MAGNETIC-ANOMALIES OF JAPAN AND ADJOINING AREAS

The magnetic anomaly map of Japan and adjoining areas was produced by compiling many data sets collected by air-borne and ship-borne surveys. In order to tie between adjoining areas, IGRF removal, upward continuation, and a linear shift were applied to each data set. Linear features are detected from the map as a special reference to suggest possible crustal structures. Two significant features in the linear map are found. The one is the existence of lineations crossing the trench. The lineations are caused by oceanic magnetic layers that lie beneath the overriding plate. The distance from trench to the end of lineation is variant. The variation of distance depends on the amplitude of anomaly, the structure of the topmost part of lithosphere, the dip angle of subducting plate, and the Curie point depth. The second is the occurrence of two types of lineations found in the back-arc spreading regions described as, (1) lineations running parallel with the arc trend on a rifting or spreading stage, and (2) lineations across the arc trend in the young back-arc rift region. They are related to igneous rocks produced by volcanic activities of back-arc rifting and spreading. Consequently, the two features express igneous rocks either (1) parallel with, or (2) across the arc trend. They are probably associated with two different spreading stages of the back-arc basin.