THERMOCHRONOLOGY OF WILDFIRE AND FAULT HEATING THROUGH SINGLE-GRAIN (U-TH)/HE AND FISSION-TRACK DOUBLE-DATING

Wildfire and shear-heating at surface or near-surface conditions can reset low-temperature thermochronometers such as the (U-Th)/He and fission-track (FT) apatite and zircon systems. Although thermal effects of both phenomena have been observed in either He or FT ages, their identification and utility for understanding the dynamics of wildfire or faulting has conventionally been limited by several complications, including uncertainty diagnosing reheating signatures in ages of either system. Here we show through both empirical observation and diffusion/annealing models, that reheating over timescales on the order of minutes to hours induces a characteristic He-FT “age-inversion,” the details of which can constrain temperatures and durations of reheating events at surface or near-surface conditions. By plucking crystals for He-dating directly from FT-mounts, we measured both He and FT ages in the same single crystals of rocks subjected to a wide range of reheating from wildfires, including detrital grains from the soil surface. Apatites from the outermost 1 cm of exposed bedrock have completely reset FT ages but only partially reset (50-90%) AHe ages. Samples from 1-3 cm depths, as well as detrital samples, have fully to partially reset AFT ages, but only slightly reset (0-25%) AHe ages. The vast majority of affected crystals have AFT ages much younger than AHe ages, an apparent contradiction of expectations based on the relative closure temperatures of the systems. However, models of He diffusion and FT annealing clearly show that, because of the contrasting kinetics of these two processes in both apatite and zircon, the He-FT age-inversion is a diagnostic feature of short-duration (< ~100 hours) heating. Further, the specifics of the age inversion and fractional resetting of each system can be used to constrain the temperature and duration of heating. We are currently using He-FT double-dating in detrital samples from hillslope and fluvial settings to explore applications in long-term erosion rate estimates, and in fault-proximal settings to elucidate shear heating dynamics.