The horizontal motions in the Tyrrhenian basin and surrounding chains, induced by subduction along the Calabrian Arc and Northern Apennines, are analyzed by means of numerical models based on finite element techniques. The driving mechanism, in 2-dimensional vertical cross--sections perpendicular to the trench, is the slab--pull due to the negative buoyancy of a stratified viscoelastic plate that models the Ionic oceanic lithosphere or Adriatic plate sinking in the upper mantle. For the modern tectonic setting of the area, we test the sensitivity of extension in the back--arc and of roll--back of the overriding plate to variations in the viscosity structure and in the geometry of subduction. Our results are compared with extensional velocities inferred from geology and SLR-VLBI data. ¿ American Geophysical Union 1994