南沙北部郑和群礁海域洋陆边界下构造层形成时代的研究

The structure of the continent ocean transition (COT) at the northern Nansha Waters is an important window to study the evolution of the South China Sea (SCS). Thick strata with dipping and closely space reflection and high angle faults below angle disconformity are found aound Zheng`he Reefs located in the northern Nansha Waters. These phenomenon are very uncommon at COT of the SCS. However, it is being debated about whether the thick strata are pre-rift marine sediment during Mesozoic or syn-rift sediment during Cenozoic. The correct identification of the stratigraphic age is helpful to understand the property of OCT, the evolution model of the SCS and the Mesozoic distribution in Nansha Waters. Formation interval velocity and strata correlation are important methods of identification of Mesozoic and Cenozoic in the margin of SCS. In this program, new high resolution velocity structure will be acquired by reprocessing and processing the original and the latest OBS data around Zheng`he Reefs. Then velocity structure characteristics will be obtained by velocity comparison between Zheng`he Reefs and northern margin of SCS, where velocity of the Cenozoic and Mesozoic is known. The structural characteristics will be obtained by comparing with near well seismic line, strata correlation among strata below angle disconformity around Zheng`he Reefs and proven syn-rift and pre-rift strata of SCS. Baseed on the characteristic of velocity and structure, the age of thick strata below angle disconformity will be determined. At last we will make a preliminary study of the formation mechanism of thick strata.

洋陆边界的地层形变特征是研究洋盆张裂模式的重要窗口。最新的973地震测线显示，南沙北部郑和群礁海域洋陆边界处向陆一侧的地层被角度不整合分为两大套地层，上构层基本水平，角度不整合遭受强烈的剥蚀，而下构造层为异常厚的密集反射层，其下构造层反射特征和构造特征与南海其它地区洋陆边界的新生代沉积结构明显不同。该处下构造层形成时代的确定有利于加强对南沙张裂边界处地层属性和南海张裂模式的认识。本申请拟通过处理该处原有的和最新的小间距海底地震仪数据，获取沉积层高精度速度结构及其速度变化特征，通过该处不整合之下地层与南海确认的中生界和裂谷期地层特征的对比，井位投影、地震剖面地层特征的对比追踪，获取该处地层构造形变特征，最后确定该处下构造层的形成时代，探讨厚层沉积层的演化过程和新生代裂谷期该处地层的张裂模式。

郑和群礁海域洋陆边界处地层被角度不整合分为上、下构造层，下构造层是南海洋陆边界处最厚的沉积体，其地震反射形态与南沙拖网采集到中生代岩样处的相似，也与南沙礼乐盆地和潮汕坳陷角度不整合之下的厚层中生界特征相似，也与廷贾断裂以南发育的新生代褶皱构造相似。为了研究郑和群礁海域洋陆边界下构造层的时代，本项目主要研究郑和群礁海域洋陆边界处地层速度变化特征、地层形变特征和深地壳速度结构。获得如下重要结果：1）长排列多道地震计算的折射波速度表明, 郑和群礁海域上覆层地震波速度为2.5～3.0 km.s-1，下伏层地震速度为3.5～5.5 km.s-1，跨不整合界面发生速度突变（其差值约1.0～2.0 km/s）；2）从礼乐滩-郑和群礁的广大海域发育大范围的中生界，且中生界厚，层内地层倾斜或者发育褶皱，层顶为角度不整合，剥蚀强烈；3）根据下构造层的挤压形变特征、下构造层顶部的强烈剥蚀、跨不整合界面的速度突变，南沙晚白垩地层和新生代早期地层的缺失，以及南沙出露的中生界样品，确定研究区下构造层的时代为中生界；4）利用海底地震仪获取了南沙深部地壳速度结构。该研究的关键数据为地层折射波速度结构和深地壳速度结构。本研究表明，南沙地壳减薄程度略低，Moho面与基底面呈镜像关系，指示张裂过程中上地壳脆性断裂；南沙洋陆边界海域角度不整合之下的下构造层为中生界，说明新生代裂谷期南沙地壳伸展较弱，拉张后中生界以断块形式残留；研究区存在中生界褶皱构造，地层速度较低，约3.5 km.s-1，可能预示较好的中生界油气储集。