History of subduction and back-arc extension in the Central Mediterranean

Geological and geophysical constraints to reconstruct the evolution of the Central Mediterranean subduction zone are presented. Geological observation s such as upper plate stratigraphy, HP-LT metamorphic assemblages, foredeep /trench stratigraphy, are volcanism and the back-are extension process are used to define the infant stage of the subduction zone and its latest, back -are phase. Based on this data set, the time dependence of the amount of su bducted material in comparison with the tomographic images of the upper man tle along two cross-sections from the northern Apennines and from Calabria to the Gulf of Lyon can be derived. Further, the reconstruction is used to unravel the main evolutionary trends of the subduction process. Results of this analysis indicate that (1)subduction in the Central Mediterranean is a s old as 80 Myr, (2) the slab descended slowly into the mantle during the f irst 20-30 Myr (subduction speeds were probably less than 1 cm year(-1)), ( 3) subduction accelerated afterwards, producing are volcanism and back-are extension and (4) the slab reached the 660 km transition zone after 60-70 M yr. This time-dependent scenario, where a slow initiation is followed by a roughly exponential increase in the subduction speed, can be modelled by eq uating the viscous dissipation per unit length due to the bending of oceani c lithosphere to the rate of change of potential energy by slab pull. Final ly. the third stage is controlled by the interaction between the slab and t he 660 km transition zone. In the southern region, this results in all impo rtant re-shaping of the slab and intermittent pulses of back-are extension. In the northern region, the decrease in the trench retreat can be explaine d by the entrance of light continental material at the trench.