HOKUSATSU BEND AND CLOCKWISE ROTATION OF THE SOUTHWEST JAPAN ARC

The Hokusatsu Bend, one of the major bends in South Kyushu, is mapped in detail. The Bend affects the Shimanto Supergroup and the Sambosan Group, and is accompanied by N-S right-lateral and NW-SE left-lateral strike-slip faults. Geometrical analysis has revealed that folds developed within the Hokusatsu Bend are conical style, like those of the Hitoyoshi Bend. They were formed by the flexural-slip folding of formerly inclined strata with vertical fold axes. The vertical cone axes of the conical folds later tilted about 30° toward west-northwest. Thus, the Shimanto Supergroup around the Hokusatsu Bend has a steeper dip than that of the other areas in Kyushu. The conical folds of the Bend were formed associated with the clockwise rotation of the Southwest Japan Arc in Miocene time. The Shimanto and Sambosan Terrains of Kyushu, to the northeast of the Hokusatsu Bend, rotated clockwise about 30°, and those to the south rotated counterclockwise about 30°-40°.