A crustal structure profile across the northern continental margin of the South China Sea

An ocean bottom seismometer (OBS) geophysical experiment was carried out across the northern continental margin of the South China Sea. Two-dimension kinematic ray tracing was used to construct synthetic records and to define the geological model along the OBS profile. According to the calculation and interpretation of refracted P-wave phases. the crustal thickness decreases from 22 km (OBS-1) in littoral area to 8 km (OBS-15) in deep sea. The lower slope structured by volcanism changes sharply in Moho depth from 22 to 9 km. The volcanoes over the lower slope were markers of ocean-continental transition zone. A high velocity layer (HVL) with an average thickness of 4 km was found in the base of the lower crust over the shelf and slope. The South China Sea was originated by the passive rifting of South China continent and seafloor spreading. However, geological and geophysical data collected in the northern margin of the South China Sea show a small amount of magma intruded into or erupted onto the crust during pull-apart and breakup. Further analysis shows that the HVZ in the base of the thinned continental crust was related to magmatism that occurred after the cessation of seafloor spreading of the South China Sea. Magmatism during the crustal extension and breakup is very weak. Therefore, the northern continental margin of the SCS is classified as non-volcanic passive margin. The southern margin of the South China Sea has a crust with a steady thickness of 18-24 km, which is different from the nor-them margin of the South China Sea with a gradually thinned crust. They constitute an asymmetrically conjugated pair of passive margins. (C) 2001 Elsevier Science B.V. All rights reserved.