Neogene dextral transpression due to oblique convergence across the Andes of northwestern Patagonia, Argentina

Between Bariloche (41°S) and El Bolsón (42°S), Neogene sediments of the Ñirihuau foreland basin and Paleogene volcanoclastic rocks have been thrust westward beneath basement rocks of the Andean cordillera. North of Bariloche (40°–41°S), Paleogene volcanoclastic rocks within the main cordillera show Neogene deformation. The large-scale Neogene tectonics of the area are revealed by superimposing geological maps with digital topographic data. Fault-slip data provide information on the relative amount of crustal thickening and strike-slip faulting. Throughout the area, major reverse faults and thrusts trend northwest, forming the edges to Cenozoic basins of foreland or ramp styles. Some of these are inverted grabens of Mesozoic age. The dominant strike-slip faults are right-lateral and trend nearly north, parallel to the cordillera. Conjugate left-lateral faults trend nearly east. At a regional scale, based on the fault-slip data, the principal direction of shortening is northeast, in areas where thrusts predominate, but swings around to the north in areas where strike-slip faults predominate. Thus the results indicate a degree of strain partitioning, but they are broadly compatible with the oblique direction of convergence between the Nazca and South American plates. This tectonic style seems to have lasted throughout the Neogene.