Pliocene to Holocene volcanic geology at the junction of Las Cruces, Chichinautzin and Ajusco ranges, southwest of Mexico City

Abstract

The geology of Mexico City's southern volcanic area is discussed on the basis of volcanological, stratigraphic and glacial criteria. Three different eruptive periods occurred from Late Pliocene to Holocene in the juncture between Las Cruces, Ajusco and Chichinautzin ranges. The oldest (Las Cruces) Eruptive Period is represented by the activity of Los Picachos polygenetic volcano with dacitic pyroclastic flows, lahars and lavas of Las Cruces Formation (Cantirnplora Pyroclastic Breccia Member and Apilulco Dacitic Lava Member), formed mostly during Late Pliocene-Early Pleistocene. During the Ajusco Eruptive Period (Middle Pleistocene), Ajusco volcano was formed by extrusion of several andesitic lava domes. The last stage is the Chichinautzin Eruptive Period of monogenetic volcanism, characterized by Strombolian-type activity during the Late Pleistocene-Holocene. Scoria and lava cones formed in this period constitute the different units of Chichinautzin Group. The Picachos, Ajusco and Panza volcanoes and related advent cones are aligned on a N 65° W fracture system (active at least since Late Pliocene). Dip slip faults are parallel to this alignment. Glaciers of the Santo Tomas and Albergue Glaciations left horns, U shaped valleys. Cirques and moraines in Los Picachos and Ajusco volcanoes. The volcanism of the Chichinautzin Eruptive Period was contemporaneous during Late Pleistocene with an interglacial period (between Santo Tomas and Albergue Glaciations).