An analysis of the geomagnetic variation field across the Rio Grande rift has identified two concentrations of telluric current flow beneath the rift caused by channeling of telluric currents in electrically conductive structures in the crust and upper mantle. A shallow conductor nearly coincides with a very strong reflection in a high-resolution seismic-reflection profile across the central Rio Grande graben which has been attributed to a lens at mid-crustal depth. The deep (>30 km) conductor is 200 km wide and may indicate anomalously high temperatures and, by inference, a thinning of the lithosphere beneath the rift.

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