Abstract
The Dewa Hills in Northeast Japan are a tectonic uplifted zone parallel to the main direction of the Northeast Japan arc, bounded by the Kitayuri Thrust Faults on the western side under a compressional stress field. We examine the uplift process of the hills, related to the active tectonic structure due to crustal shortening, on the basis of the surface deformation pattern derived from the fluvial terraces and fault dislocation models. The fluvial M1 terrace is identified as the last interglacial terrace that corresponds to oxygen isotope stage 5e, depending on the stratigraphic relation between the 112-115ka Toya volcanic ash and terrace deposits. The displacement of the M1 terrace shows asymmetrical warping with uplifting at a maximum uplift rate of 0.5mm/yr. A simple dislocation model to explain the asymmetrical surface deformation requires a reverse fault with detachment at the depth of 6km under the Dewa Hills. The calculated fault geometry suggests that the uplift of the Dewa Hills is accompanied by the growth of a fault-bend fold. The horizontal strain rate under the Dewa Hills is estimated to be 6.6×10-8/yr in late Quaternary. This strain rate indicates highly horizontal crustal shortening after 3.5Ma at the Dewa Hills in the late Quaternary. The amount of slip on the modeled fault significantly decreases in the upper crust shallower than 6km, compared with that under the depth of 6km. The characteristics of slippage of the fault plane show the difference in the crustal shortening processes between deeper and shallower part of the fault.
