Rapid rotation of southwest Japan-palaeomagnetism and K-Ar ages of Miocene volcanic rocks of southwest Japan

Miocene volcanic rocks have been sampled from the San'in district in the central part of southwest Japan. Reliable palaeomagnetic direction and K-Ar dating data have been obtained from 28 localities. A clockwise deflection in declination (D = 38.5°) is observed in the Kawai Formation having an age of 16.1 ± 1.4 Ma, whereas a northerly declination is observed in formations younger than 14.2 ± 0.6 Ma. These data indicate that southwest Japan had ceased rotational motion by about 14 Ma. When compared with the amount of rotation estimated from Cretaceous palaeomagnetic data, it is apparent that more than 80 per cent of the overall clockwise rotation occurred between about 16 and 14 Ma. We conclude that the angular velocity of southwest Japan about a rotation pole at 129°E, 34°N reached 20° Myr−1 at about 15 Ma. The corresponding rate of linear motion of the eastern part of southwest Japan was 21 cm yr−1.