Structural-controlled valley morphology of the largest Central Alpine Glacier

The susceptibility of catchment rocks to glacial erosion may control the evolution of valley morphology in high-relief mountain ranges such as the Alps. Non-uniform proneness to bedrock erosion may indeed localize knickpoints and overdeepenings characteristic of glacial valleys. Yet, little is known about the explicit influence of bedrock properties (i.e. lithology, hardness, and geological structures) on glacial erosion processes. In this study, we select the Great Aletsch Glacier (Swiss Alps) as a natural laboratory to document and investigate the relationship between bedrock properties and subglacial erosion mechanisms. The Great Aletsch Glacier with a length of more than 20km and an ice thickness of up to 800m is the largest glacier in Central Europe. The underlying bedrock consists of the crystalline basement units of the Aar massif (gneiss, granite, and granodiorite) and is dissected by a large number of steep faults and former ductile shear zones. Geological and remote sensing lineament mapping combined with 3D geological modelling allowed us to make a large-scale characterization of the lithologies and structures’ spatial frequency over the entire length of the glacier. Additionally, we performed field-based rock hardness analyses (Schmidt hammer) along the glacier’s bedrocks (intact rock and faulted/sheared domains) to testify for structure-controlled erosion behaviour. Obtained results demonstrate that: (i) the typology and distribution of faults and shear zones are not uniform over the entire length of the glacier; (ii) high-frequency structure domains correlate with overdeepenings and/or abrupt glacier flow deflection in the direction of the strike of the structures; (iii) low-frequency structure domains correlate to the absence of overdeepenings and a straight glacier trajectory. In terms of erosive resistance domains of intact rock masses show high hardness values for each of the investigated lithologies without substantial variability between the different basement rocks (rebound values ranging from 45 to 60 N/mm²). On the contrary, faulted or sheared domains show a significant drop in hardness value (rebound values ranging from 10 to 40 N/mm²). Based on these results we propose that, for the case of the Great Aletsch Glacier, differences in crystalline basement lithologies do not exert an important role in glacial erosion. We postulate instead that the non-uniform spatial distribution of geological structures imposes a major control on the development of the glacial valley. The substantially reduced bulk hardness within high-frequency structure domains renders indeed the bedrock to be more prone to efficient glacial erosion process at these sites (i.e. glacial quarrying) and therefore to the development of large-scale overdeepenings, local scouring, or changes in the glacier flow direction. By contrast, the more massive undeformed and therefore less erosive low-frequency structures domains coincide with sections with no knickpoints or overdeepenings. In times of global warming and glacial retreat, such structure-controlled bedrock incisions are prone for further enhanced surface weathering and gravitation-controlled erosion processes, such as rockfalls and landslides, providing sites of enhances natural hazard potential.