Understanding the role of structural inheritance and flat slab geometry in Central Andes

Abstract

The Sierras Pampeanas (29 - 35°S) located south of the Altiplano-Puna plateau above the Chilean subduction zone, consist of uplifted foreland basement blocks that are an expression of the eastward propagation of compresive deformation. Their presence is one of the most enigmatic features of the Andes. The formation of these ranges is considered an end member of the thick-skinned foreland deformation style, which involves the deformation of the sedimentary cover and the crystalline basement. At 33°S, the onset of compression occurs at 22Ma, and the change between thin and thick skinned deformation style at 16Ma. However, the mechanism responsible for this evolution remains controversial. Two main hypotheses have been proposed to explain this evolution. The first one atributes the change in foreland deformation style to the setting of the Pampean flat slab at 12 Ma, which is contemporanous to the southward migration and subduction of the Juan Fernandez hotspot ridge at 33S. Alternatively, it has been proposed that the reactivation of pre-existing structures inherited from pre-Neogen tectonic events could better explain the onset of deformation about 10 Ma before the arrival of the flat-slab. To resolve this controversial debate, we have developed a data-driven 3D geodynamic model using the FEM geodynamic code ASPECT. We incorporated the present-day geometrical and thermal configuration of the southern central Andes and the flat-slab from previous models. This approach allowed us to study the structural and thermomechanical factors responsible for the location of deformation in the Sierras Pampeanas (e.g., topography, temperature and composition, strength of the lithosphere and velocity of the plates).  Moreover,  we investigated the role of the geometry of the Nazca plate on the foreland deformation, and proposed a new mechanism ("flat slab conveyor)" that reconciles the timing of the main geological events (onset of shortening, change in tectonics style of deformation of the foreland, growth of the topography, cessation of volcanic activity, uplift of the basement, and propagation of the deformation). This work expands our understanding of how plates interact at convergent boundaries, in particular at the subduction zones, and how and where deformation is expressed at the surface of the the upper continental plate.