LOS-AZUFRES SILICIC CENTER (MEXICO) - INFERENCE OF CALDERA STRUCTURAL ELEMENTS FROM GRAVITY, AEROMAGNETIC, AND GEOELECTRIC DATA

Las Azufres geothermal field is located within a silicic volcanic complex in central Mexico. The complex is one of the major silicic centers in the Trans-Mexican Volcanic Belt (TMVB). Pradal and Robin (1985) first suggested the existence of the Los Azufres caldera, and Ferrari et al. (1991) recognized the existence of a collapse structure. According to Pradal and Robin this is a caldera of resurgent type. This geophysical study aims to contribute to the knowledge of the structure of the Los Azufres area. Gravity, aeromagnetic, magnetotelIuric (MT) and d.c. vertical electric-resistivity soundings were analyzed. Results show that Los Azufres is a very structurally complex setting with relatively thin crust caused by the extensional tectonics characterizing this central sector of the TMVB. Faults belonging to the E-W to NE-SW (extensional neotectonics) and NW-SE (Basin and Range province) systems are observed to affect the geologic units of Los Azufres. According to our study, the Los Azufres geothermal freld is located in a structural high located in the middle of a sub-circular depression delimited to the north-northeast by the Santa Ines Range, and to the southwest by the Mil Cumbres formation. The larger depression consists of two narrow, deep depressions that correspond to La Venta and to the Valley of Juarez. They are separated by the above mentioned structural high. These sub-depressions are believed to be the sites ofa maximum caldera collapse, and the structural high is interpreted to be at least in part the caldera's resurgent dome. Geoelectric structure of the caldera derived from d.c. resistivity indicates that the brines of the Los Azufres geothermal system ascend along faults, both bounding and internally disrupting the structural high/resurgent dome. A reasonable correlation is observed between gravity and aeromagnetic data.