Intraplate shortening and underthrusting of a large basement ridge in the eastern Nankai subduction zone

Abstract

Due to the collision of the Izu-Bonin arc with central Japan, the subducting Philippine Sea oceanic plate is affected by lithospheric shortening seaward of the eastern Nankai trough, in the Zenisu ridge area. Seismic reflection and refraction profiles across the ridge show a series of low-angle thrusts cutting through the entire oceanic crust. They also indicate that the formation of this ridge is related to a single lithospheric thrust with ∼10-km throw, that inserts a corner of upper mantle within the lower crust. We propose that the serpentinized mantle–crust transition acts as a decoupling level above which distributed shortening results in the formation of a crustal prism. Although the amount of deformation is much smaller than in a mountain belt such as the Pyrénées, a parallelism can be proposed between both systems that correspond to intraplate shortening and have in common a single mantle shear zone and overlying distributed crustal deformation. The underthrusting below the eastern Nankai accretionary prism of a similar and now inactive tectonic structure, the Paleo-Zenisu ridge, is evidenced by the deformation pattern of the margin. This ridge is probably detached on top of the subduction décollement, at least where volcanoes are impinging the continental margin. Part of the ridge may thus be accreted to the prism backstop. We show that both Zenisu and Paleo-Zenisu systems accommodate the same N–S compression related to convergence partitioning process. This subduction kinematics can be understood in terms of a steady state model, partially constrained by the trails of volcanoes that have impinged the margin. N–S shortening is accommodated by the thrusts seaward of the trough and right-lateral motion is prevalent along the subduction system. Based on these kinematics, we propose that the formation of the Paleo-Zenisu and Zenisu ridges are related to the successive collisions of volcanic massifs of the Izu-Bonin arc with the Japanese arc. The Zenisu system is a result of the collision of the Izu block since ∼2 Ma, while the Paleo-Zenisu system was the result of the earlier collision of the Tanzawa massif, between 5 and 3 Ma.