Nascent plate boundary along the eastern margin of the Japan Sea: testing a new hypothesis by geomorphological methods

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Kobayashi (1983) and Nakamura (1983) presented a new hypothesis that the boundary between the North American and Eurasian plates has jumped from Central Hokkaido to the eastern margin of the Japan Sea in the recent geologic past. In order to test this hypothesis quantitatively, we determined rate Quaternary rates of slip on major fault zones in Central Japan, where the landward extension of this nascent plate boundary is believed to pass. We revealed that the rates of slip on the Itoigawa-Shizuoka Tectonic Line and the Ina Valley Fault Zone (IVFZ) are as large as 8 mm/yr or more. Although data on slip rates on other faults are not sufficient, the total convergence rate of Central Japan seems larger than that (c.a. 11 mm/yr) predicted by the new hypothesis. We also determined the direction of slip on the IVFZ by a new geomorphological method that we developed; this slip direction (N123+10 deg E), again, is discordant with the east-west convergence direction predicted by the new hypothesis. Thus, we tentatively interpret that the deformation in the middle and southern part of Central Japan is much affected by the collision of the Izu arc, although the possibility of the nascent boundary is not ruled out. Further data on late Quaternary rates of net slip on active faults in the northern part of Central Japan are required for testing this hypothesis.