The vertical structure of the sources of lineated marine magnetic anomalies has remained poorly known ever since the recognition, more than 30 years ago, that the oceanic crust records reversals of the Earth's geomagnetic field. Several authors have suggested that some or most of the stable source might reside in the gabbroic crust based on the magnetization of dredged gabbros (Fox and Opdyke, 1973; Kent et al., 1978). However, the surficial samples have been subjected to varying degrees of hydrothermal alteration and weathering during faulting and emplacement to the seafloor that would likely significantly affect their magnetic properties. During the site survey for Leg 118 (Dick et al., 1991b), well-defined magnetic anomalies were mapped over large regions of the rift mountains of the Southwest Indian Ridge adjacent to the transform fault. Extensive dredging of these regions, including Atlantis Bank, recovered largely gabbro and peridotite, suggesting that these lithologies must be responsible for the anomalies (Dick et al., 1991b). The magnetic properties of the Leg 118 cores from Hole 735B were found to be consistent with a gabbroic source layer for the anomaly over the site (Kikawa and Ozawa, 1992; Pariso and Johnson, 1993). The Leg 176 cores, however, are not only consistent with these interpretations, but together with the Leg 118 cores demonstrate that the 1.5-km Hole 735B section is the source of the lineated magnetic anomaly over the site, to the extent that this section is representative of the crust in three dimensions.
This conclusion presents the possibility that gabbroic crust may potentially contribute to, and even dominate over, the contribution of pillow basalts and sheeted dikes to marine magnetic anomalies elsewhere. However, the rapid acquisition of thermal remanence of the Hole 735B gabbros is consistent with rapid cooling due to unroofing and uplift to sea level at the ridge-transform intersection. This may not be the case for normal Southwest Indian Ridge crust away from transforms, which would cool under a 2-km carapace of pillow basalts and sheeted dikes, and acquire its thermal remanence more slowly.
Thermal and alternating field demagnetization studies of discrete samples from Leg 176 reveal two magnetization components: a low-stability component apparently related to drilling, and more stable components with steeper inclinations. The mean characteristic inclination (Inc) for Leg 176 discrete samples is reversed, with Inc = 71.4° (+0.3°/ 3.1°), uncorrected for any hole deviation from vertical (Fig. 20). This is statistically identical to that found in the upper 500 m (71.3°, +0.4°/ 11.0°) when the latter are recalculated using the method of McFadden and Reid (1982). Since Antarctica has been relatively fixed in the plate reference frame over the last 11 Ma, the present latitude of Hole 735B is likely close to that of the paleo-ridge axis at the time of remanence acquisition. Thus, the observed inclination is steeper than the expected 52° for the site, and requires a tectonic rotation of the section of approximately 19° ±5° (depending on the deviation of the hole from vertical).
Overall, the tight cluster of stable magnetic inclinations downhole is significant to the interpretation of the igneous petrology and structure of Hole 735B, as the inclinations indicate that there has been little tectonic disruption of the section since it cooled below the Curie point at about 580°C. Moreover, there is an unexpected strong preferred orientation of declinations at around 260° in the reference frame of the core liner. This is explained by the fact that the structural geologists systematically oriented each section of core for splitting so that they were cut orthogonal to the foliation, with each half placed in the working and archive halves so that the foliation dipped consistently in one direction. Thus, the consistent declinations demonstrate that these foliations are not random, and suggest that gross reorientation of structural features in the core may be possible. Assuming a south-pointing characteristic remanence declination, the mean declination of 260° would be restored to 180° by a counterclockwise rotation of approximately 80°. Structural planar features that preferentially dip toward 090° in the core reference frame would thus dip toward the axial rift to the north.
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