Abstract
We used concentrations of in situ cosmogenic 10Be from riverine sediment to quantify the basin-averaged denudation rates and sediment fluxes in the Plessur Basin, Eastern Swiss Alps, which is a tributary stream to the Alpine Rhine, one of the largest streams in Europe. We complement the cosmogenic dataset with the results of morphometric analyses, geomorphic mapping, and sediment fingerprinting techniques. The results reveal that the Plessur Basin is still adjusting to the landscape perturbation caused by the glacial carving during the Last Glacial Maximum c. 20,000 years ago. This adjustment has been most efficient in the downstream part where the bedrock comprises high erodibility North Penninic flysch and Bündnerschist, whereas glacial landforms are still prominently preserved in the upstream region, comprising low erodibility South Penninic and Austroalpine bedrock. This geomorphic observation is supported by the 10Be based denudation rate and sediment provenance analysis, which indicate a much faster sediment production in the flysch and schist lithologies. Interestingly, the reach of fast denudation has experienced the highest exhumation and rock uplift rates. This suggests that lithologic and glacial conditioning have substantially contributed to the local uplift and denudation as some of the driving forces of a positive feedback system.
Highlights
In mountainous areas, the shape of a landscape is the expression of a complex interaction between tectonic and erosional processes over multiple temporal scales [1,2,3,4]
We focused on the Plessur Basin, situated in the southern part of this window where geodetically measured rock uplift rates are highest in the region (Figure 1b) and where the youngest apatite fission track ages have been reported (Figure 1c)
We identified the sediment source areas through provenance tracing, and we related these data to the lithotectonic architecture of the Plessur Basin and to the topographic imprint caused by the Last Glacial Maximum (LGM) and possibly previous glaciers
Summary
The shape of a landscape is the expression of a complex interaction between tectonic and erosional processes over multiple temporal scales [1,2,3,4]. One particular expression of the interaction between tectonic and denudation is a positive feedback, where erosion-driven unloading has the potential to initiate an isostatic response of the lithosphere in the form of crustal uplift [5]. Such a mechanism at work has, for instance, been proposed for the Central European Alps [6,7]. For the Alps of Eastern Switzerland (Figure 1), several authors have suggested that the high uplift rates are rather a long-lived consequence of neotectonic shortening [8] than a feedback response to erosion. The occurrence of high uplift rates was explained within the context of a shorter timescale of observations, whereby the retreat and melting of glaciers of the Last Glacial Maximum (LGM), ca. 20 ka ago, was considered to have induced an isostatic rebound in response to unloading [9,10,11]
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