11:00 〜 13:00
[SEM15-P02] 房総半島南端，千倉層群布良層の古地磁気層序
キーワード：古地磁気学、Mammoth逆磁極亜帯、極性反転
The Chikura group, consisting of the Upper Pliocene –Lower Pleistocene marine sedimentary succession, is distributed in the Southernmost part of the Boso Peninsula, Chiba Prefecture, Japan. The Group is a trench slope basin fill deposit whose source area is thought to be the Izu-Bonin volcanic arc partly. Therefore, this deposit retains a strong and stable paleomagnetic signal due to volcaniclastics, including abundant magnetic grains. We present a new paleomagnetic stratigraphy for the Mera Formation, which spans between the C2An.3n and C2An.2n chronozones, including the lower boundary of the Mammoth reversed subchronozone.
For rock magnetic and paleomagnetic measurements, we collected three mini cores from 193 horizons at every 1.0-1.5 m interval using a portable electric drill. Additionally, we also collected mini cores from 73 horizons at every 10-20 cm interval near the lower reversal boundary of the Mammoth reversed subchronozone. This study used a hybrid method of thermal demagnetization at 250 °C and progressive alternating field demagnetization.
The mean paleomagnetic direction from the C2An.3n section is in good agreement with the geomagnetic axial dipole (GAD) field at the study area. This indicates that the secondary magnetization was sufficiently removed by the hybrid method. In this section, we found three intervals showing an intermediate polarity, which might be paleomagnetic excursions. On the other hand, in the C2An.2r section, the average declination is about 7° westward from the GAD field, and the average inclination is shallower than the GAD. This result suggests that, due to a normal polarity magnetic field, the secondary magnetization acquired after structural tilting has not been sufficiently removed. In the C2An.2n section, the average declination is eastward from the GAD field. Since this is considered to be due to a tectonic rotation, the amount of rotation was assumed, and a paleomagnetic polarity was determined based on corrected declinations using the rotation angle. In this section, we found two intervals showing an intermediate polarity, which might be paleomagnetic excursions. These results will contribute to reconstruct a more detailed geomagnetic chronostratigraphy in the Late Pliocene.
For rock magnetic and paleomagnetic measurements, we collected three mini cores from 193 horizons at every 1.0-1.5 m interval using a portable electric drill. Additionally, we also collected mini cores from 73 horizons at every 10-20 cm interval near the lower reversal boundary of the Mammoth reversed subchronozone. This study used a hybrid method of thermal demagnetization at 250 °C and progressive alternating field demagnetization.
The mean paleomagnetic direction from the C2An.3n section is in good agreement with the geomagnetic axial dipole (GAD) field at the study area. This indicates that the secondary magnetization was sufficiently removed by the hybrid method. In this section, we found three intervals showing an intermediate polarity, which might be paleomagnetic excursions. On the other hand, in the C2An.2r section, the average declination is about 7° westward from the GAD field, and the average inclination is shallower than the GAD. This result suggests that, due to a normal polarity magnetic field, the secondary magnetization acquired after structural tilting has not been sufficiently removed. In the C2An.2n section, the average declination is eastward from the GAD field. Since this is considered to be due to a tectonic rotation, the amount of rotation was assumed, and a paleomagnetic polarity was determined based on corrected declinations using the rotation angle. In this section, we found two intervals showing an intermediate polarity, which might be paleomagnetic excursions. These results will contribute to reconstruct a more detailed geomagnetic chronostratigraphy in the Late Pliocene.