1 Introduction
It is widely acknowledged that the Variscan Belt of Western Europe formed as a result of the multiple collisions that occurred between Gondwana to the south and Laurussia to the north, from Early Devonian to Carboniferous times. These collisions stacked together continents and micro-blocks such as Armorica or Mid-German Crystalline Rise that had been previously drifted from Gondwana (e.g., Faure et al., 2005, 2008; Franke, 2000; Holder and Leveridge, 1986; Lardeaux et al., 2014; Matte, 1986, 2001; Pin, 1990). In the French Massif Central (FMC, Fig. 1A), a polyphase synmetamorphic deformation is well documented (e.g., Faure et al., 2009 and enclosed references). By analogy with the Himalayas, it was initially proposed that a ductile south-directed shearing, active from the Late Devonian to the Middle Carboniferous, accommodated the nappe stacking with younging from north to south (e.g., Ledru et al., 1989; Mattauer, 1974; Mattauer and Etchecopar, 1977; Matte, 1986). Top-to-the-south shearing is indeed well established in the southern part of the French Massif Central (Montagne Noire and Cévennes, Fig. 1) – where it is referred to as a “D3 event”, but more to the north and west, in the Lot, Rouergue, Lyonnais, Sioule and Limousin, the dominant structure is instead a NW–SE-stretching lineation with a top-to-the-NW shearing. The tectonic significance of this NW–SE deformation and its relations with the widespread southward D3 shearing are not clear. According to Brun and Burg, (1982) and Burg et al. (1987), top-to-the-NW shearing would be due to a combination of southward-thrusting and sinistral wrenching during the Ibero-Armorican oroclinal bending. Mattauer et al. (1988) considered this deformation as a consequence of extensional tectonics, whereas Bouchez and Jover (1986), Friedrich et al. (1988), Faure et al. (2005), and Bellot and Roig (2007) argued that it corresponds to nappe stacking. This event is referred to as the “D2 event” (Faure et al., 2009).
A. Inset showing the Variscan belt in France. NASZ: North Armorican Shear Zone, NBSASZ: North Branch of the South Armorican Shear Zone, SBSASZ: South Branch of the South Armorican Shear Zone, NPF: North Pyrenean Fault, MGCR: Mid-German Crystalline Rise. B. Simplified structural sketch of the French Massif Central (purple areas: Upper Gneiss Unit; the Lower Gneiss Unit and Para-Autochtonous Unit are not distinguished). C. Structural map of the southern part of the French Massif Central with emphasis on the compressional structures. Syn- to late-orogenic extensional structures are not considered here. Captions are the same as in Fig. 2.
Here, we present structural and dating data in the Rouergue–Albigeois (Fig. 1B) area, which allows us to discriminate and date the south- and NW-ward displacement events (D3 and D2). We report similar observations in the Armorican Massif. We propose a geodynamic interpretation of the D2 event in the two massifs in the general framework of the Laurussia–Gondwana continental collision.
2 The architecture of the southern Massif Central
The French Massif Central stack of metamorphic nappes belongs to the northern Gondwana margin. From top to bottom, the following units are recognized: (1) the Upper Gneiss Unit, (2) the Lower Gneiss Unit, (3) the Para-Autochthonous domain, (4) the Fold-and-Thrust Belt, and (5) the southern Foreland. South of the Margeride pluton, the nappe architecture exhibits several particularities, which we describe below (Figs. 1 and 2).
Crustal scale cross-section through the Montagne Noire–Rouergue area (see location in Fig. 1). The bulk architecture results from three shearing events, D1, D2, and D3, with different ages and displacement directions.
2.1 The Upper Gneiss Unit
The Upper Gneiss Unit (UGU) crops out as several klippes (Marvejols, Vibal, Lévézou, Najac, and Decazeville). It is made of an association of felsic-mafic-ultramafic rocks and sedimentary rocks, called the leptynite-amphibolite complex. This complex experienced a high pressure–medium temperature (HP/MT) metamorphism (Bodinier and Burg, 1980–1981; Burg et al., 1986, 1989; Nicollet and Leyreloup, 1978) at 415 ± 5 Ma (U/Pb) or 408 ± 7 Ma (Sm/Nd), thus in Late Silurian–Early Devonian times (Paquette et al., 1995; Pin and Lancelot, 1982). During their exhumation, most of the eclogites were retrogressed in the amphibolite facies and the melting of the Al-rich part produced migmatite. Due to its circular shape in map view and the abundance of anatexites in its centre, the southernmost Lévézou klippe has long been considered as a diapiric dome (Burg, 1987; Collomb, 1970; Collomb and Meyzindi, 1991). However, structural and gravimetric data show that the amphibolites and migmatites are not rooted in the Lower Gneiss Unit but rather overlie it (Bayer and Hirn, 1987; Matte, 1986). The UGU klippes have thus been displaced to the southwest. As a matter of fact, northwestward-overturned folds deform the UGU foliations. As we explain below, they are attributed to the D2 deformation phase.
2.2 The Lower Gneiss Unit (LGU)
The Lower Gneiss Unit (LGU) consists of metagreywackes and metapelites that never experienced any HP/MT metamorphism. Several plutons that intrude the LGU, such as the Caplongue diorite (557 ± 10 Ma, U/Pb on zircon, Lafon, 1984), the Rodez alkaline granite, and the Pinet and Comps porphyritic monzogranites have been transformed into augen orthogneiss.
2.3 The Para-autochthonous Unit (PAU)
The Para-autochthonous Unit (PAU), well exposed in the Cévennes, consists of a thrust sheet imbrication of greenschist facies metapelites, quartzites, and metagrauwackes with subordinate layers of conglomerate, felsic and mafic lava, and rare intrusions (Caron, 1994; Pin and Marini, 1993). In the Albigeois area, the PAU has been subdivided into the Saint-Sernin-sur-Rance and the Saint-Salvi-de-Carcavès nappes (Guérangé-Lozes, 1987; Guérangé-Lozes and Burg, 1990; Guérangé-Lozes and Guérangé, 1984). The lithostratigraphy of these two nappes is similar, with, from base to top, Cambrian greywacke, rhyolite and ignimbrite, Ordovician white quartzite, and black metapelite. However, a biotite–garnet ± staurolite assemblage is fairly common in the former, but is absent in the latter.
2.4 The Fold-and Thrust Belt
The Fold-and Thrust Belt, developed in the Montagne Noire and the Viganais, is composed of unmetamorphozed fossiliferous Palaeozoic sedimentary rocks terminated by a Late Visean to Serpukhovian turbiditic basin. The series have been deformed by south-directed thrust sheets and kilometre-scale recumbent folds (Arthaud, 1970; Engel et al., 1980–1981; Gèze, 1949). The Montagne Noire axial zone is a migmatitic dome that crosscuts the folds and thrusts. This area is beyond the scope of this paper.
3 Polyphase shearing in the southern Massif Central
The pervasive foliation developed in the UGU, LGU, and PAU units results from three synmetamorphic ductile deformations, D1, D2, D3, each one associated with specific kinematics and P–T conditions. The three deformation phases are described below, from the youngest to the oldest.
3.1 The D3 Carboniferous southward-thrusting phase
Spectacular kilometre-scale, southward overturned recumbent folds characterize the Palaeozoic Fold-and-Thrust Belt (e.g., Arthaud, 1970, Arthaud and Matte, 1977). In the southern Montagne Noire, folding is coeval with horizontal cleavage and rare north–south to N30°E trending mineral and stretching lineation (Fig. 1). A more dominant lineation exists, that strikes NE–SW (Echtler and Malavieille, 1990; Lee et al., 1988). This northeastern microstructure is a late feature, since it overprints the already deformed series. The NE lineation is coeval with a HT/LP metamorphic event that has been related to the doming of the axial zone (e.g., Courtillot et al., 1986; Echtler and Malavieille, 1990; Faure et al., 2014). In the northern Montagne Noire and the Saint-Salvi-de-Carcavès nappe, the submeridian stretching lineation is coeval with a top-to-the-south sense of shear (Brunel, 1974; Guérangé-Lozes, 1987). In map view, the basal thrust of the D3 Saint-Salvi-de-Carcavès nappe truncates the bedding surface (Figs. 1 and 2). The D3 event recognized in the PAU series in the Cévennes and at the base of the UGU series in Marvejols is dated to ca. 335–325 Ma and 340–335 Ma, respectively (Caron, 1994; Faure et al., 2001; Pin and Lancelot, 1982). Therefore, the D3 event is clearly younging southward. Moreover, the D3 structures are locally overprinted by a top-to-the-north shearing related to the Late Carboniferous extensional tectonics (Arnaud and Burg, 1993) that are not considered here (cf. Faure et al., 2009 for details).
3.2 The D2 Early Carboniferous top-to-the-NW shearing phase
The main deformation of the Saint-Sernin-sur-Rance nappe is characterized by a flat-lying foliation and a NW–SE-trending mineral and stretching lineation ascribed to the D2 phase. Near Naucelle (Fig. 1), top-to-the-NW shear criteria coeval with a biotite–garnet–staurolite metamorphism are observed (Fig. 3C, E, F; Burg et al., 1986; 1989). A flat-lying mylonitic fabric with NW–SE-trending folds with axes parallel to the stretching lineation developed during the ductile shearing that accommodated the tectonic superposition of the Saint-Sernin-sur-Rance nappe upon the LGU (Fig. 3A, B). Further east, the dip of the foliation increases progressively to become SW east of Naucelle, and vertical south of the Lévézou klippe. The kinematic consistency from a top-to-the-NW shearing developed in the flat-lying foliation to a dextral one observed in vertical surfaces with subhorizontal stretching lineation suggests a wrench-thrust structure. The D2 deformation phase is not recognized in the Saint-Salvi-de-Carcavès nappe and farther south, but, in contrast, it is clearly observed in the North in the LGU, and to a lesser extent, in the UGU units.
Field and thin section photographs showing D1 and D2 deformations in the Saint-Sernin-sur-Rance nappe and Lower Gneiss Unit. A. NW–SE-trending D2 fold (L2) reworking an early NE–SW trending L1 lineation. B. NW–SE-trending “a-type” fold deforming the L1 lineation. C. Low-temperature D2 top-to-the-NW shear zone reworking the foliated Pinet granodiorite. D. North–south striking intrafolial D1 fold. E. Thin section of staurolite porphyroblast with top-to-the-NW, D2, asymmetric biotite pressure shadows. F. Garnet with top-to-the NW, D2, pressure shadows.
3.3 The D1 early southwestward shearing phase
The D2 top-to-the-NW shearing event was preceded by an earlier deformation phase that is well observed in the UGU and LGU series, in the Saint-Sernin-sur-Rance and Saint-Salvi-de-Carcavès nappes in the Rouergue area. Where the units are not deformed by the later D2 and D3 events, a north–south to N30°E-trending stretching lineation is observed, associated with intrafolial folds and top-to-the-SW shearing (Figs. 1, 3D). Yet, at many places, the D2 NW–SE trending folds deformed this early mineral lineation (Fig. 3A,B D). In the Saint-Sernin-sur-Rance nappe, kilometre-scale south-verging recumbent folds appear re-deformed by the D2 deformation, and hence are attributed to the D1 phase (Guérangé-Lozes, 1987). The D1 top-to-the-south stretching lineation is observed in the Rodez orthogneiss of the LGU series, and in the UGU gneiss, amphibolite and migmatite of the Lévézou and Vibal klippes.
The foliated Pinet orthogneiss has recorded both the D1 and the D2 deformation events. Although the D2 NW–SE lineation is dominant in many places (Fig. 1, Burg and Teyssier, 1983), an older north–south-trending D1 mineral lineation is observed where the D2 overprint is lacking. Petro-structural analysis documents a top-to-the-south shearing coeval with a high-temperature deformation, whereas low-temperature top-to-the-NW shear bands are ascribed to the D2 phase (Fig. 3D, Duguet and Faure, 2004).
4 Chronological constraints
4.1 Timing of the polyphase shearing in the southeastern Massif Central
Due to their similar submeridian trend and top-to-the-south sense of shear, the D1 and D3 deformation events have been so far considered as the result of a single, long-duration intracontinental shearing period, operating during the “Himalayan thrusting” (Mattauer, 1974, Mattauer and Etchecopar, 1977). However, we have shown that the D1 and D3 events are diachronous since they are separated in time by nearly 40 Ma. The age of the D3 event is well constrained as this event deforms the Late Visean–Early Serpukhovian (330–325 Ma) syntectonic flysch basin of the southern Montagne Noire. Muscovite populations from the Mendic orthogneiss and the Saint-Salvi-de-Carcavès nappe metasandstone yield 40Ar/39Ar ages of 330 ± 3 Ma and 333 ± 4 Ma, respectively (Costa, 1990). These Visean ages are similar to those available in the Cévennes area (Caron, 1994; Najoui et al., 2000).
The biotites, muscovites and amphiboles aligned along the NW–SE lineation reveal 40Ar/39Ar ages in the range 354–346 Ma. Since the closure temperature of micas is lower than that of the metamorphic climax, the above ages provide the minimum age of the D2 event. In the Saint-Sernin-sur-Rance nappe, muscovites from quartzites yield a 345 ± 3.5 Ma age. In the UGU and LGU series, biotites, muscovites, and amphiboles yield consistent 40Ar/39Ar ages in the range 354–346 Ma (Costa, 1990). The age of the D1 event is still poorly constrained with only one 40Ar/39Ar muscovite age of 380 ± 10 Ma (Guérangé-Lozes, 1987).
Whatever the precise absolute age of the three D1, D2 and D3 events, their relative timing allows us to draw an interpretative general cross-section (Fig. 2). The top-to-the-SW D1 structures were folded in the Late Devonian–Early Carboniferous by the northwestward D2 deformation. Later, during the Middle Carboniferous, the D3 southward shearing event re-deformed the whole stack. This new interpretation makes the Pinet orthogneiss a pre-D1 pluton.
4.2 New zircon U–Pb age of the Pinet orthogneiss
The Pinet orthogneiss is a monzogranite or syenogranite foliated in a variable way, which includes both nearly undeformed facies at local spots, and mylonitic to ultramylonitic shear zones at other spots. A detailed petro-structural study of the Pinet orthogneiss has revealed that the pluton experienced two deformation phases, ascribed to the D1 and D2 events (Duguet and Faure, 2004). The early deformation phase is coeval with a pervasive post-solidus planar fabric and a north-south striking lineation. The second deformation phase is characterized by a local foliation cross-cutting the previous one, and a NW–SE-striking lineation with top-to-the-NW shearing. Zircon populations in the granite yielded a TIMS U–Pb age of ca. 360 ± 20 Ma (Pin, 1981), whereas a biotite single grain was dated to 346 ± 7 Ma using the 40Ar/39Ar method (Maluski and Monié, 1988).
We separated zircon concentrates from two samples (FR 41 and FR 42) of the porphyritic orthogneiss, located along the Tarn River, at 44°03′16.27′′/002°45′56.35′′ and 44°02′43.57′′/002°46′47.70′′, respectively. We applied analytical procedures and data reduction as in Li et al. (2009) and Do Couto et al. (2016). Cathodoluminescence images of the zircon grains show a well-developed oscillatory zoning representative of a magmatic origin of the grains (Fig. 4).
Cathodoluminescence images of representative zircons for SIMS U–Pb dating. White ellipses indicate the in situ analytical spots of 206Pb/238U age (shown nearby the spot). SIMS spots are 30 μm in length for scale.
Zircons from the two rock samples yield consistent Ordovician ages of 477 ± 5 Ma and 475 ± 4 Ma (Fig. 5); these ages are interpreted as dating the emplacement of the porphyritic granite. This new result shows that the Pinet orthogneiss is not a syn-kinematic pluton emplaced during the D1 or D2 events, but instead formed in the Early Ordovician pre-orogenic magmatism phase, which is widespread in the Variscan belt. The ca. 350 Ma 40Ar/39Ar age of post-folial biotite corresponds to the D2 event.
Zircon ion-probe U–Pb age of the Pinet orthogneiss. Samples FR 41 and FR 42 are located at 44°03′16.27′′/002°45′56.35′′, and 44°02′43.57′′/002°46′47.70′′, respectively.
5 Discussion–conclusion
5.1 Significance of the three D1, D2 and D3 tectono-metamorphic events
The D1 event is recognized in the Lyonnais, Sioule, Limousin, and Plateau d’Aigurande areas of the Massif Central. It is coeval with the Middle Devonian (385–375 Ma) migmatization, which was interpreted as the result of the exhumation of the HP rocks of the UGU series (cf. Faure et al., 2008 for details).
The Late Devonian–Early Carboniferous (360–350 Ma) D2 event with a top-to-the-NW shearing is also recognized in many places of the Massif Central (Fig. 6). In the LGU series of Marvejols (Fig. 1), biotites from metadiorite and micaschist, aligned along the NW–SE lineation, yield consistent 40Ar/39Ar ages of 352 ± 2 and 351 ± 3 Ma (Costa, 1990). The Limousin, Sioule and Plateau d’Aigurande series are affected by a consistent medium temperature–medium pressure metamorphism coeval with a top-to-the-NW shearing (e.g., Bellot and Roig, 2007; Bouchez and Jover, 1986; Faure et al., 1990, 1993, 2005; Floc’h, 1983; Friedrich et al., 1988; Roig and Faure, 2000). Since the 40Ar/39Ar dates are very sensitive to the temperature, the monazite U–Th–Pb chemical chronometer that records high temperature events has also been carried out in the UGU, LGU, and PAU metapelites of Limousin and Sioule areas (Do Couto et al., 2016; Melleton et al., 2009). The 360–350 Ma ages yielded by the syn-D2 metamorphic monazites confirm the Famennian–Tournaisian age of the D2 event. Furthermore, in the eastern Massif Central, the Brévenne ophiolitic nappe was emplaced from south-east to northwest under amphibolite facies conditions, before the unconformable deposition of the Early Visean (i.e. 345 Ma) Le Goujet, terrigenous formation (Leloix et al., 1999). Thus, the Brévenne ophiolitic nappe formed also during the D2 event.
Structural map of the French–Britain segment of the Variscan belt with emphasis on the D2 NW–SE shearing developed from Albigeois up to the Lizard nappe. The Late Carboniferous, south-directed, D3 event develops in the southern Massif Central (Cévennes and Montagne Noire). At the scale of the entire Variscan belt, a top-to-the-north-directed deformation, characterized by thrust and faults, deforms the Ardennes Massif and the northern Variscan front.
The D3 event is identified only in the southern Massif Central. The Visean to Serpukhovian southward migration from middle to shallow crustal levels is acknowledged since a long time as the consequence of the post-collisional nappe stacking (Faure et al., 2009; Ledru et al., 1989; Matte, 1986).
5.2 Geodynamic significance of the D2 event in the French Variscan segment
In the southern Armorican Massif, which belongs to the metamorphic zone of the Variscan chain, the D2 synmetamorphic shearing event is recognized in the Vendée and the South Brittany coast (Burg, 1981; Cannat and Bouchez, 1986; Fig. 6). There, an east–west- to NW–SE-striking mineral lineation develops on a flat-lying foliation, coeval with a MP/MT metamorphism characterised by a biotite–garnet–staurolite assemblage. Top-to-the-NW shearing shown by asymmetric pressure shadows and shear bands is conspicuous.
More to the north, in Central Armorica, top-to-the-NW shearing is described only in the Saint-Georges-sur-Loire and Lanvaux units (Bouchez and Blaise, 1976; Cartier et al., 2001; Cartier and Faure, 2004; Cogné et al., 1983; Diot et al., 1983; Faure and Cartier, 1998). The pre-Visean age of the deformation is attested to by the age of the terrigenous deposits of the Ancenis basin, which did not record the D2 event. Therefore, the top-to-the-NW shearing, which is widespread in the Armorican Massif and the southern Massif Central, is a major feature of the Variscan orogeny.
The MP/MT D2 metamorphism dated to ca. 360 Ma is coeval with the Devonian–Carboniferous “Bretonian phase” long recognized in the central part of the Armorican Massif, through the Tournaisian unconformity and the widespread erosion of the Late Devonian formations (e.g., Stille, 1929 in Rolet, 1982). In the 1970s and 1980s, on the basis of the gentle folding of the pre-Tournaisian formations, and the small unconformity angle (lower than 20°), the Bretonian phase was considered as a kilometre-scale upright folding coeval with vertical movements, responsible for erosion and clastic sedimentation in the Laval and Châteaulin basins (e.g., Cogné, 1965, 1974; Houlgatte et al., 1988; Le Gall et al., 1992; Paris et al., 1982; Pelhâte, 1971; Fig. 6). However, recumbent folds, ductile shear zones, and low-angle thrust faults identified in the Brest area (Rolet et al., 1986) allow us to reassess the importance of the Bretonian phase.
The Variscan orogeny is the consequence of the closure of the Rheic ocean, and subsequent collision of Laurussia that was driven by a southward oceanic and then continental subduction below Gondwana. The Rheic suture is today hidden below the English Channel, but the Lizard ophiolitic nappe of southwestern Britain that overthrusts to the NNW the Laurussia foreland at ca. 360 Ma is a remnant of the Rheic Ocean (e.g., Holder and Leveridge, 1986; Le Gall and Darboux, 1986; Sandeman et al., 1994). Therefore, we argue here that the ductile and synmetamorphic, top-to-the-NW, shearing observed in the French Massif Central and South Armorica, and more broadly the Famennian–Tournaisian deformation ascribed to the Bretonian phase, are the result of the collision between Laurussia and Gondwana. The top-to-the-NW shearing observed at the microtectonic scale results from the Variscan collision.
Although close to the Rheic suture, (Fig. 6), the synmetamorphic ductile deformation does not exist in Central Armorica. A possible explanation would be to consider the contrasted mechanical behaviour between the Massif Central–South Armorican domain with a widespread D2 event, and the Central Armorican domain where D2 is absent. Due to its crustal rigidity, possibly related to the Neoproterozoic Cadomian orogeny, and the thick subcontinental mantle lithosphere, the Central Armorican domain behaved as a rigid plate characterized by sinistral wrenching, responsible for the opening of the Châteaulin and Laval basins (Fig. 7). In its southern margin, along the Nort-sur-Erdre strike-slip fault, and the Saint-Georges-sur-Loire and Lanvaux units, the sinistral shearing accommodated the decoupling between the ductile and brittle domains (Cartier and Faure, 2004; Cartier et al., 2001; Cogné et al., 1983; Diot et al., 1983; Faure et al., 1997, 2005; Lardeux and Cavet, 1994). This interpretation, which is at variance with the previous geodynamic models (e.g., Ballèvre et al., 2009), provides a satisfactory solution for the significance of the D2 event in the French Variscan orogen.
3D Schematic geodymamic interpretative model to account for the tectonic difference between the Massif Central–Massif Armoricain areas dominated by the top-to-the-D2 synmetamorphic ductile shearing, and the Central Armorican Domain, where the D2 deformation is represented by brittle structures only. This difference in rheological behaviour can be explained by a tectonic decoupling between the two domains. The Neoproterozoic Cadomian orogeny, and the thick subcontinental lithosphere mantle may explain the rigid behaviour of the Central Armorican Domain, in which the brittle deformation is associated with the opening of the Châteaulin and Laval basins. The left-lateral Nort-sur-Erdre strike-slip fault can be seen as a transfer fault between the southern Gondwana and central Armorican domains.
Acknowledgements
This article synthetizes numerous works conducted since 20 years, in the framework of PhD and Master theses supported by ISTO and BRGM. The dating of the Pinet orthogneiss has been funded by the National Natural Science Foundation of China (grant No. 41273070), and the Ministry of Science and Technology (MOST) of China (projects 2016YFC0600401, and 2016YFC0600102). Scientific and Editorial comments by I. Manighetti that allowed us to improve the early version of this paper are greatly acknowledged.
The first author of this paper has been invited by the Editor-in-Chief, being one of the recipients of the 2016 prizes and awards of the French Academy of Sciences.