Magnetotelluric imaging of the Ceboruco Volcano, Nayarit, Mexico

Ceboruco Volcano is a Quaternary stratovolcano located in Nayarit near the Trans-Mexican Volcanic Belt western limit. It is one of the most active in the region and the only volcano in Nayarit with historically documented eruptions, the last one between 1870 and 1875. Currently, seismicity and fumarolic activity suggest hydrothermal activity. To study the geoelectrical setting at Ceboruco Volcano and its hydrothermal system, a magnetotelluric exploration campaign was carried out in November 2016, establishing 24 measuring stations at the caldera, and foothills of the volcanic edifice. Three broadband instruments were used in the sampling frequency range of 32,768–128 Hz. Recorded time series were processed to determine the MT transfer functions using a robust algorithm and consider the remote-reference station technique. Dimensionality analysis based on the phase tensor determined a shallow 1D geoelectrical structure at periods shorter than 1 s, while a 3D formation was defined at longer periods. To approximate the Ceboruco Volcano substructure, the 3D inversion of magnetotelluric data was carried out. 3D modeling revealed surficial resistivity regions that were related to young volcanic deposits. A relative conductor (10–30 Ωm) was also associated with the shallow regional aquifer. The two most intriguing results of 3D inversion were one uppermost conductive region (~1 Ωm) that established characteristics, resistivity, position, and thickness related to a clay alteration minerals zone. The other one (2–5 Ωm) corresponds to a conductive area broadening and fading with depth, centered slightly to the west of the summit at Ceboruco Volcano. The preferred interpretation of this geoelectrical feature was an envelope of high-temperature fluids, convection of steam, and brines, and they are interacting with the host-rock and surficial groundwater. This geoelectrical feature extends in depth, suggesting a permeable region at the Trans-Mexican Volcanic Belt basement that may be targeted for in-depth geothermal exploration. The geoelectrical setting identified in this study provides information that contributes to reducing uncertainty about the geological, geophysical, and hydrogeological conceptual models at Ceboruco Volcano geothermal system.