Zircon U-Pb ages of sedimentary complexes in the Hidaka Belt

: New age data on the northern and southern areas of the Paleogene Nakanogawa Group, central Hokkaido, northern Japan

Abstract

Recent reexamination of the Hidaka Supergroup (Hokkaido, Japan) has left several stratigraphic units undated, and it is therefore unclear whether these units should be considered as protoliths of the Hidaka Metamorphic Belt. Furthermore, the Erimo Complex remains uncorrelated, precluding a thorough interpretation of the Horoizumi Shear Zone. We present results from detrital zircon U-Pb dating of the sedimentary complexes around the Hidaka Mountains to clarify the tectonic and orogenic history of the area. The turbidite sandstone of the T-Unit (Horobetsugawa Complex, Idonnappu Zone; youngest age cluster of 64.3 ± 0.9 Ma, early Paleocene) is significantly older than the Nakanogawa Group (Hidaka Belt). The turbidite sandstone of the Erimo Complex (youngest age cluster of 52.8 ± 0.3 Ma, early Eocene) is significantly younger than the T-Unit turbidite sandstone and of similar age to the Bisei Formation (Nakanogawa Group, northern unit). We conclude that the Erimo Complex is closely related to the Nakanogawa Group (Hidaka Belt). This relationship implies that the Horoizumi Shear Zone (between the Hidaka Metamorphic Belt and the Erimo Complex) is not a southern extension of the Hidaka Main Thrust but rather a right-lateral or normal fault. The acidic tuff bed, Gm-01, at the base of the turbidite facies of the Hiroo Complex (Nakanogawa Group, southern unit), is dated at 64.4 ± 1.0 Ma, constraining the upper age limit of the Hiroo Complex turbidite deposits. The acidic tuff, Ym-1 (Nakanogawa Group, northern unit), was deposited in a similar hemipelagic environment to that of the acidic tuff bed, but its depositional age (57.0 +1.3/−0.5 Ma; Nanayama et al., 2018) is younger than that of Gm-01. This age differential suggests an age polarity between the northern and southern units of the Nakanogawa Group. The youngest age cluster (48.8 ± 0.4 Ma, early Eocene) of a turbidite sandstone from the northeastern extension of the previously undated Bisei Formation represents the youngest age yet obtained for the Nakanogawa Group and the Hidaka Supergroup as a whole.