Abstract
The Andes are the modern active example of a Cordilleran-type orogen, with mountain-building and crustal thickening within the upper plate of a subduction zone. Despite numerous studies of this emblematic mountain range, several primary traits of this orogeny remain unresolved or poorly documented. The onset of uplift and deformation of the Frontal Cordillera basement culmination of the Southern Central Andes is such an example, even though this structural unit appears as a first-order topographic and geological feature. To solve for this, new (U-Th)/He ages on apatite and zircon from granitoids of the Frontal Cordillera at ~33.5°S are provided here. These data, interpreted as an age-elevation thermochronological profile, imply continuous exhumation initiating well before ~12–14 Ma, and at most by ~22 Ma when considering the youngest zircon grain from the lowermost sample. The age of exhumation onset is then refined to ~20 Ma by combining these results with data on sedimentary provenance from the nearby basins. Such continuous exhumation since ~20 Ma needs to have been sustained by tectonic uplift on an underlying crustal-scale thrust ramp. Such early exhumation and associated uplift of the Frontal Cordillera invalidate the classically proposed east-vergent models of the Andes at this latitude. Additionally, they provide further support to recent views on Andean mountain-building proposing that the Andes at ~33.5°S grew firstly over west-vergent basement structures.
Highlights
Two regions have been mostly investigated and documented at the scale of the whole orogen, to the north (~18–21°S) and to the south (~33–34°S) of the Central Andes
At ~33.5°S, the Andes are composed of fold-and-thrust belts affecting Meso-Cenozoic sedimentary and volcanic rocks of the Principal Cordillera to the west (e.g.22–24,31), and of the Proterozoic to Permo-Triassic pre-Andean basement of the Frontal Cordillera to the east[27,32,33] (Fig. 1c)
The results presented here imply that total Cenozoic exhumation since its onset was not sufficient to exhume rocks with fully exhumational ZHe ages, i.e. rocks that were initially deeper than the former ZHe PRZ
Summary
Two regions have been mostly investigated and documented at the scale of the whole orogen, to the north (~18–21°S) and to the south (~33–34°S) of the Central Andes. In addition to these sedimentary archives, recent results from (U-Th)/He thermochronology on apatite from the source rocks of the Frontal Cordillera by Hoke et al.[43] (Fig. 1c) give more direct constraints on the exhumation of this basement high at ~33–33.5°S Based on their data, these authors propose that exhumation of the Frontal Cordillera initiated early by ~25 Ma at a slow rate of ≤0.1 km/Myr. Following an indirect reasoning based on the extrapolation of their apparent exhumation rate at lower elevations and with some assumptions on the geothermal gradient, they propose that exhumation accelerated sometime after ~10 Ma, related to the onset of Andean deformation of the Frontal Cordillera. The implications of these findings are discussed in terms of total exhumation and of age of exhumation onset, as well as of Andean mountain-building at this latitude
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