CUENCA USPALLATA: AN INTERMONTANE BASIN RECORDS EPISODIC UPLIFT OF THE CORDILLERA FRONTAL AND PRECORDILLERA IN THE LATE MIOCENE

The timing, rate of subsidence and stratigraphic architecture within retroarc and foreland basin successions is strongly controlled by the kinematics of the thrust system and subsequent magnitude and migration of the tectonic load. The record of west-to-east crustal shortening is well-established in the south-central Andes, with a significant decrease in the magnitude of shortening from the flat slab segment to the north (>150 km) to the normal slab segment to the south (<90 km). Synorogenic Neogene foreland basins at the latitude of 33°S (Mendoza), including the Alto Tunuyan, Uspallata and Cacheuta basins, provide a sensitive record of the spatial and temporal patterns of tectonics, magmatism and orogenic exhumation. An important question relevant to the evolution of the Andean system is whether these basins constitute the remnants of an initially contiguous foreland basin that has be subsequently cannibalized, or if they evolved independently during thrust migration.

Cuenca Uspallata (Uspallata Basin) straddles the boundary between the Frontal Cordillera and Precordillera, and contains a complex succession of conglomerate, sandstone and mudstone deposited in an arid fluvial system. Basin strata unconformably overlie Permian-Triassic volcanic rocks of the Cordillera Frontal, and basal units contain coarse conglomerates and megaclasts of volcanic debris representing a basin margin facies. Sedimentary provenance, constrained by conglomerate clast composition and detrital zircon data, records an initial influx of detritus from the Cordillera Principal and Cordillera Frontal that interfingers upsection with distinct detritus from the Precordillera. Available U-Pb age constraints from a resedimented tuff and detrital sandstone ages suggest sedimentation was active from at least 9-12 Ma, which is significantly younger (>5 Ma) than synorogenic deposits in the Cacheuta basin to the east. Sedimentologic and geochronological constraints suggest Cuenca Uspallata developed as an intermontane basin trapped between the uplifting Cordillera Frontal and the Precordillera, and was not part of the main foreland succession.