A Multi-Proxy Approach Using Zircon (U-Th)/He Thermochronometry and Biomarker Thermal Maturity to Robustly Capture Earthquake Temperature Rise Along the Punchbowl Fault, California

Abstract

During an earthquake, work done to overcome fault friction is dissipated as heat. Coseismic temperature rise, critical for identifying and constraining the magnitude of past earthquakes, is difficult to accurately quantify. To address this issue, we compare two temperature-sensitive geochemical systems, zircon (U-Th)/He (ZHe) thermochronometry and thermal maturity of organic matter (biomarkers), which respond to short-duration, high temperatures. Models of prior biomarker data from the Punchbowl fault (PF), CA, indicate coseismic temperatures of ∼465–1,065°C in the principal slip zone (PSZ; Savage & Polissar, 2019, https://doi.org/10.1029/2019gc008225) depending on prescribed thickness of the deforming zone. We resampled two PF sample sites and acquired high-spatial resolution ZHe data (n = 45 individual analyses) from the PSZ and fault core gouge, together with adjacent crystalline basement and Punchbowl Formation rocks. Results define a positive ZHe date-effective U (eU) trend from ∼10 to 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. Individual apatite (U-Th)/He dates (n = 5) from an undeformed Punchbowl Formation sample are ∼4 Ma for grains with ∼30–150 ppm eU, implying rapid cooling and exhumation at that time due to PF activity. Zircon damage-diffusivity relationships inform a suite of numerical models that collectively bracket coseismic temperatures on the PF to <725–800°C for 90% He loss. Results support general compatibility between ZHe and biomarker-derived temperature rise estimates, and spatio-temporal variability in coseismic temperatures along the PF.