Magnetic Chart Deduced from Paleomagnetism of Widespread Marker Tephras

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The Aso pyroclastic-flow deposits are divided into four eruptive units : Aso-1, Aso-2, Aso-3, and Aso-4 in ascending order. The paleomagnetic directions of welded tuff samples determined for these units, which are distributed in central Kyushu, are characteristic of the individual units, and are all of normal polarity. The unit-mean directions for Aso-1 and Aso-4 are similar to the direction of the present geomagnetic field. On the other hand, the unit-mean direction for Aso-2 is characterized by a steep inclination, and that for Aso-3 by an extremely easterly declination. It appears that the distinctive directions of Aso-2 and Aso-3 could be useful in identifying these tephras. The mean VGP for the Aso-4 co-ignimbrite ash-fall deposit is identical to those for subunits of the Aso-4 tephra : the Aso-4A welded tuff in Oita Prefecture, the lower part of the Ube volcanic ash layer in Yamaguchi Prefecture, and the distal ash-flow deposit in Miyazaki Prefecture. From the identical VGPs and … More other stratigraphic evidences, the subunits are inferred to have been deposited at the same time. This indicates that the magnetizations were acquired in a geomagnetic field dominated by the same dipole field (DF) component in all regions of the Japanese Islands ; the DF pole is assumed to coincide with the overall-mean VGP ; declinations and inclinations over the Japanese Islands calculated from the pole range between - 3.5° and - 1.5°, and between 44.0° and 59.0°, respectively.In order to clarify the configuration of the geomagnetic field at the time of the Aso-4A eruption, the surveyed area was divided into eight regions, and the local geomagnetic field (LF) direction of each region was calculated from the regional-mean VGP. Local geomagnetic anomalies, exceeding the a_<95>s of the (LF) directions, could not be detected by comparing DF and LF in all of the eight regions. Had local geomagnetic anomalies existed at the time of the Aso-4A eruption, they have been in the narrower zones than the eight regions of this study Less