Tilting Movements of the Japanese Islands Inferred from Cretaceous and Early Tertiary Paleomagnetic Data

抄録

Cretaceous and early Tertiary rocks have been collected, by many workers including the present authors, from about two hundred sites in the Japanese Islands and South Korea for paleomagnetic investigations. The area mean direction of magnetization for the Oshima Peninsula and Okushiri Island of Hokkaido was 30.0° in declination and 58.4° in inclination. It differs distinctly from the area mean direction for granitic rocks from the Kitakami and Abukuma mountains. The area mean direction of magnetization obtained from the northern Kitakami belt was extremely westerly (D=-81.1°, I=22.3°) and that from the southern Kitakami belt was northwesterly (D=-44.4°, I=51.7°). The area mean from the Abukuma belt was also northwesterly (D=-31.9°, I=54.6°). The site mean directions of magnetization for granitic rocks in the Uetsu Province were not significantly grouped in the area. In Southwest Honshu, the area mean direction of the normal samples from various kinds of Cretaceous rocks was 50.2° in declination and 60.5° in inclination and that of the reversed ones was -114.80° and -55.8°, respectively. The normal and reversed magnetizations of early Tertiary rocks have shown similar directions to those of the Cretaceous rocks. The area mean of the normal samples was D=37.6°, I=52.3° and that of the reversed ones was D=-131.4°, I=-44.8°, respectively. The site mean directions of magnetization for Cretaceous rocks in South Korea appear to be slightly northeasterly at each sampling site. The remarkable discrepancy between these area mean directions of magnetization is interpreted as being due to regional tilting movements which occurred in the Japanese Islands since the Cretaceous.