Grandes Rousses Research Articles

Changes in taxonomic and functional composition of subalpine plant communities in response to climate change under contrasting conditions of bedrock and snow cover duration

AbstractQuestionsWe assessed interactions between climate change, bedrock types and snow cover duration on the trajectories of taxonomic and functional composition of subalpine plant communities. We predict (i) an increase in species richness on siliceous bedrock due to a reduced competition and a decrease in richness on calcareous bedrock due to increasing drought stress; (ii) decreasing snow cover duration should induce a higher shrub encroachment in hollows as compared to ridges; and (iii) increasing growing season temperature should induce taller sizes and more conservative growth traits, in particular in hollows.LocationSubalpine belt of the Grandes Rousses mountain range, southwestern Alps (France).Methods189 vegetation plots were sampled in 1997 and 2017–2018. The duration of snow cover was assessed during two years in 1995–1997 and five functional traits were measured on 108 species in 2021. We performed multivariate analyses, quantified community‐weighted means (CWM) of traits and used ANOVAs to detect responses to local‐scale factors and changes in snow cover, temperature and precipitation since 1997 according to a nearby meteorological station.ResultsOverall, taxonomic composition weakly changed and changes were more dependent on the position of communities along the snow cover duration gradient than on their bedrock type. The abundance of drought‐tolerant species increased at the border of hollows and there was, over all communities, a slight increase in the abundance of dwarf shrubs and tall herbaceous species, a strong decrease in short herbaceous species and, thus, an overall decrease in species richness. There were important overall changes in CWM of size traits, in particular leaf area which increased the most in hollows irrespective of bedrock types.ConclusionIn this subalpine site the effects of decreasing snow cover duration overwhelmed the effects of bedrocks, which may explain the overall increase in competitive species and decrease in species richness.

Changes in species richness with climate change in subalpine communities are dependent on regional environmental conditions and local functional composition

AbstractQuestionsClimate change may have different effects on species richness depending on regional climates, soil types and the functional composition of local communities.LocationSubalpine belt of the Grandes Rousses (Alps) and Sancy mountain ranges (the Massif Central), France.MethodsWe compared changes in species richness in response to recent climate change in communities from two mountain ranges subjected to contrasting environmental conditions, with a more continental climate and drier soils in the Alps than in the Massif Central. Vegetation composition of 189 and 157 plots was assessed in 1997 and 2017–2018 in the Alps and 1988–1989 and 2022 in the Massif Central, respectively. Five species traits (height, lateral spread, leaf area, specific leaf area and leaf dry matter content) were measured on 108 and 144 species, respectively. Changes in vegetation composition and species richness along spatial environmental gradients were analysed with correspondence analysis at the Sancy site and canonical correspondence analysis at the Alpe d'Huez site. Changes in functional composition across sites were analysed with principal components analysis. Changes in species richness with climate change were analysed with repeated‐measure ANOVAs.ResultsIn both mountain ranges summer temperatures increased and the hydric balance decreased over the past three decades, but snow cover duration decreased in the Alps only, whereas irradiance strongly increased in the Massif Central only. Functional composition was characterized by shorter and more conservative species in the Alps than in the Massif Central, which was dominated by tall exploitative species. Species richness overall decreased with climate change by ca 10% in the Alps but overall increased by ca 10% in the Massif Central. Species richness decreased in the Alps probably because of increasing competition induced by dwarf shrub encroachment due to decreasing snow cover duration, whereas species richness increased in the Massif Central probably due to increasing irradiance in a very nebulous climate.ConclusionThe opposite changes in species richness observed in the two mountain ranges with recent climate change were likely to be explained by their contrasting climate and soil conditions driving different functional compositions and diversity–environment relationships.

Découverte d’une nouvelle population de Rhithrogena delphinensis Sowa & Degrange, 1987, dans le massif des Arves, et compléments à la description de la larve (Ephemeroptera, Heptageniidae)

Discovery of a new population of Rhithrogena delphinensis Sowa & Degrange, 1987, in the Arves Massif, and additions to the morphological description of the larva (Ephemeroptera, Heptageniidae). Rhithrogena delphinensis, described originally on the basis of four larvae from the Western Alps, south of the Arves Massif and from the northern flank of the Ecrins Massif, had not been captured again since 1986. Here, we report the discovery of a new population from river Arvan, whose drainage basin is located between the Grandes Rousses Massif and the northern flank of the Arves Massif. This newly discovered population seems abundant in numbers, and reveals the particular ecological requirements of the species as well as its dependence on glacier fed or nival streams. The morphological characteristics of the larvae are described in detail, and illustrated by photographs. The variability of some of the proposed identification criteria is discussed, and a key to the identification of the Rhithrogena species from the alpestris group of the Western Alps, to which R. delphinensis belongs, is provided. Finally, a portion of 658 base pairs of the COI gene of R. delphinensis is sequenced for the first time and compared to already existing data on the alpestris group in the Western Alps.

A meteorological and blowing snow data set (2000–2016) from a high-elevation alpine site (Col du Lac Blanc, France, 2720 m a.s.l.)

Abstract. A meteorological and blowing snow data set from the high-elevation experimental site of Col du Lac Blanc (2720 m a.s.l., Grandes Rousses mountain range, French Alps) is presented and detailed in this paper. Emphasis is placed on data relevant to the observations and modelling of wind-induced snow transport in alpine terrain. This process strongly influences the spatial distribution of snow cover in mountainous terrain with consequences for snowpack, hydrological and avalanche hazard forecasting. In situ data consist of wind (speed and direction), snow depth and air temperature measurements (recorded at four automatic weather stations), a database of blowing snow occurrence and measurements of blowing snow fluxes obtained from a vertical profile of snow particle counters (2010–2016). Observations span the period from 1 December to 31 March for each winter season from 2000–2001 to 2015–2016. The time resolution has varied from 15 min until 2014 to 10 min for the last years. Atmospheric data from the meteorological reanalysis are also provided from 1 August 2000 to 1 August 2016. A digital elevation model (DEM) of the study area (1.5 km2) at 1 m resolution is also provided in RGF 93 Lambert 93 coordinates. This data set has been used in the past to develop and evaluate physical parameterizations and numerical models of blowing and drifting snow in alpine terrain. Col du Lac Blanc is also a target site to evaluate meteorological and climate models in alpine terrain. It belongs to the CRYOBS-CLIM observatory (the CRYosphere, an OBServatory of the CLIMate), which is a part of the national research infrastructure OZCAR (Critical Zone Observatories – Application and Research) and have been a Global Cryospheric Watch Cryonet site since 2017. The data are available from the repository of the OSUG data centre https://doi.org/10.17178/CRYOBSCLIM.CLB.all.

A km-scale “triaxial experiment” reveals the extreme mechanical weakness and anisotropy of mica-schists (Grandes Rousses Massif, France)

The development of Andersonian faults is predicted, according to theory and experiments, for brittle/frictional deformation occurring in a homogeneous medium. In contrast, in an anisotropic medium it is possible to observe fault nucleation and propagation that is non-Andersonian in geometry and kinematics. Here, we consider post-metamorphic brittle/frictional deformation in the mechanically anisotropic mylonitic mica-schists of the Grandes Rousse Massif (France). The role of the mylonitic foliation (and of any other source of mechanical anisotropy) in brittle/frictional deformation is a function of orientation and friction angle. According to the relative orientation of principal stress axes and foliation, a foliation characterized by a certain coefficient of friction will be utilized or not for the nucleation and propagation of brittle/frictional fractures and faults. If the foliation is not utilized, the rock behaves as if it was isotropic, and Andersonian geometry and kinematics can be observed. If the foliation is utilized, the deviatoric stress magnitude is buffered and Andersonian faults/fractures cannot develop. In a narrow transition regime, both Andersonian and non-Andersonian structures can be observed. We apply stress inversion and slip tendency analysis to determine the critical angle for failure of the metamorphic foliation of the Grandes Rousses schists, defined as the limit angle between the foliation and principal stress axes for which the foliation was brittlely reactivated. This approach allows defining the ratio of the coefficient of internal friction for failure along the mylonitic foliation to the isotropic coefficient of friction. Thus, the study area can be seen as a km-scale triaxial experiment that allows measuring the degree of mechanical anisotropy of the mylonitic mica-schists. In this way, we infer a coefficient of friction μweak=0.14 for brittle-frictional failure of the foliation, or 20% of the isotropic coefficient of internal friction.

Linking glacier annual mass balance and glacier albedo retrieved from MODIS data

Abstract. Albedo is one of the variables controlling the mass balance of temperate glaciers. Multispectral imagers, such as MODerate Imaging Spectroradiometer (MODIS) on board the TERRA and AQUA satellites, provide a means to monitor glacier surface albedo. In this study, different methods to retrieve broadband glacier surface albedo from MODIS data are compared. The effect of multiple reflections due to the rugged topography and of the anisotropic reflection of snow and ice are particularly investigated. The methods are tested on the Saint Sorlin Glacier (Grandes Rousses area, French Alps). The accuracy of the retrieved albedo is estimated using both field measurements, at two automatic weather stations located on the glacier, and albedo values derived from terrestrial photographs. For summers 2008 and 2009, the root mean square deviation (RMSD) between field measurements and the broadband albedo retrieved from MODIS data at 250 m spatial resolution was found to be 0.052 or about 10% relative error. The RMSD estimated for the MOD10 daily albedo product is about three times higher. One decade (2000–2009) of MODIS data were then processed to create a time series of albedo maps of Saint Sorlin Glacier during the ablation season. The annual mass balance of Saint Sorlin Glacier was compared with the minimum albedo value (average over the whole glacier surface) observed with MODIS during the ablation season. A strong linear correlation exists between the two variables. Furthermore, the date when the average albedo of the whole glacier reaches a minimum closely corresponds to the period when the snow line is located at its highest elevation, thus when the snow line is a good indicator of the glacier equilibrium line. This indicates that this strong correlation results from the fact that the minimal average albedo values of the glacier contains considerable information regarding the relative share of areal surfaces between the ablation zone (i.e. ice with generally low albedo values) and the accumulation zone (i.e. snow with a relatively high albedo). As a consequence, the monitoring of the glacier surface albedo using MODIS data can provide a useful means to evaluate the interannual variability of the glacier mass balance. Finally, the albedo in the ablation area of Saint Sorlin Glacier does not exhibit any decreasing trend over the study period, contrasting with the results obtained on Morteratsch Glacier in the Swiss Alps.

Tracking glacial and high altitude Holocene alpine environments fluctuations from clastic and organic markers in proglacial lacustrine archives (Lake Blanc Huez, Grandes Rousses Massif, France)

Tracking glacial and high altitude Holocene alpine environments fluctuations from clastic and organic markers in proglacial lacustrine archives (Lake Blanc Huez, Grandes Rousses Massif, France)

Mechanisms of margin inversion in the external Western Alps: Implications for crustal rheology

The kinematics of the collisional crustal wedge in the external Western Alps is discussed in the light of new cross sections of the whole external zone at the latitude of the Belledonne–Grandes Rousses–Oisans massifs (External Crystalline Massifs, ECM), as well as a detailed analysis of the deformation of their basement and cover. The cross sections were built from new field data and published geological maps, and were restored through time (Oligo-Miocene times) to unravel the successive stages of the Dauphinois margin contractional deformation. During Oligocene times, the Variscan basement was deformed essentially by greenschist facies Alpine distributed shear zones with no significant reactivation of the inherited Jurassic normal faults or the Variscan foliation. However, the inherited syn-rift basins localized the deformation characterized by a thick-skinned style and a cover disharmonically folded over basement shear zones, with no major décollement in between (shortening of 11.5km, 20%). Those early deformations progressively localized on the frontal crustal ramp and caused shortening in the folded belt (16.5km of shortening, 23%). A similar sequence of deformation has been inferred in other Alpine ECMs. This strongly suggests that the crust of Dauphinois proximal passive margin was thin enough, and its buoyancy low enough (as a result of the Liassic rifting), to experience significant tectonic burial during the Tertiary collision. As a consequence of a weakening by both the presence of inherited basins (and their weak syn-rift sedimentary rocks) and the P–T conditions (greenschist facies), the crust was shortened and the structural style was thick-skinned.

The detailed snowpack scheme Crocus and its implementation in SURFEX v7.2

Abstract. Detailed studies of snow cover processes require models that offer a fine description of the snow cover properties. The detailed snowpack model Crocus is such a scheme, and has been run operationally for avalanche forecasting over the French mountains for 20 yr. It is also used for climate or hydrological studies. To extend its potential applications, Crocus has been recently integrated within the framework of the externalized surface module SURFEX. SURFEX computes the exchanges of energy and mass between different types of surface and the atmosphere. It includes in particular the land surface scheme ISBA (Interactions between Soil, Biosphere, and Atmosphere). It allows Crocus to be run either in stand-alone mode, using a time series of forcing meteorological data or in fully coupled mode (explicit or fully implicit numerics) with atmospheric models ranging from meso-scale models to general circulation models. This approach also ensures a full coupling between the snow cover and the soil beneath. Several applications of this new simulation platform are presented. They range from a 1-D stand-alone simulation (Col de Porte, France) to fully-distributed simulations in complex terrain over a whole mountain range (Massif des Grandes Rousses, France), or in coupled mode such as a surface energy balance and boundary layer simulation over the East Antarctic Ice Sheet (Dome C).

Monitoring spatial and temporal variations of surface albedo on Saint Sorlin Glacier (French Alps) using terrestrial photography

Abstract. Accurate knowledge of temperate glacier mass balance is essential to understand the relationship between glacier and climate. Defined as the reflected fraction of incident radiation over the whole solar spectrum, the surface broadband albedo is one of the most important variable in a glacier's mass balance. This study presents a new method to retrieve the albedo of frozen surfaces from terrestrial photography at visible and near infrared wavelengths. This method accounts for the anisotropic reflectance of snow and ice surfaces and uses a radiative transfer model for narrow-to-broadband conversion. The accuracy of the method was assessed using concomitant measurements of albedo during the summers 2008 and 2009 on Saint Sorlin Glacier (Grandes Rousses, France). These albedo measurements are performed at two locations on the glacier, one in the ablation area and the other in the accumulation zone, with a net radiometer Kipp and Zonen CNR1. The main sources of uncertainty are associated with the presence of high clouds and the georeferencing of the photographs.

Suture zones and importance of strike-slip faulting for Variscan geodynamic reconstructions of the External Crystalline Massifs of the western Alps

AbstractThis paper reviews the geodynamic evolution of the Belledonne, Grandes Rousses and Oisans massifs in the western Alps from Early Ordovician to Permian times. Three domains are distinguished. The eastern domain, which includes the NE Belledonne massif and the inner Oisans massif, records the subduction of the Central-European ocean along a NW dipping subduction zone. The western domain is marked by Cambro-Ordovician back-arc rifting (Chamrousse ophiolite) initiating the opening of the Rheic ocean. It was followed by Mid-Devonian obduction of the back-arc Chamrousse ophiolite, towards the NW in relation with the SE dipping subduction of the Saxo-Thuringian ocean. The central domain, including the SW part of the Belledonne massif, the Grandes Rousses massif and the outer Oisans massif, records the Devonian to Carboniferous orogenic activity that produced calc-alkaline magmatism, Mg-K granite intrusions and syn-collisional sedimentation related to Visean nappe stacking that we relate to the closure of the Saxo-Thuringian ocean. Based on tectonostratigraphic correlations we propose that these domains initially correspond to the northeastward extension of the Bohemian massif. During the late Carboniferous, the External Crystalline Massifs including Sardinia and Corsica were stretched towards the SW along the > 600 km long dextral External Crystalline Massifs shear zone. Offset of the Saxo-Thuringian and eo-Variscan suture zones from the Bohemian massif to the ECM suggests a possible dextral displacement of about 300 km along the ECM shear zone.

Multistage shortening in the Dauphiné zone (French Alps): the record of Alpine collision and implications for pre-Alpine restoration

Three-dimensional modelling tools are used with structural and palaeomagnetic analysis to constrain the tectonic history of part of the Dauphine zone (external Western Alps). Four compressive events are identified, three of them being older than the latest Oligocene. Deformation D1 consists of W-SW directed folds in the Mesozoic cover of the study area. This event, better recorded in the central and southern Pelvoux massif, could be of Eocene age or older. Deformation D2 induced N-NW-oriented basement thrusting and affected the whole southern Dauphine basement massifs south of the study area. The main compressional event in the study area (D3) was WNW oriented and occurred before 24 Ma under a thick tectonic load probably of Penninic nappes. The D2–D3 shift corresponds to a rapid transition from northward propagation of the Alpine collision directly driven by Africa-Europe convergence, to the onset of westward escape into the Western Alpine arc. This Oligocene change in the collisional regime is recorded in the whole Alpine realm, and led to the activation of the Insubric line. The last event (D4) is late Miocene in age and coeval with the final uplift of the Grandes Rousses and Belledonne external massifs. It produced strike-slip faulting and local rotations that significantly deformed earlier Alpine folds and thrusts, Tethyan fault blocks and Hercynian structures. 3D modelling of an in itially horizontal surface, the interface between basement and Mesozoic cover, highlights large-scale basement involved asymmetric folding that is also detected using structural analysis. Both, Jurassic block faulting and basement fold-and-thrust shortening were strongly dependent on the orientation of Tethyan extension and Alpine shortening relative to the late Hercynian fabric. The latter’s reactivation in response to oblique Jurassic extension produced an en-echelon syn-rift fault pattern, best developed in the western, strongly foliated basement units. Its Alpine reactivation occurred with maximum efficiency during the early stages of lateral escape, with tectonic transport in the overlying units being sub-perpendicular to it.

High-altitude varve records of abrupt environmental changes and mining activity over the last 4000 years in the Western French Alps (Lake Bramant, Grandes Rousses Massif)

Two twin short gravity cores and a long piston core recovered from the deepest part of proglacial Lake Bramant (Grandes Rousses Massif, French Alps), under and overlying a large slump identified by high-resolution seismic profile, allow the investigation of Holocene natural hazards and interactions between human activity and climatic changes at high-altitude. Annual sedimentation throughout the cores (glacial varves) is identified on photographs, ITRAX (high-resolution continuous microfluorescence-X) and CAT-Scan (computerized axial tomography) analyses and is supported by (1) the number of dark and light laminations between dates obtained by radionuclide measurements ( 137Cs, 241Am), (2) the correlation of a slump triggered by the nearby AD 1881 Allemond earthquake (MSK intensity VII) and of a turbidite triggered by the AD 1822 Chautagne regional earthquake (MSK intensity VIII), (3) the number of laminations between two accelerator mass spectrometry (AMS) 14C dates, and (4) archaeological data. In Lake Bramant, dark layers are coarser, contain less detrital elements, but more neoformed elements and organic matter content. These darker laminations result from calm background sedimentation, whereas the lighter layers are finer and rich in detrital elements and reflect the summer snowmelt. Traces of mining activity during the Roman civilization apogee (AD 115–330) and during the Early Bronze Age (3770–3870 cal BP) are recorded by lead and copper content in the sediments and probably result from regional and local mining activity in the NW Alps. Warmer climate during the Bronze Age in this part of the Alps is suggested by (1) two organic deposits (4160–3600 cal BP and 3300–2850 cal BP) likely reflecting a lower lake level and smaller glaciers and (2) evidence of a different vegetation cover around 2500 m a.s.l. The onset of clastic proglacial sedimentation between 3600–3300 cal BP and since 2850 cal BP is synchronous with periods of glacier advances documented in the Alps and high-lake levels in west-central Europe. This major change in proglacial sedimentation highlights the development of a larger St. Sorlin glacier in the catchment area of Lake Bramant.

A quantification of the glacial imprint on relief development in the French western Alps

The morphology of the western Alps has been strongly influenced by Quaternary glaciations. On the basis of observations of glacial morphology in the Belledonne, Grandes Rousses, Taillefer and Pelvoux-Ecrins Massifs (south-eastern France), we reconstitute the glacial trimline and Equilibrium Line Altitude (ELA) during the most extensive glaciation (MEG). Our best estimate of the MEG ELA is 1800 ± 100m. Using digital elevation models, we compare our glacial reconstruction with the relief structure of nine major catchments draining the massifs. Modal elevations of the largest catchments occur at 2000–2500m and coincide with minima in plots of mean slope angles as a function of elevation. Modal elevations and slope minima occur between the modern and MEG ELAs, confirming a strong glacial imprint on relief. In order to quantify glacial valley carving in the massifs, we isolated high-elevation, low-relief surfaces that form rock shoulders adjacent to the glacial valleys from a Digital Elevation Model and constructed an interpolated surface passing through these. Subtracting the present-day topography from this surface allows us to quantify the maximum glacial valley depths. Maximum valley depths determined in this manner are typically > 1000m, with spatial maxima occurring around the location of the MEG ELA in most valleys. These numbers do not take into account glacial valley widening and local glacial overdeepenings. The also neglect, however, potential pre-glacial fluvial valley incision, which could account for 20–50% of the measured valley depths. In spite of these problems, inferred valley depths are reasonably well correlated with the mean reconstructed ice thickness, and constitute about half of the sub-ridgeline relief of the studied catchments. These results lead us to propose a significant Quaternary increase in the relief of the French western Alps, controlled by climate and associated with the initiation of alpine glaciations. For reasonable values of the effective elastic thickness of the lithosphere, the isostatic response to glacial valley carving reaches values of ∼ 300m across the massifs. This number is insufficient to substantially offset topographic lowering due to regional denudation, and we conclude that the isostatic response to glacial valley carving has not increased peak elevations significantly.

Reconstructing recent environmental changes from proglacial lake sediments in the Western Alps (Lake Blanc Huez, 2543 m a.s.l., Grandes Rousses Massif, France)

The evolution of high-altitude glaciers and human activities in the Grandes Rousses massif is documented by high-resolution seismic reflection profiling and multiproxy analysis of short sediment cores in proglacial Lake Blanc Huez. These lacustrine data are compared with historical chronicles, geomorphological features and glaciological studies in this region of the western Alps and they allow the documentation of recent environmental changes. The specific geometry of high-amplitude reflections in the uppermost seismic unit, the lithology of short cores and the available limnological data in the lake suggest that clastic particles eroded by the glaciers and transported in suspension by glacial melt waters in early summer essentially develop homopycnal flood events in the lake. A conceptual model linking fluctuations of glacier equilibrium line altitudes in the catchment area with sedimentary facies retrieved in the lake basin is proposed. This approach allows reconstructing continuous glacier fluctuations since AD1820–1850 and suggests several phases of glacier fluctuations during the Little Ice Age (LIA). These reconstructions are based on changes in lacustrine sediment laminations, density, magnetic susceptibility, reflectance spectra, organic matter and Arsenic content. The age-depth model of short sediment cores is provided by 210Pb, 137Cs and 241Am radionuclide dating. This chronology is further supported by identifying in lacustrine sediments the impact of (i) the nearby M 5.3 Corrençon earthquake in AD 1962, (ii) the development of the ski resort at high-altitude close to the lake and (iii) the last advance of glaciers during the LIA in AD1820–1850 and the following phase of glacier retreat observed in the alpine region at the end of the LIA in AD 1880. Frequent sandy layers enriched in organic matter and associated with fluctuations in the Arsenic concentrations may result from hydraulic remobilisation of Middle Age mine tailings at the lake shore by snow melt or heavy rain fall events during the LIA.

Evolution of Mg/Fe Ratios in Late Variscan Plutonic Rocks from the External Crystalline Massifs of the Alps (France, Italy, Switzerland)

Late Variscan plutonic bodies are widespread in the External Raumer et al., 1993; Menot et al., 1994). From south to Crystalline Massifs of the Alps (Argentera, Pelvoux, Belledonne, north, they are the Argentera, Pelvoux, Belledonne, Grandes Rousses, Mont Blanc, Aiguilles Rouges, Aar, Gotthard). Grandes Rousses, Mont Blanc and Aiguilles Rouges They can be classified on the basis of their Mg/(Fe+ Mg) ratio massifs (Western Alps), and the Aar and Gotthard massifs and mafic mineral content (expressed by the B = Fe + Mg + (Central Alps) (Fig. 1). They appear as dome-like strucTi parameter). Together with available ages of emplacement, this tures of crystalline basement surrounded by a mostly classification highlights the existence of two plutonic suites, one Mesozoic sedimentary cover. As a whole, they define an early, Visean (~330–340 Ma), and highly magnesian, the other arcuate structure, possibly inherited from the Variscan later, mainly Stephanian (~295–305 Ma), and more ferriferous. orogeny (Bogdanoff et al., 1991). This evolution from a magnesian plutonism to a more ferriferous During the Variscan orogeny, the ECM were part of one, which also occurs in other Variscan massifs (e.g. Corsica), the Helvetic–Moldanubian terrane of the internal zone might be accounted for by a combination of factors related to the of the Variscides (von Raumer & Neubauer, 1993). nature of the source of the magmas, the physical and chemical Precambrian metamorphic units and Precambrian to conditions of melting, and the Late Variscan geodynamic setting. Palaeozoic sediments, interlayered with volcanic and As a basis for these considerations a comprehensive review is ultramafic rocks, display in most areas a polymetamorphic presented of all the External Crystalline Massifs and their history and underwent a highly complex pre-Variscan Late Variscan intrusions. and Variscan evolution. Although they share a number of similarities, the ECM each have also specific characteristics (von Raumer et al., 1993). In addition to orthogneisses deriving from Early Palaeozoic plutonic (and volcanic?) protoliths (Bussy & von

Evolution of Mg/Fe Ratios in Late Variscan Plutonic Rocks from the External Crystalline Massifs of the Alps (France, Italy, Switzerland)

Late Variscan plutonic bodies are widespread in the External Crystalline Massifs of the Alps (Argentera, Pelvoux, Belledonne, Grandes Rousses, Mont Blanc, Aiguilles Rouges, Aar, Gotthard). They can be classified on the basis of their Mg/(Fe + Mg) ratio and mafic mineral content (expressed by the B = Fe + Mg + Ti parameter). Together with available ages of emplacement, this classification highlights the existence of two plutonic suites, one early, Viséan (∼330–340 Ma), and highly magnesian, the other later, mainly Stephanian (∼295–305 Ma), and more ferriferous. This evolution from a magnesian plutonism to a more ferriferous one, which also occurs in other Variscan massifs (e.g. Corsica), might be accounted for by a combination of factors related to the nature of the source of the magmas, the physical and chemical conditions of melting, and the Late Variscan geodynamic setting. As a basis for these considerations a comprehensive review is presented of all the External Crystalline Massifs and theirLate Variscan intrusions.

Preservation of old palaeosurfaces in glaciated areas: examples from the French western Alps

Abstract Remnants of deeply-weathered rock have been recorded in many regions which were ice-covered during the Pleistocene. They generally belong to shields and Palaeozoic massifs. This paper presents an example from the French Alps which testifies to the good preservation of very old (> 200 Ma) landforms in a high relief area situated on the western side of the ‘Grandes Rousses’ which was entirely covered by ice 30 000 years ago. Sedimentary structures, such as ripples on sandstone bedding planes and other erosional near-shore features, dating back to the Late Triassic before the development of the north, passive, margin of the Tethys ocean, are found in this high relief area. These are remarkably well preserved and occur in close proximity to typical glacial landforms. More generally, widespread elements of a pre-Triassic erosional surface are still recognized in the present landscape. Evidently, ice was unable to obliterate these 200 Ma old features in a high mountain area which was strongly glaciated 30 000 years ago.

Variations climatiques et fluctuations glaciaires au cours du siècle écoulé

A linear regression analysis between annual mass balances of Glacier de Sarennes 1948-75 (Massif des Grandes Rousses, France) and corresponding meteorological data from Lyon-Bron station (precipitation and temperature) has been developed. The faithful regression (77 % of variance explained) is obtained in using : -total precipitation from october to may, -total precipitation of june, -july and august mean monthly maximum temperature. According to this relation, mass balances of Glacier de Sarennes have been reconstituted since 1882 and they show a 46 m average decrease of the whole surface. There is a good correlation between the annual variations of previously reconstituted values and annual variations of mass balances measured on other alpine glaciers (Mer de Glace, 1969-75, Aletsch Glacier, 1922-75). Using a series of transverse surface profiles mealace (France) since 1905, it is possible to study the correlation between annual mass balance fluctuation and glacier reactions. The results suggest a very simple model in which the climatic fluctuations produce an uniform variation of the whole glacier surface with an amplification below ice-falls. These local perturbations are then the origin of kinematic waves travelling downith four or more times the surface ice velocity.

Wind Regimes And Heat Exchange On Glacier De Saint-Sorlin

During the summers of 1969 and 1970, we recorded in the ablation zone of the Glacier de St-Sorlin (Massif des Grandes Rousses, France) temperature, air moisture, and wind profiles, as well as the radiation balance and the daily ablation. Numerous profiles characterize a katabatic flow following the line of greatest slope, and there appears to be a correlation between the speed of the “glacier wind” and the corresponding temperature gradients. Computed according to Prandtl’s theory of turbulent transfers, the flux of sensible and latent heat added to the radiation flux leads to theoretical values for the daily melting in good agreement with the measured values. The relative importance of the radiation balance on the melting of the snow is 57%; that of the sensible heat flux is 43%; the latent heat flux is very weak and negative.