The scarce Eocene geological record in the Andean region of central Chile (∼33.5°S) is represented by the Estratos del Cordón de los Ratones (ECLR) unit. This is a 450 m thick mainly volcanic continental series, with a bimodal character, that hosts numerous subvolcanic intrusions. The igneous products show compositional features indicating a cogenetic character for the whole suite, relatively juvenile mantle signatures and arc-like affinities. This along with age determinations of 42 Ma indicate that the unit corresponds to the middle Eocene arc in the region. Geochemical data obtained complement the evolving arc magmatic record throughout Andean evolution. This allows us to track: (i) the earliest Late Cretaceous orogenic event, (ii) neutral tectonic conditions from latest Cretaceous until middle Eocene times, (iii) the Oligocene-early Miocene extensional episode, and (iv) the onset of prevailing compression. The record also indicates that the extensional event started no earlier than the middle Eocene, with the deposition of the ECLR, and no later than 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.

Supplementary material: Analytical Methods (Item 1), the full data set of U-Pb age determinations by SHRIMP (Item 2) and by LA-ICP-MS (Item 3), and additional geochronological diagrams (Item 4) are available at https://doi.org/10.6084/m9.figshare.c.4710305.

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