Structural Evolution of the Eastern Nankai Trough accretionary prism, Central Japan—Subduction of Trough-Parallel Seamount Chains

Abstract The Nankai Trough, offshore of Southwest Japan, represents a plate subduction zone where a long history of devastating mega-earthquake and tsunami events has been best documented. Because of this societal relevance, the area has been extensively studied and monitored for several decades. Among the Nankai Trough region, the Eastern Nankai Trough and the Enshu Forearc Basin remain a focal point for ongoing geological and geophysical research due to their complex morpho-structural framework compared with Central and Western Nankai Troughs. In this study, twenty-five migrated Multi-Channel Seismic (MCS) profiles parallel to the subduction direction and combined bathymetric data were used to investigate the morpho-structural framework of the outer accretionary prism of the Eastern Nankai Trough. Our results show three domains (Domain 1 ~ 3) of the accretionary prism from west to east of the study area. Domain 1 is characterized by the development of an outer accretionary prism separated by a boundary thrust into an upper prism and lower prism, named Outer Accretionary Prism Boundary Thrust (OPBT). This domain also shows close similarity with the Central Nankai Trough region morpho-structural framework, where the Tokai Thrust is an equivalent of the megasplay fault. Domain 2 shows that a large part of the outer accretionary prism to the Kumano Basin edge has been deformed and eroded. Piggy-back style upper slope basins are recognized between the OPBT and the Tokai Thrust. Domain 3 represents the development of backthrusts seaward of the upper slope basin. The overall morpho-structural framework of the Eastern Nankai Trough, characterized by frontal thrust, OPBT, backthrust system, slope basins, Tokai Thrust, and forearc basin deformation caused by the Kodaiba Fault, could be best explained by subduction of a trough-parallel oceanic basement high complex (OBHC), possibly with some seamounts similar to the present-day Zenisu Ridge. We therefore confirm that the previous proposal of the subduction of the Paleo-Zenisu Ridge, which took place since around 1 Ma, is likely the most reasonable scenario for the morpho-structural evolution of this region.