Structural and morphotectonic evolution of the Sierras Pampeanas (Argentina) constrained by a multithermochronometer approach

Abstract

The Sierras Pampeanas in central and northwestern Argentina constitute a distinct morphotectonic feature above the shallow subducting Nazca Plate beneath the South American Plate between ~27 ° S and ~33° S. Uplift and deformation of the mountain ranges within this area are interpreted to be closely related to this flat-slab subduction during the Neogene. New thermochronological data, derived from application of the apatite fission-track, zircon and apatite (U-Th)/He thermochronometers as well as from K-Ar dating of fault gouges, presented in this thesis, reveal new and improved insights of the thermal, structural, and morphotectonic evolution of the Argentine Pampean ranges. In the Eastern and Western Sierras Pampeanas pronounced cooling below ~175 °C is related to a Permo-Triassic flat-slab subduction period affecting these regions, whereas the northern Pampean ranges were presumably unaffected by this process. During the Mesozoic locally restricted areas experienced burial re-heating, e.g. the Sierra de El Gigante, due to rifting along reactivated Late Paleozoic major structures, but in general cooling and exhumation continued in this phase, even though mainly decelerated. Final cooling and exhumation to near-surface temperatures in the Eastern and Western Sierras Pampeanas occurred between the Late Cretaceous and the Paleogene. This facilitates the idea of an already existing positive topography since at least that time, which was just accentuated during the Neogene, hence contradicts previous theories that the uplift of the mountain ranges in these areas is completely linked to the Andean flat-slab subduction. Contrastingly, the region of the northern Pampean ranges is characterised by a significantly different Cenozoic cooling history, which is presumably influenced by the uplift of the Puna Plateau. Final cooling in this area occurred in the Late Miocene following a period of burial re-heating during the Paleogene to Early Neogene.