[S-CG63_2PO1] Geochemistry of fault systems

[SCG63-P01] Cooling history of a fracture zone in the Kojyaku granite, Tsuruga area: Constraints from multi-system thermochronology

*Shigeru SUEOKA¹, Ken-ichi YASUE¹, Masakazu NIWA¹, Koji SHIMADA¹, Tsuneari ISHIMARU¹, Koji UMEDA¹, Ryuji YAMADA², Tohru DANHARA³, Hideki IWANO³, Chitaro GOUZU⁴ (1.Japan Atomic Energy Agency, 2.National Research Institute for Earth Science and Disaster Prevention, 3.Kyoto Fission-Track Co., Ltd., 4.Hiruzen Institute for Geology and Chronology Co., Ltd.)

Keywords:dating of a fault, fission-track thermochronology, K-Ar dating method, U-Pb dating method, Kojyaku granite

Ages of faulting are generally estimated from ages of displaced geomorphic markers, e.g., terrace surfaces, alluvial deposits, or artificial structures. However, these markers are not always available, such as for faults in basement rocks. Such faults have been attempted to date by detecting chronological anomalies (e.g., Ikeya et al., 1982; Murakami and Tagami, 2004; Yamada et al., 2013; Gansawa et al., 2013) or dating hydrothermal veins or clay minerals formed after faulting (e.g., Zwingmann et al., 2004; Watanabe et al., 2008; Siebel et al., 2009; Yamasaki et al., 2013). However, definitive procedures to determine faulting ages based on such geochronological methods have not been established because thermogenesis and mass transport along fault zones are not simple. More basic and case studies are desirable to improve these methods. We introduce an attempt to date a fracture zone observed in the northwestern part of the Tsuruga peninsula, southwest Japan, by constraining its cooling history from fission-track (FT), K-Ar, and U-Pb thermochronometries. In the northern part of the Kinki Triange, including the Kohoku and Tsuruga bay areas, NE-SW or NW-SE strike-slip faults such as the Kohokusanchi and Nosaka-Shufukuji fault zones, are dominant (e.g., The Headquarters for Earthquake Research Promotion, 2003a, b). Strike-slip faults in mountainous areas are generally difficult to date by using geomorphic markers. The fault we study is a strike-slip fault formed in the Tsuruga body of the Kojyaku granite (Kurimoto et al., 1999), along which no gromorphic marker is available. We dated 1) the fault gauge, 2) uncrushed host granitic rock, and 3) dolerite intruding within a few meters from the fault. The dispersions between zircon U-Pb ages and zircon fission-track ages of 1) and 2) are not significant at 2 sigma level and both of the zircon fission-track lengths are not shortened, implying 1) and 2) shared the cooling histories between closure temperatures of zircon U-Pb (>900 deg. C) and zircon fission-track methods (210-350 deg. C). On the other hand, apatite fission-track ages of 50.8 +/- 18.5 Ma for 2) and 28.4 +/- 13.6 Ma for 1) may be interpreted to be reflections of different cooling histories below 90-120 deg. C, closure temperature of apatite fission track method. Although the younger age of a) is attributable to the faulting during the Neogene/Quaternary or intrusion of the dolerite at 19.1-18.8 Ma inferred from plagioclase and whole-rock K-Ar ages, definitive conclusions are difficult to be drawn because of the wide error bars of the apatite FT ages and luck of apatite fission-track length data. In this presentation, we are going to give more precise discussions based on apatite fission-track length analyses.