Variability of syn-rift geometry in Pearl River Mouth Basin, China: implications for faulting patterns in two-phase rift basins.

Recent studies of two-phase non-coaxial analogue experiments and natural rifts suggest that the pre-existing faults that form in the first-phase rift could strongly influence the fault development during a subsequent phase of extension. However, related models from natural examples are still lacking. Here we compare the fault geometry and evolution with different pre-existing fault arrays in two adjacent areas (Xijiang and Lufeng sags) from the Pearl River Mouth Basin in the South China Sea. This basin has experienced two-phase rifting including the middle Eocene rift phase 1 (NW–SE extension) and a late Eocene–early Oligocene rift phase 2 (N–S extension). The Xijiang Sag developed an approximately NE-striking listric fault system, while the Lufeng Sag formed complex structure is composed of six major faults with various orientations. We demonstrate that the first-phase fault network in the Xijiang Sag is a colinear listric fault system, while that in the Lufeng Sag comprises two sets of (non-colinear) faults. In the Xijiang Sag, the second-phase fault network consists of the reactivated first-phase faults, newly formed faults abutting against or cross-cutting the pre-existing reactivated faults or occurring between the pre-existing faults. In contrast, in the Lufeng Sag, the second-phase faults include partially reactivated first-phase non-colinear faults and new non-colinear faults. The influence of first-phase faults on second-phase faults is manifested in several distinctive ways. Two sets of major non-colinear faults led to the development of more complicated oblique faults in the Lufeng Sag. The development of the second-phase faults is the result of a combination of geometry and heterogeneity of pre-existing fabrics, variation of extension direction and local perturbations set up by the geometry of pre-existing faults. The Xijiang Sag is controlled by a NE-striking listric fault system, the Lufeng Sag formed complex structure composed of six major faults with various orientations. The heterogeneous intra-basement fabrics led to a faulting viariablity between the Xijiang and Lufeng sags. The influence of first-phase faults on second-phase faults is manifested in several distinctive ways. [ABSTRACT FROM AUTHOR]