Deformation of the western Andes at ~20–22°S: a contribution to the quantification of crustal shortening

The Andes are an emblematic active Cordilleran orogen. It is admitted that mountain-building in the Central Andes, at ~20°S, started ~50–60 Myr ago, along the subduction margin, and propagated eastward. In general, the structures sustaining the uplift of the western flank of the Andes 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, in the Atacama Desert, 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 placing Andean Paleozoic basement over Mesozoic strata; and (2) the west-vergent 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 recorded main deformation can be bracketed sometime between ~68 and ~29 Ma – and possibly between ~68 and ~44 Ma – from dated deformed geological layers, with a subsequent significant slowing-down of shortening rates. Our results therefore reveal that the contribution of the structures of the West Andes can no longer be neglected, in particular at the earliest stages of Andean mountain-building.