MULTIPHASE PRE-ANDEAN DEFORMATION GUIDES CENOZOIC MOUNTAIN BUILDING IN THE CENTRAL ANDES, SOUTHERN PERU

The central Andes are the locus of the highest magnitude shortening and crustal thickening along the Cenozoic Andean margin. In southern Peru, pre-Andean shortening and extension during the Paleozoic and Mesozoic, respectively, guided subsequent deformation. Late Permian shortening is evidenced by folds and reverse faults of variable orientation, potentially reflecting fold interference or changing stress orientation. These folds are covered unconformably by volcanic and non-marine siliciclastic rocks of the Triassic Mitu Formation. Lateral variations of facies, thickness, and fault patterns within the Mitu Formation suggest it is the stratigraphic archive of crustal extension overprinting Permian shortening. Although the magnitude and geodynamic drivers of these deformation phases remain debated, inherited structures from both events were selectively reactivated. During Andean orogenesis, the deformation front jumped from the Western Cordillera to the Eastern Cordillera in the Eocene, and potentially localized near pre-existing Triassic extensional structures. Thin-skinned shortening within the Eastern Cordillera reactivated inherited normal and reverse faults. One key structure is the Ayaviri fault, which defines the eastern Altiplano margin, and preserves growth strata constraining Oligocene fault motion. This structure has been variably interpreted as accommodating reverse or strike-slip offset, a reactivated pre-Andean fault, and as a boundary between suites of different volcanic products potentially reflecting contribution from different lithospheric blocks. Structural and magmatic observations from other segments of the Andean margin support variable influence from inherited structural architecture. These results emphasize how pre-Andean deformation may condition the crust for future deformation, as well as the continued need to characterize the role of inheritance on orogenesis.