Three-dimensional magnetotelluric (MT) imaging in central Colorado revealed a set of north-striking high-conductivity tracks at lower-crustal (50−20 km) depths, with conductive finger-like structures rising off these tracks into the middle crust (20−5 km depth). We interpret these features to represent saline aqueous fluids and partial melt that are products of active extensional tectonomagmatism. These conductors are distributed over a wider region than the narrow corridor along which Rio Grande rift structures are traditionally mapped at the surface, and they consequently demarcate regions of the lower crust where accommodation of bulk extensional strain has concentrated conductive phases. Our observations reveal limitations in existing models of Rio Grande rift activity and may reflect unrecognized spatiotemporal variations in rift system evolution globally.

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