Publication of geomagnetic and gravity anomaly chart and characteristics of the magnetic anomalies in the southern waters of Japan.

Abstract

The Hydrographic Department of Japan (HDJ) has published a new magnetic map and free-air gravity map at a scale of 1 : 3, 000, 000 by compiling large amounts of data sets. The data source of these two maps are mainly composed of homogeneous and high”resolution data collected by the Continental Shelf Surveys Project which has been carried out by the HDJ since 1983, and other data sets provided by Japan Oceanographic Data Center. The maps cover vast area of southern waters of Japan, including two island arc trench systems, from East China Sea, northern Phillippine Sea to Northwest Pacific Basin. This paper introduces the outline of the above published maps and summarizes the characteristics of magnetic anomalies and tectonic features related to island arc volcanic activities and seafloor spreading. The type of volcanic activities on several remnant or active island arcs and other submarine elevations are classified into three groups in terms of the characteristics of magnetic anomalies. The difference of these types might be dependent on the distribution of zones of crustal weakness. Evolutionary process of seafloor spreading in the back arc basins and deep ocean floor are discussed by comparing the pattern of the lineated magnetic anomalies in the Shikoku Basin and Northwest Pacific Basin. Magnetic anomalies show back arc basins tend to have more complicated history of seafloor spreading.