Abstract

An extensive system of NW striking faults constitutes a major tectonic feature of the Coastal Cordillera in northern Chile, but fundamental questions remain about timing and kinematics of these structures. We present new geologic mapping and geochronology that provide insight into the structural evolution and tectonic significance of the Taltal fault system (TFS). The TFS displaces the Early Cretaceous arc-parallel Atacama fault system (AFS) with similar to 10.6 km cumulative offset across a similar to 15 km wide zone. Brittle fault data demonstrate that the TFS is vertical to steeply NE dipping with an average sinistral slip vector plunging 11 degrees from the NW, compatible with E-W shortening. Two late Early Cretaceous dikes cut the AFS but are cut by TFS faults, and synkinematic calcite on a TFS strand yielded a U-Pb calcite date of 114.1 +/- 7.0 Ma. These data demonstrate that the AFS was abandoned and deformation (re) initiated on the TFS between similar to 114-107 Ma, with continued slip after intrusion of the Tropezon (similar to 110 Ma) and Librillo (106-101 Ma) plutonic complexes. Emplacement of a similar to 146 Ma rhyolite dike along the main Taltal fault and 141 +/- 11 Ma calcite mineralization in the fault core suggests that a precursor structure influenced magma emplacement and fluid flow in the Late Jurassic/Early Cretaceous, supporting the hypothesis that the TFS reactivated long-lived inherited crustal weaknesses. The Early Cretaceous shift from arc-parallel shear to slip on the TFS and E-W shortening shortly preceded migration of the magmatic arc and records a change in the Chilean margin subduction dynamics.