Timing and drivers of exhumation and sedimentation in the eastern Peruvian Andes: Insights from thermokinematic modelling

Abstract

This study assesses the impact of fold-thrust belt driven deformation on the topographic evolution, bedrock exhumation and basin formation in the southeastern Peruvian Andes. We do this through a flexural and thermokinematically modelled balanced cross-section. In addition, published thermochronology samples from low-elevation (river canyons) and high-elevation (interfluves) and Cenozoic sedimentary basin datasets along the balanced cross-section were used to evaluate the age, location, and geometry of fault-driven uplift, as well as potential relationships to the timing of ∼2 km of canyon incision. The integrated structural, thermochronologic, and basin data were used to test the sensitivity of model results to various shortening rates and durations, a range of thermophysical parameters, and different magnitudes and timing of canyon incision. Results indicate that young apatite (U-Th)/He (AHe) canyon samples from ∼2 km in elevation or lower are consistent with river incision occurring between ∼8–2 Ma and are independent of the timing of ramp-driven uplift and accompanying erosion. In contrast, replicating the young AHe canyon samples located at >2.7 km elevation requires ongoing ramp-driven uplift. Replicating older interfluve cooling ages concurrent with young canyon ages necessitates slow shortening rates (0.25–0.6 mm/y) from ∼10 Ma to Present, potentially reflecting a decrease in upper plate compression during slab steepening. The best-fit model that reproduces basin ages and depositional contacts requires a background shortening rate of 3–4 mm/y with a marked decrease in rates to ≤0.5 mm/y at ∼10 Ma. Canyon incision occurred during this period of slow shortening, potentially enhanced by Pliocene climate change.