Precise mapping of active faults along the Itoigawa-Shizuoka Tectonic Line in the southern part of Matsumoto Basin

Abstract

Precise air-photo interpretation of tectonic landforms in the south of Matsumoto basin was made in order to clarify the overall nature of active faulting along the Itoigawa-Shizuoka Tectonic Line. The fault zone consists of both thrust and strike-slip faults. At the eastern boundary of the basin is an east-dipping thrust fault, which has become inactive progressively from the south to the north in late Quaternary time. Three to four kilometers west of the boundary fault is another west-dipping thrust fault, which, however, is still active in Holocene time. Thus, the thrust front is likely to have migrated basin-ward in late Quaternary time. A purely strike-slip fault (Gofukuji fault) exists on the upthrown side of, and parallel to, the boundary fault; the Gofukuji fault has a slip rate as high as 8.6±1.0 mm/yr and is still active. Our observations indicate that the overall movement on the Itoigawa-Shizuoka Tectonic Line at depth is transpressional and is transferred at shallow depths into both thrust and left-slip movements. We therefore interpret that significant slip partitioning is taking place on the up-dip extension of an east-dipping fault, which was originated from a normal fault bordering the west margin of the Northern Fossa Magna rift of early and middle Miocene age, and has rejuvenated as oblique slip fault since Pliocene time.