SEA FLOOR SPREADING OF SOUTH CHINA SEA AND ITS DEPOSITIONAL RECORDS OF SEA AND LAND CHANGES

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.