Palaeozoic evolution of the Variscan Vosges Mountains

Abstract A geological synthesis of the Palaeozoic Vosges Mountains (NE France) is presented using existing observations and new data. The geodynamic evolution involves: (1) Early Palaeozoic sedimentation and magmatism; (2) Late Devonian subduction triggering back-arc spreading; (3) early Lower Carboniferous continental subduction, continent–continent collision and polyphase deformation and metamorphism of the orogenic root; and (4) late Lower Carboniferous orogenic collapse driven by thermal weakening of the middle crust. The evolution is integrated within the framework of the European Variscan Belt. The Northern Vosges comprise sediments of Rhenohercynian affinity separated from Teplá-Barrandian metasediments by a Lower Carboniferous magmatic arc. The latter is correlated with the Mid-German Crystalline Rise, and is ascribed to the south-directed subduction of the Rhenohercynian Basin. The Saxothuringian–Moldanubian suture is thought to be obliterated by the magmatic arc, while the Lalaye–Lubine Fault is interpreted as the Teplá-Barrandian–Moldanubian boundary. The Central Vosges are paralleled with the Moldanubian domain of the Bohemian Massif where identical lithologies record the Devonian–Carboniferous SE-directed subduction of the Saxothuringian passive margin below the Moldanubian upper plate. The Southern Vosges represent the upper Moldanubian crust and are linked to the southern Black Forest. The presence of an oceanic domain to the south of the Vosges–Black Forest remains unclear. Supplementary material: List of radiometric ages used for probability plots is available at http://www.geolsoc.org.uk/SUP18734.

The Variscan orogen is an 8000 km-long belt which formed as a result of Palaeozoic subduction and collision events (e.g. Matte 2001;Nance et al. 2010). In Europe, it has been divided into distinct litho-tectonic domains since the early works of Suess (1926) and Kossmat (1927). This subdivision into the major Rhenohercynian, Saxothuringian and Moldanubian domains is based on the assumption that the orogen represents a juxtaposition of different litho-tectonic units formed by continuous continental belts with their surrounding basinal sequences (e.g. Behr et al. 1984;Ziegler 1984;Matte et al. 1990;Franke 2000). However, a growing number of studies tends to reveal that the Variscan domains are neither lithologically homogeneous (e.g. Oncken 1997;Chopin et al. 2012) nor laterally continuous (Franke & Zelazniewicz 2000;Lardeaux et al. 2014). These works describe the litho-tectonic domains as a more complex juxtaposition of autochthonous and allochthonous material (e.g. Guy et al. 2011) that originally belonged to different plates. It is therefore in the light of these new studies and concepts that the Variscan Orogen should be examined today.
The Palaeozoic basement of the Vosges Mountains (NE France) illustrates the lack of continuity between the Variscan domains and the noncylindricity of the orogenic belt (Fig. 1a). The Vosges Mountains are traditionally divided into a northern part of Saxothuringian affinity and a southern part correlated with the Moldanubian domain (Kossmat 1927). The boundary is traced along the Lalaye -Lubine and Baden-Baden fault zones located in the Vosges and Black Forest, respectively (Krohe & Eisbacher 1988;Fluck et al. 1991). However, Kossmat (1927) recognized that the characteristic thrust of the Moldanubian over the Saxothuringian was much less developed in the Vosges Mountains than towards the east. By comparison with the Bohemian Massif, Franke (2000) pointed out that the Teplá-Barrandian domain was missing between the Saxothuringian and Moldanubian parts of the Vosges Mountains. The presence of the Bristol Channel-Bray Fault also renders difficult the correlations with the Variscan massifs located to the west (Fig. 1a).
The possible correlations with the neighbouring Variscan massifs leave a wealth of open questions. Although the northern Black Forest seems to be a prolongation of the Northern Vosges (e.g. Montenari & Servais 2000), safely linking the Central Schwarzwald Gneiss Complex with the Central Vosges metamorphic units remains difficult, despite their geochemical resemblance (Müller 1989). Similarly, the suture zone of a south-dipping oceanic domain recognized in the southern Black Forest (Loeschke et al. 1998) cannot be directly followed in the Southern Vosges, but may be located further to the south (Maass et al. 1990). It is also problematic to connect the Southern Vosges back-arc basin with the Brévenne unit in the NE French Massif Central (Faure et al. 2009;Skrzypek et al. 2012b). Finally, the classical litho-tectonic zonation is questioned by Edel & Schulmann (2009) who propose that both the Rhenohercynian and Saxothuringian sutures could lie to the north of the presently exposed Vosges basement.
The present contribution tries to review more than one century of geological observations of the Palaeozoic basement of the Vosges Mountains. It is complemented by new data to present a synthesis of the lithostratigraphic record, igneous activity, metamorphic conditions and structural evolution of the Vosges Mountains. The different datasets are combined in order to constrain the significance of the different litho-tectonic units of the Vosges Mountains, and discuss the position and geodynamic evolution of this segment in the framework of the European Variscan Belt.

Lithostratigraphy
The Palaeozoic basement of the Vosges Mountains is formed by a wide central zone of granitoids and metamorphic rocks surrounded by Early Palaeozoic-Carboniferous (meta-)sediments to the north and Upper Devonian -Carboniferous (volcano-)sedimentary rocks to the south (Fig. 1). The Permian clastic sediments are mostly found around the massif, but locally overlay the magmatic and metamorphic rocks in the central part. The Vosges Mountains are subdivided into three parts (

Northern Vosges
The Northern Vosges correspond to a succession of NE -SW-striking sedimentary belts intruded by a magmatic suite. The latter separates the younger sediments and volcanics of the Northern succession from the older and weakly metamorphosed sediments of the Southern succession (Fig. 1).
Northern succession (Bruche unit). The base of the Northern succession is represented by basaltic lava flows, acid-volcanic rocks and coarse-grained sediments of possible Lower Devonian age (e.g. Juteau 1971; Fig. 2). They are overlain by early Middle Devonian conglomerate and sandstone (Benecke & Bücking 1898) and Givetian greywacke-pelite alternating with bimodal volcanism (Firtion 1945(Firtion , 1957. The volcanic association is composed of mostly submarine altered basalt and rhyolite ('spilite-keratophyre') with pyroclastic breccias showing a tholeiitic affinity (Ikenne & Baroz 1985;Rizki & Baroz 1988). The Givetian age is also recognized in scarce carbonate lenses, which preserve evidence of a reef environment receiving abundant siliceous material from the neighbouring continent (Jaeckel 1888;Bücking 1918;Blanalt 1969), and in the surrounding polymictic conglomerate containing Late Cambrian granitic pebbles (Dörr et al. 1992). These observations indicate that the Middle Devonian is associated with the erosion of a Cambrian substratum, coastal sedimentation and the development of reef carbonates in a relatively shallow-marine siliciclastic basin (Fig. 2).
The sedimentary succession continuously passes to thick Frasnian and Famennian sandy-pelitic deposits with radiolarite intercalations and numerous samples of plant debris (Figge 1968;Blanalt & Lillié 1973;Braun et al. 1992;Aghai Soltani et al. 1996;Fig. 2). This record reflects a quiet sedimentation in a coastal environment receiving continental flora (Blanalt & Doubinger 1973). However, late Upper Devonian sedimentary breccias with clasts of the underlying lithologies indicate subsequent sedimentary instabilities. It is further supported by the lower Visean greywacke and pelite alternations which document a synsedimentary tectonic activity and preserve characters of flyschtype turbiditic deposits (Corsin & Dubois 1932;Dubois 1946;Corsin et al. 1960). The Early Carboniferous tectonic activity probably culminates during middle Visean time, as indicated by the sedimentary hiatus, contact metamorphism (Bonhomme & Prévôt 1968) and the contemporaneous magmatism occurring to the south.
Few upper Visean deposits are found in the axial part ('Bande médiane') of the magmatic suite ( Fig. 1). They are juxtaposed with granitic rocks as a result of late normal faulting. They chiefly correspond to pyroclastic rocks and ignimbrite, but rare pelite and greywacke are also observed (Elsass & von Eller 2008). According to Rizki et al. (1992), this calc-alkaline to shoshonitic volcanism is related to the upper part of the magmatic suite and is indicative of an active margin setting in the late Lower Carboniferous.
Upper Carboniferous-Permian sediments and volcanics are present on both sides of the magmatic suite ( Fig. 1). They are represented by Stephanian-Autunian coal-bearing coarse-grained sediments (Doubinger 1956(Doubinger , 1965, Saxonian rhyolitic volcanism (Mihara 1935;Lippolt & Hess 1983;Boutin et al. 1995) and Thuringian arkosic sandstone and conglomerate (Hollinger 1969;Fig. 2). In the Stephanian -Autunian deposits, numerous pebbles of magnesio-potassic (Mg -K) granite, gneiss and schist from the Central Vosges indicate that the deep crustal levels were already close to the surface at that time. It was followed by a widespread Middle Permian subaerial acid volcanism which is also documented in the neighbouring Black Forest or Saar regions Schleicher et al. 1983) and by Upper Permian continental sedimentation.
Southern succession (Villé and Steige units). The Southern succession is represented by the two NE-SW-trending Villé and Steige units (Fig. 1). The structurally deeper Villé unit is formed by Cambrian -Ordovician pelite followed by quartzopelitic sediments with quartzite and acid tuff intercalations that have all been metamorphosed under greenschist-facies conditions (Doubinger & von Eller 1963b;Ross 1964;Reitz & Wickert 1989;Fig. 2). Although a Precambrian age has been proposed (Doubinger & von Eller 1963b), the Villé unit seems to be correlated with the similar Late Cambrian-Early Ordovician low-grade sediments found in the northern Black Forest (Montenari & Servais 2000). The Villé unit is overlain by Ordovician-Silurian sandy and chiefly pelitic metasediments of the Steige unit which show a lowgrade metamorphic overprint (Doubinger 1963;Doubinger & von Eller 1963a;Ross 1964). The lithology and major element geochemical signature of both units suggest that the Early Palaeozoic sedimentation occurred in a shallow-marine, probably platform, environment (Tobschall 1974).

Central Vosges
In the Central Vosges, it is possible to distinguish between a narrow zone of metamorphic units to the north and a larger zone of granitoids to the south (Fig. 1). The latter frequently hosts large migmatitic bodies which are derived from lithologies found in the metamorphic units.
Magmatic units. The numerous magmatic rocks of the Central Vosges can be divided into two distinct groups. The oldest I-type magmatic event is associated with elongated bodies of biotite -amphibolebearing porphyritic granitoid (CVMg -K granitoids) and occurred at 340 -335 Ma across a large part of the Vosges (Figs 1 & 2). The CVMg -K granitoids are intrusive in the Central Vosges metamorphic units as well as in the Southern Vosges sediments where microgranite sills are developed. The younger S-type Central Vosges Granite (CVG) represents a second event of widespread anatexis in the Central Vosges at 330 -325 Ma (Fig. 2). Detailed mapping of this voluminous biotite-bearing anatectic granite reveals the presence of numerous xenoliths of Mg -K granitoid, gneiss or sedimentary rocks (von Eller 1961). On the other hand, the associated biotite -muscovite-bearing leucogranites ('Thannenkirch-Brézouard-Bisltein') correspond to narrow and elongated plutons that occur along the major tectonic discontinuities (Fig. 1).

Southern Vosges
The Southern Vosges are dominantly composed of (volcano)-sedimentary successions that are divided into allochthonous and autochthonous units (Jung 1928). The boundary between the units lies close to the Klippen Belt which corresponds to discontinuous exposures of partly ophiolitic material. Towards the south, an east-west-trending Mg-K magmatic complex is intrusive in the autochthonous units ( Fig. 1).
Autochthonous units (Oderen and Thann units). The oldest autochthonous sediments ('Belfortais') are found in the southernmost part of the Vosges (Fig. 1). There, limestones of probable Frasnian age are conformably overlain by a thin Fammenian pelitic sequence (Chevillard 1866;Asselberghs 1926;Bain 1964) which preserves a fauna indicating an Upper Devonian platform environment (Fig. 2). The pelites are in turn unconformably overlain by a Lower Carboniferous conglomeratic greywacke which is also observed in the northern part of the autochthonous units (Oderen unit). There, the autochthonous succession corresponds to thick Tournaisian -lower Visean pelite and greywacke with episodic conglomerate and carbonate deposits (Corsin et al. 1956;Corsin & Mattauer 1957;Mattauer & Théobald 1957;Mattauer 1959). The sediments are locally interlayered with submarine altered basalt and rhyolite ('spilite-keratophyre') showing a tholeiitic affinity (Lefevre et al. 1994). Towards the top of the Oderen unit, acid volcanism is found together with more abundant carbonate intercalations indicating an early middle Visean age (Hammel 1996;Montenari et al. 2002), although Tournaisian fossils are also found (Hahn et al. 1981;Vogt 1981). The Oderen unit represents flysch-type turbiditic deposits (Gagny 1962;Krecher 2009) that were later affected by middleupper Visean sedimentary instabilities as indicated by the resedimented fauna occurring at its top (Schneider et al. 1989). These instabilities are the only expression of the so-called 'intra-Visean event', and no significant deformation or sedimentary hiatus is documented in the Vosges Mountains at that time (Schneider et al. 1989).
The boundary with the younger autochthonous unit (Thann unit) is marked by the emplacement of abundant andesitic lavas (Fig. 2) that have a calc-alkaline potassic affinity (Lefevre et al. 1994). They are overlain by upper Visean sandstone or conglomerate alternating with trachytic to rhyolitic volcanic rocks (Corsin et al. 1973;Coulon et al. 1975aCoulon et al. , b, 1978. Up to the Namurian, the coarsening of sediments and the increasing amount of plant debris indicate the progressive filling of small basins associated with an ultimate episode of high-K rhyolitic volcanism (Lefevre et al. 1994). The continental sedimentation is later characterized by Stephanian-Saxonian sandstone, conglomerate, pelite and rhyolitic tuff with some coal-bearing strata (Mathieu 1968;Creuzot 1983 (Jung 1928;Schneider et al. 1990;Skrzypek et al. 2012b). The klippen preserve serpentinite, ophicalcite and Famennian gabbro overlain by a conglomeratic greywacke containing Neoproterozoic gneiss blocks. This block-in-matrix formation is capped by Famennian siliceous pelite (Maass & Stoppel 1982;Fig. 2). The Klippen Belt is conformably overlain by thick lower Visean pelite and greywacke deposits (Markstein unit) with only minor conglomerate and carbonate material (Corsin & Mattauer 1957;Corsin & Ruhland 1959). The sedimentation is thought to last up to the upper Visean (e.g. Krecher 2005), but middle Visean granitic intrusions in the already folded allochthonous units make this hypothesis unlikely. To summarize, the allochthonous lithologies indicate the presence of a deep Late Devonian basin subsequently filled by lower Visean flysch turbidites with characters of a prograding system of sandy submarine fans (Krecher et al. 2007).

Magmatism
Four major magmatic associations are recognized in the Variscan Vosges Mountains. From north to south, they include: the Northern Vosges magmatic suite, the Central Vosges Mg -K granitoids, the Central Vosges Granite and the Southern Vosges Mg -K complex.

Northern Vosges magmatic suite
The Northern Vosges magmatic suite ('Champ du Feu') corresponds to NE -SW-trending or circularshaped magmatic bodies intruding the surrounding (meta-)sedimentary units (Fig. 1). It is a composite succession of I-to S-type plutons associated with subaerial to aerial volcanic rocks which were all emplaced during a short middle Visean event at 335 -330 Ma (Fig. 2).
The oldest I-type magmatic rocks correspond to the narrow belts of diorite ('Neuntelstein') and the southern granodiorite body ('Hohwald'). Al-inhornblende barometry points to an intrusion depth of c. 10 km for the diorite (Altherr et al. 2000); the southern granodiorite was emplaced at a slightly shallower level as indicated by intrusive contacts with the low-grade Steige metasediments and by the presence of numerous metasedimentary xenoliths. I-type magmatic rocks show a clear enrichment in light rare earth elements (LREE) and large ion lithophile elements (LILE) and pronounced Nb and Ti anomalies (Altherr et al. 2000). The isotopic compositions lie at 1 Nd  Fig. 3). The volcanic rocks comprise pyroclastite, tuff and ignimbrite that range from a basaltic to rhyolitic composition. Trace elements reveal Nb and Ti anomalies, and support a genetic link with the I-type plutonic rocks (Elsass & von Eller 2008). It is proposed that the diorite is derived from an enriched lithospheric mantle source, while the granodiorite originated from the melting of a meta-igneous protolith (Altherr et al. 2000).
The calc-alkaline magmatic activity is followed by the intrusion of the S-type northern granite ('Belmont'). This heterogeneous body hosts abundant xenoliths of sedimentary and volcanic rocks (Elsass & von Eller 2008) which suggest that its emplacement at a shallow depth was associated with magmatic stoping of the overlying Northern succession. The last magmatic episode is reflected by the intrusion of the circular-shaped, S-type younger granites ('Andlau', 'Natzwiller', 'Senones' and 'Kagenfels' granites) which cross-cut the NE -SW-trending bodies (Fig. 1). These high-K to shoshonitic granites are characterized by a granophyric texture towards their margins, and represent the shallowest intrusions of the magmatic suite. S-type magmatic rocks are enriched in LILE and show weak Nb, Ti anomalies compared to the I-type plutonic rocks (Altherr et al. 2000). The isotopic compositions are nearly similar for the northern and younger granites with 1 Nd  (Altherr et al. 2000). Previous works emphasize the calc-alkaline to high-K affinity of the Northern Vosges magmatic suite and interpret it as arc-type magmatism related to an Early Carboniferous subduction event (Altherr et al. 2000;Tabaud 2012).

Central Vosges Mg -K association
The Central Vosges Mg -K association exhibits porphyritic plutonic rocks ranging from amphibolebiotite syenite (durbachite) to granite ('Granite des Crêtes') emplaced between 340 and 332 Ma. The prominent geochemical feature of these rocks is a constant total alkali content with increasing SiO 2 and a significant enrichment in Mg, Ni, Cr, K, U and Th. In addition, they show a decreasing REE content from the basic to the acid endmembers. The isotopic data indicate nearly constant 1 Nd (340) ¼ 26.7 to -5.3 values with 87 Sr/ 86 Sr (340) increasing from 0.7096 to 0.7137 (Tabaud 2012;Fig. 3).
The rocks of the CVMg-K association are thought to reflect the partial melting of an enriched lithospheric mantle located above a subduction zone and the subsequent crustal contamination of the magma at a deep crustal level (Gagny 1968). It is proposed that they represent the mixing between mantle magmas and acid melts derived from the anatexis of the lower orogenic crust (Tabaud 2012).

Central Vosges Granite
The large Central Vosges Granite ('Granite fondamental'), emplaced between 335 and 325 Ma, corresponds to different textural variants of biotite or muscovite-biotite granite that locally contain cordierite or andalusite (Hameurt 1967;Tabaud 2012). These are typical S-type peraluminous granites with a high LILE content. With respect to the average continental crust, the CVG is enriched in light REE and medium REE, but shows a similar heavy REE content (Tabaud 2012). Based on isotopic data, it is possible to distinguish between the eastern and western CVG. The eastern CVG shows higher 1 Nd (320) ¼ 25.2 to -4.1 and lower 87 Sr/ 86 Sr (320) ¼ 0.7089-0.7117 values than the western CVG. The latter preserves a nearly constant 1 Nd (320) ¼ -6.8 to -6.1 with 87 Sr/ 86 Sr (320) ¼ 0.7152-0.7198 (Tabaud 2012 ; Fig. 3).
The presence of relictual Mg-K granitoid, gneiss or sedimentary rocks within the CVG (von Eller 1961) and its isotopic signature are used to interpret the CVG as a result of in situ melting of a heterogeneous orogenic middle crust (Tabaud 2012).

Southern Vosges Mg -K association
The southeastern part of the autochthonous volcano-sedimentary units is intruded by the east -west-trending SVMg-K magmatic complex ('Ballons' complex; Fig. 1). The oldest intrusions correspond to a peripheral zone of gabbro, diorite and monzogranite which were probably emplaced at c. 345 Ma. They were shortly followed by the intrusion at c. 340 Ma of a large biotite-amphibolebearing monzonite to granite with a fine-grained ('Corravillers') to porphyritic ('Ballons') texture (Fig. 2). The plutonic bodies are associated with high-K volcanic rocks ('Molkenrain') in the autochthonous Thann unit (Fig. 1).
The high amounts of U and Th (e.g. Rothé 1962) indicate that the granite is strongly similar to the CVMg-K granitoids found in the Central Vosges. As for the CVMg-K association, the SVMg -K rocks show constant total alkali content with increasing SiO 2 and enrichment in Ni, Cr, K, U and Th with a weaker Mg enrichment. The REE content also decreases from the basic to the acid end-members. By contrast, the isotopic compositions are more primitive ( Fig. 3) with 1 Nd (340) ¼ -4.8 to 1.4 and 87 Sr/ 86 Sr (340) ¼ 0.7048-0.7084 (Tabaud 2012). Previous studies show that the SVMg-K association has a high-K signature and is probably derived from a basaltic source, either of tholeiitic (André & Bébien 1983) or shoshonitic affinity (Pagel & Leterrier 1980). André (1983) proposes that the basic rocks located at the margin of the SVMg -K complex are derived from fractional crystallization of a tholeiitic basaltic magma, while Pagel & Leterrier (1980) relate the SVMg-K complex to hyperpotassic or shoshonitic series where alkaline magma was contaminated by the continental crust. The mixed mantle and crustal affinities are also recognized in the volcanic rocks and could testify for a subduction setting (Lefèvre et al. 1994).

Metamorphic record
Contrasted metamorphic conditions are documented across the Vosges Mountains (Fig. 4). They range from limited contact metamorphism around the granitoids to ultra-high-pressure conditions in peridotite slices of the Central Vosges. Nevertheless, geochronological studies demonstrate that metamorphism was restricted to a relatively short Late Devonian -Early Carboniferous period.

Contact metamorphism
The various granitoid intrusions are commonly associated with metamorphic aureoles in the neighbouring sedimentary rocks. The polyphase intrusion of the Northern Vosges magmatic suite is responsible for contact metamorphism in both the Northern and Southern successions (Fig. 4a). The sediments of the Bruche unit document a resetting of the Rb -Sr isotopic system (Bonhomme & Prévôt 1968), whereas a narrow aureole of hornfels and spotted slate was generated by the southern granodiorite and the younger Andlau granite in the Steige unit (Rosenbuch 1877;von Eller 1964).
In the Southern Vosges, the margin of the allochthonous Markstein unit and parts of the autochthonous Oderen unit are affected by contact metamorphism over a relatively large area (Fig. 4a). In the northern part, the Mg -K granitoids produce mostly hornfels while both the Mg -K and Central Vosges granites transform the southern sediments into spotted slates. Hornfels is additionally found in the autochthonous units around the SVMg-K magmatic complex.

Geochronology
The compilation of geochronological data for the Palaeozoic basement of the Vosges Mountains emphasizes the Late Devonian-Early Carboniferous thermal events. They are related to mediumto high-grade metamorphism in the Central Vosges, but also to several magmatic events which occurred across the entire massif.

Timing of the igneous activity
The synthesis of existing ages for igneous rocks indicates distinct pulses of magmatic activity between 350 and 290 Ma (Fig. 5a). The oldest igneous rocks are related to the basic plutonism which occurred at the margin of the SVMg-K complex at c. 345 Ma, as indicated by U-Pb zircon data on diorite and monzodiorite samples (Schaltegger et al. 1996;Tabaud 2012). It was shortly followed by more abundant magmatism with the intrusion of the larger SVMg-K granite at 339-336 Ma (Schaltegger et al. 1996;Tabaud 2012) and the associated aerial rhyolitic volcanism at 340-335 Ma (Boutin et al. 1995;Schaltegger et al. 1996). The SVMg-K magmatism is nearly coeval with the CVMg-K event recognized in the Central Vosges (Fig. 5a). There, durbachitic to granitic intrusions preserve U -Pb zircon ages between 340 and 332 Ma (Schaltegger et al. 1996;Schulmann et al. 2002). Nevertheless, a few zircon cores yielding ages of c. 350 Ma suggest that this magmatic event could have started earlier (Tabaud 2012).
Following both Mg-K events, arc-type magmatism took place in the Northern Vosges (Fig. 5a). The successive intrusions of I-to S-type plutons are dated by various methods, and there is a general agreement to consider this episode to be relatively short-lived and lasting from 335 to 330 Ma (Boutin et al. 1995;Hess et al. 1995;Reischmann & Anthes 1996;Altherr et al. 2000;Edel et al. 2013). However, recent U -Pb zircon and monazite data show inheritance at 360 -345 Ma (Elsass & von Eller 2008;Edel et al. 2013), indicating that the igneous activity in the Northern Vosges could have started earlier. Conversely, several 40 Ar/ 39 Ar ages of c. 320 Ma reported for the magmatic suite could represent partial resetting due to the younger and widespread Middle Permian acid volcanism estimated at 299-293 Ma in the northernmost part of the Vosges Mountains (Lippolt & Hess 1983;Boutin et al. 1995).
The latest Carboniferous magmatic event produced granitoids which cover the largest part of the Central Vosges (Fig. 1). It is associated with the emplacement of the Central Vosges Granite at 328 -320 Ma (Schaltegger et al. 1999;Tabaud 2012) and leucogranites between 330 and 323 Ma (Boutin et al. 1995;Schulmann et al. 2002;Kratinová et al. 2007). The CVG also preserves inherited zircon or monazite ages of c. 335 Ma, suggesting that the granite digested host rocks emplaced or metamorphosed during the Early Carboniferous, that is, most likely rocks belonging to the Central Vosges metamorphic units (Tabaud 2012).

Timing of metamorphism
Previous geochronological studies show that metamorphic ages cluster at 350 -330 Ma (Fig. 5b). Few data document Early Carboniferous metamorphism in the Northern Vosges. Rb -Sr wholerock analyses indicate contact metamorphism in the Northern succession at 339 + 22 Ma due to the intrusion of the northern granite (recalculated age after Bonhomme & Prévôt 1968), while the southern granodiorite affects the Southern succession at 339 + 38 Ma (recalculated pooled age after Clauer & Bonhomme 1970). In the Central Vosges metamorphic units, U -Pb and 40 Ar/ 39 Ar ages indicate a prominent event at 340-335 Ma (Fig. 5b). The monotonous unit seems to lack Early Carboniferous zircon ages (Fig. 5b), but preserves a 40 Ar/ 39 Ar biotite cooling age of 330 + 14 Ma (Boutin et al. 1995). Conversely, U -Pb zircon data in both the varied gneiss leucosomes and restites indicate that HT metamorphism and partial melting occurred from 340 to 335 Ma. In the felsic granulite unit, zircon grains similarly point to granulite-facies metamorphism at 345-335 Ma (Schaltegger et al. 1999;Skrzypek et al. 2012a), although the peak pressure event may have been slightly older. In the two high-grade units, 40 Ar/ 39 Ar ages between 340 and 325 Ma lie close to U -Pb zircon estimates, indicating rapid cooling of this deep part of the crust. In the migmatite bodies, inherited zircon ages of c. 335 Ma indicate that this domain was also affected by Early Carboniferous metamorphism before being pervasively invaded by the Central Vosges Granite (Tabaud 2012).

Structure
The synthesis of new and existing data allows the structural succession for the different lithotectonic units of the Vosges Mountains to be constrained. The observations include the planar and linear structures in sedimentary or metamorphic lithologies and the anisotropy of magnetic susceptibility data (AMS) in magmatic rocks (Fig. 6).

Northern Vosges
Northern succession. The dominant planar structure in the Bruche unit is the sedimentary bedding S 0 . This bedding is initially affected by a gentle kilometre-scale north-south folding which generates S 0 planes variably dipping to the east or west (Fig. 7). The folded S 0 is subsequently affected by a moderate kilometre-scale NE-SW folding locally associated with a spaced cleavage S 1 (Fig.  7). The NE-SW-striking cleavage S 1 steeply dips to the SE and cross-cuts the S 0 bedding at a high angle. The second deformation is responsible for the main NE-SW trend of the Bruche synform (see also Blanalt & Lillié 1973;Wickert & Eisbacher 1988), but the final deformation pattern clearly results from two quasi-orthogonal compression events (Fig. 8a).
Magmatic suite. Folowing the results of Edel et al. (2013), the AMS record in the magmatic suite shows a clear distinction between the northern granitic domain and the southern dioritic-granodioritic domain (Fig. 7). In the northern granite, the magnetic foliation strikes NW-SE and dips moderately to the NE while the lineation trends north -south to NW-SE with a variable plunge (Fig. 7). Similar orientations are observed in the late granites intruding the northern part. Conversely, the belts of volcanic rocks, diorite and granodiorite preserve magnetic structures which are nearly perpendicular to those observed in the northern domain (Fig. 8a). These are east-west-to NE-SW-striking foliations steeply dipping to the north or SE, and NE-SW-trending lineations moderately plunging to the SW (Fig. 7). All structures were acquired at a magmatic state.
Southern succession. In the Steige unit, the original sedimentary bedding S 0 is rarely visible and is more commonly affected by upright east -west folds. The folding produces a subvertical eastwest-schistosity S 1 , which is the dominant structure in this unit (Fig. 7). In addition, a variably plunging intersection lineation L 0 -1 is observed on S 1 surfaces. It suggests that S 0 was folded, most likely along a north-south axis, before the development of S 1 . At the contact with the granodiorite, magmatic veins are found parallel to the subvertical S 1 in spotted slates (Fig. 8a). A later deformation event generates subhorizontal cleavage planes, and the superposition of orthogonal fabrics gives rise to a typical pencil cleavage.
In the Villé unit, the dominant metamorphic foliation S 1 is most likely developed parallel to the   Edel et al. (2013) in the magmatic suite, Kratinová et al. (2007) in the leucogranites, Kratinová et al. (2012) in the eastern anatectic granite, Rey et al. (1992) in the western anatectic granite, Blumenfeld (1986)  sedimentary bedding (Ruhland & Bronner 1965). The S 1 was probably originally subhorizontal, but is now commonly affected by east -west-to NE-SW-trending chevron-type folds. The folding generates east -west-to NE -SW-striking S 122 planes that dip moderately to the SE or steeply to the NW, and a NE-SW axial plane cleavage S 2 steeply dipping to the SE (Fig. 7). A later vertical shortening is locally observed close to the contact with the Steige unit.
At the southern margin of the Villé unit, a narrow zone of black schist with quartz augen surrounded by sigmoidal mica-rich bands occurs. The black schist preserves a subvertical NE-SW-striking schistosity cross-cut by subvertical east-west-striking shear planes that bear a subhorizontal lineation. This fabric superposition reflects a dextral sense of shear which is compatible with the latest kinematics of the LLFZ (e.g. Bouyalaoui 1992).

Central Vosges
Metamorphic units. The structural analysis of the Central Vosges metamorphic units reveals the superposition of three main structures. The oldest structure corresponds to the metamorphic foliation S 1 which is dominant on both sides of the Mg -K granitoid (Fig. 7). In all units, the S 1 consistently strikes north-south to NE-SW and dips steeply to the NW in the eastern part and to the SE in the western part, thereby defining a fan-like structure (Fig. 8b). In the varied gneiss, coarse-grained quartz-K-feldspar -garnet leucosomes are additionally found parallel to the S 1 fabric.
The S 1 foliation is subsequently affected by millimetre-to metre-scale recumbent folds that are open to isoclinal. This heterogeneous vertical shortening event produces the new subhorizontal foliation S 2 (Fig. 7). To the west of the granitoid, the S 2 foliation is shallowly dipping to the south or SW, and is associated with concordant anatectic veins. To the east of the granitoid, S 2 corresponds to the axial planar cleavage of north -south-trending folds or, more commonly, to a continuous foliation shallowly dipping to the west. There, the S 2 is dominantly present at the contact between the varied and monotonous gneiss units and gives to this contact an apparent thrust geometry; no kinematic indicators that could support the hypothesis of an east-directed transport are, however, observed (Fig. 8b).
To the east of the Mg -K granitoid, the monotonous and varied gneiss units are weakly affected by a subsequent metre-to kilometre-scale east -west upright folding (Fig. 7). This weak deformation produces east -west-striking S 223 planes variably dipping to the north or south, and an incipient east -west axial plane cleavage S 3 steeply dipping to the north (Fig. 8c).
Leucogranites. The structural record in the leucogranites intruding the Central Vosges metamorphic units was detailed by Kratinová et al. (2007). From north to south, the Thannenkirch and Brézouard bodies preserve a northern isotropic zone and a southern anisotropic margin with a magmatic to solid-state fabric, whereas the Bilstein granite only exhibits a pervasive solid-state deformation with S-C fabrics indicating sinistral shearing. Similarly, the AMS record highlights kilometre-scale domains of north -south-to NW-SE-trending lineations cross-cut by narrow zones of east -westtrending lineations and subvertical east-west magnetic foliations (Figs 7 & 8c).
Western anatectic domain. The NE-SW-trending Sainte-Marie-aux-Mines Fault Zone (SMMFZ) divides the Central Vosges into two distinct anatectic domains (Fig. 1). The western domain comprises a large migmatitic unit surrounded by the CVG. In the western part of the migmatitic unit, Blumenfeld (1986) documented a NE-SW-striking foliation steeply dipping to the NW, whereas in the eastern part the foliation shallowly dips to the north or NE (Fig. 7).
To the north of the migmatitic unit, the Central Vosges Granite preserves a magmatic fabric which gradually evolves to a dominant solid-state foliation towards the south (Rey et al. 1992). The foliation is in continuity with the subhorizontal S 2 fabric observed in the southwestern part of the metamorphic units (Fig. 7). It shallowly dips towards the migmatitic unit, that is, it is south-or SW-dipping in the northern part and east-or north-dipping along the eastern margin of the migmatite. Close to the migmatitic unit, fault striations and other shear indicators indicate a top-to-the-SW normal displacement (see also Rey et al. 1992).
To the south of the migmatitic unit, AMS data in the CVG reveal a dominant NE-SW-striking magnetic foliation moderately dipping towards the SE or NW (Fig. 7). Close to the migmatitic unit a few lineations moderately plunge towards the NW, while most lineations plunge at variable angles to the east or NE in the rest of the area.
Eastern anatectic domain. The eastern anatectic domain comprises relictual bodies of migmatitic orthogneiss and metagreywacke surrounded by the large CVG. The metamorphic and magmatic structures in the migmatitic units were detailed by Schulmann et al. (2009a). In the larger orthogneiss body an east -west-striking foliation steeply dipping to the south or SW is preserved in the centre, but tends to have a NW-SE strike towards the margins (Fig.  7). The small orthogneiss body indicates a nearly complete reworking of the original east -west subvertical fabric into shallowly south-or SW-dipping planes.
The surrounding CVG preserves magmatic to subsolidus structures to the north, but is isotropic towards the south (Kratinová et al. 2012). To the north, the magnetic foliation dips shallowly to the south and is associated with a subhorizontal eastwest lineation (Fig. 7). In the isotropic granite, moderately east-dipping or shallowly south-dipping magnetic foliations are observed. The magnetic lineation trends north -south and gently plunges to the south or SE (Fig. 7).
Mg -K magmatism. Three distinct CVMg -K granitoid bodies occur in the Central Vosges. The two largest intrusions are found in the metamorphic units and the CVG. They both preserve a subhorizontal K-feldspar fabric. The nearly orthogonal AMS structures define an axial zone of NE-SW subvertical magnetic foliations and NE-SW lineations cross-cut by east -west-to NW -SE-trending lineations associated with subhorizontal foliations (Fig. 7). Importantly, the NE -SW magnetic structures are concordant with the S 1 foliation in the metamorphic core.
A small body of CVMg-K granitoid crops out at the northern margin of the allochthonous sedimentary units. There, numerous biotite-rich xenoliths are found and point to a clear intrusive contact with the sediments (Fig. 8c). In the granite, Kratinová et al. (2012) showed that the magnetic foliation is moderately dipping to the SW and that the north -south-trending lineation gently plunges to the south or SE (Fig. 7).

Southern Vosges
Autochthonous units. The dominant structure of the autochthonous units is the sedimentary bedding S 0 .
It is mostly visible in sediments of the Oderen unit and only rarely in the dominantly volcanic Thann unit. According to new data and observations summarized by Krecher (2005), two distinct S 0 trends can be recognized. In the central part, the bedding strikes NW-SE and variably dips to the NE or SW (Fig. 7). By contrast, narrow zones to the north and to the south show a north-south-striking S 0 . In the northern part the S 0 is subhorizontal and parallel to the roof of the underlying granite, whereas it is moderately to steeply east-dipping near the SVMg-K complex (Fig. 7).
Allochthonous units. The main structure in the allochthonous Markstein and Klippen Belt units is the sedimentary bedding S 0 (see also Ruhland 1958;Petrini & Burg 1998). As in the autochthonous units, two orthogonal trends are recognized. A first upright folding event produces north -southstriking subvertical S 0 planes that are mostly preserved along the margin of the allochthonous units. This domain of north -south-striking orientations coincides with the zone affected by contact metamorphism and points to an influence of the surrounding granitoids (Figs 4 & 7). Conversely, the central part exhibits kilometre-scale asymmetrical folds associated with NW-SE-striking S 0 planes variably dipping to the NE or SW (Fig. 7). On both sides of the Klippen Belt, the consistently NE-dipping S 0 suggests the presence of a thrust of the allochthonous units over the autochthonous units (Fig. 8d). The subvertical S 0 planes additionally bear NW-SE-trending subhorizontal striations, indicating a mostly dextral sense of shear.
Mg-K magmatic complex. The SVMg-K complex is characterized by a juxtaposition of orthogonal structures. An earlier study of K-feldspar phenocrysts proposes that a dominant east -west to NW-SE subvertical foliation is cross-cut by narrow corridors exhibiting a north -south to NE-SW subvertical fabric (Blanchard 1978). Similarly, new AMS data reveal the occurrence of orthogonal fabrics. In the centre of the magmatic body a northsouth-striking and steeply east-or west-dipping magnetic foliation is associated with a northsouth lineation steeply plunging to the north or south (Fig. 7). By contrast, the eastern and western parts of the granite exhibit east-west foliations steeply dipping to the south and subhorizontal east-west-trending lineations (Fig. 7). All structures were acquired at the magmatic state.

Geodynamic evolution
Early Palaeozoic: the pre-collisional history The Vosges Mountains: a Gondwana-derived assemblage. The record of the Neoproterozoic -Early Palaeozoic evolution is cryptic (Fig. 9). Only the metagranite found in the Klippen Belt testifies for the presence of a Neoproterozoic substratum (Skrzypek et al. 2012b). In addition, the Northern Vosges Villé unit preserves Cambro-Ordovician siliciclastic sediments with acid tuffs and quartzite indicative of a shallow-marine basin (Fig. 2). Together with the contemporaneous pelite and carbonate found in the northern Black Forest (Sittig 1965;Montenari & Servais 2000), they define a succession which partly resembles that of the margin of the Gondwana continent. Similar deposits are documented on other Late Proterozoic continental blocks which are commonly regarded as peri-Gondwana crustal fragments (e.g. Chlupač 1993;Doré 1994;Linnemann et al. 2000). The Cambro-Ordovician protolith ages for granitic pebbles in the Northern Vosges sediments (Dörr et al. 1992) and for the felsic granulite (Skrzypek et al. 2012a) also point to acid magmatism at c. 500 Ma (Fig. 9). This magmatic activity is commonly interpreted as the incipient break-up of the northern Gondwana margin and the associated opening of oceanic basins bounded by microcontinental blocks (e.g. Pin & Marini 1993;Crowley et al. 2000;Schätz et al. 2002). All these arguments confirm the Gondwana derivation of the entire Vosges Mountains.
The Central Vosges metasedimentary units: deposits of the Saxothuringian Basin? The opening of an Early Palaeozoic basin is indicated by the thick monotonous and varied metasedimentary units. The monotonous unit comprises psammitic sediments derived from a Cadomian source, and was probably deposited in the Late Cambrian (Fig.  5b). The varied unit originates from the Late Ordovician-Early Silurian sedimentation of pelitesandstone derived from a Cambro-Ordovician source, with scarce carbonate and basic magmatic rocks (Fig. 5b).
This Early Palaeozoic sedimentary record bears strong similarities to the lithostratigraphy of the Saxothuringian domain, and especially to the Thuringian facies. The Thuringian succession is characterized by Ordovician sandy-pelitic sediments, overlain by Silurian shales with intercalations of carbonates and basic lavas (Falk et al. 1995). At first glance, the monotonous unit can be seen as the base of the Thuringian facies while the varied unit resembles the overlying Silurian succession. Both units could therefore be interpreted as proximal deposits of a Saxothuringian passive margin sequence. Correlating such sediments from the Vosges up to eastern Germany is possible, since similar sedimentation ages are proposed for some medium-to high-grade metasediments of the central Black Forest (Kober et al. 2004). However, the structural and petrological data demonstrate that the varied succession was not originally located above the monotonous sediments. The varied unit presently rests over the monotonous unit, but shows a higher metamorphic overprint (Fig. 8b). It argues for a separate evolution of the two units, which should originate from different sedimentation areas. The monotonous unit was deposited close to a Neoproterozoic and Cadomian substratum which is also known to underlie the monotonous unit in the Bohemian Massif (e.g. Fritz 1996;Friedl et al. 2004;Schulmann et al. 2005). Conversely, the varied unit is related to the erosion of a Cambro-Ordovician substratum which is more abundant in the northern part of the peri-Gondwanan continental blocks (e.g. Kröner et al. 2000b;Kemnitz et al. 2002). The varied unit is therefore interpreted as a sedimentary succession from the northern part of the Saxothuringian Basin. The monotonous unit was probably deposited further south, and its link with the Saxothuringian Basin is less clear (Fig. 10a).
The Northern succession (Northern Vosges): deposits of the Rhenohercynian Basin? In the Northern succession, the oldest sediments correspond to Middle Devonian coastal conglomerate and sandstone with a few reef limestones (Fig. 2). In addition, pebbles point to the erosion of a Cambro-Ordovician granitic substratum (Dörr et al. 1992) which is also found further to the north in the Saar Basin (Sommermann 1993). The substratum of the Saar Basin is overlain by thick Middle-Upper Devonian platform carbonate (Hering & Zimmerle 1976). These scarce data indicate the presence of a Devonian sedimentary basin in the northernmost part of the Vosges. The coarse-grained character of the first sediments suggests that a probable episode of Early Devonian emersion was followed by marine transgression (Fig. 9). The correlation with the Middle Devonian deposits in the Saar Basin additionally points to a relatively shallowmarine platform environment which may be slightly deepening towards the north (Fig. 10a, b).
Nearly similar Devonian siliciclastic and/or carbonated sediments are documented in the Rhenish Massif (Franke 1995) in SW England (Leveridge & Hartley 2006) or Moravia (Hladil et al. 1999). These successions are interpreted as the filling of the Rhenohercynian Basin which started to open in the Early Devonian (e.g. Clark et al. 1998). By contrast, Devonian black shales or cherts in both the Thuringian and Bavarian facies of the Saxothuringian Basin indicate a deeper sedimentary environment (Falk et al. 1995). The Middle-Upper Devonian record of the Northern succession is therefore correlated with that of the Rhenohercynian Basin. In this view, the Northern succession could represent a proximal part of the southern margin of this basin (Fig. 10a -c).

Late Devonian: onset of collision
Across the European Variscan Belt, multiple arguments testify for the activity of subduction zones during the Late Devonian (e.g. Matte 1998). In the Vosges Mountains, evidence for a subduction setting is represented by HP granulite-facies rocks which were probably metamorphosed during Late Devonian -Early Carboniferous (Skrzypek et al. 2012a) time and by the remnants of a Late Devonian back-arc basin in the Southern Vosges (Skrzypek et al. 2012b).
Subduction of the northern Saxothuringian passive margin. In the Central Vosges metamorphic units, the contrasted sedimentological records and detrital zircon ages are used to propose that the monotonous and varied gneiss protoliths were deposited in different areas (Figs 2 & 5b). In addition, petrological data reveal that the monotonous gneiss only reached peak amphibolite-facies conditions, whereas the felsic granulite and varied gneiss units underwent HP granulite-facies metamorphism (Fig. 4c).
HP granulite-facies metamorphism is a peculiar feature of the European Variscides (e.g. Pin & Vielzeuf 1988) and its significance has been explored by various studies (e.g. Kröner et al. 2000a;O'Brien & Rötzler 2003; see also Kotoková 2007 for a review). Based on geochronological and geochemical arguments, Janoušek et al. (2004) proposed that the felsic granulites which are now found in the Moldanubian domain represent metamorphosed and partially molten equivalents of Ordovician granites located in the Saxothuringian domain (Fichtelgebirge). It was integrated in a tectonic model where the HP granulite-facies rocks are interpreted as a part of continental crust which was subducted below the Moldanubian crust Chopin et al. 2012). Such a model additionally suggests that the Mg -K granitoids frequently associated with HP granulites are the products of mixing between melt lost from the felsic granulite and the overlying lithospheric mantle (Janoušek & Holub 2007;Lexa et al. 2011).
The striking lithological, petrological and geochemical similarities between the Central Vosges metamorphic units and rocks of the Bohemian Massif argue for an identical tectonic scenario for both regions. In the Bohemian Massif, the timing and polarity of the SE-directed Saxothuringian subduction is well constrained thanks to numerous ages reflecting HP metamorphism at c. 380 Ma (Gebauer & Grünenfelder 1979;Stosch & Lugmair 1990;Beard et al. 1995) and to Late Devonian flysch sedimentation followed by the NW-directed emplacement of high-grade nappes in the Saxothuringian domain (e.g. Franke 1984). The genesis of the Central Vosges orogenic root is therefore explained by the SE-directed continental subduction of a Saxothuringian-type passive margin below the Moldanubian upper plate (Fig. 10).
In this view, the present-day Moldanubian domain is interpreted as a mixture of a Saxothuringian allochthonous continental portion and the autochthonous Moldanubian upper plate (see also Chopin et al. 2012). This leaves the question regarding the nature of the Moldanubian upper plate before continental subduction to be resolved. In the Bohemian Massif, the Moldanubian crust is thought to involve a Neoproterozoic substratum (e.g. Friedl et al. 2004) and Early Palaeozoic backarc sediments separated from the Saxothurigian Basin by the Teplá-Barrandian domain (e.g. Schulmann et al. 2009b). In the case of the Vosges, the monotonous unit is considered as the upper-plate material and probably rests on a Neoproterozoic substratum similar to that found in the Klippen Belt (Fig. 10a). The parallel between both massifs indicates that the Moldanubian upper plate represents the southern margin of the Saxothurigian Basin (Fig. 10a), but contains a varying amount of Teplá-Barrandian-type material between the Saxothuringian Basin and the Neoproterozoic substratum.
The Saxothuringian-Moldanubian (Teplá) suture in the Vosges Mountains. Invoking the subduction of the northern Saxothuringian passive margin requires the continuation of the Saxothuringian-Moldanubian (Teplá) suture to be identified within the Vosges Mountains. This suture was classically defined along the east -west Lalaye -Lubine Fault Zone (Fluck et al. 1991), although it is devoid of any ophiolitic remnants in the Vosges Mountains as well as in the neighbouring Black Forest. Several arguments are presented here to challenge this earlier interpretation.
Placing a suture along the LLFZ implies that the Southern succession of the Northern Vosges (Steige and Villé units) represents material from the Saxothuringian lower plate. Sediments involved in oceanic or continental subduction are expected to record pressure-dominated metamorphism coeval with HP conditions in the deeply subducted material (e.g. Maruyama & Liou 1988). However, such features are incompatible with the P-T -deformation-time record in the Steige and Villé units. These units show a continuous Barrovian metamorphic gradient reaching garnet grade towards the south (Fig. 4b) and kyanite grade if mica schists occurring in the northern Black Forest are considered (Wickert et al. 1990). The peak metamorphic assemblages are developed in an originally subhorizontal foliation which was transposed much later into a subvertical cleavage during Early Carboniferous north-south shortening (Fig. 6). In addition, metamorphic ages of 345-340 Ma in phyllite and mica schist (Clauer & Bonhomme 1970; unpublished electron microprobe monazite ages) are similar to those obtained in the deep orogenic root (Fig. 5b). All data indicate that the Southern succession represents a normal sequence metamorphosed along a MP/MT gradient during the Early Carboniferous.
From a structural point of view, the earliest fabrics observed in the Central Vosges strike NE -SW (Fig. 7) and are parallel to the c. 60 km-long Sainte-Marie-aux-Mines Fault Zone (Fig. 1). The consistent orientation of these structures over several tens of kilometres indicates a NW -SE bulk shortening direction, which is in contradiction with the east-west strike of the LLFZ. Several major Variscan suture zones are also characterized by a thrust of high-grade units over less-metamorphosed rocks, producing inverted metamorphic sequences (e.g. Pitra et al. 2010). For the Vosges Mountains, the structural observations do not support a thrust of the medium-grade monotonous gneiss unit over the Southern succession, as already emphasized by Kossmat (1927).
To summarize, the Saxothuringian -Moldanubian (Teplá) suture is not believed to lie along the LLFZ. The Southern succession should be regarded as autochthonous sediments resting on the Moldanubian upper plate. Given that the deposition age of the Southern succession is older than that of the more metamorphosed monotonous unit (although poorly constrained), the Southern succesion could not conformably overlay the monotonous unit sediments. Instead, it should correspond to a thin piece of Teplá-Barrandian-type material located at the northern edge of the Moldanubian upper plate. This interpretation is supported by the lithological similarities between the Southern succession and the Teplá-Barrandian deposits of the Bohemian Massif (Fig. 2). The metamorphism of the Southern succession is explained by moderate crustal thickening during the early Lower Carboniferous (Fig. 10a -c). According to this interpretation, the Saxothuringian-Moldanubian suture should lie to the north of the LLFZ. Edel & Schulmann (2009) consider that the Northern succession (Bruche) also belongs to the Teplá-Barrandian domain and trace the suture to the north of the presently exposed Vosges basement. However, the present work prefers to link the Northern succession with deposits of the Rhenohercynian Basin. The Saxothuringian-Moldanubian suture is therefore thought to lie within the Northern Vosges, where it is presently obliterated by the magmatic suite (Figs 10 & 11).
Southern Vosges: back-arc basin opening. The Southern Vosges Klippen Belt testifies for the opening of a Late Devonian back-arc basin due to the subduction of an Early Palaeozoic oceanic domain (Fig. 10b). The origin of this back-arc spreading through the SE-directed closure of the Saxothuringian basin or the north-directed closure of the Palaeotethys Ocean is still unclear (Skrzypek et al. 2012b). The arguments in favour of the first hypothesis involve the coeval timing of the Saxothuringian (or Rheic) subduction (Stampfli et al. 2013) and the doubtful existence of an oceanic domain to the south according to palaeontological data (e.g. Paris & Robardet 1990). Arguments for the second hypothesis include the east -westtrending structures of the basin (Fig. 7), which could indicate inversion controlled by its initial shape (e.g. Oncken et al. 1999) acquired during north -south opening, and the correlation with the neighbouring southern Black Forest. There, the Badenweiler-Lenzkirch zone is thought to reflect the north-directed subduction of a southern oceanic domain (Loeschke et al. 1998).
A correlation with the Brévenne unit located in the NE French Massif Central faces the same dualistic view. There, the occurrence of HP metamorphic rocks ('Monts du Lyonnais') and relicts of a Late Devonian back-arc basin ('Brévenne') was interpreted as a result of the north-directed subduction of a southern oceanic domain (Lardeaux et al. 2001). However, this idea is seriously challenged by structural (Leloix et al. 1999) and geochronological (Faure et al. 2008) data which indicate a general north-vergence of the inverted back-arc basin, and no temporal link between HP metamorphism and back-arc spreading. These observations alternatively support the origin of the Late Devonian Brévenne back-arc due to the southdirected subduction of the Rheic Ocean (e.g. Faure et al. 2005).

Early Lower Carboniferous: polyphase collisional tectonics
East-west shortening: Saxothuringian-Moldanubian collision. The earliest fabric observed in the Vosges metamorphic units is the NE -SW subvertical S 1 foliation (Fig. 7). The S 1 foliation is connected with sillimanite growth after kyanite in both the felsic granulite and varied gneiss (Fig.  4c). By contrast, the metamorphic conditions of S 1 are not known for the monotonous unit, but garnetstaurolite relicts point to a prograde evolution (Rey et al. 1992). The observations therefore indicate a vertical upwards flow of the lower crust and a possibly contemporaneous downwards flow of the middle crust. Because the NE -SW fabrics are shared by upper and lower plate rocks along a vertical section of at least 10 km (from 8 kbar to at least 12 kbar; Fig. 4c) and along a horizontal section of c. 60 km (Fig. 7), the associated NW-SE to east -west shortening is thought to be nearly parallel to the compression direction at that time, and is tentatively ascribed to the compression imposed by the SE-directed subduction of the Saxothuringian passive margin (Fig. 10a-c).
The subvertical S 1 foliation is subsequently transposed into a subhorizontal S 2 fabric (Fig. 6). The metamorphism associated with S 2 corresponds to a pervasive LP/HT overprint affecting all units (Rey et al. 1989;Latouche et al. 1992). Due to its high-temperature character the metamorphic event is correlated with zircon ages of 340 -335 Ma that are repeatedly obtained in metamorphic rocks (Fig. 5b), especially in leucosomes parallel to S 2 in the varied gneiss (Schaltegger et al. 1999). The combined observations are interpreted as a widespread vertical shortening of the metamorphic units which were previously juxtaposed at a midcrustal depth. This event is responsible for the fan-like structure of the root and the apparent thrust of the HP/HT units over the monotonous unit (Fig. 8b), formerly interpreted as a result of nappe tectonics (Fluck et al. 1991). The localized vertical shortening may be due to a continuous accumulation of allochthonous felsic material at the base of the crust, as is proposed for the ductile thinning mechanism developed in the Franciscan complex (Ring & Brandon 1999). This event still results from the collision between the Saxothuringian and Moldanubian continental margins (Fig. 10d).
North-south shortening: subduction of the Rhenohercynian Basin and Gondwana indentation. The Vosges Mountains experienced a switch from east -west to north -south shortening (Fig. 6) at c. 340 Ma. The change in the shortening direction is probably best reflected by the structural record in the Mg-K granitoids. These bodies were emplaced between 340 and 332 Ma (Schaltegger et al. 1996;Schulmann et al. 2002), and systematically preserve two orthogonal fabric sets (Fig. 7). Bulk northsouth shortening is also indicated by the east -west to NE-SW subvertical cleavage planes developed in the Northern Vosges, and by the NW -SE-trending upright folds in the Southern Vosges sedimentary units (Fig. 7). In addition, east -west upright folding of the subhorizontal S 2 foliation is observed in the eastern part of the metamorphic units (Fig. 6). The south-vergent structures developed in the Lower Carboniferous turbiditic sediments of the southern Black Forest ) also testify for a general northsouth shortening (Fig. 6).
In the Northern Vosges, the Lower Carboniferous flysch-type sedimentation indicates tectonic instabilities in the basin. This deformation event culminates with the middle Visean sedimentary hiatus and the coeval emplacement during transtension of the NE-SW-trending magmatic arc at 335-330 Ma (Altherr et al. 2000;Edel et al. 2013). Evidence for a contemporaneous south-directed subduction of the Rhenohercynian passive margin (Holder & Leveridge 1986) and associated arc magmatism in the Mid-German Crystalline Rise (MGCR; Anthes & Reischmann 2001) is described across the European Variscan Belt. Consequently, the magmatic suite of the Northern Vosges is interpreted as a prolongation of the MGCR and its emplacement is ascribed to the south-directed subduction of the Rhenohercynian Basin (Fig. 10e). The Lower Carboniferous magmatic arc intrudes a sedimentary succession of Rhenohercynian affinity to the north, and Early Palaeozoic metasediments of Teplá-Barrandian affinity to the south. This close juxtaposition of contrasted lithologies is an additional argument for tracing the former Teplá suture at the place of the magmatic suite. Such a discontinuity could have controlled the intrusion of the NE-SW-trending magmatic arc in the upper plate.
The observed tectonic switch could be explained by a rigid block rotation, since palaeomagnetic data document a c. 808 anticlockwise rotation of the Vosges Mountains during the Early Carboniferous (Edel et al. 2013). However, the structural data indicate a bulk north-south shortening in present-day coordinates. In addition, deformation-age relationships reveal that the east-west structures are progressively younger towards the north (Fig. 6). These data are in good agreement with the idea of a northwards indentation of Gondwana which was already invoked to explain the north-south Carboniferous shortening in the European Variscan Belt (Vollbrecht et al. 1989). A similar north-to NW-directed shortening due to the indentation of the Brunovistulian microcontinent is also recorded in other parts of the Moldanubian domain (SE Bohemian Massif, Schulmann et al. 2005;Sudetes, Chopin et al. 2012). The north-south shortening accounts for the general south-verging structures observed in the southern part of the Variscan orogen, and especially in the Southern Vosges and Black Forest (Wickert & Eisbacher 1988;Eisbacher et al. 1989; Fig. 10e).

Late Lower Carboniferous: orogenic collapse
Detachment systems in the Central Vosges. The north-south shortening is followed by northsouth extension at c. 330-320 Ma (Fig. 6). It is revealed by normal faulting in the uppermost crust, by the transtensional emplacement of the Northern Vosges magmatic suite (Edel et al. 2013) and by the development of detachment zones in the middle crust. The western part of the CVG preserves evidence for the activity of a late Lower Carboniferous SW-to south-directed detachment system (see also Rey et al. 1991). At the same time, the eastern CVG is emplaced along a SE-to south-directed detachment zone (Schulmann et al. 2009a) while the leucogranites are emplaced under a transtensional regime (Kratinová et al. 2007). All these structural features point to synmagmatic south-directed extensional tectonics during the late Lower Carboniferous (Fig. 10f ).
Significance of the Lalaye -Lubine Fault: a prediction of the model. The LLFZ is a subvertical shear zone separating greenschist-to amphibolite-facies phyllite from partly migmatitic paragneiss, and documents Upper Carboniferous(?) dextral strikeslip (Fig. 1). It was previously interpreted as a south-dipping suture zone (e.g. Fluck et al. 1991), but several arguments challenge this view. Alternatively, it is proposed that the LLFZ marks the boundary between metasediments of Teplá-Barrandian affinity (Southern succession) and metasediments originally deposited on the Moldanubian upper plate (monotonous gneiss unit). The Teplá-Barrandian -Moldanubian boundary in the central Bohemian Massif corresponds to a subvertical shear zone hosting sheared granitoids and documenting a significant normal movement of the Teplá-Barrandian relative to the Moldanubian domain (e.g. Scheuvens & Zulauf 2000). In the NE Bohemian Massif, this presumed boundary corresponds to a dextral strike-slip zone separating greenschist-to amphibolite-facies rocks from migmatitic orthogneiss, but it is thought to have previously operated as a detachment fault (Mazur et al. 2005;Chopin et al. 2012).
The LLFZ has numerous features in common with the Teplá-Barrandian -Moldanubian boundary described in the Bohemian Massif. The presence of sheared granitoids along the eastern prolongation of the LLFZ in the Black Forest (Baden-Baden Fault Zone, Wickert et al. 1990) adds to this list of similarities. Moreover, rocks located to the north of the Lalaye-Lubine and Baden-Baden fault zones represent a continuous metamorphic section reaching kyanite grade, while the monotonous gneiss located to the south documents peak upperamphibolite facies conditions (Fig. 4c). The relatively small metamorphic gap suggests that the Steige-Villé units could have been originally located above the monotonous gneiss unit. Consequently, it is speculated that the LLFZ represents a former north-directed detachment system (Fig.  10e). By analogy with the Bohemian Massif, the Steige-Villé units are regarded as the detached upper crust of Teplá-Barrandian affinity. This interpretation is a prediction of the present model, and further work should try to recognize the possible earlier detachment structures that did not suffer the later and pervasive strike-slip reactivation of the LLFZ (e.g. Bouyalaoui 1992).
Driving mechanisms for orogenic collapse. The late Lower Carboniferous tectonic evolution was dominated by the activity of detachment systems. They mostly developed in the upper to middle crust, but left the orogenic lower crust unaffected (Fig. 6). It indicates that lower crustal flow (e.g. Vanderhaeghe et al. 1999) was not a driving mechanism for orogenic collapse in the case of the Vosges Mountains.
The development of a detachment zone during the emplacement of the CVG points to a role of the thermal structure of the crust. During extension the zone of anatexis developed above deep crustal rocks which show no signs of melting (Fig.  10f), suggesting that the heat source was instead located at a mid-crustal level. Simulations of the geotherm relaxation show that the large and highly radioactive Central Vosges Mg -K granitoids (e.g. Rothé 1962) emplaced in the middle crust at c. 340 Ma (Schaltegger et al. 1996) can trigger partial melting of the surrounding rocks after a period of c. 10 Ma, that is, precisely when the CVG was emplaced (Tabaud 2012). Conversely, the shallower Southern Vosges Mg-K granitoids are not expected to generate a sufficient perturbation of the geotherm, since radiogenic heat production is strongly dependent on the depth of radioactive rocks in the vertical column (e.g. McLaren et al. 1999). Such a contrast emphasizes the influence of radioactive heat production on the late-orogenic tectonic evolution. Accordingly, detachments in the Central Vosges are interpreted to be activated by a thermal weakening of the middle orogenic crust (Tabaud 2012).
The north-south extension can also be correlated with the larger-scale Variscan evolution. Arc-type magmatism at 335 -330 Ma in the Northern Vosges is thought to reflect the climax of the Rhenohercynian subduction (Edel et al. 2013). Such a south-directed subduction is likely to drive extension in the back-arc region, that is, precisely in the Central Vosges (Fig. 10f). Different directions of post-thickening extension have been documented in the European Variscan Belt (Burg et al. 1994), but this event can be correlated with the Middle Carboniferous NE-SW extension and abundant plutonism which are recognized in the French Massif Central (Faure 1995). The analogy with a Cordilleran metamorphic core complex setting (e.g. Coney & Harms 1984) suggests that the late Lower Carboniferous extension reflects an interplay between extensive melting of the middle orogenic crust and far-field forces.

Conclusions
The zonation of the Palaeozoic Vosges Mountains is described considering that the major Variscan lithotectonic domains: (1) have a variable width along the orogenic belt, and (2)