Constraints on timing of faulting events associated with plastic deformations of biotites based on fission-track thermochronometry

Abstract:

We performed fission-track (FT) analyses for constraining timing of fault slip events associated with plastic deformations of biotite grains formed at high temperature (around 150–250°C or higher). We investigated crushed zones in the Tsuruga Peninsula, southwest Japan, consisting of the late Cretaceous Kojaku granite emplaced at ~68 Ma. We collected seven granite samples around the crushed zones associated with plastic deformations of biotite grains, according to the distance from a basaltic dyke intruded at ~19 Ma. Apatite FT ages were estimated at 48.4–14.3 Ma, getting younger near the basaltic dyke. By comparing the distribution of apatite FT ages with forward thermal and FT annealing calculation results, we interpreted the younger ages are basically explained by the basaltic intrusion. On the other hand, reheating event(s) occurred at 19–14 Ma is also potential since the youngest FT age is younger than 19 Ma at 2σ level. By following the thermal history above, we concluded the plastic deformations of biotite grains could occur associated with the following events: 1) primal cooling of the granitic body from 68 to around 50 Ma, 2) basaltic intrusion at ~19 Ma, and 2’) potential reheating event(s) by unknown heat source at 19–14 Ma.