Geomagnetic anomalies around Nansei-Syoto (Ryukyu Islands),Japan.

Abstract

Geomagnetic total intensity anomalies at the sea around Nansei-Syoto（Ryukyu Islands) and Senkaku Islands were compiled into three sheets of geomagnetic anomaly maps based on the geomagnetic surveys made by the Hydrographic Department from 1975 to 1982. The survey area is divided into several zones according to the feature of the geomagnetic anomalies considering the geological/topographical characteristics of the region concerned, and geotectonic implication of the geomagnetic anomalies for each zone is investigated. A complicated anomaly zone exists along the Paleo-Ryukyu Volcanic Belt and the Ryukyu Volcanic Belt (Konishi, 1965) and a magnetic smooth zone is recognized over the area of the Ryukyu Geoanticline and the continental slope located in the east of the volcanic zones. The southern end of the Quaternary volcanic front located along the Tokara volcanic chain has been recognized near Io Tori Sima up to the present (Matsumoto, 1977), and it is newly recognized that the front extends along the east boundary of the Okinawa Trough and the south boundary of the West Ryukyu knolls group from the results of geomagnetic and geological/topographical surveys (Kato, et al., 1982). The fact that the short wavelength magnetic anomalies dominate along the volcanic front in contrast to a rather long wavelength feature of the topographic undulations may indicate that the basic rocks intruding into the non magnetic Pre-Miocene basement complex form the volcanic islands and knolls of the Tokara volcanic chain. There are no magnetic anomalies over the uplift of knolls and bank inside the Tokara volcanic chain. The positive magnetic lineated anomaly zone is seen from Kume Sima to Igyo-Sone bank to the west of Okinawa Sima. The fact that there was a green tuft volcanism in the Miocene at Aguni Sima may indicate a volcanic activity in the Miocene in the area of positive magnetic anomaly. The Tokara channel and Kerama Gap have been considered to be left-lateral faults which cut the zonal structure of the geological and topographical features of this area, but this consideration is not positively supported by the magnetic anomalies.