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.

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 southern Andes were created by two main cycles of shallow to flat subduction settings that were followed by steepening subduction zones starting in Late Cretaceous times. The first wave of contractional deformation and Andean uplift migrated through the continental interior as a result of two shallow subduction zones, one developed between 36 and 39°S and the other between 40 and 46°S, associated with the expansion of arc magmatism. In latest Cretaceous to Eocene times, its northernmost segment flattened, increasing the compression and uplift of mountains in the far foreland area, whereas, to the south, a steepening subduction zone provoked extensional collapse of vast sectors of the fold and thrust belt followed by within-plate magmatism. The whole area between 36 and 44°S retreated as a large steepening zone in late Oligocene times, inducing asthenosphere injection and the formation of large within-plate plateaux in the foreland zone, as well as narrow extensional basins induced by the incipient collapse of the fold and thrust belt hinterland zone. The late Miocene was characterized by the development of three shallow subduction zones that expanded differentially between 34°30′ and 50°S. These were again associated with arc expansions and lateral construction of the fold and thrust belt. Their evolution finished in Pliocene to Quaternary times with the eruption of within-plate plateaux and widespread extensional deformation that still governs important sectors of the present retroarc area. Finally, an incipient shallow subduction setting could have been developing between 35 and 39°S in the last 3 Ma associated with renewed Andean uplift at these latitudes. Cyclic shallow subduction in the southern Andes, and therefore repeated behaviour of constructional stages followed by collapsing ones associated with voluminous volcanism, could be the consequence of the cycle imposed by the docking of seismic ridges, one achieved in latest Cretaceous (?) to Eocene times and the other in late Miocene times. Other factors, such as the collision of highly serpentinized and therefore isostatically buoyant plateaux, associated with fracture oceanic zones, are also considered to trigger shallow subduction settings.Los Andes del Sur fueron creados a través de dos ciclos principales en los cuales la zona de subducción se subhorizontalizó y posteriormente se empinó en diferentes segmentos comenzando en el Cretácico superior. La primer fase contraccional en los Andes del Sur migró desde el borde de placas hacia el interior continental en relación al desarrollo de dos zonas de subducción subhorizontal, una desarrollada entre los 36°y los 39°S y la otra entre los 40°y los 46°S, ambas relacionadas a la expansión oriental del magmatismo del arco volcánico. En el Cretácico más alto hasta el Eoceno, la sección subhorizontal más septentrional siguió acentuándose asociándose así al levantamiento de relieves montañosos lejanos al límite de placas, al tiempo que el sector meridional se empinó rápidamente lo que provocó el colapso extensio...

Constraints on the Neogene growth of the central Patagonian Andes at the latitude of the Chile triple junction (45-47°S) using U/Pb geochronology in synorogenic strata. Tecto (2018),

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ABSTRACT. The Southern Central Andes at 36ºS have been recognized as an orogenic belt where contraction, ac com modated mainly by basement structures, is associated with the inversion of a Late TriassicEarly Jurassic extensional detachment. Based on a structural crosssection, constrained by field data, 2D seismic and borehole information, and the processing of Bouger anomalies, we propose a polyphasic tectonic evolution. In the westernmost sector, along the axis of the Cordillera, NW to NNW basement structures were inverted, being a first order control in the generation of frontal narrow Ntrending thinskinned belts. This slip transfer is controlled by the Late Jurassic main detachment. These structures have low gravity anomalies that cross obliquely the main Andean trend. East of this inversion domain, beneath the frontal thinskinned belts, seismic information reveals that Late Triassic wedgelike depocenters did not experience substantial inversion. To the east doublevergent basement blocks define the Andean emergent orogenic front at these latitudes. These contractional structures truncate gravity anomalies defined by basement discontinuities, indicating that they are not related to tectonic inversion, in contraposition to the westernmost domain. Two contractional phases were distinguished. The oldest is Late Cretaceous in age, as inferred from onlap relations in Upper Cretaceous strata identi fied in seismic lines. These successions have a maximum age of 97 Ma as inferred by UPb in detrital zircons published in previous studies in the area. Contrastingly, the easternmost sector was mainly deformed in Late Miocene times as inferred from less than 18 Ma old synorogenic deposits. Moreover, contractional mechanisms varied through time for each specific sector. While Late Cretaceous contractional tectonics was generated by tectonic inversion and subordinate thinskinned deformation, it is proposed that Late Miocene deformation was controlled mainly by brittleductile transi tions at the upper crust with no major influence of previous structures. This fact can be explained by a higher thermal flux achieved in the retroarc area in the last 17 Ma due to the eastward arc expansion during a shallow subduction regime. RESUMEN. Controles estructurales variables en el tiempo en los Andes Centrales Australes (~36ºS). Los AndesCentrales Australes a los 36ºS han sido reconocidos como una faja orogénica en donde la contracción, principalmente acomodada por estructuras de basamento, está asociada a la inversión de un sistema extensional del Triásico TardíoJurásico Temprano. Basados en una sección estructural, construida a partir de datos de campo, líneas sísmicas 2D, datos de pozo y el procesamiento de anomalías de Bouger, proponemos una evolución tectónica polifásica. En el sector occidental, a lo largo del eje cordillerano, estructuras de basamento con orientación NW a NNW fueron invertidas actuando como un control de primer orden en la generación de fajas frontales de piel fina angostas y de orientación norte. La transferenci...