Journal of the Virtual Explorer

As we have seen in the previous chapter, the present-day stress field of the NA is characterised by compression in the foreland and extension in the hinterland (Montone et al., 2004). In particular, at shallow crustal levels we have compressional earthquakes in the Po Plain-Adriatic region and extensional earthquakes along the axial ridge of the Apennines. The presently active compressional and extensional belts are clearly superposed to the alignment of the compressional and extensional basins that were active in the early Pleistocene (e.g. Lavecchia et al., 1994; Scrocca et al., 2007).

This observation supports a uniformitarian view, where the long-lived tectonic process that governed the evolution of the NA is still active and is now expressed by the present-day stress field and by the on-going deformation as well. If the present is a key for the past, then also the past is a key for the present, and we can use long-term geological observation to better understand the on-going tectonic processes. In this view, the seismicity depicts a snapshot of the present-day tectonic setting, the last stage of a much longer tectonic history, active since about 17 Ma, whose main feature is the progressive eastward migration of a coupled pair of compression and extension, from the Tyrrhenian to the Adriatic region.

The outstanding similarity of the general framework, supporting the uniformitarian concept, overwhelms the local differences and irregularities in the rate of migration of the compressional and extensional waves (see chapter 3). These irregularities could be partly related to the rise and fall of the base level, induced by climatic or eustatic changes, occurred during the long-lived tectonic process, deeply affecting the onset and evolution of both hinterland and foreland basins, modifying the pattern of surface transport processes (i.e. erosion and sedimentation, see chapter 3).

Barchi et al. (2006) described the tight space and time relationships between compression and extension, highlighted by the geological data about the age of the syn-tectonic basins (long-term image) as well as by the instrumental seismicity pattern (short-term image). These relationships are expressed through three simple “rules”, controlling the switch from the compressional to the extensional regime.

The first rule describes how the tectonic regime varies in the horizontal space, and says that at any time compression and extension are contemporaneously active, in the foreland and in the hinterland, respectively.

The second rule describes how the tectonic regime varies with time, and states that at any position, extension postdates compression.

The third rule describes how the tectonic regime varies with depth, and says that extension can occur at shallow levels (in the uppermost crust) at the same time and position of compression at greater depth (lower crust and upper mantle).

These rules are strictly geometrical and they are valid independently from the adopted geodynamic model, where the space and time relationships between compression and extension are framed.

The NA are characterised by the coexistence and interaction of two different tectonic environments, which need to be understood and framed into an appropriate geodynamic setting. The AD is a typical thrust and fold belt, where a sedimentary succession, originally deposited on a continental passive margin, is involved at the periphery of a collisional orogen. The TD, with a thinned crust and lithosphere, displays all the typical geological and geophysical features of an extensional belt (Lister and Davis, 1989), such as positive Bouguer anomalies, high heat flow and strong magmatic activity.

Many different hypotheses have been proposed in the last 40 years, that can be referred to three main groups of models, i.e.: extension-dominated models; compression dominated models; and complex models, where extension and compression derive from a unique process and are of the same order of magnitude.

The first group of models assigns the main role in the dynamics of the Apennines to extension. Extension is driven by the eastward shifting of an asthenospheric rising plume, and produces active rifting in the Tyrrhenian Sea (Lavecchia, 1988; Decandia et al. 1998; Lavecchia et al., 2003). In this view, compression in the foreland would be a secondary effect, induced at the outer border of the extending region by horizontal forces (rift push), generated by the strong difference in the lithospheric thickness between the thinned TD and the unthinned AD (Lavecchia et al. 2003).

A second group of models calls for a quasi-continuous, eastward thrusting of the Apenninic crust and tends to minimize the role of extension (Boccaletti et al., 1997; 1999). Following this hypothesis, Finetti et al. (2001) proposed a crustal section across the entire NA system (from eastern Corsica to the Adriatic offshore) based on the interpretation of the deep crustal profiles M12A, CROP03 and M16. In this view, the hinterland basins were formed as compressional (thrust-top) basins, driven by out-of sequence thrusts, and the extension is a recent process, superposed on a previous compressional history (e.g. Boccaletti et al., 1997; Bonini and Sani, 2002).

The third group of models invokes a combination of extensional and compressional mechanism and suggests that the eastward shift of the Northern Apennines orogenic system was driven by the roll back of a subducting slab (Scandone, 1980; Royden et al., 1987; Doglioni 1991; Cavinato and DeCelles, 1999) and/or by delamination (Channel, 1986; Channel and Mareschal, 1989) of a crustal slice dipping westward into the upper mantle.

This hypothesis, based on the roll-back of the subducting Adriatic slab, can explain better most of the geological and geophysical features of the NA. In fact, the Apennines wedge is formed over a low-strength décollement, possesses a low topographic expression and is characterised by extension at the rear of the wedge; al these features are typical of orogens associated with the roll-back of a subduction zone. A model of west-dipping subduction and retreat (roll-back) of the Adriatic slab also explains the presence of the cold body imaged by mantle tomography and the presence of the subcrustal seismicity.

However, many uncertainties still remain: the deep seismicity is scarce and its location with respect to the supposed Adriatic slab needs to be improved (e.g. Lavecchia et al., 2003); moreover, the depth of the earthquake foci does not exceed 90 km. The location and geometry of the Adriatic slab is also matter of debate: the lateral and vertical continuity of the slab imaged by tomography is complex and controversial, possibly due to the complexity and segmentation (Royden et al., 1987; Scrocca, 2006) of the Apennines subduction. In particular it is not clear why the cold body is not present below the adjacent central Apennines, where a similar tectonic evolution is observed, including the migration of both compressional (Cipollari and Cosentino, 1997) and extensional basins (Cavinato and De Celles, 1999), and a similarity exists in both the crustal structure and the distribution of the seismicity (Ghisetti and Vezzani, 2002).

In conclusion, a satisfactory geophysical and geological model of the (Northern) Apennines is still to be produced. We need further reliable information about the geometry of the transition zone (see e.g. fig. 3). We also need to reconsider the geological constraints, in order to quantify and compare the rates of deformation related to both the extensional and compressional wave, and their variations through time.

Finally, it is important to consider that the problem of the geodynamic setting involves a lithospheric scale and cannot be addressed considering only the NA region, but needs to consider the entire Apennines belt and possibly the entire Mediterranean area.