Along‐Stream Variations in Valley Flank Erosion Rates Measured Using 10Be Concentrations in Colluvial Deposits From Canyons in the Atacama Desert. Issue 5 (9th March 2021)

• Record Type: Journal Article
• Authors: Zavala, Valeria; Carretier, Sébastien; Regard, Vincent; Bonnet, Stéphane; Riquelme, Rodrigo; Choy, Sandrine
• Is Part Of: Geophysical research letters. Volume 48:Issue 5(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-03-09
• Subjects: Andes -- Atacama -- cosmogenic -- valley widening; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2020GL089961
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: The widening and deepening of river valleys occur at different rates. This co‐evolution is largely undocumented over thousands of years. Uncertainties on incision and widening laws limit the ability of models to reproduce erosion rates and responses to tectonics and climate over geologic timescales. We studied two 1 km‐deep canyons in northern Chile, both characterized by valley widening after the upward migration of a major knickzone but with a diachronous incision initiation. We use 10 Be concentrations measured in colluvial deposits to quantify the erosion rates E of along‐stream valley flanks. We observe that E increases in the knickzone, with well‐defined relationships ( R 2 = 0.7): E increases quasilinearly with the valley‐bed slope and decreases with the valley width (exponent ∼‐0.4). Our results suggest that E decreases with time but that valley flank weathering may sustain a significant lateral erosion rate. Our results provide data for testing erosion laws governing valley widening. Plain Language Summary: A common feature of river landscapes is the widening of valleys downstream (i.e., floodplains). Several studies have explored valley width controls, considering that different processes can control it, such as lithology and river hydraulic variations downstream. However, little is currently known about how the widening of valleys, in particular, occurs. To understand these processes, we studied two 1 km‐deep canyons in northern Chile, both characterized by a different valley width downstream, a sharp change in valley slope (or knickzone), and different timings for the beginning of the incision. In an original way, we measured the concentrations of 10 Be in the colluvial deposits at the foot of the valley flanks to quantify the rate at which the flanks are eroded at different locations along the valley. Our results show that the erosion rates increase toward the knickzone, given that they are positively correlated with the slope of the valley floor and they decrease as the valley width increases. Our results suggest that the widening rate decreases with time but that the concomitant weathering of the valley flanks favors their lateral erosion. Our results give confidence in this new approach and provide the first data set to test the existing valley widening laws. Key Points: Measurements of 10 Be concentrations in colluvial deposits quantify along‐stream valley flank erosion rates Erosion rates consistently increase with the valley slope and decreases with the valley width An approach and a first data set for testing lateral erosion laws governing the valley widening rate