KINEMATICS OF THE PLATE BOUNDARY ZONE AND UPLIFT OF THE ANDEAN PLATEAU

Although many aspects of the Andes have been studied,the processes leading to Andean mountain building remain enigmatic. The past few decades have seen fast growth of observational data, yet many of these data appear to be discordant with each other, and tectonic implications of these data sets are often ambiguous. Among the most intriguing results is that while over the past 25 Ma Nazca-South America plate convergence decelerated, as shown by comparison of GPS and marine magnetic data, much of the Andean mountain building occurred during the same time, raising questions about the dynamic links between these two processes. The temporal-spatial evolution of the Andes also seems more complicated than previous model predictions. Structural, paleoelevation, and fission track data (assuming for simplicity that exhumation largely reflects uplift) jointly show that the Altiplano began rising slowly as a broad region about 30 Ma, whereas uplift and shortening in the FTB began about 10 Ma but has been more rapid, leading to the present distribution of deformation. Additional constraints result from the distribution of deformation across the boundary zone. GPS velocities relative to South America decrease smoothly across the Andes, indicating broadly distributed shortening, whereas geological and seismic data show concentrated shortening in the Eastern Cordillera and Subandean fold-thrust belt. Hence it appears that GPS velocity is partitioned into transient elastic deformation near the trench, that will be recovered during future earthquakes, and permanent deformation in the foreland thrust belt. This partioning can be described by a simple plastic-viscoelastic model for the mechanical behavior of the Andean crust: mainly elastic over short periods, viscous over long geological time scales, and plastic (sliding along faults) when tectonic stresses exceed the yield strength of the trench fault. The predicted short-term velocity field closely resembles the GPS velocity across the Andes, because the model includes both permanent deformation and elastic deformation. In contrast, the long-term velocity field, which reflects only the permanent deformation, predicts little long-term motion across the Andes but concentrated long-term crustal shortening in the Subandean FTB.