The Unjoined Arc Structure of the Japanese Islands as the Framework for the Kanto Basin: An Integrated Explanation of Its Anomalous Forearc Subsidence Driven by an Accretionary Upper Plate

The Kanto Basin exhibits anomalous forearc behavior—including 1–3 km of subsidence, a west‑shifted subsidence axis, asymmetric fault distribution, persistence of Tokyo Bay, and the formation of the Kanto Fragment—that cannot be explained by conventional forearc models or by the assumption that the Japanese arcs are fully sutured. This study proposes that an incompletely joined arc segment, or unjoined arc structure, persists beneath the South Fossa Magna and the Kanto Basin, forming what this study terms a forearc weak suture zone. This zone is defined as a region where an incompletely closed arc suture (structural weakness) overlaps with an accretionary upper plate (mechanical weakness), producing a dual‑weakness system that strongly amplifies forearc sensitivity to Pacific Plate subduction. The dual‑weakness configuration, combined with three‑sided tectonic confinement and a wedge‑shaped forearc geometry, promotes vertical deformation through the conversion of horizontal shortening, thereby enhancing forearc drawdown. This integrated framework provides a unified explanation for the Kanto Basin’s subsidence, its pronounced asymmetry, the disappearance of the Median Tectonic Line, the bending of the Japanese Islands, and the deep penetration of the Koma and Misaka Mountains following the Izu Arc collision—phenomena that would not occur if Honshu behaved as a uniformly rigid arc. While numerical modeling is left for future work, this study offers a new tectonic perspective for understanding the unique forearc response of the Kanto Basin and the South Fossa Magna, and provides a conceptual foundation for subsequent research in seismic imaging, crustal deformation modeling, and arc‑collision tectonics.