Introduction
Corsica is located in the northern part of the Western Mediterranean (Fig.1). It is subdivided into two main geological domains: a western area, mainly consisting of Late Hercynian granitoids and minor relicts of host rock-basement (Hercynian Corsica), and an eastern area characterized by continental and oceanic-derived units pervasively deformed during the Alpine orogeny (Alpine Corsica).
Figure 1. Morphostructural sketch map of the Mediterranean region
Morphostructural sketch map of the Mediterranean region showing the main orogenic systems the present day kinematics and age of opening of the different oceanic domains (modified after Faccenna et al., 2004).
Corsica and Sardinia are traditionally regarded as a microblock originally welded to southern France-Northern Iberia until Middle-Oligocene (Fig.1), when rifting and then drifting in the Liguro-Provençal basin took place (Guegen et al. 1997; Carminati et al., 1998; Speranza et al. 2002; Rollet et al. 2002 and references therein). Therefore, the western Hercynian domain is correlated with the Maures-Esterel basement of southern France whereas Alpine Corsica is regarded as the southern prolongation of western Alps, through the Western Liguria and Voltri Group Alpine units (Durand Delga, 1984 and references therein).
The eastern part of Corsica (Alpine Corsica), is made up by a stack of tectonic units, some including rocks of oceanic origin (ophiolitic basement and related cover rocks) and others of continental origin (Mattauer et al. 1981; Durand Delga 1984; Dallan and Nardi 1984; Dallan and Puccinelli 1995; Malavieille et al. 1998; Rossi et al. 2003). The maps and the schematic cross-section of Fig.2 outline the overall architecture of the belt and its major internal subdivisions. From bottom to top four different groups of units can be distinguished:
Figure 2. Tectonic map and regional cross-sections of northern Corsica
Tectonic map and regional cross-sections of northern Corsica showing the main tectonic units and the regional trend of stretching/mineral lineations with sense of shear for subduction- (black arrows) and low-pressure greenschist-related extensional fabrics.
1) “Autochthonous” Corsica, forming the western side of the island, consisting of Hercynian units (mainly Carboniferous to Permian granitoids and Permian volcano-sedimentary sequences) locally weakly affected by Alpine deformation and a reduced and incomplete Mesozoic to Mid-Eocene sedimentary cover of Briançonnais affinity (Durand Delga 1984; Michard and Martinotti 2002).
2) Corsica-derived continental-crust units, which include mildly to strongly reworked crystalline rocks of the Hercynian basement (including Permian granitoids) and their Mesozoic to Mid-Eocene sedimentary cover originally deposited on the distal part of the Corsica continental margin (Faure and Malavieille 1980; Rossi et al. 2002; Molli 2008). This group of units will be described in further detail below.
3) The ‘Schistes Lustrés’ composite nappe, comprising Ligurian Tethys-derived oceanic sequences (mantle ultramafics, gabbros, pillow lavas and associated Jurassic to Cretaceous metasediments) and relicts of the former ocean-continent transition (OCT) domain,. The latter consists of exhumed mantle, ophiolitic metagabbros and metabasalts associated with slices of continental upper crust and related metasedimentary cover rocks (Caron and Delcey 1979; Durand Delga 1984; Lagabrielle et al. 2005; Vitale et al. 2009). These groups of units show peak metamorphism ranging from blueschist to eclogite facies conditions and various degrees of retrogression under greenschist facies conditions (Dal Piaz and Zirpoli, 1979; Gibbons et al. 1986; Waters, 1990; Caron 1994; Fournier et al. 1991; Daniel et al. 1996; Molli et al. 2006; Vitale Brovarone et al. 2009; Vitale Brovarone et al., 2011).
4) The upper-Nappe system also called Nappe Supérieure includes the Balagne, Nebbio and Macinaggio units. It is formed by ophiolitic as well as continent-derived rocks mainly associated with Late-Cretaceous siliciclastic and calcareous-marly flysch deposits (Durand Delga 1984; Dallan and Nardi 1984; Marroni and Pandolfi 2003). The units are characterized by low-grade metamorphic assemblages (prehnite/pumpellyite in mafic rocks) and a geometrically high position in the nappe stack. The ophiolitic Balagne nappe is found in the external zones directly overriding the continental units (“autochthonous and external continental units” see below), while the Nebbio and Macinaggio units occupies a more internal position, resting on top of the high-pressure low-temperature Schistes Lustrés composite units.
The uppermost nappes and their contacts with the underlying units are locally unconformably sealed by lower Miocene “post-orogenic” continental or marine sediments of the Francardo, St. Florent and Aleria basins (Dallan and Puccinelli 1995; Ferrandini et al. 1998; Cavazza et al., 2001).