Abstract
A fault zone juxtaposes the allochthonous Yorii Formation and Yorii Welded Tuff overlying the Mikabu greenstones of the Sambagawa belt. This boundary fault zone, about 5 m in width, is composed of breccia and 3∼5 cm-wide gouge zone along the fault surface trending N51°E, which is different from the fault systems of this area striking WNW-ESE or N-S. The hanging wall rock, Yorii Formation, exhibits fault rock fabrics such as imbrication structures and composite planar fabrics, suggesting a dextral sense of shear. Restored strike of the boundary fault considered from the rotation of the Kanto Mountains accompanying the formation of the Japan Sea (clockwise rotation of 94°) and that in the nappe-forming process of the Yorii Formation (counterclochwise rotation of 73°) indicates about N30°E, suggesting the parallelism to the Median Tectnic Line (MTL) of the Southwest Japan before the opening of the Japan Sea. Thus, the fault zone is considered to have occurred accomanied with the activity of the MTL. The K-Ar ages were determined on fine fractions of clay minerals (<2μm) extracted from the fault gouge, resulting 23.6 ±0.6 Ma. This suggests the last motion of the fault having occurred before this age and is compared to the Akaishi phase (15-27 Ma) in the movement phase of the MTL. As a result, the boundary fault zone is considered to have formed in association with the activity of the MTL at the heimat of the Yorii Formation and the Yorii Welded Tuff in the Inner Zone of the Southwest Japan, stayed there until 23.6 Ma and thereafter was brought to the present place together with the Yorii Formation and the Yorii Welded Tuff.
