The Northern Apennines

The Apennine-Maghrebide orogen is characterized by a curved shape showing the Calabrian units in the apical zone and the Tyrrhenian extensional basin in the inner area (Johnston and Mazzoli, 2009). This first-order curved belt is divided into two arcs: the Northern Apennines Arc and the Southern Apennines-Calabrian Arc, with NE and SE convexity, respectively (Fig. 1). The two arcs are characterized by several differences in paleogeographic domains, stratigraphic successions, structural settings, the amount and sense of vertical-axis rotations, and geodynamic evolution (Boccaletti et al., 1971; Malinverno and Ryan, 1986; Carmignani and Klingfield, 1990; Doglioni, 1991; Boccaletti et al., 2005; Finetti et al., 2005; Satolli and Calamita, 2008). In the Northern Apennines, the carbonate structural units generated by the deformation of the outer Adria paleomargin crop out. Their bounding thrust faults accommodate a maximum shortening of approximately 10 km in the apical zone, as seen in length-displacement profiles across the thrust front (Mazzoli et al., 2005).

Figure 1. Tectonic sketch map.

Tectonic sketch map.

Tectonic sketch map of the Apennine-Maghrebide thrust front, showing the two main outer arcs of the Northern Apennines and Southern Apennines-Calabrian Arc.

Conversely, in the Southern Apennines, the Apulian units lie in the footwall of the basal thrust, along which the main allochthonous units are far travelled (Liguridi-Sicilidi, inner Carbonate Platform, and Lagonegro-Sannio-Molise Units). The Apulian carbonate units have been interpreted as the Apulian duplex (Mostardini and Merlini, 1986; Patacca and Scandone, 2007), the buried Apulian chain (Cello et al., 1989) with thick-skinned structural setting characterized by the reactivation of Permo-Triassic extensional faults (Shiner et al., 2004), or as the Pliocene Apennine neochain (Boccaletti et al., 2005). Geophysical studies provide evidence that deep-seated reverse faulting involves the basement in the Southern Apennines (Improta & Corciulo, 2006; Steckler et al., 2008). The deformation of the Apulian chain is characterized by limited horizontal displacement (Menardi Noguera & Rea, 2000; Butler et al., 2004; Turrini & Rennison, 2004), whereas large displacements of up to several tens of kilometers are localized at the base of the overlying allochthonous units (Mazzoli et al., 2008).

Paleomagnetic data show a separation between the Northern and the Southern Apennines. In fact, the Northern Apennines Arc is characterized by CCW rotations in the Emilia-Romagna and Marche regions passing to CW rotations in its southern sector. Similarly, the southern Apennines-Calabrian Arc is characterized by CCW rotations in the southern Apennines and strong CW rotations in Sicily.

This study focuses on the Northern Apennines Arc. This fold-and-thrust belt was formed by the convergence of the African and Eurasian plates, beginning during the Late Cretaceous Period. Starting in the Late Oligocene Period, the convergence caused the formation of the Apennines, as indicated by the age of siliciclastic deposits (Boccaletti et al., 1990). Orogenesis generated a Triassic to Miocene sedimentary multilayer characterized by a strong contrast of competence (Ciarapica and Passeri, 2002; Patacca and Scandone, 2007). From the Middle/Late Miocene to Late Pliocene Periods, coeval occurrence of normal and thrust faults characterized the western and eastern belt margins, respectively (Cavinato and DeCelles, 1999). Consequently, both syn-rift (primarily deposited within intermontane basins) and foredeep (later incorporated into the frontal thrust structures) sediments are exposed in the Apennine belt. Extension and compression continued during the Pleistocene-Holocene Period and still actively occur today, controlling seismicity in the Apennine mountain range and Po plain/Adriatic areas, respectively.