Spatial Distribution Characteristics of Illite Crystallinity and Rebound Hardness in the Mudstones of the Shimanto Belt and Kumano Group, Southern Kii Peninsula, Japan

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  • HOSONO Hinako
    Geomechanics Lab., Graduate School of Integrated Basic Sciences, Nihon University
  • TAKEMURA Takato
    Geomechanics Lab., Graduate School of Integrated Basic Sciences, Nihon University Geomechanics Lab., Dep. Earth and Environmental sciences, Nihon University
  • KIMURA Katsumi
    Fukada Geological Institute
  • KIKUCHI Teruyuki
    J-power Design Co., Ltd.

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  • 紀伊半島南部の四万十帯と熊野層群の泥質岩におけるイライト結晶度と反発硬度の空間分布特性
  • キイ ハントウ ナンブ ノ ヨンマンジュウタイ ト クマノソウグン ノ デイシツガン ニ オケル イライト ケッショウド ト ハンパツ コウド ノ クウカン ブンプ トクセイ

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Abstract

<p>In the southeast part of the Kii Peninsula, the Omine granitic body and the Kumano acidic body intrude into the Shimanto Belt and the Kumano Group of the Neogene Series. In particular, the Omine granitic body is thought to cause thermal metamorphism of the surrounding mudstone. The degree of thermal metamorphism of the mudstone can be estimated from the illite crystallinity （IC value）, which is calculated by the half-height width or full width at half maximum of the X-ray diffraction line of illite. In this study, the relationship between thermal metamorphism and physical properties of mudstones in the southern part of the Kii Peninsula was quantitatively evaluated by measured IC value, rebound hardness and true density. The spatial distribution of the IC values was used to discuss the characteristics of the distribution of landslide sites. The results showed that the rebound hardness increased as the IC value decreased. As for the relationship between IC value and true density, the true density increased with the increase of IC value. From the spatial distribution of IC values, the distribution patterns of thermal metamorphism could be classified into three categories. From the spatial distribution of IC values, the distribution patterns of thermal metamorphism could be classified into three categories. The area with the highest thermal metamorphism is influenced by the Omine granite. The most thermally metamorphosed area is considered to be influenced by the Omine granitic body, and it suggests that the granitic body is underlying the surface. A comparison between the high and low thermal metamorphism areas of the Hidakagawa Group in the Shimanto Belt shows that the high thermal metamorphism area tends to have fewer landslides.</p>

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