Abstract
Magnetic modeling of the forearc side of the Tohoku (northeastern Japan) Arc was carried out using spectral analysis of shipborne magnetic data. The original data show anomalies caused by volcanic rocks on the western, shallow side and linear marine magnetic anomalies on the eastern, deeper side throughout the forearc region. As calculated from temperature gradient data of heat flow measurements and deep drilling, the Curie depth is deeper than the Moho on the eastern, deeper side. This suggests that the Moho may correspond to the base of the magnetic layer, because analysis of upper mantle xenolith suites indicates that the upper mantle should be non-magnetic. In addition, the Conrad discontinuity may correspond to the apparent top of magnetic layer because the lower crustal layer would be more mafic than the upper layer. It was estimated by 2D spectral analysis that the magnetic base and surface lie at about 20 km and 10 km respectively. This suggests that the Moho and the Conrad may roughly correspond to the magnetic base and surface. The magnetic anomalies in the Pacific basin are traced towards and over the inner side of the trench, and show a gradual decrease in amplitude, disappearing eventually about 100 km west of the trench. This reflects the descent of the oceanic slab. The 1D spectral analysis indicates that the marine magnetic layer is descending at an angle of about 4° in the Japan Trench, this angle increasing to 20 ° degrees below the coast, where the marine magnetic layer reaches a depth of more than 30 km. This agrees with the seismic reflection data from the trench, and because the magnetic layer can be traced down to a depth of 30 km it may be suggested that the Curie isotherm deepens to more than 30 km just east of the coast.
Published Version
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