南海大陆边缘动力学:科学实验与研究进展

国家重点基础研究发展计划(973)项目(2007CB411700)首次在南海南部大陆边缘及西南次海盆开展长排列大震源多道地震、海底地震仪(OBS)折射/反射地震等的综合地球物理探测,结合地质构造、地球化学、动力模拟等的综合研究,形成如下重要认识:南海海盆新生代发生了早、晚两期海底扩张.早期扩张发生于33.5~25 Ma,经过1.5 Ma的调整,晚期扩张发生于23.5~15.5 Ma.早晚两期扩张均为由东向西传播的渐进式扩张,构造上具有南北分块、东西分带的特点.南海新生代南、北陆缘沉积盆地从断陷到坳陷的构造转型时间存在差异,并分别与海盆晚期扩张和早期扩张的结束时间相对应.南海新生代陆缘张裂变形具有随深度变化的特点,海盆的洋壳深部和上地幔顶部存在磁化现象,地幔没有明显的速度异常.南北大陆边缘均属非火山型大陆边缘,虽然北部陆缘东段存在下地壳高速层,但没有发现向海反射层,而共轭对比应沿NW-SE向进行.南海新生代南北陆缘沉积盆地形成演化存在系统差异,总体上"外油内气"呈环带分布,南部陆缘盆地具有更优的油气成藏地质条件.南海中生代南北陆缘均为华南块体的一个部分,受到古特提斯和古太平洋构造域的联合控制.

Dynamics of the continental margins of South China Sea: scientific experiments and research progresses

Comprehensive geophysical survey, including the multi-channel seismic survey with long streamer and large air-gun source and the wide-angle refraction/reflection seismic survey of OBS, has been deployed in the southern margin and Southwest Sub-basin of the South China Sea for the first time. Based on the interpretation of these geophysical data, combined with studies on tectonic structure, geochemistry, and dynamic simulation, etc, our project worked out the dynamics of the continental margin in South China Sea. Major conclusions are as follows: Two sea-floor spreading events occurred in Cenozoic. The first episode happened between 33.5~25 Ma, and after a 1.5 Ma tectonic adjustment, started the second episode of sea-floor spreading between 23.5~15.5 Ma. In both episodes the South China Sea experienced propagation rifting and spreading from east to west, resulting in a structural framework featured with blocks in north-south direction and segments in east-west direction; Cenozoic marginal basins in the northern and southern continental margins show different rift-terminated time, which correspond with the two episodic sea-floor spreading respectively. The continental crust of the South China Sea might have experienced depth-dependent stretching. The lower crust and top of upper mantle have been magnetized and no obvious velocity anomalies of the mantle have been found in oceanic basin. South China Sea margins are a kind of magma-poor rifting, although lower crustal bodies of high-velocity occur in the east of the northern margin, but without SDR. The origin and evolution of sediment basins on northern and southern continental margins are very distinct, and the hydrocarbon accumulations are distributed in a cirque with oil outside and gas inside. We believe that the hydrocarbon-bearing perspectives in southern continental marginal basins are superior to these northern continental marginal basins. In Mesozoic the northern and southern margin of the South China Sea were parts of the South China Continental Block, which was controlled by the Tethays and Pacific tectonic domains.