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

Abstract

In the Western Alps, a first Late Cretaceous to Eocene “Pyrenean-Provençal” compressive phase accommodating N-S shortening resulting from the convergence between Africa and Eurasia is classically described. 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, possibly by the subduction of the Ligurian Ocean that initiated in the Oligocene beneath the European and Iberian plates. In this paper, we present analogue models simulating the Neogene evolution of this subduction zone, 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 (Alboran, Kabylian, Peloritan and Calabrian blocks). They also show that the subduction of the Ligurian Ocean 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 Adria. 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 the 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 surrounding regions, especially in the Alpine belt: Our models show that during slab-roll back and before the Ligurian 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 contribute to 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.