Upper crustal conductivity images beneath the Colima Volcanic Complex (CVC) from 3D magnetotelluric modeling: implications on structural and geothermal anomalies

Abstract:

Colima Volcanic Complex in southwestern Mexico is a structure composed of three north-south oriented stratovolcanoes, two of which are extinct and the younger one being considered one of the most active volcanoes in America, the Colima volcano. The dynamic context of the CVC corresponds to the subduction of the Rivera (RP) and Cocos (CP) oceanic plates beneath the North American plate, which is responsible of the arc magmatism present in western Mexico. Particularly, the Colima Volcanic Complex lies about 100 km away the trench, within the Colima graben regarded as to be the limit between the RP and CP oceanic plates.

We inverted 21 high-band (10⁴-10^-2 Hz) magnetotelluric sounding data distributed broadly over the CVC region. The 3D inversion was done using a mesh of 34 x 36 horizontal cells of 2.25 km padded with 14 cells at each side with an increase in width by a factor of 1.3. The vertical mesh was constructed to properly model topography and bathymetry. It consisted of 90 cells of 50 m thickness covering the first 4.5 km with an increasing factor of 1.2 for subsequent cells. Previous the final grid was selected as a valid initial model to invert the data, we evaluated different grid sizes, background resistivity models and covariances; final resistivity and smoothness-controlling covariance values were 100 ohm-m and 0.2 respectively.

Results reveal an anomalous conductor beneath the Colima volcano at about 4 km from the surface, although the anomalous conductivity extent southwards, bounded by resistive segments of upper crustal basement rocks. The observed correlation of the anomalous conductor with low-velocity zones determined by seismic tomography and negative residual Bouguer anomaly within the studied zone is used to constrain the potential geothermal source of the active Colima volcano. We found that the regional Tamazula fault plays a central role in the evolution of the emplacement of the Colima volcano.