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doi:10.2204/iodp.proc.338.106.2014

Paleomagnetism

Hole C0021B

A total of 178 discrete samples were collected from Hole C0021B at a frequency of two samples per standard section (~140 cm) at the KCC. Magnetic measurements on the samples were then conducted using a magnetometer (2G Enterprises, model 760-3.0) at JAMSTEC, Yokosuka. Natural remanent magnetization (NRM) demagnetized directions and intensities of all samples at 0, 5, 10, 20, 30, 40, 50, and 60 mT peak fields were measured to identify characteristic remanent magnetization.

Typical demagnetization behavior of samples is shown in Figure F37. The large vertical component observed between 0 and 5 mT is generally considered a result of drilling-induced overprints (Fig. F37A) (e.g., Gee et al., 1989). Depth profiles of declination, inclination, and intensity after demagnetization at 20 mT are shown in Figure F38. The effectiveness of demagnetizing such a magnetic component is clearly demonstrated in the inclination profile as a shifting of the inclination pattern to the shallower side (Fig. F38B). In contrast, no change in the declination profile between 0 and 176 mbsf implies that only vertical components were removed by 20 mT cleaning (Fig. F38A). Less drilling-induced magnetizations are observed below 176 mbsf. At this depth, the coring method changed from HPCS to EPCS, suggesting that the EPCS method resulted in less drilling-induced magnetization.

Based on the geocentric axial dipole model, the expected inclination at Site C0021 is 52°. After demagnetization at 20 mT, inclination profiles at 0–10, 80–100, and 126–136 mbsf agree well with this expected inclination value (Fig. F38B), which means that the magnetic directions in these intervals are reliable for the polarity interpretation and structural reorientation.

Inclination in the MTD A interval (94.16–116.75 mbsf) is broadly concentrated between 40° and 90°, whereas declination is widely scattered (Fig. F38). Just below the base of MTD A, declination and inclination show a more clustered distribution. Wider scattering of both inclination and declination is also observed in the MTD B interval (133.76–176.16 mbsf). A similar signature was reported for MTD 6 in Hole C0018A (Expedition 333 Scientists, 2012b). The scattering of paleomagnetic direction in MTDs A and B likely reflects internal deformation structures of MTDs.

Other than these intervals that show scattered paleomagnetic data, two other enigmatic intervals are recognized: (1) inclinations between 117 and 125 mbsf are systematically shallower, and (2) below the base of MTD B, declination and inclination scattering is observed. Currently, causes for these intervals are unknown.

Polarity

Above the top of MTD B, all inclinations show positive values. Referring to the age model in Hole C0018, it is interpreted that the sections above MTD B belong to the Brunhes Normal Polarity Chron, although no data were obtained from the interval between 5.93 and 80 mbsf (Fig. F39). A shift of polarity 5 m above MTD 6 in Hole C0018A, accompanied with tephra occurrence of Azuki ash (0.86 Ma) below, is interpreted to be the Brunhes/Matuyama boundary (0.78 Ma) (Fig. F39A). However, such a polarity shift is not recognized above MTD B in Hole C0021B, although MTD B is correlated with MTD 6 in Hole C0018A (see “Core-log-seismic integration”). One of the possible interpretations is that the sequence overlying MTD B is much younger than that above MTD 6 at Site C0018. The paleomagnetic directions below MTD B are expected to be similar to those below MTD 6 in Hole C0018A. However, the inclination and declination scattering does not allow us to discriminate the magnetic polarity pattern below MTD B.