The Paleo-Kuril Arc in the eastern Hokkaido region of Japan, the westernmost part of the Kuril Arc in the northwestern Pacific region, shows a tectonic bent structure. This has been interpreted, using paleomagnetic data, to be the result of block rotations in the Paleo-Kuril Arc. To understand the timing and origin of this tectonic bent structure in the Paleo-Kuril arc-trench system, paleomagnetic surveys and U-Pb radiometric dating were conducted in the Paleogene Urahoro Group, which is distributed in the Shiranuka-hill region, eastern Hokkaido. The U-Pb radiometric dating indicated that the Urahoro Group was deposited at approximately 39 Ma. Paleomagnetic analysis of the Urahoro Group suggested that the Shiranuka-hill region experienced a 28 clockwise rotation with respect to East Asia. The degree of clockwise rotation implied from the Urahoro Group is smaller than that of the underlying Lower Eocene Nemuro Group (62) but larger than that of the overlying Onbetsu Group (−9). It is thus suggested that the Shiranuka-hill region experienced a clockwise rotation of approximately 34 between the deposition of the Nemuro and Urahoro Groups (50-39 Ma), and a 38 clockwise rotation between the deposition of the Urahoro and Onbetsu Groups (39-34 Ma). The origin of the curved tectonic belt of the Paleo-Kuril Arc was previously explained by the opening of the Kuril Basin after 34 Ma. The age constraint for the rotational motion of the Shiranuka-hill region in this study contradicts this hypothesis. Consequently, it is suggested that the process of arc-arc collision induced the bent structure of the western Paleo-Kuril Arc.

The Paleogene Urahoro Group crops out in the Kushiro area of eastern Hokkaido, Japan. The depositional history of the Urahoro Group probably has significant implications for interpreting Paleogene arc-arc collisional processes that are considered to have occurred on Hokkaido Island. However, the depositional age of the Urahoro Group remains uncertain. Here, we present the U-Pb zircon depositional age of an acidic tuff bed of the Tenneru Formation in the middle of the Urahoro Group in the coastal Kushiro area. Using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), we obtained a weighted mean age of zircon grains in the tuff bed of .. Ma (late Eocene). The tuff bed is stratigraphically higher than the horizon where provenance of gravels in the Urahoro Group have changed. This indicates that the tectonic event that caused uplift of the sedimentary provenance area of eastern Hokkaido occurred in the late Eocene.