东海陆架-冲绳海槽的壳幔电性结构及其构造意义

The marine magnetotelluric (MT) method is a technology using the natural electromagnetic (EM) wave as source to determine the conductivity structure beneath the seafoor. The electrical and magnetic recievers deployed in the seafoor are used to detect and record the EM signals reflected from the structure beneath the seafloor. Those EM signals are then inverted to infer the electrical conductivity structures beneath the seafloor. The electrical conductivity structures are associated with some important physical parameters of rocks such as porosity, temperature and fluid composition etc. It is also related to many geodynamics processes such as the plate coupling in subduction zone, the fluid release and evolution, the arc-magma mode, and the partial melting and serpentine in the mantle etc. The East China Sea Shelf-Okinawa Trough area is an important part of the tectonic belt of the pacific and is also an very important window to explore the tectonic activity, formation and evolution mechanism of the western pacific. The application is intended to understand the evolution and dynamics of the crust and mantle in the study area from the perspective of earth electrical conductivity structure. The forward and inversion is carried out to reconstruct the reliable electrical conductivity structure beneath the study area from the marine magnetotelluric data collected. Based on the electrical conductivity structure inferred, the possibility of the existence of fluid and its distribution characteristics and migration pattern are considered. Whether there exists the partial melting in the study area and its reasons are also investigated. Meanwhile, the relationship between the conductivity structure and the thermal structure in the study area is treated by combining the hydrothermal data observed previously by the scholars, and then the channel and way of the hydrothermal ascending process are deduced.

海洋大地电磁方法是以天然电磁波为场源的勘探方法，它利用布置在海底的电磁接收站记录自海底下方构造反射的电磁场，借助反演理论由电磁信号推断海底下方的电导率结构。而电导率结构与岩石的一些关键物理参数如孔隙度、温度及流体成分等相关，还与许多地球动力学过程如俯冲带的板块耦合、流体释放和演化及岛弧岩浆模式、地幔中的部分熔融和蛇纹石化等相关。东海陆架-冲绳海槽区域是环太平洋构造活动带的重要组成部分，是探讨西太平洋边缘海构造活动、形成及演化机制的重要窗口。本申请拟从地球电性的角度来理解研究区内的壳幔演化规律及动力学过程。针对所采集的海洋大地电磁资料，进行正反演模拟，建立研究区内的可靠电性结构模型，进而由电性结构推断研究区内流体存在的可能性及其分布特征和运移模式，探讨区内是否存在部分熔融及其成因。同时结合前人观测的热流数据，探索研究区内电导率结构与热结构的依赖关系，进而推断俯冲过程中热液上升的通道及方式。

海洋大地电磁方法(Magnetotelluric，MT) 是以天然电磁波为场源的电磁勘探方法，可以适用于海底深部大构造、海洋地球动力学及海底地壳演化规律的研究。MT方法可以有效探明地壳上地幔范围内的熔融态低阻体，可用于探测发生脱水、流体迁移或熔融的区域，也已被证明是开展地球动力学研究的有效方法。本项目拟利用海洋MT方法调查东海陆架-冲绳海槽的壳幔电性结构，为深入理解的地幔动力学过程提供深部依据，进一步深化对冲绳海槽及邻区构造特征和构造演化的研究，具有重要的科学和理论意义。同时，基于区内的深部电性结构，可以进一步判断我国海域大陆/大洋地壳和上地幔性质以及大陆架、大陆坡界限，为我国东海大陆架和专属经济区划界以及维护国家海洋权益提供有利的技术支持，因此具有重要的现实意义。. 由于原定搭乘航次方案调整及其他原因叠加，本项目未能在申请书中所述研究区域开展海洋大地电磁测量。随后，我们依托在西太平洋菲律宾海域开展的地球物理调查航次进行了海洋大地电磁的测量工作，在九州-帕劳海脊共获得了5站位的实际观测资料。针对这些实测资料，我们利用一些关键的维性分析方法对资料进行了处理和反演解释，获得了研究区内的海底一维电性结构。限于站位数量及资料水平，未能开展岩石圈电性结构与地幔动力学机制之间的关系研究。. 此外，我们还开展了大量的理论模拟研究工作，获得了一些地球电性模型的大地电磁解析解，这些模型主要包括：水平非均匀各向异性电导率模型、具有两个过渡层地球电导率模型、垂直不均匀(线性变化)及各向异性电阻率结构。这些理论模型的研究工作对于后期开展海洋大地电磁反演提供了基础。