A Magnetotelluric Study of Mantle Heterogeneities Beneath the Northeastern United States

Abstract

Analysis of magnetotelluric (MT) data across the northern Appalachian region reveals significantmantle heterogeneity. By inverting a subset of long‐period EarthScope USArray MT data, we constructed athree‐dimensional electrical resistivity model that provides new insights into the seismic low‐velocity NorthernAppalachian 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 temperaturealone and likely require the presence of volatiles, such as CO2‐rich or hydrous melts, or other volatile‐bearingphases, to reduce mantle resistivity to the observed levels. In addition, our modeling suggests that certainalternative 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 mayreflect partial melting or metasomatic enrichment of carbonated and hydrated mantle domains introduced duringpast subduction or plume interactions, potentially mobilized by edge‐driven convection at lithosphericboundaries. We also resolve a deep resistive feature in western New England, interpreted as a dry and depletedlithospheric block, though its nature remains uncertain due to limited seismic expression and the relatively lowsensitivity of MT to resistive structures. Our results suggest that the upper mantle beneath New England is bothcompositionally and thermally heterogeneous, shaped by a complex tectonic history involving subduction,metasomatism, lithospheric thinning, and ongoing asthenospheric processes.