Abstract

Over much of its length, the Andean orogen is characterized by a generally east-vergent geometry and a progressive eastward (cratonward) migration of individual arc-orogenic elements. A departure from this model 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 uplifted Precambrian and Phanerozoic basement rocks of the Sierras Pampeanas constrain the tectonic development of this terrane and indicate that deformation associated with exhumation may have propagated broadly westward since the late Miocene. Two pre-Andean cooling events—during the Carboniferous–Permian (ca. 300–280 Ma) and the early Jurassic–middle Jurassic (ca. 200–174 Ma)—have been identified. The onset of Andean deformation is represented by a cooling event during the late Paleocene–middle Eocene. This cooling was followed by a period of middle Miocene–late Miocene reheating, during foreland basin-style sedimentation. Exhumation, 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-thrust belt. The apparent convergence of deformation in these two terranes and the progressive closure of Miocene-Pliocene intermontane basins in the Sierras Pampeanas may reflect the early stages of Andean plateau uplift. The relative timing of plateau development along strike suggests that lateral thickening of the orogen is progressing southward at least from the latitude of central Bolivia (ca. 20°S). Furthermore, the time-space coincidence between basement uplift and flattening of the subducted slab beneath the Sierras Pampeanas suggests that a relationship exists between westward displacement of the terrane and the dynamics of plate interaction.

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