## 5 Discussion
Many investigations have indicated that the underlying Hainan mantle plume was one of the factors contributing to the expansion of the SCS basin [12; 24]. [PERSON] et al. (2023) [57] suggested that some of the magma in the SCS during rifting and initial seafloor spreading originated from the partial melting of the asthenosphere mantle in the northwestern SCS. The Hainan mantle plume probably facilitated the ensuing seafloor spreading and the strong post-spreading magmatism [57]. The ages of trawled basalt samples in the deep basins of the South China Sea are later than the age of the oceanic crust, and the dating results indicate that magmatic activity was mainly concentrated in the late rifting period, especially after the cessation of seafloor spreading [41]. The activity of the mantle plume in Hainan was mainly after the middle Miocene, indicating that rifting and spreading in the South China Sea were not related to the mantle plume. Numerical simulations by [PERSON] et al. (2021) [89] suggested that the Late Cenozoic magmatic activity was not associated with the deep mantle plume but was more compatible with the extensive mantle upwelling generated by the stagnation plate [89]. The early spreading of the conjugate margin in the SCS was asymmetric, with the northern margin having a significantly higher extension rate than the southern margin [90]. The spreading center moved southward in this period until a symmetric oceanic spreading formed after about 28.7 Ma. The full spreading rates of the ESB and the SWSB were 20-80 mm/a and 35-50 mm/a, respectively [31; 91]. [PERSON] et al. (2021) [82] proposed that highly faulted and thin oceanic crust was typically produced in slow or ultra-low rate spreading with a spreading rate of less than 40 mm/a [82]. The SWSB of the SCS was similar to the central part of the ESB in having an extraordinarily thin and highly faulted crust. The thin, tectonic crust in the ESB and the SWSB suggests that the final spreading phase, which stopped spreading at 20.5 Ma as proposed by [PERSON] et al. (2014) [81], hardly had a high spreading rate (half-spreading rate 38-40 mm/a) [81]. However, it supported a slower spreading rate (half-spreading rate 12.6 mm/a). The oceanic crust associated with the mantle plume was generally much thicker. However, the thickness of the oceanic crust observed at the ESB Center was only 3.7-6.0 km, and the original crust should have been thinner. Therefore, the magmatic contribution of the Hainan mantle plume during the final extension to the post-extensional phase may have been very limited [82]. The formation and tectonic evolution were influenced by continental margin spreading and plate interaction between the three plates and their microcontinents [92]. In the western and southern parts of the South China Sea, the Indo-Australian plate drifts and subtracts northward, while in the east, the Pacific and the Philippine plates drift and subduct northwestward [93]. Based on the tectonic evolution history of the SCS basin, there are several models to explain the formation of the South China Sea. (1) Back-arc extension model [30; 94]: The model suggests that the extension and opening of the Southeast Asian back-arc basin is the result of the oblique subduction of the Paleo-Pacific Ocean and the migration of the subduction zone to the ocean. (2) The collision-subduction model [12; 17; 18]: The model suggests that subduction of the crust under the North Kalimantan and Pacific plates led to the westward opening of the proto-South China Sea. In the Late Cretaceous-Moocene, the PSCS subducted under the Pacific plate and Kalimantan Island [95] and formed an open ocean basin. (3) The collision-extrusion (escape) model [19; 20]: The model implies that the Indian plate moved northward since the Paleocene and collided with the Eurasian plate, which has triggered the strike-slip shear of the main fault zones to support the extrusion movement [12; 23; 24; 25].