Detrital remanent magnetization of thick-deep-sea turbidite, off Tohoku NE Japan

Abstract:

Because a remanent magnetization of turbidite is considered to be strongly affected by hydrodynamic force, it has been generally excluded from sediment paleomagnetic study. Very few studies have been done to make it detailed. Japan trench, off Tohoku, NE Japan provide to a unique opportunity to study thick and very fine turbidites, which were induced by subduction earthquakes. A few-meter thick turbidites corresponding to huge past earthquakes were discovered in the Japan Trench. Paleomagnetic and rock magnetic studies were conducted to understand the magnetization mechanism of those turbidites. Remanent magnetic directions show characteristic changes within turbidite intervals. A sudden declination shift at the bottom of turbidites are recognized generally. It is considered that the magnetic particles are aligned reflecting the fluid motion at the bottom. The deflection angle gradually changes toward the top of the turbidite with returning to the direction of pre turbidite-deposition stage. On the other hand, the change in inclination angle tends to become shallower in the lower part of the turbidite layer as reported in previous studies. It also returns back to the magnetic direction of pre-turbidite-deposition in the top of turbidite. The shallow inclination interval corresponds to larger grain size and magnetic grain size. In these fine turbidite deposition, it is considered that the upper portion of these turbidites allow to magnetized to the ambient magnetic field, and it is considered that directional changes are caused from interaction between controls of hydrodynamic effect and the earth's magnetic field.