南沙地块深部地壳结构和南部陆缘构造性质研究

Located in the southern part of South China Sea (SCS) on a strategically important place, Nansha Block is the southern continental margin formed by the seafloor spreading of SCS, with an unique tectonic position. This project will use the multi-component Ocean Bottom Seismometer (OBS) data, newly acquired along the OBS2009-1 survey line. More deep seismic information, such as later arrivals from mid-crust and converted shear waves, will be extracted by special data processing. The best-fit inversion models of P and S wave velocity structures are going to be constructed by the methods of ray-tracing, travel-time fitting, forward and inverse modeling, of combined first and later arrivals. In the same time, model resolution test and reliability analysis will be carried out and the Vp/Vs i.e. Poisson's ratio will be calculated. By integrated geological interpretation of the obtained velocity models and structure profiles, and by careful analysis of the deep structural characteristics such as Moho interface undulations, top mantle velocity values, crustal velocity anomalies, and their lithological implications, the crustal types and tectonic natures of Nansha Block and the southern continental margin will be inferred. Comparison of the deep crustal charateristics between the northern and sourthern continental margins will lead to more reasonable interpretation of geological process, thus providding scientific evidences for better understanding the SCS formation/evolution and its oil/gas potential.

南沙地块位于南海的南部，是南海海盆扩张形成的南部陆缘，具有十分重要的构造位置和战略地位。本项目拟利用新近采集的OBS2009-1测线的多分量海底震仪(OBS)数据，通过特殊的数据处理方法提取更多的后至震相、转换横波震相等深部结构信息。采用射线追踪、走时拟合、正演和反演模拟结合、初至和后至震相联合的方法，获取P波和S波速度结构的最佳反演模型，并进行模型分辨率测试和可信度分析，计算Vp/Vs波速比和泊松比结构模型。对获得的速度模型和结构剖面进行综合地质解释，仔细分析其莫霍面起伏、地幔顶部速度值、地壳异常速度体等深部结构特征及其岩石物性含义，推断南沙地块和南部陆缘的地壳类型和构造性质。对比南海北部陆缘和南部陆缘深部地壳结构特征的异同，提出合理的地质过程解释，为南海形成演化和油气资源潜力的研究提供科学依据。

本项目按计划对南沙地块的多分量OBS数据进行分析处理，通过特殊数据处理技术，从水平分量数据中提取出转换横波震相信息，从异常台站数据中成功恢复先前丢失的深部震相信息，通过反演试算对模型的分辩率和不确定性进行了分析。在已有纵波速度结构的基础上，通过横波震相的走时拟合获得了横波速度结构模型和纵横波速比模型，通过与北部陆缘的OBS数据和结构模型进行对比分析，构建跨越南部陆缘、中部海盆和北部陆缘的地壳结构大剖面，最后探讨了南北共轭陆缘和西南次海盆的形成演化机制。项目共发表学术论文6篇，其中SCI论文3篇，培养研究生5人，项目骨干参加学术会议10人次，其中国际会议3次，充分交流了研究成果与学术思想，圆满完成项目研究计划。