TERRAPUB Earth, Planets and Space

Earth Planets Space, Vol. 59 (No. 7), pp. 763-774, 2007

Paleomagnetism of Unzen volcano: A volcanic record (Senbongi excursion) of the Iceland Basin event and the Brunhes VGP distribution for Japan

Hidetoshi Shibuya1, Yuki Iwasaki1,4 Hidefumi Tanaka2, and Hideo Hoshizumi3

1Kumamoto University, Dept. of Earth Sciences, Kumamoto 860-8555, Japan
2Kochi University, Dept. of Education, Kochi 780-8520, Japan
3Geological Survey of Japan, AIST, 1-1-1 Higashi Tsukuba 305-8567, Japan
4Central Soft Co., Ltd, Sundisk-Kanda Bdg., Kandaiwamoto-cho, Chiyoda-ku, Tokyo 101-0033, Japan

(Received January 4, 2007; Revised March 23, 2007; Accepted May 18, 2007; Online published July 20, 2007)

Abstract: A paleomagnetic study was carried out on volcanic rocks from Unzen volcano: samples were collected from a total of 69 sites with 19 sites in pyroclastic flows and 50 sites in lava flows. Ages for the flows were determined either by K-Ar methods or detailed field surveys, and indicate that all of the flows were deposited during the Brunhes chron. After demagnetization 10 pyroclastic and 48 lava flows had stable site-mean directions. One lava flow in the Senbongi area with a K-Ar age of 197±17 ka had an intermediate virtual geomagnetic pole (VGP) at 8.3°N, 21.6°E. The age and VGP position apparently correlate with the Iceland Basin event, suggesting that it is a record of the event on volcanic rocks. Our study added 53 Brunhes-aged VGPs to the overall collection for Japan, increasing it by 40% to a total of 175 VGPs. After excluding all site-mean directions having α95 > 10° and VGP latitude < 50°, the remaining 148 VGPs have a mean pole at 89.7°N, 40.9°E (A95 = 2.2°) showing no significant deviation from the geographic pole. The angular standard deviation (ASD) was calculated as 15.2° ± 1.2° (N = 148), which is compatible with paleosecular variation models from the literature. However, the data set was found to deviate from a Fisher distribution. The actual meaning of the ASD value after removing the intermediate VGPs needs to be reconsidered.
Key words: Geomagnetic excursion, Iceland basin event, PSVL, Unzen volcano.

Corresponding author E-mail: shibuya@sci.kumamoto-u.ac.jp

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