The fact that several subduction zones are not associated with back arc basins and that in a number of consuming borders back arc extension came to an end while subduction continued for millions of years, clearly indicates that subduction is not a sufficient condition for back arc extension. To overcome this difficulty, one could try to identify some correlation between the occurrence of back arc extension and one or more of the features of subduction processes, as tentatively suggested by Uyeda and Kanamori (1979). In fact, if the subduction process was in some way responsible for the opening of the back arc basin, one could reasonably expect that the related driving force depends on the features of the slab, as suggested by the results of theoretical and modelling quantifications (e.g. Scholz and Campos, 1995; Ranalli et al., 2000).
To provide evidence about this problem we have reported in a number of diagrams (Fig. 4) the distribution of 'back arc' and 'non back arc' subduction zones with respect to the major features of the related subduction systems. It can be noted that these diagrams do not show any significant correlation between the occurrence of back arc extension and particular values of the parameters considered.
Figure 4. Active back arc opening in relation to major slab features.
Distribution of subduction zones associated (dots in the upper row) or not associated (dots in the lower row) to active back arc opening with respect to some major slab features. The papers from which the related data have been taken are reported in the Table. The average dip of the slab (B) is defined as suggested by Scholz and Campos (1995). The subduction rate (E) is the trench-normal component of the absolute velocity of the subducting plate, measured in the hot-spot reference frame. In F) velocities in the upper row refer to the absolute velocities of arcs in T-A-BA systems (see the Table). The subduction systems are identified by the abbreviations given in the Table. (For enlargement)