PALEOGENE UPLIFT AND GEODYNAMICS OF THE PERUVIAN CENTRAL ANDES INFERRED FROM SEDIMENT PROVENANCE, DETRITAL GEOCHRONOLOGY, AND FLEXURAL MODELING OF ALTIPLANO STRATIGRAPHY

The Paleogene surface uplift history of the Peruvian Western Cordillera is unknown, as materials which may record surface elevation changes are altered, undiscovered, or absent. However, estimates of past elevation can be made using alternative techniques such as flexural modeling constrained by the age, thickness, and depositional environment of Paleogene Altiplano strata. We present new detrital zircon U-Pb geochronologic data in the context of ~5 km of Cenozoic siliciclastic stratigraphy from the northernmost Altiplano of southern Peru. Geohistory analysis shows cumulative tectonic subsidence of 0.2, 0.6 and 1.5 km at 50, 40 and 30 Ma, respectively. This is consistent with a tectonic model in which flexural loading from the growth of the adjacent Western Cordilleran magmatic arc produces an eastward-migrating foreland basin system and concomitant sediment sourcing from the west. We constructed a forward flexural model using a Monte Carlo approach to estimate the crustal load required to produce this magnitude of flexure-induced subsidence and sediment infilling. Results give maximum first-order estimates of paleoelevation in the Western Cordillera which increase from 1.5 ± 0.7 to 2.3 ± 0.5 km between 50 and 30 Ma. Detrital zircon age spectra < 200 Ma show apparent pulses of magmatic activity at 150 – 130, 90 – 60, and 45 - 20 Ma. Interpretation of episodic high flux magmatism on a 40 – 60 Myr time scale, when combined with protracted Paleogene uplift followed by previously documented rapid Neogene surface uplift, is consistent with predictions from models of orogenic cyclicity.