Long-term (>10 Myr) spatial and temporal patterns of magmatism and upper crustal deformation along Andean-type subduction margins are poorly understood. New geologic mapping, petrographic observations, structural analysis, and 40Ar/39Ar and U-Pb zircon geochronology of the Abanico Formation in the Rio Tinguiririca region shed light on the long-term evolution of central Chile's subduction margin, and permit testing of opposing tectonic models for the southern Andes.

Results indicate the Abanico Formation is ~3.6+ km thick in the Tinguiririca region and consists dominantly of volcanic and volcaniclastic strata, with subordinate epiclastic intervals; 40Ar/39Ar and U-Pb age data acquired for the Abanico Formation span the Late Cretaceous through Miocene (~74-11 Ma). Stratigraphic relationships within the Abanico Formation and the tight clustering of age data indicate that pulsed volcanism in the region was punctuated by long periods of emergence, erosion, and aggradation of volcaniclastic and epiclastic sediments, presumably in an intra-arc setting. A strong, generally N-S trending transpressional overprint deforms the Abanico and older map units. Radiometric dating of growth strata and an age correlated sill crosscutting fault propagation structures indicate that shortening was underway prior to the late Eocene and continued through the late Miocene, coeval with continued magmatism. Collectively, results from this study significantly revise previously proposed chronologies of magmatism, sedimentation, and deformation within this sector of the Andes.

40Ar/39Ar geochronology was critical to dating mafic to intermediate volcanic rocks examined in this study. The primary phases used in 40Ar/39Ar dating are highly susceptible to low grade and/or hydrothermal alteration that can frequently be difficult to identify in hand sample or thin section, however. Electron microscopy was employed in this study to assess the extent of alteration and the potassium distribution within six volcanic samples whose corresponding 40Ar/39Ar spectra range from slightly to strongly disturbed. Results indicate that both alteration and potassium distribution within a sample exert a first order control on the Ar isotope systematics, and that electron microscopy is highly effective for screening samples for 40Ar/39Ar geochronology.