## 2 Geological Setting  
As a special marginal sea in the western Pacific, the South China Sea is located at the intersection of the Pacific plate, the Eurasian plate and the India-Australia plate. The special tectonic position determines the complexity of its Cenozoic tectonic evolution. From the Early Paleocene to the Early Oligocene, processes of continental margin extension, lithospheric thinning, and mantle stripping, and a series of NE-directed structures formed in the northern part of the South China Sea [16; 17]. Subsequently, the sea-floor spreading of the South China Sea occurred from the Late Oligocene to the end of the Early Miocene. The expansion ridge first formed in the northern part of the South China Sea, and at the end of the Oligocene, the expansion ridge migrated to the central basin and the southwestern sub-sea basin [18; 19; 20]. During the expansion of the South China Sea, the Nansha-North Palawan Block, originally located in the northern part of the South China Sea, gradually moved southward and collided with the Northwest Borneo-Sulu Block at the end of the Early Miocene; then the expansion of the South China Sea stopped. The latest IODP349 voyage made an important contribution to the precise determination of the age of the expansion of the South China Sea. The age of the seafloor magnetic anomaly band reveals that the expansion of the South China Sea began at 33 Ma, the ridge transition occurred at 23.6 Ma, and the expansion of the central ocean basin ended at 15 Ma, slightly later than that of the southwest sub-basin, which ended at 16 Ma [21; 22]. During the expansion of the South China Sea, a series of Cenozoic sedimentary basins were formed.  
The main body of the Wan'an Basin is located in the continental shelf area in the southwest of the South China Sea, as shown in Figure 1. The whole basin is nearly NS distributed in a spindle shape with an area of about 6.6 \(\times\) 10\({}^{4}\) km\({}^{2}\) in Figure 2. It is a fault-depression compound basin formed by the extension and fracture of the crust and modified by strike-slip. Under the influence of the sea floor expansion in the South China Sea and the Wan'an strike-slip fault in the east, the faults in the basin are widely distributed in the Oligocene- Miocene strata. Most of these normal faults, mainly in the NE direction, cut the basement, resulting in the deformation and rupture of the basement in the basin. The Cenozoic tectonic evolution of the basin is broadly divided into three evolutionary phases based on the Xiwei Movement and Guangya Movement, which are the continental uplifting and local rift stretching stage in the Paleocene-Middle Eocene, the fault-depression and retransformation phase in the Late Eocene-Late Miocene and the regional subsidence phase since the Pliocene in Figure 3. The carbonate platform in this area began to develop in the Early Miocene, was active in the Middle Miocene and began to decline and submerge in the Late Miocene. There has been previous work in this area, and it is believed that the carbonate rocks in this area are high-quality hydrocarbon reservoirs.  
Figure 2: Schematic tectonic map of Wan’an Basin. The thick red lines represent the locations of seismic tracks [9; 23]. I Northern depression; II Northern uplift; III Central depression; IV Central uplift; V Southern depression; VI Eastern uplift; VII Eastern depression; VIII Northwest fault zone; IX Western depression; X Southwest slope.