Description: The Andes are the classic example of a subduction-related orogen. Segmentation of the orogenic belt is related to dynamics of the subduction zone and to upper plate thermomechanical properties. Understanding the controlling factors on deformation along the orogen requires studying cross sections at different latitudes and determining the respective roles of plate interactions, upper plate weakness zones, and crustal architecture. A newly constructed balanced cross section of the Argentinean Andes at 35°S, in the transition between a flat-slab and a normal subduction segment, shows tectonic inversion of Mesozoic normal faults and development of new thrusts during Andean shortening. Estimated shortening of 26.2 km, equivalent to 22% of the initial length, is lower than previous estimates obtained from partial cross sections using non-inversion structural models. Comparison of this estimate with crustal area balance constrained by geophysical data indicates that (1) crustal thickness was varied across the transect before Andean shortening, with a thick (~45 km) crustal block to the west related to late Paleozoic orogeny, and a thinner block (~32 km) in the east related to Mesozoic stretching; and (2) a structural model incorporating tectonic inversion is consistent with regional shortening and crustal thickness trends. Our results underscore the role of the inherited characteristics of the upper plate in subduction-related orogens, including preexisting faults and preorogenic crustal thickness variations.

Description: Deformation in the orogen-foreland system of the southern Central Andes between 33° and 36° S varies in style, locus, and amount of shortening. The controls that determine these spatially variable characteristics have largely remained unknown, yet both the subduction of the oceanic Nazca plate and the strength of the South American plate have been invoked to play a major role. While the parameters governing the subduction processes are similar between 33° and 36° S, the lithospheric strength of the upper plate is spatially variable due to structures inherited from past geodynamic regimes and associated compositional differences in the South American plate. Regional Mesozoic crustal horizontal extension generated a 〈 40-km-thick crust with a more mafic composition in the lower crust south of 35°S; north of this latitude, however, a more felsic lower crust is inferred from geophysical data. To assess the influence of different structural and compositional heterogeneities on the style of deformation in the southern Central Andes, we developed a suite of geodynamic models of intraplate lithospheric shortening for two E–W transects (33° 40′ S and 36° S) across the Andes. The models are constrained by local geological and geophysical information. Our results demonstrate a decoupled shortening mode between the brittle upper crust and the ductile lower crust in those areas characterized by a mafic lower crust (36° S transect). In contrast, a more felsic lower crust, such as in the 33° 40′ S transect, results in a coupled shortening mode. Furthermore, we find that differences in lithospheric thickness and the asymmetry of the lithosphere–asthenosphere boundary may promote the formation of a crustal-scale, west-dipping detachment zone that drives the overall deformation and lateral expansion of the orogen. Our study represents the first geodynamic modeling effort in the southern Central Andes aimed at understanding the roles of heterogeneities (crustal composition and thickness) at the scale of the entire lithosphere as well as the geometry of the lithosphere–asthenosphere boundary with respect to mountain building.