DEEP SEISMIC RESULTS IN SUB-ANDEAN BASINS OF ARGENTINA

This contribution deals with deep seismic reflection studies in three basins of northwestern Argentina: Lomas de Olmedo, Metán-Guachipas, and Southern Tucumán. The results obtained in Lomas de Olmedo Region support the asymmetry of the Cretaceous rift, with a zone of dominant thermal uplift in the northern side. Truncation of the Paleozoic beds and identification of a deep oblique discontinuity (18-21 km depth) postulate a northward-dipping detachment controlling the system asymmetry. The rift structure is mildly modified by folding related to the Cenozoic tectonic inversion in the southern sector of the basin. Metán seismic sections showed the deep structure of the reef, consisting of a series of half-grabens, filled by Cretaceous red beds of the Pirgua Subgroup; as well as the sag phase deposits consisting in limestones, marls, shales and evaporites. An east-dipping reflector at 10 to 23 km depth was interpreted as a master shear that controlled the extensional mechanism of the rift system, partially reactivated during Andean compression. Seismic reprocessing at 27ºS provided the basis of a new tectonic interpretation of the western Sierras Pampeanas. On the eastern side of Sierra de Aconquija, the structure is characterized by reverse faulting with eastward vergence, with the Guasayan and El Rosario faults exhibiting the most deformation. On the western side of the range, the deep structure is characterized by westward-vergent reverse-faults controlled by the anisotropy of the Sierra de Aconquija basement. As a whole, both fault systems depict a double-wedge active thrusting with a last detachment level at about 50-km depth.