Late Quaternary crustal shortening rates across thrust systems beneath the Ou Ranges in the NE Japan arc inferred from fluvial terrace deformation

Abstract We documented the existence of a blind thrust along the volcanic front in northeast Japan and calculated the crustal shortening rate across the Ou Ranges. We detected broad anticlinal deformation of the longitudinal profiles of fluvial terraces dated by using tephro- and cryptotephrostratigraphy. We inferred that the Daizaki flexure, located along the southern extension of the 2008 Iwate-Miyagi Nairiku Earthquake (IMEQ) fault, indicated the presence of an underlying thrust (Daizaki fault, newly named) there. Therefore, we inferred the existence of a west-dipping thrust system along the volcanic front at the eastern edge of the Ou Ranges consisting from north to south of the Kitakami Teichi Seien Fault Zone, the 2008 IMEQ fault and the Daizaki fault. To estimate rates of crustal deformation due to this thrust, we determined the uplift rate distribution from a pair of accumulation terrace surfaces, one formed during the marine isotope stage (MIS) 6–5 glacial-to-interglacial transition and the other during the MIS 2–1 transition. Because the overall uplift-rate distribution includes crustal deformation at both regional and local scales, we calculated the fault-related deformation area by subtracting the regional uplift rate (0.15–0.18 m ky −1 , obtained from a pair of fluvial terraces on the stable footwall) from the total uplift-rate distribution across the fault. A mass balance calculation of the fault-related deformation area showed the crustal shortening rate across the 2008 IMEQ fault to be 0.50 ± 0.19–0.59 ± 0.22 m ky −1 . An east-dipping thrust system along the western edge of the Ou Ranges (eastern part of the Shinjo Basin Fault Zone) has also contributed to crustal shortening during the late Quaternary at a previously estimated rate of 0.65–1.42 m ky −1 . Therefore, the shortening rates of the eastern and western thrust systems individually account for about 0.6–0.7% and 0.7–1.6%, respectively (total, 1.3–2.3%), of the convergence between the Pacific and Eurasian plates. This westward increase in the shortening rate across the Ou Ranges is in accordance with the crustal structure, which reflects the tectonic history since the Miocene backarc spreading of the Japan Sea.