K-Ar age determination and paleomagnetic measurement have been carried out on the Miocene rocks distributed in Fukui Prefecture, the northeastern part of Southwest Japan. Thirteen K-Ar whole rock ages ranging between 17.1 Ma and 7.3 Ma were obtained from the Miocene rocks, which are virtually consistent with their biostratigraphic ages. Paleomagnetic samples were obtained from sites whose ages were determined by the isotope or biostratigraphic age-determination. Their stabilities were examined by applying alternating and thermal demagnetization, and it is confirmed that 17 sites give us reliable paleomagnetic data.
The new paleomagnetic results, together with those published previously, made clear, first, that Southwest Japan was rotated clockwise through 14° with respect to Eurasia in a period between 26 Ma and 17 Ma, second, that a rapid rotation of Southwest Japan was initiated at about 17 Ma, and ceased at 15 Ma resulting in the 36° clockwise rotation. The rate of the rapid rotation is 18°/Ma. This clockwise rotation can be explained by the drift of Southwest Japan associated with the spreading of the Japan Sea during the Miocene.
