Origin of the Magnetic Anomalies in the Southern Okinawa Trough.

Two-dimensional magnetic modeling was performed to investigate the magnetic structure beneath the southern Okinawa Trough. The E-W elongating magnetic anomalies accompanied with the geologic segment of the Yaeyama Graben are explained by a simple model assuming of the seismic layers of 3.6-6.4km/s with a uniform normal magnetization of 2.5A/m as the magnetic layer. The upper surface of the magnetic layer coincides with an acoustic basement on multi-channel seismic reflection profile. The E-W elongating anomalies are caused by the V-shaped subsided pre-rifting layers. The high magnetic intensity of 2.5A/m given to the model suggests that the pre-rifting layers contain a large amount of magnetic material. Our investigating results do not support an existence of magnetic lineations of seafloor spreading origin in the southern Okinawa Trough.