Stepwise Widening of the Central Andes—The Role of the Lower Crust

Abstract

AbstractThe outward growth of many orogens occurs through pulsed migration of the strain front. During Cenozoic shortening of the central Andes, the strain front abruptly migrated ∼400 km eastward across the Altiplano, isolating the plateau interior from major deformation. In contrast to the traditional critical wedge model that focuses on shallow crust deformation, our lithosphere‐scale numerical models show the ductile lower crust plays a key role in the dynamics of the orogen. As the orogen grows upward, the lower crust flows outward and causes stepwise expansion of the orogen through landward‐migrating deformation. The step length of the strain‐front migration increases with decreasing lower crustal strength. Topographically, a weaker crust promotes the formation of a smooth plateau and underthrusting of stronger foreland lithosphere. Our results indicate that the seemly unrelated physiography, deep‐lithosphere structure, and plateau‐wide strain migration in the Altiplano may all be the result of a weak lower crust.