Interactions Between the Hainan Plume and Oceanic Lithosphere: Insights From 3D Lithospheric Structure of the South China Sea

Abstract Seismic, petrological, and geochemical studies confirm the existence of the Hainan plume in Southeast Asia, located to the northwest of the South China Sea (SCS). However, the limited number of seismic stations and lithospheric samples in the oceanic basin makes it challenging to confirm its influence on the SCS oceanic lithosphere through seismic tomography or petrogeochemistry. In the eastern sub‐basin (ESB), magnetic‐lineation offsets (including a ∼23.6 Ma ridge jump) and north–south seismic contrasts document asymmetric seafloor spreading. This pattern raises the possibility of Hainan plume involvement and motivates a focused test of lithosphere–mantle coupling across the former spreading axis. We constructed a lithospheric density model of the SCS using 3D forward and inverse gravity modeling, integrating all available geophysical and geological data constraints. This framework allows us to test the lithospheric expression of Hainan plume–SCS oceanic lithosphere interaction and to evaluate the extent to which it may have contributed to the ESB's north–south spreading asymmetry. Modeling results indicate that the north ESB basin flank (N‐ESB) is ∼10 kg/m 3 less dense and ∼50°C warmer than the south flank (S‐ESB) across the extinct spreading axis. This paired density–temperature signal is most consistent with plume–ridge interaction. During spreading, ridge suction would have drawn Hainan plume material laterally beneath the N‐ESB, modifying sub‐lithospheric flow and contributing to the north–south asymmetry of ESB seafloor spreading.