Geology of Los Azufres Caldera, Mexico, and its relationships with regional tectonics

The Los Azufres geothermal field is one of several silicic centres of the Mexican Volcanic Belt (MVB) for which a caldera structure was suggested. Geological and structural surveys in a wide area surrounding Los Azufres reveal that this complex is situated in an area of unusual concentration of acid volcanism, consisting of four pyroclastic units and several dome complexes. Although a complete caldera structure is not morphologically evident, several facts suggest a long collapse history at Los A zufres: •- the geothermal field lies at the centre of a subcircular depressed area (27 × 26 kminsize) filled by a distinctive fluviolascustrine sequence;•- Middle to Late Miocene rocks bound to the south and to the north of this depression and are encountered only at depth inside it;•- four large ignimbritic suites of latest Miocene and Pliocene age outcrop outside the depression;•- volume estimations of these pyroclastic products are comparable with the missing volume of the depressed area;•- no alternative caldera structures exist in a radius of 200 km from Los Azufres;•- dacitic to rhyolitic lavas, principally extruded as dome complexes, were emplaced inside this depression during the Pleistocene.