羌塘地块地壳电性各向异性结构研究

Tibetan Plateau exists many large-scale discontinuously distributed low-velocity and high-conductivity layers, with complicated geological structures, material compositions, rheology and deep geophysical features. Qiangtang Terrane is located in the north-central of Tibetan Plateau. In the middle and upper crust of this region, there are widespread discontinuous lateral-distributed intracrustal high-conductivity layers with various thicknesses, which show obviously electrical anisotropy features. .The magnetotelluric sounding can be utilized to study the electrical structure and the heterogeneity of lithosphere. It can also be used to detect high-conductivity layers of the molten magma within the crust and upper mantle. As the electrical anisotropy is closely related to the geodynamic processes, the study on the electrical anisotropy of Qiangtang Terrane shows great significance to understand the geological structure and the geodynamic processes of the deep Earth. .This study will adopt the improved Non-Linear Conjugate Gradient Method and Finite Element algorithm to perform anisotropic two-dimensional inversion of magnetotelluric data in the Qiangtang Terrane. And the two-dimensional high-resolution crustal structure of electrical anisotropy in the northern Tibetan Plateau will be built comprehensively based on the inversion results. The study will also integrate the research results of the two- dimensional and three-dimensional isotropic inversion results and the findings of seismic anisotropy to high-precisely determine the anisotropic features of low-velocity and high-conductivity layers. The discussion will be paid particular emphasis on the relationship between the tectonic process and the anisotropy. The geological interpretation will be completed afterwards to reveal deep dynamic features in the northern Tibetan Plateau. Furthermore, this study will contribute to a better understanding of the dynamic model and tectonic process of Tibetan Plateau.

青藏高原存在许多大规模低速高导体的不连续分布，具有十分复杂的地质结构、物质组成、流变学和深部地球物理特征。羌塘地块位于青藏高原中北部，该地区在中上地壳普遍存在高导层，且厚度不一，横向分布不连续，表现出明显的电性各向异性特征。.大地电磁测深法能研究岩石圈的电性结构及其不均匀性，并探测地壳和上地幔范围熔融态低阻体。由于电性各向异性与地球动力学过程密切相关，研究羌塘地块电性各向异性问题对了解该区地球深部地质构造和动力过程有重要意义。.本研究拟采用改进的非线性共轭梯度法和有限元算法，对羌塘地块大地电磁数据进行二维电性各向异性反演解译，全方位构造藏北高分辨率二维地壳电性各向异性结构，并整合二、三维电性各向同性反演结果及地震各向异性等研究成果，确定低速高导体各向异性特征，探讨地质构造演化与电性各向异性的关系，从而揭示藏北深部动力学特征和构造演化规律，为阐明青藏高原动力学模式和构造过程奠定基础。

电性各向异性，即介质电导率随方位变化的现象，被证实广泛存在于地球岩石圈内。大地电磁法通过测量地球表面天然电磁场的变化量，能探测地球深部电性结构，可用于研究地下介质电性各向异性特征。本项目使用自适应非结构有限元法来求解对角各向异性情况下大地电磁正演控制方程,并构建了包含模型结构惩罚项和各向异性惩罚项的反演目标函数，反演算法采取经典模型空间Occam策略，模型搜索过程稳定，能在较少迭代次数内获得合理反演结果。.项目采用NLCG法对羌塘地块5条大地电磁剖面进行了各向同性反演，结合研究区已有的地质和地球物理资料，发现羌塘地块具有南北两分的特征。南羌塘地块存在壳内双层高导层且自羌中隆起附近向南倾斜；北羌塘地块之下高导层形态似勺子状，由金沙江缝合带之下靠近地表深度向南倾斜，在北羌塘中部下延至上地幔，并开始逐渐向上延伸，在羌中隆起附近到达近地表深度。南、北羌塘的壳内高导层均在羌中隆起附近向地表延伸，形成南北对冲的异常形态。推断在三叠纪时期，松潘-甘孜混杂岩沿金沙江缝合带向南俯冲，破坏并替代了几乎整个羌塘地块的地壳结构。羌塘地块壳内富水混杂岩有利于含水熔融的发生，会导致班公湖-怒江缝合带两侧明显的地球物理差异。壳内高导体在班公湖-怒江缝合带以南30至40 km位置规模最大，具有向上地幔延伸的趋势。卫星重磁数据表明羌塘地块中部双湖地堑具有由地表向下地壳延伸的趋势。推测羌塘地块中部的双湖地堑是由于高原隆升至最大高度后，增厚的岩石圈地幔发生重力垮塌，进而形成了该南北向断裂。.最后对三条南北走向大地电磁测线进行了各向异性处理解释。结果表明羌塘地块地下介质电性分层明显，上地壳存在延展广的高阻层，中下地壳普遍发育高导层，其中南、北羌塘地块下方均发育了位置相对固定的高导体，彼此被羌中隆起区的相对高阻区域分隔。南、北羌塘地块下方高导体沿东西构造走向电阻率相较于沿南北测线走向电阻率小很多。中下地壳高导体与部分熔融有关，高导体区域的各向异性特征佐证了物质向东运移的可能性。羌塘地块中部不断上涌的幔源岩浆为维持羌塘地块不变的海拔高度和不断东流的地壳流之间的平衡提供物质支持。