Abstract

Abstract. The evolution of the drainage system in the Eastern Alps is inherently linked to different tectonic stages of the alpine orogeny. Crustal-scale faults imposed eastward-directed orogen-parallel flow on major rivers, whereas late orogenic surface uplift increased topographic gradients between the foreland and range and hence the vulnerability of such rivers to be captured. This leads to a situation in which major orogen-parallel alpine rivers such as the Salzach River and the Enns River are characterized by elongated east–west-oriented catchments south of the proposed capture points, whereby almost the entire drainage area is located west of the capture point. To determine the current stability of drainage divides and to predict the potential direction of divide migration, we analysed their geometry at catchment, headwater and hillslope scale covering timescales from millions of years to the millennial scale. We employ χ mapping for different base levels, generalized swath profiles across drainage divides and Gilbert metrics – a set of local topographic metrics quantifying the asymmetry of drainage divides at hillslope scale. Our results show that most drainage divides are asymmetric, with steeper channels west and flatter channels east of a common drainage divide. Interpreting these results, we propose that drainage divides migrate from west towards east so that the Inn catchment grows at the expense of the Salzach catchment and the Salzach catchment consumes the westernmost tributaries of the Mur and Enns catchments. Gilbert metrics across the Salzach–Enns and Salzach–Mur divides are consistent with inferred divide mobility. We attribute the absence of divide asymmetry at the Inn–Salzach divide to glacial landforms such as cirques and U-shaped valleys, which suggest that Pleistocene climate modulations are able to locally obscure the large-scale signal of drainage network reorganization. We suggest that the eastward-directed divide migration progressively leads to symmetric catchment geometries, whereby tributaries west and east of the capture point eventually contribute equally to the drainage area. To test this assumption, we have reconstructed the proposed drainage network geometries for different time slices. χ mapping of these reconstructed drainage networks indicates a progressive stability of the network topology in the Eastern Alps towards the present-day situation.

Highlights

  • By applying a set of standard morphometric analyses, we discovered several distinctly asymmetric drainage divides

  • We found divide asymmetry considering information from entire catchments, headwaters and even hillslopes

  • We suggest that the proposed drainage divide migration from west to east is inherently linked to the plan view geometry of the Salzach and Enns catchments south of the Northern Calcareous Alps (Figs. 1, 2 and 6)

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Summary

Introduction

The drainage system of a collisional orogen is inherently linked to its tectonic and climatic evolution (Beaumont et al, 1992; Willett, 1999; Montgomery et al, 2001; Willett et al, 2001; Garcia-Castellanos et al, 2003; Cederbom et al, 2004; Bishop, 2007; Miller et al, 2007; Roe et al, 2008; Champagnac et al, 2012; Herman et al, 2013; Robl et al, 2017a). In a zone of plate convergence, crustal shortening is a primary control on the horizontal and vertical metrics of the mountain range (Houseman and England, 1986; Royden et al, 1997; Robl and Stüwe, 2005a; Robl et al, 2008b, 2017a; Bartosch et al, 2017). The horizontal geometry of the mountain range reflects compression in and stretching perpendicular to the direction of plate convergence. In such a stress field, blocks of the brittle upper crust are advected along major strike-slip fault zones. This process is commonly referred to as lateral extrusion (e.g. Tapponnier et al, 1982; Ratschbacher et al, 1989, 1991; Robl and Stüwe, 2005a, b; Robl et al, 2008b)

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