Paleomagnetism of Widespread Marker Tephras and High Resolution Magnetostratigraphy

对Kikai- akahoya火山进行了古地磁测量，该火山从九州南部的Kikai火山口喷发，喷发速度约为7300 yrbp。毯层在日本广泛分布，是第四纪研究的重要标志层。我们在30个露头点采集了约600个定向样品。通过逐步交变退磁至70mT，从19个位点的样品中分离出了剩余磁化的稳定成分。利用主成分分析得到稳定分量的方向，锚定在Zijderveld图的原点上。共烟灰矿灰渣落样方向与Kikai-Akahoya断裂带焊接凝灰岩样方向一致。因此，火山灰样品的drm很可能与焊接凝灰岩样品的trm一样可靠，drm和trm代表了Kikai-Akahoya火山喷发时的地磁场方向。在不同测点测得的VGPs具有较好的一致性。这意味着它们的剩余磁化强度是在火山喷发时相同的地磁偶极子场（DF）下获得的。DF的极点可以假定位于全均值VGP上（Long =38.1°W, Lat =85.5°N, A_<95>=2.9， N=19）。目前和以前对广泛分布的麻黄菌的古地磁研究（Nakajima and Fujii, 1995; Fujii et al., 2002）基于对单个麻黄菌的多个地点的古地磁结果，得出了麻黄菌沉积时的确切地磁场。通过对这些资料的精确积累，可以完全阐明第四纪的地壳活动。

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.