摘要:
南海南、北共轭大陆边缘盆地的对比研究是深入了解南海扩张过程及古地理格局的重要途径。由于历史原因,目前对南海南缘盆地构造—沉积演化研究还非常薄弱,极大地限制了对南海扩张及海陆变迁等基础地质问题的整体认识。综合南海及其周缘盆地沉积地层和沉积环境的研究进展,对南海扩张过程和古地理格局演化进行了分析。南海南、北缘盆地破裂不整合面存在着明显的穿时性,从NE向SW逐渐变年轻,对应南海海底扩张从NE向SW渐进式打开。台湾新生代地层破裂不整合面位于33~39 Ma之间,暗示南海洋壳开始形成的时间可能在33~39 Ma之间,有部分较老的洋壳可能已经向东俯冲消减掉。南海经历了从早期"北陆南海"逐渐演变为现今"北海南陆"的过程,南海北缘早期存在一个向东开口的海湾,可能为古南海的一部分。伴随南海的扩张,海侵范围由东向西逐渐扩展,从一个狭窄的海湾形成今日的形貌。南海北缘盆地物源在~25 Ma左右发生明显的改变,早期主要为华南沿海的近源剥蚀沉积。在~25 Ma后,来自扬子地块的沉积物逐渐增多。南海南缘盆地物源在~25 Ma前与南海北缘盆地具有相似的物质来源,~25 Ma后南海洋盆阻挡扬子地块的物源向南输送,南海南缘仍以陆块内部中生代花岗岩及火山岩为主要物质来源。
Abstract:
The comparative study of the southern and northern continental marginal basins of the South China Sea (SCS) is an important approach to understand the process of seafloor spreading of the SCS and its paleogeographic pattern. Due to some historical reasons, the study of the tectonic and depositional evolution of the southern marginal basins of the SCS still remains weak up to the present. It has greatly constrained the overall understanding of the basic geological problems such as the spreading of the SCS and the changes between the sea and the land. This paper has integrated the research progress of sedimentary strata and sedimentary environment in the SCS and its adjacent basins, and retrieved the spreading process of the SCS and the evolution of its paleogeographic pattern. According to the unconformity, diachronism occurs in the southern part of the SCS just like that in the northern part. It becomes younger gradually from the northeast to the southwest, suggesting a spreading process from east to west. The seafloor spreading time was Early Oligocene (~33Ma) in the northeast, but Late Oligocene (~25Ma) in the southwest. However, they ceased almost at the same time in Early Miocene (16-20Ma). The SCS was under the marine environment in the south and under the terrestrial environment in the north in early stage. The sea spread gradually from east to west. Obvious changes occurred in the northern basins of the SCS at ~25Ma. In the early stage, sediments were mainly from the provenance of Southern China. After ~25Ma, the sediments from the Yangtze block gradually increased. Southern basins of the SCS was dominated by similar sources with the northern basins before ~25Ma and sediments were mainly from Southern China. Due to the spreading of the SCS after ~25Ma, oceanic basins blocked the sediment transportation from the Yangtze block. The Mesozoic granites and volcanic rocks in the interior of the continental block provided sediments for the southern basins of the SCS.
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