The Tertiary evolution of the Mediterranean region has been characterized by the large migration of orogenic belts (arcs) and by the opening of basins in the wake of the migrating arcs (e.g. Biju-Duval et al., 1977; Dewey and Sengor, 1979; Le Pichon and Angelier, 1979; Dercourt et al., 1986; Royden, 1993 a,b; Sengor, 1993; Mantovani et al., 1997, 2000a). This complex of interconnected tectonic processes is generally called trench-arc-back arc (T-A-BA) system. Numerous hypotheses have been advanced about the dynamics of this phenomenon, but none of them is widely accepted. Two main types of interpretations may be recognized. One postulates that arc-trench migration and back arc extension are driven by subduction-related forces, with particular regard to the negative buoyancy (slab pull) of the subducted lithosphere (e.g. Malinverno and Ryan, 1986; Royden, 1993a,b). The other type of model suggests that T-A-BA systems are closely connected with extrusion processes, induced by the interaction of buoyant structures in constrictional tectonic contexts (Tapponier, 1977; Mantovani et al., 1997, 2000a).

The outstanding problems created by the adoption of subduction-related models in the world have been pointed out in several papers (e.g., Uyeda and Kanamori, 1979; Taylor and Karner, 1983; Uyeda, 1986; Mantovani et al., 1997, 2000a, 2001a; Flower et al., 2001). In particular, it has been argued that this kind of mechanism cannot easily provide plausible explanations for a number of basic features, as e.g. the fact that back arc extension occurs in some subduction zones and not in others, that in a number of consuming boundaries subduction is still active while back arc extension ceased several My ago, that back arc basins only develop along a limited sector of the convergent plate boundary and that arcs are often characterized by a considerably arcued shape. Furthermore, the attempts at quantifying subduction-related forces by numerical and analogue modelling suggest that slab pull forces are too weak to produce extensional deformation in the overriding plate, unless it has been previously weakened and its mechanical coupling with the subducting plate is very weak (Shemenda, 1993; Hassani et al., 1997). Even in the case that the above conditions are fulfilled, a minimum slab length (about 300 km) is required to initiate the slab roll back and the consequent back arc extension (Hassani et al., 1997).

As regards the Mediterranean area, we have argued in previous papers (Mantovani et al., 1997, 2000a, 2001a) that the slab pull model cannot easily explain the space-time distribution of deformation observed in this region and that the extrusion model offers much better chances to achieve such result. However, none of our previous attempts provides a definitive proof of the reliability of the proposed model, since most of the supporting arguments are necessarily qualitative and the nature and timing of the observed deformation are affected by considerable uncertainty. Not even the quantitative arguments we have provided in support of our viewpoint, by numerical modelling (Mantovani et al., 2000b, 2001b), may overcome this problem, since they are obtained by procedures based on a number of tentative assumptions about the properties of the model adopted. Thus, the way towards a satisfactory understanding of the Mediterranean geodynamics must be patiently developed through a series of attempts, involving a progressive improvement of the recognition of the most significant tectonic events and of their consistency, as concerns timing and location, with the implications of the proposed interpretational scheme. This work describes a further effort in this direction. With respect to previous attempts, we try to provide a clearer explanation of the proposed genetic mechanism of T-A-BA systems and of the possibility to recognize its implications in the Mediterranean zones where the major back arc basins opened up. We also report new arguments about the (poor) compatibility between the expected consequences of the slab pull model and the observed deformations in the Mediterranean area.