Fig. 1

Significant scientific experiments and key data acquired during this project. Circles represent OBS recording signals caused by active airgun-shooting; blue square indicates the OBEM station; and the black lines correspond to the airgun-shooing lines. The stars are drilling sites of IODP. The orange square indicates the area of Fig. 3. The black dashed line denotes the location of seismic reflection line L1573 in Fig. 4"

Fig. 2

Vertical and depth slices in 3D Vp model. (a) The upper right image shows the seafloor, the location of the vertical slices, and the depth slice at 15 km below the seafloor; (b) The upper left image shows three vertical slices parallel to the strike of continental margin; (c) The lower image shows the vertical slices parallel to margin extension directions. The pink areas in the slices indicate the distribution of the high velocity layer in lower crust. The 3D Vp model shows that there are significant differences in the crustal structure and the distribution of the high velocity layer in lower crust in the northeast and southwest segments of study area"

Fig. 3

Scope of Continent-Ocean Transition domain in the northeastern continental margin of the South China Sea and the thickness maps for different layers, including (a) whole crust, (b) upper crust, (c) lower crust, and (d) high-velocity layer"

Fig. 4

The magmatism in multi-channel seismic reflection profile L1573. (a) Lateral variation of free air gravity anomalies shows high values in the area of dome-shaped high; (b) Line drawing and interpretation of seismic profile L1573. In terms of the data quality associated with the seismic basement the section was subdivided into four domains: well constrained, badly and moderately well constrained, and drill hole constrained. The laccolith distributed in the basement of zone 1, where the discontinuity zone is significantly developed, and caused the uplift of the top basement and syn-rift sedimentation, D1 to D9 represent dome-shaped highs formed by laccolith in sedimentary layer. C1 represents cone-shaped high; (c) For original seismic profile, see location in Fig. 1"

Fig. 5

The variation of magmatic amount during the rifting in SCS margin. The variation of magmatic amount versus extensional process in L1573. The red and blue circles show the estimates of maximum and minimum amount of magmatic additions. The green dashed line and related value represent the rate of production of the oceanic crust, referring to the half spreading rate of 30 km·ma-1 (Li et al, 2014) and an average thickness of 5.8 km"

Fig. 6

The velocity profiles and COT along L2 and L5. (a) Multi-channel seismic profile along L2; (b) Inverted velocity model along L2; (c) Multi-channel seismic profile along L5; (d) Inverted velocity model along L5. Ridges A, B and C correspond to three basement highs, see locations of L2 and L5 in Fig. 1"

Fig. 7

A rift-breakup geological evolution model of the northern margin of the SCS. (a) Rifting: magma underplating to form HVL; (b) Late rifting: magma intrusion to form several basement highs; (c) Breakup: abundant magma came up, uplifted the hyper-thinned continental crust and finally broke up; (d) Seafloor spreading: tectonism decreased and seafloor spreading gradually stabilized"

Fig. 8

Schematic diagram showing the 3D geological model of the along-strike variations in tectonism and magmatism during rift-to-drift process in the study area of SCS. See Fig. 1 for locations of L2 and L5"