Comparison of zircon (U-Th)/He and biomarker analyses to quantify coseismic temperature rise along the Punchbowl fault, CA

Emma M. Armstrong, Alexis K. Ault, Kelly K. Bradbury, Heather M. Savage, Stuart Thomson, & Pratigya J. Polissar

Submitted August 12, 2021, SCEC Contribution #11293, 2021 SCEC Annual Meeting Poster #138

During an earthquake, work done to overcome fault friction is dissipated as heat. However, coseismic temperature rise, critical for identifying past earthquakes, is difficult to accurately quantify in the rock record. To address this issue we compare two geochemical systems sensitive to short-duration high temperatures, biomarker thermal maturity and zircon (U-Th)/He (ZHe) thermochronometry, at the Punchbowl fault (PF), CA. The PF is an ancient, exhumed strand of the San Andreas fault system that juxtaposes crystalline basement of unknown age against Miocene Punchbowl Formation with a narrow fault core. The fault core architecture varies spatially at the cm-m scale and includes multi-layered cataclasite to ultracataclasite with a discrete, internal principal slip zone (PSZ), and broader zones of clay gouge. Prior biomarker data from the PF indicate coseismic temperatures of ~465-1065 ˚C in the PSZ depending on slip zone thickness (Savage & Polissar, 2019). We reoccupied two previous PF sample sites and acquired high-spatial resolution ZHe data (n = 45 individual analyses) from the PSZ and fault core gouge, as well as adjacent units. ZHe results define a positive date-effective U (eU) trend from ~10-60 Ma and ~20-700 ppm eU with a plateau at ~65 Ma at >700 ppm eU. This pattern suggests the PSZ and fault core gouge share a similar thermal history to material outside the PF and that zircon grains were not fully reset with respect to He by coseismic temperature rise. Apatite (U-Th)/He dates (n = 5) from an undeformed Punchbowl Formation sample are ~4 Ma over ~30-150 ppm eU, implying rapid cooling at that time due to PF activity. Limited apatite fission track data suggest grains are partially reset and did not experience temperatures >110 ˚C since ~12 Ma. Zircon damage-diffusivity relationships inform three different numerical models of fault temperatures, which collectively indicate peak temperatures in the PSZ of the PF are <600-750 ˚C. Thus, comparison of two fault slip paleothermometers with different diffusion kinetics allows us to refine the maximum temperature rise. New data also support spatio-temporal variability in temperatures along the PF and imply lower frictional energy than previously estimated for large earthquakes that propagated updip along this seismogenic fault.

Key Words
Punchbowl fault, low-temperature thermochronometry, coseismic heat, friction-generated heat

Armstrong, E. M., Ault, A. K., Bradbury, K. K., Savage, H. M., Thomson, S., & Polissar, P. J. (2021, 08). Comparison of zircon (U-Th)/He and biomarker analyses to quantify coseismic temperature rise along the Punchbowl fault, CA. Poster Presentation at 2021 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)