Abstract
The Lower-Middle Miocene Futamata Group, which is exposed at the southwestern-most part of the N–S trending Akaishi Tectonic Zone (ATZ), central Japan, is believed to record important information concerning the several tens of kilometers scale left-lateral motion of the ATZ. This motion was associated with the clockwise rotation of the SW Japan Arc at the eastern margin of the Asian continent, in association with the opening of the Sea of Japan and the northward bending of its eastern margin. Outcrop-scale structures of the Futamata Group in the “Futamata Graven” of the southwestern-most part of the Akaishi Tectonic Line, the western boundary fault zone of ATZ, are described. The Futamata Group has N–S to NW–SE trending, moderately to steeply westward-dipping overturned structures including some outcrop- and map-scale tight/closed folds due to horizontal shortening. Gentle strike-swings about moderately-plunging rotation axes are overprinted on them. In association with the above deformations, the strata are disrupted to varying degrees, forming broken and dismembered units in outcrop-scale. The coeval Ieta Group in the “Manze Graven” of the southeastern-most part of the Komyo Fault, the eastern boundary fault zone of ATZ, also deformed to form folds of a few meters to several hundred meters in wavelength. However, the stratal disturbances in outcrop-scale are not strong as compared to those of the Futamata Group. These sequence of deformations recorded on the Futamata and Ieta Groups might have been intimately related to the left-lateral motion of the ATZ during the latest Early to early Middle Miocene.
