West Mediterranean subductions, puppeteers of the Alps: lessons from analog models and paleomagnetic data

We present analogue models simulating subductions that occurred in the Western Mediterranean region, in order to understand how it impacted the regional tectonics. Models suggest that the tectonic evolution is largely controlled by slab roll-backs, that may be much faster than the Africa-Eurasia convergence. They reproduce the opening of the Western Mediterranean Basins and the dispersion of continental fragments that accompany slab roll-back. They show that oceanic subduction in the Western Mediterranean region favors the counterclockwise rotation of Adria. Some models reproduce the break-off of the oceanic slab that followed the beginning of continental subduction both beneath Northern Africa and Italia. 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 ~NE-SW, and that the oceanic subduction in the Western Mediterranean may explain both the post-Oligocene E-W shortening in the Western Alps and part of the Periadriatic right-lateral shear zones in the Central Alps. We compare the rotations observed in experiments with the post-Eocene rotations registered by paleomagnetic data in the Western Alpine realm, that are deduced from a synthesis of more than 55 paleomagnetic studies. We compare the counterclockwise rotations affecting internal units of the Alps evidenced by paleomagnetic data with the rotations observed in analogue experiments. The change from N-S to E-W shortening enhanced left-lateral motions in the Southern border of the Western Alps, which may explain the particularly large rotations registered in this sector. 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.