Reconstruction of the frequency of rockfalls and rock avalanchesin the Mont Blanc massif since the Last Glacial Maximum ; Reconstitution de la fréquence des écroulements rocheux post-LGM dans le Massif du Mont-Blanc

High mountain is particularly sensitive to climate variations. The raising temperature that is currently taking place due to climate change has a strong impact on the Mont Blanc massif rock walls: a higher rockfall (>100 m3) occurrence has been noticed, caused by permafrost thawing. The raising in number of rockfalls has been successfully correlated to warm periods at different timescales, e.g., during extreme warm episodes like the 2003 and 2015 heat waves, and during the last 30 years. According to the expected raising temperatures, during the 21st century rockfall occurrence should continue to rise.Rockfall frequency in the Mont Blanc massif before the Little Ice Age is still largely unknown. During Lateglacial and Holocene, high occurrence has been related to warm periods as well? In order to answer this question, this PhD thesis has four aims:i.To date several rockfalls having taken place in the central part of the Mont Blanc massif, in order to understand their frequency during Lateglacial and Holocene. Exposure age of rockfall scars is obtained using Terrestrial Cosmogenic Nuclide dating.ii.To verify possible relationships between high rockfall occurrence periods and post-glacial climate periods.iii.To quantify rockfall volumes by means of 3D reconstruction of the rockfall shapes, to explore the possible relationship between cumulate volumes and climate periods.iv.To study the relationship between exposure ages and colours of rock surfaces. Colours are quantified by reflectance spectroscopy.A total of 70 rock surfaces have been sampled during three field campaigns that took place in 2006, 2011 and 2015-2016. 63 exposure ages were obtained, ranging 30 ± 20 a to 100.50 ± 8.50 ka. Three age clusters can be correlated to warm periods, corresponding to: two Holocene Warm Periods (7.50 – 5.70 ka), the Bronze Age Optimum (3.35 – 2.80 ka) and the Roman Warm Period (2.35 – 1.75 ka). A fourth age cluster has been detected with ages ranging 4.91 – 4.32 ka. The biggest cluster, ranging 1.09 ka – recent, shows ...