Fault growth and linkage in multiphase extension settings: A case from the Lufeng Sag, Pearl River Mouth Basin, South China Sea

Oblique-slip faults are common in multi-phase rift basins. Many complex fault zones are formed by the growth and linkage of several fault segments under oblique extension settings. The LF15 fault zone is the boundary fault zone of the LF15 subsag, which is located in the southern part of the Lufeng Sag in the Pearl River Mouth Basin. Based on the 3D seismic and drilling data, we used seismic interpretation, detailed fault throws analysis, fault activity rate, and isochron map analysis to investigate the geometry and kinematic characteristics of the LF15 fault zone. The growth and linkage process, and three-dimensional evolution of the LF15 fault zone were established. The LF15 fault zone is an E-W-trending oblique-slip fault system which is composed of the lower and the upper fault systems. The lower fault system consists of a zig-zag fault with multiple strike segments, which conforms to the constant-length fault model. The upper fault system contains several overlapped and linked large-scale faults, and a series of left-stepping or right-stepping en-echelon minor faults developed in the hanging wall of the boundary fault. Most faults of the upper fault system conform to the isolated fault model, which grows by radial propagation of tip-lines and forms hard links with adjacent faults. The east side of the fault segments F15 in the upper fault system was formed by the upward propagation of the underlying fault F1, while the west side of the fault segments F11-F14 propagated downward and physically linked with the underlying fault F1. The change of extension direction in two deformation stages of the post-rift affects the strike-slip displacement components and sinistral or dextral movement of the boundary fault, which lead to the left-stepping or rightstepping patterns of the minor faults. Based on the analysis results, we speculate that the pre-existing basement faults affect the nucleation position, orientation, and scale of the lower fault system, and the lower fault system affect the orientation and the direction of oblique-slip motion of the minor faults in the upper fault system. A four-stage evolution model of the LF15 fault zone is proposed.