Evaluating high spatial resolution zircon (U-Th)/He thermochronometry as a fault slip paleothermometer

Emma M. Armstrong, & Alexis K. Ault

Published September 11, 2022, SCEC Contribution #12295, 2022 SCEC Annual Meeting Poster #148

Zircon (U-Th)/He (ZHe) thermochronometry has the potential to detect and quantify temperature rise during earthquake slip. Theoretically, He diffusion is sensitive to short-duration, high temperatures. Recently published, SCEC-supported work compared temperatures derived from ZHe and thermal maturation of organic matter data patterns in the Punchbowl fault, CA (Armstrong et al., 2022; SCEC 20153). Biomarker data within and outside the narrow slip zone are different, supporting a range of coseismic temperatures (~465-1065 deg C) depending on fault parameters. ZHe dates within and outside fault rocks define a similar, positive zircon He date-effective U pattern, revealing temperatures in the narrow principal slip zone likely did not exceed 800 deg C. Because the ZHe system does not appear completely thermally reset in the Punchbowl fault, this raises the question of whether coseismic temperatures can induce He loss from zircon crystals.

We address this question by investigating thermal resetting of the ZHe system in rocks in contact with pseudotachylyte. We target the West Salton detachment fault (WSDF), southernmost CA, where pseudotachylyte locally delineates the detachment interface. Pseudotachylyte formation is inferred to be shallow and previously reported ZHe dates from the footwall reflect exhumation. Thus, ZHe data from wall rock in contact with the pseudotachylyte have the potential to record the timing of coseismic slip. The WSDF juxtaposes La Posta-type granite against Cretaceous granite and protomylonite. Multiple, co-located cm-thick pseudotachylyte layers, which cumulatively are up to ~1 m-thick, suggest portions of the detachment repeatedly hosted earthquakes. We used a high-spatial resolution sampling approach to collect cm-scale slices of wall rock perpendicular to the pseudotachylyte interface. Mineral separation yields some zircons from these discreet domains and we will select grains for ZHe analyses leveraging any variations in visual metamictization. Comparison of ZHe dates from near the slip interface with previously published regional ZHe data, together with numerical models, will decipher the timing, magnitude, and duration of temperature rise during pseudotachylyte emplacement. These data have implications for the utility of zircon (U-Th)/He thermochronometry as a fault slip paleothermometer, the temperature and strength evolution of seismogenic faults, and the slip history SAF-related structures.

Key Words
pseudotachylyte, thermochronometry, West Salton detachment fault, friction generated heat,

Citation
Armstrong, E. M., & Ault, A. K. (2022, 09). Evaluating high spatial resolution zircon (U-Th)/He thermochronometry as a fault slip paleothermometer . Poster Presentation at 2022 SCEC Annual Meeting.

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