Zircon fission-track thermochronology and applications to fault studies

1. Zircon fission-track age and length data suggest that the GSJ 750 borehole rocks record the heat signatures at two different locations, one <25 m perpendicular to the fault and the other at the shallower depth (>25 m). The age of cooling after the heating event was estimated as ∼35 Ma for samples within ∼25 m from the fault by modeling fission-track data. 2. Zircon fission-track length data suggest that the UG 500 borehole rocks record a heat signature in the hanging wall <3 m from the fault. The age of cooling after the heating event was estimated as ∼2.5 Ma by modeling fission-track data. 3. The plausible heat source of these thermal events is heat transfer via fluids along the fault zone from the deep crustal interior, on the basis of one-dimensional heat conduction modeling, the positive correlation between the degree of fission-track annealing and deformation/alteration of borehole rocks, and the in-situ heat dispersion calculation. 4. Zircon fission-track age and length data suggest that the zircon fission-track system of the pseudotachylyte layer at the Hirabayashi trench site was totally reset (or kept reset) and subsequently cooled at ∼56 Ma. However, no signs of such cooling was found for a rock from the footwall ∼10 mm away from the fault as well as one from the hanging wall ∼30 mm from the fault. The spatial distribution of these data is approximately concordant with that of detectable zircon fission-track annealing predicted by thermal modeling of the frictional heating of fault motion. 5. The present Nojima fault system was probably formed by the Middle Quaternary reactivation of an ancient fault, which was already initiated at ∼56 Ma at the interior of crust. 6. The Nojima fault shows a temporal/spatial variation in terms of the thermal anomalies recorded in the fault rocks, implying the heterogeneous heat transfer via fluids migrated along the fault zone from the deep crustal interior. Copyright © Mineralogical Society of America.