Identification of the static backstop and its influence on the evolution of the accretionary prism in the Nankai Trough

To reveal the origin of a backstop and its influence on the evolution of the accretionary prism, we analyzed reflection seismic data acquired in the Nankai Trough off the Kii Peninsula. The deformation features of the forearc basin sequence show that the landward accretionary prism close to the coast was not deformed after the development of the forearc basin about 2-4 Ma. The surface of the landward prism can be identified as strong amplitude reflector, indicating that the landward prism has higher seismic velocity. Therefore, the landward accretionary prism inferred to be of higher strength constitutes a static backstop. Based on seismic and geologic observations, we interpret that the backstop was generated due to the large age differences of accreted material resulting from an inferred hiatus in subduction between ~13 and 6 Ma. The time-dependent processes such as the igneous activity in middle Miocene further contribute to the development of the backstop. A ridge structure beneath the forearc basin located trenchward of this backstop and running roughly parallel to it appears to reflect activity on an ancient splay fault. The strike of the ancient splay fault runs parallel to the backstop identified in this study and oblique to the current trench. This geometry suggests that location and mechanical behavior of this splay fault system is influenced by the backstop, and its distribution could be related to the coseismic rupture area.