Gran Paradiso Massif Research Articles

Reconstructing P–T paths during continental collision using multi‐stage garnet (Gran Paradiso nappe, Western Alps)

AbstractGarnet–chloritoid‐bearing micaschists from the Gran Paradiso massif (Western Alps) contain evidence of a polymetamorphic evolution. Detailed textural observations reveal that two stages of garnet growth are present in the micaschists, interpreted as: (i) relics of an early metamorphism of pre‐Alpine age and (ii) newly grown Alpine garnet, respectively. Both generations of garnet preserve growth zoning. From thermocalc‐based numerical modelling of mineral assemblages in pressure–temperature (P–T) pseudosections, we infer that garnet 1 grew at increasing temperature and slightly increasing pressure, whereas garnet 2 grew at decreasing pressure and slightly increasing temperature. Estimated P–T conditions are ∼620 °C, 6 kbar for the peak of the pre‐Alpine event, and of 490 °C, 18–20 kbar for the pressure peak of the Alpine event. Modelling of the modal proportion and chemical composition of garnet (i) shows that the subsequent decompression (to 14–15 kbar at 550 °C) must have been accompanied by moderate heating and (ii) does not support a stage of final temperature increase following decompressional cooling. This argues against a late thermal pulse associated with mantle delamination. Preservation of growth zoning in both generations of garnet and the limited amount of diffusive re‐equilibration at the boundary between the two garnets suggests that the rocks were subjected to fast burial and exhumation rates, consistent with data obtained from other internal Alpine units.

Deformation history of a subducted continental crust (Gran Paradiso, Western Alps): continuing crustal shortening during exhumation

Eclogite-facies continental basement in the Western Alps outcrops as tectonic windows below the oceanic units. In the Gran Paradiso massif, eclogite-facies assemblages in mafic rocks display a N–S stretching lineation (D A1). The main-phase foliation formed under epidote amphibolite-facies conditions (D A2) and defines a regional dome structure for the whole Gran Paradiso massif. Structural data, including new detailed mapping, microstructural analyses and metamorphic studies in the northern part of the Gran Paradiso (Cogne valley) reveal the occurrence of major thrusts in this continental basement associated with the D A2 deformation. E–W-trending stretching lineations and fold axes are associated with this second and main deformation. Nappe stacking of the different units was achieved by top-to-the-west shearing during exhumation of the eclogite-facies basement. The antiformal doming of the Gran Paradiso basement is thus produced by the nappe stacking during exhumation, under epidote amphibolite facies.

U-Pb SHRIMP data on the crystallization age of the Gran Paradiso augengneiss, Italian Western Alps: Further evidence for Permian magmatic activity in the Alps during break-up of Pangea

New U-Pb Sensitive High Resolution Ion MicroProbe (SHRIMP) ages from two augengneiss samples of the Gran Paradiso Massif in the Italian Western Alps yielded well-defined Late Permian crystallization ages of 269.0±6.5 Ma and 270.2±5.0 Ma. The ages provide further evidence that the Gran Paradiso augengneiss is not Variscan basement but reflects post-Variscan magmatic activity related to the break-up of Pangea after the Variscan orogeny eventually leading to the formation of Neo-Tethys. The Gran Paradiso Massif belongs to the so-called internal massifs, which are marked by extension-related bimodal magmatism during the Permian and Mesozoic. Furthermore, the Late Permian ages suggest that the Gran Paradiso Massif did not belong to the same pre- Alpine tectonic domain as the External massifs (as part of the European plate margin), which are characterized by Variscan basement.

Tectonic evolution of the Briançonnais units along a transect (ECORS-CROP) through the Italian-French Western Alps

Based on new structural data from an area in the Italian-French Western Alps, situated between the Petit Saint Bernard pass and the Gran Paradiso massif, the large-scale geometry of the tectonic features is established for the tectonic units derived from the Brianconnais paleogeographic domain. Based on this, and other new data on the metamorphic evolution and geochronology of the area, a consistent model for the tectonic evolution for the Brianconnais domain is proposed. A nappe stack consisting of, from bottom to top, Zone Houillere unit, Ruitor unit, Internal unit and Piemont-Ligurian oceanic unit, was affected by three deformation phases. The third phase of deformation (35–31 Ma), referred to as post-nappe folding, is responsible for the formation of two large-scale folds: the Ruitor and Valsavaranche mega-folds. An overturned nappe stack, characterized by foreland-dipping foliations, is found between the axial planes of these two mega-folds. No evidence was found for back-thrusting, postulated by many previous authors. The individual thrusts between the tectonic units were active during a second phase of deformation (43–35 Ma), characterized by top-NW to -NNW shearing. This D2 is related to nappe stacking and accompanied by substantial exhumation. Thrusting becomes relatively younger towards the external parts of the study area. We emphasize that extension played no significant role in the exhumation of the high-pressure units of the Italian-French Alps. The contacts between the tectonic units were already active during the first phase of deformation (50–43 Ma), which is related to subduction and peak pressures.

Underplating-related finite-strain patterns in the Gran Paradiso massif, Western Alps, Italy: heterogeneous ductile strain superimposed on a nappe stack

A finite-strain study in the central and northern Gran Paradiso massif of the Italian Western Alps has been carried out to elucidate whether ductile strain shows a relationship to nappe contacts and to shed light on the nature of the subhorizontal foliation typical of the gneiss nappes in the Alps. The R f /φ and Fry methods were used on feldspar porphyroclasts from 98 samples of the Gran Paradiso unit (upper tectonic unit of the Gran Paradiso massif) and 12 samples from the underlying Erfaulet unit (lower unit of the Gran Paradiso massif). Microstructures and thermobarometric data show that feldspar ductility at temperatures higher than c . 450 °C occurred only during high-pressure metamorphism, when the rocks were underplated beneath the overriding Adriatic plate. Therefore, the finite-strain data can be related to high-pressure metamorphism in the Alpine subduction zone. The augen gneiss was heterogeneously deformed and axial ratios of the strain ellipse in xz sections range from 2.1 to 69.8. The long axes of the finite-strain ellipsoids trend west–WNW and the short axes are subvertical, associated with a subhorizontal foliation. The strain magnitudes do not increase towards the nappe contacts. The data indicate flattening strain type in the Gran Paradiso unit and constrictional strain type in the Erfaulet unit and prove deviations from simple shear. We conclude that nappe stacking occurred early during subduction probably by brittle imbrication and that ductile strain was superimposed on and modified the nappe structure during high-pressure underplating in the Alpine subduction zone. The accumulation of ductile strain during underplating was not by simple shear and involved a component of vertical shortening, which caused the subhorizontal foliation in the Gran Paradiso massif.

Field and petrological evidence for a Late Palaeozoic (Upper Carboniferous–Permian) age of the Erfaulet orthogneiss (Gran Paradiso, western Alps)

In the Gran Paradiso massif (western Alps), the boundary between the Erfaulet orthogneiss and the overlying metasediments (Money Complex) is interpreted as a Late Palaeozoic intrusive contact. Major arguments in favour of this hypothesis are: (i) the obliquity of the sedimentary layering with respect to the contact; (ii) the presence of aplitic dykes within the Money Complex; (iii) the lack of a mylonitic zone; and (iv) rare relics of an early generation of garnet in the Money metasediments, interpreted as evidence of the contact metamorphism of the Erfaulet granite. To cite this article: B. Le Bayon, M. Ballevre, C. R. Geoscience 336 (2004).

A Tertiary age for Alpine high‐pressure metamorphism in the Gran Paradiso massif, Western Alps: a Rb–Sr microsampling study

AbstractThe timing of high‐pressure (HP) metamorphism in the internal basement massifs of the Western Alps has been contentious. In the Gran Paradiso massif silvery micaschists, thought to have developed from granitic precursors, contain assemblages indicative of pressures in excess of 18 kbar at 500–550 °C. This paper presents unique geochronological data for the paragenesis of the silvery micaschist HP assemblage. Rb–Sr microsampling of an apatite–phengite pair thought to have remained closed to Rb–Sr exchange since the HP paragenesis formed has yielded an age of 43.0 ± 0.5 Ma. Greenschist retrogression occurred after 36.3 ± 0.4 Ma, probably in the interval 36–34 Ma. The localised disturbance of the Rb–Sr system in phengite, apatite and allanite during retrogression means that only in situ microsampling could obtain meaningful ages from these rocks. The new data indicating a Tertiary age for HP metamorphism in the Gran Paradiso massif agree with recent data for other internal basement massifs in the Western Alps. A model fitting the Gran Paradiso massif into the Western Alpine framework is presented.

Brittle post-metamorphic tectonics in the Gran Paradiso Massif (north-western Italian Alps)

New geological mapping (1:10.000 scale) and structural analysis in the Orco valley (NW Italian Alps), give insights on the post-metamorphic brittle tectonic evolution of the Gran Paradiso Unit and adjoining Piemonte Ophiolitic Unit and Sesia-Lanzo Unit. According to existing geochronological data, the metamorphic evolution of the studied domain ended in Early Oligocene and was followed by a progressive exhumation during Neogene. This work describes the brittle fault network developed during the post-metamorphic tectonic stages coeval to exhumation. The geometry and kinematics of this fault network allow to interpret the study area as a part of a larger lowstrain, fault-bounded block, whose main bordering discontinuities are located outside the mapped region. The post-metamorphic kinematic evolution was characterised by: (a) ESE-WNW extension, attested by NNE-SSW normal faults and extensional joints; (b) dextral shearing along a major E-W shear zone (Orco shear zone). The individual faults of the studied area are considered as P- or R-shears of the Orco shear zone, which could be in turn a part of a larger NE-SW regional shear zone bounded to the N by the Chamonix-Rhône- Simplon Line.

Past and present circulation of CO2-bearing fluids in the crystalline Gran Paradiso Massif (Orco Valley, north-western Italian Alps): tectonic and geochemical constraints

In the Orco Valley, inside the Gran Paradiso Massif, 3 main fault systems are present: (a) E–W striking faults dipping at 45–60° to the N, (b) high-angle NW–SE striking faults, and (c) high-angle NE–SW to NNE–SSW striking faults. The E–W striking faults and the interposed NW–SE-faults appear to represent a cogenetic structural association related to a larger scale transtensional shear zone, while the NE–SW faults are probably inherited by an older discontinuous deformation stage. Breccia bodies or veins, mostly consisting of carbonate (siderite–ankerite±calcite)+quartz with sulphide–Au mineralisation, recording a multistage mesothermal evolution, occur along both E–W and NW–SE fault systems. Three water types are recognised: type I, Ca2+–HCO3− waters, with minor SO42−; type II, (Ca2+/Na+)–HCO3− waters varying towards Na+–(HCO3−/Cl−) waters; and type III, Mg2+–HCO3− waters. Type I and type III groundwaters are freshly recharged waters, only slightly exchanged with rocks. Type II includes waters which come into contact with carbonate fracture fillings, and geochemical modelling indicates that dissolution of carbonates along fractures is the main process controlling the groundwater chemistry. These waters evolve in a system open to uprising CO2, and their strongly negative δ13CCO2 suggests a substantial organic component in the CO2 discharge. In the past, CO2-bearing fluids were likely responsible for the formation of Fe-bearing carbonate fracture fillings. The persistence through time of the CO2 flux in the region has important implications for the reconstruction of the Alpine tectonic evolution and deep structure.

Structure and metamorphism of the Gran Paradiso massif, western Alps, Italy

The pressure–temperature-time trajectory and structural history of high-pressure rocks presently exposed in the Gran Paradiso massif provide constraints on the processes that caused their thermal evolution and exhumation. High-pressure metamorphism of the rocks is found to have culminated at temperatures around 525 °C and pressures of 12 to 14 kbar. After high-pressure metamorphism, the rocks cooled during initial decompression, while undergoing top-to-the-west shear on chlorite-bearing shear bands and larger scale shear zones. Biotite-bearing shear bands and larger shear zones related to top-to-the-east deformation affected the Gran Paradiso massif during reheating to temperatures of around 550 °C at 6 to 7 kbar. Further exhumation occurred at relatively high temperatures. A potentially viable explanation of the observed stage of reheating before final cooling and exhumation is breakoff of a subducting slab in the upper mantle, allowing advective heat transfer to the base of the crust. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00410-001-0357-6.

Tertiary age and paleostructural inferences of the eclogitic imprint in the Austroalpine outliers and Zermatt–Saas ophiolite, western Alps

The Austroalpine Sesia–Lanzo inlier and upper Austroalpine Dent Blanche, Mt. Mary and Pillonet outliers occur on top of the western-Alpine orogenic wedge and, as a whole, override the structurally composite ophiolitic Piemonte zone. Instead, the Mt. Emilius, Glacier–Rafray, Etirol–Levaz and other lower Austroalpine eclogitic outliers are inserted within the Piemonte zone, between its upper (Combin) and lower (Zermatt–Saas) tectonic elements, or within the latter. Rb–Sr dating on phengitic micas show that the eclogitic imprint in the lower Austroalpine outliers, conventionally regarded as Late Cretaceous by comparison with the Sesia–Lanzo inlier, is of Eocene age (49–40 Ma), like the underlying Zermatt–Saas ophiolite (45–42 Ma) between the Aosta valley and Gran Paradiso massif. 40Ar–39Ar plateau ages on the same mica concentrates of the ophiolitic Zermatt–Saas nappe (46–43 Ma) are consistent with Rb–Sr dating, whereas that on the Austroalpine Glacier–Rafray klippe (92 Ma) is influenced by argon excess. The lower Austroalpine outliers underwent the subduction metamorphism concurrently with the Zermatt–Saas nappe, 20–25 Ma later than the eclogitic Sesia–Lanzo inlier and blueschist Pillonet klippe. The temporal gap and present intra-ophiolitic position mean that the lower Austroalpine outliers were probably derived from an intraoceanic extensional allochthon (Mt. Emilius domain) stranded inside the Piemonte–Ligurian ocean far from the Dent Blanche–Sesia domain and Adriatic margin.

The Aosta–Ranzola extensional fault system and Oligocene–Present evolution of the Austroalpine–Penninic wedge in the northwestern Alps

Post-nappe brittle structures across the Austroalpine–Penninic wedge are representative of two principal deformation stages. The former (D1) developed during the Late Oligocene, the latter (D2) from the Miocene to the Present. Major D1 features are the E–W Aosta–Ranzola half-graben and the NE–SW Ospizio Sottile subvertical normal fault system. Oligocene extension was also characterized by hydrothermal activity and intrusion of calc-alkaline to ultra-potassic dikes. From the Miocene onwards (D2), the Ospizio Sottile system was re-activated as a SE sinistral boundary of the Pennine–Graian Alps block of nappes, bodily moving away from the Simplon detachment to the SW. Inside the escaping block, a number of D2 slightly diverging sublatitudinal splays (Aosta–Piccolo San Bernardo system) was generated, as well as a regular set of NW-trending normal faults from the Simplon to the Gran Paradiso massif. Instead, the Aosta–Ranzola system was poorly reworked in that time. More generally, the short-lived Oligocene extension (D1) was associated to slab break-off of lower plate, wedge rebound, rapid uplift and thermal perturbation. From the Early Miocene onwards the Oligocene extension was replaced by renewed compression, allowing the vertical extrusion of the Lepontine dome and lateral escape of the overlying Pennine–Graian Alps block.

Combination of Single-Grain Fission-Track Chronology and Morphological Analysis of Detrital Zircon Crystals in Provenance Studies: Sources of the Macigno Formation (Apennines, Italy)

Fission track (FT) analyses on unannealed detrital min- erals provide a powerful tool both for refining provenance models de- rived from traditional methods and for collecting information about erosion rates of the source area. Their power is increased if they are coupled with the study of zircon morphology. This combination of methods is applied to the Chattian-Aquitanian (25-23 Ma) Macigno turbidite complex. Basin-fill patterns and petrographical studies con- sistently identify the uplifting western Central Alps as the main source region for the Macigno Formation. Most zircon grains fall into a young age cluster ( ; 40-30 Ma), de- rived from a rapidly exhuming crystalline source region with a high cooling rate. Within this cluster, two age subgroups can be distin- guished at 30 and 40 Ma. In the younger subgroup, the zircon mor- phology supports the presence of two main populations: (1) from ig- neous rocks (S-type euhedral zircons), which appear to be partly de- rived from airborne tuffs; and (2) from metasedimentary units. In huge volumes of these metamorphic rocks, mica Ar-Ar and zircon fission- track thermochronometers have been reset, because of high geothermal gradients in the vicinity of the Periadriatic intrusives in mid-Oligocene times. At the present surface of the Alps, zircon FT ages around and slightly less than 30 Ma are reported in the Sesia-Lanzo zone, the Gran Paradiso Massif, the Upper Pennine nappes, the Monte Rosa Massif, and the Dent Blanche complex. The older subgroup of the Tertiary zircons (40 Ma) may have been supplied by metamorphic and mig- matitic rocks affected by an Eocene high-temperature phase. A Late Cretaceous age cluster ( ; 70-60 Ma) is related to cooling after the main Austroalpine metamorphic event at 110-100 Ma. Most of the recently exposed Austroalpine nappe complex displays mica cooling ages and zircon FT ages between 95-70 Ma and 99-55 Ma, respectively. Finally, an ill-defined Jurassic age cluster, with a mean in Late Ju- rassic times, is related to rift-shoulder heating of the Austroalpine/ South-Alpine crystalline basement due to rifting of the Pennine oceanic domain. Presently, the Silvretta nappe complex, situated at the western termination of the Austroalpine realm, and the South-Alpine basement west of the Canavese Line, display similar zircon FT ages. Therefore, a westward continuation of the Silvretta complex prior to deep Neo- gene erosion is suggested.

Extension syn-convergence, poinçonnement vertical et unités métamorphiques contrastées en bordure ouest du Grand Paradis (Alpes Franco-Italiennes)

The Piemontese zone, at the western border of the Gran Paradiso massif (western Alps) consists of the oceanic schistes lustrés units and the continental Grand Paradiso (GP) unit. In complement to previous work, tectono-metamorphic investigations allow us to establish pressure gaps between the three studied units with: the upper schistes lustrés (LS) unit, metamorphosed under blueschist facies conditions (9.5 ± 2 kbar, 340 ± 30 °C). The lower schistes lustrés (LI) unit, metamorphosed under low temperature eclogite facies conditions (12.5 ± 3 kbar, 480 ± 50 °C). The Gran Paradiso (GP) unit, metamorphosed under higher pressure and temperature eclogite facies conditions (12 to 20 kbar, 500 ± 50 °C). The whole metamorphic pile recorded a decompression under amphibolite facies conditions for the LI unit and in the greenschist facies conditions for the LS unit and GP unit, i.e. with a slight and late temperature increase. Fluid inclusion studies on quartz in late cracks and shear planes (C’) show that the three units were juxtaposed and deformed together under metamorphic conditions of 4 ± 1 kbar, and 400 ± 50 °C. A finite strain field study reveals regional scale extensional tectonics beginning under greenschist facies conditions and ending under brittle conditions. Ductile greenschist deformation corresponds to dome and basin trajectories of the foliation surface. A partition is observed between domains where a pure shear regime prevails in the core of the domes and domains where a simple shear regime prevails, towards the WNW, at the rim of the domes, at the contact between units. Brittle deformation corresponds to a continuum of the ductile deformation. Meanwhile, metamorphic contrasts suggested by pressure estimates cannot be ruled by the relative extension-related displacements. This extension corresponds to the accommodation of the vertical indentation of the high pressure units by the stacking of deep crustal slices, in front of the rigid Apulian mantle back-stop, during continuous convergence.

Syn-convergent exhumation implied by progressive deformation and metamorphism in the Valle dell’Orco transect, NW Italian Alps

Structural, metamorphic and geochronological data from the Valle del'Orco are best explained by an orogenic wedge model in which crustal slices from continent and ocean acted as passive blocks at different times in the Alpine orogeny. Deformation generally migrated westwards in this section, and metamorphic recrystallization at any given point was largely controlled by the pervasiveness of strain. The data suggest a dynamic crustal stacking system lasting from 70 to 35 Ma, rather than continental collision following long-lived B-type subduction, and offer a relevant and simple model for eclogite exhumation that requires exhumation of ductile shear-bounded slices within the orogenic wedge. The easternmost, internal unit of the Sesia-Lanzo Zone, the Eclogitic Micaschist Complex, was subducted and pervasively recrystallized under eclogite facies conditions during the early stages of convergence in the late Cretaceous. During the development of the major preserved structures, the eastern Eclogitic Micaschist Complex was undergoing general non-coaxial shear with an overall top-down to the east vorticity, suggesting a potential mechanism of exhuming an eclogitic 'pip'. Slices of pre-Alpine basement, the Vasario-Sparone Subunit, were incorporated into the lower (western) parts of this unit during the latest part of this phase, but do not show penetrative effects of the early Alpine deformation. They were passively folded during the early decompressional part of the P—T history, prior to the unit being juxtaposed against the underlying Gneiss Minuti Complex under blueschist-greenschist facies conditions. The Gneiss Minuti Complex itself was then recrystallized at high strain in the greenschist-facies in the latest Eocene. Fabrics developed at this stage indicate top-down-to-east extension which may have accommodated the exhumation of eclogitized Piemont oceanic material and the European continental material of the Gran Paradiso massif. Evidence of northwest-directed thrusting within the Gran Paradiso unit at the same time suggests again that the crustal block was transported on a deeper reverse-sense shear zone that presumably links to the more external thrusts now exposed to the west. High-pressure mineral lineations within the Piemont zone cannot be unequivocally assigned to either the burial or exhumation paths, but are consistent with the generally east-west transport directions observed in the structurally higher units.

Can crustal extension be distinguished from thrusting in the internal parts of mountain belts? A case history of the Entrelor shear zone, Western Alps

Criteria for the distinguishing between structures that accommodate crustal-scale extension or crustal-scale shortening may be applied to late orogenic shear zones developed in the internal parts of mountain belts. This is done for a large shear zone that has been mapped in the NW Italian Alps, around the internal basement massif of Gran Paradiso. The structure involved is termed the Entrelor shear zone. It carries greenschist facies European continental rocks (the Briançonnais—Grand St. Bernard unit) onto an old subduction complex of oceanic (Piemonte) and continental (Gran Paradiso) material that preserve eclogitic assemblages. Evidence for the map-scale continuity of the Entrelor shear zone is presented together with kinematic data to show how folds, stretching lineations and shear criteria relate to a regional episode of ESE-directed shearing. However, folds initiated systematically oblique to the shear direction, on a NE-SW axis. With continued shearing, the fold population remains skewed E-W of the population of stretching lineations. The shear zone may be traced for 70 km around the dome of the Gran Paradiso massif and displays a minimum displacement of 20 km. This Entrelor structure shows variable offset of metamorphic grade. However, for much of its length, greenschist facies rocks are carried in its hanging-wall onto eclogites in the footwall. Tracing these units regionally shows that this apparent extensional offset of metamorphic facies is the result of restacking of an originally ‘inverted’ metamorphic sequence (i.e. HP Piemonte on LP Briançonnais). This contractional behavior is consistent with the profile of the shear zone relative to the modern surface. The Entrelor shear climbs up from generally buried in the west to generally eroded in the east. The conclusion that the Entrelor shear zone is probably a thrust rather than an extensional structure relies on linking outcrop data into a semi-regional context. False interpretations may result from examining only small parts of the structure. However, although these map and structural criteria are consistent with contractional kinematics associated with crustal shortening, confirmation is required through linked petrological and isotopic studies. The Entrelor shear zone is thus a good illustration of the need for caution in inferring the contractional or extensional nature of structures developed late in the history of the internal parts of mountain belts.

Constraints on the late thermotectonic evolution of the western Alps: EVidence for episodic rapid uplift

Forty‐two new apatite and zircon fission track ages are presented for samples from the Western Alps in southern Switzerland, northern Italy, and southeastern France. Measured ages plotted against assumed closure temperatures yield cooling patterns for the final cooling, uplift, and exhumation of the Western Alps. Similar fission track zircon ages in the Penninic Gran Paradiso massif, Dent Blanche nappe, Sesia‐Lanzo Zone, and Ivrea Zone indicate cooling of all four units to ∼225°C by 33 Ma. Differences in apatite ages reveal differential cooling of the four blocks between 33 Ma and the present. In the Sesia‐Lanzo Zone, similarity of apatite ages regardless of elevation, together with near‐volcanic confined fission track length patterns suggest rapid cooling and uplift at ∼25 Ma compared with slow cooling of other Western Alps units around 12 Ma. Uplift is thus not continuous but episodic, often over a short time interval beyond the resolution of other methods. Such episodes of uplift, as revealed here in the Sesia‐Lanzo Zone, may be the rule rather than the exception.

Tectonic interpretation of an inverse gradient of zircon fission-track ages with respect to altitude: alpine thermal history of the Gran Paradiso basement

Extensive sampling along vertical and north-south cross-sections in the Gran Paradiso basement (internal crystalline massif, Western Alps) reveals an inverse gradient of zircon fission-track ages with respect to altitude, which is explained by an “inverted metamorphism” during Eocene time. Subsequent to the obduction of the Schistes Lustres nappe (high pressure Eoalpine phase), the Gran Paradiso massif cooled down 90-80 Myrs ago. The Austro-Alpine nappe of Monte Emilius — Dent Blanche overthrust the Gran Paradiso massif 38–40 Myrs ago, inducing in its basement an inverted metamorphism or, more precisely, a “nappe emplacement metamorphism” (greenschist facies). The last cooling below 100° C occurred as early as 35—40 Myrs ago in the southern Gran Paradiso massif; in the northern part, this last cooling occurred 20 Myrs ago, accompanied by an important uplift phase (2 mm/year).