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