南沙腹地隐伏背斜构造的成因及对古南海的指示

The South China Sea locates at the triple junction zone of the Eurasia, Indo-Australia and Pacific plates， also the tectonic transformation from Eastern Tethyan to pro-Pacific ocean region. Though there are lots of lines for the presence of a proto-South China Sea during Mesozoic, questions such as where was the subduction boundary of the proto-South China Sea and how did it extend remain less known. New seismic profiles over the hintland of the Nansha Waters, the southern part of the South China Sea, show obvious blind compressive folds featuring severe top erosion under the late Cenozoic sedimentary cover and basal decollement fault. The deformed strata may comprise sedimentary layers of the proto-South China Sea and the compressive deformation may imply subduction and convergence once occurred. To study these blind folds is significant for recognition of the extension and understanding of the closure of the Eastern Tethyan and the prospect of the Mesozoic petroleum. Considering the paucity of drilling data over Nansha Waters, it is planned to grab benthic samples of the underlying strata outcrop on the seafloor and calculate the velocity with reflection and refraction waves from long-streamer seismic records to obtain the lithologic and velocity information, so as to determine the age and environment of sedimentation. Regional geology and drilling well-tied correlation will be conducted to interpret the stratigraphy and tectonic generation of the folding, and to analyze their implication to the Tethyan proto-South China Sea and petroleum. The study is hopeful to get insights of distribution and evolution of the proto-South China Sea, and the petroleum geology there.

南海位于欧亚、印澳和太平洋三大板块的交汇作用区，也是历史上东特提斯洋与古太平洋域的此消彼长区。虽然大量迹象表明该区曾经存在过中生代古南海，其早期俯冲拼合边界及展布特征并不清楚。地震勘探显示在南海南部南沙腹地的晚新生代盖层之下存在地层形变明显、顶部剥蚀严重的隐伏滑脱背斜构造。这些褶皱地层很可能包含了古南海的沉积，挤压构造可能指示了曾经的俯冲聚合。研究这些背斜构造对于认识东特提斯的东延与关闭过程，以及中生界油气勘探具有重要意义。针对南沙海区钻井较少的现状，本项目将开展下伏地层的露头抓斗采样和反射-折射波速度计算，根据样品信息和地层速度变化，认识褶皱地层的岩性和沉积时代；结合区内和周边岩石样品测定，对比地震剖面解释褶皱构造的地层组成与构造世代关系，分析其对特提斯古南海的意义及其油气地质意义。项目研究可望在特提斯古南海的展布、演化和南沙腹地的油气构造条件方面取得新的认识。

南海位于欧亚、印澳和太平洋三大板块的交汇作用区，也是历史上东特提斯洋与古太平洋域的此消彼长区。虽然大量迹象表明该区曾经存在过中生代古南海，但其早期俯冲拼合边界及展布特征并不清楚。地震勘探显示在南海南部南沙腹地的晚新生代盖层之下存在地层形变明显、顶部剥蚀严重的隐伏滑脱背斜构造。这些褶皱地层很可能包含了古南海的沉积，挤压构造可能指示了曾经的俯冲聚合。研究这些背斜构造对于认识东特提斯的东延与关闭过程，以及中生界油气勘探具有重要意义。针对南海海区中生界钻井和样品较少的现状，本项目通过搭载多个共享航次，开展了下伏地层和基底的海底露头区采样，以及含中生界褶皱形变带的反射、折射波资料成像处理和物性分析。在东沙、南沙腹地及南海海盆的海底露头取样分别获得了疑属中生界的红层、白垩土，以及花岗岩（锆石测年128.6+-2.3 Ma）和花岗质变质岩样品，中生界露头区样品中还包括丰富的自生碳酸盐岩结核和多种底栖方式珊瑚在内的大量深水化养链生物。研究发现南沙海区中部及东沙海区都存在薄新生界、厚中生界结构特征，中生代地层经受了中、新生代两期褶皱形变，其中早期形变较弱，可能是远场俯冲汇聚背景下中生代花岗岩侵入的结果。晚期形变较强烈，形成众多浅层底辟和海底泥火山，泥火山区和附近出现异常低速带和深部高速带，分别对应含油气带和深部岩浆底侵异常。泥火山和大量生物的出现指示下伏中生界具有较强的生烃能力。泥火山活动于新生代晚期至今，动力可能源于新生代晚期局部的岩浆再次底侵。在南沙与东沙之间的海盆区，洋壳基底与地壳结构变化大，展现了在新生代构造主导的扩张过程中岩浆供应早期少、中期多、晚期少、扩张之后又增多的复杂现象；海盆区的海山上出现的中生代花岗岩代表了陆壳残留。这些结果为认识南海的演化过程、回溯古南海的地质特征，及揭示特提斯域-太平洋影响提供了新的视角和依据。