The Andes of Argentina and Chile between latitudes 33¿ and 34¿S are composed from west to east of an Oligocene to Miocene volcanic arcs and the Neogene east-vergent Aconcagua fold and thrust belt of the Cordillera Principal, and the basement-block faulted Cordillera Frontal. A regional cross section suggests that shortening across the Andes was achieved by thrusting along detachments at several levels in the crust. While thin-skinned deformation along newly formed thrusts occurred in Mesozoic sequences of the eastern Cordillera Principal, reactivation of preexisting Jurassic and Oligocene normal faults has resulted in additional hybrid thick- and thin-skinned structures in the western Cordillera Principal. Five major thrusting events are recognized in this part of the Andes: (1) Early to Middle Miocene tectonic inversion of the extensional faults in the western Cordillera Principal, (2) Middle to Late Miocene development of the Aconcagua fold and thrust belt, (3) Late Miocene uplift of Cordillera Frontal, (4) Late Miocene--Early Pliocene out-of-sequence thrusting in the Cordillera Principal, and (5) Pliocene to present deformation of the foreland.