南海西北陆缘深部异常过程的地热学与垂向均衡响应

Whether or not its development and evolution has been affected by some mantle plume is an inevitable important scientific problem when studying on geodynamic processes of the South China Sea. The northwestern continental margin of the South China Sea is an ideal region for studying the coupled relationship between present Deep Earth processes and surface processes, and investigating the possible influence of mantle plume on the development and evolution of the South China Sea. With the coupled Deep Earth and surface process in the minds, this application would employ the multi-methods of geothermics, basin analyses, thermochronology and numerical analyses to obtain the spatial variation of the background thermal regime with marine and terrestrial heat flow data, to obtain the basin’s tectonic subsidence paths of the basins by tectonic-thermal numerical analyses and the denudation histories of the continental rocks with multi-thermochronology systems, and then acquire the information of the temporal and spatial variation of the crustal vertical isostatic movement; under the constrains of heat flow, crustal vertical movement, and basaltic rock distribution, employ a dynamic numerical model to acquire information such as the temperature and evolution of the Deep Earth process; investigate the relationship between mantle plume and the evolution of the South China Sea, and its marginal sedimentation basins, analyses the roles of the Deep Earth process on earthquake genesis and occurrence, and the potentials of the hydrocarbon resources of the sedimentary basins. This research is of theoretical significance and practical application, and will contribute to the investigation of the coupled relationship between Deep Earth and surface processes in the South China Sea, the research of the development and evolution of the South China Sea and its hydrocarbon exploration, and the development of the global tectonic system.

南海形成演化有无地幔柱的贡献一直是南海研究难以回避的重要科学问题。南海西北部大陆边缘是研究现今深部过程与表层过程耦合关系，探讨南海形成演化与地幔柱关系的理想区域。申请项目拟利用地热学、盆地分析、热年代学与数值分析等多学科方法，结合海陆区热流数据，提取背景热流的空间变化；利用构造热演化数值分析和多体系热年代学方法获取盆地构造沉降史和陆区岩石剥露史，并从中获得基底垂向均衡运动的时空变化；利用耦合深部过程与表层过程的动力学数值模型，在热流、基底运动和岩体分布约束下，提取深部过程温度与演化等信息；探讨南海及其边缘盆地形成演化与地幔柱的关系，分析深部过程对强震孕育和沉积盆地油气资源潜力的影响。该项研究具有理论意义和实际应用价值，必将为探索南海深部过程与表层过程的耦合关系，为南海形成演化研究与油气勘探和全球构造理论发展做出贡献。

地质、地球物理证据表明南海西北部大陆边缘下伏有类似地幔柱特征的低速、高温异常体。项目首先利用地热学、盆地分析、热年代学等方法获取表层热状态和基底升降史，然后借助耦合表层过程和深部过程的动力学数值模型，获取深部异常体的信息，取得以下主要成果：①获得了南海北部热流分布图、热岩石圈厚度图和有效弹性厚度图，其中广东西部沿海和珠三坳陷间具有高热流（75-100 mW/m2）、薄热岩石圈厚度（75-80km）和岩石圈强度相对较弱的特点。珠三坳陷高热流站位位于坳陷北部的阳江地垒和阳江B凹，该高热流区北延与陆上的阳江新洲地热田高热区相连。②北部湾盆地和珠三坳陷晚中新世时期构造沉降速率明显降低，晚中新世后因基底隆升形成的局部剥蚀面，分布范围以雷琼火山区为中心大致呈环状，累计剥蚀厚度约为100-200m。③热年代学数据分析显示，南海西北部岩石经历了白垩纪快速冷却、晚白垩世-早始新世缓慢冷却和中始新世以后快速冷却的3个冷却阶段；白垩纪快速冷却是晚中生代快速伸展的结果，反映了早期形成的海岸山脉和高原在伸展作用下快速崩塌，中始新世以后的快速冷却是早新生代快速冷却与晚新生代周边板块相互挤压作用的结果；海南岛东北部30-50Ma具有较高的海拔，30Ma以后剥蚀和夷平作用使得海南岛东北部维持在低海拔和平缓地势，说明晚中新世以后该区没有明显的抬升。④地热学、沉积盆地分析和低温热年代分析结果表明研究区下伏低速高温异常体对现今表层热状态有重要影响，可能是晚中新世以后才作用于雷琼坳陷区，导致该区基底轻微抬升，累计剥蚀厚度很可能小于300m。⑤动力学数值模拟初步结果显示，这种轻微抬升剥蚀反映了深部异常体规模较小，很可能半径在100km左右。这些成果不仅有助于认识南海西北部热状态、盆地演化，而且对认识深部异常体、华南南部大陆边缘构造演化有重要意义。部分成果已经发表在国内外主流刊物《地球物理学报》、Gondwana Research》、《Basin Research》、《Marine Geophysical Research》等SCI 检索刊物上。