First-order Crustal Compositional Changes along the Active South American Margin Dominate the Segmented Nature of the Andean Orogen (15° -47° S)

The Andean Cordillera is a continuous feature along the western margin of South America that has been constructed by a simple common process of ocean-continent collision. However, the Andes show a remarkable continental-scale along-strike segmentation of both the current geotectonic regimen and long-term geological evolution. Here we investigate the hypothesis that differences between the Central (15° -33.5° S) and Southern (33.5° -47° S) Andes are caused by inherited compositional and hence rheological variations along the South American plate. A quantitative interpretation of elastic thickness (Te) estimates in the framework of a brittle-elasto-ductile rheology demonstrates that Te < 10 km, which characterizes the axis of the Central Andes, is only possible if the bulk crustal composition is felsic, while Te > 35 km along the Patagonian Cordillera segment (39° -47° S) of the Southern Andes implies a mafic-intermediate composition. This result is consistent with the available geological, geochemical and geophysical information, and indicates that the weak rheology of the quartz-dominated Palaeozoic basement characteristic of the Central Andean crust has been modified and rigidized toward the Southern Andes by the development of a stationary Meso-Cenozoic magmatic arc along the Patagonian Cordillera. This compositionally-caused rheological difference along the Andean margin should be consider a key factor in the understanding of the Late Cenozoic geodynamic evolution of the orogenic process.