GNGTS 2015 - Atti del 34° Convegno Nazionale

evidence and arguments presented by Mantovani et al. (2015a, 2015b) and Viti et al. (2015), which suggest that major seismicity in the peri-Adriatic zones tends to undergo a progressive northward migration through the eastern (Dinarides) and western (Apennine) boundary zones, up to reach the northern front of the Adria plate (eastern Southern Alps). The analysis of the post 1400 seismic history has allowed the recognition of a number of migrating sequences, each lasting about 200 years. The last presumably complete sequence has probably developed until about 1930. Since then, major peri-Adriatic shocks have mainly occurred in the southern peri- Adriatic boundaries (southern and central Apennines and southern Dinarides, Albanides), while only few shocks have affected the Adriatic boundary zones located more to the north (northern Apennines, northern Dinarides and eastern southern Alps). Considering this seismicity pattern and the post-seismic relaxation effects that such earthquakes have triggered in theAdria domain, one could suppose that at present the southern part of Adria is affected by higher mobility with respect to the northern sector, which is still constrained by high resistance at its boundaries (Mantovani et al. , 2015a, 2015b; Viti et al. , 2015). This hypothesis would suggest that Adria is not actually behaving as a rigid structure. For instance, the lower motion rate of the northern Adria (indicated by GPS data, 2-3 mm/yr) with respect to that expected from an Adria Euler pole coincident with the one of Nubia (Fig. 2) could be accommodated by internal deformation of the Adria continental domain, for instance an upward longitudinal flexure. Apennine belt, Calabrian Arc and Sicily. In the Apennine belt, the GPS velocity field points out a considerable variation of velocity from the outer to the inner sectors (Fig. 1). This feature is compatible with the kinematic pattern of the Apennine belt deduced by the analysis of long-term evidence (Mantovani et al. , 2006, 2009, 2015c; Viti et al. , 2006, 2011; Cenni et al. , 2012). The outer mobile portion of the Apennine belt (Fig. 3) is constituted by the Molise- Sannio wedge in the southern Apennines, the eastern sector of the Lazio-Abruzzi carbonate platform in the central Apennines, and the Romagna-Marche-Umbria and Toscana-Emilia wedges in the northern Apennines (Mantovani et al. , 2009, 2015c; Viti et al. , 2015b). The proposed geodynamic interpretation (Viti et al. , 2006, 2011, 2015b; Mantovani et al. , 2009, 2015c) suggests that some mobility also characterizes the inner sector of the Apennines, even though at lower rates with respect to the outer Apennine sectors and with a roughly north to NNW-ward orientation (Fig. 3). This long-term pattern is compatible with the short-term kinematic field delineated by GPS data. Other investigations on the present kinematic pattern in the Apennine belt, carried out by the analysis of geodetic data, are reported in literature. Notwithstanding that some of the resulting velocity fields (e.g., Bennett et al. , 2012; Devoti et al. , 2011) are similar to the one shown in Fig. 1, the proposed tectonic interpretations are drastically different from the one described above (Fig. 3), since they invoke the gravitational sinking of the Adriatic subducted lithosphere beneath the Apennine belt as the main driving mechanism of the observed surface kinematics. However, such interpretation involves some major problems, as argued in the following. First of all, it must be considered that the real development of the presumed slab roll-back and consequent trench retreat along the Apennine belt is very uncertain. Most authors (e.g., Spakman and Wortel, 2004) suggest that the evidence of subducted lithosphere beneath the Apennine belt is lacking in large sectors of the Adriatic trench zone, in particular below the central Apennines. Thus the proposed process cannot account for a basic feature of the GPS velocity field (Fig. 1), i.e. the fact that the Apennine belt moves almost uniformly roughly NE ward from the southern Apennines to the northernmost Apennines. Moreover, the NE-ward rollback of the Adriatic plate would induce a similar motion in the adjacent upper plate (e.g., Schellart and Moresi, 2013), whereas the GPS velocity vectors along the Tyrrhenian coast are north to NW-oriented. Nocquet (2012) also noted the discrepancy between the kinematics of the outer and inner sectors of the Apennines. The results of seismic surveys (CROP, Finetti et al. , 2005) and tomography (Scafidi and Solarino, 2012) show that subducted lithosphere beneath the northern and central Apennines only reaches some tens of km. Thus the proposed slab pull 124 GNGTS 2015 S essione 1.2