We have analyzed paleomagnetic data from Japan reported over the past 25 years in combination with marine geophysical and on-land geologic data to reconstruct the opening of the Sea of Japan and the bending of Honshu. From these data, we propose that the Sea of Japan opened by a two-stage back-arc spreading event. (1) In the early to middle Tertiary, Japan began to rift off the Asian coast as a single rigid block, rotated clockwise approximately 25°, and formed the Japan Basin. (2) In the Miocene, the Yamato and Tsushima basins opened, and shortly thereafter (19 Ma) the Japan arc collided with another island arc in central Hokkaido. At this stage, Honshu began to behave as two rigid blocks—southwest Honshu and northeast Honshu. Southwest Honshu continued to rotate clockwise approximately 45° as the Yamato and Tsushima basins continued to open. Northeastern Honshu rotated counterclockwise approximately 40° as Hokkaido was held fixed by the arc collision while the Yamato Basin continued to open. These two events—the opening of the Tsushima and Yamato basins and the arc collision in Hokkaido—resulted in the early to middle Miocene bending of the Japan arc observed in the paleomagnetic data. After 12 Ma, back-arc spreading ceased.

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