Figure F29. Magnetic inclination as a function of the amount of rotation around a horizontal axis for rocks with a magnetic inclination of ±28° and a declination of 0°. Inclinations of 14°, the average value in gabbro and gabbronorite from Hole 1270B, and 3°, the average value in peridotite from Holes 1270C and 1270D, are shown for reference. Relatively small rotations around a horizontal east-west axis could produce the inclinations in the gabbroic rocks and the peridotites. However, this is problematic if gabbroic rocks are reversely magnetized and peridotites are normally magnetized, as in Figure F28C. If all samples are normally magnetized, as in Figure F28B, then clockwise rotations of 42° and 31° around an east-west axis could produce the inclinations in the gabbroic rocks and the peridotites, respectively. However, there is little evidence for east-weststriking, north-dipping faults in bathymetric data from the region around Site 1270 (e.g., Fujiwara et al., 2003). Instead, geological reasoning suggests that tectonic rotations in this area are likely to be clockwise around a ridge-parallel, nearly horizontal axis striking 020°. Large rotations around a horizontal axis striking 020° could produce the observed 14° inclination in gabbroic rocks, whereas somewhat smaller clockwise rotations could produce the observed 3° inclination in peridotites. For the scenario in Figure F28C, in which gabbroic rocks are reversely magnetized (dashed curves) while peridotites are normally magnetized (solid curves), this requires ~45°50° of clockwise rotation after both rock types acquired their remanent magnetization. For the scenario in Figure F28B, in which both gabbroic rocks and peridotites are normally magnetized (solid curves), we hypothesize the gabbronorites acquired their remanent magnetization at the magnetite blocking temperature (~500°570°C) 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.
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