The Northern Patagonian Andes have been constructed through multiple mechanisms that range from tectonic inversion of extensional structures of Early to Middle Jurassic age in the Main Andes to Oligocene in the Precordilleran region. These have acted during two distinctive orogenic stages, first in late Early Cretaceous and later in Miocene times Late Oligocene extension separates these two contractional periods and is recorded by half‐grabens developed in the retroarc region. The last contractional stage coexists with an eastward foreland expansion of the late Miocene arc whose roots are presently exposed as minor granitic stocks and volcanic piles subordinately in the Main Andes, east of the present arc. As a consequence of this orogenic stage a foreland basin has developed, having progressed from 18 Ma in the main North Patagonian Andes, where the mountain front was flooded by a marine transgression corresponding to the base of the Ñirihuau Formation, to 11 Ma in the foreland area. Cannibalization of this foreland basin occurred initially in the hinterland and then progressed to the foreland zone. Blind structures formed a broken foreland at the frontal zone inferred from growth strata geometries. During Pliocene to Quaternary times most of the contractional deformation was dissipated in the orogenic wedge at the time when the arc front retracted to its present position.

A consensus on the biostratigraphic age and depositional environment of the Navidad, Ranquil, and Lacui formations exposed along the tectonic margin of central Chile has been elusive due to conflicting evidence. This study resolves this dilemma and gains further insight regarding the history of the Chilean coast. Problematic interpretations stem primarily from the remarkable similarity between the molluscan fauna of these units with those well documented for the late Oligocene to early Miocene of Peru. Planktic foraminifers, however, indicate that the Chilean sections accumulated in the late Miocene to early Pliocene interval following a regional hiatus that extends into the Eocene. The prevalence of mixed-depth bathyal assemblages of benthic foraminifers and ostracodes, the majority of which include lower-bathyal (Ͼ2000 m) indicators, reveals that downslope displacement was a primary mode of deposition in the basins. Although the molluscan assemblages are dominated by shallow marine taxa, most include species that range into or are restricted to deeper waters. Sedimentary features connote rapid subsidence and deep-water deposition of gravity flows. Although older Tertiary and Cretaceous planktic foraminifers in several assemblages indicate reworking of older units, lack of data on pre-Tortonian faunas of this region precludes recognition of other age-discordant components that could constitute a significant portion of the recovered fauna. The findings of this study revise the prevailing conception of the region's geologic history that considered these units to be early to middle Miocene shelf deposits and indicate that infilling and uplift have characterized the nearshore basins since the late Pliocene.

The North Patagonian fold-thrust belt (41º-44º S) is characterized by a low topography, reduced crustal thickness and a broad lateral development determined by a broken foreland system in the retroarc zone. This particular structural system has not been fully addressed in terms of the age and mechanisms that built this orogenic segment. Here, new field and seismic evidence of syntectonic strata constrain the timing of the main deformational stages, evaluating the prevailing crustal regime for the different mountain domains through time. Growth strata and progressive unconformities, controlled by extensional or compressive structures, were recognized in volcanic and sedimentary rocks from the cordilleran to the extra-Andean domain. These data were used to construct a balanced cross section, whose deep structure was investigated through a thermomechanical model that characterizes the upper plate rheology. Our results indicate two main compressive stages, interrupted by an extensional relaxation period. The first contractional stage in the mid-Cretaceous inverted Jurassic-Lower Cretaceous half graben systems, reactivating the western Cañadón Asfalto rift border ~500 km away from the trench, at a time of arc foreland expansion. For this stage, available thermochronological data reveal forearc cooling episodes, and global tectonic reconstructions indicate mid ocean ridge collisions against the western edge of an upper plate with rapid trenchward displacement. Widespread synextensional volcanism is recognized throughout the Paleogene during plate reorganization; retroarc Paleocene-Eocene flare up activity is interpreted as product of a slab rollback, and fore-to-retroarc Oligocene slab/asthenospheric derived products as an expression of enhanced extension. The second stage of mountain growth occurred in Miocene time associated with Nazca Plate subduction, reaching nearly the same amplitude than the first compressive stage. Extensional weakening of the upper plate predating the described contractional stages appears as a necessary condition for abnormal lateral propagation of deformation.

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The Chilean margin has been used as the model of an ocean-continent convergent system dominated by compression and active mountain building as a consequence of the strong mechanical coupling between the upper and the lower plates. The Andean Cordillera, however, shows evidence of alternating phases of compressional and extensional deformation. Volcano-sedimentary marine strata in the Aysén region of southern Chile contribute to an understanding of the causes of extensional tectonics and crustal thinning that occurred in the Andean orogeny because these deposits constitute the only reliable record of submarine suprasubduction volcanism during the Cenozoic in southern South America. In order to discern the age and tectono-sedimentary setting of these strata, referred to as the Traiguén Formation, we integrated sedimentology, ichnology, petrography, geochemistry, structural geology, foraminiferal micropaleontology, and U-Pb geochronology. Our results indicate that the Traiguén Formation was deposited in a deep-marine extensional basin during the late Oligocene-earliest Miocene. The geochemistry and petrography of the pillow basalts suggest that they formed in a convergent margin on a thinned crust rather than at an oceanic spreading center. We attribute the origin of the Traiguén Basin to a transient period of slab rollback and vigorous asthenospheric wedge circulation that was caused by an increase in trench-normal convergence rate at ca. 26-28 Ma and that resulted in a regional event of extension and widespread volcanism.