Gran Paradiso Massif Research Articles

Geology of the southern Gran Paradiso Massif and Lower Piedmont Zone contact area (middle Ala Valley, Western Alps, Italy)

ABSTRACT This work presents the structural evolution of a poorly studied key-area in the middle Ala Valley, Western Alps, where two tectono-metamorphic units are exposed. A geological map at the 1:10.000 scale, integrated with meso- and microstructural analysis, has been realised. We investigated the contact area between Gran Paradiso Massif in the footwall and Lower Piedmont Zone in the hanging wall. Both tectono-metamorphic units, with a different paleogeographic affinity, preserve similar polyphasic deformation histories, defined by four deformation phases. The Dp phase, strongly transposing previous structural relicts, is marked by a high-pressure assemblage associated with the Sp foliation. Dp controls the lithological boundary attitude. A mylonitic zone, developed during the Dp, showing kinematic indicators pointing to a top-to-the N-NW sense of shear, is responsible for the juxtaposition of the two units. Dp structural elements are deformed by Dp+1 and Dp+2 subsequent phases. A greenschist-facies overprinting was observed during the Dp+1 phase.

The Vesuvianite Gems of the Val d’Ala (Piedmont, Italy)

In Val d’Ala (Western Alps in Piedmont, Italy), the most interesting rocks for mineralogical research are represented by rodingite (rich in mineralized veins and fractures) associated with serpentinite in the eclogitized oceanic crust of Piedmont Zone, south of Gran Paradiso Massif. Among the vein-filling minerals, vesuvianite is well appreciated for its potential as gem-quality materials, even though it has never been characterized in detail. This study provides a gemological characterization of eleven vesuvianite crystals from different localities of the Val d’Ala. The refractive index (1.717–1.708) and density (1.705–1.709) values of our vesuvianite are in the range of those in the literature. Scanning electron microscopy with energy-dispersive spectroscopy (SEM/EDS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) established that the samples are pretty compositionally homogeneous in terms of major elements, while trace and rare earth elements (REE) contents are more variable. All REE patterns are characterized by pronounced positive Eu anomalies. The variations in color (from olive green to dark green with chocolate (reddish-brown color shades and polychrome bands) are due to the relevant presence of Fe and, to a lesser extent, Ti and Cr. The X-ray powder diffraction (XRPD) analyses and SEM/EDS quantitative study indicate that the other phases associated with vesuvianite are represented by diopside, garnet, clinochlore.

Landslide on glaciers: an example from Western Alps (Cogne - Italy)

In the warm summer of 2017, a landslide failed from the south-east side of the Col des Clochettes on the top of the underlying Trajo Glacier. The study area is at an elevation of about 3500 m a.s.l. in the Gran Paradiso Massif and can be hardly reached by walking from Cogne (Aosta Valley, NW Italy). Studies conducted by field surveys, photogrammetry (structure from motion) and satellite images analysis, integrated with the evaluation of data from meteorological stations have been used to reconstruct the phenomenon and infer its causes. The site is very complex to be studied especially due to logistic problems, therefore, measurements and observations that are common practice in other landslides are very difficult to apply here. So, many of the results achieved are not adequately supported by field studies. Anyway, the following factors could have affected the stability of the slope: i) the tectonic structure of the area, which is reflected on the morphology and on the geomechanics characteristics of the rock masses; ii) the meteorological conditions during 3 months before the main failure, resulting in an extremely high temperature compared to historical data. Moreover, the analysis of multitemporal satellite images allowed to recognize that it was not a single landslide but that the phenomenon is articulated over time in at least five failures in about 2 months. Moreover, several predisposing factors may have been playing an important role in causing the instability: the degradation of permafrost (probably affecting rock mass due to the circulation of warm air and water in the discontinuity systems), the alternance of the freeze-thaw cycles and the availability of a considerable amount of water from rainfalls and nival fusion infiltrating deeply in the rock mass. More common causes such as rains and earthquakes have been excluded.

Garnets from Val d’Ala Rodingites, Piedmont, Italy: An Investigation of Their Gemological, Spectroscopic and Crystal Chemical Properties

In Val d’Ala (Piedmont, Western Alps, Italy), the more interesting rocks for the mineralogical research are represented by rodingites (rich in mineralized veins and fractures) associated with serpentinites in the eclogitized oceanic crust of Piemonte Zone, south of Gran Paradiso Massif. Among the vein-filling minerals, garnets are the most appreciated as mineral specimens and, in less degree despite their vivid and rich colors, for their potential as gem-quality materials. This study provides a complete gemological characterization of five faceted samples and offers new information by means of Synchrotron X-ray computed micro-tomography imaging gem features. Electron-probe microanalysis (EMPA) and laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) established that the chemical composition of garnets from different localities, resulted both close to pure andradite, enriched in light rare earth elements (LREE) with a positive Eu anomaly, and grossular-andradite solid solution (grandite), enriched in heavy rare earth elements (HREE). X-ray powder diffraction analyses indicate the possible coexistence of almost pure grossular and andradite. A spectroscopic approach, commonly used with gem-like material, by Raman and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, completes the characterization of the samples. The new data on the textural and geochemical features of the grandite and andradite garnets suggest local growth processes under various chemical and oxidation conditions of metasomatic and metamorphic fluids interacting with the host-rocks. Garnets represent long-lasting mineral records of the complex geological history of the Val d’Ala rodingitic dikes during their oceanic- and subduction-related metamorphic evolution.

Exhumation rates in the Gran Paradiso Massif (Western Alps) constrained by in situ U–Th–Pb dating of accessory phases (monazite, allanite and xenotime)

Exhumation rates for high-pressure metamorphic rocks need to be carefully estimated to decipher tectonic processes in subduction/collision belts. In the Gran Paradiso Massif (Western Alps), the Money Unit crops out as a tectonic window below the Gran Paradiso Unit. According to previous studies, the Gran Paradiso and Money Units reached peak pressure conditions at ~ 18 to 20 kbar, 480–520 °C and ~ 17 to 18 kbar, 500–550 °C, respectively. This yields a maximum difference of ~ 9 to 10 km in the subduction depth reached by these two units during the Alpine history. Thrusting of the Gran Paradiso Unit over the Money Unit led to the simultaneous development of the main foliation under the same metamorphic conditions (~ 12.5 to 14.5 kbar and 530–560 °C) in both units. The thrust contact was subsequently folded and then both units were exhumed together. The relative timing of the growth and dissolution of the accessory phases was assessed by combining thermodynamic modelling with inclusion, textural and chemical (major and trace element) data from both major and accessory phases. The age of monazite constrained the high-pressure metamorphism in both the Gran Paradiso Unit and the Money Unit at 41.5 ± 0.3 and 42.0 ± 0.6 Ma, respectively. Allanite replacing monazite in the matrix has been dated at 32.7 ± 4.2 Ma. The late growth of xenotime associated with the crystallization of biotite pseudomorphs at the expense of garnet (at about 10 kbar) was dated at 32.3 ± 1.0 Ma. Our petrochronological data indicate about 10 m.y. between the peak pressure conditions and the crystallization of xenotime leading to an exhumation rate of the order of 2.2–5 mm/year. The new ages allow to better constrain the timing of the displacement of the thrust defining the lower boundary of the extruding wedge of eclogite-facies rocks.

Nappe stack of Piemonte–Ligurian units south of Aosta Valley: New evidence from Urtier Valley (Western Alps)

New results of structural and lithostratigraphic investigations performed in the Urtier Valley, located south of Aosta Valley at the northern flank of the Gran Paradiso Massif, shed new light on the nappe stack architecture of the Western Alps. Two distinct Piemonte–Ligurian units were recognized in the area: (a) the blueschist facies Broillot unit, with a well‐preserved lithostratigraphic succession that includes ophiolites and their metasedimentary cover and (2) the eclogitic Bardonney unit with clasts and/or slices of ophiolites embedded in a carbonate/quarzitic and/or metabasic matrix. Three major deformation stages have been recognized (D1, D2, and D3). D1 phase occurred during the peak metamorphic conditions (i.e., eclogite or blueschist facies). D2 and D3 phases are related to multistage exhumation processes, from early stacking to late refolding of former nappe contacts. Despite the intensity of deformation and metamorphism, the original lithostratigraphic succession of the Broillot unit allowed us to use a lithostratigraphic approach to reconstruct the structures in the study area. For the first time, differences in the nappe stack exposed south and north of the Aosta Valley are reported. In fact, contrary to what is observed in the north, the eclogitic units exposed in the Urtier Valley lay above and not below the blueschist units. A new model for the tectonic evolution, which ranges from stages of subduction to large‐scale nappe refolding, is proposed. It is able to explain the difference in the architecture of the north and south of the Aosta Valley.

Jadeite in the Gran Paradiso massif: the high-pressure metamorphic evolution of upper continental crust

The formation and eventual preservation of high-pressure mineral assemblages in subducted continental crust are a long-standing issue with kinetic aspects and important geophysical implications in terms of buoyancy. The issue is especially acute in the internal crystalline massifs of the Western Alps, in which the dominant rock-type, metagranite, hardly bears evidence for high-pressure transformation whereas subordinate rock types clearly point to high or very high pressure conditions. The discovery of well-preserved jadeite-bearing rocks in orthogneiss of the western Gran Paradiso massif, reported here, revives the questioning and is the opportunity to describe uncommon mineral assemblages involving nepheline, preiswerkite, as well as a new trioctahedral mica.

Constraining P–T conditions during thrusting of a higher pressure unit over a lower pressure one (Gran Paradiso, Western Alps)

AbstractIdentifying higher pressure units overlying lower pressure ones is a first order argument to determine the presence of large‐scale thrusting. For the first time, petrology is used to quantify the pressure difference between two stacked units in the Western Alps. In the Gran Paradiso Massif, the Money unit crops out as a tectonic window below the Gran Paradiso unit. The reconstruction of the Alpine evolution of these two units and the history of their tectonic contact has been achieved using a multidisciplinary approach that combines meso‐ and microstructural analysis and pseudosection calculations. In both units, four stages of deformation and metamorphism have been identified. Stage 1 reflects the phase of continental crust subduction and P–T conditions of ~18–20 kbar, 480–520 °C and of ~13–18 kbar, 500–530 °C have been estimated for the Gran Paradiso and the Money units respectively. This yields a maximum difference of ~20 km in the depth reached by these two units during the early Alpine history. Thrusting of the Gran Paradiso unit over the Money unit (stage 2) led to the development of the main foliation and occurred in the high‐P part of the albite stability field at P–T conditions of ~12.5–14.5 kbar and 530–560 °C, identical in both units. The thrust contact was folded during stage 3 together with the entire Money unit, and then both units were exhumed together (stage 4). During this polyphase evolution, detrital garnet has been partially dissolved, while the earliest Na‐bearing phases (glaucophane, paragonite) have been overprinted by the low‐P mineral associations. The uncertainties on derived pressures between the two units are unfortunately larger than hoped, and this is attributed to the muscovite solid‐solution model not incorporating a pyrophyllite component.

Derivation of P–T paths from high-pressure metagranites — Examples from the Gran Paradiso Massif, western Alps

Metamorphosed granites (SiO2≥70wt.%) are, in fact, a common rock type in high pressure (HP) and ultrahigh pressure (UHP) terrains, but these rocks were rarely used to derive metamorphic P–T paths. To test the suitability of HP metagranites for such derivations, two metagranites from the Gran Paradiso Massif were studied applying elemental mapping of phengite and garnet and calculated P–T pseudosections contoured by various chemical and modal parameters. Both rocks contain phengite with maximum Si contents of about 3.42 Si per formula unit (pfu) and 3.55Sipfu in cores, and accessory garnet which is compositionally zoned. Garnet core compositions are rich in grossular component (XCa up to 0.72). Only a rough P–T path could be derived with peak pressures below 2GPa because, for instance, Si contents in phengite become geobarometrically insensitive at HP conditions, when biotite is not anymore stable. A test of the pseudosection approach to a metagranite from the North Qaidam UHP metamorphic belt resulted in an ambiguous finding. In fact, compositions of garnet and phengite in this rock are indicative of both UHP and specific HP conditions (~1.3GPa, 530°C), but the latter conditions fit the entire mineralogical observations better.

Detrital zircon geochronology in blueschist-facies meta-conglomerates from the Western Alps: implications for the late Carboniferous to early Permian palaeogeography

In the Western Alps, the Money Complex of the Gran Paradiso Massif, metamorphosed under blueschist facies during the Alpine cycle, is considered to be Permo-Carboniferous in age, but no palaeontological or radiometric data constrain this interpretation. A revision of the lithostratigraphy of the Money Complex allows recognizing a polygenic (graphite-rich) and a monogenic (graphite-poor) meta-sedimentary formation. Detrital zircon U–Pb geochronology in both meta-sedimentary formations shows that (i) the main population is Cambrian and Ordovician in age, (ii) the youngest grains are Silurian and Lower Devonian, and (iii) Carboniferous zircon grains are lacking. A careful study of the age distributions in the Alps suggests that potential source for the detrital material in the Money Complex is the Brianconnais basement. Late Carboniferous magmatism is widespread in the Helvetic Zone of the Alps. Permian magmatism is dominant in the Brianconnais, the Austroalpine and the Southalpine basements. The lack of Carboniferous zircons in the Money Complex suggests that the detritus was not shed from the Helvetic zone, which was separated from the Money basin by the Zone Houillere basin, where the main drainage pattern was developed from south to north and where the depocenters migrated northwards from the Upper Missisippian to Upper Pennsylvanian. We suggest that the Money Complex may had been located to the east of the main river drainage inside the Zone Houillere basin or alternatively may represent a small basin, located on the east of the Zone Houillere.

Petro-structural map of the Money Unit (Gran Paradiso Massif, Valnontey valley, Western Alps)

The Gran Paradiso Massif is one of the Internal Crystalline Massifs (Penninic Zone) in the Alps. This Massif comprises the widely investigated Gran Paradiso Unit and the underlying, much less-explored Money Unit, which is a well-exposed tectonic window below the Gran Paradiso Unit. This contribution provides a new detailed petro-structural map of the Money Unit. An integrated structural and metamorphic study can relate different structures to the evolving metamorphic stages. There is also recognition of evidence of Permo-Carboniferous sedimentation, contact metamorphism and polyphase Alpine evolution, mainly developed under blueschist to greenschist facies conditions.The 1:5000 scale map (and the cross sections) display evidence for the Alpine evolution of the Money Unit and for the structural and geological setting of this area. From these data, a 3D model of the Money Unit and its folded contact with the Gran Paradiso Units is constructed.

Preservation of sub-microscopic structural relicts in micas from the Gran Paradiso Massif (Western Alps): Implications for 40Ar–39Ar geochronology

Permian igneous biotite and white mica that were re-heated at 500–550°C in the Middle-Late Eocene were investigated by laser step-heating and in situ 40Ar–39Ar techniques, in conjunction with scanning electron microscopy (SEM), electron microprobe (EMP) and transmission electron microscopy (TEM) to assess the influence of short-lived metamorphism on K–Ar systematics. Large intra- and inter-grain age variations, ranging from the Triassic to the Late Eocene, are primarily related to the extent of metamorphic re-equilibration. Brown biotite porphyroclasts from weakly re-equilibrated samples are characterized by core-to-rim zoning in major element composition, mineral structure and 40Ar/39Ar ratios. Titanium concentration and Fe/(Mg+Fe) ratios decrease from crystal cores towards the rims, which are compositionally indistinguishable from green biotite aggregates in metamorphic coronas. Compositional changes are coupled with structural modifications towards the crystal edges, where the disordered igneous 1M–2M1 stacks are replaced topotactically by highly ordered metamorphic 1M polytypes. Apparent ages up to ∼66Ma were determined in crystal cores by in situ laser-probe analyses, while ages as young as ∼45Ma are typical of crystal rims. Step-heating experiments of these zoned biotites yielded a discordant saddle-shaped age spectrum, with a concordant central segment yielding an error-weighted mean age of 44.9±0.3Ma. Biotite porphyroclasts from a more re-equilibrated specimen gave markedly different results, with uniform major elements compositional profiles, homogeneous argon distribution throughout the crystals and a flat age spectrum at 36.5±0.3Ma. TEM investigations of these optically homogeneous porphyroclasts revealed that the original igneous structure had been replaced by highly ordered 1M polytypes during Alpine metamorphism. White mica yielded comparable results, with spot ages up to ∼218Ma in domains characterized by high Na/(Na+K) ratios and low Si and Ti content, representing the best preserved igneous relicts. Significantly younger apparent ages, down to 44–39Ma, are typical of domains affected by partial metamorphic re-equilibration at high-pressure conditions, as indicated by increasing Si and Ti contents, or by exhumation-related exsolution of amoeboidal quartz. These results indicate that different intra-grain and inter-grain Ar concentration patterns arise primarily from the relative amount and distribution of relict igneous and metamorphic crystal-chemical domains. Core-to-rim age gradients in biotite result from inward-directed recrystallization from the crystal edges, eventually leading to flat age profiles as metamorphic replacement of pre-existing porphyroclasts runs to completion. This study calls for caution when estimating thermal histories from apparent age gradients or from age vs. grain size correlations, unless the presence of pre-metamorphic mineral relicts can be ruled out.

Dating deformation in the Gran Paradiso Massif (NW Italian Alps): Implications for the exhumation of high-pressure rocks in a collisional belt

The Gran Paradiso massif, situated in the internal part of the Western Italian Alps, records a complex tectono-metamorphic history involving high-pressure metamorphism and subsequent exhumation during retrograde metamorphism. The exact timing of deformation and, consequently, the geodynamic evolution of this part of the Western Alps is still debated and is addressed here by the application of Rb/Sr geochronology, 40Ar/39Ar step heating and 40Ar/39Ar total fusion dating techniques. Geochronological results are presented from shear zone samples in the core of the Gran Paradiso massif (Piantonetto Valley), and in the area closer to the contact with the overlying Piedmont ophiolitic domain (south and southwest of Pont Valsavarenche). The shear zones operated during crustal thinning and exhumation of the Gran Paradiso massif. 40Ar/39Ar step heating results from shear zones in the Piantonetto Valley show acceptable plateau ages that are interpreted to represent two events of mica growth. Similar ages, and an additional younger age cluster, are recognised in the 40Ar/39Ar total fusion analyses, indicating that specific cleavage domains operated at 39.2±0.2, 36.5±0.6 and 33.3±0.4Ma. P–T pseudosections show a progressive decrease in metamorphic conditions during deformation, suggesting that the age of incipient exhumation and the related deformation in the Piantonetto Valley is equal to or older than 39.2±0.2Ma. In the Pont area, the last increments of deformation in a top-to-W shear zone postdate 36.6±0.6Ma (Rb/Sr mineral data), whereas the present-day top-to-W contact of the Gran Paradiso massif with the overlying Piedmont domain is dated at 41.2±1.1Ma (Rb/Sr multi-mineral isochron age). We propose a model that considers exhumation of the Gran Paradiso nappe at 41–34Ma. During this period, the nappe was coupled with the Zermatt-Saas zone, forming an extruding wedge. The kinematics associated with this wedge involved top-to-W shearing within the Gran Paradiso nappe (e.g. Pont area shear zones) and top-to-E shearing at the top of the extruding wedge (e.g. Orco shear zone). Subsequent deformation (after ~34Ma) was characterised by coaxial strain involving orogenic-scale backfolding and backthrusting.

Polymetamorphic history of a relict Permian hornfels from the central Gran Paradiso Massif (Western Alps, Italy): a microstructural and thermodynamic modelling study

A petrological and thermobarometric study of the Lago Teleccio hornfelses was undertaken to reconstruct the polymetamorphic evolution and constrain the P–T conditions of Permian contact metamorphism. The Lago Teleccio metasedimentary rocks record a Variscan regional metamorphism characterized by amphibolite facies mineral assemblages including quartz, plagioclase, K-feldspar (Kfs 1), biotite, garnet (Grt 1) and staurolite; this was followed by a late-Variscan mylonitization event. Metamorphism of the Variscan metamorphic rocks at the contact with a Permian granitic intrusion produced static recrystallization and/or new growth of quartz, garnet (Grt 2), plagioclase, K-feldspar (Kfs 2), cordierite, green spinel, biotite and prismatic sillimanite (Contact 1). This thermal event, which occurred at a peak pressure of 0.23–0.35 GPa, temperature of 670–700 °C and aH2O of 0.75–1, was followed either during post-contact metamorphism cooling or, more likely, during the early-Alpine metamorphism by the breakdown of cordierite into an anhydrous kyanite + orthopyroxene + quartz assemblage. The poorly developed early-Alpine eclogite facies metamorphism (Alpine 1) was characterized by relatively anhydrous mineral associations and low strain, which locally produced coronitic and pseudomorphous microstructures in metasedimentary rocks, with scanty formation of jadeite, zoisite and a new high-pressure garnet (Grt 3). Greenschist facies retrogression (Alpine 2) was characterized by the local development of a chlorite- and muscovite-bearing mineral association, suggestive of aqueous fluid incursion. In the hornfelses, the limited extent of metamorphic overprinting is suggested by the fine grain size of the Alpine mineral associations, which formed at the expense of the Permian contact metamorphic associations, and was favoured by the anhydrous mineralogy of the hornfelses.

Geological map of the middle Orco Valley, Western Italian Alps

Please click here to download the map associated with this article. Detailed mapping of the middle Orco Valley in the Western Italian Alps allowed for the reconstruction of the lithostratigraphy and the structural evolution of different tectonic units along a geological section from the Gran Paradiso Massif to the Gneiss Minuti Complex of the Sesia-Lanzo Zone. The studied nappes can be grouped into a Lower Tectonic Element which underwent eclogite facies metamorphism and an Upper Tectonic Element pervasively equilibrated under greenschist facies conditions and lacking evidences of high pressure metamorphism. The mapping of Quaternary deposits, geomorphological features and brittle structural elements provided evidence for the occurrence of a deep-seated gravitational slope deformation on the left side of the middle Orco Valley.

Metamorphic evolution of the Gran Paradiso Massif: A case study of an eclogitic metagabbro and a polymetamorphic glaucophane–garnet micaschist

Sub-continental type gabbro preserving an eclogite facies assemblage and glaucophane –garnet bearing micaschist from the eastern border of the Gran Paradiso Massif (Western Alps) have been investigated to constrain its tectono-metamorphic evolution. Glaucophane-bearing metapelites were never reported before while metagabbros are only found in the southern part of the Massif. Growth zoning of multistage garnet in metapelites and the investigation of two different chemical systems (metapelitic and basic), allowed to reconstruct the P– T path followed by Gran Paradiso Massif during the late Variscan stage and during the Alpine orogeny. Micro-structural investigations allowed to identify different assemblages related to different tectono-metamorphic stages. Pseudosection modelling with the software PERPLE_X of the stable assemblages and of the mineral compositions has allowed to infer the P– T conditions for these different metamorphic stages. Variscan metamorphism reached 620–630 °C at 8–9 kbar. The Alpine HP stage peaked at 580–600 °C and 26–27 kbar and the first decompression stage was at 580–610 °C and 21–22 kbar with no T increase before the development of the S 1 regional foliation. S 1 represents the main tectono-metamorphic event recorded by Gran Paradiso Massif and developed at 600–625 °C and 9.5–11.5 kbar in the low T and high P amphibolite facies. From these data a different P– T path is proposed respect to previous works. The exhumation from peak eclogite facies conditions to amphibolite facies was nearly isothermal and then was followed by considerable cooling suggesting a two-stage exhumation process controlled by different mechanisms.

Local shear zone pattern and bulk deformation in the Gran Paradiso metagranite (NW Italian Alps)

The Gran Paradiso nappe of the northwestern Alps mostly consists of augen gneisses derived from the Alpine deformation of Permian granitoids. The regional foliation of the augen gneisses developed at lower amphibolite facies conditions and is associated with a top-to-west sense of shear. The granitoid protolith is preserved in the kilometre-scale low-strain domain of the Piantonetto Valley and mainly consists of a porphyritic metagranite including joints, leucocratic dykes and biotite-rich schlieren. In this low-strain domain, the Alpine deformation is mainly localized in discrete ductile shear zones within weakly foliated metagranite. The shear zones mostly dip towards S–SE in a shallow (shear zones 1) to steep inclination (shear zones 2). The shear zones show typical features that can be explained by reactivation of pre-existing joints and planar compositional heterogeneities. Palaeostress and strain analysis indicate that shear zones and the metagranite foliation both formed in the presence of a strong component of flattening. The kinematics of individual shear zones depends on the orientation of the original heterogeneities (acting as nucleation planes) and by partitioning of strain components at the kilometre-scale with concentration of the flattening component to the Piantonetto low-strain domain. The strain geometry and the kinematics of individual shear zones within Piantonetto are not directly connected to the top-to-west sense of tectonic transport observed elsewhere in the Gran Paradiso nappe. However, the bulk stress ellipsoid reconstructed for the incipient shear zone network within very weakly deformed granites is oriented consistently with the bulk direction of tectonic transport within the Gran Paradiso massif. We conclude that the shear zone network of the Piantonetto Valley is representative of the incipient stages of ductile deformation of a granite nappe. Even if its architecture is determined by the arrangement of pre-existing structural and compositional heterogeneities, aspects of the large-scale bulk strain can be derived from this local shear zone pattern.

The age of HP metamorphism in the Gran Paradiso Massif, Western Alps: A petrological and geochronological study of “silvery micaschists”

We present a petrological and geochronological study of “silvery micaschists” that crop out in the northern side of the Gran Paradiso Massif, Western Alps, with the aim of constraining P–T conditions and age of the Alpine high-pressure metamorphism. The studied “silvery micaschists”, which are the products of metasomatic alteration of granitoids along ductile shear zones, consist of Mg-chlorite, talc, chloritoid, minor phengite, and accessory minerals. Microstructural relationships indicate the following prograde sequence in the growth of U–Th bearing accessory minerals: florencite → monazite → allanite. Thermobarometric calculations indicate that the Mg-chlorite + talc + chloritoid + allanite peak assemblage was stable at P = 1.9–2.7 GPa and T = 515–600 °C, while monazite formed earlier at pressures over 2.0 GPa. SHRIMP dating of allanite yielded 33.7 ± 1.6 Ma, interpreted as the age of the high-pressure metamorphic peak. Prograde monazite yielded an age of 37.4 ± 0.9 Ma, implying a minimum duration of ∼ 2–4 Ma for the Alpine subduction event. Combining our ages with previous constraints, it can be concluded that the initial exhumation of the Gran Paradiso Massif occurred at a fast rate higher than 2 cm/year, and slowed down to ∼ 1 cm/year in the final stages. In a regional context, the new geochronological data align the subduction of the Gran Paradiso Massif with the other Internal Crystalline Massifs of the Western Alps. This supports a subduction model marked by alternating compressional events, related to the accretion of continental terranes, and extensional events, related to the episodic retreat of subduction zone hinges.

Deformation mode switches in the Penninic units of the Urtier Valley (Western Alps): Evidence for a dynamic orogen

Multiple deformation mode switches from shortening to extension have been detected in the Penninic units of the Urtier Valley, to the north of the Gran Paradiso massif. An early shortening phase, whose traces are poorly preserved, led to the tectonic burial of rock units to high pressure conditions. Subsequent extensional shearing (SZ gr 1) led to the formation of mylonitic fabrics. During this event the eclogitic Piemonte unit was exhumed to greenschist facies conditions ( P = 0.2–0.3 GPa) in the footwall of the blueschist Piemonte unit. Exotic slices of continental basement and Mesozoic continental shelf carbonates are found along their contact, implying the existence of older thrust stacks dissected by the younger extensional shear zones. The extensional structures were later affected by upright folding (Fu), indicating yet another deformation mode switch, which led to re-burial to P = 0.6–0.8 GPa. Later multi-stage deformation culminated in an episode of extensional shearing with top-to-the-west kinematics (SZ gr 2). Therefore, at least two shortening–extension cycles affected the Piemonte units of the Urtier Valley. Existing geochronological data allow less than 20 Myr for the complex deformation history discussed here. Shortening–extension cycles can be inferred also from published studies from other sections of the Western Alps. However, while the first shortening–extension cycle was widespread in the Penninic units, the lateral extent and relative timing of the second cycle in different areas are still unclear.

The nappe rule: why does it work?

The nappe rule stresses that imbrication emplaces nappes with higher metamorphic grade above those with lower grade. This means that the overriding nappes must be exhuming during nappe stacking; otherwise metamorphic grade would increase structurally downwards. The high-grade metamorphic internides of many orogens are characterized by a pervasive nappe-emplacement-related subhorizontal foliation indicative of vertical ductile shortening (thinning). By quantifying finite strain in the Gran Paradiso Massif, Italian Alps, we show that vertical ductile thinning associated with a nappe-emplacement-related subhorizontal foliation caused sufficient exhumation of the overriding nappes and can thus explain the occurrence of higher-grade rocks above lower-grade ones.