Abstract
SUMMARY Geological and geophysical constraints to reconstruct the evolution of the Central Mediterranean subduction zone are presented. Geological observations such as upper plate stratigraphy, HP–LT metamorphic assemblages, foredeep/trench stratigraphy, arc volcanism and the back-arc extension process are used to define the infant stage of the subduction zone and its latest, back-arc phase. Based on this data set, the time dependence of the amount of subducted material in comparison with the tomographic images of the upper mantle 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 as old as 80 Myr, (2) the slab descended slowly into the mantle during the first 20–30 Myr (subduction speeds were probably less than 1 cm year x1 ), (3) subduction accelerated afterwards, producing arc volcanism and back-arc extension and (4) the slab reached the 660 km transition zone after 60–70 Myr. This time-dependent scenario, where a slow initiation is followed by a roughly exponential increase in the subduction speed, can be modelled by equating the viscous dissipation per unit length due to the bending of oceanic lithosphere to the rate of change of potential energy by slab pull. Finally, the third stage is controlled by the interaction between the slab and the 660 km transition zone. In the southern region, this results in an important re-shaping of the slab and intermittent pulses of back-arc extension. In the northern region, the decrease in the trench retreat can be explained by the entrance of light continental material at the trench.
Highlights
Most of our knowledge about the subduction process comes from present-day data, such as earthquake hypocentres which trace out snapshot images of slabs
The evolution of the subduction of the Central Mediterranean represents a unique opportunity to unravel the way the upper mantle evolved over an 80 Myr time-span
We find that subduction was dominated by slab-pull in a restricted, upper mantle convection environment
Summary
Most of our knowledge about the subduction process comes from present-day data, such as earthquake hypocentres which trace out snapshot images of slabs. Mitrovica et al 1989; Gurnis 1992), and when the slab has reached the 660 km discontinuity, impedance to flow will be accompanied by a decrease in subduction velocity and trench migration rate (e.g. Christensen & Yuen 1984; Kincaid & Olson 1987; Zhong & Gurnis 1995) All of these dynamic effects will leave geological fingerprints that can interpreted and used for a reconstruction. The rate of convergence at the trench was very low and the consumption of the oceanic lithosphere was probably mostly driven by its negative buoyancy, resulting in trench rollback (Malinverno & Ryan 1986; Royden 1993; Giunchi et al 1996; Faccenna et al 1996) This absence of significant plate convergence and the availability of geophysical and geological data from decades of research make the Central Mediterranean a prime candidate to unravel the evolution of subduction zones. There is a permanent decrease of subduction speed that is most likely to have been caused by a stagnant slab and by continental material entering the trench from 30 Ma onwards
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