RESULTS

Sixteen samples were measured from Site 169, with six from the diabase and the other ten from the basement basalt (Table T1). Natural remanent magnetization (NRM) intensities were higher in the basalt section (1.9-7.3 A/m) than in the diabase (0.7-1.9 A/m). All samples were AF demagnetized. Demagnetization behavior for the basalts was generally excellent, with successive demagnetization steps tracing a smooth line toward the origin (Fig. F1). Because of this behavior, MAD angles were small, typically <2° (Table T1). Although the results from the diabase sill were not quite as consistent, as indicated by larger MAD angles (2.9°-5.6°), it was possible to define characteristic magnetization directions for all samples (Table T1).

All inclinations from the diabase sill are negative and define two distinct units. The upper four samples (17-169-6R-1, 27-29 cm, to 6R-4, 54-56 cm) yield an average colatitude of 123.7°, whereas the lower two samples (17-169-6R-4, 99-101 cm, and 6R-5, 49-51 cm) give an average of 106.8°. This difference suggests that there are two sills, probably emplaced at times separated by at least a few thousand years, long enough for secular variation to change the magnetic field inclination. If the sills were formed during the mid-Cretaceous, their magnetic polarity should be normal. The colatitude values are consistent with normal magnetization acquired south of the equator.

Two of the samples from the basalt flow section have positive inclinations. This is likely a result of inadvertent inversion of the core pieces during handling. This section could be interpreted as normal polarity formed south of the equator or a reversed polarity formed north of the equator if the inclinations of these samples are assumed to be negative. Given that the site is near Anomaly M21 and the observation that Jurassic paleomagnetic data probably indicate significantly less northward drift of the Pacific plate than mid-Cretaceous data (Cox and Gordon, 1984; Larson et al., 1992), the reversed polarity interpretation may be the correct one. The sample colatitudes fall into two groups: the upper six samples with colatitudes <113°, and the lower four samples with colatitudes >118° (Table T1). The Z-test indicates that these two groups are statistically distinct at 95% confidence.

Calculation of corrected colatitudes and confidence limits gives 115.2° ± 17.8° (this and other colatitude errors are 2 bounds, approximating 95% confidence) for the diabase sills and 117.9° ± 16.6° for the basalt section (Table T2).

Seven samples were measured from Core 17-170-16R (Table T1), recording moderate NRM intensities (1.1-4.2 A/m). Although the samples gave good demagnetization results (Fig. F1), the sample inclinations are inconsistent in sign. Four samples gave negative inclinations, whereas three gave positive values. Because the demagnetization results appear consistent, I assume the difference in signs reflects inversion during handling or transport and that the negative inclination is appropriate. This corresponds to a normal polarity magnetization acquired south of the equator. When given the same inclination signs, the samples show little variation in colatitude values, implying a single magnetic unit. Averaging of the colatitude values gives a mean of 103.5° ± 19.8° (Table T2).

Twelve samples were measured from Site 171 (Table T1). NRM values were slightly higher than those from Sites 169 and 170, ranging from 3.3 to 7.2 A/m. All produced smooth demagnetization plots (Fig. F1) and consistent negative inclinations that imply a single magnetic unit. The measurements define a mean colatitude of 101.9° ± 22.0° (Table T2).

A total of 84 samples from Site 581 was measured: 8 from Hole 581, 10 from Hole 581A, 28 from Hole 581B, and 38 from Hole 581C (Table T3). In general, NRM values were moderate to strong but display a range from 1.4 to 13.9 A/m. Only 10 samples were AF demagnetized because the samples appeared to be prone to acquisition of a spurious magnetization, perhaps an anhysteretic remanent magnetization (ARM), induced in the demagnetization apparatus at high demagnetization steps. Demagnetization behavior of Site 581 samples was not as smooth as those from Sites 169-171; although only three samples displayed unstable behavior. Commonly, the demagnetization removed a moderate to large low-temperature overprint and sometimes a separate medium-temperature overprint (Fig. F2). The characteristic magnetization direction was typically revealed at temperatures of 300°C and above (Fig. F2). Typically, four to six demagnetization steps were used to calculate the characteristic direction, producing relatively low MAD angles of less than ~8° (Table T3).

Because the four holes were drilled within ~300-400 m of one another but with relative positions that are not well known, it was necessary to compare the results from each hole and combine them. Samples from both Holes 581 and 581A were from a single core each, and from consistent positive inclinations, each appear to be a single magnetic unit (Table T3; Fig. F3). Inclinations from Hole 581C samples are likewise predominantly positive and can be subdivided into three units (Table T3). The positive inclinations in these cores imply a normal magnetization formed north of the equator. In contrast, Hole 581B inclinations are predominantly negative, implying reversed polarity. Furthermore, variations between flows imply eight statistically distinct units (Table T3). Three of the units are defined by only one sample each. Although Cox and Gordon (1984) used one-sample units in their analyses, it is possible that some of these samples gave spurious results so I took a more conservative approach and ignored these three data points.

Although Site 581 produced 13 statistically distinct magnetic groups (Fig. F3), differences between group means are not great in most sections. For calculations to determine a site mean, group means in Holes 581B and 581C were recombined. All of the means from Hole 581C were combined into a single value, whereas Hole 581B data were assimilated in two units (Fig. F3). With the two units from Holes 581 and 581A, the estimated total number of independent units is five (Table T2). Using the corresponding normal polarity colatitudes for reversed polarity data from Hole 581B, a site mean of 82.9° ± 9.8° was calculated (Table T2).

In this study, 109 samples were measured from Site 597, with 14 from Hole 597B and 95 from Hole 597C. This augments 69 measurements published by Nishitani (1986). Surprisingly, a large number of samples (27) gave scattered demagnetization results, from which it was impossible to define a characteristic direction (Table T4). Moreover, with many samples it was impossible to define the characteristic magnetization direction with more than three to four demagnetization steps (Table T4). It is not clear why the samples in this study gave erratic results: whether the problem occurred in measurement or in the 17 years of storage, or whether Nishitani, who also used AF demagnetization, simply did not mention spurious results in his report. Interestingly, all of the unstable samples are from Hole 597C, suggesting the erratic behavior is at least partly dependent on lithology.

Despite these problems, the best demagnetization data yield smooth orthogonal vector plots (Fig. F4) that define consistent magnetization vectors. Most samples give positive inclinations, indicative of reversed magnetic polarity in the Southern Hemisphere, where the site is located. NRM values are somewhat low, with most samples having magnetizations <3 A/m.

Most samples give consistent colatitude values, although it was necessary to exclude some apparent outlier values from group mean calculations (Table T4; Fig. F5). The outliers are unsurprising given the aforementioned difficulty with erratic demagnetization results. Statistical analysis indicates eight distinct magnetic groups with data from Hole 597B combined into a single unit with the upper 4 m of the Hole 597C basalt section. These data show that colatitude scatter is low from 100 mbsf downward, corresponding to the bottom two units defined by shipboard scientists. This observation implies that this part of the section was emplaced rapidly, with little time for secular variation to occur. In the site-mean calculations, the lowest two groups were combined (Fig. F5), giving an estimate of seven independent group means (Table T2). This produced a site mean colatitude of 119.5° ± 9.9° (Table T2), which corresponds to a paleolatitude of 29.5°S.

Samples from Hole 800A were the most problematic of all those measured in this study. Despite treatment using both AF and thermal demagnetization, for many samples it was impossible or difficult to define a consistent characteristic remanent magnetization direction. A few samples gave good demagnetization results (Fig. F6), but almost half (9 of 19) of the samples gave scattered results (Table T5). The fact that all of the unstable samples come from Core 129-800-58R suggests a problematic lithologic unit. This problematic behavior has been noted in studies of other mid-Cretaceous Pacific sills (e.g., Steiner, 1981).

Inclinations of the samples from Core 129-800A-57R are dominantly negative, whereas those from Cores 58R and 60R are positive. The inclinations imply a nonequatorial paleolatitude, thus implying a difference in polarity. Radiometric dates for the Site 800 sills correspond to the last polarity reversals of the M-series, so this difference in polarity implies the two groups are from sills emplaced during different polarity chrons. Other paleomagnetic data indicate that the Pacific plate has drifted ~30°-35° northward since the mid-Cretaceous (Larson et al., 1992), so the negative inclinations probably formed during a normal polarity chron at a site south of the equator.

Colatitudes from Hole 800A were averaged as two independent groups, with the reversed polarity data changed to corresponding normal polarity values (Table T5; Fig. F7). The data combine to give a site-mean colatitude of 105.1° ± 17.0° (Table T2).

Demagnetization results from the 20 Site 803 samples were excellent, with most samples showing little overprint and giving smooth demagnetization curves (Fig. F6). Characteristic remanent magnetization directions were precisely defined, with five to seven steps used in the calculation, producing MAD angles <2° (Table T5). Inclination values were consistently negative, indicating a normal polarity acquired south of the equator. Colatitudes were grouped into two independent units (Fig. F7) and defined a site-mean colatitude of 116.8° ± 16.5° (Table T2).

Data from Site 865 also displayed consistent demagnetization behavior with little overprint (Fig. F6). Almost all of the 22 samples allowed five to seven demagnetization steps for definition of the characteristic remanent magnetization direction, often giving MAD angles of <1° (Table T5). All samples gave negative inclinations, indicating normal polarity sills emplaced south of the equator. This result is consistent with the radiometric date for the sites, which falls within the Cretaceous Long Normal Superchron. Mean colatitudes for the three sampled sills were distinct, averaging to give a site-mean of 104.9° ± 13.2° (Table T2; Fig. F7).