Tectonic inversion
The tectonic model that best applies to the formation of the Apennines is widely debated (see for this topic Bally et al., 1986; Doglioni, 1991; Bigi et al., 2002; Speranza and Chiappini, 2002; Tozer et al., 2002, Scrocca et al., 2005, Mazzoli et al, 2005, among many others). We add some evidence from seismologic data: i) extension is accommodated on gently dipping normal faults interpretable as pre-existing thrusts (figure 3); ii) in the eastern side of the belt, compression is taking place in the Adria basement, the seismicity occurs on flat and ramps, the latter have dip higher than 60° and are likely pre-existing normal faults (figure 7); iii) the thrust system defined by geologic data at the surface appears to be rooted at depth, with uplifted high vp body in the hanging wall of steep thrust faults (figures 7 and 8). These onservations offer an intriguing scenario in which shallow decollements ripping the Mesozoic cover, deep steep ramps that likely invert basement normal faults and extension in the sedimentary cover exist for the gravitational readjustment within the wedge, as also proposed based on geologic data (Ghisetti and Vezzani, 1999; Mazzoli et al, 2008).
There are several studies that investigate how the pre-existing structure of the basement controls the style of thrusting (Dewey et al., 1989; Coward, 1994; Ziegler et al., 1995) favouring tectonic inversions (Tavarnelli et al., 2004; Scisciani, 2009). During the Apennines evolution, the subsequent repetition of extension and compression led to a complicated structure where pre-existing faults can be re-activated, although not optimally oriented with the active stress. Since only the sedimentary cover outcrops in the area (sometimes stacked in rootless nappes), we do not have direct observations of the basement structure, although it can be inferred by the subdivision of sedimentary facies in the Meso-Cenozoic realms. Only tomographic models and seismic profiles scan mid- to lower-crust depths. Since geophysical models are obviously not unique for both imaging and interpretation, we cannot definitely assess to which extent the pre-existing structure of the basement influenced the Apennines build up. So, our results are consistent with inverted normal faults in the basement (see figure 8), but the generalization of this evidence to a fully thick-tectonic model is still speculative.
Figure 9 shows a tentative scheme for the tectonics of the Apennines, explained through the reactivation of pre-existing structures. In this short section, we add some outlines from our seismological results
Inversion of normal faults during compression
There are examples worldwide that large thrust earthquakes invert pre-existing normal faults (El Asnam 1980, Chiarabba et al., 1997; Mid-Niigata 2004, Kato et al., 2005; Miyagi 2003, Okada et al., 2007). The northeast Japan case is exemplary - the transition from basin to island arc is accomplished by a general and complete positive inversion of the steep Miocenic normal faults (Kato et al., 2004).
In the Apennines, normal faults developed during two main and separate phases: i) the syn-rift evolution of the Tethys continental margin; ii) a diffuse Miocene extension, documented by geologic data, in the Adria region (Carminati et al., 2004; Bigi and Pisani, 2005). The two sets of normal faults are differently rooted in the Apennines structure, the former affecting the sedimentary cover, the latter prevalently the pre-Mesozoic basement. So, they were differently used during the formation of the thrust and fold belt (see Tavarnelli, 1999; Scisciani et al., 2001; Scisciani, 2009). An example of inversion of normal faults in the basement is given by both the high velocity antiforms present in the Apennines deep structure (figures 7 and 8) and the deep crustal seismicity developing along the Adriatic margin of the Apennines. The recent seismic sequence occurred during January 2010, with the greatest event of ML=4.2, is a good example of a compressional event that develops on an about 60° SW-dipping fault, as constrained by aftershock data (see figure 7 and focal mechanisms available at earthquake.rm.ingv.it).
Inversion of thrust faults during extension
The Apennines is one of the best-documented examples worldwide of active extension developing on a compressional wedge. Sometimes the term "negative inversion" has been used to describe the reactivation of thrust as normal fault (Carmignani and Kliegfield, 1990; Jolivet et al., 1998b; Thurner and Williams, 2004). Such a process is documented in active extension regions (Basin and Range, Wernicke, 1981; and Aegean sea, Gautier and Brun, 1994), and in the Apennines (Ghisetti et al., 1993; Bosi et al. 1994; Ghisetti and Vezzani, 1997; D'Agostino et al. 1998; Bigi, 2006). Analog modelling shows that the reactivation is common when the dip of the pre-existing thrust exceeds 40°, while normal faults can splay on the deep decollement or develop independently for progressive smaller dip angles of the pre-existing thrust (Faccenna et al., 1995). The high quality seismological data available for recent earthquakes indicate that extension is accommodated on gently dipping faults (around 40°-50°, see figure 3), with dip angles that are compatible with the inversion of thrust, as indicated by the analog modeling.
Since the angle and lateral continuity of the pre-existing thrust control the tectonic inversion, we expect that fault segments, which can rupture in normal faulting earthquakes, are controlled by the geometry of the old thrust system. The extreme fragmentation of the normal fault system, with individual en-echelon segments not longer than tens of kilometers, is directly reflecting the architecture of the thrust system at depths.
Reactivation of not-optimally oriented faults needs high fluid pressure (Sibson 1985; 1990; 2004). There are several examples that high pore pressure is conditioning seismicity in the Apennines. First, high pore pressure is observed to be a relevant factor modulating seismicity migration and multiple mainshocks sequences (Miller et al., 2004; Chiarabba et al., 2009b; 2009d). Second, a tight relation between Apennines seismicity and CO2 flux is documented (Chiodini et al. 2004). Third, it is remarkable that past centuries large normal faulting earthquakes in northern-central Apennines occurred in regions where intermediate-depth seismicity indicate the foundering and dehydration of the Adria lower crust. Just on top of this sinking material, low vp anomalies are found in the lower crust that might represent fluid-filled volumes.