SUBDUCTION ZONE DYNAMICS IN THE MEDITERRANEAN: THEORY AND OBSERVATIONS

The Mediterranean region contains multiple examples of young subduction systems that show rapid variation in the speed of subduction and trench retreat in both space and time. Theoretical modeling of similar subduction systems, where trench retreat rate is similar to subduction rate and upper plate – lower plate convergence rates are small, demonstrates that the predominate factor controlling subduction rate is the negative buoyancy of the subducting plate. Changes in subduction rate may occur within 1-2 m.y. following entry of a different buoyancy foreland into the subduction zone. This is well illustrated by the Hellenic subduction system where the late Eocene to present history of the subduction system, and of the overlying orogenic belt, can be largely tied to the spatially-varying density of the subducting plate. This has led to a subduction system where rates have varied both in space and in time. There is also a good correlation between observed subduction rates and negative slab buoyancy in the Apennine system. However the very cessation in subduction rate following entry of the Adriatic continental foreland into the southern Apennine trench cannot be explained solely by an increase in buoyancy of the subducted plate. In addition to an increase in slab buoyancy, modeling indicates that a pathway(s) for flow of asthenosphere must be present at depth, so that asthenosphere may flow from the foreland side to the hinterland side of the slab, facilitating rapid cessation of the last part of the subduction process. Such pathways may be due to break off of the slab, tears in the slab, or simply windows through the slab.