Controls on the Deformation of the Central and Southern Andes (10–35° S): Insight from Thin-Sheet Numerical Modeling

Abstract

What mechanisms and conditions formed the Central Andean orocline and the neighboring Altiplano Plateau? Why does deformation decrease going from the central to the Southern Andes? To answer these questions, we present a new thin-sheet model that incorporates three key features of subduction orogenesis: (1) significant temporal and spatial changes in the strength of the continental lithosphere in the upper plate;(2) variable interplate coupling along a weak subduction channel with effectively anisotropic mechanical properties; and (3) channeled flow of partially molten lower crust in the thickened upper plate. Application of this model to the present kinematic situation between the Nazca and South American Plates indicates that the deformed Andean lithosphere is significantly weaker than the undeformed South American foreland, and that channel flow of partially melted lower crust smoothes topographic relief. This channel flow is,therefore, inferred to control intra-orogenic topography and is primarily responsible for the development of the Andean Plateau since the Miocene. A parameter study shows that the decrease in shortening rates from the central to the Southern Andes can be attributed to the weakening of the orogenic Andean lithosphere and to along-strike variations in interplate coupling within the subduction zone. The current rates of deformation are reproduced in the model if:the Andean lithosphere is assumed to be 5-15 times weaker than the lithosphere of the Brazilian shield; interplate coupling is assumed to be relatively weak, such that the subduction zone in the vicinity of the Central Andes is some 10-20 times weaker than the Andean lithosphere; and coupling itself decreases laterally by some 2-5 times going from the central to the Southern Andes.