Seismic stratigraphy, tectonic structure and extension factors across the southern margin of the South China Sea: evidence from two regional multi-channel seismic profiles

Two recently acquired regional multi-channel seismic profiles across the southern margin, the South China Sea (SCS), are interpreted. One line is crossing the entire Nansha region from the deep oceanic Southwest Subbasin of the South China Sea, along the Nansha Islands to close to Borneo, and the other one runs east of the Reed Bank. Based on the interpretations of these profiles, we worked out stratigraphic sequences, tectonic structures and extension factors. Five tectonic-stratigraphic units are determined, together with 7 sequence boundaries. Detailed analyses on the extension factors based on the measurement of fault heaves reveal two episode of continent extension separated by a distinct unconformity, which likely correspond with the beginning of sea-floor spreading in the South China Sea. Early extension occurred during continental rifting (Late Cretaceous-Early Oligocene), and resulted in formation of half-grabens and rotated blocks controlled by a deeply rooted detachment system. Extension continued in our study area during the drifting phase of the Eastern Subbasin of the South China Sea (Late Oligocene-Early Miocene) until the subsequent opening of the Southwest Subbasin at about 25 Ma, but its intensity decreased markedly thereafter. Deeply rooted detachment systems evolved possibly during this second phase of extension at continent-ocean transition area. We suggest that a widespread carbonate platform developed across the Dangerous Grounds, concurrent with the drifting period of the Southwest Subbasin of the SCS. Fault-related stretching factors (beta(r)) to the west and east of Reed Bank are found to be comparable and show discrepancy with the whole crust stretching factors (beta(c)) in the study area as derived from gravity modeling. Thus we conclude that the continental crust of the southern margin might have experienced depth-dependent extension.