Metamorphic and Structural Evolution of the Maures-Tanneron Massif (SE

Bull. Soc. géol. Fr., 2009, t. 180, no 3, pp. 217-230

Metamorphic and structural evolution of the Maures-Tanneron massif (SE Variscan chain): evidence of doming along a transpressional margin YANN ROLLAND1,MICHEL CORSINI1 and ANTOINE DEMOUX2

Key-words. – Variscan Chain, Maures-Tanneron massif, Doming, Transpression, Migmatites, Exhumation. Abstract. – The Variscan metamorphic and structural evolution of the Maures-Tanneron massif is divided in two main post-collisional phases: (1) a MP-MT regional gradient is developed during nappe-piling process between 350 and 320 Ma, followed by (2) LP-HT regional gradient coeval with doming between 320 and 300 Ma. During this late phase, the tectonic context was dominated by E-W shortening, which produced crustal-scale upright folds and major strike-slip displacement along trans-crustal faults. Symmetric extensional fabrics are observed on the limbs of crustal-scale anticlines, and are ascribed to local accommodation of lower crust exhumation. Heat and magma transfer are allowed by these large vertical strike-slip faults, and are thought to be the cause of the late metamorphic evolution. Therefore, structures and metamorphism argue for a transpressional context at the SE branch of the Variscan chain. Comparisons with current collisional settings such as syntaxial domains of the Himalayan belt show that the timing and PT conditions of metamorphic events are similar. These observations lead us to propose that the situation of the Variscan chain during the period 320-300 Ma was still a syn-convergent setting similar to the current situation of the Himalayan-Tibet system, and that extensional movements are not the cause of, but the result of exhumation of the lower crust in this ongoing shortening context along a transpressional wrench boundary.

Evolution métamorphique et structurale du massif des Maures-Tanneron (SE de la chaîne Varisque) : mise en place de dômes dans une bordure transpressive

Mots-clés. – Chaîne Varisque, Massif des Maures-Tanneron, Dômes, Transpression, Migmatites, Exhumation. Résumé. – L’évolution métamorphique et structurale du massif des Maures-Tanneron est divisée en deux étapes post-collisionnelles principales : (1) une phase d’épaississement par transport de nappes entre 350 et 320 Ma, marquée par un gradient métamorphique régional de type MP-MT ; suivie de (2) une phase de développement de dômes entre 320 et 300 Ma dans un contexte de BP-HT. Le contexte tectonique de cette dernière phase était marqué par un fort raccourcissement Est-Ouest, qui a conduit à la formation de plis droits accompagnés par des failles décrochantes d’échelle crustale. De part et d’autre des axes anticlinaux, sont observées des structures cisaillantes en extension qui sont interpré- tées comme des figures d’accommodation de l’exhumation de la croûte inférieure partiellement fondue en cœur de pli. Les grandes failles décrochantes ont également permis des transferts verticaux de chaleur et de magma, et ont donc joué un rôle dans l’évolution métamorphique tardive. Ces structures sont marqueurs d’un environnement géodynamique fortement transpressif le long de la bordure SE de la chaîne Varisque. Des comparaisons avec les contextes tectoniques ac- tuels comme les domaines de syntaxes himalayennes montrent des similitudes concernant les structures tectoniques et la durée et la nature des épisodes métamorphiques. Ces observations indiquent que la chaîne Varisque était encore dans un contexte syn-convergence durant la période 320-300 Ma à l’image du système Himalaya-Tibet actuel, et que les mouvements extensifs ne sont pas la cause mais la conséquence de l’exhumation de la croûte inférieure dans un contexte en raccourcissement le long d’une bordure transpressive.

INTRODUCTION domains. Oblique motions may account for differential exhumation and lateral displacements of crustal blocks during Crustal thickening and exhumation during transpression and orogen-parallel syn-convergence plate motions. Such con- transtension in highly oblique (wrench) zones characterize texts are known and described since a long time [Fitch, many mountain systems such as the Himalayas, Alps, North 1972; Dalmayrac and Molnar, 1981; Tapponnier et al., 1982; American Cordillera, Anatolia, and Precambrian orogens Selverstone, 1988; Brown and Talbot, 1989]. Although it is [Whitney et al. 2007; Duclaux et al., 2007; and references not always easy to recognize such settings in fossil orogens therein]. These oblique zones are the site of various tectonic as due to thermal reequilibration and gravitational spread- strain fields, trans-tensional, trans-pressional, with local pure ing in the late evolutionary stages [e.g., Vanderhaeghe and shortening or extensional contexts, co-existing in restricted Teyssier, 2001], it is noteworthy that these settings present

1. Géosciences Azur, Université de Nice-Sophia Antipolis, CNRS, IRD, 28 Av. Valrose, BP 2135, 06103 Nice, France 2. Institut für Geowissenschaften, Universität Mainz, Germany. Manuscrit déposé le 18 mars 2008 ; accepté après révision le 6 octobre 2008

Bull. Soc. géol. Fr., 2009, no 3 218 ROLLAND Y. et al. distinct structural and metamorphic features such as pressure prograde metamorphism is dated between 440 and orogen-normal parallelism of fabrics [Tikoff and Teyssier, 400 Ma, and is ascribed to continental subduction [Pin and 1994] and distinct duration of metamorphism [Thompson et Peucat, 1986; Matte, 1998], recorded in peridotites al., 1997]. [Gardien, 1988; Gardien et al., 1990], eclogites [Mercier et The Variscan belt of Europe is a fossil example of al., 1991] and high-pressure granulites of acidic and mafic mountain chain, which has largely been used to infer the gneiss [e.g. Matte, 1991], which belongs to the Upper evolution of collisional orogens [e.g., Faure et al., 2008, Gneiss Unit in the Massif Central [Ledru et al., 1994]. This and references therein]. It is now largely accepted that the leptynite-amphibolite complex consists of alternations of Variscan belt of Europe underwent a stage of subduction acidic and mafic volcanic-derived layers, which could be and crustal thickening that has been followed by a phase of ascribed to rifting events in the Late Cambrian-Early Ordo- thermal relaxation leading to crustal partial melting and ge- vician, in relation with the opening of various marginal oce- neralized extension [Burg et al., 1994; Vanderhaeghe and anic basins or true oceans [Matte, 1986; Ménot et al., 1988; Teyssier, 2001]. However, such model is recently reconside- Peucat et al., 1990; Pin, 1990; Pin and Paquette, 1997]. red, as transpressional tectonics have been evidenced during The Maures-Tanneron massif consists of a large area of all the collisional history as in the Limousin area [Gebelin ~ 60 x 60 km crystalline Variscan rocks crosscut by E-W et al., 2007]. As in modern collisional orogens, structural Permian rift structures. It is made of two blocks, a western analyses have defined several tectonically superposed units mainly non-migmatised part and an eastern one, nearly to- which suggest that polyphase nappe tectonics have occurred tally migmatized, to the East of the N-S trending Grimaud under high to medium grade Barrovian metamorphism shear zone [Caruba, 1983; Vauchez and Bufalo, 1985, 1988; (350-340 Ma) [Costa, 1991-1992; Ledru et al., 1994]. It is Vauchez, 1987] (fig. 1). A synthesis of its evolution and la- commonly accepted that lower crust exhumation and gene- teral connections in the SE Variscan belt is presented in ralised HT reequilibration are related to late orogenic exten- Corsini and Rolland (submitted). sional tectonics (340-290 Ma) [Echtler and Malavieille, 1990; Malavieille et al., 1990; Burg et al., 1994; Faure, Lithologies 1995; Gardien et al., 1997; Vanderhaeghe and Teyssier, The Maures massif consists of several lithologies: (i) meta- 2001; Soula et al., 2001]. However, the reasons for such ex- pelites, widespread in the Maures massif and present as tensional tectonics are still debated: syn-convergence hea- relictual pods in the eastern Maures-Tanneron basement, ting due to slab-breakoff [Ledru et al., 2001] or self- or (ii) a layered formation of amphibolite and orthogneiss in- extensional trigger of HT metamorphism by mantle delami- terpreted as formed by a bimodal magmatism related to an nation [e.g., Faure et al., 2002]? Another reason of such extensional setting [Seyler, 1986], which has been corre- controversy is the recognition of extensional tectonics it- lated with similar complexes recognised in the French Mas- self. Does it really feature a generalized extensional setting sif Central and Belledonne massif [Burg and Matte, 1978; or does it result from a simple local scale relative accommo- Ledru et al., 1994]. However, there are clear differences dation of exhumation as can be exemplified in transpressive between leptynite-amphibolitic rocks of the western and settings such as the syn-convergent dome structures obser- central Maures. Western Maures leptyno-amphibolite suc- ved in the Himalayan syntaxes [Burg et al., 1998; Diao and cessions are very fine grained, and are similar to vol- Meier, 1998; Rolland et al., 2001; 2006a-b; Mahéo et al., cano-sedimentary interlayers that could be attributed to a 2002]? It can be noted that in the Himalayan belt, extensio- volcanic setting on the basis of major element geochemistry nal gneiss domes are widespread although in a global [Laverne et al., 1997]. In the central Maures, it consists of convergent context. A similar situation for the initiation of an amphibolite–leucogneiss layered unit that contains doming may have also been the case in the Variscan chain. decametre long lenses of spinel peridotite interpreted as However, arguments are still lacking to discriminate bet- arc-related ultramafic cumulates [Laverne et al., 1997; ween syn-convergent extension and post-orogenic exten- Bellot et al., 1999, 2000b], garnet peridotites and coronitic sion. This issue is controversial as exemplified by metagabbros with oceanic affinities [Bouloton et al., 1998]. contradictory interpretations of the Montagne Noire in the These are spatially associated with serpentinites, and could southern French Massif Central [Van den Driessche and be related to a series of obducted ophiolitic rocks. (iii) Sev- Brun, 1992; Matte et al., 1998; Mattauer et al., 1996; Soula eral intrusive bodies, which are deformed (Bormes et al., 2001]. orthogneiss) or undeformed (Plan de la Tour/Rouet) In this paper, we present a detailed metamorphic, struc- granitoids. tural and geochronological analysis of the Tanneron-Maures massif belonging to the Variscan belt in SE France. These Tectonic events new data allow us to discuss the succession of tectonic and thermal events that raise questions on the exhumation pro- Four main deformation events have been distinguished in cesses during the late evolution of collisional belts. the Maures massif [Arthaud and Matte, 1966; Bronner et al., 1971; Bard and Caruba, 1981; Crévola and Pupin, 1994; Buscail and Leyreloup, 1999; Bellot and Bronner, 2000; Bellot et al., 2002a, 2002b, 2003]. The first event D1 corre- GEOLOGICAL SETTING sponds to the emplacement of partly eclogitized ophiolitic rocks [Caruba, 1983; Bouloton et al., 1998], within the tec- The Maures-Tanneron massif belongs to the western Euro- tonic pile. No kinematics are clearly related to this event, pean Variscan belt (fig. 1), which resulted from the Silu- since they have been largely overprinted by HT deforma- rian–Carboniferous convergence and collision between tion. Further, a second event of deformation (D2) is featured Gondwana and Laurussia [e.g., Matte, 1986]. The high- by pervasive ductile deformation characterized by isoclinal

Bull. Soc. géol. Fr., 2009, no 3 METAMORPHIC AND STRUCTURAL EVOLUTION OF THE MAURES-TANNERON MASSIF 219 and sheath folds with penetrative flat lying foliation, Metamorphism submeridian stretching lineation and top-to-the-south shear criteria. The former structures are re-deformed by D3 N-S Classically, three stages of regional metamorphism are rec- trending crustal-scale folds associated to a subvertical ax- ognized in the Maures Massif [Bellot et al., 2005], but not all ial-plane S3 crenulation cleavage [Arthaud and Matte, are yet clearly related to the deformation events described 1966; Bronner et al., 1971; Bard and Caruba, 1981; Buscail above. Firstly, highly retrogressed eclogitic relicts (M2) are and Leyreloup, 1999; Bellot et al., 2000a]. Fold axes are locally described within the Eastern compartment, and mostly subhorizontal, oriented N340oE to N020oE, locally should thus feature the pre-collisional D1 subduction his- showing a subvertical to slightly W-dipping axial-plane tory [Bard and Caruba, 1981; Caruba, 1983; Bouloton et al., crenulation cleavage [Arthaud and Matte, 1966; Bronner et 1998]. An MP (LT to HT) metamorphic imprint (M2) is at- al., 1971; Bard and Caruba, 1981; Buscail and Leyreloup, tributed to a syn-thickening Barrovian event, likely coeval 1999; Bellot et al., 2000a]. The D3 event is also character- with D2 [Buscail and Leyreloup, 1999]. The P,T intensity of ized by N-S trending dextral ductile strike-slip shear zones. regional metamorphism increases eastward within the west- Coeval to, or lately post-dating this phase of folding, D4 ern Maures domain, and seems to be quite uniform in the structures are characterized by normal faulting associated eastern migmatitic part. Further, it was followed by an LP with Upper Carboniferous intramontane basin opening, (MT to HT) event (M3), which can locally be correlated to granite emplacement and dome structures [Demoux et al., dome structures (D4) described in this paper. However, it is 2008]. unclear to which metamorphic episode the D3 event would

FIG. 1. – A) General sketch map of the Variscan units in western Europe modified from Matte [1991]. B) Sketch geological map of the Maures-Tanneron massif, modified after Crevola and Pupin [1994], with location (1) of the geological cross-section of figure 3. The symbols indicate the foliation pattern within each unit. C) Synthetic W-E cross-section of the Maures-Tanneron massif. FIG. 1. –A,Carte générale schématique des unités varisques d’Europe Occidentale, modifiée d’après Matte [1991]. B, Carte géologique schématique du massif des Maures-Tanneron, modifié d’après Crévo- la et Pupin [1994], comprenant la localisation (1) de la coupe géologique de la figure 3. Les symboles indiquent l’orientation de la foliation dans chaque unité. C, Coupe ouest-est synthétique du massif des Maures-Tanneron.

Bull. Soc. géol. Fr., 2009, no 3 220 ROLLAND Y. et al. correlate. According to Bellot et al. [2003], from amphibole NEW STRUCTURAL OBSERVATIONS AND thermo-barometry, three PT cycles can be described in the METAMORPHIC DATA mafic rocks of the Central Tanneron: (1) anti-clockwise PT path at high P that is attributed to M1 ophiolite obduction; In this paper, we present new structural and metamorphic (2) clock-wise PT path at MP conditions during M2 features of the Maures-Tanneron massif, with emphasis on nappe-stacking event; (3) a M3 clock-wise PT path during its central part (figs. 1-3). These data provide clear evidence late orogenic extension at low P. for the timing of events and style of deformation. Below we will propose a simpler scenario, with only two phases of metamorphism, coeval with two main phases of deformation, always in an ongoing transpressional context.

Geochronology

The Maures massif Recycling of older Cambrian crust is suggested by dating of a felsic rock from the Leptyno-amphibolite complex at 548 + 15/– 7 Ma [Innocent et al., 2003]. Similarly, the Bormes orthogneiss was dated around 550-630 Ma, by Rb–Sr ages on whole rock [Maluski, 1971; Maluski and Allègre, 1970] as well as by 40Ar/39Ar ages on biotite [Maluski and Gueirard, 1978]. Metamorphic ages ascribed to the Variscan event are mostly comprised between 350 and 300 Ma. The HP (M1) metamorphism has not been dated yet. Concerning the M2 phase, an age of 348 ± 7 Ma was obtained on an amphibolite from the Leptyno-amphibolite complex, dated by Innocent et al. [2003]. U/Pb on monazite ages of 345±3Ma were obtained on the Bormes orthogneiss [Lancelot et al., 1998]. Further, a U-Pb monazite age of 331 ± 3 Ma was obtained on migmatites in the eastern Maures massif [Moussavou, 1998]. These ages of 340-330 Ma for M2 fit with U-Pb ages obtained on magmatic rocks, ranging from 338 ± 6 Ma (Hermitan Granite in the Central Maures), 334 ± 3 Ma (Reverdit Tonalite, eastern Maures) [Moussavou, 1998]. In central Maures, to the East of the Grimaud-Joyeuse Fault, the Plan-de-la-Tour granite yielded a Rb-Sr age of 313 ± 10 Ma [Maluski, 1971]; and a U-Pb age of 324 ± 5 Ma [Moussavou, 1998]. This phase of granite emplacement is thus coeval with M2 metamorphism. Recent studies showed detailed 40Ar/39Ar ages on horn- blende, biotite and muscovite in the Maures-Tanneron massif [Morillon et al., 2000; Corsini et al., 2009]. The ages range between 330 and 300 Ma, and show a clear segmentation of crustal-scale blocks with different ages on each side FIG. 2. – Field photographs showing: (A & E) top-to the South sense of of the Grimaud SZ. shear exhibited by large K-feldspar sigma-type porphyroclasts, wrapped by amphibolite facies foliation and migmatitic pods at Cannes-La-Bocca; (B) Flat foliation with top-to the East sense of shear exhibited by diatexites The Tanneron massif from paragneisses intruded by the Plan-de-La-Tour granite; (C) strong lineation fabric in the western part of the Bormes othogneisses, featuring A recent investigation of metamorphic rocks has revealed amphibolite facies top-to the West extensional sense of shear at the contact slightly younger ages than in the Maures massif [Demoux et with the western metamorphic units; (D) brittle-ductile S-C fabrics indica- ting top-to the West sense of shear on the W side of the central Maures anal., 2008]: (1) some juvenile island-arc crust formation is ticline structure (featured in cross-section 1, fig. 3). Similar features, but suggested by monazite U-Pb dating of 440 to 410 Ma in towards the east, are observed on the E side of the anticline structure. central Tanneron; (2) the migmatization event is recorded FIG.2.–Clichés de terrain indiquant : (A&E)cisaillements vers le sud déduits des figures sigmoïdes entourant de larges porphyroclastes d’or- by ages of 317 ± 1 Ma (central part), 309-310 Ma (eastern those dans une foliation formée en faciès amphibolite, soulignée par des part), 302-297 Ma (western Tanneron, or Rouet, dome), poches de migmatites, à Cannes-La-Bocca ; (B) foliation plate montrant which could be related to the late metamorphic evolution un sens de cisaillement vers l’est dans des diatexites dérivant de para- 40 39 et al. gneiss intrudés par le granite du Plan-de-La-Tour; (C) fabrique fortement (M3). Ages obtained by the Ar/ Ar method [Bosse , linéaire dans la partie ouest des othogneiss de Bormes, marquant un mou- 2003; Corsini et al., 2009] are very close to these above vement extensif vers l’ouest en faciès amphibolite au contact avec l’unité ages, and show the same segmentation of the Tanneron mas- métamorphique occidentale ; (D) structures S-C ductiles-fragiles indi- sif into crustal-scale blocks with different exhumation ages quant un sens de cisaillement vers l’ouest sur le flanc occidental de la structure anticlinale des Maures centrales (montré sur la coupe 1, fig. 3). as was evidenced by Morillon et al. [2000] in the Maures Des structures similaires, mais vers l’est, sont observées sur le flanc orien- part. tal du même antiforme.

Bull. Soc. géol. Fr., 2009, no 3 METAMORPHIC AND STRUCTURAL EVOLUTION OF THE MAURES-TANNERON MASSIF 221

Structure trajectories draw kilometric concentric folds with N-S trending axes. Three main large anticline structures are well From West to East, the Maures-Tanneron Massif is composed distinguished [Crévola et al., 1991], from West to East: the of two main domains: a western mostly unmigmatized do- Rouet antiform and the Cannes antiform, separated the Rey- main, and an eastern migmatized domain, divided into five ran synform. The Rouet antiform is characterized by a tona- units. lite-granite complex intrusive in sillimanite-cordierite (1) The western-central Maures domain is made of a migmatitic gneisses. The migmatitic gneisses display a folia- low to high metamorphic grade, mostly un-migmatized suc- tion trajectory with an irregular round shape all around the cession of para- and ortho-gneisses, overlain by an ophioli- tonalite-granite complex, which defines a cartographic dome tic body. The overall structure is W-plunging (fig. 1C). The structure (fig. 4). The stretching lineation trajectory is relati- Bormes Fault divides the western Maures low-metamorphic vely dispersed, but globally displays a radial pattern. In the grade para-derived cover from the Bormes unit crystalline Fontcounille area, the submeridian strike with gentle linea- basement, mostly composed of orthogneisses. The Bormes tion dips, is mostly attributed to a later dextral strike-slip mo- o Fault is steep (70-80 towards the W) and featured by in- vement of the Fontcounille fault during the final stage of tense top-to-the-West shearing displacement. To the east, emplacement of the Rouet granite [Onézime et al., 1999]. In the Bormes unit is separated from the central Maures by a the Joyeuse area, the stretching lineation associated to steep presumed vertical fault. The Central Maures is a narrow mylonitic foliation planes is moderately to steeply dipping zone on the western side of the Grimaud Fault, featured by (50 to 90o) to the SW. The shear criteria indicate a top to the an anticline structure: the Hermitan anticline, cored by dia- SW normal displacement with a slight sinistral strike-slip texites surrounded by metatexites and overlained by component. At the contact with surrounding gneisses the to- un-migmatized micaschists (fig. 3). Above the un-migmati- nalite displays a concordant vertical mylonitic foliation. This zed part, an ophiolitic body is defined by a discontinuous foliation plane bears a high-angle mineral lineation underli- serpentinite layer overlain by amphibolitized metagabbros ned by biotite and amphibole. This mylonitic foliation is pa- and leptyno-amphibolite successions. Ductile deformation rallel to the magmatic foliation in the western border of the is featured by long-wavelength folds, except at the proximi- tonalite intrusion suggesting a syn-kinematic emplacement ty of the main faults (in the central Maures) where isoclinal of the tonalite, associated to the Joyeuse shear zone [Demoux folds are evidenced. Internal deformation within the central et al., 2008]. A cataclasic deformation is superimposed onto Maures anticline structure is also featured by steep exten- the ductile foliation featuring a final movement under brittle sional shear planes, which have a symmetrical distribution conditions at shallower depth. on each side of the anticline. (2) The eastern Maures-Tanneron massif is formed by Two Late Carboniferous intracontinental basins [e.g. highly migmatized rocks. At the massif scale, foliation Toutin-Morin et al., 1994] are formed in the western and

FIG. 3. – Geological cross-section of the central part of the Maures massif, showing an anticlinal fold (the Hermitan anticline), featured by a migmatized core. Wulff stereonets are in lower hemisphere. Note that the foliation consistently plunges towards the west on the west side, and to the east on the east side of upright isoclinal folds. Extensionnal shear bands show symmetrical kinematics with respect to the anticlinal axis, and crenulation lineations are parallel to the fold axis. FIG. 3. – Coupe géologique de la partie centrale du massif des Maures, montrant une structure anticlinale (dite de l’Hermitan), avec un cœur de migmatites. Les projections stéréographiques de Wulff sont effectuées dans l’hémisphère inférieur. Notez que la foliation plonge de façon symétrique par rapport à l’axe des plis (droits). Des bandes de cisaillements montrent également des mouvements divergents par rapport au cœur des antiformes, tandis que les li- néations de crénulation sont parallèles aux axes de plis.

Bull. Soc. géol. Fr., 2009, no 3 222 ROLLAND Y. et al. central part of the massif and lie parallel to the trend of the contacts, including an oceanic crust sequence. This event is two major Grimaud-Joyeuse and La Moure faults (fig. 1). therefore due to early continental subduction and further The emplacement and structural evolution of such basins crustal thickening. The second stage of evolution of previ- are well exemplified in the central Maures, along the Gri- ous authors (D3-D4) is characterized by folding and local maud Fault. Here, a Carboniferous sandstone and conglo- doming of the nappe stack, associated with strike-slip fault- merate terrigenous basin lie in the synclinal part of the fold ing. rimming the Hermittan anticline on the western side of the Joyeuse-Grimaud Fault (fig. 5). The structure of the basin is dissymmetrical as it presents a gently dipping western side, The first deformation stage and a steep eastern side. Sedimentary rocks are fine-grained in the west and become coarser to the east of the basin, with The D1 deformation event corresponds to the superposition psammitic sandstones passing laterally to conglomerates of the main units by nappe contacts involving an obducted and unsorted breccias at the contact with the Grimaud Fault. oceanic crust sequence. Not all of the oceanic crust has The basin is deformed by N-S trending folds, and top-to-the been metamorphosed under HP conditions, as HP relics are west thrusting décollement on the unconformity surface at scarce and spatially related to serpentinite slices [Bouloton the western side of the basin. In the continuity of the Carbo- et al., 1998; Caruba, 1983]. It is therefore assumed that niferous basin, the small Pennafort basin is localized at the obduction of relatively unmetamorphosed oceanic crust western border of the Joyeuse Fault (fig. 4). It is filled with onto the W-Tanneron continental crust has occurred in early coarse to medium size detrital sediments. High-angle to ver- collisional stages, and may be a lateral equivalent of the tical brittle normal faults are well developed at different Belledonne ophiolite in the W Alps [Fernandez et al., scales. A normal sense of movement is deduced from step- 2002]. In the eastern Maures-Tanneron, presumed amphi- ped structures in the hanging wall of the fault. The La bolitized eclogites may be derived from a subducted oce- Moure basin is affected by syn-sedimentary deformation anic crust sequence [Caruba, 1983]. This event is therefore and by a later compressive event leading to N-S isoclinal due to early continental subduction (since c. 350 Ma), fol- folding [Crévola et al., 1991; Toutin-Morin et al., 1994]. lowing oceanic subduction and further crustal thickening (350-320 Ma) [e.g., Demoux et al., 2008]. Original transport direction during the nappe-piling process is unknown Evolutionary stages due to the intense deformation during the late (D3-4) tec- Field analysis shows that the evolution of the Maures- tonic phase. However, although the whole structural pile is Tanneron can be accounted by a 2-phase evolution. The first intensely folded top-to the West motion, and East-dipping stage of previous authors of section 2 (D1-D2) corresponds subduction could be inferred from the progression of to the superposition of main units on each other by nappe Barrovian metamorphism from West to East.

FIG. 4. – A) Structural sketch of the Rouet Dome (western Tanneron massif). Note the round shape drawn by foliation trajectories and the radial pattern of stretching/mineral lineations around the western Tanneron massif, which is cored by magmatic intrusions (tonalites and granites). In contrast, the deformation pattern along the Joyeuse Fault is featured by a very steep fabric parallel to the fault. The lineation dips appear to be extremely variable, but is relatively steep on average (~ 60oS). B, WSW-ENE cross-section of the Rouet antiform. FIG. 4.–A,Schéma structural du dôme du Rouet (partie occidentale du massif du Tanneron). Notez la forme arrondie des trajectoires de foliation et la ré- partition radiale des linéations minérales/d’étire- ment dessinant le dôme du Rouet, dont le cœur est intrudé par des tonalites et des granites. Par contre, la déformation le long de la faille de Joyeuse est marquée par une fabrique très raide et parallèle à la faille. Le long de cette faille, le plongement de la li- néation est variable, mais raide en moyenne (~ 60°S). B, Coupe WSW-ENE de l’antiforme du Rouet.

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Second deformation stage zone is infilled by magmatic melts dated at 303 Ma [De- moux et al., 2008]. Exposed mylonites along the fault strike The nappe pile appears to be folded in upright short show amphibolite to greenschist facies mineral lineations wave-length folds in the central Maures as it can be drawn (fig. 2C), mostly dipping 10-30oS, in agreement with a dex- on E-W geological cross-sections (fig. 3), and larger wave- tral + normal sense of shear. Locally, strike-slip motion be- length folds in the East Maures and Tanneron (fig. 1C). This comes extensional at the rim of the Rouet Dome, which is deformation pattern is clearly inferred from observations in ascribed to the effect of doming. The late offset by the fault the eastern Tanneron area. There, consistent top-to the includes a vertical motion with the eastern side of the fault south sense of shear is featured by N-S striking mineral and moving upwards [Morillon et al., 2000], and results in stretching lineations, and kinematics inferred from the anal- brittle deformation within the fault zone in the Late Carbo- ysis of K-feldspar porphyroclasts (fig. 2A,D). On each side niferous (at c. 300 Ma). of ~ 50 m wavelength N-S folds shear sense indicate either In conclusion, the structural features are in agreement left-lateral or right-lateral sense of shear, which could be in- with a mainly two-phase evolution: (1) a D1-2 nappe-piling terpreted either as (1) top-to the south sense of shear on pla- process; (2) a D3-4 transpressional context. This latter nar surfaces refolded by an E-W compressional event, (2) context is featured by a combination of folding and N-S top-to the south sense of shear on each sides of folds pro- stretching, related to E-W shortening while the crust under- duced by a constrictional deformation mode. We prefer the went intense partial melting and diapirism. Relative fold second interpretation as N-S folding is formed by a combi- amplification of the anticlines may explain the normal sense nation of N-S stretching and E-W shortening. Thus, top-to of shear, which is observed. the south shearing and folding may result from the same phase. Metamorphism The transpressive character of this deformation stage is also well examplified in the central Maures where upright The metamorphic features of the Maures-Tanneron massif folding occurs parallel to the Grimaud Fault (fig. 3). The are highlighted in figures 6-8. Regional-scale increase of stretching lineation pattern is globally parallel to fold axes. metamorphism (fig. 6) is shown by the progression from Exhumation of the anticlinal migmatized core is marked by chlorite-muscovite bearing phyllades, to garnet-chlorite, bio- brittle shear zones with normal sense with opposite motions tite-staurolite, biotite-kyanite, biotite-muscovite-sillimanite, on each limb of the isoclinal folds (fig. 2D). The most su- biotite-sillimanite, from West to East (fig. 7); with the oc- perficial micaschists and partly molten paragneisses are currence of a narrow zone of cordierite-K-feldspar bearing thinned or squeezed by vertical extrusion of the diatexite rocks in the W Tanneron (Rouet) dome. core, on top of the anticline structure. This structural fea- Pressure-temperature (P-T) evolution is featured by a ture is thus in agreement with diapiric fold amplification, polyphase evolution with (1) a Barrovian metamorphic which could preceed doming s.s. A migmatitic dome structure is evidenced in the W- Tanneron (Rouet) area (fig. 4). Here, doming appears to be in a more mature stage than in the central Maures as evidenced by the round shape of foliation trajectories and radial mineral lineation pattern in the gneiss. The geometry of the foliation trajectories shows a dissymmetrical pattern (fig. 4). This dome shape is in agreement with the superimposition of a dome on a N-S anticline at the scale of the Maures-Tan- neron massif, which is a parallel to the Joyeux-Grimaud Fault. Thus, we ascribe doming to fold amplification due to magmatic intrusions in dome core and migmatization. In this context, syn-magmatic extensional fabrics are attribu- FIG. 5. – Geological cross-section of the Plan de la Tour Carboniferous ba- ted to a vertical extrusion of the molten dome core. Howe- sin (location on figs. 1 and 3). Note that sedimentary records in the basin ver, it is noteworthy that ductile to brittle shear indicators are dissymmetrical, with fine-grained sediments (psammites) in the West and quartz C-axes are in agreement with strike-slip displa- and conglomeratic breccias to the East. This feature is in agreement with a cement on the pluton rims [Onézime et al., 1999], which paleo-slope along the Grimaud Fault during sedimentation, while the western side of the basin was gently dipping. Initial bedding was roughly pa- clearly shows that doming and pluton emplacement occur- rallel to the schistosity in the micaschist. Deformation of the basin is red in a transcurrent context. featured by flat thrusts at the base of the basin and N-S folding. These features indicate an E-W shortening context following the Carboniferous ba- The Carboniferous basins are emplaced in the synclinal sin infill, in a similar direction as the Hermitan anticline. cores, in relation to vertical relative displacement of the FIG.5.–Coupe géologique du bassin carbonifère du Plan de la Tour (lo- Joyeuse-Grimaud and La Moure faults. The Upper Carboni- calisation sur les figures 1 & 3). Notez que les dépôts sédimentaires du ferous detrital sediments themselves are folded, with fold bassin sont dissymétriques, avec des sédiments à grain fin (psammites) à l’ouest et des brèches conglomératiques à l’est. Cette dissymétrie est en axes parallel to those of the ductile structures, which agrees accord avec une paléo-pente le long de la faille de Grimaud, tandis que la with ongoing shortening after their deposition, in a trans- surface à l’ouest du bassin était basculée du fait du jeu de la faille. pressive context [following Onézime et al., 1999]. L’orientation des strates dans cette partie était à peu près identique à celle de la foliation des micaschistes. La déformation du bassin est accommodée Finally, the transpressive character of deformation is also par des chevauchements plats à la base du bassin et par des plis d’axes witnessed by the kinematics observed on the main faults. The Nord-Sud en accord avec un raccourcissement Est-Ouest succédant au Grimaud-Joyeuse Fault, separating the unmigmatized cen- remplissage du bassin carbonifère. La direction de raccourcissement est similaire à celle qui est à l’origine de l’antiforme de l’Hermitan, aussi il tral-western Maures from the migmatized eastern Maures est probable que le contexte tectonique n’ait pas significativement changé was active since the M2 metamorphic peak, as the fault entre ces deux étapes.

Bull. Soc. géol. Fr., 2009, no 3 224 ROLLAND Y. et al. gradient (fig. 8), which can be ascribed to crustal thickening DISCUSSION during collision (D1-2 / M2 stage). Further metamorphic evolution (2) is characterized by lower pressure – higher In the present state of knowledge, due to the acquisition of temperature peaks (2-3 in fig. 8) featuring thermal relaxa- numerous structural, metamorphic and geochronological tion of the previously thickened crust (D3-4 / M2 stage). constraints, it becomes possible to propose a self-consistent Greenschist facies overprint only occurs in the hanging-wall evolutive model of this SE segment of the Variscan chain. part of the major faults. We show in the following discussion that this situation at 320-300 Ma was probably very similar to the current evolu- The age of metamorphism is comprised within tion of a transpressive (wrench) collisional margin, such as 340-295 Ma. Joint thermobarometry and geochronology in the NW Himalayan syntaxis, and that it sheds light on the (fig. 8B) document that Barrovian metamorphic peak condi- tectonic context of the (still enigmatic) SE branch of the tions were attained at 340-325 Ma, followed by decompres- Variscan chain, and questions the generally accepted model sion at 318 Ma. Doming and low pressure – high temperature of generalized extension in the Late Variscan stages. In the metamorphism accordingly occurred in this late M2-M3 following discussion: phase, as evidenced by U-Pb monazite dating of the Rouet (1) we address the question of the significance of the granite of 302 ± 4 Ma by Demoux et al. [2008] in the W metamorphic and tectonic evolutions in terms of deforma- Tanneron (Rouet) dome. tion within a transpressive wrench zone;

FIG. 6. – Metamorphic map of the Maures and Tanneron massif. Isograds correspond to main structural contacts, defining a Barrovian succession from west to east. Two main domains are distinguished : the western domain was mostly not migmatized, except in rare zones, with a progressive transition from anchizonal metamorphic conditions to the West, to catazonal conditions in the East, at the proximity of the Joyeuse-Grimaud Faul; the eastern domain is largely migmatized, east of the Joyeuse-Grimaud Fault. Foliation strike is indicated by the orientation of symbols within units. Foliations are mostly parallel to unit contacts, and have steep plunge (> 70o). Foliations are folded by steep upright folds with vertical axial planes as is emphasized in figure 3. Dome shapes are locally emphasized W of the Grimaud Fault in central Maures, and West of the Tanneron massif (where cordierite is also evidenced). Stretching and mineral lineations are slightly oblique to the foliation strike, and become perpendicular to most tectonic contacts, which empha- size vertical motions (always with the exhumation of the eastern compartment). Data are from Crévola (unpublished data) and new observations. FIG.6.–Carte métamorphique du massif des Maures-Tanneron. Les isogrades correspondent à des contacts structuraux importants, et définissent une succession barrovienne d’Ouest en Est. Deux domaines principaux sont distingués : (1) le domaine occidental est essentiellement non migmatisé à part dans quelques zones étroites ; il est marqué par une transition progressive de l’anchizone à la catazone vers l’est, à proximité de la faille de Joyeuse-Gri- maud. (2) Le domaine oriental est intensément migmatisé, à l’est de la faille de Joyeuse-Grimaud. Les directions de la foliation sont indiquées par les symboles ; elles sont parallèles aux principaux contacts entre unités où elles montrent de forts plongements (> 70°). La foliation est replissée dans des plis raides à surfaces axiales verticales comme il est montré en figure 3. Des structures en dômes apparaissent localement à l’ouest de la faille de Gri- maud dans les Maures centrales, et à l’ouest du Tanneron (où il coïncide avec l’apparition de cordiérite). Les directions des linéations minérales et d’éti- rement sont faiblement obliques par rapport à la direction de la foliation et deviennent perpendiculaires à celle-ci dans la plupart des contacts importants, ce qui indique des mouvements verticaux (avec toujours la remontée du compartiment est). Les données sont celles de Crévola (données non publiées) et issues de nouvelles observations.

Bull. Soc. géol. Fr., 2009, no 3 METAMORPHIC AND STRUCTURAL EVOLUTION OF THE MAURES-TANNERON MASSIF 225

FIG. 7. – Microphotographs of representative assemblages: (A) garnet-kyanite-staurolite assemblage within micaschist from the East Tanneron massif,asa relict of M2 metamorphism within M3 migmatites emphasized by the development of K-feldspar + quartz leucocratic pods to the left of the photograph; (B) a more detailed photograph of a M2 garnet-staurolite schist; (C) M3 migmatites, featured by K-feldspar + quartz + plagioclase leucosom, and relictual biotite ± muscovite, strongly reacting to form sillimanite; (D) diatexite showing local leucogranitic texture, with relictual biotite, nucleus of a late muscovite crystal formed during cooling of the leucogranite. Fig.7.–Photographies au microscope optique des assemblages représentatifs : (A) grenat-disthène-staurotide dans un micaschiste du Tanneron oriental, relique du métamorphisme M2 dans des migmatites M3. Celles-ci sont caractérisées par le développement d’orthose + quartz dans des poches à gauche de la photographie ; (B) une photo plus détaillée d’une paragenèse M2 à grenat-staurotide ; (C) migmatites M3, marquées par des leucosomes à orthose + quartz + plagioclase, et biotite ± muscovite relictuelles réagissant fortement pour former la sillimanite ; (D) diatexite montrant une texture localement leucogranitique, avec une biotite relictuelle, nucléus d’un cristal plus tardif de muscovite formé pendant le refroidissement du leucogranite.

(2) we correlate the structure of the Maures-Tanneron On the basis of the metamorphic analysis and the pre- massif with the rest of the SE Variscan belt and other an- sence of normal faulting in the Maures massif, Bellot et al. cient or more modern settings. [2003] argue for a late orogenic phase dominated by a NW-SE extensional regime. However, if a NW-SE extension regime may be supported by some local shear zones, evi- Evidence for metamorphism in a transpressive wrench dence for pervasive extension is lacking at the scale of the zone Tanneron-Maures massif. In the structural analysis section, we argue for synchronism of upright folding in central Mau- Most metamorphic core complexes are thought to have been res and the development of km-scale folds in the eastern formed in extensional settings, mainly related to gravita- compartment (East Maures and Tanneron; fig. 9). Extensio- tional collapse or late orogenic extension [Coney, 1980; nal shear zones are formed symmetrically on the sides of Dewey, 1988]. Following observations done in the Ameri- anticlines displaying migmatized cores. As featured in the can cordillera where such domes are formed with extension Rouet and central Maures areas, doming occurred by fold in a direction roughly normal to the chain, this model has amplification. Therefore, extensional motions are likely re- been applied to the French Variscan chain [e.g., Echtler and lated to local accommodation of diapiric amplification of Malavieille, 1990; Malavieille et al., 1990; Van den anticline cores during crustal shortening and may not be as- Driessche and Brun, 1991-1992]. cribed to generalized extension. The presence of a relatively However, the metamorphism and tectonic history of the flat lying foliation can be found in numerous tectonic Maures-Tanneron massif is consistent with models for contexts. However such domains formed in close proximity prograde deformation in highly oblique tectonic settings of vertical transcurrent faults as in eastern Maures-Tanne- [Thompson et al., 1997; Whitney et al., 2007]. The orienta- ron is typical of the structure of mid-crust levels in rigid tion of the high-T mineral lineation, the P-T-t history and blocks translated along wrench or low-angle transpressive the continuous motion of the steep crustal-scale Joyeuse- margins [Teyssier and Cruz, 2004; Cagnard et al., 2006; Grimaud Fault during the collisional history all suggest that Duclaux et al., 2007]. Such a model is supported by the pre- the Maures-Tanneron Massif evolved in a transpressive high- sence of several crustal blocks, which witnessed differential ly oblique wrench context. exhumation [Morillon et al., 2000]. It is also proposed in a

Bull. Soc. géol. Fr., 2009, no 3 226 ROLLAND Y. et al. similar zone of the SE Variscan belt, laterally, in Corsica Sardinia to the western Alps. In Sardinia [Conti et al., 2001; [Giacomini et al., 2008], and in other parts of the French Elter et al., 1999; Carosi and Palmeri, 2002], and in the Variscan belt in the nothern Limousin [Gébelin et al., 2007; Alps [Grandjean et al., 1996; Fernandez et al., 2002], the Catannaz et al., 2007]. crystalline massifs are dissected into two main domains: a western domain displaying a prograde regional evolution Lateral correlations, significance of the transpressive from Lower greenschist to Upper amphibolite conditions margin at the scale of the SE Variscan Chain and from West to East, separated from an eastern widely comparisons with more modern settings migmatized domain by a steep crustal-scale contact similar The structure, metamorphic evolution and ages obtained in to the Joyeuse-Grimaud Fault. The kinematics ascribed to the Maures-Tanneron massif are in agreement with studies this vertical contact are mostly transcurrent, with a vertical undertaken along the SE margin of the Variscan chain, from relative motion which accounts for the exhumation of the eastern migmatized domain [Conti et al., 2001; Elter et al., 1999; Carosi et al., 2002; Giacomini et al., 2008]. A shortening component sub-orthogonal to the main trend of strike-slip faults is inferred from vertical upright folds parallel to this main contact. The timing of deformation events is also in the same range in various locations along the SE Variscan belt, with collisional MP-MT metamorphism ascribed to crustal thickening at 350-320 Ma and late LP-HT metamorphism at 320-300 Ma [Carosi and Palmeri, 2002; Paquette et al., 2003; Giacomini et al., 2008]. Therefore, we propose to correlate these various zones with each other and that the tectono-metamorphic evolution of the SE Variscan chain was dominated by transpression at 320-300 Ma along a major shear zone (the Grimaud- Joyeuse Fault in the Maures-Tanneron massif). To the SE of this main shear zone, it is rather speculative to infer the relative width of the continental margin and the relationships with the Paleo-Tethys Ocean. However, the width of a continental supposedly active margin might have been quite narrow regarding to most reconstructions of the Variscan chain [e.g., FIG. 8. – P-T paths in the Maures-Tanneron massif. A, Representative P-T Matte, 1998]. paths deduced from microscopic analysis of main metamorphic paragene- ses. 1. MP-MT paths (~ 25-30oC.km-1) deduced from the observation of When regarding the Variscan chain at a larger scale, the staurolite destabilization into kyanite + garnet (almandine 0.8) ± rutile, tectono-metamorphic evolution determined in the Mau- neighboured by leucosom (sillimanite absent), sample TA0 and MA0 on figure 6. 2. Lower pressure gradient, defined by the stability of sillimanite + res-Tanneron is in agreement with a main two-stage evolu- biotite + K-feldspar migmatites, defining geotherms of 50 to 70oC.km-1, tion, a first collisional stage directly following oceanic sample TA1 and MA1 on figure 6. 3. Low pressure – high temperature gra- subduction at 350-320 Ma, followed by a post-collisional dients featured by the occurrence of Fe-cordierite in W Tanneron (Rouet) transcurrent stage at 320-300 Ma. During this latter stage, dome (> 80oC.km-1), sample TA2 on figure 6. 4. Greenschist facies assemblages (chlorite-bearing) occurring within and in the hanging-wall part of the French Massif Central was in a context of extensional the Joyeuse-Grimaud Fault, samples TA3 and MA2 on figure 6. After pe- tectonics in the late M3 stage at 320-300 Ma, with low- trographic observations done by Billo [2005] and Olliot [2006]. U-Pb mo- angle extensional faults and metamorphic core-complexes nazite and zircon ages of the MP-MT phase after Moussavou [1998], U-Pb monazite ages of LP MT and HT phases by Demoux et al. [2008], [e.g., Ledru et al., 2001]. This lateral difference is well ex- 40Ar/39Ar ages after Morillon et al. [2000] and Corsini et al. [2009]. Peak plained if a comparison with a modern context such as Hi- metamorphic conditions of M2 are comprised between 338-328 Ma, and malaya-Tibet is considered. On the northern side of the 320-300 Ma for M3, following U-Pb monazite dating. Argon datings on muscovite of 320-300 Ma reflect generalized exhumation of Middle Crust, Himalayan range, the superposition of crustal thickening with some differential uplift of crustal blocks [see Morillon et al., 2000]. and lateral escape through strike-slip faults is observed. Si- FIG.8.–Chemins P-T caractéristiques du massif Maures-Tanneron. milar magmatic and metamorphic evolutions as in the Mau- A. Chemins P-T déduits de l’analyse microscopique des principales para- res-Tanneron massif are evidenced, with a relatively similar genèses métamorphiques. 1. Chemins MP-MT (~25-30°C.km-1) déduits de l’observation de staurotide se déstabilisant en disthène + grenat (alman- timing of events. The timing of Barrovian metamorphism, din 0.8) ± rutile, bordé par un leucosome (sans sillimanite) ; échantillons and nappe-piling process (M2) separated from the late TA0 et MA0 sur la figure 6. 2. Chemins à MP/BP-HT, définis par la stabili- HT-LP metamorphism (M3) observed in metamorphic do- té de migmatites à sillimanite + biotite + orthose, indiquant des gradients de 50 à 70°C.km-1; échantillons TA1 et MA1 sur la figure 6. 3. Chemins à mes by ca. 20 Ma, as the P-T intensity of M2 and M3 meta- BP-HT, marqués par l’apparition de Fe-cordierite dans le dôme du Rouet morphism, are relatively similar (Nanga Parbat and SE (Tanneron occidental; > 80°C.km-1) ; échantillons TA2 sur la figure 6. 4. Karakorum [Rolland et al., 2001; 2006a-b] and Namche Assemblages du faciès des schistes verts (chlorite-albite) apparaissant à Barwa [Diao and Meier, 1998]). The presence of a large do- l’ouest de la faille de Joyeuse-Grimaud ; échantillons TA3 et MA2 sur la figure 6. Chemins réalisés d’après les observations pétrographiques effec- main in which extensional tectonics are present (the Tibetan tuées lors des travaux de Billo [2005] et Olliot [2006]. Les âges U-Pb plateau) resembles the extensional motions observed in the monazite et zircon de la phase MP-MT sont d’après Moussavou [1998]. Velay area. The Tibetan plateau is neighboured by a trans- Les âges U-Pb monazite des phases MP/BP and BP-HT sont de Demoux et al. [2008]. Les âges 40Ar/39Ar sont de Morillon et al. [2000] et de Corsini pressive margin along the Karakorum Fault in the SW, bor- et al. [2009]. Le pic de métamorphisme M2 est compris entre 338-328 Ma, dering a N-dipping Indian slab while a south-dipping et celui de M3 entre 320 et 300 Ma, d’après les âges U-Pb sur monazite. transpressive margin is situated in the NW Pamir area in the Les datations argon sur muscovite de 320-300 Ma reflètent une exhumation généralisée de la croûte moyenne of Middle Crust, avec un soulève- same way as the Rheic subduction system at the northern ment différentiel de blocs crustaux [voir Morillon et al., 2000]. boundary of the French Massif Central.

Bull. Soc. géol. Fr., 2009, no 3 METAMORPHIC AND STRUCTURAL EVOLUTION OF THE MAURES-TANNERON MASSIF 227

FIG. 9. – Block diagram illustrating the geometrical, lithological and metamorphic features of the Maures-Tanneron massif, related to its late transpressional tectonic evolution (320-300 Ma). 1, Unmetamorphosed carboniferous conglomerates; 2, micaschists; 3, Bormes orthogneiss; 4, Leptyno-amphibolite series; 5, Phyllades (p), of metapelitic compositions; 6, ultrabasites, and metagabbros (amphibolites), initially an obducted oceanic crust sequence; 7, migmatites and aluminous granites; 8, tonalites (mantle-derived magmatism, with some crust assimilation). FIG.9.–Bloc diagramme illustrant les caractères géométriques, lithologiques et métamorphiques du massif des Maures-Tanneron, au cours de son évolu- tion tardive en transpression (320-300 Ma). 1, conglomerats carbonifères non métamorphiques ; 2, micaschistes ; 3, orthogneiss de Bormes ; 4, série leptyno-amphibolique ; 5, phyllades (p), de composition pélitique ; 6, ultrabasites, et métagabbros (amphibolites), initialement une séquence de croûte océanique obduite ; 7, migmatites et granites alumineux ; 8, tonalites (magmatisme dérivé de la fusion du manteau avec une forte assimilation crustale).

CONCLUSIVE REMARKS and we ascribe the metamorphic and magmatic evolution to deep-seated processes that occurred below these structures. HT metamorphism is not necessarily the result of Exhumation of crustal blocks bounded by these major faults post-orogenic extension, even in the case of the Variscan is evidenced by 40Ar/39Ar data [Morillon et al., 2000; belt. The initiation of such metamorphism after crustal Corsini et al., 2009]. The situation considering this point is thickening can be interpreted alternatively as the result of also similar to what is being described in Himalaya and radiogenic heating of pelitic lithologies in a convergent Karakorum where metamorphic domes comprising context [Vanderhaeghe and Teyssier, 2001], or as low-pressure migmatites develop in the core of crustal-scale deep-seated processes such as slab breakoff [Ledru et al., folds, in direct proximity of transcurrent crustal to 2001; Lardeaux et al., 2001]. Here, we consider that tec- lithospheric-scale faults [Rolland et al., 2001; 2006-a-b; tonic and metamorphic evolutions of the Maures-Tanneron Mahéo et al., 2004]. massif are in agreement with deformation along a Acknowledgements. – We thank the help of three reviewers, including M. transpressive wrench zone. Crustal to lithospheric-scale Faure, G. Mahéo and an anonymous person, who have significantly helped faults are effective drains for magmas and heat transfer improving this manuscript.

References

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Bull. Soc. géol. Fr., 2009, no 3