TERTIARY EVOLUTION OF THE ÑIRIHUAU BASIN, NORTHERN PATAGONIAN ANDES, ARGENTINA

The Ñirihuau basin is developed between 41°-43° S, in the Northern Patagonian Andes, Argentina, with a north-south trend along 200 km between the localities of Bariloche and Esquel. The basin infill is divided into two major lithostratigraphic units: El Foyel Group (EFG) of Paleogene age, and Nahuel Huapi Group (NHG) of mainly Neogene age. A new model for the basin evolution is proposed here, integrating previous and new stratigraphical and structural data. Basin evolution seems to have been strongly influenced by changes in convergence between Farallon/Nazca and Sudamericana plates.

In the Middle Eocene an andesitic arc (Cerro Bastión Fm., EFG) was developed in the western sector of the basin during a period of fast and oblique convergence. A marked decrease in the velocity of convergence around 42 Ma induced an extensional regime with negative roll-back, producing the basin opening over a thermally weakened continental crust. The resulting depressions were flooded by the Pacific Ocean and then filled by immature sediments related to fluviodeltaic systems (Troncoso Fm., EFG). A new extensional pulse in combination with climate factors favored the deposition of polymictic conglomerates (Salto del Macho Fm., EFG). In the Early Oligocene the combination of extension and sea level rise produced the catastrophic flooding represented by marine black shales of Río Foyel Fm. (EFG) and litoral sandstones of Ñorquinco Fm. (EFG) in the eastern sector of the basin. The presence of syndepositional normal faulting and progressive unconformities indicate an extensional regime.

From Early Oligocene to Early Miocene a new volcanic event took place in the basin (Ventana Fm., NHG). Geochemical characteristics of these volcanic rocks indicate a progressive cortical thinning, suggesting that extensional conditions in the Ñirihuau basin lasted until Late Oligocene.

The time between the Early Miocene and the Miocene-Pliocene boundary constituted the main period of Andean uplift in the region, during very fast and orthogonal plate convergence. The Ñirihuau basin therefore evolves in a foreland basin with load type subsidence and a synorogenic infill composed of continental volcaniclastic deposits of the Ñirihuau and Collón Curá Formations (NHG).