Abstract The southern Patagonian Cordillera south of the present location of the Chile Triple Junction (46.5°S) preserves distinctive deformational and backarc magmatic features that are a consequence of a series of northward-propagating ridge collision events that started at ca. 14 Ma. An abrupt increase of ~2000 m of topographic elevation and the exhumation and uplift of mid-Miocene to Pliocene plutons within the cordillera south of the Chile Triple Junction is accomplished by horizontal compressive deformation (both thin- and thick-skinned) within the Patagonian fold-thrust belt. Ridge-trench collisions have formed asthenospheric slab windows beneath the southern Patagonian Cordillera. Backarc magmatism associated with slab window formation includes a distinctive suite of adakites and extensive outpourings of oceanic island basalt (OIB)-like plateau basalts. The adakites formed from the partial melting of the young, hot trailing edge of the Nazca plate that preceded slab window opening, whereas the OIB-like plateau basalts formed from dynamic asthenospheric flow as the slab windows opened up beneath the backarc.

This paper integrates new field observations to summarize the evolution of the 37–39°S segment of the Andean margin during the Neogene period. The western Neuquén Andes represent a transitional segment between the high, broad Central Andes and the low, narrow Patagonian Andes. The Main Cordillera at this latitude was uplifted between 11 and 6 Ma. Since then, extension and transtension has dominated the area. South of 38°S, deformation concentrates along the Liquiñe-Ofqui fault zone, a crustal-scale dextral strike-slip system that accommodates part of the margin-parallel component of oblique subduction. The architecture of the volcanic arc is strongly controlled by this fault zone. We differentiate four main tectonic phases: (1) late Oligocene–middle Miocene extension and development of a segmented intra-arc continental rift basin and broad volcanic zone; (2) late Miocene shortening, resulting in uplift, exhumation, and inversion of the former basins and a volcanic gap in the Main Cordillera; (3) Pliocene–early Pleistocene extension of the orogenic structure, reestablishment of the volcanic arc, and transtension along the intra-arc zone; and (4) late Pleistocene–Holocene narrowing of the arc and localized extension-transtension along the axial intra-arc zone. In the Central Andes, shortening has been more or less continuous since the Miocene, whereas in the Neuquén Andes, shortening stopped at ca. 6 Ma, probably related to the increase of the slab angle triggering the extension of the former orogenic structure and the onset of arc-parallel strike-slip faulting. The episodic evolution and migration of volcanism are related to changes in dip of the subducting plate.

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.