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 integratin 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.

The project involved two parts conducted in tight cooperation between the universities of Salzburg (PI Robl) and Graz (Co-PI Stüwe), respectively. The two parts involved (a) field and laboratory work to determine the latest uplift history of the Eastern Alps and (b) the numerical description of topographic changes in mountain ranges during strong climate fluctuations as characteristic for the late Cenozoic. The two parts will be described separately below:

The field and laboratory work involved mapping of elevated low-relief surfaces and geochronological dating of cave sediments as proxies for the uplift history. Both aspects revealed extremely relevant findings for the field of tectonic geomorphology as they substantially consolidated ideas that have been around the community for the last 10 years. It could be shown that the surface uplift of the Alps did indeed occur about 5 times as fast as previously thought. This idea had been around since about 15 years, but has always been based on sketchy and few data. Here we dated some 45 caves at different elevations above base level and could show that the mean surface uplift rate was of the order of 0.2 mm/year for the last 5 Ma and that this surface uplift appears to have been more or less homogeneous across the eastern Alps.

Mapping of elevated low-relief surfaces was performed across much of the Eastern Alps and it was shown that there is no recognizable difference between these surfaces in regions that were glaciated in the Pleistocene and regions that were not. As such, we were able to contribute to a long-standing debate that discussed if these surfaces were glacially formed or if they are remnants of old valley floors (Piedmont Treppe). We can now confirm that they are indeed relics of a “Piedmont Treppe”. A detailed map of the elevated low relief surfaces combined with cave levels across the Eastern Alps is in the process of being submitted to the journal Geomorphology (Gradwohl et al., in prep). The results of the cosmogenic burial age dating of cave sediments are presented in a manuscript, which will be soon submitted to the journal Earth and Planetary Science Letters (Stüwe et al., in prep). The methods used for both manuscripts have been used in the past (geomorphological mapping in the field and cosmogenic burial age dating of caves) and are well established methods in this field. However, we used them to derive completely new data, in particular with cosmogenic 21Ne.

The numerical work involved the morphological analysis of a large portion of the formerly glaciated and never glaciated parts of the Eastern Alps to quantify elevation-dependent topographic metrics (e.g., slope vs. elevation distribution) and to describe the evolution of characteristic patterns in topography during the transition from fluvial to glacial conditions by employing landscape evolution models (iSOSIA and OpenLEM). In the last two decades, the “glacial buzz saw” hypothesis became popular in explaining hypsometric maxima (expressed by low relief surfaces) close to the ELA by a somewhat vague mechanism of cirque formation and summit decay above the equilibrium line altitude (ELA).

Thus, we tested whether glacial erosion led to, or at least significantly contributed to, the formation of the low relief surfaces observed in many places in the Eastern Alps. In all our numerical experiments, flat valley floors and steep valley flanks evolved during the fluvial to glacial landscape transformation, but we did not observe the formation of extended low relief surfaces at or above the ELA as proposed by the glacial buzz-saw hypothesis. Our results show that the observed extended low relief surfaces in the Alps cannot be explained by impact of the Pleistocene glaciation of originally fluvial steady state topography. However, we found that enhanced glacial erosion in the main valleys results in an average increase in slope below the ELA. The striking turning point in the slope elevation distribution close to the ELA can thus be explained by steepening of the terrain below the ELA, instead of the destruction of peak topography. The model results are consistent with topographic metrics of catchments in the Eastern Alps that were affected by a different degree of glacial erosion (Liebl et al., 2021).

Experiments on orogen-scale over multiple glacial-interglacial cycles pushed the model iSOSIA to its performance limit. A novel approach to describing glacial erosion based on the so-called glacial stream power law in analogy to the classical stream power law for rivers offered a solution. Since this approach (recently implemented in the model OpenLEM) has several limitations, its application has been controversial in the scientific community. To avoid acceptance problems in upcoming studies, we conducted a comprehensive benchmark study, in which we extended OpenLEM, calibrated the parameters, and compared the results with iSOSIA in detail. Results show that large-scale topographic patterns as well as their temporal evolution are well in line between the two models with differences being most pronounced at the first glacial advance and locally at the scale of individual landforms. Code enhancements and the calibration of the parameters in connection with the superior computational performance enable completely new experiments in the field of landscape evolution in active mountain ranges and holds the potential to change the way we understand mountain landscapes (Liebl et al., 2023).

Coupled models for the Eastern Alps accounting for fluvial and glacial erosion, sediment transport and deposition, orographic precipitation and flexural isostasy show a number of intriguing relationships between intensity and time of glacial occupation, isostatically driven uplift, and catchment-wide topographic metrics. This study is currently under preparation and will be submitted to Earth Surface Processes and Landforms (Robl et al., in prep.).