On the Fossa Magna disturbance of the Uetsu region and the orogenic movement of the Ou backbone range

Abstract

By the Median groove running northsouth, the Ou region may be divided into two parts: the U-Etsu region, and the Ou backbone range. The former contains the Gan-Etsu Paleozoic mountain land whose northward extension comprises the Akita region which was ever torn and depressed by the pulling force of the Fossa Magna. In the Miocene times the continuous submergence of the floor of the sea and the oil bearing deep sea deposits of the Akita region might be quite due to the pulling of the Echigo Mountain Range which wes dragged by the Kwanto Range of the Fossa Magna. But the latter comprises the Paleozoic Abukuma and part of the Kitakami range, of which the former was subjected to the dynamic activity of the Futaba fault running northsouth. In this movement the detached blocks cut by the fault havesunkrapidly within the earth, drawing the transgression sea water but at the same time spouting the Andesite lava. It formed a geosyncline which was filled in the next phase by the abundant gushing mass of Green Tuff. Since then the backbone range has kept on upheaving and the Akita region submergence. Both the Futaba fault and the Fossa Magna disturbance may have come from the same origin and be regarded as the manifestation of the Circum-Pacific orogenic energy at that time.