Late Carboniferous paleoelevation of the Variscan Belt of Western Europe

Received 9 September 2020 Received in revised form 17 May 2021 Accepted 13 June 2021 Available online xxxx Editor: J.-P. Avouac Keywords: stable isotope paleoaltimetry meteoric fluids shear zone detachment Variscan Carboniferous 1. Introduction The Variscan belt, extending from South America to East Asia through North America and Central Europe, is a Himalayan-type collision belt that resulted from protracted convergence between the Laurentia-Baltica and Gondwana lithospheric plates between ∼ 410 and ∼ 310 Ma (e.g. Matte, 2001). This extensive mountain belt exposes vast amounts of granites, migmatitic complexes and granulite facies rocks and is considered a “hot orogen” character- ized by crustal thickening, syntectonic crustal melting, high-grade metamorphism, and syn- to post-convergence gravitational col- lapse (Fig. 1; e.g. Gébelin et al., 2009; Vanderhaeghe et al., 2020). * Corresponding author. E-mail address: [email protected] (C. Dusséaux). https://doi.org/10.1016/j.epsl.2021.117064 0012-821X/© 2021 Elsevier B.V. All rights reserved. abstract We present the first stable isotope paleoaltimetry estimates for the hinterland of the eroded Variscan Belt of Western Europe based on the hydrogen isotope ratios of muscovite from syntectonic leucogranites that have been emplaced at ∼ 315 Ma. We focus on the Limousin region (Western Massif Central, France) where peraluminous granites are spatially associated with strike-slip and detachment shear zones that developed as a consequence of Late Carboniferous syn- to post-orogenic extension. In this region, we show that the north-east corner of the Millevaches massif (located at the junction between brittle and ductile fault systems) represented a pathway for Earth surface-derived fluids that penetrated the crust and reached the ductile segment of the low-angle Felletin detachment zone. Using microstructural, thermometry, hydrogen isotope and 40Ar/39Ar geochronological data, we show that these Variscan meteoric fluids interacted with hydrous silicates during high temperature deformation between ∼ 318 and 310 Ma. For paleoaltimetry purposes, we reference our hydrogen isotope record (δD) of ancient meteoric fluids from mylonitic rocks to ∼ 295 Myr-old records retrieved from freshwater shark remains preserved in the Bourbon l’Archambault basin that developed in the external zones of the orogen. A ∼ 76 difference in δDmeteoric water values between the Millevaches massif (δDmeteoric water value = − 96 ± 8) and the Bourbon l’Archambault foreland basin (δDwater value = − 20 ± 6) is consistent with paleoaltimetry estimates of 3.4 ± 0.7 km based on a modern lapse rate of ∼ -22/km for δDwater values. The rather large difference in δD values between the foreland basin and the continental interior suggests that the hinterland of the Variscan belt of western Europe was high enough to act as a barrier to moisture transport from the south-south-east and induce an orographic rain shadow to the north

Eocene and Miocene extension, meteoric fluid infiltration, and core complex formation in the Great Basin (Raft River Mountains, Utah)

Metamorphic core complexes (MCCs) in the North American Cordillera reflect the effects of lithospheric extension and contribute to crustal adjustments both during and after a protracted subduction history along the Pacific plate margin. While the Miocene-to-recent history of most MCCs in the Great Basin, including the Raft River-Albion-Grouse Creek MCC, is well documented, early Cenozoic tectonic fabrics are commonly severely overprinted. We present stable isotope, geochronological (40Ar/39Ar), and microstructural data from the Raft River detachment shear zone. Hydrogen isotope ratios of syntectonic white mica (δ2Hms) from mylonitic quartzite within the shear zone are very low (-90‰ to -154‰, Vienna SMOW) and result from multiphase synkinematic interaction with surface-derived fluids. 40Ar/39Ar geochronology reveals Eocene (re)crystallization of white mica with δ2Hms ≥ -154‰ in quartzite mylonite of the western segment of the detachment system. These δ2Hms values are distinctively lower than in localities farther east (δ2Hms ≥ -125‰), where 40Ar/39Ar geochronological data indicate Miocene (18-15 Ma) extensional shearing and mylonitic fabric formation. These data indicate that very low δ2H surface-derived fluids penetrated the brittle-ductile transition as early as the mid-Eocene during a first phase of exhumation along a detachment rooted to the east. In the eastern part of the core complex, prominent top-to-the-east ductile shearing, mid-Miocene 40Ar/39Ar ages, and higher δ2H values of recrystallized white mica, indicate Miocene structural and isotopic overprinting of Eocene fabrics

Flow of partially molten crust controlling construction, growth and collapse of the Variscan orogenic belt: 1 the geologic record of the French Massif Central

We present here a tectonic-geodynamic model for the generation and flow of partially molten rocks and for magmatism during the Variscan orogenic evolution from the Silurian to the late Carboniferous based on a synthesis of geological data from the French Massif Central. Eclogite facies metamorphism of mafic and ultramafic rocks records the subduction of the Gondwana hyperextended margin. Part of these eclogites are forming boudins-enclaves in felsic HP granulite facies migmatites partly retrogressed into amphibolite facies attesting for continental subduction followed by thermal relaxation and decompression. We propose that HP partial melting has triggered mechanical decoupling of the partially molten continental rocks from the subducting slab. This would have allowed buoyancy-driven exhumation and entrainment of pieces of oceanic lithosphere and subcontinental mantle. Geochronological data of the eclogite-bearing HP migmatites points to diachronous emplacement of distinct nappes from middle to late Devonian. These nappes were thrusted onto metapelites and orthogneisses affected by MP/MT greenschist to amphibolite facies metamorphism reaching partial melting attributed to the late Devonian to early Carboniferous thickening of the crust. The emplacement of laccoliths rooted into strike-slip transcurrent shear zones capped by low-angle detachments from c. 345 to c. 310 Ma is concomitant with the southward propagation of the Variscan deformation front marked by deposition of clastic sediments in foreland basins. We attribute these features to horizontal growth of the Variscan belt and formation of an orogenic plateau by gravity-driven lateral flow of the partially molten orogenic root. The diversity of the magmatic rocks points to various crustal sources with modest, but systematic mantle-derived input. In the eastern French Massif Central, the southward decrease in age of the mantle- and crustal-derived plutonic rocks from c. 345 Ma to c. 310 Ma suggests southward retreat of a northward subducting slab toward the Paleotethys free boundary. Late Carboniferous destruction of the Variscan belt is dominantly achieved by gravitational collapse accommodated by the activation of low-angle detachments and the exhumation-crystallization of the partially molten orogenic root forming crustal-scale LP migmatite domes from c. 305 Ma to c. 295 Ma, coeval with orogen-parallel flow in the external zone. Laccoliths emplaced along low-angle detachments and intrusive dykes with sharp contacts correspond to the segregation of the last melt fraction leaving behind a thick accumulation of refractory LP felsic and mafic granulites in the lower crust. This model points to the primordial role of partial melting and magmatism in the tectonic-geodynamic evolution of the Variscan orogenic belt. In particular, partial melting and magma transfer (i) triggers mechanical decoupling of subducted units from the downgoing slab and their syn-orogenic exhumation; (ii) the development of an orogenic plateau by lateral flow of the low-viscosity partially molten crust; and, (iii) the formation of metamorphic core complexes and domes that accommodate post-orogenic exhumation during gravitational collapse. All these processes contribute to differentiation and stabilisation of the orogenic crust

Structure of late Variscan Millevaches leucogranite massif in the French Massif Central: AMS and gravity modelling results

In the Limousin area, Variscan leucogranitic plutons are spatially associated with normal faults and major strike-slip shear zones that are a continuation of the South Armorican shear zone. Our study focuses on the large N-S-trending Millevaches granitic massif (Massif Central, France), and intends to highlight, through gravity modelling, structural and anisotropy of magnetic susceptibility (AMS), the massif structure at depth and to discuss the mode of emplacement of granites within a strike-slip tectonic context. The mica subfabric suggests that the magnetic foliations display a general NW-SE sub-horizontal pattern on both sides of the N-S Pradines dextral wrench fault zone that deforms the core of the massif on 5 km width. The magnetic lineation trend exhibits a sigmoïdal pattern, N-S in the Pradines fault zone and NW-SE on both sides of it, which are consistent with a dextral wrench component. The horizontal magnetic foliations and lineations are consistent with the thin granite laccolith model. There is no significant imprint of the extensional Variscan belt collapse on the internal fabric of Millevaches granites than the tectonic dextral transcurrent movement prevailing in this area. © 2005 Elsevier Ltd. All rights reserved

Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France: Structural and ⁴⁰Ar/³⁹Ar investigations

New structural, microstructural, and 40Ar/39 Ar data from the NW Massif Central (France) provide additional constraints on the timing and tectonic setting of late Variscan granite magmatism. Previous studies had emphasized the role of late orogenic extension in the emplacement of granite plutons in the Limousin region. In contrast, the new data set is consistent with syntectonic emplacement of magma in a dextral simple shear active from 350 to 300 Ma in a transpressional regime. As an alternative hypothesis to late orogenic extension, we propose that magmas migrated into tensional bridges between active P shears associated with a lithospheric shear zone comparable to a pop-up structure. The Galician region, in the western end of the Ibero-Armorican tectonic arc, exhibits major left-lateral ductile shear zones which can be interpreted as conjugate structures to the Limousin and Armorican shear zones. Copyright 2007 by the American Geophysical Union

Oligo-Miocene extensional tectonics and fluid flow across the Northern Snake Range detachment system, Nevada

The Northern Snake Range (Nevada) represents a spectacular example of a metamorphic core complex and exposes a complete section from the mylonitic footwall into the hanging wall of a fossil detachment system. Paired geochronological and stable isotopic data of mylonitic quartzite within the detachment footwall reveal that ductile deformation and infiltration of meteoric fluids occurred between 27 and 23 Ma. 40Ar/39Ar ages display complex recrystallization-cooling relationships but decrease systematically from 26.9 ± 0.2 Ma at the top to 21.3 ± 0.2 Ma at the bottom of footwall mylonite. Hydrogen isotope (δD) values in white mica are very low (-150 to-145 ‰) within the top 80-90 m of detachment footwall, in contrast to values obtained from the deeper part of the section where values range from-77 to-64 ‰, suggesting that time-integrated interaction between rock and meteoric fluid was restricted to the uppermost part of the mylonitic footwall. Pervasive mica-water hydrogen isotope exchange is difficult to reconcile with models of 40Ar loss during mylonitization solely by volume diffusion. Rather, we interpret the 40Ar/ 39Ar ages of white mica with low-δD values to date syn-mylonitic hydrogen and argon isotope exchange, and we conclude that the hydrothermal system of the Northern Snake Range was active during late Oligocene (27-23 Ma) and has been exhumed by the combined effects of ductile strain, extensional detachment faulting, and erosion. Copyright 2011 by the American Geophysical Union

Seismic anisotropy of the Archean crust in the Minnesota River Valley, Superior Province

The Minnesota River Valley (MRV) subprovince is a well-exposed example of late Archean lithosphere. Its high-grade gneisses display a subhorizontal layering, most likely extending down to the crust-mantle boundary. The strong linear fabric of the gneisses results from high-temperature plastic flow during collage-related contraction. Seismic anisotropies measured up to 1 GPa in the laboratory, and seismic anisotropies calculated through forward-modeling indicate ΔVP ~5-6% and ΔVS ~3%. The MRV crust exhibits a strong macroscopic layering and foliation, and relatively strong seismic anisotropies at the hand specimen scale. Yet the horizontal attitude of these structures precludes any substantial contribution of the MRV crust to shear wave splitting for vertically propagating shear waves such as SKS. The origin of the regionally low seismic anisotropy must lie in the upper mantle. A horizontally layered mantle underneath the United States interior could provide an explanation for the observed low SWS. Key Points The Archean crust of the Minnesota River Valley is strongly anisotropic The horizontally layered crust of the MRV cannot split vertical shear waves The cause of low SWS in the MRV must be in the uppermost mantle ©2014. American Geophysical Union. All Rights Reserved

Déformation et mise en place des granites (360-300Ma) dans un segment de la Chaîne Varisque (Plateau de Millevaches, Massif Central)

M. Maurice BRUNEL Professeur, Université de Montpellier II -Directeur de ThèseM. Michel FAURE Professeur, Université d'Orléans - Directeur de Thèse M. Jean-Marc LARDEAUX Professeur, Université de Nice RapporteurM. Didier MARQUER Professeur, Université de Besançon Rapporteur M. Philippe ROSSI Ingénieur BRGM, Orléans ExaminateurM. Patrick MONIÉ CR1 CNRS Montpellier ExaminateurM. Patrick LEDRU Ingénieur BRGM, Orléans InvitéM. Philippe MATTE DR, CNRS Montpellier InvitéThe Limousin area is situated in the North West French Massif central and is characterized by large granitic plutons emplaced between 360 and 290 Ma. These plutons display close relationships with normal faults and strike slip faults forming the SE extension of the South-Armorican shear zone. The large N-S trending granitic complex of the Millevaches, limited by wrench faults and normal faults, displays structures characteristic of a emplacement within a strike-slip tectonic context.Structural and microstructural analyses, AMS, gravity analysis and geochronology (40Ar/39Ar et U/Pb) allow us to propose a emplacement model for the Millevaches granite.Magma ascent proceeds through vertical narrow ducts by successive injections along the NS-oriented principal axis of thePradines fault. Magmas are then trapped and channeled in the previously-formed flat-lying micaschist foliation which constitutes a major mechanical anisotropy of the middle crust. Synkinematic plutons emplaced in the dextral wrenching Pradines fault record N-S trending deformation trajectories in the Pradines fault and NW-SE on both sides of it The magma rising towards the roof of the laccolithe induces an oblate coaxial deformation accommodated by flat-lying normal faults. The crystallization of synkinematic leucogranites of the Millevaches dated at 313 ± 4 Ma, is coeval with the granulitic metamorphism which affected the surrounding micaschists. Within the Limousin area, the onset of wrench tectonic is dated around 350 Ma and deformation ends around 300 Ma. We suggest that the two generations of granite (granodiorite-monzogranite and leucogranite) were emplaced around 350 Ma in a tectonic belt resulting from a transpressive tectonic context in response to the continental collision between Laurentia and Gondwana. Ductile shear zones constitute branch of a large and single wrenching lithospheric system similar to a NW-SE trending dextral “pop-up structure” going from the south Armorican Massif to the Limousin.Le Limousin (NW du Massif Central) est caractérisé par de larges massifs granitiques mis en place entre 360 et 290 Ma. Ils présentent d'étroites relations spatiales avec de grands accidents ductiles en faille normale et décrochement qui prolongent vers le SE la zone de cisaillement Sud Armoricaine. Le volumineux (~ 10000km3) complexe granitique N-S de Millevaches, limité par des décrochements et failles normales, est un exemple type de granite mis en place dans un contexte tectonique décrochant. Le modèle de mise en place des granites de Millevaches prend en compte l'analyse structurale, microstructurale, magnétique (A.S.M.), gravimétrique et géochronologique (40Ar/39Ar et U/Pb). L'ascension des magmas se fait par des conduits verticaux étroits sous forme d'injections successives qui se relaient le long de l'axe principal N-S des Pradines. Les magmas sont ensuite piégés puis canalisés par la foliation précoce, anisotropie mécanique sub-horizontale majeure de la croûte moyenne. Les magmas syntectoniques du décrochement dextre N-S des Pradines enregistrent des trajectoires de déformation orientées N-S dans la faille et NW-SE de part et d'autre. La poussée du magma au toit du laccolite induit une déformation par aplatissement relaxée par le développement de failles d'échappement sub-horizontales et normales. La mise en place syntectonique des leucogranites du Millevaches, datée à 313 ± 4 Ma est contemporaine du métamorphisme granulitique subi par les roches encaissantes. Le fonctionnement des décrochements du Limousin débute vers 350 Ma et finit vers 300 Ma. Nous proposons que les deux générations de granites (granodiorite-monzogranite et leucogranite) se mettent en place dès 350 Ma, dans une ceinture tectonique résultant d'un contexte en transpression. Les cisaillements ductiles constituent les branches d'un large, long (~700 km), et unique système décrochant lithosphérique analogue à une « pop-up structure » NW-SE dextre allant du Massif Sud Armoricain au Limousin

Late Carboniferous paleoelevation of the Variscan Belt: astable isotope paleoaltimetry study in the French MassifCentral

National audienceWe present the first stable isotope paleoaltimetry estimates for the internal zones of theeroded Variscan Belt of Western Europe based on the hydrogen isotope ratios (δD) ofmuscovite from syntectonic leucogranites that have been emplaced at ~315 Ma. Wefocus on the Limousin region (Western Massif Central, France) where peraluminousgranites are spatially associated with strike-slip and detachment shear zones thatdeveloped as a consequence of Late Carboniferous syn- to post-orogenic extension andmerge to the northwest with the South Armorican Shear Zone.The NE corner of the Millevaches massif is located at the junction between brittle andductile fault systems that acted as preferential pathways for Earth surface-derivedfluids. These meteoric fluids penetrated the crust at depth and reached the ductilesegment of the low- angle Felletin detachment zone. Using microstructural,thermometry, hydrogen isotope geochemistry and 40Ar/39Ar geochronological data, weshow that these Variscan meteoric fluids interacted with hydrous silicates during hightemperature deformation between at least ~318 and ~310 Ma. Based on the measuredδDMuscovite values ranging from -116 to -105‰ in mylonitic leucogranite and atemperature of hydrogen isotope exchange of 540 ± 51°C deduced from the Ti-inmuscovitegeothermometer, we calculate an average δDmeteoric water value of -96 ± 8‰.For paleoaltimetry purposes, we reference our hydrogen isotope record of ancientmeteoric fluids from mylonitic rocks to time-equivalent (~300 Ma) oxygen isotoperecords retrieved from freshwater shark remains preserved in the Bourbonl’Archambault (BA) basin that developed in the external zones of the orogen. Using ahydrogen isotope lapse rate of -22‰/km, a ~76‰ difference in δDmeteoric water valuesbetween the Millevaches massif and the BA basin (δDwater = -20 ± 6‰) permitspaleoaltimetry estimates attaining 3400 ± 700 m. Obviously, we are limited inextrapolating isotope elevation relationships into the distant past; however, the ratherlarge difference in δD values between the foreland basin and the orogeny interiorsuggests that the hinterland of the Variscan belt of western Europe acted as a barrierto moisture from the south-south-east and was probably high enough to induce anorographic rain shadow to the north

Late Carboniferous paleoelevation of the Variscan Belt: a stable isotope paleoaltimetry study in the French Massif Central

International audienceWe present the first stable isotope paleoaltimetry estimates for the hinterland of the eroded Variscan Belt of Western Europe based on the hydrogen isotope ratios of muscovite from syntectonic leucogranites that have been emplaced at ~315 Ma. We focus on the Limousin region (Western Massif Central, France) where peraluminous granites are spatially associated with strike-slip and detachment shear zones that developed as a consequence of Late Carboniferous syn- to post-orogenic extension and merge to the northwest with the South Armorican Shear Zone.Here we show that the NE corner of the Millevaches massif located at the junction between brittle and ductile fault systems represented a pathway for Earth surface-derived fluids that penetrated the crust and reached the ductile segment of the low-angle Felletin detachment zone. Using microstructural, thermometry, hydrogen isotope geochemistry and 40Ar/39Ar geochronological data, we show that these Variscan meteoric fluids interacted with hydrous silicates during high temperature deformation between at least ~318 and 310 Ma.For paleoaltimetry purposes, we reference our hydrogen isotope record (δD) of ancient meteoric fluids from mylonitic rocks to ~295 Myr-old records retrieved from freshwater shark remains preserved in the Bourbon l’Archambault basin that developed in the external zones of the orogen. A ~76‰ difference in δDmeteoric water values between the Millevaches massif (δDmeteoric water value = -96 ± 8‰) and the Bourbon l’Archambault foreland basin (δDwater value = -20 ± 6‰) is consistent with minimum paleoaltimetry estimates of 3400 ± 700 m based on a modern lapse rate of ~-22‰/km for δDwater values. The rather large difference in δD values between the foreland basin and the continental interior suggests that the hinterland of the Variscan belt of western Europe was high enough to act as a barrier to moisture transport from the south-south-east and induce an orographic rain shadow to the north