-A new structural map of the Paleozoic crust of the Pyrenees based on an extensive compilation and new kinematic data allows for the evaluation of the mechanical coupling between the upper and lower crust of the abnormally hot foreland of the Variscan orogen of SW Europe. We document partitioning between coeval lower crustal lateral flow and upper crustal thickening between 310 and 290 Ma under an overall dextral transpressive regime. Partitioning also involved syn-convergence transtensional gneiss domes emplacement during this period. Late orogen-normal shortening of the domes and strain localization in steep crustal-scale transpressive shear zones reflects increasing coupling between the lower crust and the upper crust. The combination of dextral transpression and eastward flow in the Pyrenees results from the shortening and lateral escape of a hot buoyant crust along the inner northern limb of the closing Cantabrian orocline at the core of the Iberian-Armorican arc between ca. 305 and 295 Ma. Delamination or thermal erosion of the lithosphere enhanced orocline closure and explains (1) the switch from crust-to mantlederived magmatism in the Iberian-Armorican arc and (2) the abnormally hot and soft character of the Pyrenean crust that escaped the closing syntax.Keywords: Variscan / Pyrenees / syntax / orocline / gneiss dome / transpression / hot orogen Résumé -Partitionnement vertical de la déformation dans la croûte varisque chaude : échappement des Pyrénées de la syntaxe ibéro-armoricaine. Une nouvelle carte structurale harmonisée et une série de coupes de la croûte paléozoïque des Pyrénées sont présentées sur la base d'une compilation et de nouvelles observations de terrain. Cette carte permet d'évaluer les modalités du couplage mécanique entre la croûte supérieure et la croûte inférieure de l'avant-pays anormalement chaud de l'orogène varisque. La croûte inférieure a flué latéralement pendant l'épaississement de la croûte supérieure dans un contexte transpressif dextre entre 310 et 290 Ma. Dans ce contexte convergent et grâce à un couplage crustal partiel, la croûte inférieure a pu être partiellement exhumée dans des dômes gneissiques transtensifs. Le serrage tardif des dômes et la localisation progressive de la déformation dans des zones de cisaillement transpressives d'ampleur régionale attestent de l'augmentation du degré de couplage entre croûtes supérieure et inférieure pendant le refroidissement régional. La combinaison de la transpression dextre et du fluage latéral de la croûte des Pyrénées résulte du raccourcissement et de l'échappement du matériau piégé dans la syntaxe ibéro-armoricaine lors de son amplification et la fermeture de l'orocline cantabrique en son coeur entre 305 et 295 Ma. L'érosion thermique ou la délamination du manteau lithosphérique a permis (1) la transition entre un magmatisme crustal et un magmatisme mantellique ou hybride dans la syntaxe et (2) la fermeture de l'orocline et l'échappement contemporain du matériel pyrénéen chaud qu'elle contenait.

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International audienceGarnet-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. Fromthermocalc-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

Permian granulites associated with noritic intrusions and websterites are a common feature of the post-Variscan European crust. Such granulites are common in the Southern Alps (e.g. Ivrea Zone), but occur only in the Gruf Complex in the Central Alps. To understand the geotectonic significance of these granulites, in particular in the context of Alpine migmatisation, zircons from 15 highgrade samples have been U-Pb dated by SHRIMP II analysis. Oscillatory zoned zircons from charnockite sheets, interpreted as melts generated through granulite facies fluid-absent biotite melting at 920-940°C, yield ages of 282-260 Ma. Some of these zircons contain inclusions of opx, unequivocally attributable to the granulite facies, thus confirming a Permian age for the charnockites and associated granulites. Two samples from an enclave-rich orthogneiss sheet yield Cambrian and Ordovician zircon cores. Two deformed leucogranites and six ortho-and augengneisses, which compose two-thirds of the Gruf Complex, give zircon ages of 290-260 Ma. Most zircons have milky rims with ages of 34-29 Ma. These rims date the Alpine amphibolite facies migmatisation, an interpretation confirmed by directly dating a leucosome pocket from upper amphibolite facies metapelites. The Gruf charnockites associated with metre-scale schlieren and boudins of opx-sapphirine-garnet-granulites, websterites and gabbronorites can thus be identified as part of the postVariscan European lower crust. A geotectonic reconstruction reveals that this piece of lower crust stranded in the (European) North upon rifting of the Neotethys, such contrasting the widespread granulite units in the Southern Alps. Emplacement of the Gruf lower crust into its presentday position occurred during migmatisation and formation of the Bergell Pluton in the aftermath of the breakoff of the European slab.

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We describe field occurrences of sapphirinebearing granulites, charnockites and migmatites in the Gruf complex, Central Alps and present a new geological map and a structural analysis of the entire Gruf complex for the first time. We have carried out an accurate analysis of the relationships between granulite facies metamorphism, migmatisation and deformation within the complex, in relation to the intrusion of the Bergell pluton. Granulites and charnockites display fabrics different from those defined by the regional foliation and lineation, which are, typically for migmatites, disordered on the mesoscale. On a regional scale, strike variations are also related to the structural complexity of migmatites within which no major antiform could be identified. Irregular interfingering of subparallel leucosome veins and back-veining along the contact between the Gruf migmatites and the Bergell tonalite are evidence for contemporaneous emplacement and crystallisation at about 740°C and 6.5-7.5 kbar in Oligocene times (ca 30 Ma). Metamorphic conditions in the charnockites and granulites ([920°C for 8.5-9.5 kbar) largely exceed these regional metamorphic conditions and are dated at 282-260 Ma. We propose that the ascending Bergell pluton entrained the polymetamorphic, granulitic lower crust enclosed within the peripheral migmatitic Gruf complex.

Mafic eclogites are found in many orogens as lenses embedded in quartzofeldspathic migmatites. These high‐pressure relics are interpreted either as remnants of ancient sutures and thus formed during oceanic subduction or as fragments of lower crust exhumed from the root of orogenic thickened crust. It is critical to distinguish between these two endmember scenarios as the resulting palaeogeographic and geodynamic reconstructions may significantly differ. In this contribution, we investigated eclogite relics from Lac Cornu in the Aiguilles‐Rouges massif, one of the External Crystalline Massifs of the Western Alps. Phase equilibrium modelling suggests that these mafic rocks were buried along a prograde path (M1) from ~600°C/1.2 to 1.6 GPa to peak conditions of ~630–775°C and >1.6 GPa. Zircon rims, with a rare earth element signature typical of eclogite facies zircon (no Eu anomalies, flat HREE spectrum), and rutile were dated by U–Th–Pb laser ablation inductively coupled plasma mass spectrometry (LA‐ICPMS) at c. 335–330 Ma. Prograde deformation has not been identified in the field and is only recognized thanks to the crystallographic preferred orientation (CPO) of inclusions of omphacite and rutile in garnet. Peak pressure conditions were followed by a decompression stage (M2) from ~760°C/1.4 GPa to ~600–650°C/0.9 GPa supported by the breakdown of omphacite into plagioclase–clinopyroxene symplectite and the crystallization of plagioclase–amphibole corona around garnet. The M2 retrogression stage is associated with the development of a main sub‐horizontal planar fabric and the CPO of minerals composing symplectite. This deformation stage is interpreted as the result of horizontal lower crustal flow. The final stage of exhumation (M3) is characterized by the replacement of symplectite and garnet by plagioclase and large euhedral amphibole and by the breakdown of rutile and ilmenite into titanite dated at c. 300 Ma. The CPO of titanite and amphibole are consistent with the development of vertical dextral shear zone in a transpressive regime. The combination of field observations and petrological, microtextural and geochemical analyses suggests that the mafic eclogites preserved in migmatitic rocks of the Aiguilles‐Rouges massif are remnants of a continental lower crust exhumed and juxtaposed with lower‐grade migmatites in crustal‐scale vertical transpressive shear zones.