Evidence for major shortening on the eastern edge of the Bolivian Altiplano: the Calazaya nappe

For the first time, evidence is presented of major shortening on the eastern edge of the Altiplano, in Bolivia. A tectonic nappe showing at least 40 km of horizontal displacement towards the east is revealed at the Altiplano-Cordillera Oriental transition (20°S). Analysis of the differences in stratigraphy, sedimentology and structure between allochthonous and autochthonous units confirms the 40 km of overthrust. A geometric analysis of the thrust structures has been made to interpret the three-dimensional geometry of the nappe, and a deformation sequence is suggested. The nappe emplacement is related to the eastward transport of a large allochthonous domain during the Late Oligocene-Early Miocene. The sole thrust of this allocthtonous domain is located in a stratigraphie unit whose three-dimensional basin geometry controlled the deformation. The existence of the Calazaya nappe allows us to explain the exceptional thickness of the Altiplano (55-70 km) as mainly resulting from the Neogene shortening that occurred between the Altiplano and the Cordillera Oriental.