Figure F16. Magnetic inclination as a function of the amount of rotation around a horizontal axis, for rocks that initially are normally magnetized with a magnetic inclination of 28° and a declination of 0°. Inclinations of 15°, the average value in gabbronorite from Hole 1268A, and 36°, the average value in talc-altered peridotite from Hole 1268A, are shown for reference. Small rotations around an east-west axis could produce inclinations different from 28°. However, this would require opposite senses of rotation for the gabbronorites and the peridotites. Geological reasoning suggests that tectonic rotations in this area are likely to be counterclockwise around a ridge-parallel, nearly horizontal axis striking 020°. Large rotations around a horizontal axis striking 020° could produce the observed 15° inclination in gabbronorites, whereas smaller counterclockwise rotations could produce the observed 36° inclination in peridotites. We hypothesize the gabbronorites acquired their remanent magnetization at somewhat higher temperature than the peridotite during slow cooling of the rocks as they were uplifted toward the seafloor. Peridotites acquired their remanent magnetization at a later time, during magnetite growth associated with hydrothermal alteration and serpentinization at ~300°C. Rotation began before serpentinization of the peridotites, so the peridotites record only part of the tectonic rotation. CW = clockwise, CCW = counterclockwise.
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