Preliminary rock-magnetic studies of core samples from the IODP Japan Trench Fast Drilling Project (JFAST)

During IODP Expedition 343, Japan Trench Fast Drilling Project (JFAST), boreholes were drilled through the prism and across the fault that is thought to have slipped during the 11 March 2011 Tohoku-Oki Earthquake. 74 subsamples of the core recovered from hole C0019E were subjected to rock magnetic analyses to identify magnetic minerals, determine the magnetic-grain size distribution and investigate rock magnetic changes related to fault zone processes. Magnetic susceptibility was measured with a MFK1-FA Kappabridge instrument at three different frequencies: 976 Hz (F1), 3904 Hz (F2) and 15616 Hz (F3), and frequency-dependent susceptibility (χfd%) was calculated using the equation χfd%=(χ@F2-χ@F3)/χ@F2×100%. Magnetic hysteresis curves and backfield DC demagnetization curves of isothermal remanent magnetization were measured using an alternating gradient field magnetometer. Bulk coercivity parameters (Hc, Hcr) show a significant down-core variation. Samples from the lower part of the frontal prism sediment (688-820 mbsf) showed high Hc and Hcr (15-52 mT and 45-85 mT). Samples from hemipelagic sediment we interpret was deposited on the oceanic basement (824-832 mbsf) showed slightly lower Hc and Hcr (13-26 mT and 34-65 mT). A sheared scaly clay zone between two sediment types (820-824 mbsf), inferred to be the plate boundary decollement, clearly has low Hc and Hcr (10-13 and 22-24 mT). The difference in Hc and Hcr suggests a difference in magnetic mineralogy. Saturation magnetization (Ms) and saturation remanence (Mr) show a down-core increasing trends with no obvious changes at the geologically-indicated plate boundary.Magnetic susceptibility shows different down-core variation with coercivity parameters. Generally, the scaly clay has the highest susceptibility and susceptibility of the hemipelagic sediment is relatively lower. For example, the mean values of low frequency (F1) magnetic susceptibility are 74.21, 387.83 and 30.58×10-8 m3 kg-1 for the lower part of frontal prism sediment, sheared scaly clay zone and hemipelagic sediment, respectively. The variation may be related to the difference in magnetic mineralogy and relative concentrations. In addition, magnetic susceptibility shows a strong dependence on measured frequency, and frequency-dependent susceptibility (χfd%) generally increases with depth, which may reflect an increasing contribution of the superparamagnetic components with depth. Further measurements, such as low-temperature magnetic properties, and thermomagnetic measurements of magnetic susceptibility and induced magnetization are intended.