## 2 Geological Background  
The South China Sea is spread in a NE-SW rhomboid shape, located at 5\({}^{\circ}\) N-25\({}^{\circ}\) N, 105\({}^{\circ}\) E-122\({}^{\circ}\) E (Figure 1). The SCS was formed at the convergence of the Pacific, the Indo-Australian, and the Eurasian plates, and it is located in the superposition of the Neo-Tethys (collision) and the Pacific tectonic domain (subduction). A large amount of oceanic lithosphere was subducted into the underlying mantle. Therefore, the SCS basin and its surrounding areas have been subject to intricate tectonic evolution since the Cenozoic, forming a trench-arc-basin ocean-continent subduction system [28; 33; 34; 35]. The research area has experienced four major stages: the northwesterly extrusion of the Pacific plate (Mesozoic), rifting (Late Cretaceous to Eocene), spreading of the seafloor from the Oligocene to Middle Miocene, and arc-continental subduction and collision from the Middle Miocene to the present after spreading stopped [4; 36]. The northern SCS developed atypical NE-ENE trending dominated passive continental margin rifted basins. The south is mainly a subduction-collision zone with an extrusion basin. The western South China Sea mainly developed the strike-slip pull-apart basins. The east of the SCS developed a N-S trending compressional basin zone. The central part of the SCS basin is a seafloor spreading zone [37; 38]. Figure 1: Tectonic map of the South China Sea and its surrounding areas. NWSB: Northwest Sub-basin; SWSB: Southwest Sub-basin; ESB: East Sub-basin; 1: Pearl River Mouth Basin; 2: Southwest Taiwan Basin; 3: Yinggehai Basin; 4: Qiongdongngan Basin; 5: Zhongjiannan Basin; 6: Wan’an Basin; 7: Nanweixi Basin; 8: Beiikang Basin; 9: Nansha Trough Basin; 10: Zengmu Basin; 11: Brunei-Sabah Basin. The base map is based on GMT6 [39], Global Bathymetry and Topography at 15 Arc Second: SRTM15+ [40], Mercator projection. Magnetic anomaly strip modified from [41; 42; 43]. Late Cenozoic magma modified from [41]. Subduction zones and faults are modified from [44; 45; 46]. The two yellow lines are the trace of the geological cross sections provided in Figure 2.  
The SCS conjugate mainland margins differed in both N-S and E-W directions [47]. The SCS northern passive margin showed thinned continental crust and aborted oceanic rifting [48], with large co-depositional stratigraphic thicknesses in the northern margin. The southern margin behaved as a typical undercompensated basin. The southern margin lithosphere thickness was much thinner compared to the northern margin [49]. Spatially, the east sub-basin was wider on one side and had strong post-rift magmatism [50], while the SWSB was relatively narrower and had wider continental margins on both sides (Figure 2). The data in Figure 3a are from the Earth magnetic anomaly grid EMAG2 with a resolution of 2\({}^{\prime}\)\(\times\) 2\({}^{\prime}\)[52]. The main trend of magnetic anomalies in the South China Sea Basin is N-E and nearly E-W, the trend of magnetic anomalies in the East Sub-basin is nearly E-W, and the trend of magnetic anomalies in the Southwest Sub-basin is NE-SW. The magnetic lineations are approximately parallel to the spreading center of the mid-ocean ridge, which propagated from east to west during the formation of the South China Sea Basin. The oldest identified magnetic lineations are about 32 Ma, and the latest magnetic lineations are about 15.5 Ma [41; 42]. The distribution of striped magnetic anomalies (Figure 3) suggests that the SCS basin was formed by a gradual east-west expansion and that the eastern part of the basin began to expand before the western part [53], which began at about 32 Ma and stopped at 15.5 Ma [31]. The former N-S directional spreading formed the Northwest Sub-basin (NWSB) and the East Sub-basin (ESB), and the ESB seafloor spreading began at 29.7 Ma [54]. During this stage, a series of N-S trending pull-apart basins were formed. At \(\sim\)27-23 Ma, the ESB was characterized by NNW-SSE directional extension.