Observations of the intraplate stress field, as compiled and evaluated in the context of the World Stress Map Project, can be compared with stress modelling results to increase our understanding of the dynamics of motion of the lithospheric plates. In this paper we investigate the driving and resistive plate tectonic forces acting on the South American plate in order to establish a detailed basis for future stress modelling. Our basic assumption is that the plate is in mechanical equilibrium; that is, the net torque of the forces acting on the plate vanishes. The complexity of the forces exerted along the margins of the South American plate requires the dynamical analysis to be performed step-wise. Incorporating the forces which are expected to be of major importance, a first-order model is defined. The effect of additional plate boundary forces is subsequently examined in terms of superpositions onto the first-order model. With the first-order model the relation between three resistive forces and shear stress on the base of the lithosphere is derived. The ridge push force, driving the motion of the South American plate, can be quantified independently and is included in the calculations as a known quantity. Solutions are obtained using four different determinations of the Euler vector of absolute motion of the South American plate, which is taken to define the orientation of the basal shear stress.

The results demonstrate that the concept of a basal shear stress actively driving plate motion cannot be discarded on the basis of a dynamical analysis. Further, with the assumption that basal drag is entirely concentrated below the continental part of the lithosphere, estimates of the maximum amount of continental basal shear resistance are derived. Evaluation of an alternative model for the South American plate, excluding the northernmost part of the Andes, demonstrates the significance of the resistive force exerted on the South American plate due to convergence with the Caribbean plate. It is to be expected that comparison of theoretical distributions of intraplate stress, computed on the basis of the present dynamical analysis, with observations of the actual state of stress will further constrain the range of feasible force models. ¿ American Geophysical Union 1992