Climatic and structural controls on Late‐glacial and Holocene rockfall occurrence in high‐elevated rock walls of the Mont Blanc massif (Western Alps)

Abstract

In the Mont Blanc massif (European Western Alps), rockfalls are one of the main natural hazards for alpinists and infrastructure. Rockfall activity after the Little Ice Age is well documented. An increase in frequency during the last three decades is related to permafrost degradation caused by rising air temperatures. In order to understand whether climate exerts a long-term control on rockfall occurrence, a selection of paleo-rockfall scars was dated in the Glacier du Géant basin [>3200m above sea level (a.s.l.)] using terrestrial cosmogenic nuclides. Rockfall occurrence was compared to different climatic and glacial proxies. This study presents 55 new samples (including replicates) and 25 previously-published ages from nine sampling sites. In total, 62 dated rockfall events display ages ranging from 0.03 ± 0.02ka to 88.40 ± 7.60ka. Holocene ages and their uncertainties were used to perform a Kernel density function into a continuous dataset displaying rockfall probability per 100years. Results highlight four Holocene periods of enhanced rockfall occurrence: (i) c. 7-5.7ka, related to the Holocene Warm Periods; (ii) c. 4.5-4ka, related to the Sub-boreal Warm Period; (iii) c. 2.3-1.6ka, related to the Roman Warm Period; and (iv) c. 0.9-0.3ka, related to the Medieval Warm Period and beginning of the Little Ice Age. Laser and photogrammetric three-dimensional (3D) models of the rock walls were produced to reconstruct the detached volumes from the best-preserved rockfall scars (≤0.91 ± 0.12ka). A structural study was carried out at the scale of the Glacier du Géant basin using aerial photographs, and at the scale of four selected rock walls using the 3D models. Two main vertical and one horizontal fracture sets were identified. They correspond respectively to alpine shear zones and veins opened-up during long-term exhumation of the Mont Blanc massif. Our study confirms that climate primarily controls rockfall occurrence, and that structural settings, coincident at both the massif and the rock wall scales, control the rock-wall shapes as well as the geometry and volume of the rockfall events.

Rock temperature prior to failure: Analysis of 209 rockfall events in the Mont Blanc massif (Western European Alps)

“…Volume and depth of detachments are derived from several methods (frequently cross‐checked to verify their reliability): (1) 12 rock walls of the MBM are laser scanned (LiDAR) almost annually and comparison of the 3D models obtained provides very accurate data 3,42 . (2) Another 12 rock walls have been photogrammetrically surveyed 43 to obtain accurate measurements of the dimensions of clearly identifiable rock elements (pillar ridge, overhang, etc.). For all other events, (3) data are obtained by laser telemetry or by estimating the dimensions of the scars, by transferring altitudes to terrestrial or aerial photographs 15 .…”

Évolution des parois rocheuses gelées de haute montagne sous forçage climatique

“…Les 67 écroulements de 500 à 65 000 m 3 ainsi documentés depuis 1862 présentent, comme pour les Drus, une très bonne corrélation avec les périodes les plus chaudes (Ravanel et Deline, 2011). Ainsi, 82 % des écroulements se sont produits au cours de la période 1990-2015, caractérisée par l'accélération du réchauffement climatique, et les étés les plus chauds (1947,1976,1983,2003,2015) périodes d'occurrence accrue d'écroulements (figure 5) : i) vers 7,0-5,7 ka, correspondant aux optima chauds de l'Holocène ; ii) 4,5-4,0 ka, coïncidant avec l'optimum sub-boréal ; iii) vers 2,3-1,6 ka, lié à l'optimum climatique romain ; et iv) vers 0,9-0,3 ka, période correspondant à l'optimum climatique médiéval et au début du Petit Âge glaciaire (Gallach et al, 2020). Ces résultats confirment, ici sur un pas de temps long, l'influence du climat sur le déclenchement des écroulements.…”

A new method for dating the surface exposure age of granite rock walls in the Mont Blanc massif by reflectance spectroscopy

“…An increase in rockfall frequency in the massif over the last 30 years is correlated to the permafrost degradation caused by the current air warming. In order to understand this relationship over a longer timescale, Terrestrial Cosmogenic Nuclide (TCN) surface exposure dating has been used to date a selection of paleo-rockfalls in the Glacier du Geant basin rock walls (Bohlert et al, 2008;Gallach et al, 2018Gallach et al, , 2020. Most of the resulting ages have been correlated to some warm periods of the Holocene (Gallach et al, 2020).…”

“…In order to understand this relationship over a longer timescale, Terrestrial Cosmogenic Nuclide (TCN) surface exposure dating has been used to date a selection of paleo-rockfalls in the Glacier du Geant basin rock walls (Bohlert et al, 2008;Gallach et al, 2018Gallach et al, , 2020. Most of the resulting ages have been correlated to some warm periods of the Holocene (Gallach et al, 2020). TCN dating is considered to be the most precise method available for surface exposure dating, however its complexity and expensiveness can become a limitation, and they sometimes preclude obtaining large statistically robust datasets.…”

“…In the MBM, colour change between fresh and long-exposed granite surfaces is highlighted by each rockfall as follows: light grey colour of a recent rockfall scar corresponds to non-weathered granite, whereas the surface of long-exposed rock walls has a typical orange-reddish colour, as shown, for example, by the west face of the Drus . Redness of the granite rock surfaces in the MBM was preliminarily shown to be time-dependent by Bohlert et al (2008), and was used as a proxy of relative exposure age for TCN dating sampling (Gallach et al, 2018(Gallach et al, , 2020. Evolution of the MBM granite redness was first studied by analysing five rock samples that were extracted from the Aiguille du Midi during a pilot campaign (Bohlert et al, 2008).…”