我国南海历史性水域线的地质特征

The geological basis of the China Historical Sea Lines in South China Sea

40a的海洋地质、地球物理实测研究表明，九段线不仅是显示我国南海主权的历史性水域线，而且总体上也是南海与东部、南部和西部陆区及岛区的巨型地质边界线。根据实测数据，本文将从地质成因、来源、演化的角度论述此南海历史性水域线的合理性。主要结论包括：历史性水域线的东段在地形上基本与马尼拉海沟一致，海沟西侧为南海中央海盆洋壳区，东侧为菲律宾群岛。根据国际地质研究的资料，菲律宾群岛始新世以前位于较偏南的纬度，后来于中晚中新世（距今16～10Ma）仰冲于南海中央海盆之上，因此菲律宾群岛是一个外来群岛。而黄岩岛在马尼拉海沟以西，是中央海盆洋壳区的一个岛礁，与菲律宾群岛成因不同。南海历史性水域线的南段在地形上基本与南沙海槽一致，伴随南沙地块由北部陆缘向南裂离，古南海洋壳沿此海槽以南俯冲至加里曼丹岛陆壳之下，因此南沙地块与加里曼丹陆块为两个来历不同的地块。南海历史性水域线西段的分布在地形上与越东巨型走滑断裂带基本一致，可能与西沙地块、中沙地块、南沙地块从南海北部陆缘向南滑移有关。南沙地块北缘陡直的正断层结构，突显中央海盆是拉裂形成，其基底和中新生代地层与北部珠江口盆地的地层结构可以对比，说明南沙岛礁原属我国华南大陆南缘，后因南海的形成裂离至现今的位置。

Forty years of geological and geophysical investigation and research made us clear that Chinese Historical Sea Lines （abbreviated as CHSL） in the South China Sea （SCS） not only demonstrated our sovereign right, but also a combination of giant geological boundaries between continental blocks,islands and countries. This paper will focus on the geological rationality of the CHSL in SCS, based on the investigation of geophysical features, regional geological structures,origin and evolution. The main conclusions are： the location of the eastern CHSL is consist- ent with the Manila Trench in topography. The west side of the trench is the Central Sea Basin of the SCS, with 4 km to 8 km oceanic crust. The east side of the trench is Philippine Archipelago. Based on paleomagnetic analysis, this archipelago was originally a small continental block located in the south latitude. It drifted northward to the present location along the Manila Trench since Middle to Late Eocene （about 16 Ma to 10 Ma）, and thrusted on the oceanic crust of the Central Basin of South China Sea due to the combined action of Pacific Plate and Australia Plate. Thus,the Philippine Archipelago was outside source island arc block. The Huangyan Island is located west of the Manila Trench. It＇s a volcanic island of oceanic crust of central sea basin of South China Sea. It doesn＇t be- long to the Philippine Archipelago. The location of the southern CHSLs of the SCS in topography is consistent with Nansha Trough. Accompanied with the Nansha Block splitting southward from the northern margin,the oceanic crust of proto South China Sea moved southward and subducted beneath the Kalimantan Continent along the Trough. Thus,Nansha Block was absolutely a different geological source continental block from the Kalimantan Continent. The location of the western CHSLs of South China Sea is the same in topography features as a giant East Vietnam Fault. The Fault might be related to the southward shift of the Xisha, Zhongsha and Nansha Block from north of the SCS. The north edge of Nansha block showed steep and straight normal faults,meaning that the Central basin of the South China Sea was mainly evolved by extension. Multi-channel seismic profiles, drilled re- cords as well as the dredged samples showed that the stratigraphic divisions of several Nansha basins, Zhongsha and Xisha Block were very similar to and might be fully compared with that of the Pearl River Mouth Basin on the northern continental margin and paleo South China Continental Plate, so the Xisha, Zhongsha and Nansha Block originally belonged to the South China Continent,latterly it was owing to the evolution of South China Sea. split away and moved southward to present location owing to the evolution of South China Sea.