Detachment and Transfer Fault Systems in the Northern South China Sea, Insights Into 3D Tectonic Segmentation of Rifted Margins

The 2D rifting modes interpreted in traditional "magma-poor" and "magma-rich" margins cannot explain the crustal structure and inferred rifting processes in the northern South China Sea (SCS) rifted margin. The "intermediate-type" terminology has been therefore applied to the mid-northern SCS, where a "wide-rift" model has been widely accepted. However, the tectono-magmatic processes of the SCS are still debated and at least five contrasting models exist. We present a compilation of 3-D seismic volumes and regional 2-D seismic surveys covering the entire Baiyun and Liwan Sub-basins to investigate their tectonic structure and faulting style in this "wide-rift" region. We interpret two segments with contrasting tectonic styles separated by a volcanic lineament and steep transfer faults. The Baiyun Sub-basin was controlled by a landward-dipping detachment system. The Liwan Sub-basin, however, was formed by a similar to 100 km-long oceanward-dipping, concave-up detachment fault working at a low angle of <10 degrees. The lateral boundaries of the detachment system were mechanically decoupled from surrounding tectonics by a volcanic lineament/transfer zone to the west and a > 190 km-long N-S-trending left-lateral strike-slip fault to the east. The planar low-angle detachment does not resemble classical metamorphic core complex domes interpreted previously. Our results indicate a 60-80 km-wavelength segmentation within a single "wide-rift" system, indicating complex 3D rifting during crustal extension. This study supports that the intermediate SCS margin had a kinematically complex deformation style locally dominated by the interaction between detachment and transfer fault systems that might be yet unrecognized in other margins.