Abstract

The mountain cryosphere is currently undergoing substantial modifications in an unusually short period of time. Glaciers in mountain regions are shrinking and consequently becoming less dynamic, and ice-rich permafrost features like rock glaciers have experienced increased creep since the 1990s in parallel with rising ground temperatures and widespread ice loss. Considerable research efforts have been invested to better understand specific cryospheric features that combine debris and ice, e.g., debris-covered glaciers and rock glaciers, but usually following separate research paths (glaciology and geomorphology). However, very little research has focused on areas where these elements are intertwined. In this sense, Alpine glacial and periglacial systems are mainly composed of small ice masses, partially or completely covered by debris, flowing under gravity, locally affected by significant ice melting (although at moderate rates compared to glaciers) and that can be found in permafrost environments.
The dynamics of these glacial and periglacial systems, and the glacier-permafrost interactions are even more rarely studied by the scientific community because their complex behavior is caused by the different elements that coexist. As these systems are partly decoupled from climate due to the effect of the debris cover, many questions arise concerning the fundamental mechanisms that control their dynamics. It has been shown that some of this systemsthem respond mainly to seasonal and interannual timescales. However, their long term behavior is still poorly understood. To help fill these knowledge gaps, this thesis investigates these complex environments by calling on several disciplines (glaciology, geophysics, geomorphology and remote sensing) to quantify the dynamics (i.e., variations in mass surface and surface velocities) over almost 70 years. By observing and describing the elements visible at the surface, it was also possible to characterize the internal structure and decrypt the landforms that coexist at three sites in the French Alps. Together these results and the literature review provide a comprehensive view of these fascinating systems and of the processes that govern them.
Large amounts of massive ice under a relatively thick layer of debris were found at all three study sites. The highest concentration of ice, mostly in the upper reaches, suggests a glacial origin, probably during the cold and wet LIA. Conversely, the concentration of ice decreases towards the lower margins, where debris are not efficiently evacuated in downward geomorphic systems. In all cases, relatively small surface velocities were observed with an increase in velocity (i.e., doubling) after the 1990-2000s, suggesting the delayed influence of air temperatures over the entire thickness is the main driver of variations in the creep rate by modifying the internal thermal conditions. Regarding mass changes, a general surface lowering (mass loss caused by ice melting) was observed in all three study sites. Although the debris layer has locally limited the melt rates to a few centimeters a year, mass loss has increased over the last two decades.
At the local scale of the Laurichard rock glacier, I successfully transposed a methodology classically used in glaciology based on the mass conservation or continuity equation to quantify mass balances. I discuss several poorly understood processes that nevertheless shed light on rock glacier dynamics. This method should deserves to be used at other sites to confirm the findings, even though several hypotheses regarding the accumulation of ice and debris and the ablation rate remain to be confirm.
In the other two study sites, the ice content, the thickness of the debris-cover, the development of thermokarst lakes at the surface and the circulation of water inside the landforms, have strongly influenced both their long-term behavior and their current dynamics. One of the study sites has already been the scene of catastrophic events related to a complex combination of topographical predisposition, hydro-meteorological conditions, deformation mechanisms and englacial thermal state raising concerns about the degradation of the mountain cryosphere.
Finally, this thesis provides a new methodological framework to properly address long- term monitoring and better understand changes to Alpine glacial and periglacial systems. We recommend the glaciological and periglacial community adopt a common research line to better understand these systems as a whole. Even though the response of the amount of water stored within these landforms to climate variations is complex, its local implications will increase even more with the expected rapid shrinking of the mountain cryosphere.

Keywords : French Alps, rock glaciers, debris-covered glaciers, remote sensing, photogrammetry, geophysics