Stress balance above subduction: application to the Andes

The frequent changes in tectonic and morphological evolutions of active plate margins suggest that similar processes drive apparently antagonistic tectonics. We address the problem by a thin viscous sheet approach where the governing stresses are restricted to interplate traction, buoyancy and viscous stresses. Various cases are explored, from compression to extension, leading to dismiss corner-flow-driven models and to favor interplate traction-driven models. From the topography of the Andes, we determine the magnitude and profiles of the current stresses beneath the Andes at various locations. The total transmitted force is about 9×1012 N m−1. Shear stresses at the boundaries of the deforming lithosphere range between 25 and 110 MPa and almost balance the buoyancy stresses due to crustal thickening, the remaining difference being the viscous stresses. The deviatoric stress is less than 10 MPa and the effective viscosity of the Andean lithosphere is as low as ∼3×1021 Pa s. Maximum compression is located in the Eastern side of the Andes, matching the geological observations, but we find that the current stress regime is insufficient to explain the present-day elevation. This suggests that interplate traction was slightly stronger in the past when the Neogene plate convergence was faster, and this decrease in interplate traction implies that high and steep parts of the Andes cannot be sustained anymore. The shape of the Altiplano can be explained by a weak lateral variation of the viscosity, which allows the high plateau to be constructed in the characteristic time span of Andean building.