The elevated low-relief landscapes of the Eastern Alps

The morphology of the Eastern European Alps is characterized by a bimodal slope distribution with relatively flat surface areas located at both high and low elevations. While low-slope regions in valley floors are easily explained by base-level erosion or sediment fill, low-slope surfaces at summit levels are unusual and have both been interpreted to (a) have formed by glacial erosion or (b) reflect remnants of old base levels that survived uplift and subsequent erosion. In order to contribute to this debate, we present a map of these surfaces on the scale of the Eastern Alps (mapped at roughly 1:300,000 scale). Importantly, our mapping covers both areas glaciated in the Pleistocene and areas that have remained unglaciated. Mapping results show that elevated low relief surfaces occur at roughly equal proportions in formerly glaciated and unglaciated regions, thus giving strong support to the notion that they are relicts of paleosurfaces formed at base level. As karst caves often form near the phreatic-vadose transition also at base level, we support our interpretation with a statistic analysis of passage lengths of some 13,000 karst caves in the region. Hypsometric analysis shows that the distribution of low-relief surfaces differs between formerly ice-covered and ice-free regions. We explain this by glacial overprinting of the surfaces during the Pleistocene. Based on the hypsometric analysis a correlation of the low-relief surface and caves across the Eastern Alps remains unclear, most likely because uplift varies across the region. However, plotting the elevation of the paleosurfaces relative to an arbitrary base level of the nearest 300 km2 catchment provides distinct peaks in hypsometric analysis suggesting a somewhat clearer correlation of levels between different regions. Because many of these surfaces are known to have formed at base level in the Pliocene, we therefore suggest that Pliocene surface-uplift in much of the Eastern Alps is of long wavelength. Topographic differences between major valleys and basins surrounding the Alps were created due to erodibility contrasts between Miocene sediments and basement. Tectonic dissection of the Eastern Alps causing variable uplift rates of different blocks appears to have been minor in the last 5 my of the uplift history.