## 3 Methods and Model Setup
### Model Setup  
The initial model set up an ideal undeformed continental lithosphere and simulated its extension under stress. The model did not consider co-deposition and erosion, which could affect lithospheric extension [66]. The model also did not consider the formation of a new oceanic crust at the spreading center, and possible peridotite serpentinization in the oceanic crust region could affect the rheological properties of the rocks [67], which cannot be reflected in the model. This also suggests that there is still a need for subsequent development of more complex models to correspond to the reality. The setting of the simulation model should be based on the actual situation of the study area. From the Late Cretaceous to the Eocene, the South China Sea was in the continental rift stage, and many sedimentary basins were formed [42]. In the Paleocene-Early Eocene, a continental fluvial-lacustrine depositional system developed in the northern margin of the South China Sea. And littoral and nerritic facies clastic rock deposits only occurred in the West Taiwan Basin and Southwest Taiwan Basin. During the Paleocene-Eocene, the basins along the southern margin of the South China Sea mainly developed nerritic-hemipelagic facies deposits. During the Late Eocene and Oligocene, the South China Sea began to spread, and the Nansha block separated from the South China block and drifted southeast. In the Early Oligocene, the marine transgression began to occur in the northern margin of the South China Sea, which mainly developed marine-continental interaction facies sandstone and mudstone deposits, and the marine facies in the southern margin of the South China Sea gradually expanded [68]. The drilling data show that the pre-Cenozoic basement has been drilled in the Pearl River Mouth Basin, Qiongdongnan Basin, and Beibuwan Basin at the northern edge of the South China Sea, and the lithology is dominated by granite and metamorphic rocks. The basement of the southern basins of the South China Sea mainly consists of pre-Cenozoic neutral acidic-basic igneous rocks and metamorphic rocks. Among them, the basement of the northeastern continental margin is mainly Mesozoic sedimentary rocks, the northwestern part is Paleozoic metamorphic rocks, the southeastern part is sedimentary rocks, the western land margin is Mesozoic sedimentary rocks, and the southern part is Mesozoic metamorphic rocks [69]. The crustal rocks above the Moho surface are mainly composed of felsic rocks such as gabbro and granulite, while the mantle below the Moho surface is mainly composed of peridotite [67]. The crustal density distribution in the South China Sea region calculated by [PERSON] et al. (2022) [70] using the Parker-Oldenburg method is generally distributed in the range of 2700 kg/m\({}^{3}\) to 2900 kg/m\({}^{3}\), with the density of the South China Sea basins as well as the north and south continental margins above 2800 kg/m\({}^{3}\), with a higher density in the basin, and the Southwest Taiwan Basin and Palawan Basin showing a lower density below 2760 kg/m\({}^{3}\), related to the distribution of low-density Mesozoic blocks in this region [70]. The bottom interface of the sedimentary layer is 2670 kg/m\({}^{3}\)[71], and the density of the mantle is about 3300 kg/m\({}^{3}\)[72]. The effective viscosity coefficient of the sedimentary layer in the South China Sea is 10\({}^{22}\)-10\({}^{23}\) Pa-s, the effective viscosity coefficient of the upper crust varies between 10\({}^{21}\) and 10\({}^{24}\) Pa-s, and from the northern continent to the central basin, the value of \(\eta\) gradually decreases from 10\({}^{24}\) to 10\({}^{21}\) Pa-s, the effective viscosity coefficient of the lower crust varies between 10\({}^{21}\) and 10\({}^{23}\) Pa-s, and the average viscosity of the upper mantle of the rock layer is 10\({}^{20}\)-10\({}^{21}\) Pa-s [73]. The average viscosity coefficients of the upper and lower mantle are 10\({}^{19}\) Pa-s and 10\({}^{20}\) Pa-s, respectively [74].