Faults connect the Earth’s crust with hydrosphere and biosphere and are the site of crustal deformation including earthquakes. Documenting fault zone behavior in space and time informs the mechanical, chemical, and thermal conditions of crustal deformation and illuminates interactions between plate tectonics and Earth surface processes. National Science Foundation (NSF) grant EAR-1419828 to lead PI Ault (Utah State University), with co-PIs Evans (Utah State University), Reiners (University of Arizona), and Shuster (Berkeley Geochronology Center), and with collaborators from the United States Geological Survey and institutions across the US, developed hematite (U-Th)/He thermochronometry for exhumed fault rocks to provide timing constraints on past fault slip. Hematite is a mineral that is common in fault zones and generally precipitates from oxidizing fluids circulating in higher permeability fault rocks. Hematite is amenable to (U-Th)/He thermochronometry, which is a radioisotopic technique that quantifies the temperature evolution of mineral or rock and is used to place specific ages on Earth materials and processes.

            Collective results from this project demonstrate that integrated hematite micro- to nano-scale textures and hematite (U-Th)/He thermochronometric data patterns from fault rocks can document both earthquake (fast slip) and creep (slow slip) in the rock record. Interpretation of hematite (U-Th)/He dates as directly dating deformation and inferring fault slip rates hinges on textural observations, the temperature sensitivity or closure temperature of the target aliquots, and the post hematite formation thermal history as rocks are exhumed to Earth’s surface. We developed a workflow to address this problem yields constraints on hematite aliquot grain size distribution and textural characterization using scanning electron microscopy to inform closure temperature, and compares hematite (U-Th)/He thermochronometry data patterns with apatite (U-Th)/He and apatite fission-track thermochronometry from the host rock to characterize hematite He loss during and after hematite formation. This approach has been applied to date fluid flow, hematite mineralization, and deformation in a diverse suite of fault systems including the Wasatch fault zone, UT; eastern Denali fault zone, Yukon Territory, Canada; southernmost San Andreas fault zone in Mecca Hills, CA; reactivated structures in the central Front Range, CO; the Kuh-e-Faghan fault system in central Iran; faults in the Rio Grande rift, NM; and fault-related fissure fills in Gower Peninsula, Wales.

The approach developed in this project is transportable to exhumed hematite-bearing fault zones globally. Dating brittle fault slip is a research frontier integral for characterizing numerous upper crustal processes. Direct, robust, and efficiently obtained timing constraints are necessary to reconstruct structural-tectonic histories, mountain building processes, and landscape evolution, as well as to document ancient seismicity histories to assess modern seismic hazards, earthquake forecasting, and fault mechanics. To date, results have been disseminated via (1) nine peer-reviewed publications, four of which are led by students and one led by a postdoctoral fellow; one publication in review; and five manuscripts in preparation, four of which are led by students; (2) 27 presentations, 17 of which are led by students, at national and international meetings; and (3) 10 talks at colleges and universities across the country.

This project supported an early career, female researcher, as well as provided hands-on field and analytical training for one PhD student, three MSc students, two undergraduate research assistants, and partial support for a postdoctoral fellow. Throughout the project, the research team has interacted with underserved groups including (1) annual engagement with K-12 students at USU Department of Geosciences Rock and Fossil Day through an interactive fault rock and earthquake activity, reaching >500-1300 local K-12 students and their families; (2) initial development of a middle school earthquake science program involving classroom, field, and laboratory activities that reached over 100 5^th and 5^th graders at the Promontory School for Expeditionary Learning in Perry, UT; (3) tutoring 5^th grade students at the Edith Bowen Laboratory Elementary School on Utah State University campus; (4) public engagement through nine public talks. Each of these efforts integrates research results supported by this NSF award to magnify and enhance research impact.

Last Modified: 12/31/2019
Modified by: Alexis K Ault