THE SEDIMENTARY RECORD OF INTRAMONTANE BASINS IN THE SOUTHERN CENTRAL ANDES: INSIGHT INTO TECTONIC VERSUS SURFACE PROCESSES INTERACTIONS IN THE CREATION OF THE PUNA PLATEAU

Intramontane basins within and along the margins of orogenic plateaus preserve syntectonic clastic sequences that contain unique information on the temporal and spatial evolution of such enigmatic areas on Earth. With an average elevation of ~3700 m the uplift of the Puna Plateau must have significantly influenced climate, erosion, sediment dispersal patterns, and the overall evolution of the tectonic stress field in foreland areas. Within the plateau Eocene-Oligocene sediments record the response of the sedimentary system to tectonic deformation in a broad region. In particular, apatite fission track thermochronology in the Salar de Antofalla region documents exhumation beginning ~27 Ma. Sediment composition and facies associations in adjacent basins suggest that arid climate conditions prevailed at that time, which might have been further enhanced by the creation of orographic barriers, contributing to the filling and closure of the basins. Intramontane basins along the Puna margin, such as the Bòlson de Fiambalà (BF), record the latest (Mio-Pliocene) stages of plateau evolution and constrain when it reached geomorphic conditions similar to present. Sedimentological, structural and thermochronological data in the BF record two shortening phases in the late Mio-Pliocene reflected by an evolution of the paleodrainage system from western to more northern areas. Two coarsening upward sequences are the direct response to deformation and uplift of the Precordillera to the west, and the southern Puna margin to the north. Detrital thermochronology shows that the paleo-relief of the Puna margin was at least similar to the present by the late Miocene suggesting that the plateau was already established. Our data indicate a complex two-stage evolution of the Puna plateau commencing in the Eocene-Oligocene with the compartmentalization of the proto-plateau into compressional basins and ranges in a generally arid environment. This resulted in internal drainage conditions, sediment storage, and the onset of reduction in relief contrasts that led to the present plateau morphology. A later regional plateau uplift event, possibly associated with delamination as suggested by Kay et al. (1994), occurred in the late Miocene and was responsible for establishing the present day topographic expression of the plateau.