## 5 Discussion  
The South China Sea is a passive continental margin of tensional genesis, which experienced the process of continental margin stretching, rupture, and seafloor spreading during the Late Mesozoic to Early Cenozoic. The crustal structures on the east and west sides of the South China Sea continental margin are significantly different in terms of upper/lower crustal thickness ratio and deformation characteristics [83]. The differences of tectonic activities in the southern and northern margins of the South China Sea are obvious [47]. The genetic mechanism of continental margin extensional basins is closely related to the lithospheric extensional fracture model [84]. The extensional deformation of the lithosphere is the pure shear extensional deformation, forming the basin-ridge-type fault basin system controlled by high-angle normal faults. With the continuous extension, the strength increases and strain concentration occurs, forming large detachment faults. Corresponding to different stages of the evolution of continental margin tensional rupture, the proximal zone, necking zone, distal zone, and ocean-continent transition zone are formed from the land to the ocean [49]. In the initial stages of the rift, the distal and proximal edges of the margin behave differently. In the distal zone of the continental margin, the crust is generally wedge-shaped and brittle, and a large number of extensional detachment faults are developed [85; 86]. The northern margin of the South China Sea is similar to the general continental margin. In the early stage of tensional rupture, the distal zone of the northern continental margin generates multiple high-angle normal faults, which control the development of semi-graben or simple graben. With the continuation of extension, some early high-angle faults evolved into large detachment faults, and the lower crust began to be exposed [86; 87]. In the process of the SCS underwent expansion, its southern margin was subjected to a southward pulling initiated by the PSCS block subduction, and the collision between Kalimantan Island and the southern margin changed the crustal tectonics in the southern margin, while the northern margin did not [88]. Therefore, the southward flow of the lower crust occurred, which changed the crustal spreading pattern from symmetric to asymmetric. The modification of the late western collision was also limited, and the extension pattern after the collision remained asymmetric. The apparent transformation of the eastern collision shifted the crustal extension pattern from asymmetric to generally symmetric [7]. The passive continental margins are divided into magma-rich type and magma-poor type, and magma plays a decisive role in the process of tensile cracking and rupture of the passive continental margins. Magmatism in the continental margins occurred before the thinning of the lithosphere [83], and the addition of high-temperature rock veins will lead to the weakening of the rheological structure of the lithosphere, which is conducive to greater deformation of the lithosphere under the same tensile stress. In the process of continental rifting, basaltic magma appeared rapidly in the South China Sea basins, changed from the continental margin to the ocean basin in a short time, and the rupture process was very rapid. The faults at the margins of the South China Sea reflect the close connection between tectonics and magmatism. Similar to other wide rifts, the northern margin of the South China Sea has developed core complex tectonics, which were accompanied by magma intrusion during the extension process. During rifting, the weakening process changed from mechanical weakening along faults to fault localization by serpentinization and magmatism. These melts made the lower crust more ductile, which prevented the strain concentration during the rupture period, resulting in the simultaneous development of multiple detachment faults [86; 87].