Westward expansion of the andean orogenic wedge: tectonostratigraphy and analogue modeling of the Western Cordillera of Northern Chile

Abstract

Orogeny in the Central Andes is the product of active subduction below the western margin of South America. The Cenozoic building of the Western Cordillera has been ascribed to magmatic addition and volcanism associated with subduction of an oceanic plate, but more importantly, to processes related to intracontinental, Ampherer-type, subduction. Despite the increasing geological and geophysical data published in the past ten years, the polarity of this subduction and main direction of tectonic transport (or vergence) are still strongly debated in the case of the Central and Southern Central Andes. This suggests that it is necessary to revisit this evidence in the context of mechanical models that illustrate how orogens work . These models should include the mechanisms of material distribution and accommodation operating below convergent orogens, and their relationship with crustal-scale structures and boundary conditions (direction of subduction) comparable to those operating below the Andes. In this work, a combination of tectonostratigraphy and analogue modeling of doubly-vergent subduction wedges is presented to approach the aforementioned problem. The tectonostratigraphy of the Western Cordillera of northern Chile at ~19.5°S suggests that the range is underlain by a crustal-scale pop-up, formed by tectonic shortening and subsequent thickening, in combination with magmatic addition. U-Pb ages in deformed rock units suggest that contraction took place as early as 49 Ma (middle Eocene), and that deformation was active during most of the Neogene. The synchronicity between the deformation in the Western Cordillera and the tectonic events that took place in the Subandean Sierras suggests that they were mechanically coupled. On the other hand, the analogue model provides a cross-sectional demonstration of the distribution and deformation of material accreted into the wedges within a setting similar to the Andean geodynamic scenario. In a single model, one subduction wedge is analogue to the Coastal Cordillera whilst a second subduction wedge is analogue to the Western Cordillera. Each wedge grows above a respective subduction plane, but the wedge analogue to the Western Cordillera grows by accretion of a much thicker section. A PIV (Particle Image Velocimetry) analysis applied to the model allowed the visualization of instantaneous velocity, strain rate and kinematic vorticity of the material inside the wedges. Movements inside the wedges are predominantly lateral, with decreasing velocities in the same sense of subduction. Deformation in the model is partitioned and strongly asymmetrical. Above the subduction plane (AKA pro-side), there is a zone of distributed deformation, analogue to a foreland fold-and-thrust belt. Towards the back (retro-side), a single, high-angle shear zone develops synchronously, thrusting material towards the rear at a slower rate compared to propagation of deformation towards the pro-side. This deformation zone begins to expand towards the rear when cumulative subduction and shortening on the pro-side are high, overriding a flat zone between the two wedges, which in the model resembles the forearc Central Valley of Chile. Considering all limitations of the analogue model, the results are contrasted with the first-order structure of the Central Andes. The comparative analysis shows similarities between pro-side structure development and the tectonic evolution of fold-and-thrust belts of Argentina and Bolivia. Regarding the Andes of northern Chile, the Miocene propagation of the Western Cordillera monoclines is similar to the rearward propagation of the retro-side of the modeled wedge. The correspondence allows to conclude that the Western Cordillera could be underlain by a westward propagating structure, similar to the one developed in the rear of this type of model, growing by accommodation of material incorporated below the orogen from the east.