NEW CONSTRAINTS ON 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 Mesozoic oblique subduction beneath northern Chile. The southern portion of the Paposo fault juxtaposes mylonitized granites against Jurassic volcanic and sedimentary rocks. Geologic mapping along a 1.25 km-long transect across the ductile root of the Paposo fault documents a strain progression towards the core of the fault. Moving from E to W, Early Cretaceous and Late Jurassic granites grade from unstrained to a 350 m-thick zone containing discrete mylonitic bands. This zone of variably strained granite transitions to a 650-1100 m-thick pervasive high strain zone which contains a 200-550 m-thick band of hydrothermally-altered ultramylonite and locally mylonitized cataclasite, indicating that fluids played an important role in brittle-ductile deformation. This mylonitized cataclasite contains oblate clasts and S>L tectonite fabrics, suggesting transpressional flattening. Across the transect, mylonitic foliations and lineations have a mean orientation of 043/62 SE and 18/211, respectively. Most SE-dipping mylonitic fabrics record sinistral-reverse shear, but in several areas, symmetric microstructures suggest a component of pure shear. Near the core of the fault, a 250 m-thick protomylonite zone is overprinted by a 50 m-thick illite-rich, subvertical gouge zone that strikes 018 and records sinistral S-C fabrics.

The upper age limit of deformation is constrained by zircon U-Pb ages on the mylonitized granites (138.8 ± 1.6 Ma and 151.1 ± 1.8 Ma) and a mylonitized aplite dike with ages ranging from ~137-148 Ma. A zircon (U-Th)/He cooling age of 116.6 ± 5.4 Ma from the ~151 Ma granite provides a lower age limit for mylonitic deformation. Together these data constrain deformation to the Early Cretaceous, similar to the age of deformation along other segments of the AFS. Regionally, the Paposo fault is arcuate, trending NNW-SSE in the north and NNE-SSW in the south. Previous studies have observed sinistral transtension in the north, whereas we observe sinistral transpression in the south. We propose that transtension and transpression along the AFS are controlled by the arcuate geometry, and both are compatible with sinistral simple shear along the N-S-trending magmatic arc.