The role of the upper and lower plates in the Andean orogeny: Insights from field observations to numerical modeling

BARRIONUEVO, M.; LIU, S.; MESCUA, J.F.; BABEYKO. A,; SOBOLEV, S.; GIAMBIAGI, L.B.; QUINTEROS, J.; YAGUPSKY, D.

Concepción

Congreso; XV Congreso Geológico Chileno; 2018

Sociedad Geológica de Chile

The Southern Central Andes shows a pronounced latitudinal variation in the structural pattern with a decrease in shortening and orogenic width from 30º S to 36º S. One of the most important factors invoked for controlling the deformation is the subduction dynamics with a slab angle increase from a flat slab segment between 27º-33ºS to a normal subduction angle in 33º-36ºS. Despite this main control there are other factors that can control deformation, such as the strength of the upper plate. This have been proposed previously for this zone as well as for the Altiplano-Puna region. Whilst in the plateau region previous numerical studies confirmed the hypothesis that lithospheric and sediment strength of upper plate have effects on the deformation patterns, in the 30-36ºS segment it was not tested.Based on field geological and geophysical observations along the 36º S transect we have constructed a conceptual model which reflects the present-day orogenic structure. In this zone the Malargüe fold-and-thrust belt developed under Andean contraction as a thick-skinned belt. Here the deformation is accommodated by the interaction between pre-andean faults (related to the rifting phase in Neuquén basin) which are inverted and newly created reverse faults. This rifting phase could have changed the structure of the upper plate by thinning of the crust and corresponding strengthening of the lithosphere.We firstly apply large-scale thermo-mechanical 2D models to evaluate the role of the changing subducting slab angle. In the model, the kinematic boundary condition is prescribed to the slab to mimic subduction with controlled (and changing) subduction angle. In the second part we study the influence of spatial material property variations of the South American plate on the Andean deformation. In particular, we investigate deformation behavior with different compositions and geometries of upper plate during the shortening. Since the sediment layer is much thinner than the entire lithosphere, there should be sufficient number of finite elements to track the sediment deformation in the numerical simulation. Therefore, we further explore the development of thin-skinned and/or thick-skinned belts in the uppermost crust by using high-resolution (less than 500 m per element on the grid) shortening models.