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International audience; Periglacial rock walls are affected by an increase in rockfall activity attributed to permafrost degradation. While recent laboratory tests have asserted the role of permafrost in bedrock stability, linking experimental findings to field applications is hindered by the difficulty to assess bedrock temperature at observed rockfall locations and time. In this study, we simulated bedrock temperature for 209 rockfalls inventoried in the Mont Blanc massif between 2007 and 2015 and 209 000 random events artificially created at observed rockfall locations. Real and random events are then compared in a statistical analysis to determine the results significance. Permafrost conditions (or very close to 0 °C) were consistently found for all events with failure depth > 6 m, and for some events affecting depths from 4 to 6 m. Shallower events were likely not related to permafrost processes. Surface temperatures were significantly high up to at least 2 months prior to failure with the highest peaks in significance 1.5 to 2 months and 1 to 5 days before rockfalls. Similarly, temperature significances at scar depths were significantly high, but steadily decreasing, 1 day to 3 weeks before failure. The study confirms that warm permafrost areas (> -2 °C) are particularly prone to rockfalls, and that failures are a direct response to extraordinary high bedrock temperature in both frozen and unfrozen conditions. The results are promising for the development of a rockfall susceptibility index but uncertainty analysis encourages to use a greater rockfall sample and a different sample of random events.

This study is part of the ANR 14-CE03-0006 VIP Mont Blanc and the EU ALCOTRA AdaPT Mont Blanc project. The current paper studies the dynamics and age of the Triangle du Tacul (TDT) ice apron, a massive ice volume lying on a steep high-mountain rock wall in the French side of the Mont-Blanc massif at an altitude close to 3640 m a.s.l. Three 60 cm long ice cores were drilled to bedrock (i.e. the rock wall) in 2018 and 2019 at the TDT ice apron. Texture (microstructure and lattice-preferred orientation, LPO) analyses were performed on one core. The two remaining cores were used for radiocarbon dating of the particulate organic carbon fraction (three samples in total). Microstructure and LPO do not substantially vary with along the axis of the ice core. Throughout the core, irregularly shaped grains, associated with strain-induced grain boundary migration and strong single maximum LPO, were observed. Measurements indicate that at the TDT ice deforms under a low strain-rate simple shear regime, with a shear plane parallel to the surface slope of the ice apron. Dynamic recrystallization stands out as the major mechanism for grain growth. Micro-radiocarbon dating indicates that the TDT ice becomes older with depth perpendicular to the ice surface. We observed ice ages older than 600 year BP and at the base of the lowest 30 cm older than 3000 years. Publisher PDF

International audience; This paper focuses on the visibility of glacier surfaces in the Mont Blanc Massif (Western European Alps) on TerraSAR-X/PAZ images and the identification of geometric distortions (GDs) based on the SAR acquisition geometry. Small glaciers which exist in complex topographies like steep slopes are most prone to GDs. We built a visibility map for both ascending/descending orbits by utilizing previously documented algorithms like the R-Index (RI) and the Layover Shadow (LS) simulations, combined with an analysis of the angle between the steepest slope direction (SSD) and line of sight (LOS) vectors. The visibility map allows us to identify glaciers on steeper slopes which should be considered for further analysis using TerraSAR-X/PAZ images.

International audience; In the high mountain rock walls of the Mont Blanc massif, changes in the granite surface colour are related to its exposure age. The light grey colour of fresh rock surfaces turns orange when is long exposed to weathering. In order to study this colour/age relationship, reflectance spectroscopy was performed on 73 samples, and Terrestrial Cosmogenic Nuclide (TCN) dating was used to obtain their surface exposure age. The standard deviation of the reflectance values was calculated for each wavelength of the visible spectrum to study the behaviour of each spectral region. The aim was to find two colour regions that showed opposite behaviour, and once they are combined, they could provide a representative index of the rock colour changes that are linked to the degree of weathering.As an adaptation of the Green Red Vegetation Index used to measure colour changes in vegetation, the GReen-Infrared GRanite Index (GRIGRI), a normalized difference between the granite 770 nm and 530 nm reflectance values, was developed. The GRIGRI value of a weathered granite surface has a close relationship with its exposure age (R2 = 0.85). The reflectance spectra of seven samples for which TCN dating failed and two samples for which the TCN age was considered to be an outlier were used to calculate their GRIGRI value to assess the colour-based ages, that were plausible according to rock wall morphology and the TCN exposure ages of the surrounding surfaces. We propose a new method of surface dating for the rock walls of the Mont Blanc massif using reflectance spectroscopy.

Deglaciation of high mountain rockwalls alters slope stability as rockwalls become more sensitive to modifications in environmental factors (e.g. seasonal temperature variations). In the past decades, increasing efforts focused on studying deglaciated Alpine rockwalls. Yet, currently deglaciating rockfaces remain unstudied. Here, we quantify surface area variations of massive ice bodies lying on high mountain rockwalls (ice aprons) in the French sector of the Mont Blanc massif between the end of the Little Ice Age (LIA) and 2018. Surface area estimates are computed from terrestrial and aerial oblique photographs via photogrammetry. This technique allows using photographs taken without scientific intent, and to tap into diverse historical or recent photographic catalogs. We derive an ice apron surface area model from precipitation records and the positive degree-days. The studied ice aprons shrank from 1854 to the 1950s, before expanding until the end of the 1990s. The beginning of the 21st century shows a decrease in surface area, leading to the complete melt of one of the studied ice aprons in 2017. Observed variations correlate with modeled surface area, suggesting strong sensitivity of ice aprons to changes in climatic variables. By studying site-specific correlations, we explore the importance of local drivers over the balance of ice aprons.

Glacial processes can have a strong impact on human activities in terms of hazards and freshwater supply. Therefore, scientific observation is fundamental to understand their current state and possible evolution. To achieve this aim, various monitoring systems have been developed in the last decades to monitor different geophysical and geochemical properties. In this manuscript, we describe examples of close-range monitoring sensors to measure the glacier dynamics: (i) terrestrial interferometric radar, (ii) monoscopic time-lapse camera, (iii) total station, (iv) laser scanner, (v) ground-penetrating radar and (vi) structure form motion. We present the monitoring applications in the Planpincieux and Grandes Jorasses glaciers, which are located in the touristic area of the Italian side of the Mont Blanc massif. In recent years, the Planpincieux-Grandes Jorasses complex has become an open-air research laboratory of glacial monitoring techniques. Many close-range surveys have been conducted in this environment and a permanent network of monitoring systems that measures glacier surface deformation is presently active.

Since the end of the Little Ice Age, the west face of the Drus (Mont Blanc massif, France) has been affected by a retrogressive erosion dynamic marked by large rockfall events. From the 1950s onwards, the rock failure frequency gradually increased until the large rockfall event (292,680 m3) of June 2005, which made the Bonatti Pillar disappear. Aiming to characterize the rock failure activity following this major event, which may be related to permafrost warming, the granitic rock face was scanned each autumn between October 2005 and September 2016 using medium- and long-range terrestrial laser scanners. All the point clouds were successively compared to establish a rockfall source inventory and determine a volume-frequency relationship. Eleven years of monitoring revealed a phase of rock failure activity decay until September 2008, a destabilization phase between September 2008 and November 2011, and a new phase of rock failure activity decay from November 2011 to September 2016. The destabilization phase was marked by three major rockfall events covering a total volume of 61,494 m3, resulting in the progressive collapse of a new pillar located in the northern part of the June 2005 rockfall scar. In the same way as for the Bonatti Pillar, rock failure instability propagated upward with increasing volumes. In addition to these major events, 304 rockfall sources ranging from 0.002 to 476 m3 were detected between 2005 and 2016. The temporal evolution of rock failure activity reveals that after a major event, the number of rockfall sources and the eroded volume both follow a rapid decrease. The rock failure activity is characterized by an exponential decay during the period following the major event and by a power-law decay for the eroded volume. The power law describing the distribution of the source volumes detected between 2005 and 2016 indicates an exponent of 0.48 and an average rock failure activity larger of more than six events larger than 1 m3 per year. Over the 1905–2016 period, a total of 426,611 m3 of rock collapsed from the Drus west face, indicating a very high rock wall retreat rate of 14.4 mm year−1 over a surface of 266,700 m2. Averaged over a time window of 1000 years, the long-term retreat rate derived from the frequency density integration of rock failure volumes is 2.9 mm year−1. Despite difficulty in accessing and monitoring the site, our study demonstrates that long-term surveys of high-elevation rock faces are possible and provide valuable information that helps improve our understanding of landscape evolution in mountainous settings subject to permafrost warming.

The increasing rockfall frequency in high mountain rockwalls is generally associated with global warming via the permafrost warming but long series of high resolution data on rockfall are still necessary to better appreciate the evolution of their frequencies and volumes, and to better understand their triggering factors.Here we present an inventory of rockfalls surveyed by terrestrial laser scanning (LiDAR) since 2005 in the east face of the Tour Ronde (3792 m a.s.l.) in the Géant glacial basin (Mont Blanc massif).Between 2005 and 2018, the rockwall was scanned 12 times, giving 11 comparisons of 3D models [1]. These highlighted a very intense morphodynamics with 91 destabilizations with volumes between 1 and 15,578 m3 for a total volume of 31,610 m3 (mean erosion rate: 29,8 mm.yr-1). In the first year of measurement, the Bernezat spur was affected by a collapse of more than 700 m3 [2]. Then, it was affected by rockfalls not exceeding a few tens of m3. On the other hand, in the rest of the face, there is a very strong increase in rockfall activity, especially during the hot summer 2015 at the end of which (August 27) the most voluminous collapse of the whole period occurred.The modelled surface temperature distribution at the scale of the Mont Blanc massif [3] attests to the presence of permafrost throughout the rockslope, confirmed by temperature measurements carried out at 3, 30 and 55 cm deep in the rock at the base of the Bernezat spur between October 2006 and May 2009. In addition, the main collapses left massive ice, at the level of their scar, more or less mixed with rock debris. These different elements, associated with the fact that collapses occur essentially during and following the highest summer heat, point to the role of degradation of permafrost [4]. A collapse on December 4, 2018 at the level of the small spur located at the foot of the Bernezat and whose volume is estimated at 7000 m3 reinforces this hypothesis since the detachment surface was covered - except for its margins - by massive ice. This has been sampled and its dating will perhaps confirm the age of the ice present in the cracks of the permafrost-affected rockwalls of the Mont Blanc massif. In 2017, a collapse of 44,000 m3 in the north face of the Aiguille du Midi (3842 m a.s.l.) had exposed 4060 calBP ice. In the Tour Ronde case, ice/snow cover changes and glacial debutressing could also partly explain the rockfall activity. [1] Ravanel L. et al. (2010). Revue Française de Photogrammétrie et de Télédétection, 192 : 58-65.[2] Rabatel A. et al. (2008). Geophysical Research Letters, 35: L10502.[3] Magnin F. et al. (2015). Geomorphologie, 21: 145-162.[4] Ravanel L. et al. (2017). Science of the Total Environment, 609: 132-143.

De-glaciating high mountain areas result in new landscapes of bedrock and debris where permafrost can degrade, persist or even newly form in cases, and of new lakes in glacier bed overdeepenings (GBOs) becoming ice-free. These landscapes with new lakes in close neighborhood to over-steepened and perennially frozen slopes are prone to chain reaction processes (e.g. rock-ice avalanches into lakes triggering impact waves, dam breach or overtopping, and debris flows) with potentially far-reaching run-out distances causing valley floors devastation. The frequency, magnitude and zonation of hazards are shifting, requiring integrative approaches combining comprehensive information about landscape evolution and related processes to support stakeholders in their adaptation strategies. In this study, we intend to setup an essential baseline for such an integrative approach in the Mont Blanc massif (MBM), which is a typical high-mountain range affected by de-glaciation processes. We first (i) predict and (ii) detect potential GBOs by combining the GlabTop model with a visual analysis based on morphological indications of glacier flow through over- deepened bed parts. We then (iii) determine the level of confidence concerning the resulting information, and (iv) estimate the approximate time range under which potential lakes could form. The location of the predicted GBOs and the shape of glacier beds are evaluated against currently forming water bodies at retreating glacier snouts, and seismic and ice penetrating radar measurements on the Argentière glacier. This comparison shows that the location of predicted GBOs is quite robust whereas their morphometric characteristics (depth, volume) are highly uncertain and tend to be underestimated. In total, 48/80 of the predicted or detected GBOs have a high level of confidence. In addition to five recently formed water bodies at glacier snouts, one of the high confidence GBOs (Talèfre glacier) which is also the most voluminous one could form imminently (during coming years), if not partially or totally drained through deeply incised gorges at the rock threshold. Twelve other lakes could form within the first half of the century under a constant or accelerated scenario of continued glacier retreat. Some of them are located below high and permanently frozen rock walls prone to destabilization and high-energy mass movements, hinting at possible hot spots in terms of hazards in the coming decades, where more detailed analysis would be required.

Providing quantitative constraints on late Pleistocene glacier fluctuations remains an important challenge for understanding glacier response to past and future climate changes. In most mountainous settings, paleo-glacier reconstructions are limited because they often lack precise temporal constraints. Different geochronological methods have been developed and applied to date specific geomorphological or sedimentological markers for paleo-glacier dynamics. Recently, OSL (Optically Stimulated Luminescence) surface exposure dating has been introduced and provides us with an opportunity to improve paleo-glacier reconstructions. This method is based on the sensitivity of the OSL signal from rock minerals to light, resulting in bleaching of the OSL signal within the upper first millimeters of the exposed rock surface, a process that depends on the exposure age, the rock type and the local setting (e.g. topographic shielding, bedrock orientation etc.). Here, we investigate the potential of OSL surface exposure along a vertical cross-section of polished bedrock surfaces with known post-LIA (Little Ice Age) exposure ages (from 3 to 137 years) along the Mer de Glace glacier (Mont Blanc massif, France). The infrared stimulated luminescence (IRSL) signals from rock slices exhibit increasingly deep bleaching profiles with elevation and thus exposure age, which is consistent with progressive glacier thinning since the LIA. Our results show that OSL surface exposure dating can be applied to periglacial environments, and is a promising tool for high-resolution reconstruction of ice extent fluctuations, both in space and time.

Since the Last Glacial Maximum, ∼20 k.y. ago, Alpine glaciers have retreated and thinned. This transition exposed bare bedrock surfaces that could then be eroded by a combination of debuttressing or local frost cracking and weathering. Quantification of the respective contributions of these processes is necessary to understand the links between long-term climate and erosion in mountains. Here, we quantified the erosion histories of postglacial exposed bedrock in glacial valleys. Combining optically stimulated luminescence and terrestrial cosmogenic nuclide (TCN) surface exposure dating, we estimated the erosion rate of bedrock surfaces at time scales from 101 to 104 yr. Bedrock surfaces sampled from the flanks of the Mer de Glace (Mont Blanc massif, European Alps) revealed erosion rates that vary from 3.5 ± 1.2 ⋅ 10−3 mm/yr to 4.3 ± 0.6 mm/yr over ∼500 m of elevation, with a negative correlation between erosion rate and elevation. The observed spatial variation in erosion rates, and their high values, reflect morphometric (elevation and surface slope) and climatic (temperature and snow cover) controls. Furthermore, the derived erosion rates can be used to correct the timing of deglaciation based on TCN data, potentially suggesting very rapid ice thinning during the Gschnitz stadial.

International audience

International audience; In high alpine environments, glacial shrinkage and permafrost warming due to climate change have significant consequences on mountaineering routes. Few research projects have studied the relationship between climate change and mountaineering; this study attempts to characterize and explain the evolution over the past 40 years of the routes described in The Mont Blanc Massif: The 100 Finest Routes, Gaston Rébuffat's emblematic guidebook, published in 1973.The main elements studied were the geomorphic and cryospheric changes at work and their impacts on the itinerary's climbing parameters, determining the manner and possibility for an itinerary to be climbed. Thirty-one interviews, and comparison with other guidebooks, led to the identification of 25 geomorphic and cryospheric changes related to climate change that are affecting mountaineering itineraries. On average, an itinerary has been affected by nine changes. Among the 95 itineraries studied, 93 have been affected by the effects of climate change-26 of them have been greatly affected; and three no longer exist. Moreover, periods during which these itineraries can be climbed in good conditions in summer have tended to become less predictable and periods of optimal conditions have shifted toward spring and fall, because the itineraries have become more dangerous and technically more challenging.

The study of the ice (cryostructures, cryostratigraphy, types and age) that partly fills the fractures in rockwall permafrost is necessary to better understand the increasing frequency/volume of rockfalls in the high alpine rockwalls. Typically inaccessible, this ice is visible only when a rockfall occurs, exposing previously hidden ice. Over the past two years, the two largest rockfalls of the decade in the Mont Blanc massif and a smaller one allowed to start the documentation of the rockwall permafrost ice. This poster aims to present the three cases. Up to now, dating has been carried out for only one of the three cases. How to date permafrost ice? Dating ice cores from alpine glaciers has shown the wide time span covered by these, from a few decades to several thousand years. Several dating methods are available to quantify the age. Counting the annual layers, which proved to be effective, is widely used in the study of mountain glaciers but cannot be used for the internal ice of the permafrost since it does not present these annual layers. Structure of the ice in the scar of the Trident of Tacul rockfall scar shows nevertheless a series of layers. We will therefore use radionuclide dating to obtain the absolute age of the ice. So far, only one age is available for interstitial ice from permafrost-affected rockwall: the ice sampled from a felt block that is part of the deposit of the 150'000 m 3 rockfall that occurred in Feb. 2014 at Piz Kesch (Swiss Alps

Alpine rockwalls with warm permafrost (near 0°C) are the most active rockfall detachment zones in the Mont Blanc massif (MBM, French Alps) with more than 380 recent events. Near-vertical rockwall permafrost is spatially controlled by variations in rock fractures, snow cover, and microtopography. A reliable method to validate the distribution of permafrost in critical and unstable areas does not yet exist. We present seven electrical resistivity tomography (ERT) surveys measured on five near-vertical rockwalls in the MBM from 2012 and 2013 that have been calibrated with measurements on a granite sample in the laboratory. ERT shows consistent measurements of remaining sensitive permafrost relating to inferred temperatures from 0 to −1.5°C. ERT results demonstrate evidence of topographic controls on permafrost distribution and resistivity gradients that appear to reflect crest width. ERT results are compared to two permafrost index maps that use topoclimatic factors and combine effects of thin snow and fractures, where index model spatial resolution is crucial for the validation with ERT. In cryospheric environments, index maps seem to overestimate permafrost conditions in glacial environments. As a consequence, the sensitive areas of permafrost may slightly deviate from the results from distributed models that are only constrained by topoclimatic factors and interpreted with consideration of local fracture and snow conditions. This study demonstrates (i) that the sensitive and hazardous areas of permafrost in near-vertical rock faces can be assessed and monitored by the means of temperature-calibrated ERT and (ii) that ERT can be used for distributed model validation.

To calibrate the in situ 10Be production rate, we collected surface samples from nine large granitic boulders within the deposits of a rock avalanche that occurred in AD 1717 in the upper Ferret Valley, Mont Blanc Massif, Italy. The 10Be concentrations were extremely low and successfully measured within 10% analytical uncertainty or less. The concentrations vary from 4829 ± 448 to 5917 ± 476 at g−1. Using the historical age exposure time, we calculated the local and sea level-high latitude (i.e. ≥60°) cosmogenic 10Be spallogenic production rates. Depending on the scaling schemes, these vary between 4.60 ± 0.38 and 5.26 ± 0.43 at g−1 a−1. Although they correlate well with global values, our production rates are clearly higher than those from more recent calibration sites. We conclude that our 10Be production rate is a mean and an upper bound for production rates in the Massif region over the past 300 years. This rate is probably influenced by inheritance and will yield inaccurate (e.g. too young) exposure ages when applied to surface-exposure studies in the area. Other independently dated rock-avalanche deposits in the region that are approximately 103 years old could be considered as possible calibration sites.

Rockfall is one of the main geomorphological processes that affects the evolution and stability of rock-walls. At high elevations, rockfall is largely climate-driven, very probably because of the warming of rock-wall permafrost. So with the ongoing global warming that drives the degradation of permafrost, the related hazards for people and infrastructure could continue to increase. The heatwave of summer 2015, which affected Western Europe from the end of June to August, had a serious impact on the stability of high-altitude rock-walls, including those in the Mont Blanc massif. A network of observers allowed us to survey the frequency and intensity of rock-wall morphodynamics in 2015, and to verify its relationship with permafrost. These observations were compared with those of the 2003 summer heatwave, identified and quantified by remote sensing. A comparison between the two years shows a fairly similar rockfall pattern in respect of total volumes and high frequencies (about 160 rockfalls >100m3) but the total volume for 2003 is higher than the 2015 one (about 300,000m3 and 170,000m3 respectively). In both cases, rockfalls were numerous but with a low magnitude and occurred in permafrost-affected areas. This suggests a sudden and remarkable deepening of the active layer during these two summers, rather than a longer-term warming of the permafrost body.

International audience; In the Western Alps, the External Crystalline Massifs (ECM) are key places to investigate the kinematics and thermal structure of a collisional crustal wedge, as their paleo-brittle/ductile transition is now exhumed at the surface. New (U–Th–Sm)/He data on zircon and new Raman Spectroscopy on Carbonaceous Material (RSCM) data from the Aiguilles Rouges and the Mont Blanc massifs, coupled to HeFTy thermal modeling, constrain the thermal evolution and exhumation of the massifs. In the cover of the Aiguilles Rouges massif, we found that the maximal temperature was about 320 °C (+/− 25 °C), close to the maximal temperature reached in the cover of the Mont Blanc massif (~ 350 °C +/− 25 °C). We show that, after a fast heating period, the thermal peak lasted 10–15 Myrs in the Mont Blanc massif, and probably 5–10 Myrs in the Aiguilles Rouges massif. This thermal peak is synchronous with crustal shortening documented in the basement. (U–Th–Sm)/He data and thermal modeling point toward a coeval cooling of both massifs, like other ECM, at around 18 Ma +/− 1 Ma. This cooling was related to an exhumation due to the initiation of frontal crustal ramps below the ECM, quite synchronously along the Western Alps arc.

10.1080/00357529.2012.728898-F0001 Figure 1. Smoky quartz gwindel, 12.2 cm, from the Mont Blanc massif, Les Courtes, France. Francis Benjamin specimen, Jeff Scovil photo. Crystals that are bent, tw...

International audience; One of the most glacierized areas in the European Alps, the Mont Blanc massif, illustrates how fast changes affect the cryosphere and the related morphodynamics in high mountain environments, especially since the termination of the Little Ice Age. Contrasts between the north-west side, gentle and heavily glaciated, and the south-east side, steep and rocky, and between local faces with varying slope angle and aspect highlight the suitability of the study site for scientific investigations. Glacier shrinkage is pronounced at low elevation but weaker than in other Alpine massifs, and supraglacial debris covers have developed over most of the glaciers, often starting in the nineteenth century. Lowering of glacier surface also affects areas of the accumulation zone. While modern glaciology has been carried out in the massif for several decades, study of the permafrost has been under development for only a few years, especially in the rock walls. Many hazards are related to glacier dynamics. Outburst flood from englacial pockets, ice avalanche from warm-based and cold-based glaciers, and rock slope failure due to debuttressing are generally increasing with the current decrease or even the vanishing of glaciers. Permafrost degradation is likely involved in rockfall and rock avalanche, contributing to the chains of processes resulting from the high relief of the massif. The resulting hazards could increasingly endanger population and activities of the valleys surrounding the Mont Blanc massif.

As part of a research programme into the controls on the evolution of the supraglacial debris cover of Glacier d'Estelette, a study of debris cover extent has been conducted over two timescales, décennal and annual. By comparison of aerial photographs, a reduction (1952 -1993) then extension (1993 -2006) of the debris cover could be correlated with contemporary phases of glacier advance then retreat. Its proportion of the 2/3 glacier surface area has oscillated between 31.1% and 52.2%. A comparison of oblique terrestrial photographs has confirmed the increase of debris covered glacier which characterised the last half-century, with an increase of debris cover between 2006 and 2007. These results confirm that the climatic signal recorded by the glacier 's dynamic response is the principal control on the extent of the debris cover, via the intensity of ablation and the glacier velocity., Dans le cadre d 'un programme de recherche sur les paramètres qui contrôlent l 'évolution de la couverture détritique du glacier d 'Estelette, une étude a été conduite à deux pas de temps : décennal depuis 1952 et annuel de 2006 à 2007. Par photo-comparaison d'aérophotographies, la rétraction (entre 1952 et 1993) puis l 'extension (entre 1993 et 2006) de la couverture détritique ont pu être corrélées avec les phases d 'avancée puis de retrait glaciaires contemporaines, et mesurées : son rapport à la superficie des 2/3 aval du glacier a oscillé entre un minimum de 31,1 % et un maximum de 52,2 %. L 'étude comparative de photographies obliques terrestres a confirmé l 'accroissement du détritisme supraglaciaire qui caractérisent le dernier demi-siècle, avec une augmentation des débris sur le glacier entre 2006 et 2007. Ces résultats confirment que le signal climatique enregistré par la dynamique glaciaire est le principal contrôle de l 'extension de la couverture détritique, via l 'intensité de l 'ablation et la vitesse d'écoulement du glacier., Mazué Romain, Deline Philip, Kirkbride Martin P. Suivi de l'évolution de la couverture détritique d'un glacier noir par photo-comparaison : le glacier d'Estelette (Massif du Mont Blanc). In: Collection EDYTEM. Cahiers de géographie, numéro 8, 2009. Neige et glace de montagne. Reconstitution, dynamique, pratiques. pp. 171-178.

A quantitative analysis was performed with the aim of identifying changes in the volume and thickness of the Brenva Glacier tongue (Mont Blanc Massif, Italy) in the second half of the 20th century. This analysis was based on the comparison of digital elevation models (DEMs) derived from historical records, specifically maps (1959, 1971, 1983, 2003) and photogrammetric surveys (1991, 1997). The DEMs were generated by means of a digital photogrammetric workstation, with semi-automatic and automatic procedures. Problems relating to the identification of the control points in the photos had to be resolved in order to define the external orientation. An unconventional photogrammetric methodology, based on the identification of homologous points in zones considered outside of the glacier area, was adopted to insert the surveys into a single reference system. Furthermore, along with the photogrammetric data, DEMs derived from digitized historical maps were generated and compared to define changes in the geometry of the glacier tongue. The results indicated a positive long-term glacier tongue balance. In fact, between 1959 and 2003, there was an increase of 22.6 × 106 m3, equal to an average thickness of ca.+ 34 m (+ 0.7 m a− 1 w.e.). Validation of the data obtained from comparison of the DEMs and the reliability of the results were discussed as well.

International audience; The behaviour of rare earth elements (REE) during fluid–rock interaction in mid-crustal shear zones has received little attention, despite their potential for mass balance calculation and isotopic tracing during deformation. In this study, several cases of large REE mobility during Alpine fluid-driven shear zone development in the pre-Alpine granitic basement of the Mont Blanc Massif are considered. On a regional scale, the undeformed granite compositions range within 5 wt% SiO2 (70.5–75.3 wt%) and magmatic chemical variations are of the order of 10–20%, ascribed to minor effects of crystal fractionation. Major and trace element mobility observed in shear zones largely exceeds these initial variations. Shear zones developed a range of mineral assemblages as a result of shearing at mid-crustal depths (at not, vert, similar0.5 GPa, 400°C). Five main shear zone assemblages involve muscovite, chlorite, epidote, actinolite and calcite, respectively, as major phases. In most cases, selective enrichments of light or heavy REE (and Y, Ta, Hf) are observed. REE mobility is unrelated to deformation style (cataclastic, mylonitic), the intensity of strain, and to the shear zone's major metamorphic mineral assemblages. Instead, the changes in REE concentrations are ascribed to the alteration of pre-existing magmatic REE-bearing minerals during deformation-related fluid–rock interaction and to the syntectonic precipitation of metamorphic REE-bearing minerals (mainly monazite, bastnäsite, aeschynite and tombarthite). Minor proportions (

The steep rockwalls of the Mont Blanc massif have been affected by an increase in rockfall activity in the last decades. Permafrost degradation is suggested as the most likely triggering factor. To better understand geomorphic processes we investigate permafrost distribution and address questions on its pattern in steep alpine bedrock. We use GIS-modeling to simulate Mean Annual Rock Surface Temperature (MARST) distribution. Rock temperature measurements including three 10-m-deep borehole monitoring at the Aiguille du Midi (AdM, 3842 m a.s.l) serve to estimate the temperature offset (i.e. temperature difference between rock surface and depth of negligible inter-annual temperature varibility). The estimation of the lower extent of permafrost distribution is derived from a combination of both approaches and hypotheses on permafrost occurrence are evaluated with Electrical Resistivity Tomography (ERT) measurements. The MARST model indicates that the 0 °C isotherm extends down to 2600 m a.s.l in the most shaded faces and rises up to 3800 m in the most sun-exposed areas. According to recent literature and the AdM borehole thermal profiles, we postulate that permafrost could extends down below MARST reaching up to 3°C due to temperature offset processes. ERT measurements performed along 160-m-long profiles at six different sites which the top are located from 3360 m a.s.l to 2760 m a.s.l and the MARST range from 3°C are the first of this kind. Five of sites are located in the granite area making them directly comparable. They all show high resistivity values at depth (>200 k) interpreted as permafrost bodies. Lower resistivity values (< 90 k ) are found either above the high resistivity bodies and interpreted as thawed active layer, or below MARST warmer than 2-3°C and interpreted as non-perenially frozen rock. Two sites were measured in autumn 2012 and autumn 2013 allowing for time-lapse investigation which demonstrates the change in resistivity in repeated measurements. These preliminary results could confirm that steep alpine bedrock permafrost exists below surface tempera- ture reaching up to 3 °C. A temperature-resistivity calibration will be performed in a freezing laboratory at the Technical University of Munich to better assess ERT results and their interpretation in terms of permafrost occurrence and interannual changes.

Permafrost and related thermo-hydro-mechanical processes are thought to influence high alpine rock wall stability, but a lack of field measurements means that the characteristics and processes of rock wall permafrost are poorly understood. To help remedy this situation, in 2005 work began to install a monitoring system at the Aiguille du Midi (3842 m a.s.l). This paper presents temperature records from nine surface sensors (eight years of records) and three 10 m deep boreholes (4 years of records), installed at locations with different surface and bedrock characteristics. In line with previous studies, our temperature data analyses showed that: micro-meteorology controls the surface temperature, active layer thicknesses are directly related to aspect and ranged from

The study of rockfall (volume >100 m3) in high mountains is essential to understand landscape evolution and to evaluate natural hazard. The number of rockfalls seems actually to rise in the European Alps, while exposure is increasing from high elevation areas (e.g. cable cars, huts) to valley floors (e.g. urbanization, transport). Recent rockfalls from high-Alpine steep rockwalls are hypothesized to be a consequence of the climate change through the warming of the permafrost. Given the lack of systematic data on rockfall, this relationship has however remained difficult to assess despite few evidences including laboratory tests and temperature measurements indicating permafrost degradation, while the increase of rockfall frequency and magnitude remained conjectural. Here we analyze several inventories of rockfalls acquired in the Mont Blanc massif by innovative methods in order to characterize the rockfall triggering conditions and to emphasize the role of permafrost.

The Mont Blanc Massif, the highest of the main external crystalline massifs of the Western Alps, is renowned for its extensive glacier cover, steep granite rockwalls and vertiginous peaks. Bordered by populated French, Italian and Swiss valleys, the massif has attracted both tourists and scientists over several centuries. Numerous sites are remarkable associations of landforms, for example, (a) the Aiguilles de Chamonix, a superb alignment of arêtes and peaks reaching 3,500 m a.s.l. which dominates the valley of Chamonix; (b) the Mer de Glace, the largest French glacier, which has been a major centre for tourism and glaciology since the eighteenth century; and (c) the Glacier du Miage, one of the largest debris-covered glaciers in the Alps. Since the Last Glacial Maximum, the landscape of the massif has changed significantly and its morphodynamics is evidently still very active. Glacier retreat accelerates, and permafrost degradation has triggered more rockfalls. Nevertheless two million people are still attracted each year by the 'top of Europe'.

A gradual reduction in the stability of steep bedrock slopes is recognized as one potential impact of warming in high mountain regions. Recently, consideration has turned to the potential direct role of extremely warm temperatures in triggering rockfalls. We provide here a first systematic assessment of the timing of 53 recent rockfalls relative to defined seasonal extremes of daily maximum air temperature. Rockfall observations from the Swiss Alps, Mont Blanc Massif, and Southern Alps of New Zealand, are combined with climate analyses based on the nearest available long-term records. At four high elevation climate stations in Switzerland, there has been significant warming across all quantiles of daily maximum temperature during the spring and summer months, with corresponding increases in both the frequency and magnitude of extremely warm days, and generally less warming (even cooling) during winter and autumn. In the same region, an unusually high occurrence of extremely warm days occurring in the week leading up to rockfalls has been observed over recent decades, with 14 out of 24 rockfalls preceded by one or more extremely warm days. At the neighboring Mont Blanc Massif, based on only two years of observations, few rockfalls can be linked to extremely warm temperatures, although an early and extremely warm onset of seasonal thawing in spring 2007 may have contributed to the large number of rockfalls observed that same summer. In the Southern Alps of New Zealand, the available evidence provides no basis to suggest extremely warm temperatures have triggered unusual rockfall activity.

Thermochronometric data collected across the Alps over the last three decades allows for investigation of the evolution of this orogen, which is subject to changes in climate and geodynamics. Exhumation rates are inferred from the thermochronometric ages using a statistical inversion method based on the fact that the distance a sample traveled since closure is equal to the integral of the exhumation rate from the present day to the age of the sample. Exhumation rates are assumed to be spatially correlated but are free to vary through time. This results in the quantification of exhumation rates across the Alps, since 32 Ma, along with assessments of the quality of these inferences. We find that exhumation rates are initially fast in the internal arc of the Western Alps at rates up to 0.8 km/Myr at 30 Ma, decreasing at 20 Ma to 0.3 km/Myr to remain slow to the present. At the same time, around 20 Ma, rates across the External Crystalline Massifs of Western Alps increase to 0.6 km/Myr. We also find that the onset of high exhumation rates in the Tauern Window and the Lepontine Dome occurs at around 20 Ma, a time characterized by major reorganizations in the Alpine chain. A general increase in exhumation rates at around 5 Ma over the entire Alps is not confirmed. Instead we find that the Western Alps exhibit a 2 to 3 fold increase in exhumation rate over the last 2 Ma, during a recent event not seen further east, in spite of very similar topographic characteristics. We attribute this strong signal to detachment of the European slab in the Western Alps, combined with efficient glacial erosion.

Historical and proxy records document that there is a substantial asynchronous development in temperature, precipitation and glacier variations between European regions during the last few centuries. The causes of these temporal anomalies are yet poorly understood. Hence, highly resolved glacier reconstructions based on historical evidence can give valuable insights into past climate, but they exist only for few glaciers worldwide. Here, we present a new reconstruction of length changes for the Glacier des Bossons (Mont Blanc massif, France), based on unevaluated historical material. More than 250 pictorial documents (drawings, paintings, prints, photographs, maps) as well as written accounts have been critically analysed, leading to a revised picture of the glacier’s history, especially from the mid-eighteenth century up to the 1860s. Very important are the drawings by Jean-Antoine Linck, Samuel Birmann and Eugene Viollet-le Duc, which depict meticulously the glacier’s extent during the vast advance and subsequent retreat during the nineteenth century. The new glacier reconstruction extends back to AD 1580 and proves maxima of the Glacier des Bossons around 1610/1643, 1685, 1712, 1777, 1818, 1854, 1892, 1921, 1941, and 1983. The Little Ice Age maximum extent was reached in 1818. Until the present, the glacier has lost about 1.5 km in length, and it is now shorter than at any time during the reconstruction period. The Glacier des Bossons reacts faster than the nearby Mer de Glace (glacier reconstruction back to AD 1570 available). The Mont Blanc area is, together with the valley of Grindelwald in the Swiss Alps (two historical glacier reconstructions available back to AD 1535, and 1590, respectively), among the two regions that are probably best-documented in the world regarding historical glacier data.

We describe a 0.5 Mm3 rock avalanche that occurred in 2008 in the western Alps and discuss possible roles of controlling factors in the context of current climate change. The source is located between 2410 m and 2653 m a.s.l. on Mont Crammont and is controlled by a densely fractured rock structure. The main part of the collapsed rock mass deposited at the foot of the rock wall. A smaller part travelled much farther, reaching horizontal and vertical travel distances of 3050 m and 1560 m, respectively. The mobility of the rock mass was enhanced by channelization and snow. The rock-avalanche volume was calculated by comparison of pre- and post-event DTMs, and geomechanical characterization of the detachment zone was extracted from LiDAR point cloud processing. Back analysis of the rock-avalanche runout suggests a two stage event. There was no previous rock avalanche activity from the Mont Crammont ridge during the Holocene. The 2008 rock avalanche may have resulted from permafrost degradation in the steep rock wall, as suggested by seepage water in the scar after the collapse in spite of negative air temperatures, and modelling of rock temperatures that indicate warm permafrost (T > −2 °C).

Rockfalls fundamentally affect the morphodynamics of high mountain rockwalls, and represent a great danger for both people and infrastructures, but still are poorly known. By comparing old, recent and new photographs, in addition to geomorphological field data, we propose an inventory of the rockfalls that occurred since the end of the ‘Little Ice Age’ on the north side of the Aiguilles de Chamonix (Mont Blanc massif), ranging in volume from 500 to 65 000 m 3. These 42 rockfalls occurred after 1947, of which > 70% during the last two decades, with a maximal frequency during the warm summers, especially in 2003. Average elevation of scars (3130 m a.s.l.) close to the lower modelled permafrost limit, and the topography (e.g. spurs) of the affected rock faces enhancing lateral heat fluxes, suggest that a climatically driven permafrost degradation has triggered many of the recent rockfalls in high-Alpine steep rockwalls.

International audience; Les écosystèmes aquatiques reçoivent des apports conséquents de matière organique d'origine terrestre qui pourraient, lorsqu'ils sont intégrés dans les réseaux trophiques, soutenir une part significative de la production secondaire et piscicole de ces écosystèmes ou modifier la structure des communautés. Les études relatives à l'impact fonctionnel des apports d'origine terrestre aux écosystèmes lacustres sont, jusqu'à présent, essentiellement centrées sur les lacs peu profonds des boucliers scandinaves et Nord-américains. Les connaissances sont encore sporadiques en ce qui concerne les lacs profonds, dans lesquels la contribution du carbone d'origine terrestre n'a encore été que peu quantifiée. L'étude de la part prise par les apports allochtones dans le fonctionnement du système semble plus particulièrement pertinente dans les milieux en voie de réoligotrophisation, milieux dont ils pourraient maintenir la productivité. Dans le cadre d'une étude préalable sur les possibilités de distinguer les matières organiques d'origine terrestre dans les écosystèmes lacustres, un suivi mensuel des caractéristiques des MOD aquatiques dans la masse d'eau lacustre et dans les affluents a été mis en place. Des échantillons du niveau 0-20 m ont été prélevés dans le centre des lacs d'Annecy et du Léman de mars 2007 à mars 2008. Parallèlement des échantillons ont été prélevés dans la zone littorale du lac Léman et dans un certain nombre d'affluent des deux lacs. Les échantillons prélevés et filtrés à 0.2μm ont été analysés en utilisant les outils suivants : analyseur de DOC, spectrométre UV-visible, spectroscope de fluorescences 3D, chromatographie d'exclusion stérique. Les résultats mettent en évidence que les spectres de fluorométrie 3D peuvent constituer des signatures des différentes eaux. Les caractéristiques des MO fluorescentes apparaissent en effets bien distinctes : - prédominance des substances humiques dans les apports (indice HIX élevé) - importance des pics liés à la production récente de MO (pic γ de fluorescence) dans les eaux du centre des lacs - importance de l'activité biologique dans les zones littorales directement sous l'influence d'une rivière On constate par ailleurs une très nette évolution de la taille moyenne et de la distribution des familles de tailles de molécules au cours de la saison dans les eaux des lacs (diminution de la taille moyenne lors des pics de production printanier et estivaux) mais également dans les eaux des affluents). Ces résultats préliminaires mettent enfin en évidence le besoin d'études intégrées des matières organiques dans les écosystèmes lacustres avec une nécessaire combinaison d'approches analytiques diverses (spectroscopique, chromatographique mais aussi biomarqueurs) pour comprendre les interactions entre matières organiques autochtones et allochtones dans ces écosystèmes.

International audience; Due to a lack of systematic observations, the intensity and volume of rock falls and rock avalanches in high mountain areas are still poorly known. Nevertheless, these phenomena could have burly consequences. To document present rock falls, a network of observers (guides, mountaineers, and hut wardens) was initiated in the Mont Blanc Massif in 2005 and became fully operational in 2007. This article presents data on the 66 rock falls (100 m3≤V≤50,000 m3) documented in 2007 (n=41) and 2008 (n= 25). Most of the starting zones are located in warm permafrost areas, which are most sensitive to warming, and only four rock falls are clearly out of permafrost area. Different elements support permafrost degradation as one of the main triggering factors of present rock falls in high mountain areas.

Permafrost and its change in steep high-Alpine rock walls remain insufficiently understood especially because of the scarcity of existing instrumented sites. Thus, it is very interesting to develop investigations at the Aiguille du Midi (3842 m a.s.l), where roc-kwalls with diverse aspects and slopes, and galleries are accessible year-round. In the framework of the project PermaNET, we monitor the rock thermal regime, (i) with 9 sensors with one or three thermistors installed up to 55 cm deep ; (ii) using electrical resistivity tomography (ERT), based on the temperature-resistivity relationship of the local granite ; (iii) by infrared thermography on the rockwalls. These measurements are completed by two mobile automatic weather stations, while the installation of thermistor chains up to a depth of 10 m is planned. These data will be used for physically-based model validation or construction of statistical models of rock temperature distribution and variability in the rock walls. A 3D-high-resolution DEM made using long-range and short-range terrestrial laser scanning (for rockwalls and galleries, respectively) will help for numerical modelling of (i) rock fractures and water flow in unsaturated fractures ; (ii) the 3D distribution and evolution of temperature fields in the subsurface. Finally, monitoring of the Arête des Cosmiques SE face and SE Pilastre by long-range terrestrial laser scanning, combined with cosmogenic dating of some rockwalls will document the past and present morphological activity of the site., Le permafrost et ses modifications dans les parois rocheuses de haute montagne restent insuffisamment compris, en particulier du fait de la rareté des sites instrumentés. D 'où l 'intérêt de développer des recherches à l 'Aiguille du Midi (3842 m), aux expositions et pentes variées, et dont les parois et galeries sont accessibles toute l 'année. Dans le cadre du projet PermaNET, nous y suivons le régime thermique dans la roche, (i) avec 9 capteurs à un ou trois thermistors implantés jusqu 'à 55 cm de profondeur ; (ii) par tomographic électrique résistive, sur la base de la relation température/résistivité du granite local ; (iii) par thermographie infrarouge sur les parois. Ces mesures sont complétées par deux stations météo automatiques mobiles, tandis qu 'il est prévu l 'installation de chaînes de thermistors d'une dizaine de mètres de profondeur. Ces données seront utilisées pour valider des modèles physiques ou construire des modèles statistiques de la distribution de la température de la roche et sa variabilité. Par ailleurs, un MNT à haute résolution réalisé à l 'aide de laserscans terrestres longue et courte portées (pour les parois et dans les galeries respectivement) servira de support à la modélisation numérique (i) de la fracturation de la roche et de la circulation d'eau dans les fractures non saturées ; (ii) de la distribution 3D et de l 'évolution des températures de subsurface. Enfin, le suivi de la face SE de l 'Arête des Cosmiques et du Pilastre SE à l 'aide d 'un laserscan terrestre longue portée complète des datations cosmogéniques sur certaines parois pour documenter la morphodynamique passée et actuelle du site., Deline Philip, Coviello Velio, Cremonese Edoardo, Gruber Stephan, Krautblatter Michael, Jaillet Stéphane, Malet Emmanuel, Morra di Cella Umberto, Noetzli Jeannette, Pogliotti Paolo, Rabatel Antoine, Ravanel Ludovic, Sadier Benjamin, Verleysdonk Sarah. L'Aiguille du Midi (massif du Mont Blanc) : un site remarquable pour l'étude du permafrost des parois d'altitude. In: Collection EDYTEM. Cahiers de géographie, numéro 8, 2009. Neige et glace de montagne. Reconstitution, dynamique, pratiques. pp. 135-146.

[1] The contribution of climate and tectonics to the Neogene exhumation history of the European Alps is studied in the Mont Blanc (MB) massif using low-temperature thermochronology. Apatite fission track and (U-Th-[Sm])/He data suggest that the MB massif was exhumed episodically: Rapid exhumation (2.5 ± 0.5 km/Ma) before 6 Ma is followed by an episode of slow exhumation and a period of accelerated exhumation (>1 km/Ma) after ∼3 Ma. Comparing the exhumation rates of the MB massif with reported exhumation rates of all other external crystalline massifs (ECM) shows that the MB massif is the only ECM that experienced rapid exhumation before 6 Ma, which is possibly related to NW and minor SE directed thrusting of the MB massif. The data demonstrate that the Messinian base level drop (∼5.5 Ma) and the increase in atmospheric moisture caused by an intensification of the Atlantic Golf Stream (4.6 Ma) did not affect the exhumation of the external Alps. All ECM, except the Gotthard massif, show an increase in exhumation rates at ∼3 Ma. We interpret this as the result of normal faulting along orogen-parallel faults and beginning Alpine glaciation. The relative impact of these processes on the exhumation of the ECM can vary spatially and temporally, mainly depending on differences in geology and geomorphology. In the case of the MB massif we propose that the acceleration in exhumation rates at ∼3 Ma is caused by rapid valley incision related to glaciation, and that the recent relief of the MB massif is thus a young feature.

International audience; Because of insulation by supraglacial debris, the dynamics of debris-covered glaciers differ from those of‘clean' glaciers. Thus, changes in debris cover have to be taken into account when interpreting glacier fluctuations in terms of climate forcing. Three large glaciers in the Mont Blanc massif were investigated for the period since the termination of the‘Little Ice Age' (LIA) using historical and scientific documents of the eighteenth and nineteenth centuries. Although debris cover was limited to its frontal area until the 1840s, Miage Glacier exhibited a continuous debris cover in the first decades after the LIA, increasing to its present extent by the 1930s. On the Mer de Glace, a partial debris cover formed by coalescence of two of the four medial moraines at the close of the LIA; until 1890, these moraines have formed the present Veine noire (‘Black vein'). In contrast, Brenva Glacier had a continuous debris cover at the end of the eighteenth century, possibly following a rock avalanche in 1767. For glaciers like Mer de Glace and Miage Glacier, the close of the LIA represents a threshold marked by a rapid change in state from‘clean' to debris-covered; for others, long-term debris accumulation or frequent rock avalanching (e.g., Brenva Glacier) mask the climate control of a slow post-LIA expansion of debris cover.

The authors want to underline that present-day research cannot be detached from a long tradition. Two centuries of observations made by field geologists, and new methods helped to attain our present level of knowledge. In this sense, the different sections recall the evolution of ideas accompanied by a growing sophistication of methods.

(1984). Chronology of a core sample in the Col du Dome (Mont Blanc massif) La Houille Blanche: Vol. 70, No. 6-7, pp. 491-498.

Foreland-propagating external thrust belts may be considered as essentially plane strain phenomena so that displacements can be correlated throughout their linked, three-dimensional fault geometry. This approach has been applied to part of the northwest external French-Swiss Alps, around the Mont Blanc basement massif. Imbricates of basement and cover sequences on the SW margin of this massif restore to a width in excess of 77 km with an implicit shortening of at least 67 km. These displacements can be correlated with those in the neighbouring Helvetic nappes by transferring movements, via lateral branch lines, onto the Mont Blanc thrust. By reappraising thrust geometries, the Helvetic/Ultrahelvetic nappe complex has been restored to a width of 114 km to the ESE of the Aiguilles Rouges basement massif. Displacements on the internal (SE) margin of the Mont Blanc massif, estimated by balanced sections and a restoration of the Ultrahelvetic klippen in the sub-alps, exceed 59 km. Thrust continuity, incorporating the restorations of nappes and imbricate geometries around the Mont Blanc massif, is illustrated on a crude, restored branch-line map which also serves as a preliminary palaeogeographic reconstruction. External thrust systems, to the east of the external Belledonne/Aiguilles Rouges massif, restore to a width of at least 140 km in the footwall to the Frontal Pennine thrust.

Three generations of mylonites occur in the Mont Blanc Massif :— the first one has produced mylonitic schists trending N20°E, of regional extension; the late Variscan Mont Blanc granite intruded these mylonites;— the second one has affected large areas in the Mont Blanc granite, with the same N20°E direction;— the third one is located along rather narrow bands N45°E, in the granite as well as in the older mylonitic schists.All theses mylonites have been generated under (Bi) - greenschist facies conditions. They exhibit steeply plunging schistosity and lineation. They are all ante-Alpine; paradoxically, Alpine structures within the massif do not exhibit a striking appearence.

Observations in the upper Bon Nant valley in the southwest part of the Mont Blanc massif show that klippes are more numerous than hitherto supposed. They indicate that dislocation at the base of the Tre-la-Tete and Mont-Tondu massifs involved faulting, during which tectonic slices were superposed on each other from east to west, and that large vertical faults are probably absent.

Field observations and microscope analyses of crystalline rocks of the Mont Blanc massif (France) lend support to the concept that Alpine deformation was continuous in space and regional in scale. The Mont Blanc and Aiguilles Rouges massifs are both part of the same Prealpine basement, which became separated by Alpine deformation. The general structural trend of the Aiguilles Rouges massif is north-south, but that of the Mont Blanc massif more closely corresponds to the Alpine trend. This change in orientation is directly related to mylonization of schistose crystalline basement rocks, under Alpine stresses.

Presents arguments in support of the concept that dynamometamorphic processes (recrystallization by mechanical deformation during which no material is added from sources outside the rock mass being deformed) and regional metamorphism (rock alteration accompanied by the additionof material from adjacent terrain or from depth) are parts of a same process and not distinct independent processes. The concordance exhibited by a differentially metamorphosed mass of Liassic limestones wedged in crystalline rocks of the Mont Blanc massif, exposed in a gorge near the tongue of the Tre-la-Tete glacier, France, is cited as an example of recrystallization which took place during the regional metamorphism associated with Alpine crustal movements.

Results of in situ stress measurements performed in bore-holes from the road tunnel between Chamonix and Courmayeur are given.