古第三紀における古千島弧での砕屑物供給源変化と海嶺沈み込み：砕屑性ジルコン年代分布のBayesian population correlation 解析

抄録

The origin of the Paleo-Kuril Arc (PKA) is the key to understanding the tectonic history of the Northwest (NW) Pacific Region since the Cretaceous. The PKA has been considered an intra-oceanic arc system developed between the Izanagi-Pacific Plates. In contrast, several studies estimated that the mid-oceanic ridge existed between these two plates and subducted in the Paleogene. In the latter hypothesis, the PKA is regarded as the continental magmatic arc system along the southern end of the Okhotsk Block. A lack of geochronological data in the sedimentary rocks in the PKA made it challenging to resolve this problem. Here, we measured the U-Pb ages of the detrital zircons that occurred in the Cretaceous–Paleogene Tokoro and Nemuro Belts of the PKA to identify the origin of the arc system. As a result of Bayesian Population Correlation (BPC) analysis, two clusters of age distributions were identified. Sandstone exhibiting Type 1 age distribution occurred in the Cretaceous to Paleocene strata in the PKA and contained abundant Precambrian zircon grains provided by the continental cratons. This suggests that the PKA was developed as the continental magmatic arc system.In contrast, sandstones exhibiting Type 2 age distribution contain only the detrital zircons of syn-depositional ages. The synchronous transition of the zircon age distributions implies the provenance transition from the diverse sources to the single in-situ volcanic source in the early Eocene. This isolation from the continental provenance could correlate to the significant tectonic event in the NW Pacific Region in the Paleogene.