Abstract: The conventional view suggests that the subduction of the South China Sea plate beneath Luzon occurred due to the oceanic lithosphere's high density, facilitating subduction initiation. However, before the South China Sea opened, a continental margin likely existed, meaning that Luzon was directly adjacent to the continental margin rather than the oceanic basin. This would make subduction initiation more challenging. Here, we propose a new model suggesting that during the formation of the South China Sea, extensive mafic magmatic underplating occurred along its continental margin. The high-density magmatic additions may have increased the overall density of the continental margin, potentially exceeding that of Luzon, thereby enabling subduction to proceed.