Discussion
Paleomagnetic data collected in Italy indicate that vertical axis rotations played a key role in the Neogene and Quaternary geodynamic evolution of this part of the Mediterranean Basin.
The distribution of tectonic rotations indicate that no rotations occurred before the sedimentary units were involved in the orogenic wedge, both in the Northern Apennines and the Calabrian Arc, suggesting that the measured paleomagnetic rotations totally occurred during the deformation of the orogenic belts. It is worth to note that in the Northern Apennines paleomagnetic data have been mostly collected in the more external units (i.e., Umbria-Marche and Miocene-Pleistocene foredeep units) and only few data are available for innermost units (Epiligurian and Tuscan units). This hampers the possibility to investigate a possible extension of thecounterclockwise rotation of the Corsica-Sardinia block to the Apennine orogenic wedge.
Oroclinal tests carried out for the two arcs indicate that they were progressively shaped during the main deformational phases. In the Northern Apennines the curvature of the arc was acquired mostly during early Miocene to lower Pliocene, as the Plio-Pleistocene units show any relationships between structural trend and paleomagnetic rotations. In the Calabrian Arc the curvature of the arc was acquired during the middle Miocene to early Pleistocene, being the uppermost lower Pleistocene-middle Pleistocene deposits completely unrotated.
Concerning with the geodynamic mechanism responsible for the curvature of the Calabrian and Northern Apennines arcs, one common feature observed in both the arcs is that the core of the arc is located on top of subducting slab, evidenced by seismic tomography and subcrustal seismicity (Selvaggi and Amato 1992; Selvaggi and Chiarabba 1995; Chiarabba et al. 2009; Di Stefano et al. 2009). On the other hand, the different distribution of tectonic rotations in the Northern Apennines and the Calabrian Arc suggest a distinct evolution and mode of arc formation. In particular, the different amount of curvature of the arcs can be related to the nature, length and different evolution of the subducting lithospheres.
In Northern Apennines subducting lithosphere is continental since Oligocene times, and almost homogeneous all along the arc. The correlation between subduction, nappes emplacement and paleomagnetic rotations is demonstrated by the lack of vertical axis rotations along the internal Tyrrhenian margin, during the same time span of the acquisition of the Northern Apennines curvature. In this sector, minor (second order) arcs also concurred to the observed paleomagnetic rotations. These small-scale arcs are mainly related to heterogeneities in the sedimentary successions or to the presence of magmatic bodies which influenced the geometry and development of thrust propagation.
In the Calabrian Arc, the upper mantle structure is more complex and reflects, on surface, the different structural architecture and tectonic evolution between the Calabria-Peloritane Domain, from one side, and Sicilian Maghrebides and Southern Apennines, on the other side. Earthquake distribution and seismic tomography indicate that the Calabria-Peloritane Domain is presently located on top of a narrow and steeply dipping slab (e.g., Anderson and Jackson 1987; Spakman et al. 1993; Selvaggi and Chiarabba 1995; Lucente et al. 1999; Wortel and Spakman 2000; Piromallo and Morelli 2003). The across-strike width of the slab (about 700 km) corresponds to the Ionian Mesozoic oceanic lithosphere, intervening between the Apulia and Africa continental margins. Its northeastern and southwestern boundaries match, at the surface, the boundary between Calabria-Peloritane Domain and Southern Apennines and western Sicily, respectively, which conversely, do not show any evidence of deep seismicity. According to the current geodynamic models, the evolution of the Calabrian Arc during the Neogene and Quaternary was driven by the southeastward retreat of the Ionian slab (among others, Malinverno and Ryan 1986; Faccenna et al. 2001). The fast retreat of the Ionian slab during Neogene was responsible of the southeastward drifting of the Calabria-Peloritane Domain far away from Sardinia-Corsica, to be finally juxtaposed to the Southern Apennine-Maghrebide orogenic system (Alvarez et al. 1974; Malinverno and Ryan 1986; Faccenna et al. 1997; Bonardi et al. 2001; Mattei et al. 2002). Differently from Northern Apennines, where only laterally homogeneous continental lithosphere was subducted, in the Calabrian Arc, the peculiar configuration of the subduction system (an oceanic lithosphere locked between two continental lithospheres) initially favored the fast roll-back of the Ionian oceanic lithosphere and enhanced the tight curvature of the Calabrian Arc. Later on, when the reduction of the amount of oceanic lithosphere available for subduction led to shortening of the continental lithosphere, the retreat of the Ionian subducting slab was prevented and the curvature halted, together with back-arc opening in the Southern Tyrrhenian Sea.
The different shape and tectonic evolution of the Northern Apennines and the Calabrian Arc indicate how the internal heterogeneities within the subducting lithosphere may influence the mode and the shape of arc formation, confirming that the nature of both the overriding plate and the subducting slab, their geometry, width and space-time evolution, are elements that must be necessarily taken into account to fully understand the formation and evolution of related arc-shaped belts.