Late Quaternary crustal shortening rate across the Shinjo basin, northeast Japan

摘要

The active fault and fold belt in the eastern Shinjo basin has contributed to late Quaternary crustal shortening of the northeastern Japan arc. The geometries of several deformed strata and geomorphic surfaces recorded different rates of cumulative shortening. Fluvial terraces as old as 290 ka were dated by ~(14)C and fission track dating and tephrochronology. We used published seismic profiles to constrain the deep structure of the belt. Vertical offsets of different terrace surfaces show that the slip rate of the Chojahara fault in the innermost basin has increased through time. On the basis of its seismic profile, this fault was interpreted to dip at a high angle. The Funagata anticline between the Chojahara fault and the Ou ranges was probably caused by a low-angle blind thrust at about a 2 km depth. The different geometries between the basin sediments and fluvial terraces caused by the blind thrust indicate downward propagation of the thrust through time. Synchronous development of the Chojahara fault (increased slip rate) and the blind thrust beneath the Funagata anticline (downward propagation) suggests that these faults form a piggyback fault system connecting to the deep part of the basin-bounding Kyodanbara fault. Estimated crustal shortening rates are averaged to be 0.65-1.05 and 0.79-1.42 m/kyr over the last 200-260 and 12-28 ka, respectively, accounting for about 0.7-1.6% of the convergence between the Pacific and Eurasian plates. Estimated strain rates are 0.07-0.11 and 0.080.14 ppm/yr for the last 200-260 and 12-28 ka, respectively.