The mechanism we propose (Figure 1) generally occurs along a sector of a consuming border, where the accretionary belt, under the action of a longitudinal compression, undergoes a trenchward extrusion and partly separates from the overriding plate. This separation is accommodated by crustal stretching in the back arc zone. Simultaneously, the outward migration of the deforming belt (arc) causes the roll back of the slab lying in front of it (Figure 1). The tectonic context which produces the deformation of the arc may be quite different from case to case. Most often, this phenomenon occurs when a mechanically strong and buoyant structure enters a sector of the consuming border, with a direction of motion not perpendicular to the trench. In this oblique constrictional context, the accretionary belt undergoes a longitudinal compression, which is accommodated by its outward extrusion/bending, at the expense of the adjacent lithospheric domain. The occurrence of this mechanism requires that the buoyancy of the accretionary belt is significantly higher than that of the lithospheric domain lying in front of it. For instance, the lateral extrusion of the arc is strongly favoured when it faces a very old oceanic lithosphere, since this kind of structure, as the Ionian and Levantine oceanic domains, is presumably characterized by very low, or even negative, buoyancy (e.g., Cloos, 1993). Another basic condition for the formation of a back arc basin with this mechanism is the brittle behavior of the belt, which allows the formation of relatively large crustal wedges, decoupled by major strike slip faults (Figure 1). If this condition is not fulfilled, the extruding material would tend to occupy the entire space available and, thus, it would not allow the separation of the arc from the overriding plate and the consequent back arc opening. The above property may be found, for instance, in a accretionary belt, since this kind of structure is entirely constituted by a buoyant and brittle upper crustal material, scraped off a subducting lithosphere.
Figure 1. Sketch of the extrusion model
Sketch of the extrusion model here proposed as genetic mechanism of back arc extension. Up) Structural/tectonic setting which may precede the opening of a back arc basin. Subduction (black arrows) occurs along a convergent plate border leading to the formation of an accretionary belt. Down) Dynamic conditions required for the generation of a back arc basin. Due to oblique collision with a buoyant indenter, the belt is stressed parallely to its main trend (black arrow). The related shortening is accommodated by the lateral expulsion of crustal wedges, which results in a outward bending of the arc, at the expense of the adjacent low buoyancy lithosphere. The divergence between the arc and the overriding plate causes crustal stretching in the back arc zone. The geodynamic framework which may induce a longitudinal compression in the belt may be quite variable from case to case, as discussed in the text. The trenchward migration of the arc and the consequent roll back of the slab attract asthenospheric material from the surrounding mantle. (Click for enlargement)
The importance of extrusion processes in the generation of back arc basins has already been stressed by a number of authors (e.g. Tapponnier, 1977; McCabe, 1984; Tapponnier et al., 1986; Uyeda, 1986; Lavé et al., 1996; Mantovani et al., 1997, 2000a, 2001a). The physical plausibility of this kind of mechanism has been demonstrated by analitical computations and by analogue and numerical modelling (Tapponnier et al., 1982; Peltzer and Tapponnier, 1988; Ratschbacher et al., 1991; Faccenna et al., 1996; Mantovani et al., 2000b, 2001b).
As argued by Mantovani et al. (2001d), the extrusion model may provide plausible explanations of the major features of T-A-BA systems in the world and may help to overcome the outstanding problems of subduction-related interpretations.
In the next sections we discuss on how the conditions required for the occurrence of this mechanism may be recognized in the Mediterranean zones where T-A-BA systems developed. To help the identification of the structural/tectonic elements mentioned in the discussion, with respect to their paleotectonic contexts, the evolutionary reconstruction proposed for the study area is reported in Figures 2 and 3.