Identification of Lower Pleistocene tephras under Tokyo and reconstruction of Quaternary crustal movements, Kanto Tectonic Basin, central Japan

摘要

The Kanto Tectonic Basin (KTB) originated from a paleo-forearc basin, and is filled with the Pleistocene Kazusa Group, Quaternary marine sediments with a thickness of more than 3000 m. In order to understand general process of emerging paleo-forearc basin development on active plate margins, an attempt was made to establish the tephrochronological framework of the Kazusa Group under the Tokyo metropolitan area, southwest KTB, and reconstruct changes in depositional environments and geological structure. Chemical compositions of major, trace and rare earth elements of glass shards, and refractive indices of glass shards and phenocrysts, were used to identify 12 Early Pleistocene tephras collected from five sediment cores, including seven tephras newly recognized under the Tokyo metropolitan area: Kd8B, YM, KK, Ob4e-Kd23A, Ob4d-Kd23B, 0b4c-Kd24, Ob4b-6, Ob4b-5, Ob4b-4, 0b4b-l, Omn-SK110, and SYG, in descending order. Calcareous nannofossil biostratigraphic and magneto-stratigraphic positions of 24 tephras discussed in this paper, including 17 known tephras described by previous studies, indicate that their ages range from ca. 0.9 Ma to ca. 2.0 Ma. Northward-dipping strata with a gradient of about 0.015 of the base of the Johoku Sand and Gravel Bed (1.219-1.343 Ma) indicates that south-side was uplifted, caused by crustal movements with a mean ratio of 1.2 × 10~(-5)/ky since its deposition. This movement has continuously occurred, deforming MIS 5.5 marine terrace surfaces. The depths of SYG and Ob4c-Kd24 suggest that the deformation of the southwestern part of the NE-SW trending Mizonokuchi Syncline is significantly lower than that of the northeast part. This syncline extends northeastward to the southeastern part of the Musashino Upland. Activity of the Mizonokuchi Syncline probably was terminated at the end of the Early Pleistocene to Middle Pleistocene. The Tachikawa Active Fault (TAF), trending NW-SE, has been active since the Middle Pleistocene. The termination of the Mizonokuchi Syncline and the activation of TAF simultaneously occurred roughly around 1.0-0.5 Ma. These tectonic changes most likely resulted from a change in motion direction of the Philippine Sea Plate relative to the North American Plate.