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doi:10.2204/iodp.proc.314315316.222.2015

Methods

AMS measurements were made on a total of 899 samples (~7 cm³ volume) from Sites C0004 (n = 209), C0006 (n = 255), C0007 (n = 101), and C0008 (n = 334). Samples were collected at regular intervals from every possible section and were measured with the AGICO KLY 4S Kappabridge installed at the Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology.

Magnetic susceptibility is a proportionality between the intensity of the induced magnetic field and that of the applied magnetic field. AMS represents the geometric alignment and intensity of mineral fabrics as a magnetic ellipsoid, which is commonly interpreted to reflect the strain ellipsoid (e.g., Borradaile and Alford, 1988). The magnetic susceptibility ellipsoids are expressed with the principal susceptibility axes (K1  >  K2  >  K3). The minimum axis K3 is widely regarded as the orientation of maximum shortening (e.g., Borradaile, 1991). A sensitive response of unconsolidated sediments in accretionary prisms to applied stress has been detected with AMS studies (e.g., Byrne et al., 1993; Owens, 1993), which shows the K3 axis oriented toward the maximum shortening strain.

All the raw data listed in Table T1 have been measured with a KLY 4S Kappabridge, and all the derived parameters are described in the “Appendix.” Here we present the following parameters derived from the principal susceptibility axes for discussion: the bulk magnetic susceptibility Km, the lineation parameter L ( K1/K2) and the flattening parameter F (K2/K3), the anisotropy degree (P′) and the shape parameter (T), and the inclination of K1 and K3. P′ and the T are, conceptually, amended expressions in a polar coordinate system proposed by Jelinek (1981) out of traditional lineation versus a foliation (L–F) diagram (Flinn diagram) commonly used for structural geology.

Km reflects the amount of magnetically susceptible components in the specimen and thus reflects lithology and/or mineralogy. L–F and P′–T are pairs of factors that show the shape of magnetic ellipsoids but are different in their main focus. L and F indicate the intensity of the shape components, lineation and flattening, respectively. Given both L and F, we know the shape of the ellipsoid. T provides the shape information (oblate if 0  <  T  <  1 and prolate if –1  <  T  <  0), where the intensity of distortion compared to the true sphere is presented by P′. Therefore, P′–T data are useful for discussing the general shape of the magnetic ellipsoid while L–F data are most useful for highlighting the lineation or flattening components.

For normally deposited and compacted marine sediments, it is expected that P′ shows a gradual increase with depth in association with the reduction of porosity, and T is approximately random in shallow sediments and shifts toward oblate values with depth (Kitamura et al., 2010). This compaction trend is seen as a stable L and increasing F with depth. A vertical K3 axis is expected for gravitational compaction (e.g., Kanamatsu et al., 2012).

We selected clayey samples for the AMS measurements. The magnetic properties of the samples from this expedition have been reported by Zhao and Kitamura (2011) in a study documenting that the main magnetic component is paramagnetic minerals with a diamagnetic effect and multidomain or pseudosingle domain size components. Magnetic susceptibility is carried by a comparable amount of the magnetite-titanomagnetite series mineral and clay minerals (Kitamura et al., 2010; Zhao and Kitamura, 2011). The chemical effects that could change magnetic properties appear to be minor, as there are no signs of such effects from previous results in this area (Kitamura et al., 2010; Zhao and Kitamura, 2011).

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