Paleomagnetic directions of the Aso pyroclastic-flow and the Aso-4 co-ignimbrite ash-fall deposits in Japan

The Aso pyroclastic-flow deposits, products of major Pleistocene volcanic eruptions in Japan, are divided into four eruptive units: Aso-1, Aso-2, Aso-3, and Aso-4 in ascending order. The paleomagnetic directions determined from welded tuff samples of these units, which are distributed near the source caldera 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. The direction for Aso-2 is characterized by an unusually steep inclination, and that for Aso-3 by an unusually easterly declination. The mean VGP for the Aso-4 co-ignimbrite ash-fall deposits in Chubu, Tohoku, and Hokkaido 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. According to some evidences of volcanology, these subunits are supposed to have been deposited at the same time. The regional-mean directions for these subunits are almost the same as the directions expected for the overall-mean VGP (A(95) = 1.3 degrees, N = 42) located at 22.7 degreesW, 83.8 degreesN. This indicates that its magnetization was acquired in a geomagnetic field dominated by the same dipole field (DF) component in all regions of the Japanese Islands; the DF pole is inferred to coincide with the overall-mean VGP.