Radiometric age determination on fault rocks with regard to disposal of nuclear waste

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Ito, Hisatoshi۱; Takahashi, Susumu۲; Fukuchi, Tatsuro۳; Tanaka, Daichi۳
Holding Date 08 September 2008

In Japan, high-level radioactive wastes are disposed of below 300 m in depth. The repository site should be away from active volcanoes and active faults. In order to assess fault activities of fault rocks that are encountered during drilling and/or tunnel excavation processes, reliable dating techniques that are applicable to fault rocks are necessary. In order to test the feasibility of fault rock dating, three dating techniques (K-Ar, fission-track, and ESR) were applied to fault rocks obtained at an active fault, the Sakai-toge fault, Japan. The fault was selected because it extends both Jurassic sedimentary rocks and Cretaceous granite; thereby fault rocks originated from the two different origins (sedimentary and granite) were available. The results are as follows. 1) K-Ar (illite or smectite) ages of ~30-40 Ma were obtained from fault rocks (pseudotachylyte, ultracataclasite) of granite origin. 2) K-Ar (illite) age of ~50 Ma was obtained from fault gouge of sedimentary origin. 3) Fission-track (zircon) ages of ~60 Ma were obtained from fault rocks (pseudotachylyte, ultracataclasite) of granite origin. 4) ESR (smectite) ages of ~1-3 Ma were obtained from fault rocks (ultracataclasite, gouge) of both granite and sedimentary origins. From these results, the following conclusions were obtained. (1) K-Ar ages of fault rocks are not suitable to constrain faulting events. The K-Ar age of sedimentary origin may reflect contamination from host rock of Jurassic sedimentary rocks. (2) The concordance of zircon fission-track ages of pseudotachylyte and ultracataclasite indicates that zircons in these fault rocks were completely annealed to reset fission-tracks at ~60 Ma, and hence these fault rocks were formed at this time. This conclusion is supported by the FMR (ferrimagnetic resonance) signal detected from the pseudotachylyte that indicates intense frictional heating over at least 500°C (probably more than that of the Nojima fault pseudotachylyte) occurred along the Sakai-toge fault. (3) Faulting (or hydrothermal events) in association with smectite formation occurred at ~1-3 Ma. (4) Therefore, fission-track dating is useful to infer initiation time of faulting and ESR dating can be applied to date geologically recent faulting events.