Conclusions and discussion

It is argued that the large migration and distortion of orogenic belts and the opening of several back arc basins observed in the Mediterranean region are the results of extrusion processes, driven by the convergence of the confining plates, i.e. Africa, Arabia and Eurasia. In support of this hypothesis, it is pointed out that the tectonic contexts within which the major T-A-BA systems developed were characterized by the dynamic and structural conditions implied by the extrusion mechanism (Figure 1).

As argued in the text, the proposed evolutionary reconstruction, based on the extrusion model, allows to find plausible and coherent explanations for the very complex time-space distribution of tectonic events in the study area, while such possibility is not provided by the most popular alternative model, i.e. the slab pull mechanism.

Some objections have been reported in literature about the applicability of the extrusion model to the Mediterranean back arc basins. For instance, Gueguen et al. (1997) and Jolivet et al. (1998) suggested that this model cannot be applied to the Tyrrhenian-Apennines T-A-BA system since it cannot account for the fact that the rate of trench retreat under the Southern Apennines and Calabrian Arc was considerably higher, 3-5 cm/y (e.g. Patacca et al., 1990), than the convergence rate between Africa and Eurasia, lower than 1 cm/y. However, this argument does not take into account the very important influence of the Adriatic westward motion on the formation of the Tyrrhenian basin (Mantovani et al., 1997, 2000a). Furthermore, simple geometrical arguments suggest that the velocity of an extruding wedge becomes higher than the convergence rate of the confining blocks when the lateral decoupling faults of crustal wedges form high angles with respect to the main trend of the belt. In the geodynamic context which led to the southern Tyrrhenian extension, the formation of high angle decoupling shear zones for the extruding Calabrian wedges may have been favoured by the small size of the lateral weak boundary (Ionian oceanic domain) which lay in front of the stressed Calabrian Arc. The presence of such high angle lateral guides for the extension of wedges in the southern Apennines and Calabrian arc is suggested by the results of seismic surveys (Finetti and Del Ben, 1986).

Other doubts about the feasibility of the extrusion model have been expressed for the Aegean-Hellenic T-A-BA system on the basis of the velocity field in the Aegean-Anatolian regions inferred from space geodetic data (e.g. McClusky et al., 2000). The major evidence on which these objection are based is the fact that the Aegean Arc is moving faster (roughly 30 mm/y) than the Anatolian block (roughly 24 mm/y), apparently in contrast with the hypothesis that the deformation pattern of the Aegean zone is driven by the Anatolian westward push. However, Mantovani et al. (2001c) and Cenni et al. (2001) have shown, by numerical modelling experiments, that the kinematic pattern indicated by geodetic data can also be explained as an effect of post seismic relaxation processes, triggered by the sequence of very strong earthquakes occurred along the North Anatolian fault system since 1939 (e.g. Barka, 1992). This result, along with other evidence and arguments (e.g. Anderson, 1975; Rydelek and Sacks, 1990), points out that the short-term kinematic behavior of blocks may be very different with respect to the long-term (geological) one. This possibility is also suggested by the fact that the Plio-Quaternary slip rate along the North Anatolian fault, estimated by the analysis of fault offsets and long-term seismicity pattern (Barka, 1992), is lower than 10 mm/year.

Arguments against the extrusion model in the Mediterranean area are also based on numerical and analogue modelling (Faccenna et al., 1996; Meijer and Wortel, 1997; Lundgreen et al., 1998; Wortel and Spakman, 2000). In particular, the above attempts suggest that the observed strain patterns in the Aegean and Tyrrhenian zones cannot be reproduced if a trench suction force is not assumed in the Hellenic and Calabrian arcs, respectively. However, one must consider that the results of the above experiments may be strongly influenced by the oversimplified models assumed in computations, with particular regard to the fact that only very few tectonic discontinuities, with respect to the large amount of active features recognized in the study area, have been included in models. This possibility is suggested by the fact that a satisfactory match of the strain field in the central-eastern Mediterranean area, deduced by a large amount of geological and geophysical information can be obtained adopting more realistic models, which take into account lateral heterogeneities of mechanical properties and major tectonic discontinuities in the zones considered, and assuming the convergence of the confining blocks (Africa, Arabia and Eurasia) as the only driving mechanism (Mantovani et al., 2000b, 2001b).