Recent Study (EPSL 2020)

Glacial erosion promotes high mountains on thin crust

Robl J., Hergarten S. and G. Prasicek

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Here we present the first global analysis of the morphology and distribution of more than 16,000 peaks. We spatially correlate peak height and steepness with mean elevation and crustal thickness. Our analysis reveals that the steepness of peaks increases with altitude. Comparing peaks of similar altitude, the steepness of peaks increases towards high latitudes, while the crustal thickness supporting these peaks decreases. This evidences for a progressive crustal thinning with intensity and duration of glacial occupation transforming mountain belts from a fluvial towards a glacial topography. Due to the characteristic glacial landscape geometry with very steep peaks separated by spacious glacial valleys even a relatively thin crust is sufficient to supporting very high peaks.

The destiny of orogen-parallel streams in theEastern Alps: the Salzach–Enns drainage system

Trost G., Robl J., Hergarten S., Neubauer S.

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The evolution of the drainage system in the Eastern Alps is inherently linked to different tectonicstages 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 hencethe vulnerability of such rivers to be captured. This leads to a situation in which major orogen-parallel alpinerivers such as the Salzach River and the Enns River are characterized by elongated east–west-oriented catchmentssouth 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 ofyears to the millennial scale. We employχmapping for different base levels, generalized swath profiles acrossdrainage divides and Gilbert metrics – a set of local topographic metrics quantifying the asymmetry of drainagedivides at hillslope scale. Our results show that most drainage divides are asymmetric, with steeper channels westand flatter channels east of a common drainage divide. Interpreting these results, we propose that drainage dividesmigrate from west towards east so that the Inn catchment grows at the expense of the Salzach catchment andthe Salzach catchment consumes the westernmost tributaries of the Mur and Enns catchments. Gilbert metricsacross the Salzach–Enns and Salzach–Mur divides are consistent with inferred divide mobility. We attributethe absence of divide asymmetry at the Inn–Salzach divide to glacial landforms such as cirques and U-shapedvalleys, which suggest that Pleistocene climate modulations are able to locally obscure the large-scale signal ofdrainage network reorganization. We suggest that the eastward-directed divide migration progressively leads tosymmetric catchment geometries, whereby tributaries west and east of the capture point eventually contributeequally to the drainage area. To test this assumption, we have reconstructed the proposed drainage networkgeometries for different time slices.χmapping of these reconstructed drainage networks indicates a progressivestability of the network topology in the Eastern Alps towards the present-day situation.

The effects of lithology and base level on topography in the northern Alpine Foreland

Baumann S., Robl J., Prasicek G.,  Salcher B. and M. Keil

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The evolution of topography is driven by climate and tectonics, and strongly influenced by substrate properties and different base levels. The contributions of these factors may vary in space and time and are thus difficult to disentangle. Our study area, the Hausruck - Kobernaußerwald range, has a rather uniform climatic and tectonic history but is drained by rivers with different base levels and consists of contrasting sedimentary rocks, mainly due to different sedimentation environments. This makes them an ideal location to study the effects of lithology and base level on topography.

Topographic evolution of the Eastern Alps: The influence of strike-slip faulting activity

Bartosch T., Stüwe K.,  and J. Robl

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We present the results of a numerical model that was used to investigate aspects of the landscape evolution of the Eastern European Alps in the Miocene. The model allows the consideration of strike-slip faulting, an inherent feature of the Miocene tectonics in the Eastern Alps, within a viscous medium. Mechanical deformation of this medium is coupled with a landscape evolution model to describe surface processes. For the input variables, the activity history of strike-slip faulting in the Eastern Alps was compiled from literature sources. The results present a major improvement in the predicted topographic development over earlier models in terms of the location and build-up of valleys and mountain ranges that form in response to the strike-slip faulting activity.

The Topographic State of Mountain Belts

Robl J., Hergarten S., and G. Prasicek

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The topography of mountain ranges reflects the competition of constructive and destructive processes driven by tectonics and climate, respectively. There is a vital debate whether the topography of individual orogens reflects stages of growth, steady-state or decay that is fueled by the million-year time scales hampering direct observations on landscape evolution, the superposition of various process patterns and the complex interactions among different processes. Hence, there is a demand for sophisticated analysis tools to extract constraints on the long-term evolution of orogens from their topography. We review the field of orogen-scale landscape evolution from a numerical perspective, summarize the most prominent modelling concepts and their implications for the fluvially-driven development of mountain topography, and finally evaluate their applicability for understanding real-world orogens.

The topography of a continental indenter: The interplay between crustal deformation, erosion and base level changes in the eastern Southern Alps

Robl J., Heberer B., Prasicek G., Neubauer F. and S. Hergarten

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The topography of the eastern Southern Alps (ESA) reflects indenter tectonics causing crustal shortening, surface uplift and erosional response. Fluvial drainages were perturbed by Pleistocene glaciations that locally excavated alpine valleys. The Late Miocene desiccation of the Mediterranean Sea and the uplift of the northern Molasse Basin led to significant base level changes in the far field of the ESA and the Eastern Alps (EA), respectively. Among this multitude of mechanisms, the processes that dominate the current topographic evolution of the ESA and the ESA-EA drainage divide have not been identified. We demonstrate the expected topographic effects of each mechanism in a 1-dimensional model and compare them with observed channel metrics. We find that the normalized steepness index increases with uplift rate and declines from the indenter tip in the northwest to the foreland basin in the southeast. The number and amplitude of knickpoints and the distortion in longitudinal channel profiles similarly decrease towards the east. Changes in slope of χ-transformed channel profiles coincide spatially with the Valsugana - Fella fault linking crustal stacking and uplift induced by indenter tectonics with topographic evolution. Gradients in χ across the ESA-EA drainage divide imply an ongoing, north-directed shift of the Danube-ESA watershed that is most likely driven by a base level rise in the northern Molasse basin. We conclude that the regional uplift pattern controls the geometry of ESA-EA channels, while base level changes in the far field control the overall architecture of the orogen by drainage divide migration.

Alpine topography in the light of tectonic uplift and glaciation

Robl J., Prasicek G., Hergarten S. and S. Stüwe

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In steady-state orogens, topographic gradients are expected to increase with elevation whereas the European Alps feature a transition from increasing to decreasing slopes. This peculiar pattern has been interpreted to reflect either the critical slope stability angle or a premature fluvial landscape but is also consistent with the glacial buzz-saw hypothesis. To disentangle the contributions of each of these principles we split the Alps into contiguous domains of structural units and analyze their slope–elevation distributions emphasizing glaciated and non-glaciated realms. In comparable structural units within the extent of the last glacial maximum (LGM) the transition from increasing to decreasing slopes is located at the equilibrium line altitude (ELA) of the LGM and we interpret this to be evidence for the impact of glacial erosion. Decay rates of glacial landforms towards steady-state slopes depend on lithological properties leading to a landscape characterized by different transient states. Beyond the LGM limits the slope–elevation distributions show local maxima as well, but these are located at varying altitudes implying a tectonic driver. This observation and data from surrounding basins suggests that at least parts of the European Alps experienced a pre-Pleistocene pulse of tectonic uplift. The resulting presence of premature low-gradient terrain above the ELA during the global cooling in Plio–Pleistocene times would have heavily influenced the onset and the extent of an alpine ice cap.