Seismic Structure and the Variation of Magmatic Budget in the Southwest Sub‐Basin of the South China Sea

Abstract The oceanic lithospheric structure is key to understanding the processes related to the interplay between tectonics and magmatism from the end of rifting to the onset and cessation of seafloor spreading. In the oceanic basin of the South China Sea (SCS), the Southwest Sub‐basin (SWSB) is the last‐opening segment before the abandonment of seafloor spreading. The SWSB shows a V‐shaped geometry associated with the SW‐ward propagation of the spreading center during the opening. Here, we present a 540‐km‐long wide‐angle seismic profile along a seafloor‐spreading flow‐line in the central sector of the SWSB. A joint refraction and wide‐angle reflection travel‐time tomography provides a P‐wave velocity (Vp) model of the crust and uppermost mantle. Continental break‐up produced asymmetric conjugate margins, abutted by a typical oceanic crust with a two‐layer Vp structure. The distinct vertical velocity gradients in the upper and lower crust, and a clear Moho, indicate a magmatically‐dominated oceanic system. In contrast, oceanic crust <19.2 Ma displays no Moho interface and a Vp structure indicating reduced magmatism and increased tectonic extension. Anomalously slow Vp structure under the median valley indicates serpentinized mantle resulting from enhanced faulting during the final opening phase. The spatial variation in Vp structure reflects two main temporal stages of the SWSB seafloor spreading before cessation. Previous results of plate kinematics reconstructions appear to support that the transition from magmatically robust to increased tectonics and final cessation of seafloor spreading in the SWSB might relate to the subduction of the proto‐SCS.