New insights on the seafloor spreading patterns of the northern South China Sea (16° N-23° N)

There are several models which attend to describe the spreading history of the South China Sea and its sea-floor spreading evolution. Based on the magnetic anomaly systematic study in the past, the South China Sea Basin can be divided into the southwestern and eastern parts by the several spreading orientation change stages. The eastern basin is revealed asymmetric between magnetic anomaly isochron 11 (about 32 Ma) and the ceased of the spreading of the South China Sea (magnetic anomaly isochron 5c) (Briais et al., 1993). Recent magnetic lineations showed the existence of magnetic anomaly 15-17 in the northernmost South China Sea area. However, the spreading features of the northern South China Sea (older then 32Ma) and their tectonic correlation with the whole South China Sea are still not well understood. To understand the rifting structures and their tectonic implications in the northern South China Sea, 9 multi-channel seismic profiles (ORI645, ORI654, ORI689, ORI693, ACT, EW9509, MLTW, 97304Aa and 97034Ab), the magnetic data collected off southwestern Taiwan (117° E~121° E longitude and 18.5° N - 23° N latitude), NGDC magnetic trackline data (National Geophysical Data Center from 1950 to present), CCOP magnetic data (Coordination Committee for Geoscience Programmes in East and Southeast Asia), satellite gravity anomaly data, the bathymetry (NGDC and swath bathymetry) are used. Based on the seismic data, the post-rift oldest sedimentary features were identified to be uplifted in the northeastern South China Sea (117.5° E-120.5° E and 19° N-22° N). The oldest features can be divided into a N45° trending part and a N90° trending part, separated by a possible N-S trending fracture zone. This feature is clearly absent at the north region of the LRTPB (Luzon Ryuku Transform Plate Boundary) and could be prolonged southward based on the geophysical data (gravity data , the swath bathymetric data and the magnetic data). Furthermore, in the whole northern South China Sea (112° E-121° E longitude, 16° N-23° N latitude), the orientation of the spreading has a major change between magnetic anomaly isochron 9 and 10. The orientation of the magnetic anomaly 11-17 area revealed E-W to ENE-SWS. However, the oldest sedimentation features in this area trend NE-SW. It is interesting that the anomaly 11 to 17 part of the oceanic crust is not existed at the south side of the South China Sea. There is no evidence which the northern anomaly 11-17 part was the extinct portion of the other spreading system(not belonged to South China Sea). Therefore, the anomaly 11-17 south part of the oceanic crust could be reasonably interpreted as subducted portion beneath the northwest Palawan.