Late Pleistocene to Holocene Uplift Rates in the frontal part of the Fold-and-Thrust Belts in Northern Hokkaido, Japan

In Hokkaido to Sakhalin, Late Cenozoic compressional processes have produced N-S trending fold-and-thrust belts. Active tectonics started in late Pliocene time (Ito, 1999) was poorly constrained from the previous studies on seismicity, geodesy and structural geology because of the complicated tectonic framework. In northern Hokkaido, coarsening upward Pliocene-Pleistocene basin-fill successions, which named the Koetoi, Yuchi and Sarabetsu Formations in ascending order, have been deformed by west verging stepped folds structures. The Teshio fault zone constitutes the frontal part of the fold-and-thrust belts and contributes to the history of uplifting along the western coast of northern Hokkaido. Terraces distributed in the study area are classified into Higher terraces group (Terrace I-III), Middle terraces group (Terrace IV) and Lower terraces group (Terrace V-VI). On the basis of tephrochronology and 14C dating methods, Terrace IV and VI are correlated with MIS 5e and 1, respectively (Koike and Machida eds, 2001). The author clarifies the tectonic processes of the Teshio fault zone by using the dislocation model and balanced cross-sections to determine the Plio-Quaternary shortening deformation. The N-S trending Kitakawaguchi anticline is a typical fault-propagation-fold defined by a steeply west dipping forelimb and a more gently east dipping and broader back limb. Western part of the anticline, the Yuchi Formation has up to 30° dip to the west, and Terrace II and IV tilt westward. The elevation of the shoreline angle of Terrace IV and II are about 55-60 m and 110 m a.s.l., respectively. The Kitakawaguchi anticline has started to grow in the Early Pleistocene when the Yuchi Formation deposited, and has continued to move during Late Pleistocene time, shown in the seismic reflection profile (Ogura and Kamon, 1992). The Teshio fault zone extends 5-10 km west off shore farther south of the Kitakawaguchi anticline. According to Hydrographic Department, Maritime Safety Agency (1992, 1994), Early-Middle Pleistocene successions containing stratigraphic growth architecture have thickly deposited in front of the forelimb of the flexure. In consideration of the Late Pleistocene to Holocene uplift of the coastal area, the Teshio fault zone has continued to develop during Quaternary. In order to evaluate the activity of the Teshio fault zone, Echigo et al. (2006) simulated the surface deformation for the blind thrust beneath the Teuri and Yagishiri islands using the dislocation model. Assuming a east- dipping (20°) fault for a total length of 40 km at depth of -500 to -800 m a.s.l., the amount of net slip since the Last interglacial is calculated at 50 m, and the average slip rate attains to 0.40 mm/yr. Further studies are needed to constrain the amount and rate of shortening since late Pliocene time.