Extensional Breakup Processes and Models of Conjugate Margins Constrained by Double Breakup Unconformities: Insights from the Southwest Sub-Basin, South China Sea

The southwest sub-basin of the South China Sea (SW-SCS) represents a small oceanic basin characterized by a short spreading history and high sedimentation rates, offering exceptional geological archives for investigating critical lithospheric breakup processes. Based on high-resolution multichannel seismic profiles across the conjugate margins of the SW-SCS, together with detailed tectono-stratigraphic interpretation and regional data correlation, this paper systematically analyzes the crustal architecture, stratigraphic sequence, and magmatic features. The results suggest that three first-order interfaces—the seabed, basement top (Tb), and Moho—govern the crustal architecture and morphology of the conjugate margin. Accordingly, four tectonic domains can be subdivided: the necking, hyperextended, ocean–continent transition (OCT), and normal oceanic crust domains. Within the sedimentary sequence, two key breakup-related unconformities—the crustal breakup unconformity (CBU) and the lithospheric breakup unconformity (LBU)—can be delineated based on their reflection patterns. They are defined as the top boundaries of the crustal breakup sequence and lithospheric breakup sequence, respectively. Further, multiple secondary sequences are recognized within the crustal and lithospheric breakup sequences; these hierarchically different breakup sequences thus provide a complete record of the conjugate margin evolution in the southwest sub-basin, from crustal extension and breakup to lithospheric breakup and eventual seafloor spreading. Consequently, a three-stage lithospheric extensional breakup model is proposed: crustal breakup, lithospheric breakup, and seafloor spreading. This model not only offers a high-resolution diagnosis of the breakup dynamics within the SW-SCS but also constitutes a novel theoretical foundation for understanding the genesis of conjugate margin basins and for guiding hydrocarbon exploration strategies.