Magnetic Chart Deduced from Paleomagnetism of Widespread Marker Tephras

Aso火山碎屑流沉积由大到小分为Aso-1、Aso-2、Aso-3、Aso-4四个喷发单元。对分布于九州中部的焊接凝灰岩单元所测得的古地磁方向均具有个体特征，且均为正极性。Aso-1和Aso-4的单位平均方向与现在的地磁场方向相似。另一方面，Aso-2的单位平均方向具有陡峭的倾角，而Aso-3的单位平均方向具有极偏东的赤纬。看来Aso-2和Aso-3的不同方向可能有助于识别这些tephras。Aso-4共烟灰岩灰落矿床的平均VGP与Aso-4火山的亚单位相同：大分县的Aso-4A焊接凝灰岩、山口县的宇部火山灰层下部和宫崎县的远端灰流矿床。根据相同的vgp和其他地层证据，推断亚单位是在同一时间沉积的。这表明，在日本列岛的所有地区，磁化是在由同一偶极子场（DF）分量主导的地磁场中获得的；假定DF极点与总体平均VGP重合；在- 3.5°至- 1.5°和44.0°至59.0°之间分别计算出日本群岛上的赤纬和倾斜度。为了明确Aso-4A喷发时的地磁场结构，将调查区域划分为8个区域，并根据区域平均VGP计算每个区域的局部地磁场（LF）方向。通过比较DF和LF， 8个区域均未发现超过（LF）方向a_<95>s的局部地磁异常。如果在Aso-4A火山喷发时存在局部地磁异常，那么这些异常出现在比本研究的八个区域更窄的区域

The Aso pyroclastic-flow deposits are divided into four eruptive units : Aso-1, Aso-2, Aso-3, and Aso-4 in ascending order. The paleomagnetic directions of welded tuff samples determined for these units, which are distributed in central Kyushu, are characteristic of the individual units, and are all of normal polarity. The unit-mean directions for Aso-1 and Aso-4 are similar to the direction of the present geomagnetic field. On the other hand, the unit-mean direction for Aso-2 is characterized by a steep inclination, and that for Aso-3 by an extremely easterly declination. It appears that the distinctive directions of Aso-2 and Aso-3 could be useful in identifying these tephras. The mean VGP for the Aso-4 co-ignimbrite ash-fall deposit is identical to those for subunits of the Aso-4 tephra : the Aso-4A welded tuff in Oita Prefecture, the lower part of the Ube volcanic ash layer in Yamaguchi Prefecture, and the distal ash-flow deposit in Miyazaki Prefecture. From the identical VGPs and … More other stratigraphic evidences, the subunits are inferred to have been deposited at the same time. This indicates that the magnetizations were acquired in a geomagnetic field dominated by the same dipole field (DF) component in all regions of the Japanese Islands ; the DF pole is assumed to coincide with the overall-mean VGP ; declinations and inclinations over the Japanese Islands calculated from the pole range between - 3.5° and - 1.5°, and between 44.0° and 59.0°, respectively.In order to clarify the configuration of the geomagnetic field at the time of the Aso-4A eruption, the surveyed area was divided into eight regions, and the local geomagnetic field (LF) direction of each region was calculated from the regional-mean VGP. Local geomagnetic anomalies, exceeding the a_<95>s of the (LF) directions, could not be detected by comparing DF and LF in all of the eight regions. Had local geomagnetic anomalies existed at the time of the Aso-4A eruption, they have been in the narrower zones than the eight regions of this study Less