Tectonic evolution of the western slope of the Central Andes (36°S) and the building of the Western Andean Thrust System

Résumé fourni par la source

The western slope of the Andes between 18°S to 44°S is marked by the development of west-vergent thrust systems (Western Andean Thrust System, WATS), configuring an overall doubly-vergent orogen. Contrasting hypotheses have been raised regarding the crustal-scale structural style of this system, its long-term evolution during the Neogene, and its relationship with active seismogenic structures. The Western Principal Cordillera at ∼36°S exhibits a unique opportunity to address these questions due to its long-term geological record and presence of active structures. Based on integrated structural geology and U-Pb zircon geochronology, we studied the WATS at ∼36°S, aiming to assess the role of inherited extensional basins and their subsequent inversion in the long-term tectonic evolution of present-day active structures. Our results show that, following the onset of extensional tectonics in the late Eocene–Oligocene, a short-lived rift climax developed in the Abanico Basin between ca. 23 and 20 Ma, associated with the Farallon plate fragmentation, high convergence rates, and slab rollback. This extensional pulse led to the westward expansion of the Abanico Basin to the westernmost Principal Cordillera and Central Depression, resulting in rapid subsidence, high deposition rates, and the development of geochemically primitive sequences, such as the Colbún Formation. A sharp tectonic transition to contractional conditions at ca. 20–19 Ma marked the inception of the WATS by the inversion of former extensional faults, the resumption of plutonic activity and the synorogenic deposition of the Cura-Mallín Formation. This transition correlates with a decrease in previously high convergence rates, which would have reduced heat along the megathrust interface and, consequently, promoted higher coupling. The late Early Miocene event was followed by different uplift pulses during the Neogene, assembling a pop-up crustal structure for the WATS, bounded by inverted extensional faults (Mesamávida, Ancoa, and Las Zorras faults), accompanied later by oblique strike-slip systems. Structural restoration to the pre-contractional stage ( ca. 20 Ma) indicates a ∼17% total shortening in the study area, with the WATS accounting for a ∼10%. Quaternary deformation along the frontal thrust (Mesamávida Fault) is recorded by the deposition and later displacement of the Pliocene-Pleistocene Rodados Multicolores Beds. We propose that this episodic reactivation can be explained by the location of the WATS over a ramp that transfers deformation from west-vergent domains involving deformation of the basin basement, to an east-vergent, thin-skinned domain. Our findings underscore the pivotal role of tectonic inheritance in shaping the architecture and activity of fore-arc fault systems and contribute to a refined understanding of doubly-vergent orogens in subduction settings.

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