八ヶ岳火山列の古地磁気
2002 年 47 巻 5 号 p. 435-448
詳細
2002 年 47 巻 5 号 p. 435-448
Directions, intensities and stability of the natural remanent magnetization of the Yatsugatake Volcanic Chain, central Japan were measured by the conventional paleomagnetic method. We investigated about 600 samples of the volcanic rocks with ages ranging from 0.01 to 1.3 Ma together with basement rocks younger than 3 Ma collected at 130 sites covering the entire period of the Yatsugatake activity. Total 320 reliable paleomagnetic directions from 76 sites were determined using the Zijderveld diagrams for progressive alternating-field demagnetization. Paleomagnetic polarities thus obtained are correlated to the standard magnetic chrons and subchrons with the aid of K-Ar radiometric ages of rocks. Results are summarized as follows in the order of decreasing ages; (1) Kiretto andesite and Minoto porphyrite representing the earliest volcanic products have the normal polarity correlatable to a subchron older than the Jaramillo event in the Matuyama reversed polarity chron, whereas their basement (Shiga welded tuff, Yashigamine lava and Mizuochi-kannon lava) has reversed polarity. (2) Kasuga volcanics, Kyowa-bokujyo lava, Tatamiishi lava and Honmagawa lava composing the lower layers of the old Yatsugatake stage are all reversedly magnetized and correlatable to the Matuyama reversed polarity chron. (3) Most of the upper layers overlying the Kasuga volcanics have the normal polarity corresponding to the Brunhes normal polarity chron. (4) Reversed polarities are found in several lava flows of the younger volcanic stages. They appear to be records of the Blake event and the Laschamp excursion in the Brunhes chron. (5) Most of the paleomagnetic directions of the Yatsugatake samples are clockwisely deflected by approximately 10° to 15° from the present geographical north. These deflections seem to be caused by the tectonic movement of the Fossa Magna region, since similar deviations of the paleomagnetic directions are also observed in the Quaternary volcanics of the western Izu Peninsula adjacent to the Yatsugatake region.
