## 6 Conclusions  
Numerical simulations show that the tectonic asymmetry of the South China Sea basin was influenced by the extension rate, which can affect the rock rheology, produce different structures, and control the migration of ocean ridges. The rapid expansion rate was one of the important factors in the asymmetric expansion of the model, with other factors held constant. With rapid lithospheric extension thinning and mantle upwelling, the lower crust was heated above the Moho temperature, and the layer viscosity was significantly reduced. The lower crust of low viscosity decoupled the mantle and crust, deflecting the mantle upwelling, resulting in the thinning and the highest heat flux towards one margin. The lithospheric mantle continued thinning in the stress concentration area, with the crust being pulled apart before the lithospheric mantle, eventually forming an ocean basin. However, when the extension rate was low, the model expanded almost symmetrically, and the lithosphere thinning occurred at a slow rate.  
The simulation results of Model 1 are relatively similar to the actual profile in Figure 2a. This indicates that the east sub-basin and the northwest sub-basin have higher extension rates (~60 mm/a). Model 2 showed nearly symmetrical spreading, with the center of spreading located in the center of the model, and the lithosphere ruptured to form an oceanic basin, and the resulting faults were mostly concentrated near the center of the model spreading, and were symmetrical normal faults. In Model 3, the lithosphere thinned slowly, and a large number of high-angle normal faults appeared in the model, forming a nearly symmetrical rift basin, and no ocean basin was formed. Model 2 resulted in the formation of an ocean basin within a short time (~8 Ma) of lithospheric rupture, and Model 3 developed large detachment faults, which were similar to the actual situation of the SWSB, respectively. Therefore, it is hypothesized that the SWSB expanded at a slower rate (~25 mm/a) during the early stages of spreading and that the rate should have increased slightly during the middle and late stages of spreading, with an average rate of ~40 mm/a. This also proves that the spreading rate of the SCS's ESB was faster than that of the SWSB.  
We believe that the subduction of the proto-South China Sea played a crucial role in the opening of the South China Sea, providing a more plausible mechanism. The opposite movement of the Indo-Australian plate and Kalimantan may have inhibited the formation of the southwest sub-basin of the South China Sea, resulting in a later spreading of the southwest sub-basin than the east sub-basin, as well as a lower rate of spreading than the east sub-basin.  
Conceptualization, C.L.; methodology, C.L.; software, C.L.; validation, C.L.; investigation, C.L. and J.L.; data curation, C.L.; writing--original draft preparation, C.L. and J.L.; writing--review and editing, C.L. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Key Research and Development Program of China, grant number 2023 YFC2906501 and the China National Science and Technology Major Project, grant number 2016 ZX05033001.  
Not applicable.  
Data Availability Statement: Data are contained within the article.  
The simulation in this paper was carried out using MATLAB, and the code used was modified from Gerya (2019). The base map in Figure 1 was generated using GMT6. I would like to express my sincere thanks here.  
The authors declare no conflict of interest.