Cenozoic Evolution Model of the Sea‐Floor Spreading in South China Sea: New Constraints from High Resolution Geophysical Data

Abstract

AbstractBased on the analysis of new high resolution gravity and magnetic data, combined with swath bathymetry, the Cenozoic evolution model of the South China Sea is revised. Two major sea‐floor spreading episodes were revealed with different dynamic mechanism, separated by a remarkable tectonic‐sedimentary event near 25 Ma. Sea‐floor spreading started firstly in the East and Northwest Sub‐basins from ~33.5 Ma to 25 Ma, and formed the original oceanic crust with E‐W or NEE‐oriented magnetic anomalies in the northern and southern side of the East Sub‐basin nowadays and whole Northwest Sub‐basin. Since 25 Ma the sea‐floor spreading continued with different direction until 16.5 Ma. The later NW‐SE‐oriented spreading made the NE‐oriented magnetic anomalies in the central East Sub‐basin and Southwest Sub‐basin. Different tectonic areas developed during these two episodic spreading in the oceanic basin. The whole oceanic basin could be divided into three subregions from north to south. The northern and southern sub‐regions are related to the first episodic sea‐floor spreading, while the central one was formed during the later sea‐floor spreading. And the central sub‐region could be further separated into six segments by NW‐NNW‐oriented transform faults. Evidence from magnetic anomalies showed that the sea‐floor spreading in the later episode began in the east segments with C6c anomaly (23.5 Ma), and continued with C6b (22.8 Ma) in the middle segments, and with C5e (18.5 Ma) in the west. We indicated that the central part of the East Sub‐basin and Southwest Sub‐basin experienced progressive rifting from east to west and the spreading direction changed in each segment between 25 Ma and 16.5 Ma. A NNW‐trending transform fault separated the East and Southwest Sub‐basins. It not only cuts the magnetic anomalies with different directions, but also makes a nearly 95 km sinistral offset of the spreading ridge. Before the collision of the Luzon arc with the Eurasia Plate, the South China Sea was a gulf‐shape marginal sea opening to the Pacific Ocean, resulting from westward propagation of a spreading center into the Sunda Continent, as in the case of the western Gulf of Aden.