Lithospheric Footprint of Mantle Upwelling Beneath Late Cenozoic Basalts in the Beibu Gulf Basin, Northwestern South China Sea

In the convergence area of the southeastern margin of the Tibetan Plateau and the northwestern South China Sea (SCS), the Beibu Gulf Basin (BGB) has experienced a complex geological evolution profoundly influenced by the lateral extrusion of the India-Asia collision and the rifting of the SCS continental margin. Nonetheless, late Cenozoic basalts are widespread in the BGB following the cessation of SCS seafloor spreading, with controversial dynamic mechanisms. To investigate the mantle dynamics beneath these late Cenozoic basalts, we aim to investigate the high-resolution lithospheric structure of the BGB region. Our study utilizes data from 162 seismic stations, including three temporarily deployed dense arrays, to investigate the seismic structural features of the Moho and Lithosphere-Asthenosphere Boundary (LAB). P-to-S receiver function analysis reveals a significantly thinned crust with high Vp/Vs ratios beneath the late Cenozoic basalt regions, coinciding with the occurrence of deep crustal earthquakes. Using S-to-P receiver functions, a shallow, strong, and broad LAB is observed beneath the BGB, implying the presence of ponded partial melt. These seismic structural features indicate that the mantle upwelling beneath the BGB likely interacted with the crust, leading to mafic crustal composition, deep earthquakes, and magmatic eruptions. These findings provide new evidence for mantle dynamics in the northwestern SCS, enriching our understanding of interactions between the mantle and crust in the context of intraplate magmatism and deepening seismicity.