Structural evolution of the Atacama fault system, northern Chile: Insights into rheology and strain in a magmatic arc during oblique convergence

Colorado State University, Fort Collins CO

Investigators

Abstract

This project addresses the displacement history and mechanical behavior of the Atacama fault system -- a major tectonic structure that cuts along the Early Cretaceous magmatic arc in the Coastal Cordillera of northern Chile. Intra-arc structures such as the Atacama fault system play a critical role in accommodating plate motion at convergent boundaries, yet significant uncertainty remains surrounding their development. Societal benefits of the research include the development of a new collaboration between geoscience faculty and undergraduate/graduate students from Colorado State University and Pontificia Universidad Catolica de Chile. Ten students from Colorado State University and Pontificia Universidad Catolica de Chile are participating in a joint field-mapping program that comprises a foundation for the research and provides a high-impact learning experience for the students. Funding for this project contributes to broadening of participation of underrepresented groups in an important Science, Technology, Engineering and Mathematics (STEM) discipline by providing support a female doctoral student from Colorado State University. The funding also supports the research program of an early-career principal investigator. The research provides a new framework for understanding economic copper-iron-gold deposits located along the Atacama fault system, and the results will be presented by a field trip that targets the Atacama mining community. The main scientific goals of this project are to: a) determine the timing, magnitude, and rate of slip across the Atacama fault system, b) determine the spatial and temporal relationship between magmatism and deformation along the Atacama fault system, and c) determine the rheology and deformation conditions near the brittle-plastic transition along the Atacama fault system. Reconstructing the slip histories of major geologic structures such as the Atacama fault system is essential for understanding how oblique convergence is partitioned above subduction zones, and information on fault zone rheology and the relationship between magmatism and deformation is needed to understand how and why strain localizes in the overriding plate. The Atacama fault system is one of the most important Andean Mesozoic structures, and it played a major role in controlling economic metal deposits. Despite its significance, the tectonic development of the Atacama fault system is poorly understood due to the lack of detailed geologic mapping and geochronology in the region. During their preliminary fieldwork the project researchers discovered distinct offset markers and numerous pre-, syn-, and post-kinematic igneous units that offer great potential for addressing the magnitude, timing, and rate of slip. They also identified evidence for widespread brittle-plastic deformation, allowing them to evaluate the rheology and peak strength of the fault system. In addition to establishing critical constraints on the Mesozoic tectonic evolution of the central Andean margin, this research will provide important insight into how oblique convergence is accommodated within magmatic arcs. To address the proposal objectives, the research team is using a multidisciplinary research approach that includes geologic mapping, U-Pb geochronology, 40Ar/39Ar and (U-Th)/He thermochronology, microstructural analysis, electron backscatter diffraction, Ti-in-quartz and oxygen isotope geothermometry, and Fourier transform infrared spectroscopy. This research is being carried out in collaboration with Chilean geologists from Pontificia Universidad Catolica de Chile, who are playing a major role in field mapping and structural data analysis. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.