Elevated Low Relief Landscapes in Mountain Belts

Elevated low relief landscapes are an abundant feature in mountain ranges worldwide. This peculiar topographic pattern, which is indicated by a transition from increasing to decreasing slopes with elevation, has been explained by temporal changes in climate or tectonics. This ultimately culminates in two opposing hypotheses: The hypothesis of glacial reshaping explains the large scale topographic pattern by a buzz-saw style erosion of glaciers above and localized excavation of valleys below the snowline of ice covered regions, respectively. Elevated low relief landscapes must then occur within a formerly glaciated part of the mountain range, at or above the equilibrium line altitude. In the Alps, they must have formed after the mid-Pleistocene climate transition. Elevated low relief and incised landscapes form simultaneously, whereas the degree of glacial reshaping and the size of low relief surfaces increase with the duration of glacial occupation. The hypothesis of fluvial prematurity explains the topographic pattern of low relief landscapes at high elevations and incised landscapes at low elevations by a recent uplift event, where the two contrasting landscape types represent the ancient and recent tectonic regime, respectively. In this scenario low relief landscapes are uplifted first and dissected subsequently, with the result that their size decreases with time. Within this interpretation, elevated low relief surfaces are not correlated to the glacial extent and may have formed before the mid-Pleistocene transition. In order to proof or refute these two opposing hypotheses we propose to perform a study in the Eastern Alps where both glaciated and never-glaciated regions exist in direct spatial proximity. We pose three specific questions that will be answered by this project. (1) Where do we observe elevated low-relief landscapes and incised landscapes within the Eastern Alps? (2) When did low relief- and incised landscapes form and at which rates? (3) How did the observed topographic pattern evolve over time? To reach these goals we will perform a series of analyses in two adjacent areas that were and were not covered by ice during the Pleistocene glaciations. The two key areas are perfectly complementary as they feature a similar lithological and structural inventory but differ with respect to their glacial history. We plan three major work packages: (1) We will map the regional pattern of elevated low relief and incised landscapes by compiling existing maps and analyzing digital elevation models. (2) We will apply cosmogenic nuclide dating to determine the absolute age of landforms (via cave proxies) and compute incision rates. (3) We will model multiple scenarios to constrain the time-dependent evolution of elevated low relief and incised landscapes during cold and warm climate conditions. By integrating the results of these three methodically independent work packages, we are well-positioned to proof or refute the two opposing hypotheses in order to infer drivers of landscape evolution in the Eastern Alps. Beyond the Eastern Alps, findings from this project will have far reaching implications on the understanding of relief formation and destruction in mid-latitude mountain ranges.

In many mountains of the earth, and so also in the Eastern Alps, extensive landscapes with low relief occur at medium and high altitudes. In strong contrast to the standard conception of topographic evolution, these landscapes are characterized by a transition from increasing to decreasing steepness with increasing altitude. Historically, this feature of the landscape has been explained either by intense glacial erosion during glacial periods (climatic driver) or by rapid uplift of former valley floors in the recent geologic past (tectonic driver). While both mechanisms qualitatively result in similar low relief landscapes, the spatial distribution, age or rates of formation, and temporal evolution of these topographic features differ. An international team of researchers tested the two opposing hypotheses using field, laboratory, and numerical methods. (1) The spatial distribution of low relief landscapes was mapped in the field and with digital elevation models. (2) The formation age of caves and correlated areas of planation surfaces was determined with cosmogenic nuclides to derive rates of large-scale uplift. (3) They evolution of landscapes according to the two hypotheses was calculated with models and compared with terrain and laboratory results. Large-scale mapping shows that low-relief landscapes occur both in regions that were and where not glaciated in the Pleistocene. Evaluation of this mapping revealed that extensive extended planation surfaces in both glaciated and unglaciated domains are due to uplift of valley floors that can be correlated with distinct cave levels. Dating the formation of over 40 caves yields an uplift rate for the Eastern Alps averaged over the last 5 million years in the order of 0.2 mm/year. It could thus be shown that the recent uplift of the Eastern Alps was about five times faster than previously assumed. Model results showed that glacially imprinted landscapes below the long-term snowline are steeper than those without glacial overprint and that in formerly glaciated areas a pronounced bimodal distribution in slope gradient occurs. This is expressed by broad and flat valley floors and steep valley flanks. In absence of extended planation surfaces, the relative decrease in slope gradient at mid-elevation observed throughout the Alps results from a steepening of lower-lying landscape parts along the trunk valleys. We could not find evidence for the formation of large-scale planation surfaces by cirque formation with simultaneous destruction of the peak topography. In addition to the project-specific results, a completely novel numerical approach to describe glacial erosion was further developed, calibrated, and tested. The superior computational performance enables previously infeasible experiments that can describe fluvial and glacial erosion, sediment transport, orographic precipitation, and regional isostasy at the mountain range scale over millions of years and will provide fundamentally new insights into the evolution of mountain landscapes.