The age and significance of the tenango basalt, central Mexico

Abstract

The Tenango Basalt, which forms a prominent isolated mesa 19 km south of Toluca, is made up of four associated flows of blocky andesitic basalt lava extruded from three separate centres about 8,500 years B.P. There are no associated pyroclastics or cinder cones and the viscous magma may have come up NE-trending fissures associated with a major E-W fault cutting Tertiary andesites immediately to the south. The lavas rest on re-worked pyroclastic deposits derived from the neighbouring major stratovolcano, Nevado de Toluca, and locally overstep onto the fault-scarp. The stream pattern resulting from this blocked drainage is described. The fault, part of a widespread system associated with basaltic volcanism in central Mexico, is probably early Pleistocene in age.