New paleolatitudes for the CKP (Site 1138) and NKP (Site 1140) (Fig. F1) coincide with previous paleolatitudes obtained from Kerguelen Plateau and Ninetyeast Ridge rocks (Antretter et al., 2002). All of the paleolatitudes differ from one current option for the location of the Kerguelen hotspot at 49°S beneath the Kerguelen archipelago (Fig. F1). Motion between the Earth's mantle and rotation axis (i.e., true polar wander) cannot explain the difference. Numerical modeling of plume conduit motion in a large-scale mantle flow predicts southward motion of the Kerguelen hotspot of 3°-10°, which is consistent with paleomagnetic results (Antretter et al., 2002).
Igneous rocks from the seven Leg 183 basement sites (Figs. F1, F2) display variable rock magnetic properties (Zhao et al., this volume). Most subaerial basalt samples from Sites 1136, 1137, 1138, 1141, and 1142 underwent high-temperature oxidation deuterically, which is responsible for the high Curie temperature. These subaerial basalts are most likely good paleomagnetic recorders that preserve original and stable magnetic remanences. In contrast, basalt flows at Site 1139 underwent low-temperature oxidation. Substantial alteration at this site may have reset the original magnetization. At Site 1140, the fine grain size, inferred from rock magnetism, indicates rapid cooling of the pillow basalt samples. From the high-field magnetic moment curves and Curie points, it may be inferred that Ti-rich titanomagnetites are present in these submarine lavas, and they are expected to give accurate paleomagnetic results. Basalt recovered from Site 1142 was provisionally interpreted as pillow basalt in shipboard petrological descriptions. Results of rock magnetic investigations on a limited set of these rocks, however, suggest that they were most likely erupted in a subaerial environment, similar to their counterparts at Site 1141 (Zhao et al., this volume). The generally good magnetic stability and other properties exhibited by titanomagnetite-bearing rocks support the inference that the characteristic directions of magnetization isolated from Cretaceous rocks were acquired during the Cretaceous Normal Superchron (Shipboard Scientific Party, 2000). Thus, the stable inclinations obtained from these samples should be reliable for tectonic studies. Rock magnetic investigations of sediments from Sites 1138 and 1140 show that titanomagnetites with varying Ti content are the main magnetic minerals (Antretter et al., this volume).
Magnetic properties of subaerial and submarine basalts differ in opposite ways. In subaerial basalt, altered flow tops typically have high magnetic susceptibility and natural remanent magnetization (NRM) in the massive flow interiors, whereas submarine pillows and flows typically show the opposite (Delius et al., in press). Cooling and alteration are the main factors influencing the generation and distribution of oxide minerals, and the effects of low-temperature alteration are most noticeable in the distribution of more mobile elements (e.g., potassium). The main signal of the spectral gamma ray log is determined by potassium concentration and is therefore an excellent proxy for alteration. The strong positive correlation between total magnetic field and spectral gamma ray log response, both determined from downhole measurements, suggests that alteration can explain why submarine basalts show opposite magnetic properties to subaerial basalts (Delius et al., in press).