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.

Deforming Alps

 

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.

Drainage System Alps

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.