Tectonic processes of a Pliocene-Pleistocene sedimentary basin in the uplift belt of inner arc: Case of the Nishime Sedimentary Basin in the Dewa Hills, Northeast Japan arc

In the Northeast Japan Arc, uplift ranges and subsided sedimentary basins form topographic rows parallel to the north-trending arc and trench, and their arrangement reflects the neotectonic movement of the arc-trench system due to the horizontal shortening stress field in the east-west direction. To reconstruct the detailed tectonic processes of the Dewa Hills, one of the remarkable uplift belts in the inner zone of the Northeast Japan Arc, this paper aims at elucidating the Latest Cenozoic succession of the Pliocene-Pleistocene Nishime Sedimentary Basin in the hills. In the Nishime Basin accompanying the Nishime Syncline, the Pliocene Tentokuji and Sasaoka Formations of marine origin, the Nishime Formation of latest Early to early Middle Pleistocene age, deposited under neritic to fluvial environments, and the Yurihara Debris Avalanche Deposits flowing down the Chokai Volcano in middle to late Middle Pleistocene time, are well-exposed in ascending order. The Last Interglacial thalassostatic terrace surface, namely Hj-M1 Surface, is fragmentarily present in the north of the basin. The Nishime Sedimentary Basin continued to exist as a relative subsided area accompanying the movement of the Nishime Syncline from the earliest Pliocene, and filling by a sequence of basin-fills was completed before the deposition of the Yurihara Debris Avalanche in the middle to late period of Middle Pleistocene age, caused by the extinction of the syncline. Since then, the regional uplift has predominated in the whole of the Nishime Basin up to present, and the basin is no longer an active sedimentary basin. The Nikaho Thrust Faults situated on the western fringe of the basin, partly promoted the growth of the Nishime Syncline, but are inferred to have also moved since Late Pleistocene time. From these and some data on the other sedimentary basins in the Dewa Hills, it is concluded that both the uplifted and subsided areas were concurrent during the time from the early Pliocene to the Middle Pleistocene in the Dewa Hills. In contrast, the hills are considered to have grown since Middle Pleistocene time until present as a single uplift zone restricted at its western margin by the Kitayuri Thrust Fault System.