A Study on the Morphogenetic Movements Associated with the Formation of the Sekita Mountains and the Iiyama Basin, Central Japan

We studied the morphogenetic movements associated with the formation of the Sekita Mountains and Iiyama Basin situated at the boundary between the Niigata and Nagano prefectures in central Japan. As a result, we obtained following conclusions. (1) The morphogenetic movement of the Sekita Mountains is a semicylindrical upwarping originating from faulting at both sides of the mountains. The origin was an inclined thrust of a basal block. In addition, it is assumed that the upheaval of the mountains due to a change in isostasy caused by erosion resulted in the four-thousand meter thick strata that constitutes the mountains, and erosion originated from a fold in the strata occurring in the middle Pleistocene. (2) The morphogenetic movement constituting the Iiyama Basin is a reverse fault, which originated from tilting and drawing of the basal crust as a result of a semicylindrical upwarping of the Kato Mountains situated at the east side of the Iiyama Basin. The upwarping was caused by magma ascending from a deep part of the crust. Therefore, the Iiyama Basin was formed as a fault-angle basin. The mountain body of Kenasi Volcano tilted toward the west through the process of forming the Iiyama Basin. We think the Iiyama Basin continued to subside at the rate of about one millimeter a year in the Holocene, based on radiocarbon dating of drilling core samples collected from sediments in the alluvial lowlands.