Tania Habel, Martine Simoes, Robin Lacassin , Daniel Carrizo, Germán Aguilar

The Andes are an emblematic active Cordilleran orogen. It is admitted that mountain-building in the Central Andes at ~20°S started by Late Cretaceous to Early Cenozoic along the subduction margin, and propagated eastward. In general, the structures sustaining the uplift of the West Andean flank are dismissed, and their contribution to mountain-building remains poorly solved. Here, we focus on two sites along the western Andes at 20–22°S, where structures are well exposed. We combine mapping from high-resolution satellite images with field observations and numerical trishear forward modeling to provide quantitative constraints on the kinematic evolution of the western Andes. Our results confirm the existence of two main structures: (1) the Andean Basement Thrust, a west-vergent thrust system placing Andean Paleozoic basement over Mesozoic strata; and (2) a series of west-vergent folds pertaining to the West Andean Fold-and-Thrust-Belt, deforming primarily Mesozoic units. Once restored, we estimate that both structures accommodate together at least ~6–9 km of shortening across the sole ~7–17 km-wide outcropping fold-and-thrust-belt. This multi-kilometric shortening represents only a fraction of the total shortening accommodated along the whole western Andes. The timing of the main deformation recorded in the fold-and-thrust-belt can be bracketed sometime between ~68 and ~29 Ma – and possibly between ~68 and ~44 Ma – from dated folded geological layers, with a subsequent significant slowing-down of shortening rates. Even though negligible when compared to total shortening across the whole orogen, the contribution of the structures of the West Andes has been likely significant at the earliest stages of Andean mountain-building before deformation was transferred eastward.