Abstract
The areal distribution of Green Tuff district is re-examined, forcusing mainly on the tectonic characters of eastern Hokkaido. Miocene volcanism in eastern Hokkaido began with hornblende-bearing andesite at 14 Ma. The intrusions of ilmenite-series granitoids are slightly earlier than it. Volcanism at 14 Ma, associated with ilmenite-series granitoids, prevails through the outer zone of Southwest Japan. It is proposed that the Miocene igneous belt in eastern Hokkaido extends down to the outer zone of Southwest Japan. Igneous rocks, such as 14 Ma-andesitic basalt at Toyokoro, SE Hokkaido, and 22 Madacitic volcanic conglomerate in Holes 438 and 439, DSDP, and as a heat source for Jyouban hot spring, located off the NE Japan coast, occur between both Miocene igneous belts. On the contrary the Miocene NE volcanic belt, typical of Green Tuff belt, passes through the Oshima Peninsula and attains to the Sea of Japan. There was the zonal arrangement of middle Miocene to Pliocene igneous belts along the Japanese islands from the inner (back-arc) side to outer (trench) one: the arc, monogenic and forearc volcanic belts. The trench during this period was located less than about 100 km far from the forearc volcanic belt, probably through the present Abashiri City area in eastern Hokkaido.
