Active Tectonics along the Western Margin of Kitakami Lowland: Tectonic Geomorphology and Subsurface Structure of the Nanshozan Fault Group, Northeast Japan

The Kitakami Lowland fault zone (KLFZ) is an active reverse fault zone that extends about 70 km eastward along the Quaternary volcanic front of the northeast Japan arc. The activity of this fault zone in the Quaternary is mostly recognized as a fault reactivation of Miocene normal faults in the area. We conducted a high-resolution seismic reflection profile survey, Nanshozan 09, to define the geological structure of the Nanshozan fault group, northern portion of the KLFZ. The Nanshozan fault group is composed by three reverse faults, F1, F2, and F3 faults, and deforms late Quaternary fluvial terraces and alluvial fans in the Kitakami Lowland. Several isolated andesite hills such as Jonaisan and Kitayachi-yama, line the eastern edge of the hanging-wall of the F3 fault. The high-resolution seismic reflection profile shows that the fault scarps and tilted fluvial terraces are traced down to a depth of 2 km along the Nanshozan fault group, formed by an west-dipping high-angle reverse fault (F1 fault) and along two of the thrusts that have a flat-ramp geometry (F2 and F3 faults). Furthermore, the results of K-Ar dating and field observations show that the Jonaisan hill is certainly composed of Pliocene andesitic rocks, not Quaternary intrusives. Cross-section restoring across the Nanshozan fault group implies that F2 fault activity occurred after the initiation of F3 fault activity. Hence, isolated hills near F3 fault, such as Jonaisan and Kitayachi-yama, are recognized as volcanic blocks that survived long periods of erosion while being deformed by F3 fault thrusting.