南海东北部三维精细地壳结构及其对岩浆活动和构造属性的约束

The magmatism and structural feature in northeastern South China Sea (SCS) is an important foundation to solve extensional rifting evolution and seafloor spreading process of SCS, and the high-velocity layer developed in lower crust caused a great deal of controversy for the tectonic attribution of continental margin of SCS. Furthermore, some special tectonic bodies such as high magnetic anomaly belt, Mesozoic strata, north-dipping fault zone and magnetic quiet zone are the carrier to discuss tectonic evolution of SCS from late Mesozoic to early Cenozoic. This research treats these special tectonic bodies as a structural system based on previous achievements, and carry out more detailed three-dimensional ocean bottom seismometer surveys and wide-angle refraction and reflection seismic tomography of crust, mainly solve three problems: (1) The detailed three-dimensional structure, thickness and the vertical and lateral variable characteristics of high-velocity layer in the lower crust of northeastern SCS, and determine the intensity, intrusion mode and tectonic origin of the magmatism. (2) The P and S wave velocity structure of special tectonic bodies, crustal thickness and the plane distribution features of Moho and their coupling relationship, and reveal tectonic attribution of the crust for these tectonic bodies, and determine whether the existence of Mesozoic volcanic arc and palaeo-oceanic crust. (3) The contrasting analysis among the whole structural system of northeastern SCS, modern subduction zone and oceanic crust, and discuss the existence of palaeo-subduction zone and their roles in the seafloor spreading of SCS.

南海东北部的岩浆活动和构造特征是解决南海张裂演化和海底扩张过程的重要基础，其下地壳高速层的发育造成了南海陆缘构造属性的极大争议，而高磁异常带、中生代地层、北倾断裂带和磁静区等特殊构造体又是探讨南海中生代晚期到新生代早期构造演化过程的重要载体。本项目拟在前人工作的基础上，将下地壳高速层、高磁异常带、潮汕坳陷、北倾断裂带和磁静区作为一个构造体系，开展更为详细的三维OBS探测和地壳精细结构的研究，重点解决：（1）南海东北部下地壳高速层的三维结构、发育厚度及其纵横向变化特征，判断岩浆活动的强度、侵入方式和构造成因；（2）南海东北部特殊构造体内部的纵横波速结构、地壳厚度与莫霍面平面展布特征及彼此之间的耦合关系，揭示各构造体深部地壳的构造属性，判断是否存在火山弧、古洋壳等中生代残留构造体；（3）南海东北部整个构造体系与现代俯冲带、洋壳的对比分析，探讨古俯冲带的存在性及该构造体系在南海扩张中扮演的角色。

为了揭示南海东北部下地壳高速体的三维结构及其成因机制、确定晚新生代岩浆活动规模和通道模式、阐明南海北部独特的构造属性特征，我们以项目申请书提出的技术路线为指引，从野外航次、数据处理、壳幔三维结构成像、地质构造信息解译等方面重点对南海东北部以及整个南海北部陆缘开展了深入研究。详细开展了壳-幔三维层析成像结构的研究；勾画了中生代残留盆地、下地壳高速体的速度结构、发育厚度、纵横向变化特征；探明了研究区海底火山的分布范围和几何学特征；首次定量计算了南海东北部晚新生代岩浆活动的规模；提出了海南地幔柱深至下地幔的物质运移通道的三维空间展布和形态特征；阐述了南海东北部陆缘是一个受控于先存构造单元且被裂后期岩浆活动强烈改造的贫岩浆型被动大陆边缘，为研究贫岩浆型大陆边缘的形成演化及后期破坏提供了重要案例。共发表学术论文19篇，其中SCI论文14篇（二区以上论文7篇），培养博、硕士研究生15名（毕业6名）；项目负责人和1名主要参与人员晋升为研究员，1人晋升为副研究员，项目负责人作为PI组建了海底岩浆活动与地幔动力学研究团队；参加国际学术会议12人次，国内学术会议20多人次。有关南海北部岩浆活动的系列研究成果被科学网、中科院和南海所等官网报道。综上所述，我们很好地完成了项目计划书所提出的各项任务和目标，圆满地实现了各项成果指标。