A Unified Forearc Subsidence Model Based on the Three-Arc Unjoined Structure of the Honshu Arc

Yoshikazu Hiraoka

This study reinterprets the subsidence, uplift, fault activity, and geomorphic development observed in the Nōbi Plain, Ise Bay, Lake Biwa, Wakasa Bay, and Osaka Bay through a new structural framework in which the Honshu Arc is understood as an incompletely joined system of three arcs. The Southwest Japan Arc (narrow sense), the Hida–Tamba Terrane, and the Northeast Japan Arc approached each other during the opening of the Japan Sea but never fully sutured. Their unjoined boundaries are exposed on the forearc side, forming Forearc Weak Suture Zones, where the weakness of accretionary complexes and the overlap of major fault zones concentrate subsidence, seismic shaking, and geomorphic reorganization. The Nōbi Plain lies within a Dual Weak Surface Structure, defined by the Yōrō Fault (inner weak surface) and the Biwa Western Margin–Rokko Fault Zone (outer weak surface), and is therefore interpreted not as a depositional basin but as a subsidence-dominated forearc basin. Ise Bay represents its southern continuation, forming a single forearc subsidence system. Lake Biwa occupies the northwestern end of the unjoined arc structure, while uplift along the Wakasa Bay coast is explained by three-sided tectonic confinement imposed by rigid crustal blocks. The continuous subsidence belt from Lake Biwa through the former Ogura Lake and Funo Basin to Osaka Bay defines the Kinki Subsidence Line, a western counterpart to the South Fossa Magna, effectively a “Second Fossa Magna.” This study demonstrates that the South Fossa Magna–Kanto Basin system and the Nōbi–Biwa–Osaka system are governed by the same forearc weak suture zone processes, thereby establishing a unified tectonic model for forearc subsidence in the Japanese Islands.