The Calabrian Arc
The Calabrian Arc defines a mountain belt encircling the Tyrrhenian Sea, from Southern Apennine to Sicilian Maghrebides. It is characterized by two sectors: the Calabria-Peloritane Domain in the central part, and the Southern Apennines and Sicilian Maghrebides thrust and fold belts at the edges of the curved-shape belt, characterised by different tectonic evolution, structural architecture, and deep lithospheric structure (Fig. 4).
Figure 4. Simplified geological map of the Calabrian Arc.
The Southern Apennines and Sicilian Maghrebides are mainly composed of Mesozoic to Tertiary shallow-platform and deep basin sedimentary units of the African and Adriatic continental margins, which form the Apennine fold-and-thrust system together with sediments from Neogene-Quaternary foredeep and thrust-top basins. The main compressional phases in Southern Apennines and Sicily started in the late Miocene and were almost finished during the early Pleistocene (Argnani et al. 1987; Butler and Grasso 1993; Patacca and Scandone 2001, 2004). The emplacement of the thrust nappe in Southern Apennines was followed by extensional tectonics, which progressively disrupted the fold-and-thrust belt. Extensional processes started during late Pliocene-early Pleistocene (Ascione and Romano 1999) along the Tyrrhenian coast and migrated during the Quaternary to reach the axial part of the chain. Here, active deformation is mostly accomplished by NW-SE oriented normal faults, which are responsible of the largest historical earthquakes recorded in this part of the Italian territory (Valensise and Pantosti 2001; Montone et al. 2004).
Juxtaposed between Southern Apennines and Sicilian Maghrebides, the Calabria-Peloritane Domain is located in the core of the Calabrian Arc. The Calabria-Peloritane Domain mostly consist of a Hercynian basement, Alpine polymetamorphic rock successions related to the deformation of the Southern Tethyan margin, and Mesozoic sedimentary units. These units are unconformably overlain by Tertiary sedimentary sequences, deposited in different tectonic provinces of the Calabrian Arc. In particular, upper Tertiary-to-Quaternary forearc basin sedimentary sequences developed along the Ionian side of the Calabria-Peloritane Domain (Cavazza et al. 1997; Bonardi et al. 2001) at the rear of an accretionary wedge located offshore in the Ionian Sea (Cernobori et al. 1996). Conversely, Miocene-to-Quaternary deposits cropping out along the Tyrrhenian side are interpreted as extensional back-arc basins, associated with opening and expansion of the Southern Tyrrhenian Sea (Sartori 1990; Mattei et al. 2002; Cifelli et al. 2007a).
Starting from the 70’s, in Southern Italy more than 500 sites were sampled either for paleomagnetic or magnetostratigraphic investigations. The age of the investigated rocks range from middle Jurassic to Pleistocene (for a recent review, see Cifelli et al., 2007a).
Paleomagnetic data collected in Southern Apennines and Sicily show a different distribution in time and space compared to Calabria-Peloritane Domain. In Southern Apennines and in Sicily paleomagnetic data from Mesozoic and lower Tertiary sedimentary sequences come from different paleogeographic units of the southern Tethyian passive margins, which presently form the fold and thrust belt of the Southern Apennines and Maghrebian chain. Paleomagnetic results indicate that such units record large magnitude, opposite sense, rotations either in Sicily or in Southern Apennines, clockwise and counterclockwise, respectively (Fig. 4). Paleomagnetic data from upper Miocene and Pleistocene deposits come from foredeep and piggyback basins lying in the external part of the Maghrebian and Southern Apennines thrust belts. In Sicily, most of the data show a significant magnitude of clockwise rotation until lower Pleistocene (Butler et al. 1992; Scheepers and Langereis 1993; Duermejier et al. 1998; Speranza et al. 1999; Speranza et al. 2003). In Southern Apennines, only one result is available from upper Miocene foredeep siliciclastic sediments outcropping along the Tyrrhenian side of the chain, which show about 40° of counterclockwise rotations, whereas most of the data have been obtained from Pliocene strata collected in claystones from the Potenza, Calvello, and Sant’Arcangelo foredeep and piggyback basins (Sagnotti 1992; Scheepers et al. 1993). Data from the Sant’Arcangelo basin and Bradanic foredeep lower Pleistocene sediments indicate a counterclockwise rotation of 25° (Sagnotti 1992; Scheepers et al. 1993; Scheepers and Langereis 1994). Finally, both in Sicily and Southern Apennines, uppermost lower Pleistocene and middle Pleistocene sediments are substantially no rotated (Scheepers et al. 1994; Cifelli et al. 2004; Mattei et al. 2004).
In the Calabro-Peloritane Domain, paleomagnetic data have been mainly collected in upper Miocene to Pleistocene sedimentary sequences cropping out both in the post-orogenic basins along the Tyrrhenian coast and in the forearc basins located along the Ionian side of the region. Notwithstanding the large amount of paleomagnetic results, no data are available from Mesozoic and lower Tertiary units, hindering the possibility to reconstruct the older rotational history of the Calabro-Peloritane Domain crustal block. With the exception of the northwestern sector of the Crati basin, where counterclockwise rotations were measured (Cifelli et al., 2008), upper Miocene to Pleistocene paleomagnetic sediments show a general CW rotational pattern along the entire Calabro-Peloritane Domain. Serravallian to upper Tortonian sediments sampled from the Tyrrhenian and Ionian side of Calabria show similar 20° clockwise rotations. This same value has been also obtained from Pliocene to lower Pleistocene strata in Calabria and Peloritani area (Tauxe et al. 1983; Aifa et al. 1988; Scheepers et al. 1994; Speranza et al. 2000; Mattei et al. 2002; Cifelli et al. 2004). Similar to Southern Apennines and Sicily, in the Calabro-Peloritane Domain the uppermost lower Pleistocene–middle Pleistocene sediments generally do not show appreciable rotations. This is well documented in the Crati Basin, where paleomagnetic data from the uppermost lower Pleistocene–middle Pleistocene sedimentary strata are not affected by tectonic rotations (Cifelli et al., 2007b), These data indicate that no significant vertical axis rotations affected sediments of that age, suggesting that the major episodes of the Calabrian Arc bending were almost completed by that time.
The Calabrian Arc has long been considered a classic example of an orocline, which assumes that its curvature was acquired through bending of an originally almost linear orogenic chain (e.g., Eldredge et al. 1985). From a structural point of view, the arcuate trend of the Calabrian Arc is defined by the regional variation in the strike of fold axes, striking from NW-SE with a NE vergence in Southern Apennines to E-W with a southern vergence across Sicily. Paleomagnetic data show a correlation between the changes in paleomagnetic declinations and the changes in the trend of the orogen, indicating that the present-day shape of Calabrian Arc is a secondary feature, achieved throughout circa symmetrical opposite rotations along the two limbs of the belt, with counterclockwise rotations in southern Apennines and clockwise rotations in the Calabria-Sicilian Maghrebides (e.g., Channell et al. 1980; Channell et al. 1990; Speranza et al. 1999; Gattacceca and Speranza 2002; Speranza et al. 2003). A recent reanalysis of the large paleomagnetic database collected in the last 20 years demonstrates that the orocline model is inappropriate for explaining the spatial distribution and temporal evolution of paleomagnetic rotations along the arc (Cifelli et al., 2008), and that, as we will discuss in this paper, the Calabrian Arc is better described as a progressive arc, following the kinematic classification proposed by Weil and Sussman (2004).