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doi:10.2204/iodp.proc.330.104.2012

Paleomagnetism

The natural remanent magnetization (NRM) intensity of samples from Hole U1373A ranges from 0.08 to 20.46 A/m (median = 2.99 A/m), with the highest values exclusively associated with Unit VII at the base of the hole. Relatively well defined principal component directions with misfit values of ≤3.42 were obtained for 576 intervals from archive-half core measurements (for pieces longer than 9 cm). These directions are generally consistent with stepwise alternating-field (AF) and thermal demagnetization results from 34 discrete samples. This agreement suggests that it may be possible to obtain reliable inclinations for 10 in situ cooling units.

Archive-half core remanent magnetization data

The remanent magnetization of archive halves from Cores 330-U1373A-1R through 13R was measured at 2 cm intervals using the cryogenic magnetometer. All data acquired within 4.5 cm of either piece end were filtered out prior to further processing, and thus only data from pieces longer than 9 cm were considered (see “Paleomagnetism” in the “Methods” chapter [Expedition 330 Scientists, 2012a]).

The downhole variation of NRM intensity and magnetic susceptibility is shown in Figure F55B and F55C. Samples from Unit VII have the highest NRM intensities (median = 6.22 A/m). There is no evidence for any systematic trend in NRM intensity with lithology or depth. For example, median NRM intensities are comparable in the peperitic lavas of Unit VI (median = 2.0 A/m) and the sedimentary units (Unit I, median = 2.2 A/m; Unit III, median = 1.1 A/m). The pronounced variations in susceptibility in Unit VI and the uppermost part of Unit VII presumably reflect variations in magnetic mineral content in the individual peperitic lava flows. However, there is no clear pattern of susceptibility or remanent intensity variations relative to any flow margins in Unit VI.

Archive halves were progressively AF demagnetized, and principal component analysis (PCA) directions were calculated automatically over a range of demagnetization levels, where the lowest misfit value was used to identify the most reliable linear segment (see “Paleomagnetism” in the “Methods” chapter [Expedition 330 Scientists, 2012a]). The resulting characteristic remanent magnetization (ChRM) directions and intensities (1436 intervals) are shown in Figure F55D and F55B, respectively, where more robust PCA picks with misfits of ≤3.42 (40% of all data; n = 576) are emphasized by black and dark red circles.

The archive-half AF demagnetization data reveal a pronounced difference in magnetic stability for the lowermost thick lava flow in Unit VII (lithologic Unit 15). The average median destructive field (MDF′, the alternating field required to reduce the vector difference sum to half its initial value) in lithologic Unit 15 is 11.6 mT (Fig. F55E). The low coercivity of the remanent magnetization in this unit is also evident in the nearly one order-of-magnitude difference in the NRM and PCA intensities (note the difference between purple and black circles in Fig. F55B). Interestingly, the inclination of the low-coercivity (0–15 mT) component of magnetization throughout Hole U1373A is broadly distributed but with a mean centered near 15°. This shallow-inclination low-coercivity component is therefore unlikely to be related to drilling, which normally results in a subvertical low-coercivity component of magnetization. Although radially oriented magnetization components (related to drilling or sawing) have been noted during some previous hard rock legs (e.g., Fig. f116 in Dick, Natland, Miller, et al., 1999), we see no compelling evidence for such radial magnetizations.

The mean MDF′ value of lithologic Units 6–13 is 34.9 ± 14.7 mT, and the more stable nature of the remanent magnetization is evident in the AF demagnetization data (Fig. F56). In these units, most intervals display relatively simple behavior during demagnetization, and in general the automatically picked PCA direction spans the highest AF demagnetization range.

The resulting ChRM inclinations from Units I–III are scattered, whereas those from Units IV–VII have consistent moderate to steep negative inclinations indicative of normal polarity in the Southern Hemisphere (Fig. F55D). Lithologic Units 6–15 from the lower half of the hole (33.91–66.17 mbsf) were all assigned the highest in situ confidence index (ISCI = 3; see “Igneous petrology and volcanology” in the “Methods” chapter [Expedition 330 Scientists, 2012a]), whereas units in the upper part of the hole, including the two sedimentary horizons (Units I and III), were classified as units that cannot have retained their original orientation since cooling (ISCI = NA). Paleomagnetic data support this division on the basis of the increased spread in inclinations determined for the uppermost units compared to lithologic Units 6–15.

Discrete sample remanent magnetization data

Remanent magnetization data were obtained for 34 discrete samples from Hole U1373A (Table T12), including basalt flows and basalt clasts from the upper portion of the hole. NRM intensities for these samples range from 0.34 to 13.5 A/m, and the mean Königsberger ratio (Qn) is 20.9, indicating that the contribution of the induced component to the magnetization is minimal (Fig. F57).

Twenty-three discrete samples were subjected to stepwise AF demagnetization, and the remaining samples were thermally demagnetized. Samples from the volcanic breccia of Unit II may exhibit different ChRM directions over small spatial scales and may also have multicomponent remanent magnetization (Fig. F58B), presumably reflecting variations in the remanence direction of individual clasts. Larger clasts in the volcanic breccia, as well as in the sedimentary conglomerate (Fig. F58A, F58C), generally have nearly univectorial magnetization following removal of a lower stability component (by 5–15 mT or 400°C). Most samples from basalt flows or peperitic flows display relatively simple demagnetization behavior (Fig. F58D–F58F). As noted above, samples from lithologic Unit 15 have low-stability magnetization, with MDF values of <10 mT and much of the magnetization removed by 200°–300°C (Fig. F58G).

Anisotropy of magnetic susceptibility

The anisotropy of magnetic susceptibility (AMS) was measured for all discrete samples (Table T13). The average degree of anisotropy (P′; Jelinek, 1981) is 1.04, and the anisotropy tensor shapes range from strongly oblate to strongly prolate. The lava flows from Units IV and VII are the most anisotropic (P′ values as high as 1.11), and most have prolate magnetic fabrics. The maximum eigenvectors from Unit VII samples are all subhorizontal, whereas those from the lavas in Unit IV are more variable. Magnetic fabrics and structural observations agree in some cases (e.g., both show shallow, eastward-dipping foliations in Core 330-U1373A-11R). However, the AMS data show few near-vertical foliations in lithologic Unit 15, in contrast to structural observations (see “Structural geology”). Shore-based studies may provide a more systematic comparison of AMS and magmatic fabrics.

Discussion

Archive-half and discrete sample inclination data from Hole U1373A are summarized in Figure F59 and Table T14. Ten in situ lava flows (lithologic Units 6–15) were identified with the highest ISCI (ISCI = 3; see “Igneous petrology and volcanology”). Discrete samples were obtained from only 9 of 10 in situ lava flows. The archive-half measurements provide the most comprehensive view of inclination variations in these flows, and we calculated average inclinations using inclination-only statistics (Arason and Levi, 2010) for each lithologic unit in two ways (Fig. F59C–F59E) (see “Paleomagnetism” in the “Methods” chapter [Expedition 330 Scientists, 2012a]).

For lithologic Units 6–15, the various inclination estimates from the archive halves agree well (Fig. F59). The piece-average inclinations (Fig. F59D) from lithologic Unit 15 show considerable scatter, resulting in part from the fact that the ChRM direction represents only a small fraction of the initial remanence and from the tendency for spurious anhysteretic remanent magnetization to be acquired during AF demagnetization at high fields in these low-coercivity samples. The archive-half data sets also agree well with results from discrete sample demagnetization (Fig. F59F). The discrete sample inclinations show a significant serially correlated variation from ~38 to 45 mbsf. These variations are paralleled by changes in the piece-average directions.

Summary

The range of inclinations recorded in Hole U1373A is broader and generally shallower than that found in Hole U1372A, as illustrated in archive-half data at both the 2 cm interval and piece-average levels (Fig. F60C, F60D). The more dispersed inclination distribution from Hole U1373A relative to that from Hole U1372A is a result of the significant number of positive inclinations recorded in the uppermost units comprising sediment and breccia lithologies (Units I–III) and the increased scatter in lithologic Unit 15, which has lower magnetic stability (see earlier discussion). The more reliable (ISCI = 3) discrete data have only negative inclinations (Fig. F60B). Although these 25 inclinations are more clustered, the inclination-only mean of –51.9° ± 6° (α₉₅) is nonetheless consistently shallower than that in Hole U1372A (Fig. F60E). If the proportionally larger number of discrete measurements corresponding to lithologic Unit 15 (n = 11) are first averaged using inclination-only statistics to give a flow mean, then the subsequent inclination-only mean of the nine distinct flow units with good paleomagnetic data becomes –55.2° ± 10.6° (α₉₅). However, the particularly small number of lava flows recovered at this site may not allow averaging of paleosecular variation and therefore limits any paleolatitude determination.

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