Zircon U–Pb Dating of the Urahoro Group and Atsunai Formation in the Shiranuka Hills of Eastern Hokkaido, Northeast Japan: Implications for Tectonic Development

Abstract

Zircon U–Pb dates for detrital grains in the Shakubetsu Formation of the uppermost Eocene Urahoro Group and the Upper Miocene Atsunai Formation distributed in the eastern part of Hokkaido have been determined for the first time. Both weighted mean 48.8 ± 1.4 Ma (n = 80) for < 70 Ma zircons and youngest grain 36.5 + 1.8, −2.5 Ma (Middle to Late Eocene) ages of the Urahoro Group are consistent with the sedimentary ages and Paleogene magmatic arc origin of the detritus clarified in the previous studies. Although the Zircon U–Pb ages (n = 24) of the Atsunai Formation range between 5 and ~1700 Ma, a weighted mean age of 7.05 ± 0.57 Ma (n = 10) for < 10 Ma zircons (youngest cluster) could constrain the maximum sedimentary age, which is consistent with the previous ages from diatom biostratigraphy and K-Ar method. However, while the K-Ar age was obtained from an Ol-Cpx-Opx basalt in tuff breccia, the dated zircons were probably derived from felsic tuff including many pumice grains. Therefore, the bimodal volcanism could have occurred at c. 7 Ma in the present forearc region, indicating that the volcanic front at this age migrated south significantly, correlated to the one which also occurred coevally in the seaward side of the present volcanic front in the NE Japan arc. Further, it has been found that the eastern boundary of the Shiranuka Hills is characterized by an ESE verging, large-scale monocline based on the present field study and previous geological maps. Since the Atsunai Formation was involved in the formation of the large-scale monocline, the structure started to form after the sedimentation at c. 7 Ma. The fact implies that the ongoing collision of the Kuril forearc sliver with the NE Japan arc has propagated to this area since the latest Late Miocene.