Magnetostratigraphic and Neotectonic Study of Plio-Pleistocene Widespread Tephra Deposits

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We made paleomagnetic studies of widespread tephra deposits aiming to obtain magnetostratigraphic and neotectonic data for the Plio-Pleistocene sediments in central Japan. Magnetic measurements of the Ebisutoge-Fukuda tephra, which occurs just above the Olduvai subchronozone, provided the following results :1.The Fukuda volcanic ash layer and its correlative ash deposits in the Osaka-Kyoto area, near Lake Biwa and in the Mie and Niigata areas yielded identical site mean declinations of approximately -170°, while the source volcanic unit, Ebisutoge pyroclastic deposits in the Takayama area, showed a mean declination of approximately -l55°. These results suggest that no significant rotation occurred between the Osaka-Kyoto, Mie and Niigata areas, but that the Takayama area suffered a clockwise rotation in respect to the other areas during the Quaternary.2.The Kd38 and Kd39 ashes in the Boso Peninsula both showed reversed magnetic polarity, suggesting both postdate the Olduvai subchron. These volcanic ash layers showed discordant site mean directions even after tilt correction, which may be explained by differential tectonic rotations near the active plate margin.3.The O2 volcanic ash in Oyabe-Kanazawa area, The Gokenchaya volcanic ash of the Kobiwako Group, and the Ide volcanic ash of the Osaka Group provided paleomagnetic data, which are consistent with the Fukuda tephra.4.A stable magnetic component of normal polarity was found from the Ide volcanic ash. This component is carried by a high-coercivity magnetic mineral, which was formed secondary during a normal polarity period.