Title of Oral/Poster Presentation

Hematite (U-Th)/He dating as a tool for reconstructing million-year earthquake chronologies on the Wasatch Fault, Utah.

Class

Article

Department

Geology

Faculty Mentor

Alexis Ault

Presentation Type

Poster Presentation

Abstract

Evidence for seismicity in the rock record is rare, yet documenting the occurrence of past earthquakes and their relation to long-term fault zone evolution is critical in modern day seismic hazard assessments. The Wasatch Fault zone (WFZ) is a ~400 km-long, segmented normal fault extending through eastern Utah. The 2014 seismic hazard assessment of the WFZ has indicated that the probability of rupture within the WFZ has changed by ±20%, thereby further motivating attempts to document past seismicity. Prior study of hematite-coated fault surfaces in the Brigham City segment of the WFZ suggests that the high gloss and iridescent nature of these surfaces is indicative of elevated temperatures from shear heating during seismic slip. Furthermore, internally reproducible, but significantly different, preliminary hematite (U-Th)/He (HeHe) dates from the same surface may be indicative of thermal resetting during localized shear heating at fault surface asperities. These surfaces are thus targets for dating seismic slip events on million-year timescales. This study incorporates HeHe and apatite (U-Th)/He (AHe) dating, geologic mapping, and thermomechanical modeling to both test the utility of this novel method in documenting ancient seismicity and to link this information into a chronology for long-term fault zone evolution. The AHe system is sensitive to temperatures of 30-90°C and can constrain the timing of exhumation through the upper ~1-3 of Earth's crust. This provides a measure of the long-term, averaged deformation rate. Preliminary AHe dates from a vertical transect between 1512 m and 1770 m elevation overlap within 2σ uncertainty at ~4.6 Ma, indicating rapid exhumation. Ongoing geologic mapping constrains the structural architecture of the fault zone and thus provides a framework that further informs the interpretation of HeHe dates. Thermomechanical modeling then restricts the number of frictional heating scenarios that can explain HeHe dates.

Start Date

4-9-2015 3:00 PM

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Apr 9th, 3:00 PM

Hematite (U-Th)/He dating as a tool for reconstructing million-year earthquake chronologies on the Wasatch Fault, Utah.

Evidence for seismicity in the rock record is rare, yet documenting the occurrence of past earthquakes and their relation to long-term fault zone evolution is critical in modern day seismic hazard assessments. The Wasatch Fault zone (WFZ) is a ~400 km-long, segmented normal fault extending through eastern Utah. The 2014 seismic hazard assessment of the WFZ has indicated that the probability of rupture within the WFZ has changed by ±20%, thereby further motivating attempts to document past seismicity. Prior study of hematite-coated fault surfaces in the Brigham City segment of the WFZ suggests that the high gloss and iridescent nature of these surfaces is indicative of elevated temperatures from shear heating during seismic slip. Furthermore, internally reproducible, but significantly different, preliminary hematite (U-Th)/He (HeHe) dates from the same surface may be indicative of thermal resetting during localized shear heating at fault surface asperities. These surfaces are thus targets for dating seismic slip events on million-year timescales. This study incorporates HeHe and apatite (U-Th)/He (AHe) dating, geologic mapping, and thermomechanical modeling to both test the utility of this novel method in documenting ancient seismicity and to link this information into a chronology for long-term fault zone evolution. The AHe system is sensitive to temperatures of 30-90°C and can constrain the timing of exhumation through the upper ~1-3 of Earth's crust. This provides a measure of the long-term, averaged deformation rate. Preliminary AHe dates from a vertical transect between 1512 m and 1770 m elevation overlap within 2σ uncertainty at ~4.6 Ma, indicating rapid exhumation. Ongoing geologic mapping constrains the structural architecture of the fault zone and thus provides a framework that further informs the interpretation of HeHe dates. Thermomechanical modeling then restricts the number of frictional heating scenarios that can explain HeHe dates.