The Izanagi Plate was an ancient oceanic plate in the northwest (NW) Pacific region predicted by the age distribution of the oceanic crust of the Pacific Plate1. There must have been a mid-oceanic ridge between these two oceanic plates. It has been suggested that the Izanagi Plate vanished when this ridge subducted into the continental plates in the North Pacific region2–4. Many studies considered such ridge subduction as one of the most significant tectonic events in this region1–3, which may have caused the cessation of volcanism and uplifting around the subduction zone.
However, a recent study2 suggested that the boundary between the Izanagi and Pacific Plates was the subduction zone and, thus, that ridge subduction did not occur in the Paleogene. One of their primary pieces of evidence is that the Paleo-Kuril Arc (PKA) system in Hokkaido Island in northern Japan (Fig. 1a) originated as an intraoceanic arc system, which was indicative of the existence of a subduction zone between two plates2. Meanwhile, paleomagnetic studies suggested that the PKA could have developed in low-latitude areas. Thus, Bazhenov et al.5 interpreted the PKA as an intraoceanic magmatic arc system between the Izanagi and Pacific Plates.
The recent geochronological analysis of Harisma et al.7 in the Late Cretaceous–Paleogene Nemuro Belt implied that the sediment provenance of the PKA was the Precambrian continental crust, suggesting that the PKA originated along the southern end of the Okhotsk Block as a continental arc system during the Late Cretaceous. In contrast, another geochronological study suggested that the PKA could have developed as an immature oceanic island arc system in the Late Cretaceous8 because it did not detect detrital zircon (DZ) grains before 85 Ma in the basal successions of the Tokoro Belt, which is part of the PKA system. The geochemical analysis of the igneous rocks in the Nemuro Belt also suggested that the PKA may have developed as an immature magmatic arc system9. Thus, the discussion on the origin of the arc has yet to be settled.
Contradictory interpretations of the origin of the PKA have been proposed probably because the areas interpreted as the PKA may have not been a single tectonic terrain initially. For instance, the Nemuro and Tokoro Belts (Fig. 1b and 1c) had been interpreted as tectonic belts that belonged to the PKA system2,5,6. However, Bazhenov et al.5 argued that these two tectonic belts belonged to different intraoceanic island arc systems and that they were accreted to the NE Japan Arc system at different timings. This hypothesis can be verified by examining whether the Tokoro Belt recorded a sediment provenance transition similar to that in the Nemuro Belt, indicated by the DZ U–Pb age distributions. Harisma et al.7 reported that the sediment provenance of the Nemuro Belt changed from multiple-source (continental crusts and magmatic arcs) to single-source (local magmatic arc) regions. If the Tokoro Belt was part of the PKA, the provenance transition observed in the Nemuro Belt should have also been recorded in this tectonic belt.
Aiming to conclude the discussions on the origin of the PKA, we report the U–Pb age distributions of the DZ grains newly obtained from sandstone samples of the Late Cretaceous Saroma and the Late Cretaceous–Paleogene Yubetsu Groups in the Tokoro Belt distributed in eastern Hokkaido. We then conducted statistical analyses with integrated datasets using existing DZ data from the Nemuro and Tokoro Belts7,8,14 to provide new insights into the link between the two tectonic belts and the evolution of the PKA.
Tectonic Evolution of the PKA
The PKA system is exposed in the eastern part of Hokkaido and consists of two tectonic belts: the Nemuro and Tokoro Belts (Fig. 1a). The Nemuro Belt comprises Upper Cretaceous to Paleogene strata (the Nemuro, Urahoro, and Onbetsu Groups). The Nemuro Group has been interpreted as comprising forearc basin deposits15,16. Meanwhile, the Tokoro Belt is another tectonic belt of the PKA composed of forearc basin deposits and Cretaceous accretionary complexes (the Nikoro, Saroma, and Yubetsu Groups). This belt is located in eastern Hokkaido (Fig. 1a) and extends from north to south, exceeding 150 km long and up to 40 km wide17.
The tectonic evolution of the PKA before the Miocene can be summarized as follows according to geophysical and geological studies. The PKA developed as a magmatic arc in a subduction zone in the Late Cretaceous. An accretionary complex (the Nikoro Group of the Tokoro Belt) and forearc basin deposits (the Saroma and Nemuro Groups) were formed along the subduction margin. However, as described earlier, the locations of the subducting plate and the subduction zone are yet controversial2,3,7. The unconformity between the Nemuro and Urahoro Groups also developed from the Paleocene to the Early Eocene (62 to 54 Ma), which could be attributed to oceanic ridge subduction3. In the Middle Eocene to the Oligocene, a clockwise rotation of more than 70° occurred in the western part of the PKA, implying the tectonic bending of the magmatic arc 19,20. Katagiri et al.20 suggested that the oblique collision of the PKA to the eastern margin of the Eurasian Plate caused this bending of the tectonic belts21–23.