南海海底地震仪三维深地震探测的进展及挑战

Current Status and Challenges for Three-Dimensional Deep Seismic Survey in the South China Sea

获取深部速度结构是认识南海形成演化模式的基础.海底地震仪(OBS)二维与三维深地震探测技术是获取深部速度结构最成功的方法之一.OBS三维探测结果揭示:西南次海盆横跨残留扩张脊两侧洋壳不对称增生的速度结构特征;东部次海盆具有4种不同洋壳类型的速度结构;珍贝-黄岩海山链是在海盆停止扩张后6～10 Ma岩浆活动形成的;东沙隆起区显示了张裂期后的岩浆活动及其上涌通道;南海东北部马尼拉俯冲带前缘是受到张裂期后岩浆活动影响的减薄陆壳.IODP367-368钻探区实施的OBS三维探测将以建立南海洋陆转换带(COT)张裂-破裂机制模型为科学目标.OBS三维探测方法在南海各向异性、岩浆活动、形成演化等方面将继续发挥着不可替代的作用.

The deep velocity structures are the basis for building the formation and revolution of the South China Sea（ SCS）. Two-dimensional（ 2 D） and three-dimensional（ 3 D） OBS surveys are successful technique to acquire the deep seismic structures. The results from 3 D OBS surveys in the SCS have enhanced the knowledge for theoretical concepts. In the Southwest sub-basin,the asymmetrical velocity structures have been presented across the extinct spreading ridge（ ESR） due to the development of detachment faults. In the central part of the east sub-basin,four types of crust have been defined： thin oceanic crust（ 〈5 km）; typical oceanic crust（ 5-6 km）;thick oceanic crust hosting post-spreading volcanoes（ 〉 6 km） with significant intrusive roots; and thick oceanic crust with enhanced spreading features（ 〉 6 km） but without significant roots. The post-spreading volcanoes（ Zhenbei-Huangyan seamounts chain） were formed 6-10 Ma ago during a N-S tensional episode,several millions of years after seafloor spreading ceased in the SCS,and emplaced where the crust was the weakest,might have been influenced by the Hainan plume activity through a buoyancy-driven partial meltingmechanism. The study on Dongsha uplift highlights magmatic activities through upwelling channels after rifting ceasing. The velocity structure in the Manila subduction zone（ northeastern SCS） shows the nature of the crust and its relationship with the Manila subduction slab. A 3 D OBS survey will be carried out in 2018 in the area of the IODP Legs 367-368,aiming at building a model of lithospheric rifting and breakup mechanism in the continent-ocean-transition（ COT） zone of the SCS. The joint 2 D and 3 D survey method will play a major role in the understanding of the SCS geodynamics by integrating IODP drilling results with information on the deep structures.