Thin- vs. thick-skinned tectonic styles

The structural setting of the central Mediterranean region has been widely investigated by deep seismic reflection and refraction profiles. However, contradictory readings of seismic reflection/refraction data still lead to different interpretations of the thickness involved during orogenesis (Bally et al., 1986; Mostardini and Merlini, 1986; Finetti, 2005). During the 1980s, a thin-skinned tectonic style was proposed for the Apennines (Fig. 7a), based on seismic profiles (Bally et al., 1986; Mostardini and Merlini, 1986; Calamita and Deiana, 1988; Hill and Hayward, 1988). Moreover, the first magnetic anomaly map of Italy (AGIP SpA. Italia, 1981) showed a SW-dipping regional monocline in the axial northern Apennines at a depth of 10-15 kilometers (Arisi Rota and Fichera, 1987). This monocline supported the interpretation of the foreland Apennines as a thin-skinned orogen (Bally et al., 1986; Mostardini and Merlini, 1986). During the 1990s, the interpretation of different arrays of seismic profiles (CROP 03, Menichetti et al., 1991; Barchi et al., 1998; Morgante et al., 1998; Finetti, 2005), and the integration of surface and subsurface datasets made available by the oil industry (Coward et al., 1999; Butler et al., 2004) lead to thick-skinned models of the Northern Apennines (Mirabella et al., 2008; Boccaletti et al., 2005; Finetti et al., 2005; Fig. 7b, c) that appealed to structural inheritance (Tavarnelli et al., 2004; Scisciani et al., 2010; Calamita et al., 2011; Fig. 7d, e). Furthermore, interpretation of a recent magnetic anomaly map of the Apennines-Adriatic foreland system showed that the basement is incorporated in the foreland Apennine thrust structures and is involved in high-angle thrust ramps, likely reactivating pre-existing extensional faults (Speranza and Chiappini, 2002).

Gravimetric models can help complete the picture of the geologic setting of the Northern Apennines. Gravity analyses in the Northern Apennines are complicated by the similar densities of Mesozoic shelf limestones, Triassic evaporites, and upper basement rocks (Mostardini and Merlini, 1986). Gravity-magnetic modeling confirms that reflectors associated with the basement in seismic interpretation correspond to the bottom of sedimentary cover. The duplication of the Moho beneath the hinterland of the Apenninic chain (Scarascia et al., 1998) has been explained as a consequence of the westward subduction of the Adria lithosphere (Royden et al., 1987; Doglioni, 1991). An alternative interpretation is that the Moho is imbricated along a main lithospheric thrust fault (Finetti et al., 2005) that, in the outer part of the Apennines, could be an expression of the Neo-lithospheric chain (Boccaletti et al., 2005). Different amounts of shortening have been proposed for the OAS thrust, depending on structural style. The thin-skinned style supposes large-scale displacements and duplication of the sedimentary sequence, decoupled from the undisturbed basement along Triassic evaporites. Conversely, the amount of shortening is more conservative in a thick-skinned context, especially if it is involved in inversion tectonics (Fig. 7).