RHEOLOGICAL BEHAVIOR OF INTRA-ARC STRIKE-SLIP FAULTS: INSIGHTS FROM THE ATACAMA FAULT SYSTEM IN NORTHERN CHILE

Intra-arc strike-slip faults play a critical role in accommodating oblique convergence between oceanic and continental lithosphere. Evaluating intra-arc fault zone rheology and the relationship between magmatism and strain is needed to understand how and why strike-slip deformation localizes within arcs. We present new geologic mapping, geochronology, and microstructural data from the Atacama fault system (AFS), which accommodated sinistral shear along ~1,000 km of the Early Cretaceous arc in northern Chile. Mylonitization along the AFS between 24.8°S and 27.5°S is spatially associated with synkinematic plutons ~139–119 Ma, whereas segments that cut Jurassic and older rocks lack penetrative strain. Along-strike variations in geothermal gradients set up by Early Cretaceous plutonism resulted in a spatially heterogeneous rheology and development of a segmented fault system. The highest strain AFS fabrics are localized in Paleozoic metasedimentary rocks adjacent to synkinematic plutons and within pluton alteration zones dominated by fine-grained polyphase mixtures. Where quartz forms an interconnected weak phase in AFS mylonites, brittle-plastic deformation is associated with a quartz dynamically recrystallized grain size of ~7–8 µm, suggesting peak stresses lower than those previously documented in strike-slip faults outside of arcs. The strength of the AFS is consistent with a rheological model for a strike-slip fault with normal (Byerlee) friction, hydrostatic fluid pressure, and high geothermal gradients (~50°C/km) associated with synkinematic magmatism and a shallow (~5–7 km) brittle-plastic transition. Parts of the fault system distal from synkinematic plutons likely record peak strength that was 30–40% greater. We propose that high geothermal gradients and the presence of weak zones such as metasedimentary rocks and pluton alteration zones govern the strength of intra-arc faults and strain localization within arcs.