Kutcharo caldera is situated in the eastern part of Hokkaido, Japan. It is subcircular with a diameter of 20-26 km and was formed by repeated violent rhyolitic explosive eruptions from 340 to 35 ka. The caldera has three post-caldera volcanoes: Atosanupuri, Nakajima and Mashu. Our new tephrostratigraphical survey suggests that a pyroclastic fall deposit (Nakajima pumice), which was extruded from Nakajima, is exposed at the western slope of Atosanupuri. The deposit comprises dacitic pumice clasts (< 9 cm across) in a fine-grained matrix. The pumice clasts are coated with fine-grained ash, suggesting the deposit was produced by a phreatomagmatic eruption. The deposit directly overlies a scoria-fall deposit, which was ejected from Mashu volcano at 17-12 ka, and is overlain by Ma-k tephra, which was extruded from Mashu volcano at 10 ka. The stratigraphy of the deposit suggests that a phreatomagmatic eruption occurred at Nakajima volcano between 17 and 10 ka.
Abstract The Akan‐Shiretoko volcanic chain, situated in the Southwestern Kurile arc, consists mainly of nine subaerial andesitic stratovolcanoes and three calderas. The chain extends in a SW–NE direction for 200 km, situated oblique to the Kurile trench at an angle of 25 degrees. Thirty‐seven new K–Ar ages, plus previous data, suggest that volcanic activity along the Akan‐Shiretoko volcanic chain began at ca 4 Ma at Akan, at the southwestern end of the chain, and systematically progressed northeastward, resulting in the southwest‐northeast‐trending volcanic chain. This spatial and temporal distribution of volcanoes can be explained by anticline development advancing northeastward from the Akan area, accompanied by magma rising through northeast‐trending fractures that developed along the anticlinal axis. The northeastward development of the anticline caused uplifting of the Akan‐Shiretoko area and changed the area from submarine to subaerial conditions. Anticline formation was likely due to deformation of the southwestern Kurile arc, with southwestward migration of the Kurile forearc sliver caused by oblique subduction of the Pacific plate. The echelon topographic arrangement of the Shiretoko, Kunashiri, Etorofu and Urup was formed at ca 1 Ma.
We found high-Mg andesite (56.5 wt.% SiO2 and 7.2 wt.% MgO) from Mikasayama in Wassamu town, northern Hokkaido. Its K-Ar age is 11.1±0.8 Ma. The high-Mg andesite is characterized by co-existence of Fo-rich olivine (Fo90-85) and An-poor plagioclase (An64-38) phenocrysts. The mineralogical evidence suggests that the high-Mg andesite from Mikasayama was produced by mixing of primitive basalt magma, containing Mg-rich olivine and clinopyroxene phenocrysts, and hornblende dacite magma.
