Paleointensity measurements of pyroclastic flow deposits co-born with widespread tephras in Kyushu Island, Japan

Abstract

We report the results of paleointensity determination with welded pyroclastic flow deposits co-born with widespread tephras derived from Kikai (K-Ah, 7.3 ka), Aira (AT, 28 ka), and Aso (Aso-1, 270 ka; Aso-2, 140 ka; Aso-3, 120 ka; Aso-4, 90 ka) calderas in Kyushu Island, Japan. Since the widespread tephras are commonly used as good key layers, their ages and oxygen isotope stages have been well established. Thermal demagnetization and several rock magnetic experiments showed that the pyroclastic flow samples had stable remanent magnetizations and are suitable for paleointensity measurements. Using Thelliers’ method, we obtained reliable paleointensities from six eruptive units, and calculated corresponding virtual dipole moment (VDM) values ranging from 2.7 to 9.7 × 10 22 Am 2 . VDMs for the youngest units, K-Ah and AT, are consistent with the global trend of increasing dipole moment for the past 40 kyr. Aso-4, -3 and -1 yield moderate VDMs similar to the mean paleointensity for the past 1 Myr, and especially, the results from Aso-4 and -3 show good agreement with the VDM variation between 70 and 130 ka obtained from La Reunion Island [J. Geophys. Res. 96 (1991) 1981; Earth Planet. Sci. Lett. 140 (1996) 173]. A low VDM value and an abnormal VGP position obtained from Aso-2 suggest a possibility that Aso-2 records an excursion-like event occurred around 140 ka between Blake and Jamaica events. © 2002 Elsevier Science B.V. All rights reserved.