To explain the actual crustal thickness of the southern Central Andes a balanced cross-section at 21¿S latitude was modeled. A forward modeling procedure was used on a crustal scale to explain the gross crustal structure obtained by seismic refraction investigations. In the model, 320 km of shortening since late Cretaceous, based on balanced geological cross sections, are assumed for the whole Andes from the trench to the foreland, and the initial crustal thickness varies between 30 and 40 km. It results that about 20% of the actual crustal volume must be explained by other sources than crustal shortening. Structures of high-velocity material in the upper crust of the Eastern Cordillera can be explained by detached and overthrust material from the crustal base thus producing a crustal doubling in the back arc. Underplating of continental material that was removed by subduction erosion and magmatic addition of derivates of the subducted Nazca plate may contribute to the lower crust of the forearc and arc regions. Crustal thickening of the Eastern Cordillera and parts of the Altiplano was undoubtedly produced by tectonic thickening, whereas the sources of thickening in the Western cordillera and forearc region are uncertain.