Discrete samples from Hole 1173B were demagnetized during Leg 197 using an alternating field (AF) of 0 to 80 mT, at 5-mT steps for the first 50 mT and at 10-mT steps for the remaining treatments, to isolate stable remanence components in the samples. Characteristic remanent magnetizations (ChRMs) were obtained using principal component analysis (Kirschvink, 1980), and the stability of remanence was determined using Zidjerveld (1967) plots and equal-area projections.
All samples showed a relatively strong magnetic overprint, which was likely imparted by the drill string and/or Earth's present-day magnetic field. AF demagnetization spectra (Fig. F40) show that the samples have a rather low bulk coercivity, with median destructive fields ranging between 5 and 12 mT. In five of the six samples analyzed, the overprint appeared to be removed after demagnetization to 15-35 mT (Fig. F40A, F40B, F40D, F40E, F40F). For these samples, a stable reversed-polarity ChRM could be fit by principal component analysis. However, departures of linear fits to the data from the origins of orthogonal vector plots indicate the presence of high-coercivity magnetic phases that were not demagnetized by the applied AF treatments. For Sample 196-1173B-1R-1, 61-63 cm, the AF demagnetization applied was insufficient to remove the overprint and a ChRM could not be isolated (Fig. F40C). In Sample 196-1173B-3R-2, 14-16 cm, the definition of the ChRM declination after removal of the overprint was scattered, resulting in a fit with a very large median angular deviation (Table T12).
The three stratigraphically highest samples yielding ChRMs have inclinations between -18° and -23°. The two stratigraphically deepest samples analyzed have ChRM inclinations of -38°. Because of this stratigraphic relationship, the magnetizations suggest that the recovered core might record two polarity intervals. However, because the linear fits deviate from the origin of orthogonal vector plots, the paleomagnetic inclination differences might instead arise from the differential presence of small amounts of high-coercivity magnetic minerals and/or the variable effectiveness of AF demagnetization in removing magnetic overprints. Because of these possibilities, the uncertainties of the reported ChRM directions (the median angular deviation; Table T12) should be considered to be minimum values. Detailed shore-based paleomagnetic and rock magnetic analyses employing thermal demagnetization are needed to further evaluate these issues.
The average inclination of the two groups of ChRM directions, following the inclination averaging technique of McFadden and Reid (1982), is -30.2°. (The small number of paleomagnetic units prevents the application of formal uncertainty analyses, which are valid when the number of units is three or greater.) The average inclination value can be used to calculate a nominal paleolatitude using the geocentric dipole relationship:
where I is the mean ChRM inclination and l is the paleolatitude. This is a nominal value because the recovered basalt does not contain enough time-independent inclination units to adequately average geomagnetic secular variation (see discussion in Tarduno and Sager, 1995). Nevertheless, the nominal paleolatitude (16.2°N) is useful for discussing the potential meaning of the measured magnetic data.
Hole 1173B is located at 32°14.66´N, 135°01.51´E. The present-day geocentric axial dipole (GAD) inclination at this site is 51.6°. The ChRM inclinations are substantially shallower than the present-day GAD value. The age of the basalt sampled at Site 1173 is thought to be 15.6 Ma (Siena et al., 1993) and therefore plate motion might explain part of the difference between the measured data and the present-day GAD value. However, inadequacies of the demagnetization to isolate ChRM directions, insufficient averaging of secular variation, and tectonic complexity (tilting) offer sufficient explanations of the preliminary data available from the limited penetration and core recovery.
The magnetic polarity of the basalt is a stronger interpretation that can be made from the preliminary shipboard data. The reversed polarity is consistent with a number of reversed-polarity chrons between 15 and 20 Ma (Cande and Kent, 1995; Fig. F41).
4This section was written during Leg 197. Leg 197 contributor addresses can be found under "Leg 197 Contributors" in the preliminary pages of the volume.