Comparing two sections through the Apennine subduction zone (Italy), the slab is far deeper (>500 km) beneath the Calabrian arc than beneath the northern-central Apennines (<300 km). In contrast, topographic relief and cross-sectional area of the belt are larger in the northern-central Apennines. This paradox can be explained considering the depth of the basal decollement, which is deeper in the northern section (on average 10 km) than in the southern (on average 3 km). The two sections occur respectively along subduction of continental lithosphere in the north (Adriatic sea) and oceanic lithosphere in the south (Ionian sea). It is concluded that, in the Apennines, the effects of sediment thickness on elevation and width of the belt can overcome those of subduction depth.