Forces controlling the present-day state of stress in the Andes

The present-day state of stress in the Andes is expected to be controlled primarily by two different types of forces: (1) the resistive force exerted on the western overriding margin of the South American plate, and (2) forces that arise from the thickened crust of the Andes (i.e., the effect of topography and its compensating crustal root). We have studied these forces on the basis of a model for the dynamics of the entire South American plate in which the Andes are embedded. In this model a given set of forces is constrained by the criterion that the net torque on the plate should vanish. A thin elastic shell representation is used to calculate the intra-plate stress field associated with the various force distributions. We define a reference model for the present study that incorporates a uniform magnitude for the resistance associated with convergence along the western plate margin (F_pcr, plate contact resistance) and does not include the effects of topography. Subsequently, we investigate the effect of lateral variation in the magnitude of F_pcr and add the topography-related forces. The main results are: (1) A uniform magnitude of F_pcr leads to a better match with the observations than a magnitude that is a function of the dip of the lithosphere subducting below the western plate margin. (2) The amount of horizontal compression across the Andes, found in the case that ridge push is considered to be the only force driving the South American plate, is small compared to the value required to "sustain" the Andes.