Quaternary geomorphic trends within Southwest Japan: Extensive wrench deformation related to transcurrent motions of the Median Tectonic Line

Abstract

Large-scale transcurrent faults contribute to anelastic deformations along convergent plate boundaries. The Inner zone of Southwest Japan has been subjected to shearing deformations during the Quaternary, associated with dextral motions of the Median Tectonic Line, which was activated as a right-lateral fault by oblique subduction of the Philippine Sea plate. Considering the neotectonic framework, the continental crust of the Chugoku block in the western part of the Inner zone is simplified as a granitic test-piece under a simple shear. Geological and geographical data in the block are incorporated to produce a quantitative analysis of the deformation mode. Together with an empirical model of the rheology of an orogenic crust, the present results indicate that the Chugoku block behaves as a Newtonian fluid, and flows with a strain rate of about 10 −14 s −1 under a tectonic stress of 10 MPa. The mode of wrenching of the study area is compared with confined deformations of other plate margins in the same tectonic regime. Unique extensive wrench deformation (as wide as 150 km) along the Median Tectonic Line is attributed to the absence of weak zones in the continental crust, such as ductile melange belt or arc volcanism which is a common phenomenon in the area of active convergence. The Inner zone of Southwest Japan is immune from these major factors reducing the shearing strength of the crust because of the intensive intrusions of Cretaceous granites and the regional Quaternary thermal event.