MAGNITUDE AND TIMING OF EARLY CRETACEOUS SLIP ALONG THE ATACAMA FAULT SYSTEM, NORTHERN CHILE

The Atacama Fault System (AFS) in northern Chile formed during intra-arc sinistral shear due to Mesozoic oblique subduction. Displacement estimates along this >1,000 km-long structure vary widely due to a lack of clear piercing points. Here we combine newly recognized field relationships and geochronometric data on pre-, syn-, and post-kinematic intrusions to constrain the magnitude and timing of slip along the El Salado segment of the AFS.

New detailed geologic mapping documents an association of quartzite mylonites intruded by variably mylonitized tonalitic plutons separated on either side of the eastern branch by 20-24 km. Zircon U-Pb ages on the tonalite plutons overlap within error (124.3 ± 1.9 Ma and 123.2 ± 1.8 Ma) and whole-rock XRF data show major element compositions match to within 0.5-1.0%, suggesting these tonalites are offset equivalents. Older plutons in the area (188.3 ± 0.8 Ma, 194.1 ± 2.1 Ma, and 195.8 ± 1.0 Ma) and quartzite bodies adjacent to these plutons lack penetrative strain. Localization of mylonitization at the edges of the tonalitic plutons suggests heat from pluton emplacement facilitated ductile quartzite deformation. As such, we propose the quartzite mylonite-tonalite association records syn- to post-intrusion displacement along the AFS, constraining Early Cretaceous sinistral displacement on the eastern branch to 20-24 km. Two syn-kinematic andesite dikes (119.5 ± 0.9 Ma and 117.5 ± 1.0 Ma) located ~30 km S of the quartzite-tonalite associations crosscut mylonitic foliation in a separate metasedimentary unit but are cut by the brittle core, marking the transition from ductile to brittle deformation. New zircon (U-Th)/He ages along the AFS record cooling through 180°C by ~100 Ma, most likely providing a lower bound on Early Cretaceous arc magmatism and the end of slip on the AFS.

The association of synkinematic plutons with mylonitization suggests a correlation between Early Cretaceous arc magmatism and AFS deformation. The low displacement magnitude for a fault system accommodating oblique plate motion suggests significant sinistral slip was either partitioned along other structures such as the western branch, subsidiary faults, and trench or distributed through the forearc. Alternatively, these data may suggest the convergence angle was less oblique than previously thought.