CENOZOIC EXTENSION VS. COMPRESSION IN THE CENTRAL NEUQUEN BASIN (S37° - S36°, ARGENTINA)

The tectonic evolution of the central Neuquen Basin during the Cenozoic is still a matter of debate. The inversion of the Mesozoic Neuquen basin started during the Eocene (Peruvian phase) and lasted at least until the Miocene. Extension during the Cenozoic has been deduced from geochemical data but no clues have been put forward from direct observations. Here, we present evidences of such extension and we propose a new model of deformation for the Cenozoic. Our study is based on the morphostructural analysis of remote sensing data (satellite images and digital elevation models) implemented by field work. We have evidenced different families of normal faults which were sorted according different criteria including occurence of hydrothermalism along fault planes, superposition of striae, consistency of the stress tensor from site to site and mapping of the faults according to the basin geometry. We have observed N160°E-trending normal faults in the Cordillera del Viento, parallel to the Loncupué through and consistent with E-W extension during the Oligocene. Folding and thrusting in the Agrio fold and thrust belt is consistent with E-W compression that took place during the Miocene (Quechua phase) to Quaternary. It is described as the major tectonic event in the area. The last tectonic stage corresponds to a strike-slip tectonic regime with s1 trending at N50°E. It is responsible for the initiation of normal faults trending N-S to N60°E cutting Quaternary lavas. These fractures seem to be associated to the emplacement of volcanic vents. The alternance between periods of extension and compression in the Central Neuquen basin during the Cenozoic may be related either to variation in the geometry of the Nazca slab or/and strike-slip movement along the Liquiñe-Ofqui fault, well displayed more to the South in Chile, and whose effects may be recorded in the Neuquen basin.