Andean Cross Section at 30ºS: A Window Onto the Tectonic Evolution of a Non-Collisional Orogen

At 30º South, the western Central Andes are comprised by the Coastal Cordillera, a seemingly little-deformed zone containing Mesozoic volcanic arc and back arc-related rocks . To the east, the Vicuña fault separates the Coastal Cordillera from the Principal Cordillera and the Frontal Cordillera. Those two units  display relatively small areas of Mesozoic sedimentary cover that have been preserved despite the uplift which exposes mainly Carboniferous to Triassic plutonics. Further east in the Precordillera, deformation involves increasingly recent Cenozoic sedimentary units.

Based on 2025 campaign field data acquisition and the revision of previous geological maps, we present a structural cross section along a 30ºS, E-W transect and its restoration . During the Mesozoic, extension in the Coastal Cordillera was mainly accommodated by the west-dipping Vicuña fault from the Jurassic to Late Cretaceous. Constraints from pluton emplacement depths and stratigraphic relationships suggest that significant uplift and topographic growth might have interrupted this extension in the Early Late Cretaceous in the Coastal Cordillera. Uppermost Cretaceous syn-orogenic deposits mark the onset of contraction. Upon shortening, the Vicuña fault was folded and reactivated as a west-vergent thrust during the uplift of the Principal Cordillera. This shortening episode also created the present-day relief in the Coastal Cordillera although the timing of this uplift is not well constrained. Subsequently, shortening propagated eastwards into the foreland of the orogen, forming the folds and thrusts of the Precordillera.

Our cross section suggests successive phases of extension and compression which can alternate at variable timescales and operate in different locations. This tectonic evolution raises numerous questions: Which geodynamic factors drive the occurrence of contraction or extension? What is the relationship between surface and deep crustal structures? In a non collisional context, what controls the localization of deformation?...