The belt boundary thrust within the Cretaceous–Neogene accretionary complex of the Shimanto Belt, southwestern Japan, extends for more than ~ 1 000 km along the Japanese islands. A common understanding of the origin of the thrust is that it is an out of sequence thrust as a result of continuous accretion since the late Cretaceous and there is a kinematic reason for its maintaining a critically tapered wedge. The timing of the accretion gap and thrusting, however, coincides with the collision of the Paleocene–early Eocene Izanagi–Pacific spreading ridges with the trench along the western Pacific margin, which has been recently re‐hypothesized as younger than the previous assumption with respect to the Kula‐Pacific ridge subduction during the late Cretaceous. The ridge subduction hypothesis provides a consistent explanation for the cessation of magmatic activity along the continental margin and the presence of an unconformity in the forearc basin. This is not only the case in southwestern Japan, but also along the more northern Asian margin in Hokkaido, Sakhalin, and Sikhote‐Alin. This Paleocene–early Eocene ridge subduction hypothesis is also consistent with recently acquired tomographic images beneath the Asian continent. The timing of the Izanagi–Pacific ridge subduction along the western Pacific margin allows for a revision of the classic hypothesis of a great reorganization of the Pacific Plate motion between ~ 47 Ma and 42 Ma, illustrated by the bend in the Hawaii–Emperor chain, because of the change in subduction torque balance and the Oligocene–Miocene back arc spreading after the ridge subduction in the western Pacific margin. 　白亜系−第三系四万十帯を二分する境界スラストは総延長1000キロメートルを超える。この断層は，後期白亜紀以来継続的に成長した付加体の臨界尖形維持という力学的理由によって形成された順序外スラストであると一般的には考えられている。しかし，付加の中断とスラスト形成のタイミングは西太平洋縁辺での暁新世−前期始新世のイザナギ−太平洋海嶺の海溝との衝突時期に一致する。イザナギ−太平洋海嶺沈み込みは，西南日本弧のみならず，北海道，サハリン，シホテアリンなどのアジア大陸縁辺での火成活動の中断や前弧海盆の不整合，アジア大陸下のトモグラフィーをよく説明できる。イザナギ−太平洋海嶺沈み込みによるトルクバランスの変化はハワイ−天皇海山列の屈曲に現れるような47~42Maの太平洋プレートの広域的な運動の変化に現れ、その後の漸新世−中新世の背弧拡大へ繋がったと言えそうである。 The ridge subduction hypothesis is consistent with the cessation of magmatic activity along the continental margin and the presence of an unconformity in the forearc basin in the western Pacific margin. A Izanagi–Pacific ridge subduction model revises the classic hypothesis of a great reorganization of the Eocene Pacific Plate motion and the Oligocene–Miocene back arc spreading after the ridge subduction in the western Pacific margin.