TECTONO-STRATIGRAPHIC EVOLUTION OF THE ÑIRIHUAU BASIN, NORTH PATAGONIAN ANDES

A long segment of the southern Central Andes underwent extensional tectonics during the Oligocene to Early Miocene. As a result, a series of complex extensional basins filled with volcanic products and marine and/or continental sedimentary rocks developed along the margin in either fore-, intra-, and retro-arc positions. The infill of some of these basins records a transition to a compressive tectonic regime that became predominant all along the segment by the Late Miocene. The Ñirihuau basin developed in an intra- to retro-arc position in the North Patagonian Andes between 41° and 43°S, and it constitutes an excellent case study where to analyze this tectonic switching because of its relatively continuous and well-exposed infill. In this contribution, we integrate sedimentologic (facies and provenance studies), geochronologic (U-Pb ICP-MS dating of detrital and igneous zircons) and structural data (surface and subsurface data) in order to constrain the tectono-sedimentary evolution of the basin infill and its relation with the alternating tectonic regimes.

In the northern sector of the basin, the Miocene Ñirihuau Fm represents an exceptional record of sedimentation during different tectonics regimes. This unit starts with a very coarse alluvial sedimentation which rapidly pass into lacustrine facies with extensional synsedimentary features, representing synrift deposits. A big change in the sedimentation occurs in near the middle part of the unit with the sudden development of a sandy braid plain. We interpret this forced regression due to the tectonic inversion and the beginning of the uplift at west. Even though that it could be produced by an increase in volcaniclastic input to the basin as the explosive volcanic activity is continued and important during the deposition of all the unit, the abundant synsedimentary deformation structures and changes in the provenance sources indicates a major tectonic change. Structural information reveals that sedimentation of the basal section of the Ñirihuau Fm is controlled by normal faults, while the upper section shows growth strata related to thrust faulting. According to our geochronologic constraints, the tectonic switching between extension and shortening took place between 15 and 13 Ma.