Late Quaternary Geomorphological Development of the Mikata Five Lakes Lowland in Central Japan Based on the Subsurface Geology and Fault Structures of the Mikata Fault Zone

The Mikata Five Lakes and their surrounding lowlands in the northern part of the Kinki district, central Japan, have been formed due to the activity of the Mikata Fault Zone extending north to south along the eastern margin of the lake area. The Nakayama lowland northeast of Lake Mikata is located closest to the fault zone and its detailed subsurface geology may be a key to understanding the late Quaternary activity and structural development of the Mikata Fault Zone. Three new cores drilled in the lowland are analyzed and sedimentary facies analysis, radiocarbon dating, tephra analysis, and sulfur analysis are conducted to establish the chronology and correlation of the core sediments, and to reconstruct paleoenvironmental changes since the Marine Oxygen Isotope Stage (MIS) 6. Based on sedimentary facies and results of the analysis, four sedimentary environments are deduced as follows: 1) fluvial deposition by the Paleo-Hasu River flowing through the lowland during MIS 6, 2) coastal conditions due to marine transgression during MIS 5e, 3) fluvial deposition until MIS 3, and 4) marshy environment after the beginning of MIS 3 at around 50 ka. Isochronous horizons of subsurface sediments in the lowland indicate accumulated deformation due to the activity of the blind fault, the strike of which is evaluated to be WNW–ESE, as opposed to the previous estimation of N–S. The upper limits of marine-influenced deposits during MIS 5e reveal large differences in relative elevation of 71-75 m between Lake Mikata and the Nakayama lowland. This may have been caused by the activity of another blind fault between the two areas. The two blind faults are considered to be the southeastern extension of the Hiruga fault, suggesting the structural development of the Mikata Fault Zone since MIS 6.