Tephrochronology of Late Quaternary Strath Terraces and Their Implications to Neotectonic Movements in the Shitada and Tochio Regions of the Niigata Basin, Central Japan.

Abstract

Late Quaternary strath terraces younger than 50ka were investigated in the Shitada (Ikarashi River) and Tochio (Kariyada River) regions, central eastern margin of the Niigata Basin, in order to clarify their chronology and implications to neotectonic movements. The terraces were classified based on geomorphologic configuration and tephro-stratigraphic relation between marker tephra horizons and the terraces. Six tephra horizons (ST 1-ST 6) were found in the loam and black soil, which overlie terrace deposits. ST 4 is correlated with the widespread tephra As-K (13ka), ST 5 with AT (25ka) and ST 6 with Dkp (50ka). The terraces are in chronologic order Sh-D, Sh-C, Sh-B, Sh-A3, Sh-A2 and Sh-A1 terraces for the Shitada region and To-D, To-C, To-B, To-A4, To-A3, To-A2 and To-A1 terraces for the Tochio region.The presence of strath terraces since the post-glacial transgression indicates that intermittent local and/or regional uplift initiated terrace development. Same aged terraces had different relative heights from the present riverbed across NNE-SSW and NW-SE trending active faults. These discontinuities cannot be simply explained by tilting due to active folding. We propose that the NNE-SSW and NW-SE trending fault systems have been active as the result of block segmentation, which characterize the uplift movement of the Shitada and Tochio regions since 50ka.