Differences between Alps and Apennines

As outlined above and discussed by Carminati et al. (2004a), Alps and Apennines show distinct characters (e.g., Figs. 7, 8, 9, 17). They have, respectively: 1) subduction hinge moving towards vs. moving away from the upper plate; 2) double vs. single vergence; 3) high vs. low morphological and structural elevation; 4) deep vs. shallow basement rocks involved; 5) higher metamorphic grade vs. lower metamorphic grade; 6) the basal décollement involves the crust and the LID of both upper and lower plates in the Alps whereas only the shallow crust of the lower plate contributes to the accretionary prism in the Apennines; 7) shallow vs. deep foredeep; 8) low vs. high dip of the foreland monocline; 9) thickened vs. thinned crust under the ridge; 10) the Alps have both in the upper and in the lower plate a pre-subduction Moho, whereas the Apennines have in the footwall plate a pre-subduction Moho, but in the upper plate they present a new forming Moho; 11) thickened lithosphere vs. a shallow asthenosphere in the upper plate; 12) no back-arc basin and syn-subduction magmatism vs. very wide back-arc basins and abundant syn-subduction arc-tholeiitic to calcalkaline and potassic/ultrapotassic magmatism; 13) >99% of the igneous products in plutonic facies, with relatively important shallow crustal interaction of mantle melts in the Alps vs. >99% of the igneous products in effusive to pyroclastic facies and essentially mantle-derived melts in the Apennines and Apennine-related igneous activity; 14) SiO2-oversaturated vs. SiO2-saturated to strongly SiO2-undersaturated compositions of igneous rocks; 15) low vs. high gravity and heath flow anomalies (in particular in the internal part of the Apennines). These changes reflect, at least in part, the first order differences between “west” and “east” directed subduction zones, as widely discussed by Doglioni et al. (1999; 2007).

The Italian orogens represent two prototypes of subduction zones whose main characters can be exported throughout the Mediterranean and the entire Earth. The basic difference is the asymmetric behaviour of the decoupling zones. The fundamental decoupling is the one separating lithosphere and the underlying mantle, occurring in the low-velocity layer atop of the asthenosphere. Along W-directed subduction zones, this decoupling is folded and subducted, being the slab dragged eastward by the relative motion of the mantle, since the lithosphere has a net westerly directed rotation (Crespi et al., 2007). Along E- or NE-directed subduction zones, i.e., along the slabs following the mantle flow in the direction of the tectonic equator, the basal decoupling is rather uplifted toward the surface, allowing exhumation of deep seated rocks, and wide shortening of the upper plate. In other words, the main difference between the two subduction end-members is the trajectory of the decoupling between lithosphere and mantle. Moreover, where the subduction hinge is retreating relative to the upper plate as it is in the Apennines, there is not an upper plate actively converging over the lower plate, which is instead the case of the Alps.