Deciphering the driving forces of erosion rates on millennial to million‐year timescales in glacially impacted landscapes: An example from the Western Alps

Abstract

[1] In many regions, tectonic uplift is the main driver of erosion over million-year (Myr) timescales, but climate changes can markedly affect the link between tectonics and erosion, causing transient variations in erosion rates. Here we study the driving forces of millennial to Myr-scale erosion rates in the French Western Alps, as estimated from in situ produced cosmogenic 10 Be and a newly developed approach integrating detrital and bedrock apatite fission-track thermochronology. Millennial erosion rates from 10 Be analyses vary betweeñ 0.27 and~1.33 m/kyr, similar to rates measured in adjacent areas of the Alps. Significant positive correlations of millennial erosion rates with geomorphic measures, in particular with the LGM ice thickness, reveal a strong transient morphological and erosional perturbation caused by repeated Quaternary glaciations. The perturbation appears independent of Myr-scale uplift and erosion gradients, with the effect that millennial erosion rates exceed Myr-scale erosion rates only in the internal Alps where the latter are low (<0.4 km/Myr). These areas, moreover, exhibit channels that clearly plot above a general linear positive relation between Myr-scale erosion rates and normalized steepness index. Glacial erosion acts irrespective of rock uplift and thus not only leads to an overall increase in erosion rates but also regulates landscape morphology and erosion rates in regions with considerable spatial gradients in Myr-scale tectonic uplift. Our study demonstrates that climate change, e.g., through occurrence of major glaciations, can markedly perturb landscape morphology and related millennial erosion rate patterns, even in regions where Myr-scale erosion rates are dominantly controlled by tectonics.Citation: Glotzbach, C., P. van der Beek, J. Carcaillet, and R. Delunel (2013), Deciphering the driving forces of erosion rates on millennial to million-year timescales in glacially impacted landscapes: An example from the Western Alps,

Tectonic controls of Holocene erosion in a glaciated orogen

“…As a result of Cenozoic climate change, the relationships between topographic metrics and observed Holocene (last ∼ 12 kyr) erosion rates in glaciated mountain ranges are more complex than in purely fluvial settings (Moon et al, 2011;Godard et al, 2012;Glotzbach et al, 2013). These poorly understood relationships are likely caused for two reasons: (1) glaciers reorganized previously fluvial channel networks and relief to create a landscape with their preferred geom-etry, radically changing the orogen topography (Whipple et al, 1999;MacGregor et al, 2000;Brocklehurst and Whipple, 2002, 2004Anderson et al, 2006Adams and Ehlers, 2017), and (2) Holocene erosion rates may be dominated by transient signals as surface processes remove the topographic disequilibrium imposed by glacial erosion (Moon et al, 2011).…”

Tectonic controls of Holocene erosion in a glaciated orogen

“…6). In tectonically active and previously glaciated mountain ranges there are three common orogenic processes that are most often suggested to dominate Holocene erosion rate patterns: climate gradients (Carretier et al, 365 2013;Olen et al, 2016), glacial modification of the landscape (Moon et al, 2011;Glotzbach et al, 2013), and patterns of tectonic rock uplift (Adams et al, 2016;Scherler et al, 2013;Godard et al, 2014). In the following we explore the relevance and applicability of these explanations to our data set.…”

“…For our calculations, we use q = 0.45. This value has been shown to describe the concavity of fluvial systems in the Olympic Mountains (Adams and Ehlers, 2017 (Montgomery, 2001;Hassan, 2006, 2007;Brocklehurst and Whipple, 2007;Robl et al, 2008;Hobley et al, 2010;Glotzbach et al, 2013;Adams and Ehlers, 2017). Many assumptions generally that are adopted in purely fluvial settings do not apply in mixed glacial-fluvial landscapes.…”

“…hillslope steeping or channel shallowing) have changed the activity of extant surface processes as compared to before the onset of glaciation. There are many examples of ranges where there have been significant changes to topography [Adams and Ehlers, 2017;Brardinoni and Hassan, 2006;Simon H Brocklehurst and Whipple, 2007;Glotzbach et al, 2013;Hobley et al, 2010;D. R. 410 Montgomery, 2001;Robl et al, 2008] and erosion during and after glaciation (Moon et al, 2011;Reiners et al, 2003;Christeleit et al, 2017), and others where such changes are not clearly observed (Thomson et al, 2010).…”

“…As a result of Cenozoic climate change, the relationships between topographic metrics and observed Holocene (last ~12 kyr) erosion rates in glaciated mountain ranges are more complex than in purely fluvial settings (Moon et al, 2011;Godard et al, 2012;Glotzbach et al, 2013). These poorly understood relationships are likely caused for two reasons.…”