Some facts on activity of southern part of the Kamogawa Lowland fault zone, Boso peninsula, central Japan

Abstract

　 The Kamogawa lowland fault zone had been recognized as a highly active fault zone in the southern Boso peninsula. Many researchers had tried to make clear the activity of this fault zone, however no one could get definite evidence of activity of this fault zone during the Quaternary era. The author tried to describe fundamental features of this fault zone, so carried out re-interpretation of fault landscapes, field geological survey and observation of minor faults in the Neogene deposits around the southern part of the Kamogawa lowland. Four topographic lineaments occur in the study area, they are characterized by series of knick points on hill slopes, and partly characterized by ill-systematic left and partly right-lateral bend of streams. The northern two major lineaments facing north are coincident with geological faults cutting the Neogene deposits with north-side upheaval displacement. The other two lineaments locate south of the Kamogawa lowland, and have left-lateral bend of streams. There are many low-dipping to horizontal slickenlines on the minor fault plain. Many minor faults accompany with hard consolidated fault gauge, but some of minor faults accompany with unconsolidated fault clay and sand. These facts suggest that the two major northern lineaments have contrary sense between topographic features and geological structure, the former suggest north-side subsidence motion in the latest geological age, and the later indicate north-side upheaval movements since the Neogene. Other southern two lineaments have harmonious sense between lateral bend of streams and low angle slickenlines on the minor faults in the Neogene deposits. The author thinks that more researches have to be done to make clear the activity of southern two lineaments during the late Quaternary.