year in 3D model results, ～0.3mm/year in 2D model results） are consistent with geological observations and deformation survey across those faults. In 3D models calculation, the stress field in depth of 10～15 Km shows that the maximum and the least principal stress axis is nearly horizontal, the principal tensile stress is about 2～8 times of the principal compressive stress, and their orientations are NNW and NEE, respectively, which are consistent with focal mechanism results and field stress survey. The calculated strain rate in the whole region is rather low, ～10^-9/year, which is in accord with former study. Besides, there are three north-east trend high gradient shear strain zones along Shanxi fault zone, Tangshan-Hejian-Cixian fault zone and Tanlu fault zone, respectively. However, in 2D result, the predicted horizontal slip velocity of Zhangjiako-Bohai fault zone is 1.35～ 1.45 mm/year, more higher than that of other faults and 3D model results. 2D model and 3D model calculations imply that the present-day horizontal lithosphere deformation movement pattern in North China region is controlled by the movement of large tectonic blocks surrounding the region. And the influence from the vertical and lateral heterogeneity of lithosphere is secondary. At the same time, Controlled by large tectonic blocks around, the slip features of the main faults within the region inherit their fashions of geological time scale and assume rather low slip velocity, except Zhangjiakou-bohai fault zone which needs to pay more concern.Based on 3D model （3DCM_I/3DCM_II）, taking power law constitutive relation of rock into account, and using elastic-creep static structural finite element analysis method, the tectonic process during later geological era （～4Ma） is simulated to study the dynamic environment and driving forces of the present-day crustal movement evolvement in North China. In the calculation, the GPS observations surrounding the model area are taken as boundary condition, and the rheology of lithosphere, gravity factor, displace-strain nonlinearity induced by large deformation are also considered. Through choosing boundary conditioning for side faces and bottom of model, together with adjusting rheologic parameters of model element, the influence factors such as the plate tectonics, surrounding block movement, faults activity, and the lithosphere heterogeneity are analyzed to find the most plausible calculation that fits GPS observations and tectonic stress information within the region. The thesis gives 22 model calculations. The results shows that when the nodes on upper side faces of the models move little faster than that of the lower side faces, coupled with the drag movement on the model bottom, the simulation brings its better accordant with observation. Under such conditioning, the lowest average discreteness between the predicted surface velocities and GPS observations is 1.1529mm²/year （model 3DCM_I）, 1.1451mm²/year （model 3DCM_II）, respectively. It’s lower than elastic structure finite element analysis result. And the predicted fault slipping rates is consistent with geological information. Under the same boundary conditioning, 3DCM_I model result is better than 3DCM_I model and the simulation including fault slipping are better than the opposite one. The predicted stress field in the depth of 10-15km shows that the least compressive principal stress axis is NNW trend, nearly horizontal. The maximum compressive principal stress axis is NEE trend, nearly horizontal in south area and nearly vertical in north area. The orientation of the predicted principal stress axis is accord with existing information. However, the state of the predicted stress axis is different with the existing cognition to some extent, which favors the maximum and least principal stress axis all being horizontal. The predicted stress field variation feature with depth is obviously controlled by boundary conditioning （ dynamic setting ）. The influence of faults on the predicted stress field only focuses on local area or within fault itself.In order to study the surface deformation influenced by local construction movement, especially by the activity of blind faults, finding the method to obtain such movement message, the thesis develops GPS differential deformation analysis method. Using this method, the GPS observations in North China region and China Continent are analyzed, respectively. The differential velocity field between the simulation and GPS observations in North China shows that Zhangjiako- Bohai fault zone and Tangshan-Hejian-Cixian fault zone are recent intensively active area, and need closely observe against strong earthquake in future. As for China continent, three models （homogenous model, block model and multi-driving model） are tested for GPS differential deformation analysis. The results show that the present-day tectonic movement in China continent is controlled by multi-driving forces. Among them, the northward indention of India plate and the mantle convection induced dragging force on the bottom of lithosphere play important roles. The former determines the tectonic fashion and surface movement pattern in west continent including Tibet and surrounding region. The latter exerts significant influence on east continent including North China region and South China block. In addition, the influence from the main fault zones, such as Altyh fault zone and Zhangjiakou-Bohai fault, exert their influence on the local surface horizontal movement and should not be ignored.Assembling the digital simulations above, we can draw the conclusion as follows. The regional dynamic environment and driving forces of lithosphere deformation in North China is very completed. In one hand, under the subduction of Pacific slab and the northward indention of India plate, the movement of Ordos active block, South China active block and East-North active block controls the surface movement pattern of North China region. In another hand, mantle convection process may directly influence the surface deformation movement of the region. At the same time, the influence from present-day large active fault systems such as Zhangj iakou-Bohai fault and lithosphere heterogeneity, especially the existent of mid-crustal shear zone, should not be ignored.更多还原