抄録
Miocene Osuzuyama volcano-plutonic complex 30km across in diameter around 15Ma reveals intracaldera debris, newly named Kanaiso debris avalanche deposit. The solidified deposit involves irregular blocks reaching 50m across as a megablock and displays sedimentary characteristics clearly instead of the lost topography, such as blocks showing peculiar plastic deformation, disjunction and displacement along the joints as well as jigsaw-puzzle cracks and patchwork texture with the matrix. Imbrication and flame-shaped tail of the blocks develop near the boundary with the basement, indicating the flow direction of avalanche not from the inner caldera. Also, a concentration in orientation of the long axis develops. The avalanche deposit consists of debris derived from the basement Shimanto supergroup of Cretaceous to Early Miocene, Iorigawa conglomerate and Welded tuff 1 of rhyolite in the lower units of the complex. But no debris derived from Welded tuff 2 of dacite in the upper unit of the complex appears in the deposit, except a few obsidian blocks in the same chemical composition of the Welded tuff 2. Neither pumice nor volcanic ash exists in the deposit obviously. The avalanche deposit occupies two horizons in detail. The lower one overlies the basement and the solidified Welded tuff 1 and is overlain by the Welded tuff 2. Another one intercalates in the lower part of the Welded tuff 2. These facts suggest that the Kanaiso debris avalanche deposit derived from the unstable caldera wall most probably soon after the caldera collapse of the complex. No debris avalanche appeared during the first pumice eruption to issue the Welded tuff 1. But putting a cooling lag after the first pumice eruption, avalanches occurred before the main phase in the second pumice eruption producing the Welded tuff 2.
