Building the Andes at 35°S latitude: Late Cretaceous-Cenozoic volcanism and transpressive crustal shortening in the Principal Cordillera, central Chile.

Abstract:

Thick volcanic arc successions composing the Abanico Formation are exposed in the Principal Cordillera and have been linked to building of the Andes inboard of central Chile’s subduction margin near 35°S. Previous models suggest these volcanic and volcaniclastic deposits accumulated in extensional basins that were later inverted during Miocene shortening. New mapping results and radiometric age data from the Río Tinguiririca region provide surprising new insights into long-term volcanic arc dynamics and upper crustal shortening history that challenge previous models of Cenozoic mountain building at this latitude. The Abanico Formation, exceeding ~3.6 km in local composite thickness, is composed of rhyolitic tuffs, intermediate to mafic lavas, and interbedded volcaniclastic deposits. ⁴⁰Ar/³⁹Ar and U-Pb zircon age data from volcanic intervals span the Late Cretaceous through Miocene (~74-11 Ma). The lower Abanico Formation (~74-36 Ma) records the slow accumulation of volcanic deposits (~35 m/Myr) with little evidence for syndepositional faulting, suggesting strata did not accumulate in active, tectonically-controlled basins. In contrast, the upper Abanico Formation (~36-11 Ma) locally preserves growth strata recording contractional deformation that initiated by the late Eocene and continued through the late Miocene, coeval with continued volcanism. A system of dextral and sinistral oblique-slip faults locally displaces the Abanico Formation and older map units, revealing a previously unrecognized strike-slip component to faulting in this region. Results of this study greatly extend the age of the Abanico Formation into the Late Cretaceous, forcing significant revision to its long-accepted Oligo-Miocene age and associated arc volcanism. The data also significantly revise previous models of Miocene inversion tectonics, identifying an Eo-Miocene transpressive component to Andean orogenesis tied to oblique plate convergence near 35°S latitude.