The Baiyun and Liwan Sags: Two supradetachment basins on the passive continental margin of the northern South China Sea

Abstract

Detachment faults have been reported on the northern and southern margins of the South China Sea (SCS) but have not been recognized in the Paleocene-Eocene Baiyun and Liwan Sags of the Pearl River Mouth Basin (PRMB). This is despite the well-known, very thin crust beneath the two sags. In this study, we use 2D and 3D seismic data to interpret the sequence stratigraphy, as well as identify faults and other important structural features in the Baiyun and Liwan Sags. We show that high-amplitude seismic reflectors, corresponding to detachment faults in the two sags, occur in multistep ramp-flat geometries associated with important structures including: (1) alternating sub-sags and sub-uplifts developing on the hanging walls, (2) secondary faults, (3) dome structures, and (4) extensional allochthons. The detachment faults dip landward and seaward in the Baiyun and Liwan Sags, respectively, indicating paired detachment systems. Hence, integrated with thin crust and subsidence centers offset at great distances from the fault-bounded margins, the Baiyun and Liwan Sags are demonstrated to be supradetachment basins seated on the distal domain of the northern SCS's passive continental margin. The well-correlated sequence stratigraphic framework in all sub-sags shows that the detachment faults fit the standard detachment model in which faults are continuously active. Furthermore, two detachment basins are inferred to have developed over hot ductile crust resulting from plate subduction in the Mesozoic. Since the upper crust stretches far less than the middle/lower crust, the fault-controlled extension is proposed to have had a smaller contribution to the thinned crust than a ductile extension did within the middle/lower crust in the study area. The recognition of detachment faults and supradetachment basins is a significant contribution towards understanding the formation of the northern passive continental margin of the SCS.