The magnetic fields in the deepest oceanic drill hole, DSDP/ODP Hole 504B, and in the neighboring Hole 896A, have been logged continuously with a calibrated three-axis magnetometer so that horizontal and vertical field anomalies could be determined. The extrusive basalts produce the largest anomalies (up to 5000 nT) but the amplitudes vary strongly with short wavelengths, whereas the sheeted dikes are associated with 500 to 1500 nT anomalies. The horizontal anomalies are predominantly negative and confirm the inferred inverse polarity of the drilled oceanic crust. At the base of the extrusives (oceanic Layer 2A) there are positive anomalies, presumably from basalts with normal magnetization. However, steeply dipping layers with inverse polarity could also cause positive anomalies. The vertical extent of negative anomalies in Layer 2A is only 200 m for Hole 896A and 475 m for Hole 504B, indicating a strong lateral variability in the thickness of this source layer. The fine structures of the anomalies in both holes bear little resemblance to each other but the average amplitudes of the horizontal anomalies are both around 2000 nT in the upper parts. Simple forward models as horizontal cylinders allow us to determine average magnetization values of 3 A/m for the extrusive basalts and 1.7 A/m for the sheeted dikes (Layer 2B). The former is only half the average measured rock magnetization, whereas sheeted dike values agree. At depths that correspond to the transitions in drilling from Legs 111/137 and 140/148 there are clear changes in the horizontal anomalies. These are interpreted as a result of drilling-induced magnetizations caused by strongly magnetic drill pipes used during Legs 137 and 140. The deduced average magnetization intensities have been used as parameters to model surface anomalies. Layers 2A and 2B contribute approximately equally to the surface anomalies with maximum amplitudes of 160 nT that agree well with the measured anomalies; thus gabbros are unlikely to contribute to marine lineated magnetic anomalies.
Date of initial receipt: 17 August 1994
Date of acceptance: 24 February 1995
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