Geomagnetic study on seamounts Daiiti-kasima and Katori with special reference to a subduction process of Daiiti-kasima.

Abstract

As part of the “Basic Maps of the Sea” project and partly for the “Earthquake Prediction Program”, detailed magnetic, hydrographic, seismic and gravity surveys were carried out by the Hydrographic Department, Maritime Safety Agency, Japan, in the southern part of the Japan trench during the period from 1976 to 1977. Using these results, magnetic anomalies over two seamounts, Daiiti kasima and Katori, were analyzed by means of Talwani's method. Besides, further studies based on two-dimensional modeling were conducted for Daiiti-kasima with special reference to the hypothesis of subduction of this seamount.As the results of above analyses, the author presents a magnetic structure of Daiiti-kasima and suggests “an accretional model” of the seamount during the subduction of an oceanic plate to explain the geomagnetic and topographic features of Daiiti-kasima.In addition to the above results, the following have also been concluded.1) A considerable part of the top of Daiiti-kasima must be non-magnetic, and the depression of the magnetized layer of Daiiti-kasima should be taken into account to explain the long distance between the positive and negative magnetic anomaly peaks for the size of the seamount.2) The VGP (virtual geomagnetic pole) for Daiiti-kasima is located at N62°, E349° and the VGP for the top portion of Katori at N60°, E354°. These results which are consistent with the mean VGP from Cretaceous Pacific seamounts support the hypothesis of the northward drift of the Pacific plate.3) As for Katori seamount, magnetization of the top (4200m to 4750m) and that of the bottom (4750m to 7500m) seem to be considerably different in their directions and magnitudes.