A Study of Active Faults Related to Folding, Estimated from the Deformation of Fluvial Terraces in the Tokamachi Basin, Central Japan

Abstract

Active faults related to active folding and active folds are discussed based on aerial photographs interpretation, detailed topographic analysis, and geologic surveys in the Tokamachi Basin on the middle reaches of the Shinano River. A flight of terraces formed since 0.5Ma is widely distributed in this area. Terraces are underlain by fluvial gravel beds, 2-10m thick, resting on the Pliocene-Pleistocene Uonuma Group. The terrace deformation of their cross profiles is concordant with the asymmetric folded structure of the Uonuma Group. Mainly five active reverse faults (Konegishi fault, Shimojyo fault, Miyaguri fault, Tamagawa fault, and Tsunan fault) are distributed in this area. Shimojyo fault, Miyaguri fault, and Tsunan fault can be classified based on the type of fault-related folding as follows. Shimojyo fault is a bending moment fault that is produced by shortening on the concave side of a folded layer. Miyaguri fault and Tsunan fault are main thrust faults whose activation has been independent from the growth of folding in Tokamachi Basin. The folded structure of the Uonuma Group and its terrace deformation indicate that active folding, which has continued for at least the last 0.5Ma in Tokamachi Basin under compressive stress, has been more active in the northern part than in the southern part. The horizontal compressive rate in this area is presumed to be 1.4-2.7mm/yr based on the netslip rate of Miyaguri fault.