Structure, metamorphism, and plutonism

Lithological map supports a westward-dipping monoclinal structure for western and central Maures (Figure 3). However, this relatively simple structure reflects a complex tectonics history that involves several tectonics phases (Arthaud and Matte, 1966; Maluski, 1968; Bronner et al., 1971; Chabrier and Mascle, 1975; Seyler, 1975; Le Marrec, 1976; Maquil et al., 1976; Conti, 1978; Olives-Banos, 1979; Bard and Caruba, 1981; Seyler and Crévola, 1982; Caruba, 1983; Goldberg, 1983; Vauchez and Bufalo, 1985; Seyler, 1986; Vauchez and Bufalo, 1988; Morillon, 1997). Petro-structural investigations have inferred the relationships between three of them and a polyphased regional metamorphism (Maluski, 1968; Conti, 1978; Seyler, 1975; Caruba, 1983; Goldberg, 1983), and demonstrate that the D2 tectonic phase is coeval with regional IP metamorphism (Buscail, 2000 and references therein). Many authors agree that the Variscan Maures massif has experienced late-orogenic extension marked by brittle-ductile normal faults (Gaubert, 1994; Ciancalleoni, 1995; Buscail, 2000; Morillon et al., 2000; Bellot et al., 2002), but debate continues about the mode of extension, and the relative importance of one or several pre-extension tectonics that structured this Variscan outcrop (Dumoulin-Thiault et al., 1996). Petro-structural investigations inferred the close relationships between three metamorphism and ductile tectonics (Seyler, 1975; Conti, 1978; Golberg, 1983; Séno, 1986; Gaubert, 1994; Ciancaleoni, 1995; Buscail, 2000). Based on previous and new data, we attempt to propose a sequence of Paleozoic tectonic, metamorphism, and plutonic events of the Maures massif. Western, central, and eastern Maures have possibly suffered early specific tectonics, but a common Carboniferous polyphased tectonics is inferred from superimposed metamorphic fabrics, composite foliations and various lineations.

D1 deformational phase

The D1 deformational phase is responsible for the S1-L1 rock fabric, sheath F1 folds (Figure 4A), isoclinal F1 folds (Figure 4B), and and top-to-the WNW shearing (Figures 4C and 4D). It affects western and central Maures. The main shear zone occurs within the Collobrières Unit, the uppermost Bormes orthogneiss dated 345 ± 3 Ma, the lowermost Loli Unit interpreted as a Tournaisian syntectonic deposit, and the Cavalaire Unit those sheared amphibolites are dated 348 ± 7 Ma. Because D1 shearing ceased before intrusion of the Hermittan granite (338 ± 6 Ma), it can therefore be assigned to the Tournaisian (~350-340 Ma). D1 shearing was associated with IP/HP-LT prograde metamorphism (Pmax) evidenced from garnet inclusions (Buscail, 2000). D1 structures and mineral assemblages are well-preserved in the western Maures where S0-1 composite foliation and L0-1 intersection lineation are commonly observed in flysch-like meta-sediments (Figure photo). In the northern Cavalaire Unit, lenses of metagabbros, orthogneiss, and metaperidotites forming a tectonic melange indicate a WNW-ESE stretching axis and preserved top-to-the WNW shearing in amphibolites facies conditions. In the northeastern Cavalaire Unit, a WNW-directed, eastward dipping shearing is described at the boundary between Cavalaire and Cap Nègre Units and interpreted as a WNW-directed thrust (Morillon, 1997). WNW-directed shearing is likely to be WNW-verging thrusting responsible for stacking of units leading to crustal thickening during the Early Carboniferous.

Figure 4. Superimposed fabric and kinematic criteria

Superimposed fabric and kinematic criteria

Superimposed fabric and kinematic criteria for NW-verging thrust tectonics in the western-central Maures. A. N110°E-trending sheath F1 fold (Cavalaire Unit). B. Meta-aplite foliated and WNW-trending folded during D1 tectonics (Cavalaire Unit). C. Development of F1 foliation and shear bands associated with syntectonic growth of garnet, staurolite, biotite and kyanite, indicating a top-to-the WNW sense of shear (Cap Nègre Unit). D. Sigmoid-shaped quartz developed from isoclinal folds, indicating northwestward shearing (Fenouillet Unit). E. Low-angle, superimposed S1 and S2 foliations in the Porquerolles island (Fenouillet Unit). F. Composite S1-2 foliation reworked by an upright N-S-trending F3 fold (Maurette Unit) that the axis is parallel to the stretching L3 lineation.

D2 deformational phase

The D2 deformational phase is responsible for the regional S2-L2 rock fabric (Figure 4E), F2 isoclinal and sheath folds and top-to-the SE shearing. It affected the central Maures and is particularly intense at top and bottom of the Cap Nègre Unit (Bellot and Bronner, 2000; Buscail, 2000). Observed asymmetric microstructures are: sigma-type garnet (Buscail, 2000) and staurolite with asymmetric strain shadows of biotite (Gaubert, 1994), preferred orientation of quartz sub-grains in ribbons (Séno, 1986), red biotite strain-shadows around plagioclase porphyroblasts (Bellot and Bronner, 2000), -type plagioclase with inclusions of muscovite-biotite-garnet-sillimanite (Bellot et al., 2002a). Following these observations, D2 shearing is assumed to be associated with IP-HT metamorphism (Seyler, 1975; Conti, 1978; Golberg, 1983; Gaubert, 1994; Ciancaleoni, 1995; Buscail, 2000) that decreases from 0.8-1.0 GPa/500-600°C to 4-5 kb/600-700°C in the Central Maures (Bellot et al., 2002a). D2 metamorphism increases eastward, i.e. downward the nappes pile, from anchizonal to amphibolites facies conditions.

D3 deformational phase

The D3 deformational phase is responsible for orogen-parallel sinistral shearing of the S1-2-L2 rock fabric. It affected the whole Maures massif and increases eastward. It was associated with IP-HT retrograde metamorphism (Tmax), and is likely to have accommodated exhumation of the nappes pile (Bellot et al., 2000a). In the eastern Maures, orogen-parallel sinistral shearing associated with sheath folds deformed the nappes pile (Vauchez and Buffalo, 1988) during its partial melting (Le Marrec, 1976) at c.a. 334 ± 5 Ma (Morillon, 1997). In the central Maures, a similar strain pattern has been found in the Cavalaire Unit in which the asymmetric shape of peridotites lenses and the en echelon pattern of amphibolites lenses both indicate sinistral shearing at a regional-scale (Bellot et al., 2000a; 2002b). In the western Maures, a high-angle sinistral shearing is superimposed on the boundary between Fenouillet and Maurettes Units (Bellot, 2004). These shear zones are likely to reflect crustal-scale sinistral shearing centered on the Grimaud fault that produces asymmetric folding on both sides (Vauchez and Buffalo, 1988). Sinistral shearing combined with regional-scale, orogen-parallel upright folding (Figure 4F), as the Porquerolles synform (Bellot, 2004) and synforms and antiforms of the eastern Maures (Vauchez and Buffalo, 1988) and the Tanneron (Crévola, 1977).

The D3 orogen-parallel sinistral tectonics was coupled with intrusion of syntectonic granites in the middle crust (3-6 kbar; Amenzou, 1988) close to the Grimaud fault during its sinistral movement (Vauchez and Bufalo, 1988; Bellot et al., 2002b). They are the Hermittan granite in the western Maures (338 ± 6 Ma, zircon U-Pb; Moussavou et al., 1998) and the Réverdi quartz diorite in the eastern Maures (Gueirard, 1964; 333 ± 4 Ma, zircon U-Pb; Moussavou et al., 1998). They are likely sheet-dykes complexes rather than sills emplaced in the core of an orogen-parallel upright antiform of the nappes pile (Morillon, 1997; Buscail, 2000). Shearing of amphibolites along the Grimaud fault (330 ± 2 Ma and 328 ± 3 Ma) likely reflect lastest stages of the D3 tectonics. By their relative age and their kinematics, SE-directed shearing and sinistral shearing are likely to be either two stages of a progressive transpressional tectonics, or combined into a single transpressional tectonics of Upper Visean age (340-330 Ma).

D4 deformational phase

The D4 deformational phase is responsible for top-to-the NNW flat-lying shearing, NNW-post metamorphic folding, brittle-ductile normal faulting and that crosscut the nappes pile in the central Maures, combined to dextral shearing close to and along the dextral Grimaud fault. This deformation is well dated Namurian (325-315 Ma) by biotite 40Ar-39Ar dating on western boundary of the sheared Bormes orthogneiss (~325 Ma; Gaubert, 1994), and by amphibole, biotite, muscovite 40Ar-39Ar dating on mylonites of the Cap Nègre Unit (319-321 Ma; Morillon, 1997). The Plan-de-la-Tour porphyric monzogranite (Serment and Triat, 1967), dated Namurian (324 ± 5 Ma, zircon U-Pb; Moussavou, 1998; 325 ± 10 Ma, whole rock Rb-Sr; Roubault et al., 1970a; 325 ± 10 Ma, whole rock Rb-Sr; Maluski, 1972), was emplaced in the eastern Maures in a relay of two branchs of the Grimaud fault during late stages of its dextral movement (Onezime et al., 1999). According to our field investigations, the root zone seems to be close to the center pluton. Shearing of amphibolites along the dextral Grimaud fault (312 ± 3 Ma and 314 ± 5 Ma) possibly reflects latest stages of the D4 tectonics.

The D4 deformational phase was associated with LP (0.2-0.3 GPa) metamorphism those the T increases eastward in both metapelites (Buscail and Leyreloup, 1999) and metabasites (Bellot et al., 2003). Involved mineral assemblages in metapelites are chlorite-quartz-muscovite in the western Bormes Unit (Gaubert, 1994), andalusite-biotite in the Cap Nègre Unit (Buscail and Leyreloup, 1999), and sillimanite-quartz-K-feldspar in the Cavalaire Unit (Buscail, 2000). Retrogression of previous garnet, staurolite and kyanite occurs too (Bellot et al., 2002a). Sills of syntectonic leucogranite and pull-apart pegmatite in the northern and southern Cavalaire Unit took place along normal/dextral shear zones. Centimeter-thick pseudotachylite layers and/or a cataclastic strain along high-angle normal faults are superimposed on ductile structures (Morillon, 1997; Bellot et al., 2002a). They indicate that extensional deformation has continued to brittle-ductile conditions. These data support rapid cooling and exhumation of the Central Maures during the Middle Carboniferous (Morillon et al., 2000).

The component of wrench tectonics increases eastward and culminates along the dextral Grimaud fault that produces chlorite-muscovite-bearing ultramylonites (Morillon, 1997; Onezime et al., 1999). These relationships suggest that the Grimaud fault plays a role of transfer fault of deformation permitting to the eastern Maures to be less affected by extension. By all its features, the D4 deformational phase typifies synorogenic extension experienced by internal zones of the Variscan belt during the Middle Carboniferous.

D5 deformational phase

The D5 deformational phase is responsible for fracturing along orogen-parallel Grimaud and La Mourre faults, deposition of coal-interbedded conglomerates and arkoses in the Plan-de-la-Tour basin (Lower Stephanian) and Reyran basin (Upper Westphalian-Lower Stephanian) along these faults. It was also coupled, in the eastern Maures, with posttectonic emplacement of granites (~300 Ma) in the upper crust close to the Grimaud fault: cordierite-bearing Moulin Blanc granite (301,6 ± 0,8 Ma on biotite and muscovite), Camarat granite (300,2 ± 0,2 Ma on biotite and 299,4 ± 0,5 Ma on muscovite), and dykes of pegmatite (303,6 ± 1,5 Ma on muscovite).

Amphibole, biotite, and muscovite 40Ar-39Ar dating on a wide range of igneous and metamorphic rocks of the eastern Maures yielded a reduce and very concordant time span (305-300 Ma; Morillon et al., 2000). 40Ar-39Ar ages obtained on metamorphic rocks and syntectonic granites are likely to be cooling ages: amphibolitized eclogite (303 ± 3 Ma), migmatitic gneiss (304 ± 1,5 Ma, 306 ± 2 Ma, 301 ± 0,6 Ma on biotite), Reverdi quartz diorite (300,5 ± 0,6 Ma on amphibole, biotite, muscovite), St. Pons les Mûres granodiorite (302,5 ± 2 Ma on biotite and muscovite), and apatite U-Pb age on migmatitic gneiss (301 ± 2 Ma; Lancelot et al., 1998). Similarly, ages obtained on sheared samples of the Plan-de-la-Tour granite located along the Grimaud fault (301,6 ± 1,6 Ma on biotite; 301,7 ± 1 Ma and 295 ± 0,5 Ma on muscovite; Morillon et al., 2000) are similar to those of undeformed samples of the same granite more eastward (304,4 ± 2,7 Ma on muscovite; Morillon et al., 2000) and are therefore interpreted as cooling ages, rather than "deformation" age. As a summary, 40Ar-39Ar dating reflects a major thermal overprint and/or rapid cooling (~305-300 Ma) of the eastern Maures associated with magmatism (~300 Ma) during the Lower Stephanian (Morillon et al., 2000). The D5 deformational phase can therefore be assigned to the Upper Westphalian-Lower Stephanian (310-300 Ma). This phase typifies postorogenic extension experienced by internal zones of the Variscan belt during the Upper Carboniferous, as well-described in the French Massif Central (Malavieille, 1993; Burg et al., 1994; Faure, 1995; Ledru et al., 2001; Roig et al., 2002; Bellot et al., 2005).

Geometry of the Plan-de-la-Tour basin and its precise tectonic setting in relation with the Grimaud fault is a matter of debate: syncline (Demay, 1927a and b; Bordet, 1967; Edel, 1998), pull-apart developed during a late stage of Grimaud dextral faulting (Onezime et al., 1999), deposition in a basement flexure during Grimaud sinistral faulting and refolding during Grimaud dextral faulting (Toutin-Morin and Bonijoly, 1992), graben lately refolded during Grimaud dextral faulting (Morillon et al., 2000), hemi-graben formed during the Lower Stephanian and refolded during the Upper Stephanian (Basso, 1985), continuous deposition and deformation in a basement flexure in relation to Grimaud sinistral faulting (Castanet, 1998). Our field investigations point out a monoclinal structure formed during folding of basement and fracturing along the Grimaud fault, and support the model of Castanet (1998) in relation to a sinistral movement of the brittle Grimaud fault. The Reyran basin, which also displays a monoclinal structure (Basso, 1985), was interpreted here to be a hemi-graben developed in relation to a sinistral movement of the brittle La Moure fault.