MAGNETIZATION OF THE PACIFIC-OCEAN LITHOSPHERE DEDUCED FROM MAGSAT DATA

Magsat anomalies over the Pacific Ocean are related to volcanic plateaus and the edge effect of the Pacific Cretaceous Quiet Zone (PCQZ). Four prominent plateaus, Shatsky Rise, the Mid-Pacific Mountains, Hess Rise, and Manihiki Plateau, are considered in this study. The major lateral magnetization contrasts associated with these plateaus are considered to arise from the topographic swells and the crustal roots, with root thicknesses estimated from topography using an Airy isostatic model. Shatsky Rise and the Mid-Pacific Mountains are older than the Cretaceous Quiet Period and their natural remanent magnetization (NRM) is not a significant component of the magnetization contrasts that cause the Magsat anomalies associated with these plateaus. Their magnetic properties are characterized by effective magnetic susceptibility contrasts of 0.07 (+/- 0.01, S.I., per unit volume) averaged over their topographic swells and crustal roots, equivalent to 0.04 if averaged over the entire crustal thicknesses (swell plus root plus intervening layer). This value agrees well with the estimates of previous investigators for these plateaus and for similar plateaus in other oceans. Two plateaus of Cretaceous Quiet Period age within the PCQZ, Hess Rise and Manihiki Plateau, have origins similar to those of Shatsky Rise and the Mid-Pacific Mountains, and they probably have similar effective susceptibility contrasts but they may also carry Cretaceous Quiet Period NRM. Magnetic anomalies due to Hess Rise and Manihiki Plateau, calculated from the effective susceptibility contrast deduced for Shatsky Rise and the Mid-Pacific Mountains, are substantially larger than the observed Magsat anomalies of these plateaus. There are also Magsat anomalies over parts of the Pacific where no plateaus exist. Some of these anomalies are demonstrated as probably due to a contrast in NRM between the PCQZ and its surroundings. The total remanent magnetization contrast (magnetization contrast in amperes per meter times thickness in meters, equivalent to the dipole moment per unit area) of the PCQZ is 14,000-20,000 amperes (+/- 30%), which cannot be simply explained by the absolute values of remanent magnetization predicted by conventional models of oceanic crustal magnetic properties. Modelling of the Cretaceous quiet zones in the northern Atlantic indicates a similar total remanent magnetization contrast. These results may be explained by including uppermost mantle remanent magnetization, with an intensity of about 1-2 A m-1 if magnetization persists to a depth of almost-equal-to 7 below the seismic Moho, i.e, down to the depth of the crustal roots of the plateaus. Including NRM in the uppermost mantle of the PCQZ also serves to reduce the calculated amplitude of the Hess Rise and Manihiki Plateau Magsat anomalies if the NRM of the crustal roots is less than the NRM of the surrounding uppermost mantle. On this basis the estimated intensity of NRM in the uppermost 7 km of the PCQZ mantle is almost-equal-to 1-3 A m-1, in very good agreement with the intensity estimated from the PCQZ edge effect anomalies. These estimates of uppermost mantle NRM are probably a maximum limit, recognizing that other sources may also contribute to the total remanent magnetization contrasts of the Cretaceous quiet zones.