The geological basis of the China Historical Sea Lines in South China Sea

Forty years of geological and geophysical investigation and research made us clear that Chinese Historical Sea Lines（abbreviated as CHSL) in the South China Sea（SCS) not only demonstrated our sovereign right,but also a combination of giant geological boundaries between continental blocks,islands and countries. This paper will focus on the geological rationality of the CHSL in SCS,based on the investigation of geophysical features,regional geological structures,origin and evolution. The main conclusions are: the location of the eastern CHSL is consistent with the Manila Trench in topography. The west side of the trench is the Central Sea Basin of the SCS,with 4 km to 8 km oceanic crust. The east side of the trench is Philippine Archipelago. Based on paleomagnetic analysis,this archipelago was originally a small continental block located in the south latitude. It drifted northward to the present location along the Manila Trench since Middle to Late Eocene（about 16 Ma to 10 Ma),and thrusted on the oceanic crust of the Central Basin of South China Sea due to the combined action of Pacific Plate and Australia Plate. Thus,the Philippine Archipelago was outside source island arc block. The Huangyan Island is located west of the Manila Trench. It's a volcanic island of oceanic crust of central sea basin of South China Sea. It doesn't belong to the Philippine Archipelago. The location of the southern CHSLs of the SCS in topography is consistent with Nansha Trough. Accompanied with the Nansha Block splitting southward from the northern margin,the oceanic crust of proto South China Sea moved southward and subducted beneath the Kalimantan Continent along the Trough. Thus,Nansha Block was absolutely a different geological source continental block from the Kalimantan Continent. The location of the western CHSLs of South China Sea is the same in topography features as a giant East Vietnam Fault. The Fault might be related to the southward shift of the Xisha,Zhongsha and Nansha Block from north of the SCS. The north edge of Nansha block showed steep and straight normal faults,meaning that the Central basin of the South China Sea was mainly evolved by extension. Multi-channel seismic profiles,drilled records as well as the dredged samples showed that the stratigraphic divisions of several Nansha basins,Zhongsha and Xisha Block were very similar to and might be fully compared with that of the Pearl River Mouth Basin on the northern continental margin and paleo South China Continental Plate,so the Xisha,Zhongsha and Nansha Block originally belonged to the South China Continent,latterly it was split away and moved southward to present location owing to the evolution of South China Sea.