Tectonics and volcanism of Sierra Chichinautzin: extension at the front ofthe Central Trans-Mexican Volcanic Belt

Because of its recent activity and position at the southern magmatic front of the Trans-Mexican Volcanic Belt (TMVB), the Sierra Chichinautzin volcani c field (SCN) is a key area for the understanding of this controversial vol canic province. Volcanic activity has built more than 220 monogenetic volca noes (shields, scoria cones, thick lava flows, and hydromagmatic structures ) during the last 40,000 years, for a total volume of about 470 km(3). The SCN basalts are geochemically similar to OIBs, while the intermediate and f elsic volcanic rocks show a calc-alkaline trend and abundant evidence for m agma mixing. The structural analysis of this volcanic field and surrounding areas has been based on field data, satellite images, and a method for det ecting volcanic center alignments. The tectonic data, together with geophys ical evidence, confirm active general N-S extensional conditions with a str ike-slip component for the SCN area, the same structural setting that preva ils in the rest of the Central TMVB. Extensional tectonics, a negative regi onal Bouger gravity anomaly, a low-velocity mantle, high heat flow, and sha llow seismicity suggest a rift-type setting involving the upwelling of anom alous mantle beneath the Central TMVB. The combined petrological, structura l and geophysical arguments support that the SCN volcanism is rift-related, and rule out processes involving the subduction of the Cocos plate, which casts further doubts on the standard subduction model for the TMVB volcanis m. (C) 1999 Elsevier Science B.V. All rights reserved.