Magnitude, timing, and rate of slip along the Atacama fault system, northern Chile: implications for Early Cretaceous slip partitioning and plate convergence

Abstract

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 map the distribution of plutons and mylonitic deformation along the northern c. 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 c. 134–132 Ma Cerro del Pingo Complex are separated by 34–38 km, and mylonites associated with a synkinematic c. 120–119 Ma tonalite are separated by 20.5–26 km. We interpret leucocratic intrusions to be separated across the western branch by c. 16–20 km, giving a total slip magnitude of c. 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 c. 133–110 Ma at a slip rate of c. 2.1–2.6 km Myr –1 . 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: Full U-Pb, (U-Th)/He, petrographic, and structural data with locations is available at https://doi.org/10.6084/m9.figshare.c.5262177 Thematic collection: This article is part of the Isotopic dating of deformation collection available at: https://www.lyellcollection.org/cc/isotopic-dating-of-deformation