Topography of the Central Andes: Effects from Above and Below

The topography of the central Andes is the product of an interaction between solid earth and atmospheric processes -- tectonic processes tend to build topography and climatic ones erode and redistribute it. Here we can assess the relative significance of these effects because tectonic and climate patterns have very different spatial characteristics relative to the high plateau. The stable Nazca-South America plate motions of the mid-Tertiary to present, and the consequent dynamics of the modern Andes, have a high degree of spatial order -- principally a bilateral symmetry on a vertical-ENE trending plane across the plateau, parallel to particle motions of both plates. This pattern is strongly violated by climate -- wet in the north on the east side of the plateau, and dry in the south -- and consequent erosion. To resolve these effects on the topography, we consider vertical profiles across the mountain belt and subduction zone. Parallel sections equidistant from but on opposite sides of the symmetry plane experience similar kinematics but different climate effects. Although precipitation amounts vary considerably north to south along the eastern edge of the plateau, the topography there is very similar on both sides of the symmetry plane. On the other hand, the geometry of the subducted Nazca plate varies north to south, though the plate kinematics do not vary. These and other observations suggest that the overriding and subducting plates are partially coupled by viscous stresses across a narrow asthenospheric wedge, such that the mountain belt and subduction zone dynamically respond to variations in loading at the earth's surface (caused by erosion in the north versus the lack thereof in the south). Both the South America and Nazca plates deform as they slide past one another nearly face-to-face across the asthenospheric wedge. The incremental deformation of the overriding plate is probably less than that of the subducting plate; however, the former is stationary and accumulates deformation (to build the plateau) while the latter is constantly refreshed. Indeed the upper plate must be relatively strong (i.e., resists deformation) and the mantle beneath deforms owing to drag on its stable upper surface to accomodate the flow induced by subduction. Thus, minor climate effects, causing uneven erosion of the plateau, may affect the geometry and motion of the subducted slab.