A Magnetotelluric Study of Mantle Heterogeneities Beneath the Northeastern United States

Abstract Analysis of magnetotelluric (MT) data across the northern Appalachian region reveals significant mantle heterogeneity. By inverting a subset of long‐period EarthScope USArray MT data, we constructed a three‐dimensional electrical resistivity model that provides new insights into the seismic low‐velocity Northern Appalachian Anomaly (NAA). Comparison with empirical conductivity models indicates that the low‐resistivity anomalies along the northern and western edges of the NAA cannot be explained by temperature alone and likely require the presence of volatiles, such as ‐rich or hydrous melts, or other volatile‐bearing phases, to reduce mantle resistivity to the observed levels. In addition, our modeling suggests that certain alternative lithologies, particularly hydrous clinopyroxenites, may also contribute to the observed conductivity, implying that compositional heterogeneity plays a role alongside fluids or melt. These conductive features may reflect partial melting or metasomatic enrichment of carbonated and hydrated mantle domains introduced during past subduction or plume interactions, potentially mobilized by edge‐driven convection at lithospheric boundaries. We also resolve a deep resistive feature in western New England, interpreted as a dry and depleted lithospheric block, though its nature remains uncertain due to limited seismic expression and the relatively low sensitivity of MT to resistive structures. Our results suggest that the upper mantle beneath New England is both compositionally and thermally heterogeneous, shaped by a complex tectonic history involving subduction, metasomatism, lithospheric thinning, and ongoing asthenospheric processes.