Subducting passive continental margins with crustal (ultra)mafic intrusions: An underappreciated mechanism for recycling water back into the mantle

It is well known that outer rise bending-assisted oceanic plate hydration is an important mechanism for transporting substantial amounts of water into the mantle. A key question is: Are there other equally or more important water transport mechanisms? Here we propose, for the first time, that subducting passive continental margins, particularly those with crustal (ultra)mafic intrusions, play a critical role in recycling water back into the mantle. Evidence for this mechanism is the exceptionally high outer rise seismicity observed in a subducting passive continental margin (i.e., the northeastern South China Sea continental margin) near the northern Manila trench, characterized by a high-velocity lower crust that has been attributed to (ultra)mafic intrusions. Our interpretation of this correlation between high outer rise seismicity and lower crust (ultra)mafic intrusions is that (ultra)mafic intrusions alter the crustal rheology and increase brittle deformation in the lower crust in this region, thereby promoting lithospheric fracturing and plate hydration, which is evidenced by increased outer rise seismicity.