No paleomagnetic measurements were made from sediments or sedimentary rocks recovered from Holes 1213A and 1213B. Soft sediment cores from Hole 1213A were deemed to be too disturbed for reliable measurements. Cores from below the Tertiary-Cretaceous unconformity had low recovery and consisted of small pieces of chert with associated limestone and other types of sedimentary rock. These rock pieces are too small and lack the continuity in recovery to make pass-through measurements useful.
Igneous rocks from the six cores at the bottom of Hole 1213B were measured on the pass-through magnetometer because many of the core sections contained large pieces of contiguous basaltic rock. These cores were measured three to seven times, recording the natural remanent magnetization (NRM) and the magnetization at several different levels of alternating-field (AF) demagnetization. Typically, the AF steps used were 10, 20, 25, and 30 mT, although a few sections were taken as high as 35 mT. During analysis, measurements from zones with no oriented pieces, with small fragments, and near the ends of the larger fragments were excluded to remove inaccurate values resulting from the averaging of nearby pieces with different orientations.
NRM intensities for the igneous rocks fall within a narrow range close to ~1-10 A/m. After demagnetization to 30 mT, the intensities dropped to ~2.5 x 10-2 to 4 x 10-1 A/m. It was noted that magnetic inclinations changed between the 20, 25, and 30 mT demagnetization steps, implying that the characteristic remanent magnetization has not been completely isolated. Therefore, more detailed shore-based measurements will be necessary to obtain reliable magnetization directions. After demagnetization at peak fields of 30 mT, magnetic inclinations in Cores 198-1213B-28R to 32R are all positive (Fig. F26), implying normal polarity if magnetized north of the equator. The positive inclinations are all much steeper than expected for the near-equatorial position that Shatsky Rise is thought to have occupied during the Late Jurassic and Early Cretaceous (Larson et al., 1992; Sager et al., 1999). Most of the section averages for Cores 198-1213B-28R through 32R are between 50° and 70°. The cause of the steep inclinations is unclear. Measurements from Core 198-1213B-33R show negative inclinations (Fig. F26), indicating reversed polarity if the magnetization was acquired north of the equator. The inclination values are ~-30°, a value more in line with a low paleolatitude (~13°N). The apparent reversed polarity implies that this particular sill (Subunit IVC) was not emplaced during the Cretaceous Long Normal Superchron (i.e, Cretaceous Quiet Period; 83-121 Ma; see "Paleomagnetism" in the "Explanatory Notes" chapter for timescale), when many other similar sills were formed in the western Pacific (Schlanger and Moberly, 1986; Floyd et al., 1992).