Stable continental-scale drainage of North America reveals hydrated upper mantle anomaly due to long-lived oceanic subduction

Seismic tomography provides critical insights into Earth’s evolution, yet the origin of deep-mantle seismic velocity anomalies—particularly slow anomalies—remains debated. Here we constrain the nature of the slow anomalies within the mantle transition zone (MTZ) beneath eastern North America by quantifying their dynamic impact on continental-scale drainage evolution and offshore sedimentation since the Miocene using coupled mantle–surface process modeling. We show that reproducing the observed stability of the Mississippi River basin, the long-term subsidence of the eastern North American margin, and the sedimentary record of the Gulf of Mexico requires a dynamic-topography scenario consistent with neutral net buoyancy of these slow anomalies. Independent geophysical observations further support this interpretation: the MTZ slow anomalies spatially correlate with the remnant Farallon slab within the lower mantle, and coincide with regions of elevated electrical conductivity. This implies that the slow seismic anomalies beneath eastern North America are best explained by hydratedcompositional heterogeneity associated with long-lived Farallon subduction, rather than by a purely thermal origin. Our results further support regional buoyancy compensation, in which dense melts above the MTZ are offset by buoyant hydrous and/or thermal contributions, yielding neutral buoyancy at long wavelengths despite strong seismic velocity reduction. Finally, the predicted trajectories of subducted slabs and mantle flow from data assimilation models indicate that the MTZ slow anomalies mostly likely represent dehydration of the Mesozoic Farallon slab within the lower mantle, providing a long-lived source ofmantle volatile circulation.