CENOZOIC TECTONICS AND DYNAMICS OF BASIN GROUPS OF THE NORTHERN SOUTH CHINA SEA

Abstract

There are a lot of Cenozoic basins from west to east in the northern margin of the South China Sea,including the Beibu Gulf,the Qiongdongnan,the Pearl River Mouth and the Taixinan basins.Previous studies suggested that these basins are derived gradually during the rifting of the southeast continental margin of the South China Block and the formation of the northern passive margin of the South China Sea.However,a large number of seismic profiles revealed that some major basin-controlling faults in the northern margin of the South China Sea are landward-dipping,which are clearly inconsistent with the major seaward dipping faults of a typical passive continental margin basin.Thus,the Mckenzie's extension model of passive continental margin remains controversial for the formation mechanism of the northern marginal basins of the South China Sea.This article,based on the comparison between a large number of field observation and marine seismic data,reveals that the northern continental margin of the South China Sea is not a passive continental margin before 34Ma.The early faults were well developed in NE—NNE-striking,corresponding to the strike-slipping faults in the southeast South China.These dextral,right-step strike-slipping faults are NE—NNE striking,controlling the secondary faults with near-EW or NEE strikes and the distribution of tectonic units in the pull-apart basins.Therefore,a series of Cenozoic basins in the northern margin of the South China Sea is dextral,right-step pull-apart basin group,having close relation in dynamics.This pull-apart model is consistent with the migration of sedimentary-subsidence centers and depocenters,tectonic jumping and magmatic migration within Cenozoic basins.The northern continental margin of the South China Sea actually became a typical passive continental margin since 15 Ma,when the South China Sea oceanic crust terminated spreading.At last,all structures in the northern continental margin of the South China Sea are offset by the late,diffusive NWW-striking faults resulting from the gradual indentation of the Philippines Plate to the Eurasian Plate along the Luzon Arc and the Taiwan Orogenic Belt between 10 Ma and about 5 Ma.Basin dynamics in northern continental margin of the South China Sea is mainly related to the dynamics of the Pacific Plate.