Two Stages of Rifting Control the Crust Thinning and Basin Evolution: Insights From the Southern Qiongdongnan Basin, NW South China Sea

In rifted margins or rifted basins, the structure of the basin is often complex due to the multistage development of fault systems. As we can only see the result of the basin evolution, it is difficult to judge the influence of early fault activity on the later stages. Here, we present a case study from the southern part of Qiongdongnan Basin (QDNB) where the influence of the early stage can be clearly recognised. Using newly acquired high-resolution 3D seismic datasets, we analyse the prototype and temporal evolution of the study area since the Cenozoic. Three fault systems (FS1, FS2 and FS3) were identified according to fault activities and fault strike. Observation results show a rotation of the stress field at the end of the Eocene, dividing the whole rifting into two stages. FS1 initiated in NW-SE extension at rift stage I (42.5–33.9 Ma), showing a typical basement-involved structure while FS2 and FS3 developed in N-S extension at rift stage II (33.9–23.03 Ma). The faults in FS1 were either crosscut to form a zigzag plane geometry by FS2 and FS3 faults, or became long-lived active faults throughout the rifting period, resulting in a localisation of strain and extremely thinning of the crust. It can also be compared with adjacent basins which have undergone the same regional tectonic evolution history at the northern margin of the South China Sea. The long-lived NE–SW trending faults cause significant variation in the subsidence history, basin structure, and crust thickness along strike in the study area, indicating that faults developed in rift stage I play a significant role in basin evolution at rift stage II.