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Recent advanced chronological studies for the Tertiary volcanic rocks from the Northeast (NE) Japan arc revealed three volcanic fronts which differed in temporal and spatial distribution. These fronts were (i) the Matsumae-Shizukuishi-Shiogama line of 22-25 Ma which is obliquely across the Quaternary volcanic front (QVF) ; (ii) the Tomari-Shiogama line of 13-16 Ma which exists 30-50 km east of the QVF and (iii) a line of 0-8 Ma which is the same as the QVF. The first shifting of the 22-25 Ma line to the 13-16 Ma one was due to the counterclockwise rotation of the NE Japan arc during 20-12 Ma as proposed by Otofuji et al. (1985), and the second shifting of the 13-16 Ma line to the 0-8 Ma line could have contributed to a decrease in the dip of the slab of the Pacific plate which subducted beneath the NE Japan arc during 13-8 Ma.

K-Ar ages were determined for Miocene Ryozen basalts from east of Fukushima City. Northeast Japan. The Ryozen basalts mainly comprise primitive basalt lava flows of the island are tholeiite series with subordinate amounts of pyroclastic flows. Ten basalt specimens gave almost identical K-Ar ages of 12.4-16.0 Ma, corresponding to Middle Miocene age. If we combine the data of previous studies with the present ones, we can conclude that the Ryozen district have two periods of activities in Miocene. A weak andesitic volcanism took place about 22 Ma, whereas very intence basaltic activity occurred about 13-16 Ma. The volcanic front of 13-16 Ma was estimated to lie 30-50km east of the Quaternary volcanic front.

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