CONICET | Buscador de Institutos y Recursos Humanos

The Northern Patagonian Andes between 38º and 40ºS are flanked by an extensional system, which determines the orogenic front at these latitudes. This trough has affected part of a Cretaceous to Miocene fold and thrust belt reactivating transtensionaly Late Triassic basement structures. Preliminary gravimetric analysis show that its western half lays beneath a 4-5 kilometer thick succession composed of volcaniclastic rocks. This sequence corresponds to the development of two successive intra to retro arc basins which span from Late Oligocene to the Present. The last stage of orogenic collapse occurred since 5 Ma has defined three well discernible domains, which from west to east are: i) a series of east-tilted half grabens associated with profuse mafic monogenetic activity, acid dome complexes, more than 15 caldera structures, and the eastern part of the arc ; ii) a morphologically deep trough filled by pyroclastic successions corresponding to distal equivalents of thick ignimbritic sheets o the west, covered by monogenetic flood basalts; and iii) a series of west-tilted half grabens associated with dome complexes an fissural mafic floods that constitute the oldest volcanic stage in the entire area linked to the extensional trough. Kinematic indicators indicate that extension was accompanied by substantial amounts of left lateral displacements, which are geometrically compatible with widespread rhomboedric features that form the main depocenters through the basin. Late Miocene shallow subduction in the area proposed by different studies and subsequent last 5 Ma slab retreat are the more plausible mechanisms to explain widespread extension at the Andean front. Oblique convergence between Nazca and South American plates and strain partitioning along NNW Late Triassic rift structures would have controlled left lateral displacements associated with this orogenic collapse.

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