Non-Uniform Stress Field of the Forearc Region in Middle Miocene Southwestern Japan Inferred From the Orientations of Clastic Dikes and Mineral Veins in the Tanabe Group

Abstract

The Middle Miocene was a period of tectonic transition in the southwest Japan arc. The cessation of backarc spreading at 15 Ma has been thought to have caused a pronounced change in the overall stress state from tension to compression in the arc-perpendicular direction. However, the spatial variation in stress throughout the arc, especially in the forearc, which reflects the characteristics of plate subduction, has yet to be delineated. This study investigates the stress field of the forearc region and its temporal change by applying a stress tensor inversion technique to outcrop-scale dilatant fractures. We measured the attitudes of clastic dikes and mineral veins in the Lower–Middle Miocene Tanabe Group, the paleo-forearc basin deposits in the southwestern Kii Peninsula. Results of paleostress analyses show that the Tanabe Group was influenced by (i) stress with an E–W minimum compressional axis before 15 Ma and (ii) stress with a NW–SE maximum horizontal compressional axis after 15 Ma. In contrast to conventional interpretations, we find that the former stress differed from arc-perpendicular tension in the backarc region before 16 Ma. The latter stress differed from trench-parallel tension on the southernmost Kii Peninsula. Such spatial non-uniformity of the forearc stress can be explained by the occurrence of a dynamic backstop, such as a landward area of the out-of-sequence thrust, beneath the Tanabe Group.