Result of magnetic analyses on Seamount Zenisu-Oki and a consideration about the negative magnetic anomaly in the southern part of Okhotsk Sea.

Abstract

Since 1967 the Hydrographic Department has been carrying out the marine magnetic surveys on the continental shelves around Japan as a part of the project of the Basic Maρ of the Sea. Some analyses using these data have been already published by Oshima et al. (1975) and Segawa et al. (1976). Among various interesting features found in the magnetic survey data, the present paper deals with analytical studies on the anomaly caused by Seamount Zenisu Oki, located in the northern part of the Izu-Bonin Island Arc, and on the remarkable negative anomalies in the southern part of Okhotsk Sea (Oshima et al., 1975〕． Talwani’s (1965) method of the arbitrary shaped model is applied to evaluate the magnetization vector J together with the regional bias B. Test calculation for this method is made for Seamount Ryohu to yield satisfactory agreement with the result by Uyeda and Richard (1965), (Table 1). Then, analysis is made for Seamount Zenisu Oki, which is approximated by a polygon of five layers (Fig. 2), taking the depth of the seamount top to be 2200, 2700，……， 5200 m, successively (Table 2, Fig. 3). Tendency of G-value obtained from the analysis suggests that the anomaly source locates at the depth of 3300 m, i.e. about 1000 m below the actual top of the seamount. Comparing the inclination and declination derived from the most probable value of J with those at present, it may be considered that the seamount has drifted for about 15°～20° northward from its birth place to the present location. However, it remains another possibility that the seamount was formed in the west, because the position of the paleo-magnetic pole derived from the value of J deviates considerably from that by Nagata (1961). The negative anomaly in the southern part of Okhotsk Sea (Fig. 5) can be divided into regional and local properties. The regional anomaly is the one distributing from Japan Sea to Okhotsk Sea (Ueda, 1976). Model calculation is made to interpretate the local anomaly. It is found that a prism shaped body. having the magnetization vector of declination=0° and inclination= -45° or -90°, shows similar feature to the actual local anomaly. This fact suggests that the basic rocks in this area were magnetized in a reversal epoch during Neogene Tertiary.