Quantitative Sediment Provenance Modelling in the Paleogene Northern Altiplano Basin, Southern Peru Using Detrital Zircon Geochronology



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Opposing models of orogenic growth in the central Andes propose different timing and loci of crustal shortening, exhumation, and topographic growth. Discriminating between Eastern and Western Cordilleran sources provides a critical test of these models. However, southern Peru experienced several phases of basin formation and subsequent sediment recycling, rendering unique identification of sediment sources difficult. I examine this question using measured stratigraphic sections and multiple provenance methods from locations spanning the magmatic arc to the fold-thrust belt in southern Peru. Correlation of stratigraphic sections based on detrital zircon maximum depositional ages allows identification of coeval depositional systems and elucidation of their sediment dispersal patterns. Lithofacies analysis indicates predominantly fluvial–alluvial fan depositional environments, with minor floodplain deposits. Paleocurrent data indicate that these fluvial networks flowed transversely north–northeastward, implicating sources in the Western Cordilleran magmatic arc. Conglomerate clast counts also show abundances of volcanic and plutonic clasts suggesting a Western Cordilleran influence. However, point count data indicates sourcing primarily from the fold-thrust belt and craton, east of the study area. In order to resolve this apparent inconsistency, I apply both top-down and bottom-up mixture modelling approaches to detrital zircon U-Pb ages from sediments in the Paleogene Muñani Formation and Puno Group.
Both of these methods indicate that sources exposed in the Eastern Cordillera contributed to basin strata. Differences in the modelled relative contributions highlight the difficultly in discriminating sediment sources in the central Andes. Whereas top-down modelling indicates dominantly Western Cordilleran derived sediment (~50%) with minimal Eastern Cordilleran sources (<5%), bottom-up modelling suggests Eastern Cordilleran sediments contribute up to 30%. Integration of point count, detrital zircon, and paleocurrent data is consistent with a scenario in which exhumation of the Western Cordillera starting in the Late Cretaceous provided the majority of the sediment to basins to the east while localized exhumation of Eastern Cordilleran strata in the late Paleocene–early Eocene shed sediment eastward, dominating basins immediately to the east. This scenario reconciles the basin history with thermo-kinematic models and suggests that the southern Peruvian Altiplano Basin developed as a partitioned basin, disrupted by internal uplifts, since at least the Eocene.



U-Pb Geochronology, Detrital zircon, Mixture modeling