Integrating Magnetotelluric and Seismic Observations to Constrain Mantle Composition, Hydration, and Melt Distribution Beneath the Western United States.

The western United States is characterized by an exceptional diversity in volcanic and tectonic styles within a relatively compact region, including active subduction beneath the Cascade Arc, potential hotspot volcanism along the Yellowstone–Snake River Plain track, and widespread extension across the Basin and Range Province. Such diversity settings, along with excellent geophysical data enables quantitative studies on how lithospheric architecture and mantle properties control magma generation. Yet, most existing geophysical studies have treated these provinces in isolation and relied on single-method interpretations that struggle to resolve temperature, composition, melt fraction, and volatile content. Multi-scale electrical conductivity models of the United States based on the recently completed USArray magnetotelluric survey have reached a level of maturity and a resolution comparable to that of seismic tomography models offering unique insights into fluids, melts, and compositional variations. In this contribution, we present an integrated analysis of mantle structure across the western US, spanning from the Yellowstone Plateau to the Pacific margin, using complementary constraints from electrical conductivity and seismic tomography models as well as thermal constraints independently-derived from xenolith thermobarometry. Combining these datasets within a probabilistic framework enables us to separate the effects of temperature, water content, composition, and partial melt that individually may produce non-unique signatures.