Pure and simple shear plateau uplift, Altiplano-Puna, Argentina and Bolivia

The concepts of pure and simple shear strain provide a meaningful way of discussing the alternative modes of lithospheric shortening which produced uplift of the central Andean plateau. Since 10 Ma, discrete segments of the plateau exemplify pure or simple shear modes. Characteristics of the simple shear mode are a flanking thin-skinned thrust belt, and subdued relief and lack of significant Late Miocene to Recent deformation within the plateau. The pure shear mode is characterized by more rugged internal relief and active neotectonic deformation within the plateau and thick-skinned foreland deformation involving seismic deformation of the crust to 30 km or more. These modes of crustal thickening correlate with changes in lithosphere thickness, modes of isostatic compensation, broad wavelength topography and back-are magmatism. The correlation of the boundary between pure and simple shear segments with major changes in pre-existing anisotropies in the foreland of the orogen suggests that events which long pre-date the Andean orogeny indirectly controlled the dong-strike variation in the magnitude and style of shortening in the central Andes and may have influenced the development of flat subduction.