Magnetic Anomalies in the Sea of Japan

Abstract

Geomagnetic anomaly profiles obtained by Isezaki and Uyeda are reanalysed to infer the mode and time of opening of the Sea of Japan, especially in the Japan Basin. The anomalies show a roughly ENE-WSW trend which is subparallel to the coastlines of Honshu island and Sikhote Alin. It is inferred from the analysis that Vine-Matthews type magnetic anomalies are present at least in the Japan Basin. Possible reasons why the anomalies could not be correlated in the previous analyses with the geomagnetic reversal time scale are also examined. One possibility is that the anomaly patterns in the Sea of Japan is not a product of simple sea-floor spreading but is obscured by the complexity in the mode of opening or in the tectonic evolution. However, the most plausible explanation is that the assumption of symmetric spreading about some spreading center was wrong. The shape analysis shows that a better fit is obtained for one-sided spreading with a spreading center near the coast of Sikhote Alin. A satisfactory fit is obtained for a magnetic profile with a model generated for the period of 10-15Ma.This timing for the opening of the Sea of Japan is broadly consistent with the paleomagnetic data from the northeastern part of Honshu island. It is suggested that the original Vine-Matthews anomaly pattern was “contaminated” by the off-ridge volcanism which is also responsible for the creation of the abundant seamounts in both the Japan Basin and Yamato Basin.