Matching Relationship Between Pre-existing Faults and Regional Stress Field Controls the Differential Development of the Cenozoic Fault System in the Zhu I Depression

As the proximal zone of the passive continental margin in the northern South China Sea, the Zhu I Depression developed on the complex basement of the pre-Cenozoic active continental margin. It has long been located at the intersection of the West Pacific and Paleo-Tethys tectonic domains, characterized by a complex basin-forming setting, multiple stages of tectonic evolution, and significant spatiotemporal differences in its fault system. Based on seismic data interpretation, tectonic physical modeling, and regional tectonic analysis, this study systematically explores the controlling effect of the matching relationship between pre-existing faults and late-stage Cenozoic regional stress field on the fault system. Results show that: (1) Multiple superimposed "compression-extension-strike-slip" tectonic movements during the Mesozoic Indosinian and Yanshanian periods generated NE- and NW-trending pre-existing faults in the Cenozoic basement of the Zhu I Depression. The distribution density and strike of these basement faults controlled the spatial pattern of the Cenozoic fault system, with pre-existing faults being "dense in the east and sparse in the west" and Cenozoic faults "abundant in the northeast and scarce in the southwest", showing an orderly strike transition from NE to E-W to NWW from southwest to northeast. (2) The Cenozoic regional stress field of the Zhu I Depression underwent a clockwise transition: NW-SE extensional (Eocene) → nearly N-S extensional (early Oligocene) → NE-SW extensional (post-late Oligocene). Correspondingly, the dominant strike of the fault system changed from NE (Eocene) to nearly E-W (early Oligocene) and then to NW (post-late Oligocene). (3) Tectonic physical modeling reveals that stage-specific and direction-selective reactivation of NE- and NW-trending pre-existing faults under varying regional stress conditions caused the spatiotemporal differences in the Cenozoic fault system. Under Eocene NW-SE extension, NE-trending pre-existing faults were reactivated as depression-controlling faults, while NW-trending basement faults acted as weak vertical transfer zones, leading to segmented NE-trending faults with slight strike-slip components.During early Oligocene nearly N-S extension, NE-trending faults continued normal faulting with weakened intensity, while NW-trending faults intensified. Under oblique extension, both showed combined strike-slip-extensional characteristics, with new nearly E-W extensional faults or "arc-shaped" faults formed at conjugate positions. Post-late Oligocene, with NE-SW extension, NW-trending faults became dominant with enhanced activity, while NE- and NEE-trending faults diminished and became extinct.This indicates that selective reactivation of NE- and NW-trending basement pre-existing faults during the clockwise transition of the regional stress field from the Wenchang to Enping periods is the primary factor controlling the differential development of the Cenozoic fault system in the Zhu I Depression.