関東の第四紀地殻変動

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Quaternary crustal movements in the Kanto district, the area where Tokyo is situated in that central part, are overviewed with newly compiled maps (Figs. 1-3), profiles (Fig. 4), and sequence diagrams of vertical crustal movements in twelve parts of the Kanto (Fig. 5). Relations between types of Quaternary crustal movement and types of plate convergence in the area concerned (Fig. 6) are briefly mentioned.<BR>Figure 1 shows the depth distribution of the base of Neogene and Quaternary deposits in the Kanto plain by contours (drawn by halftone lines and figures in km), and the distribution of pre-Neogene rocks in the mountains around the plain and in the continental shelf. Profiles in Fig. 4 show that the plain consists of thick Plio-Pleistocene strata (the Kazusa Group), whose base is marked with K, and also fairly thick middle to late Pleistocene strata (the Sagami-Shimousa Group), whose base is marked with S. These strata were deposited in tectonic basins, called the Kazusa trough and the Kanto tectonic basin. The outline of the Kazusa trough is shown by an oval line with teeth in Fig. 1. This trough-making down-warping seems to be formed as a forearc basin above the subducted Philippine Sea plate (PHS) beneath the Kanto plain, denoted by c in Fig. 6.<BR>The Kazusa trough changed its shape to conform with the Kanto tectonic basin, shown in Figs. 2 and 3, by the uplift of its eastern part during 1.0-0.5 Ma BP. Figure 2 shows vertical crustal movements during the past about 1 Ma by contours (with figures in meters) with late Quaternary faults lines, and Fig. 3 shows approximate vertical crustal movements since the last interglacial period, using the contour lines of the last interglacial depositional surface formed nearly at the interglacial sea level. In Fig. 3, distribution of late Quaternary faultings and foldings are shown : anticlinal and synclinal axes are shown by rows of solid and open circles, respectively. Distribution of Holocene uplift during the past 6 ka is also shown in Fig. 3 by thin dotted lines (height in meters) for the southern coastal area.<BR>The sequence diagrams of vertical movements, deposition, and erosion are illustrated for twelve parts of the Kanto in Fig. 5. Diagrams (1) and (4) show the uplift and erosion of western mountains of the Kanto during the Quaternary, while diagrams (7) and (11) show rapid subsidence and later uplift of southeastern parts of the Kanto during the same period. Diagrams from central parts of the plain, (2) and (6) indicate continuous subsidence through the Quaternary.<BR>Rapid uplift of the western mountains, especially of the Tanzawa Mountains shown by diagram (4) suggests that a collision is occurring at b in Fig. 6 between the northern tip of the Philippine Sea plate (PHS) and a collisional zone (f) between the Northeast Japan arc (NEJ) and Southwest Japan arc (SWJ). The subsidence and later uplift at the southeastern part of the Kanto suggest some relation between the subduction of PHS and that of the Pacific plate (PAC) and their changing features during the Quaternary under the Kanto district.
社団法人東京地学協会の論文

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貝塚爽平
東京都立大学

関東の第四紀地殻変動

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