Paleomagnetic secular variation study using young volcanic rocks in Kyushu Island

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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