Late Quaternary evolution and activity of the northern part of the western marginal fault zone of Nagano Basin, central Japan

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The western marginal fault zone of Nagano Basin has been a tectonically active in late Quaternary, appearing as a reverse fault by strongly crustal shortening in the northern Fossa Magna area, central Japan. The Iiyama Basin is a northern part of the tectonically subsided basin by such faulting. However, two groups of evenly running faults have migrated forward since middle Pleistocene, accumulatively deforming the fluvial terraces, and the previous basin area around the Chikuma River changed the upthrown side. Average vertical slip rate is evaluated to be> 0.2 m/ka in the eastern group of migrated faults, and around lm/ka in the western group, based on the displacement and age of fluvial terraces. This indicates that both migrated fault groups have been active in late Quaternary, possibly partitioning the fault slip by horizontal crustal shortening.