The effects of differential shortening and competent blocks on the structural development of Andean fold-and-thrust belts at 32°-34°S: Insights from analogue models

Abstract

Andean fold-and-thrust belts (FTBs) in the western border of South America show important along-strike variations in their structural style (thin- or thick-skinned) and shortening magnitudes, among others. These variations can arise from multiple factors, such as depth-to-detachment, geometry, and thickness of underlying basins, structural highs, the competence of the units involved in deformation, pre-orogenic structure of the basement, the rheological state of the upper crust or latitudinal gradients in shortening, among others. We carry out a set of new analogue models to understand how two of these factors affect the development and the geometry of the thrust systems within the Andean Principal Cordillera and adjacent fold-and-thrust belts at 32°-34°S (La Ramada and Aconcagua FTBs). In this contribution, we present the results of analogue modeling of doubly-vergent thrust systems with a built-in heterogeneity that allow us to evaluate the influence of differential shortening and presence of competent blocks (of relatively higher cohesion) in the generation, evolution, and structural setting of Andean FTBs. We highlight that despite the similar pattern of differential shortening in the models, the presence of the competent block has a significant influence on the structural configuration and leads to greater topographic uplift. Differential shortening plays a role in accommodating deformation, but when a competent block is present, the faults bounding the block control the deformation, producing a notable curvature of the FTBs. Summarizing, the presence of a competent block significantly influences the structural configuration within a doubly-vergent thrust system, emphasizing the role of inherited architecture in the orogenic growth of the Southern Central Andes.