桂东北地区岩石圈导电性结构研究

Tectonically, the northeast region of Guilin is located at the west part the Nanling metallogenic belt which also coincides with the Yangtze plate and the South China folded zone. Due to the distinct tectonic and geography settings, the mineralization in this area has been studied for a long time. The region has undergone several tectonic superposition, transforming displacement, and magma intrusion since the Proterozoic age. There exist several different perspectives for the basic geology in this area, such as the location and of the strike of the plate junction between Yangtze and Huaxia plate, due to the complex tectonic settings. The dynamic mechanics, forces and the tectonic evolution for the region is also controversial. In this project, we propose to construct a 3D high resolution conductivity distribution for the crust and mantle beneath the northeast region Guilin based on the broadband magnetotelluric array data and regional geological information. The forward and inversion of magnetotelluric data will be solved using parallelized 3D finite element method with domain decomposition. We expect that the 3D magnetotelluric data can provide a constraint for the position of the Yangtze-Huaxia plate junction. The 3D magnetotelluric inversion results can also be used to analyze the distribution of rigid bodies in Yangtze and Huaxia paleosome, and also to explain the dynamic mechanism collision orogeny between these two plates. The conductivity distribution recovered from 3D magnetotelluric inversion can reveal the deep tectonic attribute of the Nanling and Qin-Hang metallogenic belt and its relationship with the mineralization. Moreover, it can provide scientific evidence for the tectonic division and the second space mineral prospecting.

在大地构造位置上，桂东北地区位于南岭成矿带的西段，扬子地块与华南褶皱带的过渡部位，具有独特的构造地理位置，一直是华南大地构造及其成矿作用研究的重点地区。自元古宙以来，桂东北地区历遭多期构造叠覆与改造变位以及各时期岩浆侵入破坏，使得该地区的许多基础地质问题一直存有不同观点，尤其是关于扬子、华夏板块结合带位置及走向、碰撞造山的动力学机制和驱动力、构造演化过程，等等。本项目拟利用宽频带大地电磁阵列观测数据，采用适合复杂地质条件下的区域分解并行混合有限元三维正反演算法，结合区域地质研究，建立桂东北地区壳、幔深部导电性三维精细结构，对扬子-华夏板块拼合带（钦-杭结合带）的西延位置提供重要限定，厘定扬子、华夏残剩古板块上地幔刚性块体的空间分布形态，探索两板块碰撞造山的动力学模式，揭示南岭成矿带和钦-杭成矿带的深部构造属性及其与成矿作用的关系，为华南大地构造划分、实现第二空间找矿突破提供一定的科学依据。

桂东北地区位于南岭成矿带的西段，扬子地块与华南褶皱带的过渡部位，具有独特的构造地理位置，一直是华南大地构造及其成矿作用研究的重点地区。利用大地电磁测深对桂东北地区岩石圈电性结构进行探测，可以厘清扬子、华夏板块结合带位置及走向，探究碰撞造山的动力学机制和驱动力、构造演化过程等基础地质问题。项目组系统开展理论研究、实地测量和综合解译工作。从频率域Maxwell方程出发，推导了电磁场耦合微分方程，研究有限单元法计算区域的外边界条件，通过加权余量法建立变分问题；研究基于非结构化三角形网格剖分的有限单元法数值解格式，分析有限单元法中稀疏系数矩阵存储技术，实现了基于非结构化网格剖分的大地电磁二维正演；针对大地电磁反演的不适定性，引入Tikhonov的正则化方法，采用主动约束平衡方法（ACB）确定正则化因子，实现了带地形的大地电磁二维反演；研究矢量有限单元法，考虑各向异性介质中的边值问题，实现了各向同性和任意各向同性介质中基于矩形块单元的矢量有限单元法大地电磁三维数值模拟。采集桂东北地区大地电磁数据，严格按照规程操作，获得了可靠的原始数据。大地电磁数据经处理获得三维反演结果；通过综合研究，给出了研究区深部电性结构特征和地质地球物理解译。