Upper-plate Shortening and Mountain-building in the Context of Mantle-driven Oceanic Subduction | τeκτoniκa

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A contribution to the quantification of crustal shortening and kinematics of deformation across the Western Andes ( ∼ 20–22° S)

<p><strong class="journal-contentHeaderColor">Abstract.</strong> The Andes are an emblematic active Cordilleran orogen. Mountain building in the Central Andes (<span class="inline-formula">∼20</span><span class="inline-formula">^∘</span> S) started by the Late Cretaceous to early Cenozoic 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 constrained. Here, we focus on two sites along the Western Andes at <span class="inline-formula">∼20</span>–22<span class="inline-formula">^∘</span> 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 investigated field sites. When upscaling our local field interpretations to the regional scale, we identify 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 thrusts pertaining to the West Andean Thrust System, deforming primarily Mesozoic units. From our interpreted sections, we estimate that both structures together accommodate at least <span class="inline-formula">∼6</span>–9 km of shortening across the sole investigated <span class="inline-formula">∼7</span>–17 km wide field sites. This multi-kilometric shortening represents only a fraction of the total shortening accommodated across the whole Western Andes. The timing of the main deformation recorded in the folded Mesozoic series can be bracketed between <span class="inline-formula">∼68</span> and <span class="inline-formula">∼29</span> Ma – and possibly between <span class="inline-formula">∼68</span> and <span class="inline-formula">∼44</span> Ma – from dated deformed geological layers, with a subsequent significant slowing-down of shortening rates. Even though the structures forming the Western Andes only absorbed a small fraction of the total shortening across the whole orogen, their contribution was relatively significant at the earliest stages of Andean mountain building before deformation proceeded eastward.</p>