Preliminary study of the geomagnetic structure of the Japanese islands arcs from inferred aeromagnetic anomalies.
Hydrographic Department of Japan (hereafter referred as JHD) conducted airborne magnetic surveys from 1984 to 1985 a part of whole Japan magnetic survey project. The magnetic anomaly profiles crossing the Japanese islands and their adjacent areas reveal the characteristic feature of anomalies related with crustal structures, thermal conditions, and subducting plates etc. Automated analysis method of magnetic anomaly profile is developped based on an inversion method outlined by Kato (1975, a, b). This method is capable of estimating top and bottom undulations directly from anomaly profile. In magnetic analyses, magnetic anomalies of wavelengths longer than 160 km are ascribed to the undulation of the lower magnetic boundary from the fact that the terrains whose wavelengths are longer than 160 km should be correlated with isostasy of crust. The magnetic models of the representative cross section are obtained using the above method and their geophysical and geological implications are considered. The conspicuous magnetic massives are estimated beneath the forearc areas of the Kuril, Japan, and Izu-Bonin arcs. In southwest Honshu, the large magnetic massif comparable to them is estimated beneath the inland area of Sanin district. The large amplitude linear anomalies over the forearc area of the northeast Honshu, and southwest Honshu coincide well in distribution with that of granitic bodies of magnetite series (Okubo et. al., 1985). The similarity of the magnetic structures of NE Honshu and SW Honshu may imply the continuity of lineation in the original form, and then, are separated by the spreading of Japan Sea. The origin of these magnetic massives occuring in the continental crust are significant for considerations of tectonic development and the process of crustal formation. Two alternative explanations for the origin of these magnetic massives are presented. One is to ascribe them to igneous activity occurring in the Cretaceous time, which may be correlated with ridge descent as proposed by Kinoshita and Ito (1988), and the other is to the formation of the lower crustal layer due to accretional process of the oceanic crust, seamounts, and overlying edifices during the plate subduction.
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| Format: | Journal Contribution biblioteca |
| Language: | Japanese |
| Published: |
1990
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| Online Access: | http://hdl.handle.net/1834/16082 |
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