FLUID CIRCULATION AND STRUCTURAL DISCONTINUITIES ON THE SANTA MARIA - CERRO QUEMADO - ZUNIL VOLCANIC COMPLEX (GUATEMALA), FROM SELF-POTENTIAL AND SOIL CO2 SURVEYS

Our study aims to identify major structural discontinuities and fluid circulation zones on the Santa Maria - Cerro Quemado - Zunil volcanic complex in NE of Guatemala. Structural discontinuities and areas of hydrothermal activity can be important zones of weakness and thus are critical to delineate as part of volcanic hazard studies and hydrothermal energy exploration. We coupled two geophysical methods: self-potential (SP) measurements (sensitive to fluid flow) and CO2 flux measurements, in order to constrain the permeability variations within the volcanic complex. We conducted a survey sampling every 20m for both CO2 flux and SP along a 17kmlong profile from the top of Santa Maria to near the top of Zunil.

Our results show that in terms of fluid circulation, there is an active shallow hydrothermal system in the east flank of Santa Maria but its limits are not very clear. Additional investigations are necessary on the entire Santa Maria volcano to better constrain the nature and extent of underlying hydrothermal activity.

In terms of structural discontinuities, the flanks of the Cerro Quemado lava dome show a succession of changes in slope of the SP signal, which we interpret as a fault system created in the outer reaches of the viscous lava dome during its several stages of emplacement. In addition, the location of a regional fault, used as a preferential zone of downward water circulation, is identified by a negative self-potential anomaly between Cerro Quemado and Santa Maria volcanoes. This zone has a higher permeability than the rest of the studied volcanic area. It thus has potential as a locus of future structural weakness, gas emission, and perhaps even eruptive activity for either the Cerro Quemado or Santa Maria magmatic systems. The relationship between this newly identified fault and the well-known regional fault network, called the Zunil fault zone, is equivocal.

The very low CO2 flux values we obtained show that there is no significant CO2 degassing along our profile although there is an extensive hydrothermal activity in the Zunil geothermal field, between Cerro Quemado and Zunil volcanoes.