Paleomagnetic Directions of the Aso-4 Ash and the Aso Pyroclastic Flows

Abstract

Paleomagnetic measurements were made of the Aso pyroclastic flows of middle to late Pleistocene age in central Kyushu, and of the Aso-4 ashes in the Chubu-Kanto-Tohoku district. The Aso pyroclastic flows are divided into four major cycles: the Aso-1, -2, -3, and -4 flows in ascending order. The paleomagnetic directions obtained are characteristic of the individual cycles, and are all of normal polarity. The mean directions of the Aso-1 (Dm=3.4°E, Im=49.0°, α₉₅=5.2°) and the Aso-4 (Dm=5.2°W, Im=42.0°, α₉₅=5.2°) flows are similar to that of the present geomagnetic field (D=6°W, I=45°). On the other hand, the direction of the Aso-2 flow is characterized by a very steep inclination (Dm=2.5°W, Im=72.7°, α₉₅=13.6°), and that of the Aso-3 flow by an extremely easterly declination (Dm=35.7°E, Im=51.2°, α₉₅=3.7°).
The Aso-4 pyroclastic flow in Oita Prefecture is subdivided into two units: the Aso-4A and the Aso-4B flows in ascending order. The mean inclination of the Aso-4B flow is shallower than that of the Aso-4A flow by 5 degrees. Assuming that this directional difference was caused by geomagnetic secular variation (GSV), we can estimate the time interval between the Aso-4A and the Aso-4B flows to be about 50 years or more based on the standard GSV curve for southwestern Japan over the past 2, 000 years.
The directions of the Aso-4A welded tuffs in Oita Prefecture are consistent with those of the Aso-4 non-welded pyroclastic flow deposits in Miyazaki Prefecture, and with those of the Aso-4 co-ignimbrite ashes in the Chubu-Kanto-Tohoku district. This comparison was made on declination and inclination values for each flow reduced from their virtual geomagnetic poles (VGPs) to a reference point at 131°E, 33°N. The declination values have a range limited between 0.5°W and 5.4°W, and the inclination values are between 44.4° and 45.9°. Identical paleomagnetic directions for the widespread Aso-4 tephra indicate that the magnetizations were acquired in a field dominated by the same geomagnetic dipole field (DF) component in all three regions of the Japanese islands; the pole of the DF at the time of eruption is inferred to coincide with the mean VGP (A₉₅=2.1°, N=21) located at 22.8°W, 83.3°N.