Did subduction in the western Mediterranean drive Neogene alpine dynamics? Insights from analogue modeling

Abstract

We present analogue models simulating the Neogene subduction that occurred in the Western Mediterranean region, in order to understand how it impacted the regional tectonics. Although models do not include the lithospheric plate overriding the subduction zone, their surface deformations share many similarities with the Neogene tectonics of Western Europe and Iberia. We observe that the tectonic evolution is largely controlled by the roll-back of the slab, that occurred much faster than the Africa-Eurasia convergence. Models reproduce the opening of the Western Mediterranean Basins and the dispersion of the AlKaPeCa continental fragments. They also show that oceanic subduction favors the counterclockwise rotation of Adria. In more elaborated models, we introduced a pre-existing weakness along the Africa and Adria margins, to reproduce the break-off of the oceanic slab that followed the beginning of continental subduction both in Northern Africa and Italia. Slab break-off is followed by the exhumation of the subducted continent. We observe that the influence of subduction on the kinematics of Adria largely decreases following slab break-off. In models, the total counterclockwise rotation of Adria varies between 7° and more than 30°, depending on the timing of slab break-off. Since the process of subduction modifies the displacement of Adria, it also impacts the tectonic evolution of the regions that bound this plate, especially in the Alpine belt: in the Western Alps, an older Late Cretaceous to Eocene “Pyrenean-Provençal” tectonic phase accommodating N-S shortening is classically described resulting from the convergence between Africa and Eurasia. It is followed by the Neogene “Alpine phase” accommodating E-W shortening. Since this major tectonic change is not explained by a modification of the global Africa-Eurasia convergence, it should be explained instead by more local causes. Our models show that during slab-roll back and before slab break-off, the azimuth of convergence between Adria and Europe shifts from ~N-S to ~ENE-WSW. Hence, they suggest that the oceanic subduction in the Western Mediterranean may explain the “Oligocene revolution” described by Dumont et al. (2011), leading to E-W shortening in the Western Alps and to the activation of the Periadriatic right-lateral shear zones in the Central Alps. We conclude that the western Mediterranean region is a spectacular example showing how the tectonics of mountain ranges and plate boundaries may be controlled by distant subduction processes.