Timescales and interplay of complex mass movements in a periglacial alpine rock slope revealed by geomorphological, InSAR and thermal data

Steep alpine rock slopes in periglacial environments are complex systems, due to the strong interplay between weathering and sediment production, mass movements with different dynamics, and associated hazards. In the climate change context, permafrost degradation can trigger slow and fast mass movements (rockslides, rockfalls, debris slides), as well as destabilise rock glaciers on steep terrain. It is thus essential to clearly differentiate between these interplaying processes, along with their mechanisms, rates and controlling factors, to assess potential geohazard scenarios. In this perspective, we selected a rock slope in Val Cedec (Central Alps, Lombardy, Italy) as a natural laboratory. The slope is a 750-m-high glacial valley flank made of phyllitic mica-schists, covering approximately 5 km², with maximum elevations of 3000 m.a.s.l. and likely hosting permafrost above 2500.

We performed a conventional geomorphological survey based on photointerpretation of aerial images, fieldwork and analysis of DEM-derived products. We applied spaceborne InSAR products derived from C-band Sentinel-1 images (2017-2021) using data from different processing techniques (dual-pass DInSAR, multitemporal) and coherence maps to decouple the kinematics and timescales of the observed processes. We also applied thermographic techniques, combining Landsat-8 satellite images (2017-2021) with time-lapse thermograms captured by a high-resolution thermal camera during field surveys (July 2021 and August 2023).

Our preliminary observations reveal a complex interplay of mass movements, where shallow periglacial processes are coupled with deep slope deformations. The deep-seated movement is outlined by a double-crested ridge, and hosts shallower nested rockslides, whose scarps and fronts are source areas for rockfalls. Two rock glaciers occur in the upper-middle slope sector, one of which shows evidence of segmentation and destabilisation. In the lower part of the slope, at least three solifluction lobes have been identified, that redistribute the abundant debris produced by frozen rock masses disrupted by the deep-seated movement, and by rock glacier destabilisation. From the different temporal baselines of wrapped interferograms (6 and 12 days, 1 and 3 months, 1 year), we inferred a significant temporal variation in the displacement and coherence of rock glacier, rockslides and solifluction processes. Time series of ground surface temperature obtained by thermal images allowed mapping of slopes sectors likely to host permafrost. By combining this information with precipitation and air temperature data, we analysed the controlling factors of the different mass movements. Our preliminary results suggest that periglacial conditions favour the development of cascading mass movement processes, involving slow deep-seated and fast shallow movements that result in enhanced debris production feeding periglacial landforms prone to destabilisation. Accurately defining these processes and their interplay is crucial to define potential hazard scenarios.