Development of a steep erosional gradient over a short distance in the hyperarid core of the Atacama Desert, northern Chile

Although generally considered to exist under hyperarid conditions over the long term, landscapes in many parts of the Coastal Cordillera of northern Chile have undergone fluvial erosion. Small-scale drainage systems in this mountain range are mostly isolated from river networks and associated processes originating in the Precordillera or the High Andes to the east, thus providing natural laboratories to investigate the interplay between erosion, atmospheric deposition, tectonics and local (micro-) climatic conditions. In this study, we present a set of cosmogenic 10Be and 26Al derived, catchment-wide erosion rates along a short (2.5 km) E-W transect on the northern rim of the Río Loa Canyon in the Coastal Cordillera of northern Chile (latitude 21.4°S). Here, a flat sedimentary gravel surface, which was deposited before the Middle Miocene, becomes increasingly dissected and changes into a badland-like topography to the west. The 10Be erosion rates increase by approximately an order of magnitude from east to west, reflecting (1) localised tectonic movements, (2) geologically recent base level lowering, (3) time-integrated (micro-) climatic gradients and (4) the presence/absence of gypcrete. These findings are corroborated by analysis of geomorphologic parameters, which point towards the presence of two fundamentally different erosional regimes in this small study area. These regimes are sharply delineated along a topographically modest tectonic ridge. To the west, a detachment-limited erosion regime prevails, while in the east a transport-limited regime is dominant. The presence or absence of gypcrete, whose prevalence is governed by (micro-) climatic conditions, generally appears to reflect the respective erosional regimes. The erosion rates we infer point to a long-term process of differential drainage evolution in the study area, likely on timescales of millions of years.