Colored Magnetic Map of Japanese Islands

Abstract

We created the 1:1 000 000 colored aeranagnetic map of the Japanese Islands and the colored aeranagnetic map of the northern part of Japan superimposed on bird-viewed topography. The existing data were merged to make a CQ~pletely continued map through upward continuation and trend removal. OVerall, the magnetic maps contain a wealth of information which will be useful as a significant tool in helping to unravel the ccrnplex geological history of Japan. The magnetics clearly delineate the volcanic chains of the active islands arcs. The lateral and regional variation of magnetic properties of granitic rocks can be identified in the map. Pre-Neogene tectonic divisions below alluvial plain are defined in scrne places. Scrne weak lineations correspond to tectonic lines. High magnetic belts reveal deep seated cretaceous ultramafic intrusions(Ogawa and Suyama, 1975). Marine magnetic lineations crossing the Japan trench are clear on the map on bird-viewed topography. The lineations disappear at a distance of al:xJut 100 krn frcrn the Japan trench to the 'Japanese Islands side because they were memorized on the subducted oceanic plate and the reoords are subducting below the continental plate(Makino and Okubo, in press). Using the magnetic data, we calculated Curie depth which describes the depth to the inferred Curie point transition of magnetite(Okubo et al., 1985a). The map delineates regional thernal structure of the Japanese Islands(Okubo et aL, 1985b). As we do not know the exact value of the Curie point and cannot calculate the value by our algorithm, we ccrnpare the Curie depth with the borehole temperature data which indicate conductive temperature in order to estimate the Curie point(Okubo et al., in press). The comparison results that the temperature gradients in the shallow zone(top 4 kilcrneters or so) are (435 "C)/(Curie depth) on the backarc side and (250 "C)/(Curie depth) on the forarc side. If we assume constant heat flow with increasing depth, we can easily estimate heat flow by the temperature gradients and thermal conductivity of shallow zone. Thermal conductivity laterally change, but 2-3 W/m· K is reasonable average value of granitic rocks of shallow zone(Matsubayashi, personal communication). Figure 1 shows the heat flow map calculated frcrn the Curie depth map when temperature gradient is (435 "C)/(Curie depth) and thernal conductivity is 2 W/m·K on the whole area. The map reveals approximate total heat quantity frcrn deep sources, energy of radioactive substances, and energy discharged from subducting oceanic plates.