Geomorphological significance of the Atacama Pediplain as a marker for the climatic and tectonic evolution of the Andean forearc

The Atacama Pediplain (AP) is a major element in the Atacama Desert landscape, commonly used as an indicator of climatic and tectonic stability. We choose the El Salado Canyon area (26°S) that longitudinally exposes the AP to study its relationship with the Andean forearc evolution, combining geomorphologic and stratigraphic observation with new geochronological data (40Ar/39Ar biotite ages on ignimbrites, and 21Ne exposure ages on quartz-clasts). The AP alluvial deposition occurred in two episodes of alluvial backfilling spatially and temporally separated from each other (Early Miocene - Mid Miocene in the Precordillera, and Mid Miocene - Early Pliocene in the Central Depression), and both triggered by semiarid to arid conditions restricted to the higher Precordillera. Both episodes occurred despite the regionally recorded Mid Miocene onset of hyperarid conditions, revealing the local control of the upstream humidity in the downstream sedimentation. Clustered exposure ages from the Pliocene reveal the abandonment of the alluvial plains due to the incision of the El Salado Canyon, whereas younger and individual exposure ages from the Early Pleistocene, explained by minor surficial activity, reveal a climatic control on the surfaces after the incision that ended with the increase of the Pleistocene hyperaridity. Therefore, the development of alluvial deposition despite the regional Mid Miocene hyperaridity, and the development of a deep canyon incision after the last recorded Andean uplift, reveals that the AP has been strongly influenced by local conditions that do not allow its use as a general marker of a single climatic or tectonic event.