Effects of serpentinized mantle on subduction dynamics and water influx estimates: Numerical modeling of the Mariana Trench

The serpentinized mantle in subduction zones plays a key role in transporting water into Earth’s interior and influencing subduction dynamics. However, estimates of serpentinization vary significantly across and within different trenches. Using 2-D thermomechanical numerical models, we investigate how variations in subduction angle, thickness, and serpentinization degree influence subduction processes in the southern and central Mariana Trench. Our models reveal three distinct subduction regimes: stable subduction, crustal exhumation, and crustal accretion, which depend on the thickness and degree of serpentinization. Stable subduction occurs in the southern and central Mariana regions where the serpentinized mantle is <13 km and 14 km thick, respectively. In the southern Mariana Trench, buoyancy-driven crustal exhumation occurs with a serpentinized mantle thickness of 14−17 km, consistent with the presence of high-pressure metamorphic rocks in the forearc. Crustal accretion, while theoretically possible, is not observed in the Mariana Trench, suggesting the serpentinized mantle is either thin or only partially serpentinized (e.g., ≤4.4 wt% at a thickness of 21 km). By integrating our model predictions with geophysical observations, we estimate the maximum thickness of fully serpentinized mantle at 17 km in the southern Mariana and 13 km in the central Mariana Trench, implying up to 30% higher water influx in the southern region. These findings refine constraints on water input variations along the Mariana Trench and highlight the significance of serpentinization in interpreting geophysical anomalies in subduction zones.