FOLD-THRUST BELT IN THE WESTERN CORDILLERA OF SOUTHERN PERU: EVIDENCE FOR POLYPHASE DEFORMATION, REACTIVATION, AND VARIED STRUCTURAL STYLES

The Andes are considered the modern analogue of a classic cordilleran system in which deformation is predicted to advance from the trench toward the craton. Despite such predictions, the presence of a fold-thrust belt in the Western Cordillera (WC) of the Central Andes remains debated largely due to widespread Cenozoic volcanic cover. Subduction of the Farallon-Nazca plate has been ongoing since the Jurassic; however, the timing and style of deformation in the WC remains debated. Regardless of uncertainty of deformation style in the WC, deformation in the Eastern Cordillera was ongoing by the Eocene, 100’s kms inboard of the trench, and advanced to the Subandes in the Miocene. This observed crustal shortening in the eastern half of the Central Andes is inadequate to explain observed crustal thickness values. Here, we explore the timing and style of deformation in the WC and whether this region accommodated significant crustal shortening and thickening. This work presents new geologic mapping, cross-sections, and geochronology to determine the style and timing of deformation in the WC of S. Peru. We observe multiple phases of deformation beginning with Jurassic rift-related normal faulting that was later inverted during the Cenozoic. Additionally, fold-thrust belt structures have been identified throughout the WC that confirm fold-thrust belt deformation before, during, and after initiation of inboard deformation in the Eastern Cordillera. Multiple sites throughout the WC exhibit a spectrum of structural styles including basement-involved reverse faults, duplexes, fault propagation folds, fault bend folds, intraformational chevron folds accommodated by varied mechanical stratigraphy, fault reactivation, and normal faulting. We utilize widespread Cenozoic igneous activity to date cross-cutting relationships and constrain deformation timing using U-Pb geochronology. This suite of evidence for polyphase deformation with varied structural styles in the WC suggests a greater complexity to this “classic” cordilleran system than previously thought. Such complexity has implications for existing crustal shortening estimates, the spatiotemporal distribution of deformation, surface responses to changes in slab geometry, paleoelevation models, and the crustal thickening budget for the Central Andes.