Magnetostratigraphic and Neotectonic Study of Plio-Pleistocene Widespread Tephra Deposits

我们对广泛分布的火山灰沉积物进行了古地磁研究，旨在获得日本中部上新世-更新世沉积物的磁性地层学和新构造数据。通过对奥杜威次臭氧层上方的Ebisutoge-Fukuda火山灰层的磁性测量，得到以下结果：1.大坂-京都地区、琵琶湖附近、三重和新泻地区的Fukuda火山灰层及其相关火山灰沉积物的平均磁倾角约为-170°，而高山地区的源火山单元Ebisutoge-Fukuda火山碎屑沉积物的平均磁倾角约为-155 °。这些结果表明，大坂-京都、三重和新泻地区在1980年代没有发生明显的自转，而高山地区相对于其他地区发生了顺时针自转。2.房总半岛的Kd 38和Kd 39火山灰都显示了相反的磁极性，表明它们都晚于奥杜威亚时。这些火山灰层即使经过倾斜校正后，也显示出不一致的场地平均方向，这可能是由活动板块边缘附近的差异构造旋转所解释的。3. Oyabe-Kanazawa地区的O2火山灰、Kobiwako群的Gokenchaya火山灰和Osaka群的Ide火山灰提供了古地磁数据。4.在井出火山灰中发现了稳定的正极性磁性成分。该成分由高磁性磁性矿物携带，该矿物在正常极性期间次生形成。

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.