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S U M M A R YMiocene 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 f 1.4Ma, whereas a northerly declination is observed in formations younger than 14.2 f 0.6Ma. These data indicate that southwest Japan had ceased rotational motion by about 14Ma. 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-' at about 15 Ma. The corresponding rate of linear motion of the eastern part of southwest Japan was 21 cm yr-'.

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Paleomagnetic studies facilitate an understanding of the evolution of the Japan Arc in Cenozoic times from the perspective of tectonic movement. The Japan Arc rifted from the Asian continent in the middle Miocene, while East Asia, including the Japan Arc, moved northward at the same time. The rifting phenomenon of the Japan Arc is described by differential rotation of Southwest and Northeast Japan. Southwest Japan was rotated clockwise through about 45" and Northeast Japan was rotated counter-clockwise through about 40". This differential rotation occurred concurrently at about 15 Ma. Eighty percent of the rotation was completed during a period of 1.8 million years. These factors lead us to propose a 'double door' opening mode with a fast spreading rate of 21 cm/yr for the evolution of the Japan Sea, suggesting that the asthenosphere with a low viscosity was injected beneath the Japan Sea area. The large northward motion of East Asia in relation to Europe is expected from the apparent polar wander path constructed from the paleomagnetic data of the Japan Arc. East Asia may have moved northward by more than 1700 km between 20 Ma and 10 Ma accompanied by a slightly clockwise rotation of 10". The eastern part of the Eurasian plate was subjected to extreme geodynamic conditions in late Cenozoic times.

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