GEOCHRONOLOGY OF THE CHINCHÍN FORMATION IN SOUTHERN ECUADOR

The subduction of the Nazca Plate underneath the South American Plate has led to the uplift of the majority of the Andes Mountains in South America. In Ecuador, the Nazca Plate subducts underneath the North Andes Plate, which collides with the South American Plate. This tectonic setting has formed two parallel mountain chains: the Cordillera Occidental and the Cordillera Real, as well as a complex series of basins between them. However, the chronology of the formation of these basins is poorly understood. This study focuses on the Chinchín Formation, an approximately 3.5 km thick, extrusive igneous formation that serves as the bedrock of the Quingeo Basin in southern Ecuador. The Late Cretaceous Yunguilla Formation is believed to underly the Chinchín Formation and provides a maximum age constraint on the timing of crystallization. A previous study assigned the Chinchín Formation an Eocene crystallization age, which was determined using zircon fission track (ZFT) dating of a single sample from the northern upper third of the Formation. This study employs zircon U-Pb geochronology to determine a crystallization age of the Chinchín Formation in order to better understand the history of basin formation in this region. Multiple bedrock samples from the southern portion of the Chinchín Formation are being analyzed. If the ZFT date corresponds to the original crystallization of this formation, the expected result of this study is a U-Pb age of between approximately 65 and 42 Ma. A younger date would indicate that the Chinchín Formation may have been emplaced or overturned during the deformation associated with mountain uplift.