TERTIARY STRUCTURAL EVOLUTION OF THE EASTERN BORDER OF THE NORTHERN PATAGONIAN ANDES FROM 41º TO 41º30′ S, ARGENTINA

The study area is located between 41º and 41º30′ S in the eastern border of the Northern Patagonian Andes, Argentina. This segment of the Andes is characterized by the presence of the Neogene Ñirihuau fold and thrust belt (ÑFTB). In its eastern sector, the deformation involved part of the Oligocene to Miocene infill of the Ñirihuau basin. Here we present a model for the structural evolution of the area based on previous and new structural data.

Normal faults affecting the structural basement of the Ñirihuau basin were identified in several seismic lines, indicating that the opening of the basin in the study area was related to an extensional regime. These faults controlled the distribution of the initial infill of the basin, conformed by Upper Oligocene alluvial and lacustrine deposits. The presence of the ÑFTB makes it difficult to observe the extensional architecture of the Ñirihuau basin during Upper Oligocene times. However, some normal faults can be identified because of their control on the orientation and location of some compressive structures. Main normal faults have a NNW strike and controlled the general trend of the basin. These faults are segmented by accommodation faults of NE trend, which are interpreted as reactivated basement anisotropies. The northern limit of the basin is an abrupt lineament of NW strike, probably related to the reactivation of a Paleozoic shear zone during extension.

In the western sector of the study area basement rocks were uplifted by east-vergent reverse faults, showing a thick-skinned structural style. On the other hand, in the eastern sector the ÑFTB shows a thin-skinned style, characterized by tight asymmetrical folds and low angle thrusts affecting the Upper Oligocene to Middle Miocene sedimentary infill of the Ñirihuau basin. A basement wedge was identified in the area located between the two different structural styles, indicating that basement faults propagated to the east through the sedimentary cover, transferring displacement to shallower levels. Subhorizontal sediments of Middle to Upper Miocene age (16 to 10 Ma) overlie the folded deposits with an angular unconformity, restricting the age of the compressive deformation in the study area between the Early and Middle Miocene.