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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Kikai-Akahoya火山喷发物进行了古地磁测量，该火山喷发物于距今约7，300年前从九州南部的Kikai破火山口喷发。火山灰在日本分布广泛，是第四纪研究的重要标志层。我们在火山灰露头的30个地点收集了大约600个定向样品。经70 mT的交变磁场分步退磁，从19个样品中分离出了剩磁稳定分量。用主成分分析法确定了稳定成分的方向，并与Zijderveld图的原点相吻合，结果表明，在Kikai-Akahoya火山碎屑岩中，熔结凝灰岩样品的稳定成分方向与熔结凝灰岩样品的稳定成分方向一致。因此，火山灰样品的DRMs和熔结凝灰岩样品的TRM一样可靠，DRMs和TRM代表了基海-赤穗屋火山喷发时的地磁场方向，不同地点的VGP相互吻合。这意味着它们的反射磁化强度是在火山喷发时同一地磁偶极场（DF）下获得的。可以假设DF的极点位于所有平均VGP上（长=西经38.1°，纬度= 85.5°N，A_<95>=2.9，N=19）。关于广布火山灰的古地磁研究（Nakajima and Fujii，1995 ; Fujii et al.，2002年）根据来自多个地点的单个火山岩的古地磁结果，得出火山岩沉积时的精确地磁场。通过这些资料的积累，可以对第四纪的GSV进行全面的解释。