Geomorphological significance of the Atacama Pediplain as a marker for the climatic and tectonic evolution of the Andean forearc, between 26° to 28°S

Abstract

Pediplains are classically identified as flat landscape surfaces in arid regions linked to tectonic quiescence, whereas deep incision of a pediplain is attributed to tectonic uplift. In the Atacama Desert, pediplains are generally used as morphotectonic markers to define the chronology of episodes of Late Cenozoic Andean uplift from their erosion and incision patterns and timings. The Atacama Pediplain (AP) extends over >12,000 km2 (26° to 28°S Lat) through the Central Depression and Precordillera of the southern Atacama Desert. In this study we perform geomorphologic and stratigraphic observations on the AP in the Salado Canyon area, combining new geochronological results derived from 40Ar/39Ar biotite ages from volcanic layers interbedded within the alluvial deposits, and 21Ne exposure ages on quartz-clasts on alluvial plains, to determine the chronology of the AP evolution. Results show that the evolution of the AP is a long-term and continuous process (from >20 to ~2.3 Ma) of alluvial deposition and subsequent alluvial plain formation developed by interplay between the climate variability of the Atacama Desert and Andean uplift. The AP alluvial deposition occurred in two spatially and temporally separated episodes of alluvial backfilling: 1) shortly before ~20.14 Ma and to prior to ~9.4 Ma, a timespan that allows for the drainage capture of the eastern Precordillera, and considerable landscape rearrangement; 2) post ~9.4 Ma, with a re-positioning of alluvial backfilling from the Precordillera towards the Central Depression. This occurs after the Mid Miocene onset of the hyperarid conditions in the Central Depression and is more likely due to late Miocene surface uplift. Exposure ages reveal the cessation of basin-scale deposition and the abandonment of the alluvial plains during ~5.24 to ~3.8 Ma linked to the incision of Salado Canyon. Subsequent climate conditions modulated the surfaces by the development of lag deposits until ~2.69 to ~2.3 Ma when hyperarid conditions reach a threshold that limits surficial activity. Additionally, the drainage capture of the Precordillera by headwards erosion of the Salado Canyon explains marked deep incision depth of this canyon without the need for an increase in surface uplift or a change in climate conditions. The AP is not a general marker of a single climatic or tectonic event/period but a composite paleosurface formed by a complex concatenation of extrinsic and intrinsic geomorphic processes over more than ~17 myr.