Magnetostratigraphic and Neotectonic Study of Plio-Pleistocene Widespread Tephra Deposits

我们对广泛分布的火山灰沉积进行了古地磁研究，目的是获得日本中部上更新世沉积的磁性地层学和新构造数据。对位于Olduvai次同步臭氧上方的EBisutoge-Fukuda Tephra的磁测量结果如下：1.大阪-京都地区、琵琶湖附近以及Mie和Niigata地区的福田火山灰层及其相关的火山灰沉积产生了相同的地点平均偏角约为-170°，而源火山单元高山地区的EBisutoge火山碎屑沉积的平均偏角约为-L55°。这些结果表明，大阪-京都、Mie和Niigata地区之间没有发生明显的旋转，但高山地区在第四纪期间相对于其他地区经历了顺时针旋转。2.博索半岛的Kd38和Kd39骨灰都显示出反转磁极，表明它们都晚于Olduvai亚时。这些火山灰层即使在倾斜校正后也表现出不一致的位置平均方向，这可能是由于活动板块边缘附近的不同构造旋转所致。3.大谷部-金泽地区的O2火山灰、Kobiwako群的Gokenchaya火山灰和大阪群的Ide火山灰提供了古地磁数据，与福田火山灰一致。4.从Ide火山灰中发现了一个稳定的正极性磁性分量。这种成分是由一种高矫顽力磁性矿物携带的，这种矿物是在正常的极性期间二次形成的。

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.