The pre-Alpine tectonic history of the Austroalpine continental basement in the Valpelline unit (Western Italian Alps)

The Valpelline unit is a large slice of continental crust constituting the Austroalpine Dent Blanche nappe (NW Italy). The pre-Alpine evolution of this unit holds important clues about the Palaeozoic crustal structure at the northern margin of the Adria continent, about the history of rifting in the Alpine region, and thus about the thermomechanical conditions that preceded the Alpine convergent evolution. Several stages of the deformation history and of partial re-equilibration were identified, combining meso- and micro-structural analyses with thermobarometry. Reconstructed pre-Alpine P-T-t-d paths demonstrate that the Valpelline unit experienced an early stage at pressures between 4.5 and 6.5 kbar followed by migmatite formation. A subsequent stage reached amphibolite to granulite facies conditions. This stage was associated with the development of the most penetrative fabrics affecting all of the Valpelline lithotypes. The pre-Alpine evolution ended with a weak deformation associated with a local mineral-chemical re-equilibration under greenschist facies conditions at ≈ 4 kbar and T < 450°C. A Permo-Mesozoic lithospheric extension is thought to be responsible for asthenosphere upwelling, thereby causing high temperature metamorphism at medium pressure and widespread partial melting, which led to upper crustal magmatic activit

Permian magmatism and metamorphism in the Dent Blanche nappe: constraints from field observations and geochronology

In the Dent Blanche Tectonic System, the Mont Morion biotite-bearing granite is a km- scale intrusion preserved in a low-strain volume. Zircon saturation thermometry suggests that it crystallised from a melt that reached about 800 °C. U–Pb zircon and allanite geochronology indicates crystallization of the magma in the Permian (290 ± 3 Ma; 280 ± 8 Ma, respectively). Migmatitic biotite-gneiss and amphibolite are found as xenoliths within the Mont Morion granite and constitute its country-rocks. In two samples of migmatitic biotite-gneiss zircon has metamorphic overgrowths that yield U–Pb ages of 285 ± 3 Ma and 281 ± 4 Ma, and are thus contemporaneous with the intrusion of the granite. The Mont Morion granite with its country-rocks of migmatitic biotite-bearing gneiss and amphibolite was thus emplaced at middle crustal levels while amphibolite facies metamorphism affected its country rocks. The magmatic and metamorphic record in the Mont Morion area reflects the high-temperature regime and lithospheric thinning of the Adriatic continental margin during Permian

The tectonometamorphic evolution of the Sesia-Dent Blanche nappes (internal Western Alps): review and synthesis

This study reviews and synthesizes the present knowledge on the Sesia-Dent Blanche nappes, the highest tectonic elements in the Western Alps (Switzerland and Italy), which comprise pieces of pre-Alpine basement and Mesozoic cover. All of the available data are integrated in a crustal-scale kinematic model with the aim to reconstruct the Alpine tectono-metamorphic evolution of the Sesia-Dent Blanche nappes. Although major uncertainties remain in the pre-Alpine geometry, the basement and cover sequences of the Sesia-Dent Blanche nappes are seen as part of a thinned continental crust derived from the Adriatic margin. The earliest stages of the Alpine evolution are interpreted as recording late Cretaceous subduction of the Adria-derived Sesia-Dent Blanche nappes below the South-Alpine domain. During this subduction, several sheets of crustal material were stacked and separated by shear zones that rework remnants of their Mesozoic cover. The recently described Roisan-Cignana Shear Zone of the Dent Blanche Tectonic System represents such a shear zone, indicating that the Sesia-Dent Blanche nappes represent a stack of several individual nappes. During the subsequent subduction of the Piemonte-Liguria Ocean large-scale folding of the nappe stack (including the Roisan-Cignana Shear Zone) took place under greenschist facies conditions, which indicates partial exhumation of the Dent Blanche Tectonic System. The entrance of the Briançonnais micro-continent within the subduction zone led to a drastic change in the deformation pattern of the Alpine belt, with rapid exhumation of the eclogite-facies ophiolite-bearing units and thrust propagation towards the foreland. Slab breakoff probably was responsible for allowing partial melting in the mantle and Oligocene intrusions into the most internal parts of the Sesia-Dent Blanche nappes. Finally, indentation of the Adriatic plate into the orogenic wedge resulted in the formation of the Vanzone back-fold, which marks the end of the pervasive ductile deformation within the Sesia-Dent Blanche nappes during the earliest Miocene

Geometry and kinematics of the Roisan-Cignana Shear Zone, and the orogenic evolution of the Dent Blanche Tectonic System (Western Alps)

The Dent Blanche Tectonic System (DBTS) is a composite thrust sheet derived from the previously thinned passive Adriatic continental margin. A kilometric high-strain zone, the Roisan-Cignana Shear Zone (RCSZ) defines the major tectonic boundary within the DBTS and separates it into two subunits, the Dent Blanche s.s. nappe to the northwest and the Mont Mary nappe to the southeast. Within this shear zone, tectonic slices of Mesozoic and pre-Alpine meta-sediments became amalgamated with continental basement rocks of the Adriatic margin. The occurrence of high pressure assemblages along the contact between these tectonic slices indicates that the amalgamation occurred prior to or during the subduction process, at an early stage of the Alpine orogenic cycle. Detailed mapping, petrographic and structural analysis show that the Roisan-Cignana Shear Zone results from several superimposed Alpine structural and metamorphic stages. Subduction of the continental fragments is recorded by blueschist-facies deformation, whereas the Alpine collision is reflected by a greenschist facies overprint associated with the development of large-scale open folds. The post-nappe evolution comprises the development of low-angle brittle faults, followed by large-scale folding (Vanzone phase) and finally brittle extensional faults. The RCSZ shows that fragments of continental crust had been torn off the passive continental margin prior to continental collision, thus recording the entire history of the orogenic cycle. The role of preceding Permo-Triassic lithospheric thinning, Jurassic rifting, and ablative subduction processes in controlling the removal of crustal fragments from the reactivated passive continental margin is discussed. Results of this study constrain the temporal sequence of the tectono-metamorphic processes involved in the assembly of the DBTS, but they also show limits on the interpretation. In particular it remains difficult to judge to what extent pre-collisional rifting at the Adriatic continental margin preconditioned the efficiency of convergent processes, i.e. accretion, subduction, and orogenic exhumation

Permian magmatism and metamorphism in the Dent Blanche nappe: constraints from field observations and geochronology

In the Dent Blanche Tectonic System, the Mont Morion biotite-bearing granite is a km-scale intrusion preserved in a low-strain volume. Zircon saturation thermometry suggests that it crystallised from a melt that reached about 800 °C. U–Pb zircon and allanite geochronology indicates crystallization of the magma in the Permian (290 ± 3 Ma; 280 ± 8 Ma, respectively). Migmatitic biotite-gneiss and amphibolite are found as xenoliths within the Mont Morion granite and constitute its country-rocks. In two samples of migmatitic biotite-gneiss zircon has metamorphic overgrowths that yield U–Pb ages of 285 ± 3 Ma and 281 ± 4 Ma, and are thus contemporaneous with the intrusion of the granite. The Mont Morion granite with its country-rocks of migmatitic biotite-bearing gneiss and amphibolite was thus emplaced at middle crustal levels while amphibolite facies metamorphism affected its country rocks. The magmatic and metamorphic record in the Mont Morion area reflects the high-temperature regime and lithospheric thinning of the Adriatic continental margin during Permian.Financial support from the Swiss National Science Foundation (Projects PZ00P2_161202, 200020-126946 and -146175) is acknowledged

Permian high-temperature metamorphism in the Western Alps (NW Italy)

During the late Palaeozoic, lithospheric thinning in part of the Alpine realm caused high-temperature low-to-medium pressure metamorphism and partial melting in the lower crust. Permian metamorphism and magmatism has extensively been recorded and dated in the Central, Eastern, and Southern Alps. However, Permian metamorphic ages in the Western Alps so far are constrained by very few and sparsely distributed data. The present study fills this gap. We present U/Pb ages of metamorphic zircon from several Adria-derived continental units now situated in the Western Alps, defining a range between 286 and 266 Ma. Trace element thermometry yields temperatures of 580-890°C from Ti-in-zircon and 630-850°C from Zr-in-rutile for Permian metamorphic rims. These temperature estimates, together with preserved mineral assemblages (garnet-prismatic sillimanite-biotite-plagioclase-quartz-K-feldspar-rutile), define pervasive upper-amphibolite to granulite facies conditions for Permian metamorphism. U/Pb ages from this study are similar to Permian ages reported for the Ivrea Zone in the Southern Alps and Austroalpine units in the Central and Eastern Alps. Regional comparison across the former Adriatic and European margin reveals a complex pattern of ages reported from late Palaeozoic magmatic and metamorphic rocks (and relics thereof): two late Variscan age groups (~330 and ~300 Ma) are followed seamlessly by a broad range of Permian ages (300-250 Ma). The former are associated with late-orogenic collapse; in samples from this study these are weakly represented. Clearly, dominant is the Permian group, which is related to crustal thinning, hinting to a possible initiation of continental rifting along a passive margin

Multistage garnet in high-pressure metasediments: Alpine overgrowths on Variscan detrital grains

International audienceGarnet is commonly found in metamorphic and magmatic rocks, where its growth records the pressure-temperature history of the rock. Petrologists pay particular attention to multistage garnets, because they document the superposition of successive metamorphic stages and/or cycles in orogenic belts. Due to its considerable resistance to mechanical and chemical weathering, garnet is also commonly present as detrital grains in modern and ancient sediments. Therefore, some multistage garnets could potentially result from overgrowths on detrital grains. We report the first discovery of this type of metamorphic overgrowth on detrital garnet grains in a high-pressure metasediment (Money Unit, western Alps). The studied garnet crystals are found in metamorphic rocks derived from matrix-supported conglomerates. Garnet cores display morphological characteristics of the detrital grains and testify that crystals have been corroded as a consequence of abrasion and dissolution. Furthermore, they display a range of compositions incompatible with their growth in a single rock type, implying their provenance from diverse sources. Garnet overgrowths developed on detrital grains during a blueschist facies event, as indicated by chloritoid and rutile inclusions

I travertini post-glaciali della bassa valle del Buthier,a nord di Aosta, con note sulle caratteristichegeologiche e morfologiche dell’area

National audienceDans la basse vallée du Buthier, à l’E de Variney, affleurent au bord du torrent des travertins contenantdes plantes et des mollusques fossiles. Ces travertins résultent de la précipitation encore active de carbonatede calcium, essentiellement autour des frondes de mousses dans une zone de résurgence d’eaux souterrainesayant percolé sur le versant ouest de la vallée dans les calcschistes de la zone du Combin

Exhumation of HP/UHP rocks by normal ductile shearing on top of the Eocene extruding wedge

International audienceA popular model for the exhumation of HP-UHP rocks is the ‘extruding wedge’ model, where a crustal slice is bounded at its base by a ‘thrust shear-sense’ fault and to the top by a ‘normal shear-sense’ fault. In the Western Alps, the late Eocene Combin Shear Zone (CSZ) allowed extrusion of a wedge made by the Briançonnais-Piemonte-Liguria (‘Penninic’) stack.Geological mapping has established the geometry and continuity of the CSZ from the frontal part of the Dent Blanche Tectonic System to the western boundary of the Sesia Zone. The CSZ has been cut during the Miocene by the brittle Aosta-Ranzola Fault, with an estimated downthrow of the northern block of c. 2.5 km with respect to the southern block. Consequently, the sections observed north (Monte Rosa) and South (Gran Paradiso) of the Aosta Fault display different structural levels in the Alpine nappe stack. The CSZ has been folded (Vanzone phase) during the final part of its history (i.e. when displacement along the CSZ was no more taking place), due to the indentation of the Adriatic mantle. This offers us the unique opportunity to study the change in deformation mechanisms along the shear zone (for a distance parallel to its displacement of about 50 km).Salient characteristics of the CSZ are the following. (i) The thickness of the ductile shear zone increases from NW (frontal part of the Dent Blanche) to SE (frontal part of the Sesia Zone), from a few hundred metres to several kilometres. The type of lithologies pervasively reworked by the ductile shear changes along strike (dominantly calcschists from the topmost oceanic units in the Combin Zone, possibly up to the whole of the ‘Gneiss Minuti’ in the frontal Sesia Zone). (ii) The main ductile deformation along the CSZ was taking place at greenschist-facies conditions, overprinting eclogite-facies to greenschist-facies deformations of Cretaceous to Middle Eocene age. The CSZ is cutting and reworking eclogite-facies structures developed in its hangingwall (Sesia) as well as in its footwall (Zermatt). (iii) Ductile displacement along the CSZ is associated with the development in its footwall of south-east-verging, kilometre-scale, folds (Mischabel phase). The sedimentary sequences of the Pancherot-Cime Bianche-Bettaforca Unit may be used to estimate the minimum amount of ‘normal shear sense’ displacement of the order of 15-20 km.A kinematic model integrating slab roll-back, ‘thrust shear-sense’ at the base and ‘normal shear-sense’ displacement on top of the Eocene eclogite-facies stack will be presented

Ductile shearing on top of the Eocene extruding wedge: the Combin Shear Zone

International audienceA popular model for the exhumation of HP-UHP rocks is the ‘extruding wedge’ model, where a crustal sliceis bounded at its base by a ‘thrust shear-sense’ fault and to the top by a ‘normal shear-sense’ fault. In theWestern Alps, three main ‘normal shear-sense’ shear zones may have bounded extruding wedges at differenttimes during the Alpine orogeny, namely the Late Cretaceous-Palaeocene Fobello-Rimella Shear Zoneduring extrusion of the Cretaceous Sesia-Dent Blanche stack, the late Eocene Combin Shear Zone duringextrusion of the Briançonnais-Piemonte-Liguria (‘Penninic’) stack, and, finally, the Frontal Penninic Faultduring Oligo-Miocene extrusion of the Helvetic stack.The available knowledge on the Combin Shear Zone (CSZ), the best exposed of these three shear zones, willbe reviewed based on a synthetic structural map of the NW Alps. A general agreement has been achieved onthe following points.1. Geological mapping has established the geometry and continuity of the CSZ from the frontal part of theDent Blanche Tectonic System to the western boundary of the Sesia Zone.2. The CSZ has been cut during the Miocene by the brittle Aosta-Ranzola Fault, with an estimateddownthrow of the northern block of c. 2.5 km with respect to the southern block. As a consequence, thesections observed north (Monte Rosa) and South (Gran Paradiso) of the Aosta Fault display differentstructural levels in the Alpine nappe stack.3. The CSZ has been folded (Vanzone phase) during the final part of its history (i.e. when displacementalong the CSZ was no more taking place), due to the indentation of the Adriatic mantle. This offers us theunique opportunity to study the change in deformation mechanisms along the shear zone (for a distanceparallel to its displacement of about 50 km).4. The main ductile deformation along the CSZ was taking place at greenschist-facies conditions,overprinting eclogite-facies to greenschist-facies deformations of Cretaceous to Middle Eocene age.More controversial are the following issues.5. The thickness of the ductile shear zone increases from NW (frontal part of the Dent Blanche) to SE(frontal part of the Sesia Zone), from a few hundred metres to several kilometres. The type of lithologiespervasively reworked by the ductile shear changes along strike (dominantly calcschists from the topmostoceanic units in the Combin Zone, possibly up to the whole of the ‘gneiss minuti’ in the frontal Sesia Zone).6. A major consequence of the ductile displacement along the CSZ is the development in its footwall ofsouth-east-verging, kilometre-scale, folds (Mischabel phase). The sedimentary sequences of the Pancherot-Cime Bianche-Bettaforca Unit may be used to estimate the minimum amount of ‘normal shear sense’displacement of the order of 15-20 km.7 The CSZ is cutting and reworking eclogite-facies structures developed in its hangingwall (Sesia) as well asin its footwall (Zermatt). Two examples of kilometre-scale overprinting structures (Ollomont and Cignana)will be discussed in detail