Tectonic controls on magmatic systems: Evidence from a three-dimensional anisotropic electrical resistivity model of Ceboruco Volcano

Abstract This paper presents results from a magnetotelluric survey at Ceboruco Volcano in Mexico. The observed data require an anisotropic 3-D electrical resistivity model which links a near-surface geothermal reservoir to the magmatic system and finally to the local tectonic setting. Ceboruco is an active stratovolcano, which is located in the Tepic-Zacoalco Rift and is part of the Trans-Mexican Volcanic Belt. Together with Chapala and Colima, the Tepic-Zacoalco Rift forms a triple rift system, which developed as a consequence of the subduction of the Rivera and Cocos oceanic plates beneath the North American continental crust. Ceboruco is the largest and most active volcano in the region. Broadband magnetotelluric data were collected at 25 sites distributed on and around Ceboruco Volcano during a two-week campaign. The survey focused on the electrical resistivity distribution to characterize the geothermal potential and the deep structure beneath the volcano. The data quality is very high in a wide period range between 10−4 and 102 s and there is a consistent data pattern at all sites. For stations on top of the volcano, topographical effects dominate the shortest periods, whereas a highly conductive layer is implied for periods below 1 s. A spatially constant phase split and coincident small induction vectors are observed to the long period end. The 3-D isotropic inversion of the data yields an excellent data fit for short and intermediate periods and reveals a shallow magma storage and an extensive hydrothermal reservoir between 1 km a.s.l. and approximately 2 km b.s.l. At greater depth, the inversion algorithm generates large-scale high resistivity contrasts, reaching far outside the area of investigation. The resulting structures are inconsistent with the known geological setting and are considered to be artifacts of the isotropic inversion. We present an alternative model, which includes two anisotropic layers below 10 km depth. The conductive principal axes of the anisotropy are consistent with the local rifting direction in the Tepic-Zacoalco Rift and relate to a ductile deformation of the host rock and a large mush zone at mid- to lower-crustal depths.