Opening of the South China Sea marginal basin: Insights from the tectonic evolution of the ENE-striking Littoral Fault Zone

The northern South China Sea (SCS) continental margin has a very complex geological history within a multiplate convergent setting and has experienced multiphase tectonic movements, during which the largest significant petroliferous basin, the Pearl River Mouth Basin (PRMB), developed. Along the northern boundary of the PRMB, a prominent wide ENE-striking intraplate deformation zone, the Littoral Fault Zone (LFZ), stretches from west to east and is considered to be the major fault structure that formed during the spreading of the SCS and rifting of the northern continental margin. However, a lack of direct constraints on stratigraphic records and fault intersections due to difficulties with offshore geophysical exploration has resulted in uncertainty about the nature of the LFZ, especially the location, geometry and deep–shallow structural coupling, which are still unclear. In this paper, we use newly collected enhanced images provided by 2D long-offset multichannel seismic reflection profiles across the LFZ Pearl River Mouth (PRM) segment to analyse the deep structural and stratigraphic elements along the LFZ. Combining these data with previous findings, we attempt to advance the understanding of the NE–ENE-striking LFZ nature, evolutionary history and controlling effect on the tectonic and sedimentary processes that led to the formation of the PRMB. The results of this work shed new light on the geodynamic processes surrounding the PRMB. We suggest that the LFZ acted as a northern boundary fault with a three-phase transition over time. The first phase of fault activity initiated as a low-angle basement decollement (phase 1), followed by high-angle SE-dipping normal faults, which were mostly inherited from basement faults and reactivated during the Zhuqiong movement (phase 2). Above these two structures, the faults are characterized by strike-slip faulting, which occurred in the Oligocene (phase 3). The overall geometry of the LFZ shows right-step dextral features in the PRM segment that are obviously segmented by NW–WNW-trending faults. Our results highlight the significant role of this ENE-striking fault system in the basin sub-basin (sag) formation on the northern continental margin of the SCS.