The Andean Thrust System—Latitudinal Variations in Structural Styles and Orogenic Shortening

Abstract

The different segments of the Andean thrust system have distinctive topography and inferred crustal roots. These two characteristics both depend upon crustal shortening, and on this basis they provide independent constraints for evaluating estimates of Cenozoic shortening obtained by balanced structural cross-sections of different segments of the fold-and-thrust systems. Three transects in the Central Andes are analyzed: a northern (22–23S), a central (32–33S), and a southern segment (37–39S). Each segment shows different amounts of orogenic shortening, generated through a complex combination of thin- and thick-skinned thrusting. Based on known age constraints, different shortening rates are calculated for each segment. Estimates of crustal shortening derived from gravity and seismic-refraction data are used to evaluate interpretations of the structural style. In some segments, where alternative styles were proposed, the crustal-shortening estimates are used to identify the more realistic models. Crustal shortening, shortening rates, and the resulting topography decrease progressively from north to south. These variations cannot be fully explained by differential fore-arc rotation, as in the Bolivian orocline model. Instead, a close correlation is suggested between the age of oceanic crust being subducted and the amount of shortening and propagation of the orogenic front toward the foreland. This fact becomes more important than fore-arc rotation farther south of the Bolivian orocline. On this basis, the present topography of the Andes, along the Nazca plate boundary, can be correlated with the age of adjacent oceanic crust.