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Accueil du site → Doctorat → Allemagne → 2020 → Alluvial fans along the coastal Atacama Desert - landforms, processes, and evolution

RWTH Aachen University (2020)

Alluvial fans along the coastal Atacama Desert - landforms, processes, and evolution

Walk, Janek

Titre : Alluvial fans along the coastal Atacama Desert - landforms, processes, and evolution

Schwemmfächer entlang der Küste der Atacama-Wüste - Landformen, Prozesse und Entwicklung

Auteur : Walk, Janek

Université de soutenance : RWTH Aachen University

Grade : Doktors der Naturwissenschaften 2020

Résumé partiel
Alluvial fans represent the dominant sedimentary systems within and along the margins of mountain regions where laterally confined, sediment-rich flows enter open plains or broader valleys. Successive aggradation creates a fan-shaped depositional landform featuring a complex internal architecture. Due to the direct coupling of alluvial fans to the source area, they are principally simple sediment routing systems and ideal archives to study local fluvial morphodynamics but also long-term landscape responses to (palaeo) climatic variability, tectonic activity, and base level changes. In most settings, however, it remained challenging to decipher these allogenic environmental controls. Along the coast of the hyperarid Atacama Desert in northern Chile (20.5°S-25.5°S), alluvial fans radiate from the steep western flank of the Coastal Cordillera, presenting the source area, over the narrow Coastal Plain, the sink. In the distal part, the coastal alluvial fans (CAF) interact with the marine environment, which forms an additional external control. In contrast to the last major alluvial activity in the interior Atacama Desert, dating back to the Miocene to Pliocene, the onset of CAF formation is much younger and CAF show a higher recent activity. However, the timing of their activity beyond the Holocene has barely received any attention yet. A major influence of climate on CAF formation has been proposed for a small part of the coastal Atacama ; however, the interplay of climate, lithology, and tectonics in conjunction with strong, marine-driven base-level changes is still insufficiently explored. Hence, the CAF of the Atacama Desert are investigated in this comprehensive study as archives for recent to past sediment transport dynamics under coupled allogenic forcing. For this purpose, the CAF morphology as well as sedimentology, primary processes constructing the alluvial fans, relationships between the CAF systems’ hydromorphometry and the prevailing environmental characteristics (climate, tectonics, and source-area lithology), and the timing of their Quaternary evolution were assessed. While detailed field studies were concentrated on five key sites, overarching large-scale analyses of regional climatic and geologic geodata as well as geomorphometric analyses included 123 CAF systems.Results reveal that alluvial fans along the coastal Atacama Desert are in an overall advanced evolutionary state featuring voluminous aggradation and in approximately half of all cases also significant to deep entrenchment. Detailed geomorphological mapping and morphometric terrain analysis at the CAF complex Guanillos (21.97°S), enabled - besides the introduction of a multi-phase morphostratigraphic model comprising the fan’s prograde evolution and successive incision - to infer debris flows as the main depositional process in this hyperarid coastal setting. The morphological evidence is further supported by the CAF stratigraphy described in detail at five sites. Secondarily, hyperconcentrated flows were found to contribute to a large extent to the CAF aggradation. In contrast, sheetfloods are of negligible relevance, which is in accordance with previous sedimentological observations. Functional relationships between hydromorphometric catchment characteristics, the frequency of extreme precipitation events, fault density, and source-area lithology suggest a primarily climatic control on the CAF morphodynamics.

Mots clés  : Atacama Desert (frei) ; alluvial fan (frei) ; arid environment (frei) ; geomorphology (frei) ; geochronology (frei) ; landscape evolution (frei) ; palaeoclimate (frei) ; tectonic uplift (frei)

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Page publiée le 7 janvier 2021