RHEOLOGY AND STRAIN LOCALIZATION ALONG THE INTRA-ARC ATACAMA FAULT SYSTEM, NORTHERN CHILE

Intra-arc strike-slip faults play a critical role in accommodating oblique subduction convergence, and exhumed intra-arc faults may provide key insight into how and why strike-slip deformation localizes within arcs. We present a detailed macro- and microstructural analysis of the brittle-plastic, intra-arc Atacama fault system (AFS) in northern Chile, which records sinistral shear associated with Early Cretaceous oblique subduction. Mylonitic strain along the AFS between 24.7°S and 27.5°S is localized in synkinematic Early Cretaceous granitoids and Paleozoic metasedimentary rocks along pluton margins, whereas segments that cut Jurassic granitoids and volcanic rocks record entirely brittle deformation at exposed structural levels. Synmylonitic veins and hydrous phases (mica, epidote, actinolite) are abundant along the AFS, indicating widespread fluid involvement during mylonitization. Mylonitic granitoids record strain softening associated with hydrothermal alteration and breakdown of feldspar, which resulted in development of a fine-grained polyphase matrix (feldspar+quartz±mica). Strain in granitoid protomylonites is focused in quartz lenses that record dislocation creep, with minor feldspar deformation accommodated primarily by fracturing. By contrast, strain in granitoid mylonites and ultramylonites is typically focused in a fine-grained polyphase matrix that deformed via diffusion creep mechanisms and is notably weaker than pure quartz domains. Mean quartz dynamically recrystallized grain size is typically ≤40 µm, and brittle-plastic deformation is associated with a grain size of ~7–8 µm, suggesting peak stresses lower than those previously documented in strike-slip faults outside of arcs. Along-strike variations in geothermal gradients set up by Early Cretaceous plutonism and discontinuous exposures of metasedimentary rocks resulted in spatially heterogeneous rheology and development of a segmented fault system. Our work demonstrates that high geothermal gradients and the presence of weak metasedimentary rocks and pluton alteration zones largely govern the strength and strain localization along the AFS near the brittle-plastic transition.