Abstract

The scarce Eocene geological record in the Andean region of Central Chile (c. 33.6 degrees S) is represented by the Estratos del Cordon de los Ratones unit. This is a c. 450 m thick, mainly volcanic, continental series with a bimodal character that hosts numerous subvolcanic intrusions. The igneous products show compositional features indicating a co-genetic character for the whole suite, relatively juvenile mantle signatures and arc-like affinities. This, along with age determinations of c. 42 Ma, indicate that the unit corresponds to the middle Eocene arc in the region. The geochemical data obtained complement the evolving arc magmatic record throughout Andean evolution. This allows us to track: (1) the earliest Late Cretaceous orogenic event; (2) neutral tectonic conditions from the latest Cretaceous to mid-Eocene times; (3) the Oligocene-early Miocene extensional episode; and (4) the onset of the prevailing compression. The record also indicates that the extensional event started no earlier than the mid-Eocene, with the deposition of the Estratos del Cordon de los Ratones, and no later than the earliest Oligocene. In addition, orogenic processes developed from early Miocene times are highlighted relative to those developed during the early Late Cretaceous, at least in the arc region.