The magmatic system of the Colima Volcano from magnetotelluric and ambient noise data

Abstract

Here we present the results obtained from the 3D inversion of the full impedance of 21 magnetotelluric soundings in the frequency range of 1000–0.05 Hz. The results confirm the existence of a flat lying conductor at a depth of 2–4 km bmsl under the Colima Volcano (CV) acting as a natural cap seal of the Colima Volcanic Complex hydrothermal system. Model results along five EW vertical sections extracted from the 3D inverted impedance show that the upper crust (<15 km) in the northern sector of the CVC is far more resistive (>1000 ohm·m) and compact than in the southern zone, where it becomes more conductive and apparently more fragmented. The combined results of horizontal and vertical resistivity slices and sections reveals a vertical conductor (10–20 ohm·m) of ellipsoidal shape under the Colima Volcano, which extend from about 5 km bmsl down to at least 15 km bmsl. The approximate dimensions of the major and minor axis of the elliptical conduit along the fault are about 20 km and 5 km respectively. The major axis of the ellipsoidal enhanced conducting pathway is parallel to a NS fault segment off the main NE-SW fault system, which suggests that this fault segment is currently controlling the magmatic fluids ascent. Down to about 15-18 km bmsl the crust becomes widespread conductive, coinciding with a flat body of low shear-wave velocity (<3.2 km/s) under the Colima Volcano at similar depths. This low density and low resistivity flat lying mush reservoir contains H2O-rich melts prone to be polarized electrically and to polarize the surrounding medium. The supporting evidence suggest that the recent and near-future activity of the CV is closely related to a deep flat magmatic source rather than to a shallow magmatic chamber, and that the triggering mechanism of recurrent magmatic activity of the CV may have an electrical component.