Date of Award:

8-2021

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geosciences

Committee Chair(s)

Alexis K. Ault (Committee Chair), Kelly K. Bradbury (Committee Co-chair)

Committee

Alexis K. Ault

Committee

Kelly K. Bradbury

Committee

Heather Savage

Abstract

Earthquakes produce heat along a fault surface from friction created as two blocks of rock move past each other. The amount of heat generated depends on a variety of factors, including rock type, stresses, and thickness of the fault zone. Identifying evidence for and quantifying this earthquake (coseismic) temperature rise are essential for identifying past earthquakes in the rock record. Indirect methods, such as textures and geochemical signatures that change with temperature, can serve as paleothermometers. Here we compare two paleothermometers, biomarkers and thermochronometry, from two transects across the Punchbowl fault (PF), California. The PF is an ancient fault strand of the San Andreas fault system and is similarly a strike-slip fault that experienced past earthquakes. Biomarkers are organic materials in rocks whose chemical character changes with temperature, such as coseismic friction-generated heat. On the PF, biomarkers indicate temperature rise of ~460-1060 °C (Savage & Polissar, 2019, Geochemistry, Geophysics, Geosystems). Minerals such as zircon are amenable to (U-Th)/He thermochronometry, where He is produced from the radioactive decay of isotopes of U and Th and can escape zircon crystals as a function of temperature.

(U-Th)/He results, reported as dates, from zircon crystals extracted from the PF itself and dates away from a fault are similar, ~10 to 65 million years old, and define similar patterns between date and mineral chemistry. This implies that temperatures in the fault zone were insufficient to cause He loss from zircon crystals. These results, together with numerical models, refine the temperature rise estimates to less than 600-750 °C. Our data imply there is variable temperature rise on the PF in space and time. Due to the abundance of zircon crystals in fault rocks, thermochronometry methods are potentially useful for quantifying coseismic temperatures in other fault zones worldwide.

Ongoing outreach and education activities related to scientific drilling along the San Andreas fault complement PF research. Activities include an interactive poster, lecture series, and an informational video with an associated assignment targeted for distinct audiences, including Cache Valley residents, and non-geoscience-majors. The objective of these activities is to increase awareness of geosciences, fault zone research, and scientific drilling.

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