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 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.