SCEC Award Number 20153 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Creating a multi-proxy approach to robustly capture earthquake temperature rise at the Punchbowl fault
Name Organization
Alexis Ault Utah State University Heather Savage University of California, Santa Cruz Pratigya Polissar University of California, Santa Cruz
Other Participants Kelly Bradbury (Utah State University), Stuart Thomson (University of Arizona), Ema Armstrong (MSc candidate, Utah State University)
SCEC Priorities 3d, 2d, 1d SCEC Groups FARM, Geology, SDOT
Report Due Date 03/15/2021 Date Report Submitted 03/15/2021
Project Abstract
Intense friction-generated heat occurs along thin, localized slip layers during an earthquake. However, documenting temperature rise in the rock record is challenging. To address this issue, we apply a multi-proxy approach of two different fault slip paleothermometers: thermal maturation of organic material and zircon (U-Th)/He (zircon He) thermochronometry to the Punchbowl fault (PF), CA. Radiation-damaged, or metamict, zircon is more susceptible to He loss from frictional heating than pristine zircon. Recent SCEC-supported work using biomarkers demonstrates temperature rise occurred along localized principal slip zones (PSZs) in the PF (Savage and Polissar, 2019, G-cubed), making the PF a well-constrained site for this inter-method comparison. Initial sampling involved reoccupying the two locations of outcrop 1 of Savage and Polissar (2019). At each site, we collected crystalline basement, Punchbowl Formation, and PF gouge domains over a distance of a ~10 cm. Mineral separation yields whole, dominantly pristine and faceted, zircon crystals amenable (U-Th)/He analysis in all domains including the highly deformed PSZ. To date, we have acquired 45 individual zircon He dates from eight samples. Individual zircon He dates range from ~10-60 Ma across the dataset and define a positive date-eU relationship. Data from the PSZ and gouge do not deviate from this pattern. To first order, these results likely reflect the long-term thermal history of low to moderate damage zircon crystals. On-going numerical modeling, which leverages the disparate kinetics between biomarker and zircon He systematics, is refining the magnitude of coseismic temperature rise in the PSZ of the PF.
Intellectual Merit Mapping past temperature rise from paleoearthquakes is critical for understanding fault mechanics, frictional energy dissipation, and slip histories, which in turn can inform future earthquake patterns. Documenting temperature rise in the rock record is challenging, as many chemical reactions are dependent on the magnitude and duration of heating. By directly comparing two different fault slip paleotemperature proxies with different kinetics, low-temperature thermochronometry and thermal maturation of organic material (biomarkers), we aim to overcome limitations of individual methods to more robustly capture past temperature rise that can be used to calculate frictional energy and the magnitude of past earthquakes. Developing tools to link fault slip temperatures and the width of the deforming fault zone informs the interplay between strain localization and slip velocity is seismogenic fault systems such as the southern San Andreas fault system. This research aligns with SCEC research objectives P3d, P2d and P1d.
Broader Impacts This SCEC project has developed a new research partnership between female scientists at USU (Ault) and UCSC (Savage) and enhanced science and educational connections between all PIs and collaborators across multiple institutions. Research activities and funding support the education and training of female USU MSc student Ema Armstrong. This work served as the underpinnings of Armstrong’s successful NSF Graduate Research Fellowship and, as a result, Armstrong will continue her PhD at USU. This project also supported USU MSc student Alexandra DiMonte, who accompanied Armstrong, PI Ault, and collaborator Bradbury, as part of an all-female Punchbowl fault field team in November 2020. Armstrong is implementing Punchbowl fault research in a multi-tiered, virtual earthquake and scientific drilling learning module, supervised and supported by Bradbury, aimed at engaging K-12 and college learners in rural Utah.
Exemplary Figure Figure 3. Individual (A) zircon and (B) apatite (U-Th)/He dates as a function of eU from the Punchbowl Fault, CA. Dated plotted with 2 sigma analytical uncertainties. PSZ = principal slip zone; PB Fm. = Punchbowl Formation. Inset in A shows photographs of analyzed zircon crystals from sample 1E.