石川/尚人

講演予稿:
　国際深海掘削計画第180次航海の1118地点の堆積岩コア試料から採取した試料 片に対して帯磁率異方性と残留磁化の測定を行った。異方性の度合いは概ね 1.02-1.12、形状値は0.5以上であった。最大および最小異方性軸の伏角はほ ぼ水平および垂直であった。このような異方性は堆積物の圧密効果に支配さ れていると考えられる。また、断層や堆積物の変形が見られる層準では上記 の通常値からのずれが認められた。安定な残留磁化方向を用いコアの回転を 補正した結果、タービダイトが多く含まれる層準の試料片の異方性軸の方向 には特異な方向に集中する傾向が見られた。これは堆積時の古流向を示唆す るものと考えられる。

abstract:
　Discrete samples were taken from sedimentary cores recovered during ODP Leg 180 in order to clarify downcore changes of magnetic susceptibility and its anisotropy (AMS) as well as to assess the stability of the remanent magnetization. Preliminary results of the AMS measurements are presented. At Hole 1118A, RCB cores were drilled from 205 to 927 mbsf, which represented eight lithological units. Cubic samples were taken from claystone and siltstone units of middle Pliocene to Pleistocene age in the upper five units to 850 mbsf. Magnetic susceptibilities (volume) of samples between 205 and 520 mbsf were about 3 x 10-3 SI. Below 520 mbsf, susceptibility values decreased gradually with depth to about 2 x 10-4 SI by 850 mbsf. The trend of susceptibility changes was broadly similar to trend of the changes of the remanent intensity, which implies a large contribution of remanence-carrying ferrimagnetic minerals to the susceptibility. Within Lithologic Unit I, samples between 205 and 300 mbsf showed scatter in the degree of anisotropy with Pj values ranging between 1.02 and 1.14, while samples between 300 and 380 mbsf had low Pj values below 1.05. Below 380 mbsf, Pj values ranged generally between 1.05 and 1.12. High Pj values up to 1.18 were obtained from samples in a fault zone between 580 and 620 mbsf. Values of the shape parameter (T) throughout the hole were dominantly positive and greater than 0.5 with a bias toward values higher than about 0.7, which indicates the predominance of oblate fabrics. The relatively high scatter in T values between about 260 and 340 mbsf may be related to soft-sediment deformation observed in this interval. Inclinations of Kmax and Kmin axes were strongly clustered about 0 degrees and 90 degrees, respectively. Scattering of Kmax axes towards relatively steeper values, and of Kmin axes towards relatively shallower values, correlates with the scatter in T values in the interval of soft-sediment deformation. Oblate magnetic fabrics with vertical inclinations of Kmin and horizontal inclinations of Kmax suggest that the AMS was controlled mainly by sediment compaction Most samples yielded a stable magnetic component in the AF demagnetization steps between 15 and 25 mT. Magnetic fabrics were corrected for core orientations estimated from the declinations of the stable remanence assuming a geocentric axial dipole field. Corrected magnetic fabrics from Lithologic Unit III between 490 and 680 mbsf, which contained turbidites, showed Kmax axes aligned E-W to ESE-WNW with Kmin axes clustered about vertical. These data imply an E-W to ESE-WNW paleocurrent direction.

キーワード:
帯磁率異方性, 古地磁気, ウッドラーク海盆

Keywords:
nisotropy of magnetic susceptibility, paleomagnetism, Woodlark Basin