Argentera-Mercantour Massif Research Articles

On the occurrence of a Variscan eclogite in the Argentera-Mercantour Massif, Western Alps: Implications for the evolution of the southern Variscan belt

The newfound Bois de Sélasse eclogite in the eastern Argentera-Mercantour Massif (External Crystalline Massifs, Western Alps) is crucial for better constraining the tectonic evolution of the southern part of the European Variscan belt. The whole-rock composition of this eclogite aligns with that of a basaltic protolith with a normal mid-oceanic-ridge affinity, and U-Pb dating on igneous zircon cores reveals an emplacement age of 524 ± 5 Ma. The emplacement may have occurred either in the oceanic lithosphere to the north of the active Gondwana margin or within a back-arc basin during the subduction beneath Gondwana. Exceptionally preserved prehnite−pumpellyite to eclogite facies minerals provide evidence of prograde metamorphism along a standard oceanic subduction geotherm (≤10 °C/km). Peak eclogite facies conditions are constrained at 610−660 °C and 1.9−2.3 GPa by thermodynamic modeling combined with Ti-in-zircon and Zr-in-rutile thermometry. A minimum age for eclogite facies metamorphism is established at 339 ± 6 Ma by U-Pb dating on metamorphic zircon rims. The protolith of the Bois de Sélasse eclogite is indeed older than the Variscan oceans, but it was similarly affected by Variscan subduction. We discuss the implications of this new finding in the context of the European Variscan belt.

Evidence of Tethyan continental break-up and Alpine collision in the Argentera-Mercantour Massif, Western Alps

In the Argentera-Mercantour Massif, swarms of high-K calc-alkaline lamprophyres intruded into Carboniferous migmatites and early Permian granites, likely in Permian-Triassic times. The dykes collected multiple magmatic injections, the latest of which of alkaline affinity. Syn-intrusive vesicles and straight chilled margins suggest that the lamprophyres emplaced at a shallow crustal level.Lower greenschist facies mineral assemblages replaced lamprophyre igneous minerals as a result of late-intrusive hydrothermal circulation that occurred soon after the dyke emplacement. The hydrothermal event is constrained at T = 300–350°C and P < 0.1 GPa by matching thermobarometry and pseudosection results. During the Alpine collision, the lamprophyres were intersected by upper greenschist facies mylonitic shear zones developed at T = 420–450°C and P = 0.2–0.4 GPa, which are the metamorphic peak conditions of the Argentera-Mercantour Massif in Alpine times.

Variscan eclogites from the Argentera–Mercantour Massif (External Crystalline Massifs, SW Alps): a dismembered cryptic suture zone

We document structural, geochemical, petrological and 40Ar/39Ar geochronological data performed on Variscan eclogites from the Argentera–Mercantour Massif, southwestern Alps. Based on the high-resolution field mapping, we present new eclogite occurrences and discuss the relationships between eclogites and surrounding migmatites. We recognized for the first time preserved eclogite facies assemblages. Trace elements and REE patterns establish that eclogites protoliths are MORBs contaminated by continental crust. Standard thermobarometry and thermodynamic modeling are consistent with P–T values of 640–740 °C for 1.5 ± 0.25 GPa, coherent with paleo-geotherms predicted for warm subduction of oceanic crust. We interpret these eclogites as a dismembered cryptic suture zone. 40Ar/39Ar dating on amphiboles yields an age of 339.7 ± 12 Ma for eclogite retrogression under amphibolite facies conditions. All these data are combined to link the Argentera–Mercantour Massif in the tectonic framework of both pre-Mesozoic Alpine basement and European southern Variscides.

Structure of lamprophyres: a discriminant marker for Variscan and Alpine tectonics in the Argentera-Mercantour Massif, Maritime Alps

Structural and microstructural analyses are carried out in two sites of the Argentera-Mercantour Massif, Valscura and Val du Haut Boréon, where swarms of lamprophyres intruded into Variscan migmatites and early Permian granitoids. Efforts aim at defining the structural relationships between lamprophyres and country rocks, and at constraining the structural and metamorphic evolution the dykes record. Mesoscale structural data are synthesised in geologic maps originally surveyed at 1/10 000 scale, supported by form-surface maps at 1/100 scale. The lamprophyres are magnesian, calc-alkalic to alkali-calcic, and metaluminous; they emplaced at very shallow crustal levels intersecting three generations of ductile structures in the host migmatites (D1, D2, D3). Epidote- and actinolite-bearing mineral assemblages result from late-intrusive hydrothermal circulation that has not affected the host rocks. Mylonitic shear zones of Alpine age (D4) are continuous through migmatites, granites, and lamprophyres: in these latter, they are supported by albite, actinolite, biotite, chlorite, epidote, phengite, and titanite. This detailed multi-scale structural analysis, coupled with major and trace elements geochemistry, highlights two main results: i) the lamprophyres, which post-date both the late- to post-collisional “high-Mg” and the “low-Mg” granitoids, reflect the last magmatic event in the Argentera-Mercantour Massif related to the Permian-Triassic lithospheric thinning; ii) the metamorphic assemblages that support the Alpine shear zones in the lamprophyres are consistent with the transition between the greenschist and amphibolite facies conditions.

Kinematic and geochronological constraints on shear deformation in the Ferriere-Mollières shear zone (Argentera-Mercantour Massif, Western Alps): implications for the evolution of the Southern European Variscan Belt

In the Western Alps, a steeply dipping km-scale shear zone (the Ferriere-Mollieres shear zone) cross-cuts Variscan migmatites in the Argentera-Mercantour External Crystalline Massif. Structural analysis joined with kinematic vorticity and finite strain analyses allowed to recognize a high-temperature deformation associated with dextral transpression characterized by a variation in the percentage of pure shear and simple shear along a deformation gradient. U–Th–Pb dating of syn-kinematic monazites was performed on mylonites. The oldest ~ 340 Ma ages were obtained in protomylonites, whereas ages of ~ 320 Ma were found in mylonites from the core of the shear zone. These ages indicate that the Ferriere-Mollieres shear zone is a still preserved Variscan shear zone. Ages of ~ 320 Ma obtained in this work are in agreement with ages of the dextral transpressional shear zones occurring in the Maures-Tanneron Massif and Corsica-Sardinia. However, transpression in the Argentera-Mercantour Massif started earlier than in other sectors of the southern Variscan Belt. This is possibly caused by the curvature of the belt triggering the progressive migration of shear deformation. Our data allow a correlation between the Argentera-Mercantour Massif and other segments of the Southern European Variscan Belt, in particular with Maures-Tanneron Massif and Corsica-Sardinia, and contribute to fill a gap in the age of activity and in the kinematics of the flow of the system of dextral shear zones of the southern portion of the EVSZ.

Morphological analysis of deep-seated gravitational slope deformation (DSGSD) in the western part of the Argentera massif. A morpho-tectonic control?

The western part of the Argentera–Mercantour massif (French Alps) hosts very large currently active landslides responsible of many disorders and risks to the highly touristic valleys of the Mercantour National Park and skiing resorts. A regional scale mapping of gravitational deformations has been compared to the main geo-structures of the massif. A relative chronology of the events has been established and locally compared to absolute 10Be dating obtained from previous studies. Two types of large slope destabilisations were identified as follows: deep-seated landslides (DSL) that correspond to rock volumes bounded by a failure surface, and deep-seated gravitational slope deformations (DSGSD) defined as large sagging zones including gravitation landforms such as trenches and scarps or counterscarps. Gravitational landforms are mainly collinear to major N140°E and N020°E tectonic faults, and the most developed DSGSD are located in areas where the slope direction is comparable to the orientation of faults. DSL are mostly included within DSGSD zones and located at the slopes foot. Most of DSL followed a similar failure evolution process according to postglacial over steepened topographies and resulting from a progressive failure growing from the foot to the top of the DSGSD that lasts over a 10 ky time period. This massif-scale approach shows that large-scale DSGSD had a peak of activity from the end of the last deglaciation, to approximately 7000 years bp. Both morphologic and tectonic controls can be invoked to explain the gravitational behaviour of the massif slopes.

Dating low-temperature deformation by 40Ar/39Ar on white mica, insights from the Argentera-Mercantour Massif (SW Alps)

In order to date low-temperature deformation, intensely strained muscovite porphyroclasts and neocrystallized shear band phengite from greenschist-facies shear zones have been dated by 40Ar/39Ar method in the Argentera-Mercantour massif. Shear zones are featured by gradual mylonitization of a Variscan granite, gneiss and Permian pelite protolith (300–315Ma) during the Alpine orogenic event. Mineralogical and textural observations indicate that phengites and chlorites developed from biotite and plagioclase in fluid system during deformation following dissolution–transport–precipitation reactions of the type biotite + plagioclase + aqueous fluid = chlorite + albite + phengite + quartz + titanite + K-bearing fluid in the granite-gneiss mylonite. Contrariwise, phengite developed at the expense of clays following substitution reaction in pelite mylonite. Based on conventional thermobarometry on phengite and chlorite and Pressure–Temperature-aqueous fluid (P–T-MH2O) pseudosections calculated with shear zone bulk compositions, the conditions during shear deformation were estimated at 375±30°C and 4.8–7±1kbar in an H2O-satured system. In this low temperature environment, 40Ar/39Ar analysis of the Variscan muscovite for various grades of ductile strain intensity shows a limited 40Ar/39Ar isotopic resetting, all ages scattering between 296 and 315Ma. Under conditions of intense ductile deformation and large-scale fluid circulation, muscovite grains formed during the Variscan retain their much older ages. 40Ar/39Ar dating of very fine grained synkinematic phengite grains, neoformed during the Alpine history, give consistent plateau ages (34–20Ma) for each shear zone. In detail, 40Ar excess can be detected in the pelite mylonitic sample where phengites crystallized by substitution process while the other mylonitic samples where phengites grow from fluid-induced reactions do not evidence any 40Ar excess. These results demonstrate that the 40Ar/39Ar dating of neocrystallized synkinematic white mica allows the determination of precise ages of deformation and fluid activity. Together with precise thermobarometry undertaken on the basis of mineral chemistry and whole-rock composition, 40Ar/39Ar dating of white mica leads to the reconstitution of precise depth-deformation history of low-grade (<400°C) metamorphic units. At the Argentera-Mercantour massif scale, several stages of shear zone development at 15–21km depth are dated between 33 and 20Ma. In the SE part of the massif shear zone ages are well constrained to be either (1) 33.6±0.6Ma or in the range (2) 26.8±0.7Ma–26.3±0.7Ma. In the West of the massif, younger shear zone ages range between (3) 22.2±0.3Ma and (4) 20.5±0.3Ma.

Exhumation controlled by transcurrent tectonics: the Argentera–Mercantour massif (SW Alps)

Terra Nova, 23, 116–126, 2011AbstractNew apatite fission track (AFT) and (U‐Th)/He ages from the Argentera–Mercantour External Crystalline Massif of the SW Alps document an exhumation history along transcurrent dextral faults. Significant AFT age differences across the strikes of the main faults have been obtained from the NE (12.9 Ma) to the SW (5.2 Ma) and are linked to vertical uplifts along these right‐lateral transpressional structures. Such segmentation is not observed in AHe ages. AHe ages have preserved younger ages (4–5 Ma) on the eastern hangingwall side of the High Durance extensional system. Such an exhumation history is interpreted to result from a transition of transpressional to transtensional regimes at 8–5 Ma during continuing Adria–Europe convergence. These data show that recent extensional deformation in the SW Alps can be related to the development of a transtensional domain in the Ubaye–Embrunais depression linked to ongoing strike‐slip deformation along the Argentera–Mercantour.

Temporal evolution of weathered cataclastic material in gravitational faults of the La Clapiere deep-seated landslide by mechanical approach

After a few years of research, the observation and the analysis of the deep-seated landslides suggest that these are mainly controlled by tectonic structures, which play a dominant role in the deformation of massif slopes. The La Clapiere deep-seated landslide (Argentera Mercantour massif) is embedded in a deep-seated gravitational slope deformation affecting the entire slope, and characterized by specific landforms (trenches, scarps…). Onsite, the tangential displacement direction of the trenches and the scarps are controlled by the tectonic structures. The reactivation of the inherited fault in gravitational faults create a gouge material exposed to an additional mechanical and chemical weathering as well as an increased of leaching. The displacement of these reactivated faults gets increasingly important around the area of the La Clapiere landslide and this since 3.6 ka BP. In this study, mechanical analysis and grain size distributions were performed and these data were analysed according to their proximity the La Clapiere landslide and times of initiation of the landslide by 10Be dating. Triaxial test results show that the effective cohesion decreases and the effective angle of internal friction increases from the unweathered area to the weathered area. The whole distribution of the grain size indicates that the further the shear zone is open or developed, the further the residual material loses its finest particles. This paper suggests that the mechanical evolution along the reactivated fault is influenced by the leaching processes. For the first time, we can extract from these data temporal behaviour of the two main mechanical parameters (cohesion and angle of internal friction) from the beginning of the La Clapiere landslide initiation (3.6 ka BP) to now.

Regional-scale relief evolution and large landslides: Insights from geomechanical analyses in the Tinée Valley (southern French Alps)

Recurring large rock-slope failures of the Argentera–Mercantour massif (southern French Alps) over the past 20,000years were identified in the field and analyzed using a continuum mechanics approach based on material weakening in a three-dimensional finite-difference model. We compared a mountain-slope failure model neglecting the effect of rock strength loss with a model simulating the effect of strength loss and integrating cohesional weakening and frictional strengthening processes. The good correspondence between the relative chronology and location of observed and simulated deformations shows that (1) the gradual loss of rock mass strength related to stress release effects following the Pleistocene deglaciation can be as long as several thousands of years, and (2) the causes and triggering of the present-day active landslides are related to this long failure history. Our study suggests that the episodic nature of massive rock slides can be explained through the gradual development of tensile and shear failure, and time-dependent strength degradation. Our numerical model is a good reproduction of large landslides at the regional-scale of a mountainous massif.

Origine tectonique du pli supportant le glissement de terrain de la Clapière (Nord-Ouest du massif de l'Argentera–Mercantour, Alpes du Sud, France) d'après l'analyse de la fracturation

A tectonic origin for the fold underlying the Clapiere landslide (NW Argentera-Mercantour massif, Southern Alps, France) deduced from an analysis of fractures. The Clapiere landslide is underlain by a folded structure, usually described as the result of the gravitational toppling that triggered the sliding. A mechanical interpretation of conjugate shear fractures formed under (at least) 4-5 km of overburden demonstrates that these fractures overprint the fold. Therefore, the fold cannot be a present-day gravitational structure. On the contrary, it has a tectonic origin and could be related to Oligocene-Miocene thrust tectonics. The fold has no direct link with current slope instability. To cite this article: Y. Gunzburger, B. Laumonier, C. R. Geoscience 334 (2002) 415-422.  2002 Academie des sciences / Editions scientifiques

Approche chimique et isotopique de l'origine des eaux en transit dans un grand mouvement de terrain: Exemple du glissement de La Clapière (Alpes-Maritimes, France)

Groundwater flowpaths of La Clapière landslide are studied by chemical and isotopic water analysis of the versant springs. Seepage appends on a recharge area larger than the sliding area (average altitude from 1 567 to 1 780 m ± 150 m). Vertical evolution of springs chemistry!, marked by sulfate ion downwards enrichment (10 mg/L from top of the versant to 800 mg/L at the base), shows gypseous occurrence under or close by the landslide. One hypothesis is that gypsum could originate from evaporites of Triassic synclines pinched into the basement of the Argentera-Mercantour Massif along the dextral strike-slip N 140 °E fault corridor of the Tinée valley. Another hypothesis is that gypsum could originate from sulfated masses percolated into the basement before the alpine sliding of its Mesozoic and Cenozoic sedimentary cover.

Schema structural des Alpes maritimes franco-italiennes [with discussion

Abstract The structural pattern of the Franco-Italian Maritime Alps is outlined. Autochthonous units include the Argentera-Mercantour massif and a folded sedimentary zone which locally represents the dislocated cover of the massif. The Col de Tende unit is considered parautochthonous. Between the Tende col and the sea is a series of rootless overthrust units which rest either directly, or with an intermediate unit of Nummulitic (Tertiary) schists on the outer margin of the so-called imbricate Ligurian Pennine complex. The paleogeographic development of the region is sketched.

La formation a Microcodium au pourtour de l'Argentera-Mercantour

Abstract Discusses the stratigraphic position of Microcodium- bearing formations in the border zone of the Argentera-Mercantour massif in the French Alps, and argues in favor of an organic (algal) origin for Microcodium, which appeared in the upper Senonian (Cretaceous) and persisted, in places, until the middle or upper Lutetian (Eocene).