CONTRIBUTION OF INDUCED AND REMANENT MAGNETIZATION TO LONG-WAVELENGTH OCEANIC MAGNETIC-ANOMALIES

The Magsat mission has provided very precise measurements of Earth's magnetic field which have been used to derive accurate maps of the long-wavelength magnetic anomaly field. Analyses of this field over localized areas have shown that induced magnetization is mainly responsible for the anomalies and that remanent magnetization is rarely detectable at Magsat altitude. A careful analysis of the data, taking into account the various effects due to non lithospheric sources, allows this field to be better described and, consequently, suggests two observations: (1) long-wavelength oceanic anomalies display many similarities with the distribution of seafloor topography, suggesting a relationship between crustal thickening and magnetization enhancement; (2) age-related pattern (i.e., ridge parallel) of anomalies is observed in two of the larger oceanic basins (Indian and North Atlantic oceans). Several forward models are constructed to investigate these correlationss. In the first model, the effect of variations in crustal thickness was modeled assuming uniform susceptibility and that thickness varies proportionally to the seafloor topography. In the second model, the field due to remanent magnetization acquired during the Cretaceous Long Normal period was modeled and shown to contribute significantly to the Magsat field. In order to make the modeled and observed maps fully comparable, several effects must be taken into account: a global magnetization contrast exists between oceans and continents and induces widely distributed anomalies because of the truncated spherical harmonic analysis; the spectral content of the observed map in the north-south direction is reduced because of the along-track filtering due to the polar orbit of Magsat. When these effects are taken into account, the model combining the contribution of induced and remanent magnetization display remarkable similarity to observed data. This comparison constrains both induced and remanent magnetization of oceanic lithospheric rocks.