The origin of curvature in mountain belts is an important topic in tectonic and geodynamic research and represents, for the Northern Apennines and in the Calabrian Arc, a long debated paleomagnetic subject (Fig. 1). The kinematics of an arc and the timing of its curvature are crucial factors for understanding the geodynamic process governing arc formation. In active convergent margins, slab roll back process is considered the most likely candidate for the formation of arcs (e.g. Schellart and Lister 2004). In this geodynamic context, the geometry and nature of both the overriding plate and the subducting slab, their width and space-time evolution, are elements that must be taken into account to fully understand the formation and evolution of related arc-shaped belts (Faccenna et al. 2004; Schellart and Lister 2004; Morra et al. 2006).
Figure 1. Schematic map of the Mediterranean area.
Depending on the relationship between the timing of thrusting and vertical-axis rotations, arcs can be classified in: (i) primary curves, where no vertical-axis rotations are required; secondary curves, where vertical-axis rotations post-date thrusting and (iii) progressive curves, where rotation and thrusting occur simultaneously (among others, Carey 1955; Elliott 1976; Sanderson 1982; Marshak 1988; Wirkerson 1992; Hindle and Burkhard 1999; Sussman et al. 2004; Weil and Sussman 2004). This classification of curved belts provides two end-members, which can be distinguished by comparing paleomagnetic declinations with structural data (Fig. 2). In primary arcs, paleomagnetic declinations remain parallel along the arc and do not correlate with changes in thrust and fold-axis trend (Fig. 2a). In secondary arcs, paleomagnetic declinations change direction along the arc and follow changes in thrust and fold-axis trend with a one-to-one correlation (Fig. 2b). In the last few decades, increasing evidence has been found that these two end members cannot describe the kinematic evolutions of most curved orogens. In fact, detailed paleomagnetic investigations from well-dated syntectonic sediments show that in several orogens, vertical axis rotations occurred neither before nor after, but during thrust activity (Fig. 2c) (among others, Allerton et al. 1993; Mattei et al. 1995; Gray and Stamatakos 1997; Pueyo et al. 2002; Sussman et al. 2004; Barke et al. 2007). Furthermore, information from thrust tectonics and stratigraphy of syntectonic sediments shows that fold-thrust belts evolve over a long time span, with complexities in the evolution of deformation (e.g., Elliot, 1976).
Figure 2. Classification of arcs.
Since the first formulation of tectonic models in the 1970’s for the Mediterranean area, paleomagnetic data have been collected in Italy as key information for constraining kinematic and geodynamic reconstructions. First paleomagnetic studies were focused on Mesozoic-lower Tertiary sequences and interpreted Italy as a single block rotated counterclockwise since late Cretaceous (Channell and Tarling 1975; Klootwijk and Van den Berg 1975; Lowrie and Alvarez 1975; Vandenberg et al. 1978). In the following years, this early tectonic interpretation was drastically modified by the increasing distribution of paleomagnetic data. Channell et al. (1978) firstly suggested that the different amount of paleomagnetic rotations observed both in Umbria and western Sicily were related to thrust emplacement and should be considered as related to the kinematics of allochtonous units which were deformed during the Neogene. This tectonic interpretation has been confirmed in the recent years by a large amount of data, which has been collected in the Neogene and Quaternary units. The analysis of sequences coeval with the main geodynamic events suggested for Italy a more complex rotational history than a single rigid rotation, and allowed to constrain the kinematics and timing of curvature of the Northern Apennines and Calabrian arcs (Sagnotti 1992; Scheepers et al. 1994; Mattei et al. 1995; Speranza et al. 1999; Speranza et al. 2000; Cifelli et al. 2007a, among many others). Paleomagnetic studies detected a different sense and amount of vertical-axis rotations over distinct geodynamical provinces of the Italian Peninsula, with tectonic rotations occurring at different geological times, sometimes as pulses with surprisingly fast rates in short geological time intervals (e.g., Scheepers et al. 1993; Mattei et al. 2004).
This paper is intended to be an updated review on the contribution of paleomagnetism to the reconstruction of the Neogene geodynamic evolution of the Apennines. In this study we reanalyze the available paleomagnetic database for the Northern Apennines and the Calabrian Arc. This quantitative analysis represents a unique opportunity to constrain the tectonic and kinematic evolution of the arcs during the Neogene, in the general framework of the geodynamic evolution of the central Mediterranean area. In particular, the curved shape of the Northern Apennines and the Calabrian Arc will be quantitatively analysed and compared in order to define whether the shape of these two arcs can be referred to a simple oroclinal model or a more complex evolution has to be invoked.