Reconstructing The Seafloor Spreading History Of The South China Sea:Insights From Stratigraphic-sedimentary Responses

Based on regional 2D seismic profiles and IODP drilling sites in the deep sea basin and continental margins,we analyzed the stratigraphic and sedimentary responses of the South China Sea in order to better understand its seafloor spreading history.Combined with the interpretation of magnetic anomalies,we investigated the range of oceanic crusts,the location of the Zhongnan Fault,and fossil ridge locations of each sub-basin.After that,we further explored the age and pattern of the seafloor spreading of the South China Sea and reconstrued the spreading history.Discussion on the spreading mechanism was made.Our major results are listed as the following.During 30-32 Ma(corresponding to T7 horizon),seafloor spreading commenced within the East and Northwest Sub-basins.The spreading direction was in N-S or NNW-SSE at that time and the spreading way of basins was obscure,we infer its spread at the same time rather than a progressive process.During this time,the breakup unconformity behaved as rifting-depression unconformity in the proximal region and an intra-rifting unconformity in the distal region.The distribution of the latter unconformity covers the Pearl River Mouth Basin,Reed Bank and Palawan.The opening of the South China Sea was speculated to be mainly affected by extensions,which was caused by subduction of Paleo-South China Sea or activity of the East Pacific Plate.At ca.28 Ma(T6-T7),the northwest sub-basin stopped spreading while east sub-basin continued its spreading,which were separated by the Zhongnan Fault.During ca.23-25 Ma,the north fossil ridge of East sub-basin stopped spreading.In the meanwhile,a new fossil ridge called south fossil ridge,away from the north fossil ridge about 30km developed in the south.The direction of this ridge began changing in NNW-SSE while the spreading mode of this stage is obscured and needing further discussion.Meantime,Southwest Sub-basin began its spreading with a direction of NW-SE and the spreading mode was progressive from NE to SW.At this time,the continental margins including north,west,south all show typical regional unconformity.Close to the ocean basin in north continental margin,there was an obvious unconformity separated the rifting and drifting deposits,implying that the whole South China Sea experienced active tectonism during this time.Its might respond to a comprehensive effect of the Red River strike-slip fault,subduction of proto-South China Sea,and interactions between the Indo-Australian and Eurasian plates.A passive spreading status was present at this time.At ca.16 Ma(T5).the Southwest Sub-basin stopped spreading when the continental margins experienced uplifting and denudation.At ca.13Ma,the East sub-basin gradually stopped spreading,when uplifting and denudation still dominated the northern margin.At this time,south continental margin was characterized by a northeastern prolonged collision with the Borneo.It was inferred that the South China Sea basin experienced a SW-NE oriented and progressive closure,when stopped by the collision between the southern margin and the Borneo.At ca.10 Ma,the Philippine plate gradually move northward and collided with the South China Sea basin,leading to the formation of typical trench-arc system.Taiwai island developed in the beginning of the Quaternary,and the modern tectonic pattern has formed since then.