The eastern slope of the Andes between 36°S and 39°S shows contrasting behavior north and south of 37.5°S to 38°S. The region is notable for a mixed contractional and extensional tectonic regime in the last 5 m.y. that has led to the formation of four broad extensional depocenters and the Guañacos fold-and-thrust belt. The post–late Miocene tectonic evolution of the eastern slope of the Andes south of 37.5°S has been dominated by the development of the Bío Bío–Aluminé and the Loncopué troughs, which have undergone extensional collapse in association with abundant mafic volcanism. The formation of these troughs postdates the middle to late Miocene contractional deformation that inverted the early Miocene Cura Mallín basin. At the same latitude, the forearc has been uplifted by underthrusting and basal accretion. The distribution of late Pliocene to Quaternary deformation along the forearc, arc, and retroarc in this region is typical of a subduction system governed by a negative roll-back velocity. North of 37.5°S, post–late Miocene extensional collapse has occurred in the Las Loicas trough, which extends southeastward from the frontal arc near 35°S to the Tromen region in the retroarc near 37.5°S, and in the Sierra de Reyes trough in the eastern retroarc. The extensional collapse of these basins occurred in a more restricted region than in the troughs south of 37.5°S. The retroarc extension, which is locally accommodated by transtension, postdates late Miocene deformation in the Chos Malal and Malargüe fold-and-thrust belts. To the west, uplift related to shortening has produced the young Guañacos fold-and-thrust belt in the Main Cordillera, which is antithetic to the Benioff zone. This contractional belt is considered to be out-of-sequence in the sense that deformation occurred to the west of the Cretaceous to Miocene fold-and-thrust belt. The structural contrasts north and south of 37.5°S to 38°S are difficult to explain in the context of the modern tectonic setting in which relative convergence parameters, subducting oceanic slab age, and climate are similar. The differences are better explained as responses to contrasting changes in the late Miocene to Holocene geometry of the Benioff zone north and south 37.5°S to 38°S.