On the origin of west-directed subduction zones and applications to the western Mediterranean

Abstract West-directed subduction zones show east-verging arcs of 1500–3000 km. They are usually younger than 50 Ma and are characterized by a frontal accretionary wedge and a back-arc basin propagating together toward the east. The accretionary wedge scrapes off superficial layers of the downgoing plate (thin-skinned tectonics) whereas the back-arc extension cross-cuts the entire subduction hanging wall (thick-skinned tectonics). The slab of this type of subduction is steep to vertical and the hanging wall of the subduction has a mean elevation of 1000 m below sea level. Trenches and foredeeps are the deepest basins of the Earth and the mean depth is of 5000 m below sea level. West-directed subduction occurs both in case of the highest E-W convergence rates among plates (e.g. W Pacific examples) and no or very low convergence (e.g. Carpathians). Following Atlantic W-directed subduction examples, the W-directed subductions seem to develop along the back-thrust belt of former E-directed subduction zones, where oceanic lithosphere occur in the foreland to the east with the narrowing of the American continents. This could be applied to the onset of the Apennines subduction along the back-thrust belt of the Alpine-Betic orogen where Tethys oceanic crust was present. The Alpine orogen was stretched and scattered in the Apennines back-arc basin. The back-arc extension is internally punctuated by necks (sub-basins) and boudins (horsts of continental lithosphere). Asymmetric extension in the back-arc basin appears controlled by differential drag between the eastward mantle flow and the overlying passively transported crustal remnants. Compression in the accretionary prism may be interpreted as the superficial expression of the shear occurring between the downgoing lithosphere and the horizontally moving mantle which compensates the slab roll-back. The area of the Apennines appears lower than the area of the sedimentary cover before subduction: this favours the idea that not significant crustal slices have been involved in the Apenninic accretionary prism, and the basement thrust sheets included in the western part of the belt are mainly relicts of the Alpine-Betic orogen.