Spatio-temporal variability and controlling factors for postglacial erosion dynamics in the Dora Baltea catchment (western Italian Alps)

Disentangling the influence of bedrock erodibility from the respective roles of climate, topography and tectonic forcing on catchment denudation is often challenging in mountainous landscapes due to the diversity of geomorphic processes in action and of spatial/temporal scales involved. The Dora Baltea catchment (western Italian Alps) appears the ideal setting for such investigation, since its large drainage system, extending from the Mont Blanc Massif to the Po Plain, cuts across different major litho-tectonic units of the western Alps, whereas this region has experienced homogeneous climatic conditions and glacial history throughout the Quaternary. We acquired new 10Be-derived catchment-wide denudation rates from 18 river-sand samples collected both along the main Dora Baltea river and at the outlet of its main tributaries. The inferred denudation rate results vary between 0.2 and 0.9 mm/yr, consistent with values obtained across the European Alps by previous studies. Spatial variability in denudation rates was statistically compared with topographic, environmental and geologic metrics. 10Be-derived denudation records do not correlate with the distribution of modern precipitation and rock geodetic uplift. We find, rather, that catchment topography, in turn conditioned by bedrock erodibility (litho-tectonic origin) and glacial overprint, has the main influence on denudation rates. We calculated the highest denudation rate for the Mont Blanc Massif, whose granitoid rocks and long-term tectonic uplift support steep slopes and high relief and thus favour intense glacial/periglacial processes and recurring rock fall events. Finally, our results, in agreement with modern sediment budgets, demonstrate that the high sediment input from the Mont Blanc catchment dominates the Dora Baltea sediment flux, explaining the constant low 10Be concentrations measured along the Dora Baltea course even downstream the multiple junctions with tributary catchments.

DISTRIBUTION AND EVOLUTION OF ICE APRONS IN A CHANGING CLIMATE IN THE MONT-BLANC MASSIF (WESTERN EUROPEAN ALPS)

Ice Apron (IA) is a poorly studied ice feature, commonly existing in all the world’s major mountain regions. This study aims to map the locations of the IAs in the Mont Blanc massif (MBM), making use of the very high-resolution optical satellite images from 2001, 2012 and 2019. 423 IAs were identified and accurately delineated in the MBM on the images from 2019, and their topographic characteristics were studied. We generated our own Digital Elevation Model (DEM) at 4 m resolution since the freely available products predominantly suffer from significant inconsistencies, especially in steep mountain areas. Results show that most IAs exist at elevations above the regional Equilibrium Line Altitude (ELA), on steep slopes, on concave surfaces, on northern and southern aspects and on the most rugged terrains. They are also commonly associated with steep slope glaciers as 85% of them occur on these glaciers’ headwalls. A comparison between 2001 and 2019 shows that IAs have lost around 29% of their area over a period of 18 years. This is significant and the rate of area loss is very alarming in comparison with the larger glacier bodies. We also studied the effect of topographic parameters on the area loss. We found that topographic factors like slope, aspect, curvature, elevation and Terrain Ruggedness Index (TRI) strongly influence the rate of area loss of IAs.

Investigation of a cold-based ice apron on a high-mountain permafrost rock wall using ice texture analysis and micro-14C dating: a case study of the Triangle du Tacul ice apron (Mont Blanc massif, France)

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.

Première mention en France et en Italie du Lépidoptère alticole Dahlica wehrlii (Müller-Rütz, 1920) dans le massif du Mont-Blanc (Lepidoptera, Psychidae)

First record in France and Italy of the high-altitude-living moth Dahlica wehrlii (Müller-Rütz, 1920) in the Mont-Blanc massif (Lepidoptera, Psychidae). The Lepidoptera Psychidae Dahlica wehrlii (Müller-Rütz, 1920), a species inhabiting high-altitude biotopes and up to now recorded only from Switzerland, is mentioned from the Mont-Blanc massif, both in France and in Italy. Observation conditions as well as biological elements are given.

Evaluating the temperature dependence of bedrock hillslope erosion in the Mont Blanc massif using in situ cosmogenic 3He-10Be-14C

<p>The erosion of cold bedrock hillslopes in alpine environments depends not only on rates of frost weathering and accumulated rock damage, but additionally on the removal of the weathered material from the bedrock surface. In the Mont Blanc massif, steep bedrock faces with exposure ages sometimes much older than 50,000 years sit in close proximity to actively-eroding rockwalls, suggesting a more complex relationship between temperature and erosion rates than encompassed by the proposed “frost-cracking window.” Stochastic events like rockfalls and rock avalanches, despite their rarity, contribute a non-trivial proportion of the total sediment budget in alpine permafrost regions, adding to the contribution from background “steady-state” erosion. Employing a methodology based on the combination of in-situ cosmogenic nuclides <sup>3</sup>He -<sup>10</sup>Be-<sup>14</sup>C, we test the temperature-dependence of high-alpine erosion while taking into account erosional stochasticity.</p><p>From cosmogenic <sup>10</sup>Be concentrations of amalgamated samples collected on the Aiguille du Midi (3842 m a.s.l.) in the Mont Blanc massif, we find an order of magnitude difference in erosion rate across the peak’s surface. Our preliminary measured erosion rates, ranging between appx. 0.03 mm yr<sup>-1</sup> and 1.0 mm yr<sup>-1</sup>, correlate neither with modern temperature measurements from borehole thermistors, nor with our current estimates of bedrock cosmogenic <sup>3</sup>He-derived paleotemperatures. The corresponding cosmogenic <sup>14</sup>C/<sup>10</sup>Be ratios (between 1.70 and 4.0) for these erosion rates indicate that our measurements are not biased by recent stochastic rockfall events. Our current results therefore suggest that on geomorphic timescales, bedrock hillslope erosion rates are not set solely by rates of frost-cracking, but rather by the combined effects of frost-cracking and permafrost thaw-induced rockfalls. These insights are relevant both for short-term monitoring of alpine permafrost and associated geohazards under a warming climate, as well as studies of proposed “buzzsaws” operating on glacial-interglacial timescales.</p>

Fluid circulations associated with the necking of the crust: the example of the Mont-Blanc detachment fault

<p><strong> </strong>The external crystalline massifs of the Alps, which include the Mont-Blanc massif, are found in between the external and internal parts of the orogen. The external parts correspond to the proximal domain of the Alpine Tethys (Helvetic domain), whereas the internal part corresponds to the former distal domain of the margin (Penninic domain). Therefore, the Mont-Blanc massif is a key place for understanding the proximal-distal transition during Jurassic rifting of the Alpine Tethys. </p><p>Despite numerous seismic observations at modern passive margins, the tectono-sedimentary and fluid evolution recorded in these domains called necking zone remain poorly understood. Many questions remain concerning the thermal evolution, the origin and composition of the fluids, their link to large-scale hydrothermal systems, and the impact of element transfer on the diagenesis of syn-rift sediments.</p><p> </p><p>Here we focus on the Col du Bonhomme (southern Mont-Blanc massif near Bourg St-Maurice, France), where late Triassic / early Jurassic to late Jurassic sediments preserve pre-Alpine contacts between the sediment and the basement.  The syn-rift sedimentary tract is composed of Sinemurian to Pliensbachian sandstones called “Grès Singuliers”, lying unconformably above the pre-rift and over an exhumed fault plane corresponding to the top basement.</p><p>Characterization of the faults and overlying sediments requires a multi-scale and multi-disciplinary approach combining field observation, petrography, sedimentology, structural geology, and geochemistry. The protolith of the fault rocks is a Variscan migmatitic gneiss. The damaged zone consists of cataclasites and the core zone is made of black gouge. The gouge is overlaid conformably by Liassic sandstones that contain reworked clasts of cataclasite. The observations that the top basement fault is cut by a Pliensbachian high-angle normal fault and Triassic clasts occur in the gouge enables to date this fault as Early Jurassic. </p><p>At the micro scale, the basement shows hydratation leading to chloritization of biotite and sericitisation of feldspaths (orthoclase and plagioclase). A strong hydration-assisted deformation with increase of deformation toward the fault core leads to the formation of cataclasites. They are composed of quartz, sericite with small remnants of orthoclase, chlorites with secondary pyrites and rutiles. The fault core is a black gouge with grain size comminuition and mineral neoformation.</p><p>Evidence for fluid flow is observed in the fault leading to the hydrothermal alteration of the basement (sericitisation of feldspath and corrosion of quartz)  and the formation of syn-gouge quartz and quartz-adularia veins in the black gouge (datation using the Rb-Sr an adularia and U-Pb on calcite method is in progress) . </p><p>Based on our observations we interpret the fault observed at Col du Bonhomme as a Jurassic exhumation fault associated with the necking of the European crust during Jurassic rifting. This preliminary work shows that the fault acted as an important pathway for crustal fluids with important transfer of silica and at least K, Fe and Ti.  The Col du Bonhomme area gives an opportunity to study fluid circulation and basement alteration along a rift-related detachment fault in the necking domain and therefore to understand fluid-mediated element mobility during rifting.</p><p><strong>Keywords :</strong> Detachment fault, Mont-Blanc massif, Fluid circulation , Alpine Tethys, Necking zone</p>

Understanding the impacts of climate change on high mountain practices: the case of the Mont Blanc massif through an interdisciplinary approach

<p>The intensity of the current climate change has strong consequences on high mountain tourism activities. Winter activities are currently the most studied (ski industry). However, the consequences of environmental changes are also strong in summer, as geomorphological processes are enhanced at high elevation. The Mont Blanc Massif (Western Alps) is a particularly favourable terrain for the development of research about these processes. Emblematic high summits (28 of the 82 peaks > 4000 m of the Alps), dozens of glaciers, strongly developed tourism with summer/winter equivalence, active mountaineering practice, etc. all contribute to the interest of studying this geographical area. A lot of work has been carried out on glaciological and geomorphological issues. These studies, which deal with "physical" impacts of the climate change on the high mountains, are also supplemented by studies of their consequences on human societies, as its impacts on practices such as mountaineering or glacier tourism. Risk-related issues are also taken into account with, for example, the stability of infrastructure (huts, ski lifts) or the impact of glacial shrinkage on the formation of new and potentially hazardous lakes. Accordingly, the aims of our presentation are to show the extent of the research developed on climate change in the Mont Blanc massif and how social and environmental sciences are interlinked to provide a holistic vision of the issues of this territory. As these experiments are not exactly interdisciplinary experiments, this presentation also aims to discuss the points that need to be further developed in order to promote inter- and trans-disciplinary research.</p>