Formation and preservation of low-relief surfaces by Pliocene-Quaternary glaciations 

Abstract

The Plio-Quaternary period is characterized by a cold and variable climate with the periodic advance and retreat of glaciers and ice sheets in many mountain areas. As such, mountainous topographies have undergone episodic changes from fluvial to glacially dominated erosion processes in both space and time. How these continuous changes in the dominant surface processes impacted erosion rates and topographic relief remains unclear, and in particular the role of glacial erosion. Indeed, while previous work has shown that Plio-Quaternary glaciations increased topographic relief in many mountain areas, others have argued that glaciations are capable of efficiently removing area above the mean position of the equilibrium line of glaciers limiting the topographic relief (i.e., the glacial buzzsaw mechanism). In some high latitude glaciated passive margins, it has also been suggested that glaciations could have reduced topographic relief and formed extensive low-relief surfaces, mostly during the early stages of glaciation. This view challenged previous ideas of extrapolating cold-based, non-erosive ice conditions observed during the most recent glacial cycle on these elevated plateaus to the entire Plio-Quaternary period. If true, this means that glaciations have a larger impact on topography, erosion, and the sediment budget than previously thought. However, the glacial origin of these low-relief surfaces (LRS) remains debated. Here, we present a new modelling study designed to explore the impact of Plio-Quaternary glaciations on topography. Specifically, we investigate how climatic parameters such as temperature, precipitation, and the nature of climatic cycles control the development of topographic relief. We use iSOSIA, a glacial landscape evolution model, to simulate periodic advance and retreat of glaciers to mimick Plio-Quaternary glaciations at the mountain range scale. We define our climatic scenario into two stages. The first stage is represented by symmetrical 41 kyrs glacial cycles, whereas the second stage imposes asymmetrical 100 kyrs cycles. Our model framework considers fluvial, glacial, and hillslope erosion processes. From the models we assess the production of LRS facilitated by the combination of 1) protective non-erosive ice at intermediate elevations and 2) focused erosion on ice-free summits and in main valleys, mostly during the first climatic stage. The extent of LRS depends on the efficiency of glacial erosion and climatic parameters, with simulations suggesting that the most extensive LRS are found in colder/wetter settings. However, the final preservation and extent of these LRS is significantly influenced by erosion during the second climatic stage. Indeed, former LRS can be dissected by headward propagation of erosion promoted by the higher amplitude of the asymmetric 100 kyrs cycles. This reworking of LRS thus leads to a preservation bias that is expected to occur in most alpine settings. Our model results provide new insights into the impact of glaciations on topography and bring a plausible new comprehensive framework that explains both the presence of LRS and their absence in glaciated areas.