Rigidity of the Andes from flexural modelling and admittance and coherence techniques

Abstract

Flexural modelling studies have previously been used to infer the rigidity of the Central Andean lithosphere. The results of these studies suggest important along-strike and orogen-perpendicular variations in crustal structure. The along-strike variations probably reflect differences in bulk crustal composition. These compositional differences can partly explain the contrasting geological evolution of different parts of the Andes. The Central Andes (~15?35°S), are dominated by the high and broad Altiplano?Puna plateau, but the Southern Andes are characterised by a narrower and more subdued mountain range. Lithospheric rigidity can also be determined from the admittance technique, a statistical approach that examines the correlation between topography and gravity anomalies. The results of a simple 1D admittance analysis applied to the Central and Southern Andes show some general similarities to those derived from flexural studies, but important differences remain. The admittance results predict higher rigidity for the non-plateau part of the Andes and a Moho depth under the plateau that is extremely shallow compared with observations. These discrepancies reflect either the simplicity of the admittance method applied or the complications induced by the unusually thick crust associated with the Altiplano?Puna plateau.