FORCES CONTROLLING THE PRESENT-DAY STATE OF STRESS IN THE ANDES

The present-day state of stress in the Andes is expected to be control led primarily by two different types of forces: (1) the resistive forc e 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 hav e 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 n et torque on the plate should vanish. A thin elastic shell representat ion is used to calculate the intra-plate stress field associated with the various force distributions. We define a reference model for the p resent study that incorporates a uniform magnitude for the resistance associated with convergence along the western prate margin (F-pcr, pla te contact resistance) and does not include the effects of topography. Subsequently, we investigate the effect of lateral variation in the m agnitude of F-pcr and add the topography-related forces, The main resu lts 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 th e lithosphere subducting below the western plate margin. (2) The amoun t of horizontal compression across the Andes, found in the case that r idge push is considered to be the only force driving the South America n plate, is small compared to the value required to ''sustain'' the An des.