Controls on the shape and kinematics of the Central Andean plateau flanks: Insights from numerical modeling

The flat surface of orogenic plateaus requires a weak detachment at the base of the crust that prevents the formation of large-wavelength topography in the centre of the orogen and decouples the deformation of crust and mantle. This study uses numerical distinct-element simulations of plateau orogeny to show that the mass flux across the lateral tips of this detachment controls the shape and kinematics of the plateau flanks. If the mass flux is directed towards the plateau, e.g. foreland crust moves into the detached section, it forms a low-slope, small-taper pro-flank. If the mass flux is directed outward, thrusting towards the foreland generates a steep-slope, large-taper retro-flank. The mass flux itself is controlled by the shear strength distribution at the base of the crust. The two general types of orogenic flanks from three combinations that can be used to classify the evolution of plateau orogens in general. Application of this concept to the Andes allows to decode the lower crustal shear strength distribution during the formation of the Altiplano-Puna plateau. Their western slope is a retro-flank localized by the underlying oceanic subduction zone. On the eastern side a pro-flank accretes foreland crust, which is progressively decoupled from the moving mantle below.