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Hainan Mantle Plume and the Formation and Evolution of the South China Sea

YAN Quan-shu1,2 and SHI Xue-fa1   

  1. 1. Key Lab of Marine Sedimentary and Environment Geology, The First Institute of Oceanography, State Oceanic Administration,Qingdao 266061,China;2. Institute of Oceanology, Chinese Academy of Sciences,Qingdao 266071, China
  • Received:2007-06-20 Revised:2007-06-20 Online:2007-06-20 Published:2007-06-20

Abstract: Seismic tomographic images obtained from the mantle under the southeast Asia region indicate there may exist a mantle plume beneath and around the Hainan island. A sub-vertical low-velocity column is imaged beneath the Hainan and the South China Sea, and extends from shallow depths to 660-km seismic discontinuity (i.e., the interface between upper mantle and lower mantle), and continuously to a depht of 1900 km. There is a large quantity of Cenozoic alkali basalts distributed in the South China Sea and its adjacent areas which include Leiqiong Peninsula, Hainan Island, Beibuwan Basin, Weizhou Island in Guangxi province and Indochina block. The geochemical data for these basalts show the characteristics of OIB-type basalt and DUPAL-like isotopic anomaly, and imply its deepseated origin. In addition, the average value of Tp (mantle potential temperature) for the South China Sea inferred from olivine-fluid equilibrium, is 1661℃, which is higher than that of MORB and lies between the corresponding values of Hawaii hotspot and Iceland hotspot. Based on evidences mentioned above, combined with numerical model experimental data, it shows that there does exist a mantle plume beneath the Hainan Island and adjacent areas. Until recently, scholars have developed many models about the formation and evolution of the South China Sea, and the debating issue is the geodynamic source. We suggest that the Hainan plume may be a significant geodynamical source for the formation and evolution of the South China Sea. Here the Hainan plume is introduced into our preliminary model about the formation and evolution of the South China Sea. The model is as follows: (1) 50-32 Ma, Integrated effects of collision between thd Indian Ocean plate and Euro-Asian plate resulted in retrogression of Pacific plate, created a extensional tectonic setting, and provided a channel for ascent of the mantle plume; (2) 32-21 Ma. When the head of mantle plume arrived at asthenosphere, it immediately interacted with the spreading center of the South China Sea by lateral material flow, which enhanced spreading spead. During 26-24 Ma, there took place a ridge jump, which adjusted the spreading center from nearby 18。N (i.e., present-day center of NW sub-basin) to nearby 15.5。N (i.e., present-day center of East sub-basin); (3) 21-15.5 Ma. With the mantle plume effect gradually enhancing, the hotspot-spreading center interaction became more and more intensive, and at about 21 Ma, there took place a ridge jump again, and induced the opening of SW sub-basin; (4) 15.5-0 Ma. Due to collision between the Indo-Australian plate and the Sunda continent, the spreading stopped. Subsequently, the earlier formed oceanic crust subducted along Nansha trench and Manila trench. However, the mantle plume still existed up to now. An actual evidence is: since the Pliocene a large amount of alkali basalt erupted in the South China Sea and its adjacent areas.