Geologic mapping of the Atacama and Taltal Fault Systems, Northern Chile

Abstract

The Atacama fault system (AFS) is a major brittle-ductile structure in northern Chile that accommodated left-lateral displacement during Mesozoic oblique convergence. The AFS is sinistrally offset by the NW-striking Taltal fault system (TFS) near the town of Taltal, but the absolute timing of slip along both fault systems is unclear. A well constrained deformation chronology of these structures is essential for understanding the Mesozoic tectonic evolution of the central Andean margin. Here we present new geologic mapping, structural data, and geo/thermochronometric data that constrain the kinematics and timing of deformation along both the AFS and TFS. Our geologic mapping clarifies the extent and generations of ductile deformation along the AFS. Mylonitic deformation is most strongly expressed in Paleozoic quartzite adjacent to locally protomylonitic Early Cretaceous plutons that intruded along the AFS. We observe sparse mylonite in Jurassic plutons with variable fabric orientations, while the mylonite near Early Cretaceous plutons is systematically parallel to the AFS and records sinistral shear sense indicators. A plutonic complex ranging from 131 Ma to 124 Ma is associated with different pulses of mylonitic deformation along the AFS. Along the brittle core of the AFS, mylonites are overprinted by cataclasite and gouge recording sinistral slip on NNW-SSE striking subvertical faults. An andesite dike (~110 Ma zircon U-Pb), cuts the easternmost AFS strand but is offset 148m in a left-lateral sense by an AFS strand further west; we interpret this dike to have intruded in the waning stages of AFS brittle deformation. Other dikes that cut AFS cataclasite (106.7 ± 8.5 Ma zircon He) are offset by NW-striking brittle faults of the TFS. We identify previously unmapped strands of the TFS that indicate cumulative offset of ~11.5 km of the fault system and revise previous displacement estimates to 8km of offset on the main strand of the Taltal fault. Brittle kinematic data shows that TFS strands dip steeply NE, with shallowly raking slickenlines that plunge slightly NW on average, indicating a component of transtension. We interpret that the transition from left-lateral deformation on the N-S striking AFS to the NW-SE striking TFS indicates a change in the kinematic regime to accommodate a more E-W component of shortening.