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

Abstract

Ferrari, L., Gardufio, V.H., Pasquar6, G. and Tibaldi, A., 1991. Geology of Los Azufres caldera, Mexico, and its relationships with regional tectonics. In: S.P Verma (Editor), Calderas: Genesis, Structure and Unrest. J. Volcanol. Geotherm. Res., 47: 129-148. 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 km in size) filled by a distinctive fluviolaeustrine 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 Pliocenc 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. Los Azufres is therefore interpreted as a nested caldera of latest Miocene and Pliocene age, The recent regional tectonic evolution of the central sector of MVB comprises a Late Miocene-Early Pliocene left-lateral transcurrent phase, followed by a Late Pliocene-Quatern ary transtensional one. The silicic volcanism occurring between 6.1 and 2.8 Ma can be linked to the first phase, while in the adjacent areas of MVB a volcanic hiatus can be recognized. The following transtensional phase reached the Los Azufres area only during the Pleistocene and disrupted the caldera structure. Normal faults, developed during this period, controlled the uprising of basic magma which partly interacted with the remaining differentiated one and produced part of the recent intracaldera cycle.