NEOGENE EXHUMATION AND UPLIFT OF THE ANDEAN MAIN CORDILLERA FROM APATITE FISSION TRACKS BETWEEN 33º30' AND 34º00'

The Main Cordillera, located south of the flat-slab subduction zone, can be divided into two major belts: the western belt (WMC) consists of late Eocene-early Miocene volcanic and sedimentary rocks of the Abanico Formation overlain by the Miocene volcanic Farellones Formation; the eastern belt (EMC) consists of Mesozoic back-arc sedimentary and volcanic rocks.

Fourteen apatite fission tracks (AFT) ages were collected from Cenozoic and Mesozoic rocks. In the western flank of the WMC on the Farellones Formation, the AFT ages agree with radiometric ages of this formation indicating that burial of these rocks was not enough to warm and erase the AFT. In the central part of the WMC, AFT ages obtained in lower stratigraphic levels of Farellones Formation are compatible with warming in the Partial Annealing Zone. The limit between WMC and EMC exhibits a more complex situation: The AFT ages obtained in the Abanico Formation show that low stratigraphic levels were cooled before its upper levels during the Miocene, and that the AFT of Mesozoic rocks are constrained to the Late Pliocene and Early Pleistocene. This is compatible with exhumation related to thrust fronts and deformation of isotherms, where the higher rates of denudation are located in the neighborhoods of deformation fronts, bounding major structures, as it happens in this study area.

Two main deformation stages can be recognized during an apparently countinous process of tectonic inversion for the Cenozoic Formations. The first one took place in Late Oligocene to Early Miocene times where most of the deformation occurred on the basins borders, enclosing a new basin where the Farellones Formation was deposited, with evidence for syn-tectonic depositation. The second stage took place in Late Miocene – Pliocene times, coeval with the main stage of surface uplift of the Main and Frontal Cordilleras between 33º - 34º S.