EVALUATING MODELS FOR FORELAND TO HINTERLAND BASIN TRANSITION IN THE CENTRAL ANDEAN PLATEAU USING DETRITAL ZIRCON GEOCHRONOLOGY

Basin models for Upper Cretaceous–Miocene strata in the Eastern Cordillera (EC) and Altiplano propose deposition in an eastward-migrating retroarc foreland basin system that extended from the Western Cordillera (WC) to the modern Chaco Basin. However, it is uncertain whether the Altiplano and Chaco Basin were once a unified depocenter, and if so, when the depocenter became independently evolving basins. Nevertheless, the proposed model makes specific predictions regarding sediment sourcing that are testable with detrital zircon geochronology. We present 10 DZ U-Pb geochronology samples from the eastern flank (Camargo) and 3 samples from the western flank (Ubina) of the EC spanning Upper Cretaceous-middle Miocene strata. The data will help discriminate whether the Altiplano and Chaco basins were once unified and, if so, when they separated.

Samples from Camargo are dominated by 0.5–0.7, 0.9–1.2, and 1.6–1.8 Ga zircons, with minor 0.2–0.3 Ga age modes. Most of these age modes are present in sources from the craton, EC, Altiplano, or WC. Nevertheless, their presence, absence, and relative abundance allow testing of potential sediment sources and unroofing scenarios using quantitative provenance modelling approaches.

Provenance interpretations are grounded by EC derivation of the upper Eocene–Oligocene strata based on eastward paleocurrent orientations and thermochronological evidence for coeval exhumation in the EC. Upper Eocene EC derivation is preceded by two changes. The first occurs between the Cretaceous and Paleocene, marked by dilution of a 600 Ma unimode via admixture of ~550 and 1080 Ma age modes. We interpret this as the result of recycling from the west. The second is introduction of 0.2–0.3 and 1.7–1.8 Ga zircons between the Paleocene–Eocene and the Eocene. We interpret this as exhumation of Permo-Triassic strata in the EC mixed with recycled cratonic sources.

The new provenance data are consistent with a contiguous foreland basin extending from the WC to the interior in the Late Cretaceous–Paleocene. However, only one sample yields Late Cretaceous–Cenozoic zircons, which is difficult to reconcile with derivation from the magmatic arc. Quantitative sediment provenance modelling suggests that orogenesis and separation of the Chaco and Altiplano basins was ongoing since at least the late Eocene.