Crustal structure of the Andean thrust front at 30° S latitude from shallow and deep seismic reflection profiles, Argentina

The geometry of the entire crust of the Andean thrust front zone and its associated foreland basin at 30° S latitude is imaged for the first time on a seismic profile obtained from the extended correlation of Vibroseis data. The entire region is dominated by pre-Andean structures which probably formed during Proterozoic and Phanerozoic time, related to the assemblage of Rodinia and Gondwana supercontinents. The west-verging structures of the Eastern Precordillera are controlled by basement fabrics that dip between 30° and 60° to the east, and by a 13–20 km deep décollement, probably formed during the Early Ordovician–Late Devonian orogeny. This structure was subsequently covered by at least 12 km of Mesozoic-Tertiary sediments and then reactivated during the Andean orogeny. The Ambato lineament is a lateral ramp in the décollement that produced a subtle change in the strike of the Eastern Precordillera structures. The northern end of the Sierra de Pie de Palo is formed by an east-dipping basement fault associated with a minor west-dipping fault that crops out on the eastern side of the range. Seismic stratigraphic analysis shows that this range experienced the first uplift episode between 5.4 and 2.7 Ma. The Grenville suture exposed at the western side of the range probably continues to the northeast, where it is imaged on the seismic profiles as the high amplitude reflector now associated with the Andean décollement. The intermediate crust beneath the Bermejo valley is highly structured, showing imbricated reflectors which may be related to crustal thickening mechanisms. The lower crust appears to be layered, its fabric probably being inherited from the Triassic or Cretaceous extensional periods that produced magmatic intrusions. The base of the crust appears to be characterized by a transitional Moho.