Abstract

Paleomagnetic data from Tertiary sedimentary strata in the Yubari and Haboro areas of central Hokkaido indicate consistently clockwise-deflected magnetic declinations. The mean declination of middle middle Miocene rocks deviates 21° ±5° from the Eurasian reference direction, and that of the Oligocene to lower middle Miocene rocks deviates 30° ±21°. This suggests that these sedimentary basins have rotated 10°-30° clockwise about a vertical axis since the middle Miocene. These clockwise rotations can be explained by a right-lateral displacement of a shear zone in central Hokkaido. This study demonstrates that central Hokkaido is not simply a single tectonic unit of northeast Japan characterized by counterclockwise rotations from early to middle Miocene time.

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