TOPOGRAPHY AND SUBDUCTION GEOMETRY IN THE CENTRAL ANDES - CLUES TO THE MECHANICS OF A NONCOLLISIONAL OROGEN

The central Andean orogen between 12-degrees and 32-degrees-S latitude exhibits a high degree of spatial order: principally an extraordinary bilateral symmetry that is common to the Earth's surface, the underlying Wadati-Benioff zone, and the Nazca/South America plate kinematics, which has been stable since the mid-Tertiary. This spatial order must reflect the physical mechanisms of mountain building in this noncollisional orogen. The shapes of the topography and subduction zone can be reduced to symmetric and antisymmeric components relative to any vertical symmetry plane; the particular plane which minimizes the antisymmetry (and maximizes the symmetry) is well-resolved and is essentially coincident with the stable Euler equator of Nazca/South America relative motion since the mid-Tertiary. That the topography, subduction geometry, and persistent mid-Tertiary plate kinematics share common spatial and geometric elements suggests that the distribution of topography in this orogen depends strongly on the dynamics of subduction. Other factors that might affect the topography and underlying tectonics, such as climate and inherited structural fabric, which have different spatial characteristics, must be of less significance at a continental scale. Furthermore, the small components of asymmetry among the various elements of the orogen appear to be mutually related in a simple way; it is possible that this coupled asymmetry is associated with a late Tertiary change in plate kinematics. These observations suggest that there is a close connection between plate tectonics and the form of the Earth's surface in this noncollisional setting. It follows that the distribution of topography near convergent plate boundaries may provide a powerful constraint for understanding the dynamics of subduction.