Alpine topography in the light of tectonic uplift and glaciation

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.