National Institute of Advanced Industrial Science and Technology (AIST) This page is a page of the former research institute. We stopped updating on March 31.2001.
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National Institute of Advanced Industrial Science and Technology (AIST) This page is a page of the former research institute. We stopped updating on March 31.2001.
E-mail to webmaster (Japanese) E-mail to webmaster (English)

Tectonics

The Japanese islands have been locetad on the northwestern Pacific region, where oceanic plates are subducting beneath the Asian continent. Recent paleomagnetic investigations revieled that the back-arc openning of the Japan Sea took place during Miocene. In addition, collision of the Izu-Ogasawara arc on the subductiong Philippine Sea Plate deformed central part of Japan on the Eurasian Plate. Recent E-W trending compressional tectonism induces the buckling deformation of Northeast Japan and subducting of the Japan Sea crust under the northeast Japan arc.

Rotation of the Kanto Mountains

One of the most characteristic features in geologic structure of the Japanese islands is the cuspate sturucture of basement rocks in central Japan. This bent structure was already noticed since last century and it was named as the Kanto Syntaxis. Between the Akaishi Mts. (west-wing) and the Kanto Mts. (east-wing), northern tip of the Izu-Ogasawara volcanic arc on the Philippine Sea Plate is located. This strongly suggests that the northward convex was formed by the collision of the Izu-Ogasawara arc related to the northwestward migrating Philippine Sea Plate.


As for the research concerning the rotation of Kanto Mountains, a quantitative discussion became possible by measuring the paleomagnetism of the strata during recent years. The earth's magnetic field was acquired as a fossil of magnetism when the rock was formed, which has been preserved for long years. Therefore, if the paleomagnetic direction and the age of the strata are measured, we can understand the tectonic rotation of wide area.
Hyodo & Niitsuma (1986) first applied paleomagnetic measurements for the sedimentary rock in the Chichibu Basin, central part of the Kanto Mts. They obtained eastward deflected paleomagnetic directions from 11 sites, and concluded that the Kanto Mts. rotated clockwise of abouut 90K safter formation of the Chichibu sedimentary sequence. As the age of the strata in the Chichibu Basin is approximately 15 Ma, based on the planktonic microfossils yielded, the rotation of the mountains took place after 15 Ma.


Takahashi & Watanabe (1993) performed the paleomagnetic investigation onthe Middle Miocene volcanic rocks in the Uchiyama area, northwestern corner of the Kanto Mts., and showed easterly deflected paleomagnetic directions of about 35K. Based on the K-Ar dates, the eruption of these volcanic rocks occurred at around 12 Ma. Therefore, it can be concluded that the Kanto Mts. rotated clockwise of about 50K to 60Kbetween 15 and 12 Ma, and 35K after 12 Ma.
Paleomagnetic investigation of the Chichibu Quartz Diorite, intruded in the central part of the Kanto Mts., gave a younger limit of the cease of rotation. The paleomagnetic directions of the Chichibu Quartz Diorite show N-S trending, indicating that the rotation after intrusion of this diorite is negligible. As the K-Ar ages of the diorite indicate that the Chichibu Quartz Diorite intrudedand cooled at ca. 6Ma, the Kanto Mts. has never rotated since 6 Ma. Therefore, about 35K clockwise rotation of the Kanto Mts. took place between 12 and 6 Ma.
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