Cenozoic plate tectonic activities in the Great South China Sea area
YAO Bo-chu, WAN Ling, WU Neng-you(Guangzhou Marine Geological Survey, Guangzhou 510075, Guangdong, China)
There existed some blocks (micro-plates) in the oceans between Australia and Asia in the Cenozoic, when some blocks were separated from the Australian plate and moved northward and collided and sutured with some blocks that were separated from the Eurasian plate. In this period small ocean basins such as the South China Sea, Sulu Sea, Celebes Sea and Andaman Sea formed as a result of block separation and seafloor spreading, and finally the present tectonic framework formed in the Great South China Sea area. After a study of the Cenozoic tectonic history of the Great South China Sea area, the authors believe that Cenozoic tectonic activities in the Great South China Sea were not only related to collision between the Indian Plate and Eurasian Plate but also to subduction of the Pacific Plate beneath the Eurasian Plate and were also affected by the northward movement of the Australian. Plate. In the South China Sea Basin there occurred two events of seafloor spreading in the Cenozoic. The first seafloor spreading, which was oriented in a NW-SE direction, occurred before 42-35 Ma BP under the influence of the southeastward mantle flow beneath the Eurasian continent caused by India-Eurasia collision. The first seafloor spreading gave rise to the Southwest Basin of the South China Sea. The second seafloor spreading took place before 32-17 Ma BP. As the Pacific plate was subducted beneath the Eurasian plate to 700 km depth, the SE-directed flow of the upper mantle of the Eurasian continent was blocked and then turned toward the south, thus causing N-S-trending seafloor spreading in the South China Sea area, i.e. the second seafloor spreading. The second seafloor spreading resulted in the formation of the Central Basin of the South China Sea. After the Cenozoic South China Sea Basin was produced, collision between the blocks and seafloor spreading continued in the Great South China Sea area, and under the compression of these northward blocks the south margin of the South China Sea, sediments in the area were deformed, thus producing the Wanan movement (at about 10 Ma BP) on the south margin of the South China Sea.