Transition from continental- to ocean-verging crustal-scale normal faults in the northwestern South China Sea

The South China Sea (SCS) formed in a forearc and post-orogenic environment related to the Mesozoic subduction of the Paleo-Pacific plate. It started rifting at >65 Ma and ended at 32 Ma in the eastern part and 23–19 Ma in the southwestern part, respectively. The driving mechanisms for the rifting of the SCS mainly include two models. One is the pull-apart basin model, which attributes the driving force to the collision between the Indian and Eurasian plates, inducing block extrusion along the Red River Fault System (RRFS) with sinistral strike-slip motion, while the other is the rifted basin model, which emphasizes the pull from the southward subduction of the Proto South China Sea (PSCS) plate, leading to dextral motion of the RRFS. In this study, we interpret a ~300 km-long seismic line with velocity structure by combining multi-channel seismic (MCS) and wide-angle seismic (WAS) data in the Qiongdongnan Basin (QDNB), which is located in the northwestern SCS and close to the RRFS. We identify a hyper-thinned continental crust with southward (oceanward)-dipping detachments cutting through the crust and sometimes offsetting the Moho vertically up to 8 km. This contrasts with most observations in the SCS that indicate northward (continent-ward) vergence of extensional crustal structures. Based on the thermo-mechanical model of Zhou et al. (2025), we interpret these structures as the products of reactivated orogenic inherited crustal structures. Combining other seismic observations in the northwestern SCS, there is an east to west transition in crustal vergence from northward to symmetric and then to southward. Furthermore, considering the location of our seismic line between the RRFS and the QDNB, we also propose that this reflects the influence of  strike-slip motion along the RRFS, leading to preferential activity of southward-dipping crustal structures under the effect of a horsetail structure. This indicates a dextral activity of the southern RRFS during the SCS rifting, in agreement with the PSCS hypothesis.