Oldest record of brittle deformation along the Median Tectonic Line: fission-track age for pseudotachylyte in the Taki area, Mie Prefecture

Abstract

Fission-track (FT) dating of pseudotachylyte (PST) associated with the Median Tectonic Line (MTL) in the Taki area, Mie Prefecture, SW Japan, provides new constraints on the timing of movement upon this fault. A PST vein with a thickness of about 5 cm yields a zircon FT age of 60.0 ± 3.5 Ma (1σ). In contrast, a weighted average of zircon FT ages obtained for protolith samples (cataclastic mylonitized Hatai Tonalite) collected 10 cm and 15 m from the PST vein boundary is 69.8 ± 1.2 Ma, which is significantly older than the age of the PST. Decomposition of feldspars in the PST suggests that the temperature exceeded 1100°C, which is sufficient to completely erase previous fission tracks in zircon within several seconds. The distribution of fission-track lengths in zircon from the PST vein also supports the interpretation that the zircon FT age was completely reset during frictional fusion of the PST vein. Considering an apatite FT age of 38.0 ± 1.5 Ma for the host rock, the age of the PST indicates that the frictional fusion was occurred during cooling of the Ryoke granite at temperatures between 250°C (closure temperature of the zircon FT system) and 100°C (closure temperature of the apatite FT system). This PST age is comparable with the oldest K-Ar age obtained for fine fractions of MTL fault gouge derived from both the Sanbagawa pelitic schist and the Izumi Group in Shikoku, indicating that the initiation of brittle fault movement associated with formation of the PST and/or fault gouges along the MTL had occurred by 60 Ma.