A simple viscous flow model of a subduction zone is used to calculate the deformation within contential lithosphere above a subducting slab. Our formulation accounts for two forces that dominate the deformation in the overriding lithosphere tectonic forces and buoyancy forces. The tectonic forces arise from the subduction of oceanic lithosphere and produce minor extension in the lithosphere near the trench and compression farther inland. Buoyancy forces can induce horizontal pressure gradients due to variations in crustal thickness, which cause lithospheric extension where the crust is thick. Numerical solutions, obtained by using a finite element technique, are compared with observations from the central Andes (15¿--27¿S). The model predicts the observed deformation pattern of extension in the forearc, compression in the Western Monocline (corresponding to magmatic activity), extension in the Altiplano, compression in the Eastern Monocline and Subandes, and no deformation in the Brazilian Shield. By comparing the calculated solutions with the large-scale tectonic observations we are able to evaluate the forces that govern the deformation in the central Andes. The approximately constant subduction velocity in the past 26 m.y. suggests that the rate of crustal shortening in the Andes has decreased with time due to the thickening of the crust.