We made pass-through magnetometer measurements and magnetic susceptibility measurements on all split-core archive sections at 2-cm intervals. In order to isolate the characteristic remanent magnetization (ChRM), we subjected the cores to alternating-field (AF) demagnetization. The half cores were demagnetized up to 60 mT. We analyzed the results in Zijderveld diagrams (Zijderveld, 1967) and calculated the ChRM directions using principal component analysis (Kirschvink, 1980). In addition, we determined magnetic susceptibility on all whole cores from Site 1277 at 2.5-cm intervals as part of the multisensor track (MST) analysis and measured split-core sections at 2-cm intervals with the point-susceptibility meter (archive multisensor track). No discrete shipboard paleomagnetic samples were taken because of time limitations at the end of the leg.
Paleomagnetic data obtained at Site 1277 exhibit significant variations in demagnetization behavior among various recovered lithologies (Fig. F15; Tables T2, T3, T4). Variations in magnetic susceptibility generally parallel the variations in natural remanent magnetization (NRM) intensity (Fig. F15). Drilling-induced remagnetization is observed, but it has much less effect than it did for sediments in Hole 1276A. In most instances, we were able to remove this remagnetization with 10- to 20-mT AF demagnetization and to isolate the ChRM direction using higher field intensities (Fig. F16). Paleomagnetic measurements indicate that the greenish gabbro cataclasite in Core 210-1277A-2R has the lowest NRM intensity (~0.02–0.3 A/m) in contrast with the relatively fresh aphyric basalt in Cores 1W, 3R, and 5R (~1–4 A/m), gabbro at the base of Core 4R and in the lower part of Core 5R (0.2–0.5 A/m), and serpentinized peridotite in Core 9R (1–9 A/m).
The basalts appear to record a stable component of magnetization with normal inclinations (~45°). On the other hand, the cataclasite in Core 210-1277A-2R displays variable inclinations (from positive shallow to even negative shallow), consistent with the fact that it is tectonized (see "Igneous and Metamorphic Petrology"). The gabbros and adjacent sediments in Cores 210-1277A-4R and 5R have the same stable inclination values of ~40°, which agree with the inclination values observed from the basalt. This is consistent with expected Cretaceous inclinations for this region.
The serpentine peridotites in Core 210-1277A-9R show a variable degree of overprinting, but AF demagnetization easily removed the overprint and allowed isolation of the ChRM direction (Fig. F16). ChRM inclinations obtained from different parts of long coherent core pieces generally agree within a few degrees and cluster around a mean of 40°, similar to inclination values in the basalt and gabbro. The fact that the NRM intensities of the serpentinized peridotite are typically 1–9 A/m suggests that they could significantly contribute to the regional magnetic anomaly. We also observe a distinct difference between the gabbro, which has relatively low magnetic susceptibilities and high median destructive fields (MDFs), and the serpentinized peridotite, which has high susceptibilities and low MDFs (see Table T4). This difference can be explained by either the finer magnetic grain size of the gabbro, its higher degree of low-temperature alteration, or a combination of these factors. Shore-based rock magnetic studies on discrete samples will investigate the cause of the downhole variation in rock-magnetic properties.