The influence of variations in crustal composition and lithospheric strength on the evolution of deformation processes in the southern Central Andes: insights from geodynamic models

Abstract

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-kmthick 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, westdipping 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.

Controls of the Foreland Deformation Pattern in the Orogen‐Foreland Shortening System: Constraints From High‐Resolution Geodynamic Models

“…The values of thermal parameters are within the range expected for crustal and mantle materials (e.g., Barrionuevo et al., 2021; Sobolev et al., 2006). Radiogenic heat production is 1.0 μW m −3 in the felsic crust and 0.3 μW m −3 in the mafic crust.…”

Across-strike asymmetry of the Andes orogen linked to the age and geometry of the Nazca plate

“…Changes in the slab shape and age accompany changes in the force balance of the subduction zone as well as the shape of the mantle wedge and lithosphere-asthenosphere boundary underneath the orogen, all of which form a bottom-up control on the direction of tectonic transport and orogen structure (Schellart, 2017;Barrionuevo et al, 2021). Together with our results, this mechanism suggests that the orogenic wedge asymmetry in non-collisional orogens is subject to subduction zone dynamics.…”

Distribution of Temperature and Strength in the Central Andean Lithosphere and Its Relationship to Seismicity and Active Deformation

“…South of our study region, Rodriguez Piceda et al (2020) found that the highest horizontal surface strain rates in the southern Central Andes occur in areas with abrupt variations in crustal density, suggesting an influence of crustal composition and associated rheological properties on short-term deformation. Furthermore, Barrionuevo et al (2021) have shown by means of geodynamic modeling how such variations in crustal composition influence the deformation mode the orogenic wedge.…”