We measured the anisotropy of magnetic susceptibility (AMS) to determine the magnetic fabric in this evolved portion of the prism. The AMS is expressed by a symmetrical second-order tensor and is geometrically represented as an ellipsoid that is commonly coaxial with the strain ellipsoid (e.g., Borradaile, 1988). The AMS ellipsoid has three principal susceptibility axes: maximum susceptibility (Kmax), intermediate susceptibility (Kint), and minimum susceptibility (Kmin). The plane normal to Kmin commonly expresses the magnetic foliation. The AMS ellipsoid reflects a statistical alignment of magnetic minerals, and the most magnetically susceptible minerals can have distributions of shape orientation or lattice orientations influenced by the kinematic history of the fabric, which leads to a relationship between AMS and structural fabrics. However, AMS can also be influenced by the magnetic mineral composition of rocks or sediments rather than by their fabric or strain (Borradaile, 1988).
Detailed rock magnetic tests identified that the carrier of AMS at Site 1178 is greigite (Hisamitsu et al., unpubl. data). Although the AMS of greigite-bearing sediments remains poorly understood, the AMS data of the accreted sediments at Site 1178 show a good correlation with the structural data (Fig. F4A). The magnetic foliation is parallel to bedding and bedding-parallel foliation (fissility), showing variable dips with depth. A distinct magnetic foliation is well correlated with the steeply dipping bedding-oblique foliation in the deformed interval between 400 and 506 mbsf. Magnetic foliation dips decrease rapidly across the base of the deformed interval at 506 mbsf. In contrast to the magnetic foliation, magnetic susceptibility is fairly uniform throughout the accreted sediments at Site 1178 (Fig. F4A). This strongly suggests that the changes in magnetic fabric with depth do not reflect mineralogical changes but instead represent the development of structures in the accreted sediments.
Based on the paleomagnetic directions of the samples, the orientation of AMS axes can be restored to their original positions before drilling. Despite the variable magnetic foliation dips, the corrected Kmax axes are subhorizontal and dominantly oriented northeast-southwest, whereas the corrected Kint axes are shallowly to steeply inclined and oriented perpendicular to the Kmax axes directions (Fig. F4B). The corrected Kmin axes are shallowly to steeply inclined and dominantly oriented southeastward, which is consistent with the distribution of the poles to bedding and the foliation below 400 mbsf (Moore, Taira, Klaus, et al., 2001).