徳島県祖谷川上流域の御荷鉾帯と秩父帯 : 地質・地形の総合的研究

The Mikabu and Chichibu belts in the Iya-gawa area are studied with respect to their composition, boundaries and the relation with landslides. The Kyobashira-Sugeoi Fault bound the Mikabu Greenstone Belt from the Sambagawa Belt. The Kubo Greenstone body, a part of the Mikabu Greenstone, distributes as an axial part of antiform in the southern margin of the Sambagawa Belt. The Mikabu Greenstone of the area is composed of tholeiitic and alkaline rocks. The Mikabu Greenstone thrust to the south with the Okoyatoko-Nagoro Fault upon the Jurassic accretionary complex of the North Chichibu Belt. Even the phillite generally dipping towards the north in the northern part of the North Chichibu Belt, the oceanic-plate successions of chert-mudstone sequences are stratigraphically upper towards the south. The radiolarian age, estimated at the lithological transition from chert to mudstone in the oceanic-plate sequences, is Early Jurassic. The landslides concentrate in the areas of the Mikabu Greenstone and valley walls along the Iya-gawa River. The linear depressions and ponds, arrange on the mountain ridge between the Miune and Tengu-Zuka along the southern border of the Mikabu Greenstone, are regarded as a sign of large-scale collapse by long-period gravitational mass rock deformation of the mountain body. The weathered zone of the Mikabu Greenstone often includes swelling clay that causes landslides in the area.