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[SGL23-P05] Tectonic slices at the Anko outcrop of Median Tectonic Line, Oshika Village, Nagano Prefecture, Japan
Keywords:Median Tectonic Line, fault gouge, Anko outcrop, tectonic slice
The Median Tectonic Line (MTL) divides low-grade Sanbagawa schists and high-grade Ryoke metamorphic rocks with large amount of granitic rocks. The fault rocks along the MTL in the Anko outcrop in Oshika Village, Nagano Prefecture, is highly deformed and altered. Therefore, there was some ambiguity about the exact location of the MTL, it was determined by chemical analysis and microscopic observations conducted by Kawamoto et al. (2013). When the black gouge of the MTL is designated as G1, there are three black gouge on the Ryoke side of the MTL, which the authors call G2 (1.4 m thick), G3 (1.6 m thick), and G4 (Anko West outcrop in Takagi et al., 2015: 5.0 m thick) in order of northwestward direction. In this report, we determined the attribution of the gouge (G2, G3) using small rock fragments that had been randomly collected in the gouges, and conducting microscopic observations of the plagioclase fragments there. The plagioclase fragments derived from the Sanbagawa schist are rather fresh albite with inclusions, whereas those from the Ryoke rocks are highly altered because of original Ca content in the plagioclase.
As a result of above observations, albite with inclusions was observed in all five samples of the gouge G3, and in three samples in the gouge G2, but the attribution was uncertain for other two samples, because the fracturing and alteration were particularly severe. In addition, the cataclasite between G2 and G3, which is considered to belong to the Ryoke Belt, showed significant alteration of plagioclase porphyroclasts and was distinctly different from that from the Sanbagawa schist. From the observations mentioned above, it is clear that the black gouge in G2 and G3 is almost exclusively attributed to the Sanbagawa Belt. At G4 (Anko-west outcrop), located 40 m northwest from the MTL, rocks derived from the Sanbagawa schist have been recognized (Ishikawa et al., 2014), and at Anko-Sawa outcrop, about 24 m northwest from the MTL, rocks derived from the Sanbagawa belt are also recognized in the Ryoke belt.
In the Anko outcrop, at least three gouge zone derived from the Sanbagawa schist are intercalated in the Ryoke Belt near the MTL. In this outcrop, variations of strikes exist among the gouges including exact MTL gouge (G1: N25°E) and another gouges such as G4 (N10°E ; Takagi et al., 2015) which involves Quaternary activity history. The cataclasites and a slightly consolidated fault gouge shows sinistral shear sense, while the G4 gouge shows younger dextral slip sense. These lines of evidence suggest that the Sanbagawa schist were intercalated into the Ryoke Belt as tectonic slices due to the repeated strike-slip activities of the MTL in the study area.
References
Ishikawa, T., Hirono, T., Matsuta, N., Kawamoto, K., Fujimoto, K., Kameda, J., Nishio, Y., Maekawa, Y., Honda, G, 2014, Planets and Space, 66: 36.
Kawamoto, K., Ishikawa, T., Matsuta, N., Hirono, T., 2013, Natural history reports of Inadani, 14, 1-17.
Takagi, T., Sugiyama, K., Kawamoto, K., Kitazawa, N., 2015, Abstracts, the 122nd Annual Meeting of the Geological Society of Japan, R14-O12.
As a result of above observations, albite with inclusions was observed in all five samples of the gouge G3, and in three samples in the gouge G2, but the attribution was uncertain for other two samples, because the fracturing and alteration were particularly severe. In addition, the cataclasite between G2 and G3, which is considered to belong to the Ryoke Belt, showed significant alteration of plagioclase porphyroclasts and was distinctly different from that from the Sanbagawa schist. From the observations mentioned above, it is clear that the black gouge in G2 and G3 is almost exclusively attributed to the Sanbagawa Belt. At G4 (Anko-west outcrop), located 40 m northwest from the MTL, rocks derived from the Sanbagawa schist have been recognized (Ishikawa et al., 2014), and at Anko-Sawa outcrop, about 24 m northwest from the MTL, rocks derived from the Sanbagawa belt are also recognized in the Ryoke belt.
In the Anko outcrop, at least three gouge zone derived from the Sanbagawa schist are intercalated in the Ryoke Belt near the MTL. In this outcrop, variations of strikes exist among the gouges including exact MTL gouge (G1: N25°E) and another gouges such as G4 (N10°E ; Takagi et al., 2015) which involves Quaternary activity history. The cataclasites and a slightly consolidated fault gouge shows sinistral shear sense, while the G4 gouge shows younger dextral slip sense. These lines of evidence suggest that the Sanbagawa schist were intercalated into the Ryoke Belt as tectonic slices due to the repeated strike-slip activities of the MTL in the study area.
References
Ishikawa, T., Hirono, T., Matsuta, N., Kawamoto, K., Fujimoto, K., Kameda, J., Nishio, Y., Maekawa, Y., Honda, G, 2014, Planets and Space, 66: 36.
Kawamoto, K., Ishikawa, T., Matsuta, N., Hirono, T., 2013, Natural history reports of Inadani, 14, 1-17.
Takagi, T., Sugiyama, K., Kawamoto, K., Kitazawa, N., 2015, Abstracts, the 122nd Annual Meeting of the Geological Society of Japan, R14-O12.