MULTIPLE PHASES OF CONTRACTIONAL AND EXTENSIONAL DEFORMATION IN THE WESTERN CORDILLERA OF PERU: IMPLICATIONS FOR CRUSTAL BUDGET AND UPLIFT

Constraints on the timing, position, and magnitude of regional deformation in the central Andes are disproportionately representative of the eastern Andes, as much of the western Andes is obscured by expansive volcanic cover. New field mapping, geochronology, and thermochronology in southern Peru exploit exposures in deeply incised canyons and provide insights into deformation of the Western Cordillera and forearc. Evidence for Jurassic normal faults confirms that a pre-Andean extensional back-arc architecture was selectively inverted during Andean development. Contrary to some reports of the Western Cordillera being dominated by extensional structures throughout the Cenozoic, this study recognizes the presence of a mixed thick- and thin-skinned, ramp-flat style fold-thrust belt spanning the forearc, Western Cordillera and western Altiplano. Cross-cutting relationships bracket the timing of multiple structures, demonstrating contractional deformation that spanned at least the Eocene to mid-Miocene, and suggesting protracted shortening across the Western Cordillera. This timing matches documented large-magnitude shortening in the Eastern Cordillera, revealing that shortening was active across much of the Andes for 10s of Myr in the Cenozoic. New detrital thermochronologic results from an Altiplano basin section near Cusco, Peru document decreasing lag time upsection from the Eocene to mid-Miocene. Much of the sedimentary rock in the Cusco basin was sourced from the Western Cordillera. The decreasing lag time upsection is consistent with unroofing in the Western Cordillera, further supporting field evidence for unappreciated crustal shortening in the Western Cordillera and forearc. These results have implications for crustal shortening budget for the Andes, the drivers of surface uplift, responses of Cordilleran systems to subduction zone geometry, and the behavior of convergent margin systems.