11:00 AM - 11:15 AM
[SEM16-08] Paleomagnetic directions from pyroclastic flow deposits at the eastern foot of Fuji volcano,Japan
Keywords:Paleomagnetic secular variation, Fuji volcano, East Asia
Paleomagnetic secular variation (PSV) records have been obtained from archaeomagnetic studies of kilns, and therefore have been used as a dating method since the 1980s. However, it is difficult to secure sufficient records from the Yayoi to Jomon periods, the archaeomagnetic secular variation curve of old kilns in Japan is limited to the last 1600 years. For volcanic products from Fuji volcano, about 100 units have been identifield in the past 3500 years (e.g. Takada et al., 2016). In recent years, several pyroclastic flow deposits, which are dated about 100 CE by 14C, were also found to be distributed at the eastern foot of Fuji volcano (MFRI Annual Report 2017). Here, we present a paleomagnetic study to reinvestigate the paleodirections before 400 CE.
We reconducted geological survey of the watershed of Oyamasano river. Samples for paleomagnetic measurements were collected several blocks and matrix from each pyroclastic flow deposits, measured using a spinner magnetometer with thermal demagnetization (ThD) in Okayama University of Science. As a result of ThD, these mean directions show roughly two distinct range, estimated ca. 70 CE and 230 CE by 14C dating. The mean direction of the lowest pyroclastic flow deposit shows the direction of D=6.7, I=36.5, coinciding with the mean directions of Norikawa lava flow and the pyroclastic flow deposit just above the lava flow. The mean directions of the middle pyroclastic flow deposit shows the direction of D=22.0, I=52.9, the accessory materials of the this deposit have high and low temperature components bounded at below 350 degrees C. Sung and Hirooka (2000) proposed Korean archaeological directions shows a value of around D=25 which deviates significantly eastward by the more than 10 degrees of the Japanese archaeological directions for the period of the 2nd and 3rd Centuries. Our findings suggest that the directions of pyroclastic flow deposits coincide with the Korean archaeological directions, PSV curve has a large clockwise loop through to the 3rd Century in East Asia.
We reconducted geological survey of the watershed of Oyamasano river. Samples for paleomagnetic measurements were collected several blocks and matrix from each pyroclastic flow deposits, measured using a spinner magnetometer with thermal demagnetization (ThD) in Okayama University of Science. As a result of ThD, these mean directions show roughly two distinct range, estimated ca. 70 CE and 230 CE by 14C dating. The mean direction of the lowest pyroclastic flow deposit shows the direction of D=6.7, I=36.5, coinciding with the mean directions of Norikawa lava flow and the pyroclastic flow deposit just above the lava flow. The mean directions of the middle pyroclastic flow deposit shows the direction of D=22.0, I=52.9, the accessory materials of the this deposit have high and low temperature components bounded at below 350 degrees C. Sung and Hirooka (2000) proposed Korean archaeological directions shows a value of around D=25 which deviates significantly eastward by the more than 10 degrees of the Japanese archaeological directions for the period of the 2nd and 3rd Centuries. Our findings suggest that the directions of pyroclastic flow deposits coincide with the Korean archaeological directions, PSV curve has a large clockwise loop through to the 3rd Century in East Asia.