GNGTS 2014 - Atti del 33° Convegno Nazionale

cross sections one can infer that, after the complete consumption of the Ionian Tethys oceanic domain, shortening mainly developed in response to the collision between the westward underthrusting Adriatic crust and the lower crustal layers of the eastward migrating Corsica- Sardinia and Etrurian continental fragments (arc). The shallowest layers of the Etrurian block underwent a much lower shortening since they overrode, instead of colliding with, the westward migrating Adriatic lithosphere. A tentative evaluation of this overriding can be inferred from the eastward transportation of the units (Ionides) which lay over the Ionian oceanic crust before its subduction. The fact that all upper crustal layers in the arc (Corsica-Sardinia and Etrurian blocks) systematically show an eastward shift with respect to the lower layers is consistent with the above interpretation. c) In the Pliocene, trench retreat in the Southern Apennines and Calabria consuming boundaries occurred with a relatively high rate [5-10 cm/y: e.g., Patacca et al. (1990); Finetti and Del Ben (2005a)]. This evidence cannot easily be explained as an effect of slab’s sinking as the retreat rate predicted by various modelling attempts ranges from less than 1 cm/y (e.g., Negredo et al. , 1999) to 2.5 cm/y (Buiter et al. , 2001) d) The slab-pull model predicts that extension rate in the basin is fully controlled by the difference between the rate of trench retreat and the convergence rate of the confining blocks. Since in the application of that mechanism to the Tyrrhenian basin the second factor is only represented by the slow (roughly 0.5 mm/y) Africa-Eurasia convergence, one would expect an extensional rate in the central Tyrrhenian basin very similar to the trench retreat rate mentioned above. However, considering that the Magnaghi-Vavilov basin has developed in about 4 My (from the late Miocene to the early Pleistocene) an extension rate of 5-10 cm/y, would imply a lengthening of the basin ranging between 200 and 400 km, which is much higher than the dimension of the oceanized zone in the Magnaghi-Vavilov basin (70-80 km). e) The slab pull model cannot account for amajor tectonic event which developed in the study area during the opening of the Tyrrhenian basin, that is the formation of transtensional features in the Sicily Channel since the late Miocene-early Pliocene (e.g., Finetti and Del Ben, 1986; Reuther et al. , 1993; Finetti and del Ben, 2005b). Thus, an independent driving mechanism able to account for such tectonic process must be identified. f) Considering the structural/tectonic setting of the study area, one could wonder why the trench suction exerted by slab’s sinking (invoked as the driving mechanism of Arc’s migration) caused no eastward migration of the Corsica-Sardinia microplate. Since at that time such block was already decoupled from any stable major plate by the Ligure- Provencal basin, one can hardly neglect the possibility that the work necessary to move eastward the whole Corsica-Sardinia fragment was lower than the one required to break that block and move eastward only a portion of it (as actually occurred). g) It is widely recognized that in the Apennines belt a significant change of tectonic style has occurred since the middle Pleistocene (e.g., Hyppolite et al. , 1994; Galadini, 1999; Catalano et al. , 2004; Viti et al. , 2006). This evidence can hardly be explained by the slab pull model, which does not involve any significant change of driving mechanism at that time. Discussion and conclusions. The advance of knowledge in several fields of Earth Sciences would considerably benefit from the availability of a reliable reconstruction of the present tectonic setting. For instance, attempts at assessing seismic hazard and predicting the spatio- temporal distribution of future earthquakes would have much higher chances of success if they were carried out by taking into account the present tectonic setting and its short term evolution in the zones involved. As concerns the first problem, it must be considered that the hazard map presently adopted in Italy has been elaborated by taking into account the geodynamic model here discussed (Meletti et al. , 2008). Thus, the reliability of such evaluation would be significantly undermined if such model resulted to be unreliable, as we have argued in this note. 154 GNGTS 2014 S essione 1.2