Problems on correlations of structures

The main structure of the Maures massif, i.e. the Grimaud fault (Demay, 1927a and 1927b; Vauchez and Bufalo, 1985; Morillon et al., 2000; Bellot et al., 2002b), prolongs likely to the South to the Posada-Asinara fault zone in northern Sardinia (Vauchez and Bufalo, 1988; Elter et al., 1990; Cappelli et al., 1992; Carmignani et al., 1994; Onézime et al., 1999; Carosi and Palmeri, 2002). In both cases, the shear zones (1) have experienced a ductile dextral slip at middle to low temperatures during which syntectonic granite emplace, and (2) separates two contrasted metamorphic and lithological blocks. The western block displays regional metamorphism increasing eastward from anchizonal to high-temperature amphibolites facies conditions (Arthaud and Matte, 1977; Vai and Cocozza, 1986; Buscail, 2000; Elter et al., 2000; Carosi and Pameri, 2002), although the eastern block is made of eclogite, granulite rocks and migmatites. However, the Grimaud fault has experienced a polyphased tectonics history (Vauchez and Bufalo, 1988; Onezime et al., 1999) including an Upper Visean sinistral movement, a Namurian dextral movement, and a Stephanian sinistral movement, although the Posada-Asinara fault zone seems to have experienced only dextral movement during the 330-300 Ma interval (Elter et al., 1990; Carosi et al., 2002). The Grimaud fault prolongs to the North to orogen-parallel strike-slip shear zones in the Alpine External massifs (von Raumer et al., 1989 and references therein), and the Central Bohemian shear zone in Bohemian (Rajlich, 1987) interpreted as a NW-verging thrust reactivated as a dextral fault during the Westphalian (Matte et al., 1990).

Top-to-the N to NE ductile shearing are found along the Bohemia-Sardinia branch and are interpreted as Middle Carboniferous thrust in the Belledonne massif (Fernandez et al., 2002), or Upper Carboniferous detachment faults in central Bohemia (Matte et al., 1990) and central Sardinia (Conti et al., 2001; Carosi and Palmeri, 2002). They are also found in the eastern Maures (Vauchez and Bufalo, 1988) but interpreted as syntectonic of the sinistral movement of the Grimaud fault, and thus probably associated with Middle Carboniferous S-verging thrusting in central Maures (Bellot et al., 2002b). Because of their contrasting interpretations and their unknown relationships with regional metamorphism, the significance of top-to-the N to NE ductile shearing is uncertain.

Plate-scale geodynamic model

The southern Bohemia-Sardinia Variscan branch probably formed a 1000 km-long and N-S to NE-SW-trending branch should be considered as a fundamental structure that should be included in the reconstitution of plate motions and the geodynamic history of the Variscan belt. This Variscan branch has experienced Precambrian back-arc spreading then Late Cambrian rifting. These small oceanic and continental domains were subducted, probably eastward, during the Late Ordovician-Silurian. Continental collision was initially frontal and absorbed by westward thrusting leading to crustal thickening. It rapidely became oblique, leading to southeastward back thrusting combining or evolving into sinistral orogen-parallel shearing that marks the waning stages of collision (Bellot et al., 2002b). Synorogenic extension leads to crustal thinning, but it is largely transferred along by orogen-parallel dextral movements. Postorogenic extension leads to crustal reequilibration but it is also transferred along by orogen-parallel sinistral movements. These fast changes likely reflect the progressive involvement of the north irregular margin of Gondwana in the collisional process.