Long and short time evolution of deep seated gravitational slope deformation int the Queyras massif / south east France

Deep Seated Gravitational Slope Deformation (DSGSD) is defined as a set of rock mass characterized by a very slow movement (mm.yr-1) affecting large portions of slopes of a mountain range. These typical slope instabilities must not be neglected and need to be better identified and characterized to anticipate related hazard (e.g. landslides). Characterize them requires first of all to locate them as for example recently the inventories carried out for the European Alps or in France. These specific processes, which can lead to hazard and disasters (e.g. La Clapière landslides), should not be neglected and need to be better identified and characterized to anticipate related hazard). Documenting the DSGSDs requires first of all to locate them as for example the recently published inventories initiated for the European Alps and France. These studies initiated approaches aiming at defining the factors controlling their evolution in time and space.

The research developed in this study targets a better understanding the short- (<100 yrs) and long-term (> 100 yrs) evolution of DSGSDs developed in the sedimentary rocks of the Queyras Massif (South-East French Alps). The main objective is to propose models of DSGSDs evolution with key interpretations of future developments to locate possible new landslide prone areas. The Queyras Massif was chosen because it represents an under-studied area of DSGSDs. The massif is characterized by Cenozoic marine sedimentary rocks accreted and metamorphized by the Alpine orogen. The massif is characterized by a regional schistosity plunging to the West and complex and active fault networks mark the landscape (Tricart et al., 2004). The highest summits reach an altitude of 2500m a.s.l. and are separated by deep valleys incised by the Riss and Würm glaciers and currently by torrential streams.

The method is based on a geomorphological analysis of the landscape and landforms, field observations and image interpretation of remote sensing data. Results allow locating the DSGSD, estimating their degree of activity, and characterizing their structure. Several dating methods (¹⁴C, ¹⁰Be or ³⁶Cl) complete the interpretations in order to reconstruct the history of the slopes and understand the factors that control their evolution.

At the scale of the massif, the DSGSDs were first identified using the approach proposed by Blondeau (2018). Visual remote sensing revealed the occurrence of thirty DSGSDs. These slopes were detected as they associate six common features commonly observed in DSGSDs. Eight DSGSDs were selected in order to investigate at the local scale their geomorphology, geology and hydrogeology and reconstruct their historical (millennial) and recent (last 50 years) evolution from dating methods and field observations. Through this multidisciplinary approach, present the observed bedrock and gravitational structural features and determine the predisposing factors of the formation of DSGSD. The research is part of the Program “Référentiel Géologique de la France / RGF – Chantier Alpes” which targets to update the geological knowledge of the Alpine basement, surficial formations and associated hazards in three dimensions and in digital format.