Interaction between the Nazca and South American plates and formation of the Altiplano-Puna plateau: Review of a flexural analysis along the Andean margin (15°-34°S)

The results of a two-dimensional flexural analysis applied to the Andean margin, which is based on the correlation between topography and Bouguer anomaly, are here reviewed in order to characterize rigidity variations across and along the forearc-arc transition of the Central Andes and to understand the role of the forearc in the formation of the Altiplano Plateau. The forearc has maximum rigidities between 15° and 23°S. Forearc rigidity decreases gradually southward and sharply toward the plateau. The main orogen (elevations higher than 3000 m) is very weak along the entire Central Andes. A semi-quantitative interpretation of these trends, based on the relationship between flexural rigidity and the thermo-mechanically- and compositionally-controlled strength of the lithosphere, allows the following conclusions to be made: (1) across-strike rigidity variations are dominated by the thermal structure derived from the subduction process; (2) the forearc constitutes a strong, cold and rigid geotectonic element; (3) southward weakening of the forearc is directly related to the decreasing thermal age of the subducted slab; (4) very low rigidities along the main orogen are caused by the existence of a thick, quartz-rich crust with a low strain rate-to-heat flow ratio; (5) the strength of the plateau lithosphere is localized in an upper-crustal layer whose base at ∼15 km could be correlated with a P-to-S seismic wave converter (TRAC1 of Yuan et al., 2000 [Yuan, X., Sobolev, S., Kind, R., Oncken, O. et al. 2000. Subduction and collision processes in the Central Andes constrained by converted seismic phases. Nature, V 408, 21/28 Diciembre, p. 958-961]); (6) the forearc-plateau rigidity boundary corresponds to a zone of changing thermal conditions, eastward-increasing crustal thickness and felsic component in the crust, and low strain-rate deformation, which correlates with a west-verging structural system at the surface. These conclusions suggest that the rigid forearc acts as a pseudo-indenter against the weak plateau and allows the accumulation of ductile crustal material that moves westward from the eastern foreland. This pseudo-indenter is geometrically represented by a crustal-scale triangular zone rooted at TRAC1. This model allows the integration of existing contradictory ideas on the dynamics of forearc-plateau interaction that are related to the relative importance of upper-crustal compressive structures and lower crustal accumulation below the forearc.