A Horizontal Buckle Model as a Dynamic Mechanism for Back Arc Spreading of the Japan Sea

Abstract

A horizontal buckle model is proposed for the geodynamics of Neogene clockwise rotation of the SW Japan arc and the back arc spreading of the Japan Sea. The model explains the dynamics and kinematics of the formation of megakink bands developed extensively in the Shimanto belt of the Outer Zone of SW Japan (Yanai 1986a, 1986b). Using the megakink bands as indicators of stress orientation and shortening magnitude, the regional $\sigma_{1}$ axial trajectory ($\sigma_{1} > \sigma_{2} > \sigma_{3}$) is longitudinal and almost parallel to the long axis of the SW Japan arc, and the $\sigma_{2}$ axis is directed nearly vertical. The shortening magnitude increases up to 22%, crossing SW Japan from Japan Sea to the Philippine Sea. These data suggest that the rotation mode of SW Japan is analogous to buckle folding with vertical rotation axes, in which the external compressional force is exerted parallel to the arc trend. The SW Japan wing was moved with reference to the nearly fixed Ryukyu wing, forming the inner ...