ANDEAN REACTIVATION OF THE CRETACEOUS SALTA RIFT, NORTHWESTERN ARGENTINA

Throughout the Andes, foreland geometries are correlated with the orientation of the subducting Nazca Plate: fold-and-thrust belts with steep subduction and basement uplifts with flat. The geometries observed in the southern Cordillera Oriental and northern Sierras Pampeanas do not fit this pattern. Instead, inversion of the Cretaceous Salta Rift Basin and mechanical differences between rift and non-rift domains are proposed as the primary controls on both the timing of late Tertiary uplift and deformation, and foreland geometries. The influence of the rift basin is documented through field observations of structures and lithologies, kinematic analysis of minor fault data, and published data on local stratigraphy. The southern Cordillera Oriental developed within the southwestern subbasin of the Salta rift and is a basement-involved fold-and-thrust belt. The Sierras Pampeanas developed to the south of the rift and are basement uplifts. Dominant structures in both regions are N/S-trending reverse or thrust faults. They are cut by oblique strike-slip faults. Older deformation is Mio-Pliocene in age and is characterized by thrust kinematics with E-W to NW-SE shortening. Younger deformation is Plio-Quaternary in age and is characterized by strike-slip kinematics with NE-SW shortening, except along the boundary between the Cordillera Oriental and the Sierras Pampeanas where thrust kinematics with N-S shortening prevail. The similar kinematics but different geometries in the two provinces during Mio-Pliocene deformation and the anomalous thrust kinematics observed during Plio-Quaternary deformation suggest that the Salta rift is the main control on structural geometries. A rift inversion model is developed and applied to the southern Cordillera Oriental.