Renewed Volcano-Stratigraphc Studies of Calderas with Geothermal Potential in Mexico

During the past six years we have carried out volcanologic fieldwork either in active geothermal fields in Mexico (Los Azufres, Tres Vírgenes, and Cerro Prieto) or in potential sites in which some geothermal exploration studied had been done by the National Power Company (CFE). These studies have been very successful in reassessing the location of the geothermal reservoirs within the volcanic successions through detailed mapping of the volcanic units using high resolution topography and satellite imagery to produce 3-D imagery in conjunction with field work to produce preliminary geologic maps. Detailed stratigraphy of volcanic units, assisted with 40Ar/39Ar and radiocarbon geochronology have redefined the evolution of some of these complexes. For example, our studies at Los Azufres geothermal field located in the State of Michoacán indicate that the volcanic complex of the same name sits upon a structural high transected by E-W faults related to the youngest structures of the Trans-Mexican Volcanic Belt. The volcanic complex has been emplaced during the past ~1.5 Ma. During this time, magmas evolved from basaltic to rhyolitic in composition with the emplacement of circa 100 vents. Several landforms have undergone intense hydrothermal alteration and, in some cases, generated debris avalanches. The revised stratigraphy based on drill holes and new dates of cores suggested that the geothermal reservoir is hosted in Miocene rocks bracketed between the Miocene Sierra de Mil Cumbres volcanics (17-22 Ma) and the products of the volcanic field itself. Similar studies will be carried out at four other Pleistocene calderas (Acoculco, La Primavera, Aguajito and Reforma) attempting to refine their volcanic stratigraphy, evolution, and the location of the geothermal system, and those results will help in the design of exploration strategies for geothermal sources.