南海张裂过程及其对晚中生代以来东南亚构造的启示--IODP建议书735－Full介绍

Opening of the South China Sea and its implications for Southeast Asian tectonics since the late Mesozoic

南海的形成揭示了大陆边缘张裂和盆地形成的复杂模式,尽管已经进行了广泛研究,但是关于基底岩石和深海盆沉积层的精确年代数据还很缺乏,这使得对南海张裂年代的估计存在很大的误差,对张裂机制和历史的各种假设没有得到验证。同时只有对南海的张裂过程有了精确地分析与刻画,才能更好地理解西太平洋边缘海盆地的形成以及它们在印支块体受印度—欧亚板块碰撞而向东南挤出、青藏高原隆升中可能起到的作用。2009年正式提交的国际综合大洋钻探计划(IODP)建议书735-Full建议在南海深海盆内的4个站位上实施钻探。这4个站位分布在南海盆地4个不同的次级构造单元上(南海东北部、西北次海盆、东部次海盆和西南次海盆),这样的站位设计会确保完成本建议书的整体研究目标,即揭示南海的张裂历史和它对晚中生代以来东南亚构造的启示。位于南海盆地最东北部的站位有助于确定该区域地壳的属性和验证古南海是否存在,位于西北次海盆的站位可能会提供南海的最早张裂年代,另外2个分别位于东部次海盆和西南次海盆的站位将重点确定2个次海盆的绝对年龄、基底矿物成分与磁化率以及2个次海盆的相对张裂次序。这些站位的水深大约在2910～4400m,钻探深度预计到海底以下大约700～2200m,总的钻透深度为5959m,其中5359m穿透沉积层,另外600m或400m钻入基底。所有这些站位的位置是由已有的地球物理观测数据所确定,目前计划收集更多的地质与地球物理数据以满足IODP对井位调查数据的要求。

The opening of the South China Sea （SCS） reveals complex patterns of continental margin break-up and basin formation. Despite extensive studies in the area, accurate dating on basement rocks and overlying sediments in the deep basin is currently lacking. This leaves a large margin of error in estimated opening ages and renders various hypotheses regarding its opening mechanism and history untested. Further, only with an accurate account on the opening of the SCS can we better understand the formations of western Pacific marginal sea basins, as well as the possible roles it played in accommodating the southeast extrusion of the Indochina block due to the collision between the India plate and the Eurasian plate and the uplift of the Tibetan Plateau. We for the first time propose drilling at four sites in the deep SCS ocean basin. These four drilling sites are located in four different subunits （the northeasternmost SCS, the northwest sub-basin, the east sub-basin, and the southwest sub-basin） of the SCS Basin, and are designed strategically to fulfill the overall research objective, which is to unravel the opening history of the SCS and its implications for Southeastern Asian tectonics since the late Meso- zoic. The site in the northeasternmost SCS basin will help determine crustal affinity of that part of crust and verify the existence of the proto-SCS. The site in the northwest sub-basin will give us the likely earliest opening age of the SCS. The two sites in the east and southwest sub-basins will focus on determining the absolute ages, basement mineralization and magnetization, and the relative opening sequences of these two sub-basins. The water depths at these sites range from about 2 910 m to about 4 400 m, and the drilling depths are estimated to be from about 700 mbsf to about 2 200 mbsf. The total penetration depth is 5 959 m, of which 5 359 m is through sediment and 600 m or 400 m into basement. Each site is located using existing geophysical site survey data, and an effort is being made to acquire additional site survey data.