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[HDS07-05] Pelitic schist with epigenetic deformation, as a geological factor of deep-seated landslides
Keywords:deep-seated landslide, crystalline schist, deep-seated gravitational slope deformation, axial plane cleavages, fold, Sanbagawa metamorphic complex
I report the geological conditions of a DGSD (deep-seated gravitational slope deformation) that rapid landslide concerned in Aruse, Nishi-Iyayama, Miyoshi Tokushima, and its similar geological conditions often formed rapid deep-seated landslide in the west area of Tokushima, Sanbagawa metamorphic complex.
The area of active mass is about 150 m in width and about 230 m in length by investigation of Tokushima prefecture and MILT. But its topographical boundary is not clear by observing 1 m-DEM topographical map. The noticeable activity begun in 2018, then it is moving down at less than 2 cm/month but accelerates to at about 2 cm/24 hours after rainfalls. A 50 m- wide frontal part of the mass collapsed in August 2019 after rain fall (Matsushita et. al., 2021). The geology of mass and host rock consists of pelitic schist mainly. The dip of schistosity planes is less than 10 degrees according to the drill hole investigation by MLIT.
The mass has a lot of open cracks that form along axial plane cleavages. The cleavages are formed with outcrop scale upright folds. Pelitic schist with upright folds, laying on slope is fragile and easily to flake along schitosity planes. This characteristic rock facies is likely to concern instability of the slope, due to forming large flow path of penetrating water and exfoliative rock property.
In the south marginal area of Sanbagawa metamorphic complex, pelitic schist with upright folds and axial plane cleavages is common. Aoya and Yokoyama (2009) explained that such rock facies was formed at the epigenetic shear deformation stage after main metamorphism.
Matsushita, K., Takagawa, S. and Sasai, Y. (2021) Mitigation works the Aruse I-3 block Landslide in Miyoshi, Tokushima prefecture. Excursion guide for deep-seated landslides in crystalline schist area, The Japan Landslide Society, Kansai. 42-59. (in Japanese)
Aoya, M. and Yokoyama, S. (2009) Geology of the Hibihara district. Quadrangle Series, 1:50,000, Geological Survey of Japan, AIST, 75 p. (in Japanese with English abstract, 3 p.).
The area of active mass is about 150 m in width and about 230 m in length by investigation of Tokushima prefecture and MILT. But its topographical boundary is not clear by observing 1 m-DEM topographical map. The noticeable activity begun in 2018, then it is moving down at less than 2 cm/month but accelerates to at about 2 cm/24 hours after rainfalls. A 50 m- wide frontal part of the mass collapsed in August 2019 after rain fall (Matsushita et. al., 2021). The geology of mass and host rock consists of pelitic schist mainly. The dip of schistosity planes is less than 10 degrees according to the drill hole investigation by MLIT.
The mass has a lot of open cracks that form along axial plane cleavages. The cleavages are formed with outcrop scale upright folds. Pelitic schist with upright folds, laying on slope is fragile and easily to flake along schitosity planes. This characteristic rock facies is likely to concern instability of the slope, due to forming large flow path of penetrating water and exfoliative rock property.
In the south marginal area of Sanbagawa metamorphic complex, pelitic schist with upright folds and axial plane cleavages is common. Aoya and Yokoyama (2009) explained that such rock facies was formed at the epigenetic shear deformation stage after main metamorphism.
Matsushita, K., Takagawa, S. and Sasai, Y. (2021) Mitigation works the Aruse I-3 block Landslide in Miyoshi, Tokushima prefecture. Excursion guide for deep-seated landslides in crystalline schist area, The Japan Landslide Society, Kansai. 42-59. (in Japanese)
Aoya, M. and Yokoyama, S. (2009) Geology of the Hibihara district. Quadrangle Series, 1:50,000, Geological Survey of Japan, AIST, 75 p. (in Japanese with English abstract, 3 p.).