Flat subduction near Peru occurs only where the thickened crust of the Nazca Ridge subducts. Furthermore, the South-America continent shows a westward absolute plate motion. Both the overriding motion of South-America and the subduction of the Nazca Ridge have been proposed to explain the flat slab segment below South Peru. We have conducted a series of numerical model experiments to investigate the relative importance of both mechanisms. Results suggest that the average upper mantle viscosity should be about 3.5 ¿ 1020 Pa s or less and basaltic crust should be able to survive 600 to 800¿C ambient temperature before transforming into eclogite to explain the slab geometry below Peru. The effect of the overriding plate is estimated to be as large or twice as large as that of the plateau subduction.