The Tectonics and Seafloor Spreading Mode of the Eastern South China Sea

Abstract:

The South China Sea could be separated into several sub-basins. Among all, the eastern subbasin of the South China Sea occupies the largest portion. The importance of this basin is that the eastern basin holds the main key of understanding the tectonic evolution of the South China Sea. Besides, its subduction southeastward beneath the Philippine islands along the Manila Trench may generate big earthquakes and/or tsunamis affecting the coastal countries surrounding the South China Sea. To better understand the tectonics of the eastern South China Sea basin and the Manila Trench, we have conducted a marine geophysical survey from June 23 to July 13, 2014. Our results show that the extinct mid-ocean-ridges of the basin are displayed en echelon. The ridge subduction beneath the Luzon Island probably does not occur or is just in the early beginning. For the seafloor spreading of the eastern South China Sea, there was probably no ridge jump in magnetic lineation 7 as suggested by Taylor and Hayes or Briais et al. Based on the analysis of our new reflection seismic profiles, bathymetric and magnetic data, the tectonic evolution of the eastern South China Sea basin could be described in three stages. Before magnetic lineation 7, the oceanic crust spread in N-S direction. Between magnetic lineations 7 (~25Ma) and 6c (~20 Ma), the seafloor spreading was in NW-SE direction. However, the supply of the upwelling magma along the mid-ocean-ridges was increasing during this second stage, especially in the eastern side. The abnormal supply of magma thus caused the fan-shaped seafloor spreading fabrics of the eastern South China Sea. Even after the cessation of the seafloor spreading, a supply of the upwelling magma had continued between 20 and 16 Ma, which caused the disturbed sedimentary layers, mainly in the middle area of the basin.