We measured the natural remanent magnetization (NRM) of all archive halves from Hole 1137A with the pass-through cryogenic magnetometer using measurement intervals of 5 and 2.5 cm for sediment and basement sections, respectively. Subsequently, all sediment and basement sections were demagnetized up to 20 and 60 mT, respectively. Discrete sediment samples were stepwise demagnetized with an alternating field (AF) of up to 30 mT. Discrete basalt samples were stepwise AF demagnetized in a peak field of 60 mT or thermally demagnetized in temperatures of up to 620°C. We determined the anisotropy of magnetic susceptibility (AMS) of discrete basement samples to obtain information about the magnetic fabric.
We obtained stable remanent directions from the sediments of Hole 1137A. Reliable paleomagnetic results in undisturbed sediment cores with high recovery were used to correlate normal and reversed segments with biostratigraphic zones (see "Biostratigraphy"). We measured magnetic susceptibility and remanent magnetization to determine the magnetic properties of basement units. We found an 8° difference between the paleolatitude and the present latitude of Elan Bank.
After measuring the NRM, all sediment cores were demagnetized with a peak AF of 20 mT. One archive section from each core was stepwise demagnetized up to 30 mT. We took two discrete samples per section, and 28 samples were stepwise AF demagnetized up to 30 mT to confirm the reliability of whole-core measurements. We obtained reliable paleomagnetic results in undisturbed cores with high recovery, and correlated normal and reversed segments with biostratigraphic zones (see "Biostratigraphy"). The sediments of Hole 1137A generally have a stable magnetization, which was obtained after AF demagnetization at 20 mT, especially for sediments with a high median destructive field (MDF) (Fig. F75). For magnetostratigraphic studies of Hole 1137A (Fig. F76), we used the data selection criteria described in "Paleomagnetism" in the "Explanatory Notes" chapter). The selection criteria were that (1) the intensity of remanent magnetization after AF demagnetization at 20 mT was >5 × 10-4 A/m and hence above the noise level of the magnetometer in rough-sea conditions, (2) the inclination was > ±40°, (3) at least two consecutive values (which corresponds to a 10-cm length of split core) had the same polarity, and (4) there was no significant core disturbance. We calculated characteristic remanent magnetic directions of discrete samples using component analysis. Characteristic inclinations from discrete samples generally agree well with selected inclinations from whole-core measurements (Fig. F76). We observed geomagnetic reversals within seven sections.
Correlation of biostratigraphic data and polarity reversals (Fig. F76) suggests that the reversed and normal chrons in Hole 1137A are Pleistocene to late Eocene in age (see "Biostratigraphy"). We propose the following correlations with paleontological data from the core catcher of each core (see "Biostratigraphy"). We correlate reversed polarity intervals at ~6 and ~11 mbsf to Chrons C1r and C2r, respectively. Normal and reversed segments between 100 and 107 mbsf and between 113 and 123 mbsf are Chrons C5-C6 and C7-C8, respectively. The underlying normal and reversed segments between 125 and 173 mbsf correspond to the chrons between C8 and C13. We correlate the reversed polarity interval at ~190 mbsf to Chron C13r or C15r.
We obtained a reliable paleomagnetic record from lithologic Units I and II (see "Lithostratigraphy"). In the upper part of Unit II, especially between 30 and 80 mbsf (nannofossil ooze), we observed negative susceptibilities and weak NRM intensities (Fig. F76). Remanent magnetization is less stable, and magnetizations of discrete samples from this interval are too weak for reliable measurements with the shipboard magnetometer. We observed weaker positive susceptibilities and stronger NRM intensities in the lower part of Unit II than in the upper part. Progressive AF demagnetization and measurement of discrete samples were successful and provided a reliable paleomagnetic record (Fig. F75). We observed high susceptibilities (Table T14) in Unit III (sandy packstone). We could not obtain a reliable magnetic record from Unit III as a result of either weak magnetization or core disturbance.
We determined the magnetic properties of each basement unit (see "Igneous Petrology" and "Physical Volcanology") and the variation of magnetic properties within each unit (Fig. F77). Three independent types of susceptibility measurements (MST, AMST, and discrete samples) show consistent results. We observed no significant differences in the average susceptibility and NRM intensity from the seven lava flows (basement Units 1, 2, 3, 4, 7, 8, and 10; Table T14). However, susceptibilities and NRM intensities did vary significantly within Units 2, 7, and 10. The brecciated flow tops have higher susceptibilities and stronger NRM intensities than the massive interiors. Differences in susceptibility and NRM intensity are caused by variations in magnetic mineral content, the size of magnetic mineral grains, and/or different magnetic minerals. We observed no significant variation within Units 1, 3, 4, and 8. The crystal-lithic volcanic siltstones and sandstones (Unit 5) and volcanic conglomerates (Unit 6) show lower susceptibilities and weaker NRM intensities than the lava flows. However, the volcanic clasts and grains produced higher average susceptibilities and stronger NRM intensities than in typical nonvolcanic sediments. Among the basement units, the tuff layer (Unit 9) has the lowest susceptibilities and the weakest NRM intensities as well as significant variations in susceptibilities and NRM intensities. Susceptibilities and NRM intensities increase downward in Unit 9 except for the uppermost ~1 m. We observed the highest susceptibilities and strongest NRM intensities at the top of the tuff layer. This variation is probably related to variable abundances of magnetic minerals or the type and size of magnetic minerals. All basement units except Unit 6 have steep negative inclinations corresponding to normal polarity. Scattered remanent directions from the conglomerate layer of Unit 6 suggest that the basement rocks at Site 1137 have not been reheated since their formation.
We measured the susceptibility of eight discrete samples in basement Unit 1 in 15 different directions to determine AMS and magnetic fabric. The average ratio between maximum and minimum axes (degree of anisotropy) is only 1.027. As the shape parameter of different samples has negative as well as positive values, both oblate (disk) and prolate (rod) shapes of fabric are present. We found no grouping of the axis directions (Fig. F78) and can make no conclusions about the flow direction for this unit.
We chose two discrete samples from each lava flow (basement Units 1, 2, 3, 4, 7, 8, and 10) for stepwise thermal demagnetization up to 620°C and AF demagnetization up to 60 mT. We measured susceptibility of the samples after each heating step to detect changes of their magnetic minerals. Most samples have high MDFs and stable single-component remanent magnetization (Fig. F79A), except for one sample from Unit 7 with a low MDF (8 mT; Fig. F79B). Therefore, we expect a stable and primary remanent magnetization in the lava flows. Even the sample with a low MDF shows a difference of ~10° between its characteristic inclination (-61.5°), calculated by component analysis, and the recent (Brunhes normal epoch) inclination (-71.9°) at Site 1137, assuming a geocentric dipole field. Therefore, the remanent magnetization of this sample does not represent a recent overprint. Low MDFs are observed in most parts of Subunit 7B from whole-core measurements (Fig. F77B). We found two magnetic phases during stepwise thermal demagnetization and susceptibility measurements. The high-temperature phase is characterized by an unblocking temperature of ~580°C (Fig. F80A) and the low-temperature phase by an unblocking temperature of ~300°C (Fig. F80B). The high-temperature phase probably corresponds to magnetite or titanium-poor titanomagnetite and the low-temperature phase to (titano)maghemite (see "Paleomagnetism" in the "Site 1136" chapter). Samples 183-1137A-40R-4, 50-52 cm (Subunit 8B), and 46R-1, 77-79 cm (Subunit 10B), show only high-temperature components. Sample 183-1137A-38R-4, 107-109 cm (Subunit 7B), exhibits only a low-temperature component. The other four samples from Units 1, 2, 3, and 4 have both high- and low-temperature components.
We calculate characteristic inclinations of discrete samples using component analysis (Table T15). Mean inclinations of each flow unit range from -59° to -74°, and differences between the inclinations of samples treated with AF or thermal demagnetization within each flow unit range from 1° to 8°. The maximum difference between mean flow inclinations is 15°, and the differences of inclinations between units are larger than differences within each unit. Variations of inclination between units are probably caused by geomagnetic secular variation. Because of basement penetration through seven flow units, interbedded sediment, and tuff, we believe that our paleomagnetic data are adequate for averaging secular variation. This is necessary for the accurate determination of a paleomagnetic direction. We calculated a mean inclination of -66°, which corresponds to a paleolatitude of 48°S assuming a geocentric dipole field. The difference between Elan Bank's present latitude of 57°S (Site 1137) and its paleolatitude during the Cretaceous (see "Biostratigraphy") is 8°, suggesting that either Elan Bank has moved south or the basement has been tilted since Cretaceous time. The top of basaltic basement has an apparent dip of 1.5° to the east and an intrabasement reflection has an apparent dip of 4.8° to the east (see "Background and Objectives"). Our preliminary interpretation is that Elan Bank has moved ~10° south since its formation. The southward movement of Elan Bank seems consistent with the southward movement of the southern Kerguelen Plateau (Inokuchi and Heider, 1992); however, limited measurements preclude definitive paleolatitude estimates at this time.