THREE PHASES OF JURASSIC-CRETACEOUS SINISTRAL SHEAR IN THE EARLY ANDEAN MAGMATIC ARC, NORTHERN CHILE

The Coastal Cordillera of northern Chile exposes the upper crustal remnants of the Jurassic and Cretaceous arc emplaced during the early stages of the Andean Orogeny. Near Taltal, Chile (25.4°S), a polyphase deformation history is recorded by multiple episodes of ductile and brittle deformation in arc rocks. We use geologic mapping, structural analysis, and geochronology to untangle how the style of deformation evolved and how this deformation may have connected to plate margin dynamics.

The ductile Matancilla shear zone (MSZ), of steeply WNW-dipping mylonite with shallowly-plunging stretching lineations, is as much as 1.4 km thick and is restricted to the thermal aureole of a Middle Jurassic pluton (~169 Ma). Sinistral kinematic indicators in the MSZ may record partitioning of the strike-slip component of oblique subduction and could constrain the Phoenix-Farallon spreading center to north of latitude ~25° S at that time. The MSZ is overprinted by the N-striking, subvertical Atacama fault system (AFS) that accommodated margin-parallel sinistral shear in the Early Cretaceous (~133–110 Ma). A total 54 ± 6 km of AFS sinistral shear is split between a brittle-only western branch and a brittle-ductile eastern branch, where mylonite is restricted to aureoles of plutons that intruded synkinematic to the fault system and cataclastic brittle deformation is prevalent away from these plutons. As magmatism waned in the Coastal Cordillera in the late Early Cretaceous, the AFS was overprinted by the Taltal fault system (TFS) of distributed NW-striking brittle faults with a cumulative ~10.6 km of sinistral displacement (after ~110 Ma). TFS fault kinematics suggest margin-oblique shortening, which may reflect a change in plate boundary dynamics as magmatism migrated eastwards from the Coastal Cordillera and plate convergence shifted towards the dextral obliquity of the modern Andean arc.

Our findings demonstrate that sinistral shear was partitioned into the early Andean magmatic arc throughout the Middle Jurassic to late Early Cretaceous, but the style of deformation changed through time. At least three phases of sinistral shear partitioning are recognized in the Coastal Cordillera during this timeframe, and the shifts between deformation styles may reflect changes in subduction zone dynamics as the margin evolved.