AGE-DEPENDENT 3-DIMENSIONAL MAGNETIC MODELING OF THE NORTH-PACIFIC AND NORTH-ATLANTIC OCEANIC-CRUST AT INTERMEDIATE WAVELENGTHS

Three-dimensional magnetic modeling of the North Atlantic and northeas t Pacific is performed at intermediate wavelengths using three models for the acquisition of a natural remanent magnetization. It is shown t hat a remanent magnetization which is dependent on the crustal age is the dominant source for the intermediate-wavelength pattern in both ba sins. However, a pure thermoremanent magnetization of layer 2 alone is insufficient to model the intensity and shape of the observed magneti c anomalies at satellite altitude. We conclude that the best fitting m agnetization model for both basins is a combination of a chemical rema nent magnetization in the altered upper crust and a thermoviscous rema nent magnetization of the slowly cooling lower crust and upper mantle. The North Pacific requires a bulk magnetization which is 50% higher t han that of the North Atlantic in order to fit the Magsat field. Geolo gical processes associated with a faster spreading rate such as a fast er hydrothermal alteration and the growth of a thicker gabbro layer at the expense of a weakly magnetized sheeted dike layer are plausible e xplanations for the higher North Pacific magnetization. The lineated p ositive magnetic anomaly observed over the North Atlantic spreading ce nter is not well reproduced by our models. This anomaly is likely due to a highly magnetized body along or in the vicinity of the spreading center. This highly magnetic body could be an unstable serpentinized l ens of crustal material younger than 20 m.y. which develops within the zone cooled by hydrothermal circulation.