Based on a high‐resolution seismic reflection data set, we describe and map rift domains at the tip of the V‐shaped NW‐South China Sea (NW‐SCS) Basin. We compare our rift domain map with seismic refraction, magnetic and gravity field data, and previously mapped Continent‐Ocean Boundaries (COB). We also define and map syn‐breakup sedimentary and magmatic sequences across the sediment‐rich NW‐SCS, which enables us to reconstruct the tectono‐stratigraphic evolution of the area. This work enables us to describe and discuss the 3D architecture and nature of the crust at the tip of the NW‐SCS, investigate how extensional and magmatic processes interacted during the breakup, and propose a kinematic model for late rifting, breakup, and finally stalling of the seafloor spreading system. This study provides exceptionally well‐documented insights into the spatial and temporal evolution at the tip of a so‐called “rift propagator,” and enables us to show that breakup in the NW‐SCS may rather correspond to a “retrogrador,” in which the extension zone retreats due to the reorganization of a failing rift system.

Crustal thinning and hyperextension have been clearly imaged and well‐documented in the Qiongdongnan Basin (QDNB), located in the NW South China Sea. Integrated analysis of newly acquired 3D seismic data, covering the majority of the QDNB, reveals the presence of two distinct fault systems: a NE‐trending set and an E‐W‐trending set, both of which played significant roles in controlling sedimentation patterns and crustal deformation during Cenozoic rifting. The NE‐trending faults were primarily active during a first stage (∼42.5–33.9 Ma) and led to crustal stretching. In contrast, the E‐W‐trending faults developed during a second stage (∼33.9–25.5 Ma) and induced crustal necking and hyperextension, resulting in the formation of metamorphic core complexes and associated detachment faults. Kinematic restoration of seismic profiles indicates that detachment fault activity and crustal thinning progressed through a gradual westward migration during the second stage. This evolution followed a core complex necking model, wherein ductile lower crustal material flowed into the footwall region beneath the detachment fault, thereby expanding the exhumation domain until the lower crust in the hanging wall was fully attenuated. Subsequently, fault deformation transitioned from decoupled to coupled behavior, with strain progressively localizing within the brittle upper crust of the hanging wall, ultimately resulting in the formation of a hyperextended crustal architecture.

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.