Role of the Wan-Na fault system in the western Nansha Islands (Southern South China Sea)

Abstract

Since 1987, the South China Sea Institute of Oceanology, Chinese Academy of Sciences, has acquired about 20,000 km of seismic data in Nansha (Spratly) Islands waters, southern South China Sea (SCS). Interpretation and correlation of the profiles crossing the western part of the waters, indicate that the Wan-Na Fault Zone is a dextral strike-slip system composed of the following: (1) Ranai-Sarawak strike-slip-contractional imbricate fan at its southern segment, (2) a strike-slip-extensional imbricate fan at its northern segment in the southwestern end of the Southwest subbasin (SWSB) of the South China Sea, and (3) a strike-slip-pull-apart duplex of the Wan'an basin in the central segment. The northern segment is characterized by the faults that tend to converge toward the side of the Wan-Na fault zone to the southwest and splay toward the Southwest subbasin to the northeast, down-faulted step by step. Lithospheric delamination beneath the South China-Indochina continental margins and seafloor spreading of the South China Sea led to the development of the Wan-Na fault zone as a plate-bounding tectonic element in the region. As a whole, its major activity occurred during the Eocene to Early Miocene. The strike-slip system and pull-apart duplex directly resulted in the generation of the Wan'an basin.