Abstract

Along the coast of the hyperarid Atacama Desert, late Quaternary alluvial fans emerge from the Coastal Cordillera to the Pacific Ocean between 20.5 degrees S and 25.5 degrees S. Coastal alluvial fans (CAF) show, in comparison to the interior fans of the Atacama Desert, pronounced recent activity. However, the complex interplay between climate, lithology, and tectonics affecting the CAF morphodynamics in such hyperarid coastal settings needs to be better understood. We therefore aim at assessing the major factors driving CAF activity and evaluate their effects along gradients. We conducted an extensive study relating climatic, lithologic, and tectonic characteristics to fan and catchment geomorphology of 123 CAF. Geomorphometric analyses are based on the 12.5 m TanDEM-X WorldDEM (TM), catchment lithology and faults are extracted from 13 regional geological maps, and the frequency of heavy rainfall events capable of activating CAF is derived from a Weather Research and Forecasting (WRF) regional climate model. Our results point to a primary climatic control on CAF morphodynamics shown in functional relationships with catchment hydromorphometric characteristics reflecting a high susceptibility to debris flows - the main CAF-constructing process. Catchment properties along the latitudinal gradient reflect the source of significant precipitation events: frontal systems and cut-off lows which mainly originate in the extratropics and become increasingly rare towards the north. The frequency of precipitation extremes can only be correlated to the hydromorphometry and geomorphic maturity of catchments that feature a high degree of sediment transport potential and generally do not cut back far into the Coastal Cordillera. Related to the strongly reduced influence of the Pacific precipitation source towards the east, this additional climatic transition is governed by the orographic effect of the steep W-E gradient in topography. In contrast, source-area lithology is of negligible relevance for CAF catchment morphometry and fan activity. An important indirect influence of tectonics, however, can be seen in the long-term (neo)tectonic activity within the Coastal Cordillera, which in turn shapes catchments and controls the topography.