EAR-PF: Fault-rock trapped-charge and 4He/3He thermochronometry: new paleothermometers to assess scales and rates of fault slip

Odlum, Margaret, Austin TX

Investigators

Abstract

Dr. Margaret Odlum has been awarded the NSF EAR Postdoctoral Fellowship to carry out research as well as a professional development plan at Utah State University (USU) under the mentorship of Professors Alexis K Ault and Tammy Rittenour. The goal of the project is to develop new approaches to fingerprint past earthquakes in fault rocks. Pinpointing evidence of ancient seismicity is important for understanding earthquake processes and mechanics and building a more complete record of fault activity, which collectively inform hazard assessment in seismically active fault systems. Most of the energy during an earthquake is given off as heat, which imparts a thermal and/or chemical signature in fault rocks. Proposed research uses coupled luminescence properties of minerals and He isotope thermometry integrated with textural analysis to access the thermal record of earthquake events across the full range of seismic slip rates, as well as temporal and spatial scales to inform fault slip processes. Complementary age information provided by these methods will provide a more continuous record of deformation over historical to geological timescales which is essential to understand earthquake cycles in a particular fault. This research will be conducted on the Wasatch and Hurricane faults in Utah. These are young, seismically active faults that are adjacent to Utah’s two largest population centers along the Wasatch Front and in the Saint George region, respectively. New approaches developed here will create pathways for researchers to quantify the spectrum of fault slip behavior in other fault systems. Dr. Odlum’s professional development plan is aimed at growing STEM workforce and engaging women, an underrepresented group in Utah in STEM, in earthquake science research and geotechnical skill-building. A seismic hazards module developed in partnership with the Utah Geological Survey will impact students enrolled in the new USU GeoPaths program, and reach a wider undergraduate community by sharing the module with other universities located in the Intermountain Seismic Zone and Wasatch Front region, as well as online through the USU Geosciences webpage and GeoMinutes YouTube channel, thus extending the reach and impact of this work. The goal of this research is to develop new approaches to identify, quantify, and date frictional heating caused by seismic slip in the rock record across the full range of seismic slip rates (micrometers/sec to tens of meters/sec). Much (~90%) of the energy budget during an earthquake goes into work done overcoming frictional resistance and dissipates as heat. The magnitude of temperature rise across a fault is a function of slip velocity, material properties, and strain localization. Frictional heat, in turn, impacts these factors and the interplay between variables controls fault strength during the seismic cycle. Transient, elevated temperatures impart textural and/or geochemical signatures in fault rocks. This project will combine trapped-charge analysis on quartz and feldspar with hematite and apatite 4He/3He thermochronometry of fault surfaces and underlying host-rock to recover paleotemperatures associated with seismic slip. The seismically active Wasatch and Hurricane faults in Utah will be the first field sites for applying this novel approach. Both seismogenic normal faults have textural evidence of past coseismic frictional-heat, bulk hematite (U-Th)/He dates that reflect thermal resetting or mineralization during earthquake slip within the viable age range for trapped-charge thermochronometry (<0.5 Ma), and pose seismic risk to large population centers in Utah. Coupled paleotemperature and textural information from fault surfaces will inform dynamic weakening mechanisms operative during coseismic slip and improve understanding of rheological changes to fault materials that accompany fault thermal evolution and in-situ earthquake mechanics and fault strength in the upper seismogenic zone. Complementary temporal information on fault slip from trapped-charge and 4He/3He thermochronometry will fill a critical temporal data gap between historical, geodetic, and paleo-seismic (1 - 10,000 years) and low-temperature thermochronometric (100,000 - 1,000,000 years) records of deformation, with direct applications to seismic hazards assessment. The new thermochronometric approach to extract coseismic fault temperatures and temporal constraints on deformation applied here is transferable to other fault systems, giving researchers new tools to access the record of fault slip across seismic slip rates. 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.