Metamorphic and Structural Evolution of the Maures-Tanneron Massif (SE
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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, struc- tures 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 shorten- ing 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 rac- courcissement 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 for- tement 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 mouve- ments 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 ex- humation 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