The impact of inherited crustal features on the evolution of the Southern Central Andes: insights from numerical modelling

BARRIONUEVO, MATÍAS; LIU, SIBIAO; MESCUA, JOSÉ; YAGUPSKY, DANIEL; QUINTEROS, JAVIER; GIAMBIAGI, LAURA; SOBOLEV, STEPHAN V.; RODRIGUEZ PICEDA, CONSTANZA; STRECKER, MANFRED R.

Puerto Madryn

Congreso; XXI Congreso Geológico Argentino; 2022

AGA

RESUMEN ACEPTADOThe non-collisional Southern Central Andes show a latitudinal variation in structural patterns, where second-order variations in the localization and amount of shortening occur between 33° and 36°S. However, the origin of this structural diversity is still not clear. It has been proposed that besides subduction dynamics, the strength of the upper plate imposes an important control on deformation styles of the mountain belt (i.e., Ramos et al., 2004; Oncken et al., 2006). We focus on a segment where subduction parameters (i.e., slab dip of ~30°, age of the subducting oceanic plate, plate-velocity, and climate) are similar throughout the tectonic province; thus, possible effects due to variations of subduction-related parameters can be neglected. The modeling setup and configuration were thus designed to simulate inherited lithospheric-scale anisotropies from previous deformation events, which, we argue, play a pivotal role in guiding deformation. These previous tectonic events include a regional Mesozoic extensional phase that generated a thinner-than-normal crust with more mafic composition south of 35°S (Giambiagi et al., 2012). As a result, this region is more resistant to deformation and experienced less shortening during the Cenozoic Andean compressional regime than the area north of 35°S. The shortening mode in the southern region (Fig. 1 d, e) is dominated by a decoupled shortening mode (simple-shear sensu Allmendinger and Gubbels, 1996) where the deformation in the upper crust is shifted toward the east with respect to lower crustal deformation (Giambiagi et al., 2012). North of 35°S (Fig. 1 a, b), a more felsic lower crust is inferred leading to a coupled or pure-shear shortening mode (Giambiagi et al., 2012). To assess the control of the upper-plate configuration on deformation patterns, we develop geodynamical models representative of two E-W-transects (33°40´S and 36°S), which are based on geological and geophysical information of the area. Our numerical models demonstrate that the composition of the lower crust is an important factor controlling the crustal deformation mode. Particularly, in contrast to the decoupled or simple-shear scenario with dominant lower crustal mafic lithology (south of 35°S; Fig. 1 f), a more felsic composition of the lower crust results in pure-shear deformation (north of 35°S; Fig. 1 c). Furthermore, in our models we aimed at replicating the asymmetry in the lithosphere-asthenosphere boundary due to subduction corner flow, with a thicker lithosphere in the western part of the model, which represents the forearc as a rigid indenter (Farías et al., 2010), and with a thinner lithosphere in the eastern model domain, where the lithospheric mantle is eroded thermally by the corner flow. This setup promotes the establishment of a principal east-vergent crustal-scale detachment (Fig. 1 c and f), which is in accordance with previous models (Ramos et al., 2004; Farias et al., 2010; Giambiagi et al., 2012).