IODP Proceedings    Volume contents     Search

doi:10.2204/iodp.proc.314315316.124.2009

Paleomagnetism

Paleomagnetic directions

Remanent magnetization of Holes C0002B and C0002D was measured in order to construct a paleomagnetostratigraphy. Different procedures described below were employed for measurement on two holes.

Hole C0002B

Remanent magnetization of archive-half core sections from Hole C0002B was measured at 5 cm intervals. We measured natural remanent magnetization (NRM) demagnetized directions and intensities at 0, 5, 10, 15, 20, 25, and 30 mT peak fields to identify characteristic remanent magnetization (ChRM). Profiles of declination, inclination, and intensity after demagnetization at 30 mT with depth are shown in Figure F15. Distribution of declination on samples taken with the RCB should be randomly scattered, as can be observed below 620 m CSF; however, declination in the interval above 620 m CSF shows highly clustered directions. We doubt that those directions are a natural signal. Although the mechanism is not clear so far, the quality of cores taken with the RCB is inadequate in the interval above 642 m CSF (above Core 315-C0002-20R). Apart from this problematic issue, magnetization in the other intervals is relatively stable (Fig. F16A, F16B), despite numerous biscuits induced by the RCB. Paleomagnetic directions are grouped by continuous coherent pieces, and mean directions are calculated to provide orientation for structure data (Table T12).

Hole C0002D

Remanent magnetization of Hole C0002D was measured using discrete samples at the Kochi Core Center. One discrete sample was carefully collected from an undisturbed part of each working half. NRM demagnetizated directions and intensities at 0, 5, 10, 15, 20, 25, 30, 40, 50, and 60 mT peak fields were measured to identify ChRM using the magnetometer (2G Enterprises, model 755R). A typical demagnetization behavior of magnetic vectors is shown in Figure F16C. The steeper NRM inclination in the diagram between 0 and 5 mT levels is probably due to drilling-induced overprints, and this character is recognized in the most samples of Hole C0002D.

Magnetic reversal stratigraphy

Inclination after alternating-field (AF) demagnetization at 30 mT was used to determine the polarity pattern for Holes C0002B and C0002D. The magnetic polarity record was then identified by referring to biostratigraphic datum (see “Biostratigraphy”) and correlated with the geomagnetic polarity time-scale of Gradstein et al. (2004). The result is summarized in Table T13. An inclination profile of Hole C0002D after demagnetization at 30 mT with depth is shown in Figure F17. The inclination of the interval between Sections 315-C0002D-1H-1, 110 cm, and 10H-3, 125 cm (1.20–85.625 m CSF), can be correlated to the Brunhes Chron. Although the polarity in the interval between Sections 315-C0002D-10H-4, 108 cm, and 13H-7, 115 cm (86.853–118.09 m CSF), is not clear, we interpret that the interval which is dominant in negative inclination is correlated to C1r Chron (0.781–0.988 Ma) in reference to the biostratigraphic datum (see “Biostratigraphy”). The top of the normal polarity at Section 315-C0002D-13H-8, 130 cm (119.58 m CSF), can be correlated to the top of the Jaramilo event (0.988 Ma). Because the depth recovered by Core 315-C0002D-15X (129.15 m CSF) is uncertain (see “Operations”), the correlation below this depth in Hole C0002D is unknown.

Inclination after AF demagnetization at 30 mT was used to determine the polarity pattern for Hole C0002B. The interval between 533 and 620 m CSF is excluded from the interpretation. Clear polarity change from positive to negative inclinations upward is observed between Cores 315-C0002B-40R-4, 20.0 cm (837.02 m CSF), and 41R-4, 85.0 cm (847.59 m CSF). Referring to biostratigraphic datum, this reversal can be correlated to the top of the Olduvai Subchron (1.778 Ma) and reversed polarity above this horizon can be correlated to the Matuyama Chron (Fig. F18).

Inclination and bedding dipping

Inclination value in the interval from 605 to 840 m CSF in Hole C0002B is close to the expected inclination (52°), which can be calculated from the site latitude (33°N) (Fig. F19). The interval of Unit III, however, reveals a significantly steeper inclination (mean inclination = 65°) than expected. Unit IV inclination shows large variation. Structural data obtained by logging at this site and bedding dip observed by core inspection (see “Structural geology”) show that Unit III has an inclined bedding plane and the degree of bedding dip in Unit IV becomes scattered. Inclination variation is obviously in conjunction with bedding dip variation. The steeper inclination observed in Unit III, especially, has a constraint for the dip direction. Steepening inclination requires that dip direction is subparallel to the direction of sample magnetization, based on the assumption that the sample was magnetized during normal polarity. In the case of Unit III, northward bed dipping (from northwest to northeast) is accountable for steeper inclination. Interestingly, Unit IV inclination shows a wider distribution in the shallower and steeper sides. If the original magnetic inclination of Unit IV is positive, shallower inclination can be explained by dipping either south, east, or west. Because of this, the azimuth of bedding of Unit IV may not be constant.