STRUCTURAL EVOLUTION OF THE INTRA-ARC ATACAMA FAULT SYSTEM DURING OBLIQUE CONVERGENCE

The sinistral Atacama fault system (AFS) is a major Andean forearc structure that occurs within the Mesozoic Coastal Cordillera arc; however, fundamental questions regarding its development remain. Here we seek to establish the slip history of the AFS and understand how deformation relates to magmatism. We mapped along the northern ~70 km of the El Salado segment of the AFS, documenting the distribution of arc plutons and style of deformation. Petrology, geochemistry, and geo/thermochronology were used to characterize and correlate plutons, and structural data were analyzed to understand progressive changes in the style of deformation.

New zircon U-Pb ages document a major pulse of magmatism from 150–120 Ma, with the plutons most directly tied to AFS ductile deformation intruding between 135–124 Ma. Mylonitic fabrics along the AFS are uniquely associated with the margins of Early Cretaceous plutons, and are cut by late kinematic intrusions at 120–110 Ma. Mylonitic fabrics with sinistral shear sense indicators strike ~8–12° clockwise of the AFS strands, indicating deformation occurred during progressive ductile to brittle sinistral strain. The distinctive syn-kinematic Cerro del Pingo tonalite was mapped on both sides of the El Salado segment. Petrography, geochemistry, and geochronology all overlap within error, and therefore we interpret this pluton is an offset marker along the AFS. The sinistral slip magnitude along the El Salado segment is ~49–60 km and occurred almost entirely between ~134 and ~110 Ma, for a slip rate of ~1.6–2.1 km/Myr. We postulate that thermal softening as a result of Early Cretaceous pluton intrusion into the shallow crust locally elevated geothermal gradients, allowing for ductile deformation at ~5–7 km depths. Spatially variable Early Cretaceous pluton emplacement set up a heterogeneous rheology that produced a segmented system that never evolved into a single regional-scale fault. Zircon (U-Th)/He ages record cooling through ~180°C by 116–99 Ma and relaxation of elevated gradients coeval with the end of slip along the El Salado segment. Together, these data document the development of the AFS as a highly segmented fault system that slipped at a slow rate over ~20 Myr, and was abandoned as plate motion vectors shifted in the middle Cretaceous and arc magmatism migrated eastward.