抄録
K-Ar dating and paleomagnetic measurements were performed for a lava flow in the Minatomachi Formation, Rishiri Island, off north Hokkaido, Japan. The newly obtained K-Ar age of 18.2±1.7 Ma and normal paleomagnetic polarity of the lava suggest that the Minato-machi Formation was formed during an interval of normal paleomagnetic polarity in the late Early Miocene. The Minatomachi Formation corresponds to 19–17 Ma volcanism in northern central Hokkaido, which is related to the onset of spreading in the Japan Sea Basin.
