Age data and estimates on upper-crustal shortening together with observations on the present-day deep-seismic structure, magmatic activity, and geomorphology indicate that the central Andean arc and back arc (lat 20°–22°S) reflect two major thickening mechanisms active during crustal convergence. The first process (from ca. 27 to 5 Ma) involved crustal underthrusting and pervasive lower crustal shortening and thickening. This caused thickening and uplift in the Altiplano, and shortening in the Eastern Cordillera and Interandean zone. The second process (from ca. 5 Ma to the present) involved crustal stacking and caused thickening and uplift in the Eastern Cordillera and shortening in the Subandes. This concept of two thickening mechanisms is synthesized in an area-balanced model describing the crustal evolution from the late Oligocene to the present along a profile from the Western Cordillera to the Brazilian Shield.