Journal of Geography (Chigaku Zasshi)
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
The Nankai Trench and the Block Movement of Shikoku
Shingo EHARA
Author information
JOURNAL FREE ACCESS

1953 Volume 61 Issue 4 Pages 139-146

Details
Abstract

The Nankai Trench
On the bathymetric chart of the offing of Shikoku, one may distinguish the Nankai Trench running from WWS. to EEN., which lies between the continental slope and the oceanic basin. The trench is enclosed in the 4, 600m. isobath, whose width is 60km. to W, but it tends a little narrower eastward. The bottom is rather flat and shallow, containing subellipsoidal or suborbicular deeps enclosed in the 5, 000m. isobath, which stand in a line parallel to the trench.
To the south of Ise Bay the trench, however, turns to NE, and maintaining the direction, reaches the south of Suruga Bay, and, while approaching the Bay, it becomes narrower in width, but particularly increasing the inclination of the bottom. At the terminal, where the 3, 600m. isobath passes, the trench becomes 10km, in width, and the bottom is lifted to 1, 000m. higher than in the south of Shikoku.
Concerning the tectonic character of the Nankai Trench, it should be necesary to denote the 3 phases of the tectonic movement of the Fossa Magna : 1) The pushing of the Shichito batholith into the outerzone (Akigawa series) of Middle Honshu at the neck of the Izu Peninsula, by the lateral thrust of the Shichito Trench, and the consequent occurrence of the great curving of the Median Line, which is manifested by the Toyohashi-Suwa-B5s5 arc. 2) The making of the Itoigawa-Nirazaki-Hakone line, along which the occurrence of the 60 km. shifting of the eastern wing of the arc by the continual thrust of the Shichito Trench. 3) On the western wall of the Fossa Magna, the building up of the Echelon of Akaishi, Kiso and Hida Mt. ranges which are subjected to the shearing pressure of the 60 km. shifting of the eastern wing.
The time of the tectonic movement is assigned to the beginning of the Miocene. The orogenesis seems, hewever, to have twice *occurred in history, the first, in the beginning of the Miocene, and the 2nd in the early Pleistocene. The latter movement is considered to be the repetition of the movement of the Miocene ; that is, 'the pushing of the Shichito batholith into the Misaka series containing Lepidocyclinl, which formed an arc of the Misaka-Miura-Boso, while the eastern wing of the Fossa Magna is subjected to the lateral thrust from the Shichito Trench, which is manifested by the geologic structure of the Neogene series deposited in the Fossa Magna and the Boso Peninsula.
The Nankai Trench takes part in the tectonic movement of the Fossa Magna : the trend of the trench is parallel to the Median Line and accords with the Echelon of Akaishi, Kiso and Hida Mt. ranges; the narrowing of the terminal and the upraising of the bottom due to the movement of the eastern wing of the Fossa Magna, which moved to NW. along the Itoigawa-Nirazaki-Hakone line.
According to the chart, the Nankai Trench appears to take the course to SE. to the south of the Straits of Bungo and to run along the Kyushu-Palau geanticline of Dr, Hess, though it ought ordinarily to stretch SW. and connect with the Ryukyu Trench.
The oceanic basin which delimits the southern side of the Nankai Trench appears to be the true ocean floor corresponding to that of the Pacific, It extends to SE. to unite itself to the Pacific there is, however, in the way Sulpher volcanic I. which erupted at the beginning of the Recent time. 40 years ago, the oceanic basin is properly termed “South Japan Sea” by Dr. B. Koto.
The thrust under the Nippon Trench from the Pacific bottom may act upon the floor of the South Japan Sea through the Shichito submarine ridge, and may be dynamic source pressing on the Nankai Trench and southern Shikoku.
The Block Movement of Shikoku

Content from these authors
© Copyright (c) Tokyo Geographical Society
Previous article Next article
feedback
Top