Heterogeneous Magmatism at the Northern Margin of the South China Sea: Insights From Joint Inversion of Refraction and Reflection Travel‐Time Tomography and Melting Modeling

Lower‐crustal magmatic modification has been identified along the northern South China Sea (SCS) margin, yet its controlling factors remain unclear. We analyze the OR1‐999‐6 profile across the continent‐ocean transition using joint refraction‐reflection travel‐time tomography with adaptive importance sampling, yielding an ensemble‐constrained crustal velocity structure. The model resolves three discrete lower‐crustal high‐velocity layers (HVLs), rather than a uniformly thick layer. By combining the covariation of crustal thickness (H) and lower‐crustal velocity (Vp) with melting models incorporating regional mantle source compositions and lithospheric lid thickness, we quantitatively assess the mantle and lithospheric controls reflected in the segmented HVLs. The preferred models indicate a proximal‐to‐distal transition from melting beneath a thicker inherited lithospheric lid with stronger lower‐continental‐crust contamination to depleted‐mantle‐dominated melting with negligible lid restriction at moderate temperatures. These results link the segmented HVLs to progressive source‐lid reorganization, providing a quantitative basis for interpreting possible magmatic modification during rifting‐to‐spreading evolution.