Paleomagnetism of Widespread Marker Tephras and High Resolution Magnetostratigraphy

广泛分布的标记火山灰的古地磁学和高分辨率磁性地层学

基本信息

批准号：
16500637
负责人：
NAKAJIMA Tadashi
金额：
$ 2.3万
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：
2004
资助国家：
日本
起止时间：
2004 至 2005
项目状态：
已结题

项目摘要

Paleomagnetic measurements have been carried out on the Kikai-Akahoya tephra, which erupted at about 7,300yrBP from Kikai caldera in Southern Kyusyu. The tephra is distributed widespreadly in Japan, and is the important marker bed in the Quaternary research. We collected about 600 oriented samples at 30 sites of outcrops of the tephra. The stable component of remanent magnetization was isolated from the samples of 19 sites through stepwise alternating field demagnetization up to 70mT. The direction of the stable component was obtained using principal component analysis, anchored to the origin of Zijderveld diagram.The directions of co-ignimbrite ash-fall samples are identical to those of welded tuff samples in the Kikai-Akahoya tephra. It is therefore likeky that DRMs of the ash samples are as reliable as TRMs in the welded tuff samples, and that the DRMs and TRMs represent the geomagnetic field direction at the time of the Kikai-Akahoya eruptin.The VGPs obtained from different sites agree well with one another. It means that their remanent magnetizations were acquired under the same geomagnetic dipole field (DF) at the time of eruption of the tephra. The pole of the DF can be assumed to be located on the all-mean VGP (Long.=38.1°W, Lat.=85.5°N, A_<95>=2.9, N=19).The present and the previous paleomagnetic studies regarding widespread tephra (Nakajima and Fujii, 1995 ; Fujii et al., 2002) yield the exact geomagnetic fields at the times the tephras were deposited, based on the paleomagnetic results from multiple sites for individual tephras. It can be expected that GSV in the Quaternary would be comple-tely elucidated by accumulating those data with the accuracy.

对距今约 7,300 年从九州南部的 Kikai 火山口喷发的 Kikai-Akahoya 火山灰进行了古地磁测量。该火山灰在日本广泛分布，是第四纪研究中的重要标志层。从 30 个火山灰露头的样品中分离出剩余磁化强度的稳定成分。通过逐步交变磁场高达 70mT 的 19 个位置，通过主成分分析获得稳定成分的方向，锚定到 Zijderveld 图的原点。共凝灰岩落灰样品的方向与焊接凝灰岩样品的方向相同。因此，灰烬样品的 DRM 与熔结凝灰岩中的 TRM 一样可靠。 DRM 和 TRM 代表 Kikai-Akahoya 喷发时的地磁场方向。从不同地点获得的 VGP 彼此吻合，这意味着它们的剩磁是在相同的地磁偶极场下获得的。 DF）在火山灰喷发时可以假设 DF 的极点位于全均值 VGP 上。（长=38.1°W，纬度=85.5°N，A_<95>=2.9，N=19）。目前和以前关于广泛分布的火山灰的古地磁研究（Nakajima 和 Fujii，1995；Fujii 等，2002））根据各个地点的多个地点的古地磁结果，得出火山灰沉积时的精确地磁场可以预期，通过准确地积累这些数据，第四纪的GSV将被完全阐明。