Chevauchement et détachement dans les Maures occidentales (Var, France) : géométrie, cinématique et évolution thermobarométrique de la zone de cisaillement polyphasée de Cavalaire

Abstract

The most recent studies of the Maures massif (Fig. 1) provide evidence of an extensional tectonism that accounts for the late-orogenic exhumation of the deeper units (Morillon, 1997). Based on thermo-chronometric data, the proposed model can explain most of the ductile structures (S1) postdating the Silurian continental subduction, which is reflected by high-pressure relics (Leyreloup et al., 1996; Bouloton et al., 1998; Matte, 1998). The extensional structuring of the Maures massif was accommodated by ductile to ductile-brittle top-to-the-NW shear zones, which separate successive northwestward-dipping units. The described model interprets all post-Sn deformation as increments of the late-orogenic extension during a period of retrograde metamorphism (Morillon, 1997; Morillon et al., 2000). Nevertheless, parts of the post-D1 deformation are synchronous with the regional MT-MP metamorphism (D2) —this indisputable prograde metamorphism, marked by a steeply westward dipping schistosity, is necessarily disconnected from the late-orogenic extension (D3) and inevitably associated with a strong horizontal shortening. This paper exposes a new map, kinematic and petrostructural data acquiered on a key area of the Maures massif, where the compressional structures have escaped the effects of late extension. It proposes different arguments in favour of polyphased processes that can explain the structuring of this part of the Variscan belt. Geological setting The Maures massif (60 km x 60 km) is located in the southern part of the Variscan belt of western Europe. It is composed of two well-contrasted parts (Fig. 1): (1) the northwestward-dipping Western and Central Maures units (Lower Paleozoic metasedimentary and metabasic units) and (2) the vertical Eastern Maures units (granite and migmatite; Le Marrec, 1976), which are separated from the Central Maures by the N-S trending Grimaud - Moulins de Paillasse Fault (Vauchez, 1987). In the Western Unit the gently to moderately dipping N20°E foliation bears a NW-SE stretching lineation (Fig. 2). The series here shows evidence of a MT-MP metamorphism synchronous with the D2 deformation and prograding from west to east (Buscail et al., submitted). The D2 deformation is characterized by a reworking of the S1-0 axial-plane cleavage in the isoclinal folds, by open asymmetrical similar A-folds (Arthaud and Matte, 1966; Bronner et al., 1971; Bard and Caruba, 1981) with a decametre- to decimeter-scale wavelength. The fold axes are commonly subhorizontal, with dips ranging from NNW 15° to SSE 15°. They are everywhere parallel to the regional lineation (N150°E to N0°E) observed in the surrounding gneiss and mica schist, with an axial-plane dipping steeply to the west. A westward-dipping S2 strain-slip cleavage is axial-plane to these open folds. Structural analysis A new structural sketch map A major abnormal contact, marked by numerous mylonitic sheets and a major thrust fault in the Gassin – La Croix Valmer area, separates two unconformable structural units (Figs. 3, 4a, 4b, 4c). These are, from NW to SE, are (i) the Tuiles allochtonous Unit (metasedimentary formations) with a gently westward dipping N20°E-trending foliation, and (ii) the Cavalaire parautochtonous Unit (the LAC), separated by small sheets of mylonitic gneiss with a tabular foliation plane (Pierrugues Unit). The former is characterized by a N-S-trending low-angle schistosity S2, whereas the latter mainly exibits a NW-SE-trending steeply-dipping schistosity S2. The thrusts are confirmed not only by a general obliquity between the thrust traces and isogrades, but also by an inverted metamorphic zonation —the Tuiles Unit belongs to the sillimanite-muscovite or kyanite-biotite zone, whereas the Cavalaire Unit belongs to the staurolite-chlorite zone (Buscail et al., submitted). Low-angle faults are truncated by the late sinistral strike-slip Grimaud Fault (Vauchez and Bufalo, 1988). The Pierrugues Unit The major shear zone occurs as a low-angle fault marked in the field by a more intense deformation and an accordance of the regional foliation (Fig. 5a, 5b) between the two units. The major tectonic contact, situated at the base of the Tuiles Unit, is characterized systematically by phyllonitic facies and mylonite; i.e. a tectonic mélange of mica schist, orthogneiss and layered gneiss, structured by a tabular mylonitic foliation with a regular NW-SE stretching lineation. The structural map shows that the thrusting process is characterized by many “top and base” truncations (Fig. 6), and that all the thrust planes are cross-cut by the late N-S mylonitic Grimauld Fault. Microstructural analyses S-L fabric Observations in the XZ and YZ sections of the finite-strain ellipsoid attest to an essentially planar mylonitic fabric with a well-marked linear anisotropy on the foliation plane (Fig. 6). However, evidence of an L>S fabric is observed on small patches of mylonite located at the base of a major tectonic contact. Under the microscope, the foliation is outlined by monocrystalline ribbons of quartz in a fine-grained recrystallized matrix of plagioclase and abundant mica; i.e. the rock has been completely recrystallized. The NW-SE lineation has a composite nature, the orientations of the crystallized micas (mineral lineation), the boudinaged kyanite or staurolite (stretching lineation) and the microfold axes (microcrenulation) being parallel. Kinematic analyses The plane of observation is perpendicular to the foliation and parallel to the lineation (XZ section of the finite-strain ellipsoid). Macroscopic kinematic criteria are very uncommon. However, shear bands and the sigma-type systems of blastite in the gneiss on the major contact (Fig. 7d), along with the concordance of the two stacked units (Fig. 6), attest to a top-to-the-SE shearing —the faults are thus reverse faults. Under the microscope, mylonite sampled from the Cavalaire shear zone provides evidence of rare southeastward shearing: i.e. a sigma-type system of staurolite with crystallization traits of quartz-muscovite (Fig. 8a and 8b) and asymmetrical pressure shadows of quartz-muscovite around staurolite blasts indicate a southeastward kinematics (Fig. 8c). In the field, as under the microscope, the XZ sections of the mylonite reveal a foliation cross-cut by several shear bands marked by recrystallized muscovite-quartz-chlorite and indicating a NW normal motion (Fig. 7a). Other classical asymmetrical microstructures indicate a northwestward kinematics: i.e. asymmetrical pressure shadows of chloritized muscovite around staurolite blasts (Fig. 8d); muscovite shear bands retrograde metamorphosed staurolite and quartz-plagioclase sigmoids (Fig. 8e); retrograde metamorphosed garnet in a chlorite-quartz assemblage and chlorite C and C’ planes, which attest to a high strain in the mylonite and ultramylonite (Fig. 8f); fish-shaped monoclinic boudinage of staurolite or muscovite. This deformation is penetrative but located within a restricted shear zone where the S-L fabric is overprinted. Petrostructural evolution The petrostructural study, based on observations on sections parallel to the lineation and normal to the foliation (XZ plane), indicates a complex metamorphic evolution of the mylonite sampled from the base of the Tuiles Unit. According to the blasts developed by the rock, three cases can be distinguished: fine-grained mylonite, fine-grained ultramylonite and coarse-grained mylonite. The coarse-grained mylonite shows the most complete metamorphic evolution and is therefore described in detail (Table 1). The coarse-grained mylonite contains blasts of pretectonic euhedral staurolite (Fig. 9a) with numerous oriented inclusions of garnet, biotite, quartz, rutile, tourmaline (Fig. 9a, 9b). Another corona is observed between these phases and the host-mineral, indicating that the staurolite-garnet-rutile-biotite-quartz assemblage (A1 assemblage) is in equilibrium. The garnet displays numerous inclusions of biotite, quartz and opaque minerals. Kyanite occurs as elongate relict crystals in the matrix or in plagioclase blasts (Fig. 9c); as it is clearly a pretectonic mineral (pre-D2), it can be associated with the relict pretectonic staurolite-garnet-rutile-biotite-quartz assemblage. In places, staurolite is included in more developed and spectacular pluricentimetre-size porphyroblasts of plagioclase with inclusions of: (i) K feldspar-sillimanite-biotite-muscovite-quartz with a preferential orientation parallel to the elongation of the host-mineral; (ii) magnetite-rimmed destabilized garnet; (iii) subhedral unzoned garnet in contact with the plagioclase; and (iv) rutile rimmed by successive ilmenite and sphene (A2 assemblage) (Fig. 9d). In this case, the kyanite-staurolite-garnet-rutile-biotite-quartz assemblage is partly destabilized and constitutes a relict paragenesis. The crystallization trails of theses blasts suggest sigma-type structures that indicate a top-to-the-SE reverse shearing. The porphyroblasts are limited by quartz-muscovite-chlorite±brown biotite top-to-the-NW shear bands (A3 assemblage) (Fig. 8f) and more rarely by biotite-muscovite shear bands. This indicates a progressive, continuous retrograde metamorphism associated with normal shear zones. The P-T-t-d path obtained for the mylonite from the Tuiles shear zone illustrates a clockwise evolution with Pmax followed by Tmax and a subsequent isobaric cooling (Fig. 10). Discussion-Conclusion Whereas the high-grade amphibolite assemblage (A1) is the witness of crustal thickening, the subsequent decompression with partial melting of the HP-assemblages was synchronous with southeastward thrusting of the Tuiles Unit onto the Cavalaire Unit (A2) at the peak of regional metamorphism. Lastly, the late exhumation of the HP unit occurred through a reworking of the major contact in a ductile normal fault (A3 and A4) evolving to a ductile-brittle normal fault during an isobar cooling. Successive tectonometamorphic events in this part of the Maures massif can be interpreted as reflecting syn- to late-orogenic processes: Visean to Early Namurian compressional tectonism succeeded by Late Namurian extensional tectonism during the progressive thinning of a previously thickened Variscan crust prior to its restoration to normal thickness.

Key-words: Geometry, Kinematics, Shear zones, Microtectonics, Mylonites, Polyphase deformation, Hercynian Orogeny, Regional metamorphism, Geothermobarometry, Thrust faults, Var France, Maures Massif