Chabrier & Mascle (1975) first pointed out that the pre-Oligocene geological history of Provence and Sardinia is similar. In particular, the following correlations can be made with the units defined for the Maures Massif and for the Variscan Corsica and Sardinia (Figure 9):
Figure 9. Geological relationships between Sardinia-Corsica and Maures Massifs
Geological relationships between Sardinia-Corsica and Maures Massifs (modified from Elter et al., 1999)
The migmatitic complex of the Eastern Unit of the Maures Massif (Vauchez & Bufalo, 1998; Bellot et al., 2003) and of the HGMC of Sardinia (Elter et al., 1999, 2004) record a similar P-T path along a Barrovian clockwise trajectory (cfr. Figs. 2 and 4).
Like the HGMC of Sardinia (Punta de li Tulchi - Punta Tittinosu eclogites: Elter, 1987) and Corsica (Belgodere eclogites: Palagi et al., 1985), the migmatitic Complex in the EB of the Maures Massif contains lenses of amphibolitic eclogites (e.g. La Cartelle eclogite).
The migmatitic complex of the Maures Massif is characterized by a composite secondary foliation associated with heterogeneous ductile deformation related to non-coaxial deformation (Vauchez & Bufalo, 1998) with a "top to North" sense of shear. A similar structural evolution is described by Corsi & Elter (2005), for the HGMC of Sardinia: its composite migmatitic foliation is related to a polyphase non-coaxial deformation, an earlier with a "top to Northwest" sense of shear and an younger with a "top-to-South/Southeast" shear component.
In both areas, migmatization processes are recognizable and they are substancially coeval with the non-coaxial deformation. However in Sardinia, Corsi & Elter (2005) point out the presence at least of two migmatization events as in the Variscan French Central Massif (Leloix et al., 1999, Faure et al., 2002). The migmatization is dated to 345-339 Ma in the Maures Massif (U/Pb data in Moussavou et al., 1998), while in Sardinia, an age of 344 Ma is pointed out for the second migmatization event (Ferrara et al., 1978).
In both areas the polyphase emplacement of the granitoid bodies occurred mostly in the same time interval: at 334 to 297 Ma ago in the Maures Massif and at 346 to 289 Ma ago in the Sardinia - Corsica block.
There are not only analogies between the migmatitic complexes, but also between other geological elements. The La Garde-Freinet Unit plays the same structural role of the Posada Valley Shear Zone: in fact, they both link the lower metamorphic units to the highest ones. In addition, there is a close correlation between the lithology and structural-metamorphic evolution of these two units: e.g. staurolite-biotite and kyanite-biotite micaschists are found in both units (Elter et al., 1986, Buscail & Leyreloup, 1998).
On the contrary meta-gabbros, meta-serpentinites and garnet-spinel meta-peridotites with an oceanic affinity (Bellot et al., 2003) found in the La Garde-Freinet Unit are not present in the Lower Unit of the Posada Valley Zone, but the N-T MORB Torpè orthoamphibolites (Cappelli et al., 1992) found in the Posada-Asinara Shear Zone suggest a possible correlation with the oceanic basic and ultrabasic rocks of the Mauri Massif.
The relationship between the La Garde-Freinet Unit and the Migmatitic Complex in the EB of the Maures Massif is characterized by the interposition of the Grimaud Fault, just like the Posada Valley Zone in Sardinia which separates the Internal Nappe Zone from the High Grade Metamorphic Complex. The Grimaud Fault shows a composite tectono-metamorphic evolution characterized by two events: sinistral strike-slip faulting followed by dextral strike slip faulting associated with the intrusion of the syn-kinematic Rouet-Plan de la Tour granite (Onezime et al., 1999). Also this fault separated two blocks (EB and WB) characterized by diachronous cooling (Morillon et al., 2000). Also the Posada Valley Shear Zone (Elter et al., 1990, 1999, 2004) is characterized by a composite tectono-metamorphic evolution, consisting in two events: the first in an extensional event under amphibolite facies conditions, followed by the second, dextral strike slip event under greenschist facies conditions. During the latter event, thermo-metamorphism, related to the emplacement of 320-300 Ma granodiorites, also occurred (Muzio, 2003). Therefore, the later greenschist facies evolution of the Grimaud Fault and of the Posada Valley Shear Zone is similar, in spite differences in the initial metamorphic evolution of these shear zones. Finally, lithological affinities can be pointed out between the ortho- and paragneisses, micaschists and amphibolites of the Middle Bormes Unit in the Maures Massif and those of the Upper Unit of the Posada Valley Zone (e.g. the orthogneisses of Lodè-Mamone Complex).
As it regards the correlation of the Variscan stratigraphic successions of the Northern Apennines with those of the Corsica-Sardinia block (Figure 10), the strong similarities in stratigraphy and in the pre-Alpine metamorphic grade between the greenschist Facies, ?Late Cambrian-Devonian Tuscan successions and the units of the External Nappe Zone, i.e.Gerrei, Sarrabus, Sarcidano, in Sardinia (Conti et al., 1991; Pandeli et al., 1994 and references therein) are evident. Also the rocks of the Micaschist Complex and of the Gneiss Complex have their equivalents in the Variscan units of Sardinia. In fact, rocks similar to Larderello micaschists, amphibolites and gneisses crop out in central and NE Sardinia. In particular, paragneiss and garnet- and staurolite-bearing micaschists characterized the deepest unit of the Nappe Zone (M.Grighini Unit) (Carosi et al., 1992), of the Low- to Medium- grade Complex of the Axial Zone (Franceschelli et al., 1982; Oggiano & Di Pisa, 1992) and of the Posada-Asinara Shear Zone (Elter et al., 1990; Carmignani et al., 1982, 2001). Further analogies can be found with the gneisses of the sillimanite+muscovite zone in the HGMC (Oggiano & Di Pisa, 1992). The syn-collisional Barrowian metamorphism evolution of the Low- to Medium- grade Complex of the Axial Zone (dated 336-350 Ma: Rb/Sr and Ar/Ar radiometric ages in Del Moro et al., 1991) shows evident analogies with respect to that of the Larderello gneisses and micaschists. For example, the plagioclase and garnet porphyroblasts, which include schistosity relics, is a typical feature not only of the Larderello rocks, but of those of the Garnet+albite+oligoclase and Staurolite+biotite Zones in the Sardinian Low- to Medium- grade Complex (Carmignani et al., 1982, 2001; Franceschelli et al., 1982; Oggiano & Di Pisa, 1988; Elter et al., 1989; Di Vincenzo et al., 2004) or LU in the Posada Valley Zone (Elter et al., 2004 and this paper). In addition, the garnet porphyroclasts show a prograde zoning (decrease of spessartine from core to rim: Franceschelli et al., 1982; Di Vincenzo et al., 2004) which is also present in the pre-Alpine garnets of the Larderello subsurface (Pandeli et al., 2005) and of the Cerreto Pass (Molli et al., 2002). The main foliation of the considered Sardinian rocks is overprinted by the thermometamorphic mineral assemblage connected to the Uppermost Carboniferous-Lower Permian, peraluminiferous magmatism (Granitoid Complex in Oggiano & Di Pisa, 1992; Ricci, 1992; Carmignani et al., 2001) of Northern Sardinia. This magmatic event is also likely testified, in the crystalline basement of Tuscany, by the crystallisation of andalusite+muscovite at 285 Ma in the Larderello micaschists.
Figure 10. Geological relationships between the Variscan units of Sardinia and of the Northern Apennines
Geological relationships between the Variscan units of Sardinia and of the Northern Apennines.
A further analogy is given by the oceanic affinity of the orthoamphibolite levels within the Micaschist Complex and the Gneiss Complex (Puxeddu et al., 1984; Molli et al., 2002) that recalls the Variscan N-T MORB orthoamphibolite intercalations within the Posada-Asinara Shear Zone in Sardinia (cfr. Elter et al., 1990; Cappelli et al., 1992), whereas the other Paleozoic and Triassic metabasites of Tuscany are continental WP(=Within Plate) or calc-alkaline metavolcanites (Puxeddu et al., 1984; Conti et al., 1988; Pandeli et al., 1994; Pandeli, 2002 and references therein).
During the Carboniferous-Permian times, the paleogeographic reconstruction of the southern part of the Variscan Belt consider Provence, Corsica-Sardinia, Tuscan part of Adria and Iberia as discrete or composite terranes (e.g. Stampfli & Mosar, 1999; Stampfli et al., 2003; Von Raumer et al., 2003; Rau & Tongiorgi, 1981 and references therein; see also web site: www.sst.unil.ch) interposed between the stable Europe and Gondwana. According to some authors (e.g. Rau & Tongiorgi, 1981; Von Raumer et al., 2003), in the pre-Carboniferous times, these terranes were mostly far from each other, but they were adjacent during the Variscan Orogeny or, at least, not too far (e.g. Scotese et al., 1979; Rau & Tongiorgi, 1981; Gelmini, 1985; Vai & Cocozza, 1986; Neugerbauer 1989; Vai, 1991; Dercourt et al., 1993; Von Raumer et al., 2003; see also web site: www.scotese.com/earth.htm). Stampfli & Mosar (1999) and Stampfli et al. (2003) instead located the Calabria-Peloritani terrane between the Corsica-Sardinia and the Adria blocks. During the Late Paleozoic the Variscan architecture of the terranes was modified by strike-slip tectonics. In particular, in Provence and Corsica-Sardinia segments, the data point to a syn-extensional shearing during the Late Carboniferous-Early Permian (e.g. the Grimaud Fault and Posada-Asinara line respectively) which was probably connected to the well-known dextral strike-slip megashears. The latters, characterized by hundreds of kilometres throws, repeatedly crosscut the southern part of the Variscan Belt. In particular, during Late Carboniferous-Permian, the North Pyrenean Fault (e.g. Arthaud & Matte, 1975; 1977; Ziegler, 1984; Rau, 1990; Vai, 1991 and references therein) or the High Atlas Fault (e.g. Neugerbauer, 1989) approached Provence, Corsica-Sardinia and Adria.
The post-Permian tectonic events didn't substantially change the Permian relationships of these Variscan segments: the Jurassic opening and Late Cretaceous-Tertiary closing of the NeoTethys (Abbate et al., 1986; Bortolotti et al., 1990; Argnani, 2002 and references therein) as well as the syn- to post-collisional events of the Alpine Orogeny. The latter produced the Apenninic tectogenetic shortening and the development of the Ligurian-Balearic and Northern Tyrrhenian basins due to rotation of the Corsica-Sardinia block from Provence and the Italian Peninsula from the former respectively (Montigny et al., 1981; Rehault et al., 1984; Boccaletti et al., 1990; Vigliotti & Langenheim, 1995; Carmignani et al., 1995; Sartori, 2001; Vai & Martini, 2001; Argnani, 2002 and references therein).
Therefore, during the Late Carboniferous-Permian, Maures, Corsica-Sardinia and the Tuscan part of Adria can be located in the same sector of the Variscan Belt, as supported by the similar sedimentary (e.g. the strong analogies between the Paleozoic successions of the Northern Apennines and the Central Sardinia: Pandeli et al., 1994; Pandeli et al., 2005 and references therein), tectono-metamorphic and magmatic evolution and by the possible correlation of isopic tectono-metamorphic lineaments (Pandeli & Elter, 1996; Vai, 2001; Carmignani e al., 2001; Elter et al., 2004 and previous paragraphs).
So, we think that the Posada-Asinara Line and the Grimaud Fault don't represent different shear boundaries between terranes, but a single main shear zone (see also Carmignani et al., 2001 and Elter et al., 2004). Its possible prosecution in the Tuscan Variscan segment can be tentatively hypothesized by the presence of rocks similar to those of the Posada Valley Zone.