Paleomagnetic secular variation study using young volcanic rocks in Kyushu Island

地磁场的磁电流体动力学模型与古地磁资料之间的差距仍然很大。一些古地磁参数的全球分布，将是弥合这一差距的关键观测。多年来研究的参数之一是虚拟地磁极（VGP）的角标准差（ASD）。由于它是一种研究古气候变化强度的方法，且通常以熔岩为样品，因此被称为PSVL研究。在PSVL的标准模型中，ASD是纬度的函数，但不依赖于经度。然而，在整个研究历史上一直存在异议。该项目的首席研究员提供了新西兰北方的ASD，发现它明显小于标准模型。如果能很好地确定日本在不同半球的近经度带和近纬度上的ASD，那将是很有趣的。我们已经有了 关于我们 在日本，有一些年轻火山岩的古地磁年代测定，但远不能下定论。在九州岛，在日本群岛的西端，我们有许多不同年龄的火山。我们对这些年轻火山的熔岩和熔结凝灰岩进行了古地磁测量。我们主要关注的是阿索、九州和汤布火山，因为这些地区的构造似乎不如九州南部活跃，但也从九州南部采集了一些非常年轻（小于0.1Ma）的焊接凝灰岩。在它们的古地磁方向上测量了大约60个独立的时间层。同时，我们尝试从当地的期刊和论文中整理出该地区的古地磁数据，建立了一个120多个独立层位的数据集，其中相当一部分具有一致的多站点。该数据集的ASD和正常、反向、Brunhes、pre-Brunhes等的子集给出了17°至21°的ASD。所有这些ASD值都明显大于新西兰，11.5°和太平洋其他岛屿。这一事实意味着PSV随纬度或半球而变化。少

The gap between the magnetoelectoric fluid dynamic models_ of the geomagnetism and the paleomagnetic data is still large. The global distribution of some paleomagnetic parameters, would be the key observations bridging over the gap. One of those parameters studied for years is the angular standard deviation (ASD) of the virtual geomagnetic poles (VGPs). As it is, in another word, the study of the intensity of the paleosecular variation, and usually uses lavas for the samples, it is called as the PSVL studies. In the standard models of the PSVL, the ASD is function of the latitude, but does not depend on the longitude. However, there have been objections throughout the research history. The head investigator of this project had provided the ASD in northern New Zealand, and found it significantly smaller than the standard models. It would be interesting if the ASD in Japan, in the close longitudinal band and close latitude in different hemisphere, is well determined. We already had had s … More ome paleomagnetic date of young volcanic rocks in Japan, but was far less to be conclusive.In Kyushu Island, at the western end of Japanese Islands, we have many volcanoes of various ages. We performed the paleomagnetic measurements of lavas and welded tuffs of those young volcanoes. We primarily focused on the Aso, Kuju and Yufu volcanoes, as those areas seemed to be less active in tectonics than those in southern Kyushu, some of very young (less then 0.1Ma) welded tuffs were also taken from southern Kyushu, though. About 60 independent time horizon were measured in their paleomagnetic directions. Mean while, we tried to compile paleomagnetic data in this area from local journals and theses, and build a dataset of more than 120 independent horizon, significant portion of which have concordant multiple sites. ASD of this dataset and subsets of normal, reversed, Brunhes, pre-Brunhes, etc. gave ASDs from 17° to 21°. All of those ASD value are significantly larger than that of New Zealand, 11.5° and other islands in the Pacific Ocean. This fact implies the PSV varies by the latitude or the hemisphere. Less