Four main ductile and syn-metamorphic phases can account for the tectonic evolution of the French Paleozoic Massifs. The earliest one is coeval with the high (to ultra high) pressure metamorphism recorded in the eclogite facies rocks of the UGU (Figure 6). Although most of high pressure rocks are mafic ones, jadeite orthogneiss are locally found (e. g. Lasnier et al., 1973; Ballèvre et al., 1994). Similar physical conditions of 18-20 Kb and 650-750°C are computed from UGU eclogites both in the Massif Central and in the Massif Armoricain (Figure 7, cf Lardeaux et al., 2001, and Godard, 2001 for reviews of the Massif Central and Massif Armoricain eclogitic metamorphism respectively). Available radiometric dates range around 415 Ma (Pin and Peucat, 1986). Since the high pressure rocks are blocky relics within amphibolite facies rocks or migmatites, the structural information is quite rare. The following three events are better documented.
Figure 6. Synthetic map of the Massif Central
Synthetic map of the Massif Central and Massif Armoricain showing the structural, metamorphic and geochronological elements related to the Late Silurian HP metamorphism and the following Devonian D1 event. In the Upper Gneiss Unit and Léon Block, the available radiometric dates, stretching lineation and related top-to-the-SW kinematics are presented (see text for discussion).
This tectonic-metamorphic event is coeval with the crustal melting of the Si-Al rich rocks (i. e. pelites and granitoids) of the Upper Gneiss Unit, whereas mafic rocks remain as amphibolite restite in migmatites. The migmatisation is dated of Middle Devonian : 384+/- 16 Ma by Rb/Sr method on whole rock in the Lyonnais (Duthou et al., 1994); 383+/-5 Ma by U/Pb method on zircon fractions in Limousin (Lafon, 1986); and 386+/-6 Ma by chemical U/Th/Pb method on monazite single grain in the Champtoceaux Complex (Cocherie et al., in press; Figure 6). P-T estimates from garnet-plagioclase and garnet-biotite pairs provide metamorphic conditions of 7+/- 0.5 kb and 700+/-50°C respectively (Mercier et al., 1991; Roig and Faure, 2000, Figure 7).
Figure 7. Various P-T-t paths
Various P-T-t paths showing the two metamorphic-tectonic cycles experienced by the Massif Central and South Massif Armoricain rocks. A: Silurian-Devonian cycle due to continental subduction and collision of N. Gondwana margin below Armorica. Crustal rocks forming the Upper Gneiss Unit are metamorphosed in eclogite facies around 420-400 Ma (i. e. Late Silurian-Early Devonian) and exhumed around 390-380 Ma (Early-Mid Devonian). During this exhumation, the mafic protoliths are retrogressed into amphibolites whereas the metapelites and orthogneiss are easily melted to produce migmatites. B: P-T paths related to the Carboniferous cycle in different units. TPU: Thiviers-Payzac Unit, UGU: Upper Gneiss Unit, LGU: Lower Gneiss Unit. Petrology and geochronology indicate 360-350 Ma age for the prograde intermediate pressure-intermediate temperature metamorphism. Retrogression of these units is poorly documented, except for the 340-325Ma anatexis.
Structurally, the flat-lying migmatitic foliation exhibits a NE-SW trending (N30 to N60E) mineral and stretching lineation marked by fibrolitic sillimanite or biotite in migmatites and biotite or amphibole in mafic rocks. Shear criteria indicate top-to-the-SW displacement (Floc'h, 1983; Faure et al., 1997; Roig and Faure, 2000). Locally, the migmatitic texture is totally overprinted by a mylonitic fabric. 40Ar/39Ar ages of 381+/-5 Ma and 389+/-8 Ma from hornblende in amphibolite in the Plateau d'Aigurande (Boutin and Montigny, 1993) or 385+/-8 Ma and 383 +/- 8 Ma on amphibolite and biotite respectively from gneiss recovered in the Couy borehole (Figure 6; Costa and Maluski, 1988) correspond to the age of this syntectonic metamorphism. In agreement with thermal modelling (e. g. England and Thompson, 1986) these radiometric ages support an isothermal decompression leading to crustal melting nearly 30 Ma after the development of the high pressure metamorphism. A high thermal gradient of ca. 50°/km and a 3-4 mm/y exhumation rate are computed.
As already pointed out, (Faure et al., 1997), in Morvan (Figure 2), the nearby occurrence of Middle Devonian unmetamorphosed and undeformed rocks (Delfour, 1989) and eclogitic relics in migmatite (Godard, 1990) suggests that, at least in North part of the Massif Central, the high pressure rocks were already exhumed before Middle Devonian. Other undeformed Middle to Late Devonian sedimentary rocks crop out from Vendée to Vosges (Figure 6) however their structural setting along late faults does not allow us to observe an unconformable contact. The lack of large Devonian clastic deposits implies that the exhumation of the HP Variscan metamorphics was dominantly a tectonically assisted process.
Since structural and metamorphic D2 features differ in the North Gondwana margin and in the Central Armorican Domain, these two areas are presented successively.
i) In the Massif Central and the South part of the Massif Armoricain, the major structure related to this stage is a NW-SE trending stretching lineation (Figure 8) widespread in the Lower Gneiss and Para-autochthonous Units but less developped in the Upper Gneiss Unit. The NW-SE lineation is also observed in the Thiviers-Payzac and Brévenne Units. Whatever the tectonic unit, the foliation along which the NW-SE lineation develops is flat-lying or dipping at a low-angle. This Early Carboniferous lineation must not be mistaken with another NW-SE stretching lineation coeval with the emplacement of synkinematic Namurian-Westphalian plutons during the syn-orogenic extension (cf. section 5.5).
Figure 8. Synthetic map of the Late Devonian-Early Carboniferous D2 event
Synthetic map of the Late Devonian-Early Carboniferous D2 event in Massif Central and Massif Armoricain characterized by top-to-the-NW ductile shearing and pre-Late Visean Guéret-type plutonism. T and SG are Tréban and St-Gervais granites respectively. These two plutons are the continuation of the Guéret massif east of the Sillon Houiller fault. Available U-Pb, Ar-Ar and Rb-Sr ages for granitoids and metamorphic rocks are also shown. A. M. B. P.: Magnetic Anomaly of Paris Basin.
Thermo-barometric constraints are well established in the Limousin and Rouergue (Floc'h, 1983; Feix, 1988; Burg et al., 1989; Bellot, 2001, Duguet, 2003; Figure 7). In the Upper Gneiss Unit, syn-D1 metamorphic minerals, such as kyanite, are found either as inclusions in garnet or plagioclase or as cataclazed grains boudinaged along the NW-SE stretching lineation. Syn-D2 minerals, such as biotite, garnet, plagioclase crystallized along the NW-SE lineation indicate 7-10 kb and 600-700°C. Paragneiss belonging to the Lower Gneiss Unit provide nearly similar conditions of 8-10 kb and 550-600°C. In the LGU, amphibolites which never experienced the high-pressure event indicate 8-10 kb and 700-800°C (Santallier, 1981). In the uppermost Thiviers-Payzac Unit, biotite-garnet +/- staurolite commonly crystallize along the NW-SE stretching. P-T conditions of 4-6 kb and 400-500°C respectively are computed. At the scale of the whole Massif Central, the thermobarometric constraints support a prograde middle pressure/middle temperature metamorphism coeval with the NW-SE stretching.
Top-to-the-NW ductile shearing is dominant from southeast Massif Central up to the South part of the Massif Armoricain (Figure 8; Brun and Burg, 1982; Bouchez and Jover, 1986; Burg et al., 1987; Friedrich et al., 1988). Moreover, in the Rouergue area, a kilometer-scale flat lying synmetamorphic ductile shear zone, called the “Naucelle thrust”, (Figure 8) transports to the NW the Para-autochthonous Unit of the Albigeois area upon the Lower Gneiss Unit. This northwestward shearing reworks D1 structures, such as NE-SW trending isoclinal folds and stretching lineations (Duguet and Faure, 2004). However, in some places such as Marvejols or S. Limousin, top-to-the-SE displacements are also recognized but the timing of these displacements remains poorly documented. For instance, in Limousin, it has been shown that this top-to-the-SE shearing is a late deformation related to the Late Carboniferous syn-orogenic extensional tectonics (Roig, 1997).
Top-to-the-NW shearing is also well developped in the Brévenne ophiolitic nappe, in particular, at the nappe base, a mylonitic shear zone with isoclinal folds parallel to the NW-SE stretching lineation develops (Leloix et al., 1999). This deformation is dated as pre-Visean (i. e. older than 345 Ma) by the Early Visean unconformity (the so-called Le Goujet conglomerates). The syn-D2 middle pressure/middle temperature metamorphism is dated by 40Ar/39Ar method on biotite, muscovite and amphibole between 360 and 350 Ma (Maluski and Monié, 1988; Costa, 1989, Costa, 1991-92; Figure 8). A general interpretation of the Late Devonian-Early Carboniferous D2 event will be discussed in section 5.5.
ii) In the Central Armorican Domain, several lines of evidence argue for the existence of a Late Devonian-Tournaisian event known as the “Bretonian phase”. From Brest area up to the Laval basin, stratigraphic and sedimentological works show that erosion of the Ordovician to Devonian series was very active during Early Carboniferous (e. g. Darboux et al., 1988; Paris et al., 1982; Rolet, 1982; Rolet et al., 1994). At depth, the western part of the Central Armorican Domain experienced a regional middle pressure /middle temperature metamorphism with biotite-garnet-staurolite assemblage older than the thermal contact metamorphism related to the Carboniferous Pontivy granite which is dated at 344+/-8 Ma (Rolet et al., 1994). This “Bretonian metamorphism” is attributed to crustal thickening, however, its age and associated structures are still unknown. Thin skin thrusting is described in Brest and Plourac’h areas (Rolet et al., 1986, 1994; Darboux and Le Gall, 1988, Figure 4).
More to the SE, the weakly or unmetamorphosed Devonian block-in-matrix series of the St-Georges sur Loire Unit overthrust the Lanvaux Unit to the North (Cartier et al., 2001). Top-to-the-NW shearing and left-lateral wrenching on flat-lying and vertical foliations respectively is recognized in the Lanvaux orthogneiss and its metasedimentary host rocks (Faure and Cartier, 1998). This event is older than the Middle Carboniferous dextral strike-slip of the S. Armorican Shear Zone and younger than Middle Devonian which is the age of the deformed sedimentary rocks. In the Central Armorican Domain, wrench tectonics lasting from Late Devonian to Middle Carboniferous are the main structural feature. The boundary between the N. Gondwana margin and the Central Armorican Domain, i. e. the Nort-sur-Erdre Variscan suture zone, (cf section 2) is reworked by a left-lateral shearing along which the Ancenis basin opens as a sinistral pull-apart in Tournaisian and Visean times (Diot, 1980; Dubreuil, 1986; Figure 9).
Figure 9. Schematic block diagram showing the geometric and kinematic relationships
Schematic block diagram showing the geometric and kinematic relationships between the Central Armorican Domain and the N. Gondwana margin in the southern part of the Massif Armoricain.
A consequence of the Late Devonian-Tournaisian D2 event is the thermal resetting of the geochronometers. Sm-Nd measurements of eclogites in the Champtoceaux Complex (Figures 3,4) yield a 360 Ma age (Bosse et al., 2000). This date does not comply with other ages of the high-pressure metamorphism of the Upper Gneiss Unit. Moreover, the migmatite that forms the upper part of the Champtoceaux Nappe and encloses mafic restites derived from eclogites is dated at 386+/-5 Ma (Cocherie et al. in press; cf section 3.1). Therefore, this 360 Ma age does not corresponds to the crystallization of the HP minerals. The geological significance of the 360 Ma ages recovered in the S. Brittany eclogites remains unclear. A possible interpretation would be a resetting duing the D2 tectonic-metamorphic event.
Ductile syn-metamorphic D3deformation is only recognized in the southern parts of the Massif Central and Massif Armoricain (Figure 10). In the southern third of the Massif Central, the Para-autochthonous Unit of Cévennes and Albigeois area experienced its first syn-metamorphic deformation at ca 340-335 Ma (e. g. Arthaud and Matte, 1974; Caron 1994; Faure et al., 2001 and therein references, Figure 10). More to the south, southward verging recumbent folds and thrusts of the Paleozoic series are dated of Late Visean-Early Namurian (ca. 325 Ma) by the syn-orogenic sediments in a flexural type foreland basin (e. g. Engel et al., 1981; Feist and Galtier, 1985). This progressive southward younging of the shearing tectonics coeval with the development of flat-lying foliations, N-S trending stretching lineation and top-to-the south displacement have been put forward for an Himalayan-style nappe stacking (Mattauer, 1975). This model accounts only for the Visean tectonic evolution of the southern outer part of the belt. However, the transition between the Early Carboniferous top-to-the-NW shearing and Middle Carboniferous top-to-the South shearing remains unclear. The D3 event is also recognized in the Vendée area of the southern massif Armoricain where a Tournaisian olistostrome is involved in southward shearing (Colchen and Poncet, 1989).
Figure 10. Middle Carboniferous D3 Event
NNE-SSW compression is active in the northern (SW England, Ardenne) and southern (Montagne Noire) forelands whereas NW-SE synorogenic extension develops in the northern part of the Massif Central. In the Massif Armoricain, left-lateral wrenching accommodates the extensional regime. Middle Carboniferous migmatites of north Cévennes, Montagne Noire, south Brittany are also pictured. A. M. B. P.: Magnetic Anomaly of Paris Basin
In the Central Armorican Domain, the Middle Carboniferous D3 event is recorded by the syn-tectonic chaotic sedimentation of the Châteaulin, Laval and Ancenis basins which are located along wrench faults (Figures 4, 10). Most of authors assume that these basins are dextral pull aparts (e. g. Darboux and Le Gall, 1988; Houlgatte et al., 1988; Le Gall et al., 1992; Rolet et al., 1994). If a Late Carboniferous dextral reworking is demonstrated by sheared sedimentary rocks and plutons (e. g. Berthé et al., 1979), a Visean dextral wrenching is not supported by data. Indeed, the opening of these basins probably begins already in Tournaisian and continues with the same characteristics in Visean. The left-lateral ductile shearing older than the Late Carboniferous dextral one is well documented for the Ancenis basin (see above section 3.2) and also described along the Lanvaux orthogneiss (Cogné et al., 1983; Faure and Cartier, 1998). Therefore, in the present state of knowledge, a syn-sedimentatry left lateral strike-slip motion controlling the sedimentation of the Châteaulin and Laval bassins would be in agreement with the whole kinematic picture of the Massif Armoricain (Figure 10). In the northern Massif Central, the D3 event corresponds to the onset of syn-orogenic extension. Since it is coeval with magmatic activity, it will be presented in the following sections 4.3 and 5.6.