THE GLOBAL CONTINENT - OCEAN MAGNETIZATION CONTRAST

A schematic global model for the distribution of magnetization in the lithosphere is devised. This model is characterized by a uniform stepwise change of radially integrated susceptibility at continent-ocean boundaries and an axial dipolar inducing field. A spherical harmonic expansion of the magnetic field produced by this source distribution is computed. Then, the standard method of truncation at degree n = 13 is used to determine the respective contribution of this field to the main field and to the lithospheric anomaly field. The latter is shown to display significant anomalies near continental margins as well as within continents and oceanic basins. The energy density spectrum of this field is shown to be very similar to the spectrum of the Magsat-derived lithospheric anomaly field. Furthermore, several features of this field are shown to account for unexplained anomalies of the Magsat field. This supports the hypothesis that such an abrupt magnetization variation may occur at several continental margins and contribute to the signal in the Magsat-derived long-wavelength anomaly maps. This hypothesis provides an order of magnitude estimate for the magnetization difference between oceanic and continental lithospheres. Finally, it is argued that the synthetic anomaly field produced by this model provides an upper bound to the energy density spectrum of the lithospheric field at low harmonic degrees and concluded that the lithospheric field does not contribute significantly to the Earth's field below degree 13.