The thick (up to 65 km) crust of the high-elevation regions of the Andean orogenic belt results from structural thickening of the felsic upper crust of South America. The great thickness implies shortening or loss of several hundred kilometers of lithospheric mantle and lower crust. Through numerical simulation of the thermal and mechanical processes of crustal thickening, we test a model of lithospheric-mantle removal by ablative subduction. The model combines kinematic assumptions regarding mantle motion with dynamic calculations of crustal deformation. Predictions include the formation of an outwardly growing orogenic plateau—analogous to the Altiplano—that results from high-temperature, low-viscosity flow of the lower crust due to thickening of the high-heat-production upper crust.