APATITE FISSION-TRACK THERMOCHRONOLOGY OF THE SIERRAS PAMPEANAS, CENTRAL-WESTERN ARGENTINA - IMPLICATIONS FOR THE MECHANISM OF PLATEAU UPLIFT IN THE ANDES

Over much of its length, the Andean orogen is characterized by a gener ally east-vergent geometry and a progressive eastward (cratonward) mig ration of individual arc-orogenic elements. A departure from this mode l occurs in the Sierras Pampeanas region of central western Argentina where a terrane of active basement uplifts is currently developing to the east of the main Cordillera, Apatite fission-track data from uplif ted Precambrian and Phanerozoic basement rocks of the Sierras Pampeana s constrain the tectonic development of this terrane and indicate that deformation associated with exhumation may have propagated broadly we stward since the late Miocene. Two pre-Andean cooling events-during th e Carboniferous-Permian (ca. 300-280 Ma) and the early Jurassic-middle Jurassic (ca, 200-174 Ma)-have been identified. The onset of Andean d eformation is represented by a cooling event during the late Paleocene -middle Eocene, This cooling was followed by a period of middle Miocen e-late Miocene reheating, during foreland basin-style sedimentation, E xhumation, and possible westward migration of the exhumation ''front'' in the Sierras Pampeanas, commenced during the late Miocene-Pliocene to the east of the dominantly east-vergent Precordillera fold-and-thru st belt. The apparent convergence of deformation in these two terranes and the progressive closure of Miocene-Pliocene intermontane basins i n the Sierras Pampeanas may reflect the early stages of Andean plateau uplift. The relative timing of plateau development along strike sugge sts that lateral thickening of the orogen is progressing southward at least from the latitude of central Bolivia (ca. 20 degrees S). Further more, the time-space coincidence between basement uplift and flattenin g of the subducted slab beneath the Sierras Pampeanas suggests that a relationship exists between westward displacement of the terrane and t he dynamics of plate interaction.