Fig. 1

U-shaped boundary in the South China Sea and regional tectonic-geological map of the South China Sea. The color belt represents the U-shaped boundary, while the light yellow areas denote the peripheral basins of the South China Sea. The numbers indicate five sectional intervals of the U-shaped boundary: 1 for the U-NW section, 2 for the U-W section, 3 for the U-S section, 4 for the U-E section, and 5 for the U-NE section. The red dashed line depicts the nine-dash line, drawn based on the topographic map of the South China Sea (Luo, 2023)"

Fig. 2

Three-dimensional depth map of the U-shaped boundary in the South China Sea"

Fig. 3

Map of the U-shaped boundary in the South China Sea and the structural geological units of the Yinggehai Basin, Zhongjiannan Basin, and Wan’an Basin. The red dash-dotted lines are the U-shaped boundary, the blue areas represent depressions, and the yellow areas represent uplifts. Modified from Xu (2018), Wang et al (2022), and Xiao et al (2021)"

Fig. 4

Gravity anomaly of five sections in the U-shaped boundary. (a) Gravity anomaly of the U-NW section; (b) gravity anomaly of the U-W section; (c) gravity anomaly of the U-S section; (d) gravity anomaly of the U-E section; (e) gravity anomaly of the U-NE section; (f) 3D view of the U-shaped boundary gravity anomaly"

Fig. 5

Magnetic anomaly of five sections in the U-shaped boundary. (a) Magnetic anomaly of the U-NW section; (b) magnetic anomaly of the U-W section; (c) magnetic anomaly of the U-S section; (d) magnetic anomaly of the U-E section; (e) magnetic anomaly of the U-NE section; (f) 3D view of the U-shaped boundary magnetic anomaly"

Fig. 6

Planar structural characteristics of the U-NW section of the U-shaped boundary in the South China Sea. The red dotted line represents the U-shaped boundary, the black lines denote faults, and the blue dots indicate diapiric structures, as identified by Lei et al (2015)"

Fig. 7

Direction and structural characteristics of the U-W section and Western Margin Fault, South China Sea. (a) Distribution of the fault; (b) gravity anomaly of the fault. Adapted from Fyhn et al (2009), with numeral 2 indicating the U-W section of the U-shaped boundary"

Fig. 8

Shapes of the U-S and U-E sections of the U-shaped boundary and shelf breaks in the southern South China Sea. The grey shade is the area with water depths from 100 to 200 meters. Numerals 3 and 4 are the U-S section and the U-E section of the U-shaped boundary, respectively"

Fig. 9

Tectonic-geological map of Manila Trench and the U-NE section of the U-shaped boundary"

Fig. 10

Seismic profiles perpendicular to the U-shaped boundary. The position of the survey line is shown in Fig. 1. The red star represents the U-shaped boundary location. L1 is the seismic profile of the Yinggehai Basin of the U1 section (adapted from Sun et al, 2003; Lei et al, 2015). L2 is the seismic profile of the negative flower-shaped structure of the Western Margin Fault, with the white frame indicating the negative flower-shaped structures (adapted from Vu et al, 2017); L3 is the seismic profile of the Nansha Block (adapted from Yao et al, 2023); L4 is the seismic profile of Manila Trench in the U5 section (adapted from Chang et al, 2012)"

Fig. 11

Gravity anomaly of South China Sea. The white line is the U-shaped boundary"