Characteristics and genesis of the Zhongnan Fault Zone in the South China Sea oceanic basin: insights from an integrated analysis of multibeam bathymetric and two-dimensional multi-channel seismic data

Abstract

The Zhongnan Fault Zone (ZFZ) is a large-scale tectonic belt in the South China Sea (SCS) oceanic basin, playing an important role in the formation and evolution of the basin. Nevertheless, debates persist regarding its location, orientation, nature, time of activity, and genesis. In this study, we investigate the characteristics and origin of the fault zone through an integrated analysis of multibeam bathymetric and two-dimensional (2D) multi-channel seismic data. Our results reveal the ZFZ as a fault zone approximately 400 km long and 50-90 km wide, situated between the east (ESB) and southwest (SWSB) sub-basins. The ZFZ is oriented N8°W, roughly perpendicular to and laterally displacing the relict spreading center and related spreading lineaments. Bounded by discontinuous linear seamounts, the ZFZ comprises two V-shaped sub-parallel pull-apart basins and a separating basement high. Steeply dipping (>60°) normal basement-involved faults bound these pull-apart basins, forming typical negative flower structures. Numerous NE-oriented en-echelon linear bathymetric highs within the ZFZ are identified as secondary antithetic shears. These shears are characterized by their orientation relative to the principal displacement zones defined along the pull-apart basins. The ZFZ exhibits differences from adjacent sub-basins in water depth, basement burial, stratal thickness, and seismic stratigraphic characteristics. Five seismic sequences (S1-S5 upwards) in the ZFZ and nearby ESB and SWSB are defined, dating to Early Miocene syn-spreading (S1) and Middle Miocene to Recent post-spreading (S2-S5) stages, respectively. The difficulty in correlating seismic facies in sequences S1-S3, compared to the comparable seismic facies in sequences S4-S5 between the ZFZ and adjacent sub-basins, suggests a horizontal displacement during the syn-spreading and early post-spreading stages. We propose that the ZFZ functioned as a right-lateral transform fault zone during the syn-spreading period (∼24-16 Ma) of the SWSB and transitioned into a left-lateral strike-slip fault zone during the successive early post-spreading period (∼16-5.3 Ma).