Discussion
The structural and metamorphic evolution of the Corsica-derived continental units may be used to provide first order constraints on the kinematics of the Corsican orogen. The Corsica-derived continental units and continental slices from the former OCT show a variable high pressure/low temperature metamorphic imprint. It ranges from lower grade (high pressure greenschist to lower blueschist facies) in the external and presently geometrically lowermost units, to eclogite facies in the more internal and uppermost ones (see page for more details; Bézert and Caby1988; Egal and Caron 1988; Bézert 1990; Caron 1994; Lahondère 1991; Tribuzio and Giacomini 2002; Molli and Tribuzio 2004; Malasoma et al. 2006; Molli et al. 2006; Molli 2008; Chopin et al. 2008; Vitale Brovarone et al. 2009, 2011a and 2011b).
Pervasive composite mylonitic foliation with relicts of eclogite fabrics can be observed in the uppermost slices. Stretching lineation and shear sense indicators associated with eclogite fabrics provide rare evidence of top north/northwest shearing (Lahondère 1996; Lahondère and Caby 1997), whereas the blueschist retrograde fabric formed the mappable main foliation associated with an east-west stretching lineation and a dominant top-to-west kinematics (Fig. 1.2) (Faure and Malavieille 1981; Mattauer et al. 1981; Malavieille 1983; Harris 1985 a, b; Jolivet et al. 1991; Fournier et al. 1991; Daniel et al. 1996).
The Tenda and Centuri units are characterized by a heterogeneous deformation pattern with low strain domains without metamorphic mineral fabrics (i.e. isotropic) or with a magmatic grain-shape fabric surrounded by mylonitic orthogneisses and/or mylonites. The dominant fabric in the Centuri unit is represented by blueschist facies assemblages (Malavieille 1983) whereas in the Tenda unit greenschist facies exhumation-related tectonites are widespread (Gibbons and Horack 1984; Molli and Tribuzio 2004). Kinematic criteria in the blueschist assemblages in the Centuri unit suggest both top-to-east as well as top-to-west shearing (Malavieille 1983; Harris 1984; Malavieille et al. 1998) while the preserved blueschist relict domains in the Tenda unit witness dominant top-to-west shearing (Mattauer et al. 1981; Lahondère et al. 1999; Daniel et al. 1996; Molli and Tribuzio 2004; Molli et al. 2006).
The external continental unitsare also characterized by high pressure/low temperature metamorphic overprint with structures associated with an east-west direction of transport and dominant top-west kinematics which is observable all along the boundary between Alpine and Hercynian Corsica (Bézert and Caby1988; Egal 1988; Bézert 1990; Malasoma et al. 2006, Molli 2008).
As a whole, the present geometry, internal structure, deformation style and metamorphic peak conditions of the Corsican continental units within the nappe stack can provide first order constraints on the role of the Corsican continental margin during the early stages of the Alpine orogenic history. These data sets indicate a progressive underthrusting of the upper Corsican crust in a framework of east-dipping continental subduction (Mattauer and Proust 1975; Mattauer et al. 1981; Gibbons et al. 1986; Gibson and Horack 1984; Malavieille et al. 1998; Molli et al. 2006; Molli 2008; Molli and Malavieille 2010).
The age of the earliest stages of Alpine metamorphism in Corsica is still under investigation (Handy and Oberhänsli 2004; Berger and Bousquet 2008). Despite these limitations, available geochronological data, coupled with independent geological observations, provide a coherent regional framework for the overall evolution of the belt. In the Schistes Lustrés, eclogites yielded a Sm-Nd whole rock Grt-Gla-Cpx isochron age of 84 ±5 Ma (Lahondère and Guerrot 1997). Phengite from continent-derived eclogitic gneisses of the Farinole unit yielded a 40Ar/39Ar discordant age spectrum with ages increasing from 55.3 ±4.3 Ma to 65,3±0,7 Ma, whereas post-eclogitic phengites give ages from 54.3 ±0.5 Ma to 37.4 ±0.4 Ma (Brunet et al. 2000). Similar retrogression-related ages were previously obtained by Maluski (1977) and Lahondère (1996).
For the Tenda unit, celadonite-rich phengites (Si = 3.5 apfu) from a deformed granitoid, (see also Maluski 1977; Cohen et al. 1981) yielded a discordant 40Ar/39Ar spectrum increasing from about 25 Ma to 47 Ma (Brunet et al. 2000). This suggests that high-pressure metamorphism in the Tenda Massif has a minimum age of 47 Ma (Molli and Tribuzio 2004). The more recent U/Pb TIMS study of Maggi et al. (2011) yielded ages between 48 ±18 Ma (MSWD 7.3) and 54 ±8 Ma (MSWD = 48).
On the base of stratigraphic observations, underthrusting and subduction of the Corsican continental crust is documented to last until at least the Bartonian (40-37 Ma). This conclusion is based on the presence of a sedimentary cover containing Nummulites biarritzensis and Discocyclina sp. (Bézert and Caby 1988), dated to early Mid-Eocene in the External Continental Units, which are affected by high-pressure greenschist facies to high pressure/low-temperature metamorphism (Bézert and Caby 1988; Bézert 1990; Malasoma et al. 2006; Malasoma and Marroni 2008; Molli 2008) contain