Lower Quaternary of Kyushu Island, with special reference to its tectonic Framework

Abstract

The Oita Group and its equivalent strata, extensively developed in the middle Kyushu, are documented with the tremendous volcanic products and Villafranchian to early classical Quaternary fauna and flora. Regional geologic evidence shows that the strata occupy a part of the Second Setouchi basin, which represents the final collapse of the axial zone of the Honshu geanticline at the last phase of the geosynclinal cycle. However the strata in the middle Kyushu markedly differ from those of other parts of the Setouchi by the extensive volcanism and great thickness.The basin of the middle Kyushu is divided into two parts by the Kunizaki-Chikugo upheaval zone of NEE-SWW trend. The southern part is a half graben bordered by the Oita-Kumamoto fault line at the south. The half graben and its north and south upheaval areas are corresponded respectively with the area of negative and positive Bouguer anomaly. This fact suggests that the early Pleistocene events have the relation to the geophysical character of present time in the middle Kyushu. Hence the early Pleistocene basin is examined in regard to the present geophysical features. There is a distinctive zone of the negative isostatic anomaly along the east coast of Kyushu, where the crust is still sinking with deeper MOHOROVICIC plane. The focal plane of the medial- and deep-focused earthquakes intersects with the earth surface in this zone and the intersection coincides with the zero line of the Bouguer anomaly. It should be noted about the earthquake that the more north the position, the shallower the foci. These features seem to be the expression of the Ryukyu arc at its northern extremity. Thus the overlap of the activities of the younger Ryukyu arc to the final collapse of the Honshu geosyncline results the unique geologic events in the middle Kyushu.