Abstract: The Baiyun sag in the deepwater area of the Pearl River Mouth basin is characterized by complex tectonic features and low exploration levels. There is still a need for in-depth research on the fault structures within the sag, and the activity of faults during the rifting stage and the dynamic evolution of the regional stress field remains unclear. To reveal the fault activity characteristics and dynamic mechanisms during the rifting stage in the Baiyun sag of the northern margin of the South China Sea, this paper takes the Baiyun sag in the Zhu Ⅱ depression as an example. Based on the latest 3D seismic data and stratigraphic interpretation schemes, methods such as coherence attributes and ancient fault throw analysis are used to analyze the tectonic characteristics of the Baiyun sag. The main active faults in the Baiyun sag are classified into three levels: First-order basin-controlling faults, second-order sag-controlling faults, and third-order zone-controlling faults. Based on high-resolution 3D seismic data, five fault types were identified in the study area: Flower-like faults, imbricate faults, step faults, Y-shaped faults, and nested V-shaped faults. Four fault combination patterns were identified: Parallel faults, en-echelon faults, oblique faults, and broom-like faults. By quantitatively analyzing the characteristics of 507 faults across different periods, the dynamic evolution of the regional stress field and its associated dynamic mechanisms were further explored. The study shows that from the Late Eocene to the Early Oligocene, the fault orientations underwent a clockwise rotation from NE-NEE to EW. Based on the analysis of ancient fault throw, the activity intensity of the first-order faults gradually weakened, and significant differences were observed in the activity of different faults during the same period, as well as in different segments of the same fault. Three phases of fault activity were identified in the study area: The Late Wenchang, Early Wenchang, and Enping Formations. Finally, the paper discusses the dynamic mechanisms during the rifting stage of the Baiyun sag, which corroborates previous views that it was a result of the coupled effects of the collision between the Indian and Eurasian plates, the rollback of the Pacific Plate subduction, and the continuous southward migration of the ancient South China Sea. This formed a typical extensional tectonic background in the study area. By revealing the fault activity characteristics and dynamic mechanisms in the Baiyun sag, this study contributes to Eocene rifting-stage oil and gas exploration in the northern South China Sea and provides important insights for analyzing fault activity in similar basins.

Key words: northern margin of the South China Sea, Pearl River Mouth basin, Baiyun depression, regional stress, fault activity, fault types, combination patterns