Abstract

A detailed understanding of the tectonomagmatic processes that produced and deformed the thick and geographically widespread volcanic successions exposed across much of the southern Andes is only beginning to emerge. Findings from this study reveal that the Abanico Formation in the Principal Cordillera of central Chile (35°S latitude) provides an unprecedented (64+ m.y.) record of volcanism and crustal deformation related to South America’s subduction margin and growth of the southern Andes. Here, we utilized detailed field mapping and U-Pb and 40Ar/39Ar geochronology to document the stratigraphy and structure of the Abanico Formation in the upper Río Tinguiririca drainage, thereby clarifying the tectonomagmatic history of the Principal Cordillera. The Abanico Formation exceeds ∼3.6 km in composite thickness and is composed primarily of intra-arc deposits, including intermediate to mafic lavas, block-and-ash flows, volcaniclastic sedimentary deposits, and rhyodacitic tuffs. The lower Abanico Formation (ca. 75–37 Ma) records the slow accumulation (∼46–49 m/m.y.) of volcanic and volcaniclastic strata within an arc platform without obvious structural control. In contrast, the upper Abanico Formation (ca. 37–11 Ma) preserves volcanogenic rocks intercalated with thick intervals of epiclastic accumulations synorogenic with crustal deformation. Well-dated growth strata and a system of dextral-reverse faults indicate that transpressive crustal shortening occurred in the Principal Cordillera from the late Eocene to the middle Miocene. Collectively, results from this study demonstrate that the Abanico Formation spans far more time (Late Cretaceous–Miocene) than long assumed (Eocene–Oligocene) and thus significantly revises the geologic record of arc magmatism in the Principal Cordillera. The data also reveal a previously unrecognized transpressive component to Andean orogenesis near 35°S latitude, plausibly tied to oblique plate convergence during the Cenozoic.

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