Abstract
Disentangling the influence of bedrock erodibility from the respective roles of climate, topography and tectonic forcing on catchment denudation is often challenging in mountainous landscapes due to the diversity of geomorphic processes in action and of spatial/temporal scales involved. The Dora Baltea catchment (western Italian Alps) appears the ideal setting for such investigation, since its large drainage system, extending from the Mont Blanc Massif to the Po Plain, cuts across different major litho-tectonic units of the western Alps, whereas this region has experienced homogeneous climatic conditions and glacial history throughout the Quaternary. We acquired new 10Be-derived catchment-wide denudation rates from 18 river-sand samples collected both along the main Dora Baltea river and at the outlet of its main tributaries. The inferred denudation rate results vary between 0.2 and 0.9 mm/yr, consistent with values obtained across the European Alps by previous studies. Spatial variability in denudation rates was statistically compared with topographic, environmental and geologic metrics. 10Be-derived denudation records do not correlate with the distribution of modern precipitation and rock geodetic uplift. We find, rather, that catchment topography, in turn conditioned by bedrock erodibility (litho-tectonic origin) and glacial overprint, has the main influence on denudation rates. We calculated the highest denudation rate for the Mont Blanc Massif, whose granitoid rocks and long-term tectonic uplift support steep slopes and high relief and thus favour intense glacial/periglacial processes and recurring rock fall events. Finally, our results, in agreement with modern sediment budgets, demonstrate that the high sediment input from the Mont Blanc catchment dominates the Dora Baltea sediment flux, explaining the constant low 10Be concentrations measured along the Dora Baltea course even downstream the multiple junctions with tributary catchments.
Highlights
The use of in-situ 10Be concentrations measured in river sediments to quantify catchment-wide denudation rates over centennial to millennial time scale is well-established (e.g. Granger and Schaller, 2014). 10Be concentrations are measured at the outlet of the studied basin and are inversely correlated to mean catchment denudation rate. 30 Widespread research investigation has used this technique to estimate catchment denudation around the globe and in mountain belts such as the European Alps (Delunel et al, 2020 and references therein), with the aim of illustrating the controlling mechanisms on recent (102-105 years) erosion dynamics and assessing the respective roles of climate, tectonics or even anthropogenic forcing
All the Dora Baltea (DB) tributaries catchments have average slope comprised in the threshold range of 25-30°, with the exception of DB01 whose average slope is higher than 30°. 330 Our results show a correlation between catchment denudation rates and bedrock litho-tectonic classification (Fig. 6), which has been proposed to govern erosion through rock mechanical strength
Our 10Be-derived catchment-wide denudation rates obtained in the Dora Baltea (DB) catchment vary between 0.2 and 0.9 mm/yr and fit within literature values across the European Alps (Delunel et al, 2020)
Summary
The use of in-situ 10Be concentrations measured in river sediments to quantify catchment-wide denudation rates over centennial to millennial time scale is well-established (e.g. Granger and Schaller, 2014). 10Be concentrations are measured at the outlet of the studied basin and are inversely correlated to mean catchment denudation rate (von Blanckenburg, 2005). 30 Widespread research investigation has used this technique to estimate catchment denudation around the globe (see reviews in Portenga and Bierman, 2011; Willenbring et al, 2013; Codilean et al, 2018) and in mountain belts such as the European Alps (Delunel et al, 2020 and references therein), with the aim of illustrating the controlling mechanisms on recent (102-105 years) erosion dynamics and assessing the respective roles of climate, tectonics or even anthropogenic forcing. In the European Alps, the large-scale compilation of catchment-wide denudation rates by Delunel et al (2020) highlighted (1) the first-order correlation between topographic slope (derived from glacial impact on Alpine topography) and denudation rate, 50 (2) the absence of control of modern climate on denudation patterns and (3) a significant correlation between rock uplift and denudation for >100-km catchments This compilation pointed at a rather weak control of lithology on denudation, with the lowest rates in the low-elevation foreland areas (with clastic sedimentary lithology) and highest rates in the high-elevation crystalline parts (with gneissic, granitic or metamorphic lithologies) within the core of the Alps. Similar climatic conditions and glacial history but variable bedrock lithology and geodetic uplift within the DB catchment and its tributaries allow us to assess how spatial variability in bedrock erodibility between litho-tectonic units may participate in controlling catchment topography and 10Be-derived denudation rates
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