Magmatic arcs may play a major role in the initiation, behavior, and abandonment of intra-arc strike-slip systems. Here we present zircon U-Pb and (U-Th)/He geochronology/thermochronology with new mapping to relate Coastal Cordillera arc magmatism to sinistral shear along the Atacama fault system (AFS) in northern Chile. New dates from 18 intrusions along the AFS between 24.6°S and 27°S compiled with published data record a minor Early Jurassic magmatic pulse , broad latest Jurassic to Early Cretaceous (150-120 Ma) pulse, and a minor younger (120-105 Ma) pulse. Mylonitization occurred only along the margins of Early Cretaceous plutons and surrounding Paleozoic metasedimentary rock, whereas Jurassic plutons and metasedimentary rocks away from Early Cretaceous plutons lack mylonitic fabrics. Early Cretaceous magmatism facilitated AFS deformation by thermally weakening the crust with elevated geothermal gradients that enabled mylonitization to take place at~5-to 7-km depths and low stresses. Spatial variability of pluton emplacement produced significant rheological heterogeneity, giving rise to a highly segmented fault system that did not originate as a regional-scale shear zone. Synkinematic dikes (~120-117 Ma) cut mylonitic fabrics, and a postkinematic dike (~110 Ma) records the end stages of slip. The cessation of slip coincided with cooling below~180°C at~116-99 Ma as arc magmatism migrated eastward and geothermal gradients relaxed, coeval with a major reorganization in plate motion and the onset of seafloor spreading in the south Atlantic.

An extensive system of NW striking faults constitutes a major tectonic feature of the Coastal Cordillera in northern Chile, but fundamental questions remain about timing and kinematics of these structures. We present new geologic mapping and geochronology that provide insight into the structural evolution and tectonic significance of the Taltal fault system (TFS). The TFS displaces the Early Cretaceous arc-parallel Atacama fault system (AFS) with~10.6 km cumulative offset across a~15 km wide zone. Brittle fault data demonstrate that the TFS is vertical to steeply NE dipping with an average sinistral slip vector plunging 11°from the NW, compatible with E-W shortening. Two late Early Cretaceous dikes cut the AFS but are cut by TFS faults, and synkinematic calcite on a TFS strand yielded a U-Pb calcite date of 114.1 ± 7.0 Ma. These data demonstrate that the AFS was abandoned and deformation (re) initiated on the TFS between~114-107 Ma, with continued slip after intrusion of the Tropezón (~110 Ma) and Librillo (106-101 Ma) plutonic complexes. Emplacement of a~146 Ma rhyolite dike along the main Taltal fault and 141 ± 11 Ma calcite mineralization in the fault core suggests that a precursor structure influenced magma emplacement and fluid flow in the Late Jurassic/Early Cretaceous, supporting the hypothesis that the TFS reactivated long-lived inherited crustal weaknesses. The Early Cretaceous shift from arc-parallel shear to slip on the TFS and E-W shortening shortly preceded migration of the magmatic arc and records a change in the Chilean margin subduction dynamics.

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Displacement estimates along the Atacama fault system (AFS), a crustal-scale sinistral structure that accommodated oblique convergence in the Mesozoic Coastal Cordillera arc, vary widely due to a lack of piercing points. We mapped the distribution of plutons and mylonitic deformation along the northern ∼70 km of the El Salado segment and use U-Pb geochronology to establish the slip history of the AFS. Along the eastern branch, mylonitic fabrics associated with the synkinematic ∼134–132 Ma Cerro del Pingo Complex are separated by 34–38 km, and mylonites associated with a synkinematic ∼120–119 Ma tonalite are separated by 20.5–25 km. We interpret leucocratic intrusions to be separated across the western branch by ∼16–20 km, giving a total slip magnitude of ∼54 ± 6 km across the El Salado segment. Kinematic indicators consistently record sinistral shear and zircon (U-Th)/He data suggest dip-slip motion was insignificant. Displacement occurred between ∼133–110 Ma at a slip rate of ∼2.1–2.6 km/Myr. This slip rate is low compared to modern intra-arc strike-slip faults, suggesting (1) the majority of lateral slip was accommodated along the slab interface or distributed through the forearc or (2) plate convergence rates/obliquity were significantly lower than previously modeled.Supplementary material including full U-Pb, (U-Th)/He, petrographic, and structural data with locations is available at https://doi.org/10.6084/m9.figshare.c.5262177.