Rock glacier activity over Holocene to modern timescales : insight from a western alp site

Active rock glaciers are among the most common cryospheric landforms in high-altitude mid-latitude mountain ranges. Over both short (years to decades) to long (centuries to millennia) time scales, their activity strongly influences the hydrology and geomorphology of alpine environments. Consequently, rock glaciers reflect paleoclimatic conditions and can be seen as an important player in erosion processes affecting high mountains slopes. Because they represent a visible expression of mountain permafrost and a considerable water reserve in the form of ground ice, rock glaciers are important landforms in the geomorphological and hydrological evolution of mountain systems, particularly in context of climate crisis. However, our understanding of rock glacier dynamics and its evolution at different time scales still need to be improved.

In this study, we present a multi-method approach, including field observations, remote sensing and geochronology, to study the rock glacier system of the Vallon de la Route (Combeynot Massif, western French Alps). Remote sensing images and correlation techniques are used to document the rock glacier movement field on time scales ranging from days to decades. In addition, to estimate displacement over periods ranging from centuries to millennia, we use surface exposure dating with terrestrial cosmogenic nuclides (10Be quartz) on boulder surfaces along the longitudinal line of the rock glacier, targeting different positions from the headwall to the terminus.

The remote sensing analysis processed between 1960 and 2018 agree with the geomorphological observations: the lower two units of the rock glacier are stationary/relict, the transition unit presents small displacement and not over its entire area, and the upper two active units above 2600 m elevation show integrated velocities between 14 and 15 cm a-1.  10Be surface exposure ages are ranging from 13.10 ± 0.51 to 1.88 ± 0.14 ka and their spatial distribution reveals an inverse first-order correlation between surface exposure age and elevation, and a positive correlation with horizontal distance to the headwall. These observations support the hypothesis that boulders fall from the headwall and remain on the surface of the rock glacier as they are transported down the valley. Our results also suggest that the rock glacier is characterized by two major phases of activity. The first phase, beginning around 12 ka, has a 10Be age gradient, following a quiet period between ~6.2 and 3.4 ka prior to the emplacement of the two present-day upper active units. Rock glacier started to be active again by 3.4 ka and still is now above 2600 m a.s.l. These results allow to quantify headwall erosion rates of between 1.0 and 2.5 mm a-1, greater than the watershed-integrated denudation rates estimated on millennial time scales. This suggests that the rock glacier system supports the maintenance of high rock wall erosion by acting as a conveyor of debris and allowing freshly exposed bedrock surfaces to be affected by erosional processes.