Magnetic Chart Deduced from Paleomagnetism of Widespread Marker Tephras

从广泛分布的标记火山石的古地磁学推导出的磁图

• 批准号: 12680158
• 负责人: NAKAJIMA Tadashi
• 金额: $ 2.05万
• 依托单位: Fukui University
• 依托单位国家: 日本
• 项目类别: Grant-in-Aid for Scientific Research (C)
• 财政年份: 2000
• 资助国家: 日本
• 起止时间: 2000 至 2001
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-12680158/
• 关键词: Paleomagnetism; Aso-4 Tephra; Aso-4 Co-ignimbrite Ash-fall Deposit; Magnetic Chart; Age Determination; Aso Pyroclastic-flow Deposit; Late Pleistocene; 阿蘇4火山灰; Paleomagnetism; Aso-4 Tephra; Aso-4 Co-ignimbrite Ash-fall Deposit; Magnetic Chart; Age Determination; Aso Pyroclastic-flow Deposit; Late Pleistocene; 阿蘇4火山灰

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

ASO火山碎屑流沉积物分为四个喷发单元：ASO-1，ASO-2，ASO-3和ASO-4按升序顺序排列。针对这些单元确定的焊接凝灰岩样品的古磁方向，这些单位分布在京都中部，是单个单元的特征，都是正常的极性。 ASO-1和ASO-4的单位方向与当前的地磁场的方向相似。另一方面，ASO-2的单位均值方向的特征是钢倾斜度，而A​​SO-3的特征是极端东方的偏斜。看来ASO-2和ASO-3的独特方向在识别这些tephras中可能很有用。 ASO-4共同烟灰 - 灰烬矿床的平均VGP与ASO-4 Tephra亚基的平均VGP相同：ASO-4A the the oita县的ASO-4A焊接凝灰岩，Yamaguchi县的UBE火山灰层的下部以及Miyazaki Prefutector的Ash-flow Propectoctor。从相同的VGP和…其他地层证据中，可以推断出亚基同时沉积。这表明磁化是在日本群岛所有区域以相同的偶极场（DF）组件为主的地磁场中获取的；假定DF极与整体均值VGP一致；从极点计算出的日本岛上的下降和倾向分别在-3.5°和-1.5°之间，分别在44.0°和59.0°之间。为了阐明在ASO -4A爆发时澄清地震磁场的配置VGP。无法通过比较所有八个区域的DF和LF来检测到（LF）方向的A_ <95> s的局部地磁异常。如果在ASO-4A爆发时存在局部地磁异常，则它们在狭窄的区域中比本研究的八个区域更少