Magnetotelluric imaging of lithospheric modification due to late Cenozoic extension in the Rio Grande Rift, New Mexico, USA

We present crustal and upper mantle electrical resistivity models from two-dimensional (2D) anisotropic and three-dimensional (3D) isotropic inversions of broadband and long period magnetotelluric (MT) data collected in the Rio Grande Rift in New Mexico, USA. Previous geophysical studies including seismic tomography and geodetic observations suggest that the Rio Grande Rift is a broad tectonic feature of the upper mantle responsible for minimal crustal modification with the exception of modest thinning and a narrow zone of brittle faulting near the rift axis. The data used in this study consist of over 75 broadband MT soundings (period range from 100 Hz to 10,000 s) collected by the University of Colorado Boulder and the United States Geological Survey during the summers of 2012 and 2013. Additional data used in the 3D inversion were selected from the MT database of the Summer of Applied Geophysical Experience, which consists of over 100 wideband (100 Hz to 1000 s) MT soundings collected in and around the Española Basin in north-central New Mexico over the past 25 years. We compare our 2D anisotropic models to both 2D and 3D isotropic models in an attempt to identify artifacts that result from improper modeling of higher dimensionality MT data, in particular the effect of 3D resistivity structure projecting onto 2D profiles. A key feature that we identify in our 2D and 3D resistivity models presented here is a broad ( 200 km wide) zone of enhanced conductivity (<15 Ωm) in the mid- to lower-crust that is imaged beneath profiles crossing the rift both in northern New Mexico (latitude of Taos) and southern New Mexico (latitude of El Paso, TX) and extends well beyond the geologically and structurally defined surface expression of the rift. We interpret this lower crustal conductor as a combination of melt and fluid accumulation that is a direct result of a broad thermal anomaly in the upper mantle.