Miocene volcanism in the central Chilean Andes (31°30′S-34°35′S)

The name Farellones Formation is currently used to designate continental Miocene volcanic and volcaniclastic deposits that form a 400 km long and 26-65 km wide, N/S-oriented belt of outcrops located along the central Chilean Andes. These deposits are mainly located east of an Oligocene-Miocene volcanic belt and west of the present volcanic arc. The lava flows and pyroclastics of the Farellones Formation vary in composition from andesitic to rhyolitic and, in lesser proportion, from dacitic to basaltic. Major element geochemistry and some trace elements confirm a typical calc-alkaline nature of the continental margin. The Miocene volcanic activity that formed the Farellones Formation occurred between 19.3 and 7.4 Ma, according to available K/Ar data, but this activity was not totally synchronous along the belt. It is estimated that 15.000 km³ of effusive material was extruded during this volcanic episode. The main Miocene volcanic activity can be related to an increase in the normal convergence rate between the Nazca and South American plates, which occurred between 26 and 9.6 Ma. The Farellones volcanic belt is not related to the present segmentation of the Nazca plate and thus represents an older Andean segment with a continuous and compositionally homogeneous volcanism possibly related to segmentation of the paleo-subduction zone.