The geometry, kinematics, and timing of deformation along the southern segment of the Paposo fault zone, Atacama fault system, northern Chile

The Paposo fault zone is a major brittle-ductile strand of the Atacama fault system (AFS), which records sinistral shear associated with Cretaceous oblique subduction beneath northern Chile. New detailed geologic mapping, macro- and microstructural data, and zircon geo/thermochronology reveal insight into the structural evolution of the southern portion of the Paposo fault. The core of the Paposo fault is defined by a ~50 m-thick zone of illite-rich gouge that dips steeply ESE and juxtaposes fractured but unfoliated Early Jurassic tonalite west of the fault against mylonitic Late Jurassic to Early Cretaceous granitoids east of the fault. The mylonite zone east of the fault is ~0.7–1 km thick and includes a ~100–500 m-thick band of hydrothermally-altered ultramylonite derived from Latest Jurassic (146.5 ± 1.5 Ma) to Early Cretaceous (138.5 ± 1.6 Ma) granodiorite. West of this ultramylonite zone, a ~150–350 m-thick zone of younger tonalite (136.0 ± 1.8 Ma) parallels the Paposo fault and grades from protomylonite to mylonite L tectonite fabrics most likely record a significant component of zone-normal flattening. The upper age limit of deformation is constrained to be younger than the Late Jurassic/Early Cretaceous granodiorite. Hydrothermal alteration and development of high-strain zones in the Late Jurassic/Early Cretaceous granodiorite are locally associated with mafic dikes that do not cut the younger protomylonitic tonalite, indicating that most of the hydrothermal alteration and mylonitic strain occurred between ~139 Ma and 136 Ma. The Paposo fault gouge zone formed between 150 °C and 200 °C based on clay mineralogy and the illite Kübler index. The timing of gouge formation most likely overlaps with cooling below ~180–190 °C recorded by a zircon (U-Th)/He date of 116.6 ± 6.2 Ma from the Late Jurassic/Early Cretaceous granodiorite. Together these data constrain brittle and ductile deformation to the Early Cretaceous, similar to the age of deformation along other segments of the AFS and coeval with co-spatial ar