The geology of the western Mediterranean region (continued)

Corsica and Sardinia

The islands of Corsica and Sardinia consist of a Hercynian basement and a Mesozoic-Tertiary cover which are overthrust (in northern Corsica) by slivers of Alpine origin (Mattauer et al. 1981)(Figure 6). The External Units probably originated in the southern margin of Europe, and are usually reconstructed to a position adjacent to southern France or northeast Spain (Figure 2). The separation of these terranes from Europe is attributed to the post-Oligocene rifting in the Gulf of Lion and the opening of the Ligurian Sea, in which the islands underwent ~30-° of counterclockwise rotations (Montigny et al. 1981).

Figure 6 Geological map of Corsica and Sardinia after Cherchi and Montadert (1982) and Jolivet et al. (1990).

In the nappe stack of Alpine Corsica, high-pressure ophiolitic rocks and non-metamorphic rocks are overthrust on top of external (mainly granitic) units. Radiometric ages of the high-pressure rocks in Alpine Corsica indicate that metamorphism took place during the Late Cretaceous and Tertiary (Jolivet et al. 1998, Brunet et al. 2000 and references therein), at the same time when high-pressure metamorphism occurred in the western Alps. Structural evidence shows that Alpine Corsica was thrust over the Hercynian basement with a westward sense of movement and that these structures are overprinted by younger extensional structures with eastward sense of shear (Fournier et al. 1991). Extensional deformation occurred at greenschist-facies conditions and is defined in localised shear zones along extensional detachments (Fournier et al. 1991, Daniel et al. 1996). This style of extension led to isothermal exhumation of the metamorphic core complex now preserved in northeast Corsica (Jolivet et al. 1990, Fournier et al. 1991). An age of ~32 Ma has been estimated as the commencement of extensional deformation in Corsica (Brunet et al. 2000).

The island of Sardinia consists of a Hercynian basement and a Mesozoic to Eocene sedimentary cover, overlain by syn and post-rift sediments and volcanics (Cherchi & Montadert 1982)(Figure 6). The island is transected by a N-S striking rift that forms a succession of tilted blocks filled with Oligocene-Miocene continental to marine sediments (Cherchi & Montadert 1982). Rifting probably commenced in the middle Oligocene (~30 Ma), and at the end of the Oligocene the trough was deep enough to allow invasion by sea (Cherchi & Montadert 1982). The latest syn-rift sediments are Aquitanian in age, indicating that rifting ceased at 23-24 (Cherchi & Montadert 1982). A second phase of extension that postdated the formation of the Sardinian rift took place at mid-Aquitanian to early Burdigalian (23-20 Ma) (Sowerbutts 2000). Letouzey et al. (1982) have reported Burdigalian (21-17 Ma) NE-SW contractional structures, which seemed to occur simultaneously with a third extensional phase associated with the reactivation of N to NNW trending normal faults (Sowerbutts 2000).

Calabria

Calabria is a nappe-structured belt in the southernmost part of the Italian Peninsula and the northeastern corner of Sicily connecting the Apennines and the Maghrebides (Figure 7). The northern and the southern boundaries of the Calabrian block are major strike-slip faults, the sinistral Sagiento Line and the dextral Taormina Line; their sense of motion indicates that the whole Calabrian block underwent eastern displacement relative to the Apennines and Maghrebides (van Dijk & Scheepers 1995). The rocks in Calabria are remarkably different from those in the Apennine-Maghrebide belts indicating prolonged tectono-metamorphic evolution associated with Alpine orogenesis.

Figure 7 Geological map of the Italian Peninsula and Sicily, modified after Channell et al. (1979) and Patacca et al. (1993). SL = Sagiento Line; TR = Taormina Line.

The Calabrian nappes are divided into three major tectonic units (Amodio-Morelli et al. 1976). The lowermost unit consists of a Mesozoic carbonate platform that belongs to the margin of Adria. It is overlain by two ophiolitic nappes composed of Mesozoic and Cenozoic sedimentary and ophiolitic rocks partly metamorphosed under high-pressure conditions (Knott 1987, Cello et al. 1996). The uppermost unit consists of a Paleozoic Hercynian basement and a Mesozoic to Cenozoic sedimentary cover with a strong Alpine signature (Knott 1987). The latter allochthonous terrane, thereafter referred as the Calabrian block, was progressively empvVan Dijk & Okkes 1991).

The age of high-pressure metamorphism in the Calabrian block is ambiguous and varies between 60-35 Ma (Schenk 1980, Rosseti et al. 2001). During the Oligocene and the Miocene, isothermal exhumation of the high-pressure rocks took place (Rosseti et al. 2001). This led to the emplacement of weakly metamorphosed and nonmetamorphosed rocks on top of high-pressure rocks in tectonic contacts of low-angle extensional detachments (Platt & Compagnoni 1990, Rosseti et al. 2001). The commencement of extensional tectonics has been inferred as ~30 Ma from ⁴⁰Ar/³⁹Ar data (Rosseti et al. 2001).

The Apennines

The Apennine belt is a Late Cenozoic fold-and-thrust belt striking parallel to the Italian peninsula from Calabria to the western Alps (Figure 7). The nappes of the Apennines predominantly consist of non-metamorphic or weakly metamorphosed Late Triassic to Neogene marine sediments probably deposited on the passive margin of the Adriatic foreland. The autochthonous crystalline basement is rarely exposed and is only found in the area of Alpi Apuane tectonic windows (northern Apennines). In these outcrops, low-angle normal faults juxtaposed the allochthonous cover above a Hercynian basement, which was metamorphosed at high-pressure conditions at ~25 Ma (Carmignani & Kligfield 1990, Carmignani et al. 1994, Jolivet et al. 1998, Brunet et al. 2000). Extensional tectonics during the Oligocene-Miocene has been considered to play an important role in the exhumation of the Hercynian basement (Carmignani et al. 1994, Jolivet et al. 1998).

Deformation in the Apennines commenced in the Oligocene in the Northern Apennines (Boccaletti et al. 1990) and has propagated southward since the Late Miocene. During deformation, the front of the Apennine thrust system migrated eastward and is presently located predominantly in the Adriatic Sea (Jolivet et al. 1998, Brunet et al. 2000). While ongoing thrusting took place in the (eastern) external Apennines, the (western) internal domain was subjected to crustal extension (e.g. Carmignani et al. 1994, Ferranti et al. 1996) forming extensional sedimentary basins that become younger towards the east (Patacca et al. 1990).