2:15 PM - 2:30 PM
[HDS10-03] Geological background of rapid deep-seated rock slides in characteristically deformed crystalline schist area, Shikoku range, Japan
Keywords:rapid deep-seated rock slide, landslide, crystalline schist, Sanbagawa metamorphic complex, foliation, fold
Foliation, including schistosity and cleavage is not only a major component of metamorphic rock texture, but also a key factor of rock slides. The foliation of rock brings remarkable strength anisotropy within rock mass. Those rocks are sensitive to gravitational differential stress stress on slope. Gravitational stress forms and grows rupture surfaces along planes parallel to foliations and causes flexural slip with bending deformation in foliated rock. As a result, deep-seated gravitational slope deformation (DGSD) is common in foliated rock on mountainous slope. The term DGSD is used for both rock slides that have continuous rupture surface cutting across mountain edifice and slow slope deformation without the well-defined rupture surface, which is called mass rock creep. Although there are a few evidences that the latter evolves rockslide, its evolution is reasonable. Rock slide occurs more often on cataclinal slope than other type slope, is reasonable that a result of focusing of shear on a foliation plane in development of mass rock creep. Many slow-moving rock slides have thick sliding zone that is a result of repeated shear. Once such shear zone formed, the rock slides become steady state slip, it brings cohesive body suggesting week degradation process. Most of rock slides in crystalline schist area in Shikoku range has so smooth on surface that land use has been available.
It is thought that DGSD is a factor of rapid deep-seated rock slide that often transforms rock avalanche as well. The rapid rock slides do not always have thick rupture zone. The geological factors of rapid rock slides are characterized by steep rupture surface, small asperity between moving mass and the base, weak material on the base, and keeping abundant water inside the rock mass and so on. As for non-metamorphic sedimentary rock mass, buckling which is a feature of DGSD on toe of cataclinal slope, was observed on earthquake or rain induced rockslide. Buckling forms wide weak and porous damage zone at the basal part of DGSD slope. Other than buckling deformation, “pre-existing” fragile fracture zones also play role in rapid rockslide due to weak material property and low permeability to keep abundant water on it. Those geological features could play role in metamorphic rocks, but I did not find buckling and pre-existing fragile fracture zones in some case of a metamorphic belt, Shikoku range.
By the way, metamorphism forms not only recrystallized minerals but also brings ductile deformation. Folding and its associated cleavages also brings multidirectional rock strength anisotropy depending on the wavelength. Mesoscopic folds (MF) for example the wavelength is less than 10 m, can affect in-situ rock strength. Sanbagawa metamorphic complex, which is typical high P/T metamorphic belt and typical crystalline schist area in Japan. The belt has experienced a number of metamorphic stages with tectonic shear process. As a result, overlapped shear deformations form folds deforming primal schistosity. The MF bearing rock and area dominate considerable extent in the Sanbagawa complex.
In this presentatin, I discuss the relationships of MF, DGSD and rapid-deep seated rockslides in Oboke south area from geological and geomorphological aspects that analyzed by modern high-resolution topographical representation map. Also I found that river erosion process is important for the initiation of rapid rock slide.
It is thought that DGSD is a factor of rapid deep-seated rock slide that often transforms rock avalanche as well. The rapid rock slides do not always have thick rupture zone. The geological factors of rapid rock slides are characterized by steep rupture surface, small asperity between moving mass and the base, weak material on the base, and keeping abundant water inside the rock mass and so on. As for non-metamorphic sedimentary rock mass, buckling which is a feature of DGSD on toe of cataclinal slope, was observed on earthquake or rain induced rockslide. Buckling forms wide weak and porous damage zone at the basal part of DGSD slope. Other than buckling deformation, “pre-existing” fragile fracture zones also play role in rapid rockslide due to weak material property and low permeability to keep abundant water on it. Those geological features could play role in metamorphic rocks, but I did not find buckling and pre-existing fragile fracture zones in some case of a metamorphic belt, Shikoku range.
By the way, metamorphism forms not only recrystallized minerals but also brings ductile deformation. Folding and its associated cleavages also brings multidirectional rock strength anisotropy depending on the wavelength. Mesoscopic folds (MF) for example the wavelength is less than 10 m, can affect in-situ rock strength. Sanbagawa metamorphic complex, which is typical high P/T metamorphic belt and typical crystalline schist area in Japan. The belt has experienced a number of metamorphic stages with tectonic shear process. As a result, overlapped shear deformations form folds deforming primal schistosity. The MF bearing rock and area dominate considerable extent in the Sanbagawa complex.
In this presentatin, I discuss the relationships of MF, DGSD and rapid-deep seated rockslides in Oboke south area from geological and geomorphological aspects that analyzed by modern high-resolution topographical representation map. Also I found that river erosion process is important for the initiation of rapid rock slide.