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.

中文翻译：

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古磁分析和U-Pb年龄确定的古古里尔弧西部碰撞弯曲

日本北海道东部地区的古千岛弧，是西北太平洋地区千岛弧的最西端，具有构造弯曲构造。使用古磁数据将其解释为古柯里尔弧中块体旋转的结果。为了了解古库里尔弧-沟系统中这种构造弯曲结构的时间和起源，在古近代Urahoro组中进行了古磁测量和U-Pb放射性测年，该组分布在北海道东部的Shiranuka-hill地区。U–Pb放射性测年表明，Urahoro组的沉积时间约为39 Ma。对Urahoro组的古地磁分析表明，Shiranuka-hill地区相对于东亚经历了顺时针旋转28°。Urahoro组隐含的顺时针旋转度小于下面的始新世Nemuro组的旋度（62°），但大于上层的Onbetsu组的旋度（-9°）。因此建议，Shiranuka-hill地区在Nemuro组和Urahoro组（50-39 Ma）之间经历了约34°的顺时针旋转，而在Urahoro组和Onbetsu组（沉积之间）之间经历了约38°的顺时针旋转（ 39–34 Ma）。古-库里尔弧弯曲构造带的起源以前是通过34 Ma后的千岛盆地的开放来解释的。本研究中，Shiranuka-hill地区旋转运动的年龄限制与这一假设相矛盾。因此，建议弧-弧碰撞的过程引起了西部古-库里尔弧的弯曲结构。