抄録
Geomagnetic survey by means of the proton magnetometer and the three component magnetometer was carried out by the RN 83 Cruise of the Nagasaki Maru to reveal Pleistocene igneous activities along the axial median valley (central rift) of the middle Okinawa Trough in 1983.
Preliminary results are as follows:
(1) Magnetic anomaly lineations were clearly recognized in the studied area, having approximately N80°E-S80°W trend nearly parallel to the narrow topographic depression (central rift).
(2) Amplitudes of anomalies (peak to peak) are about 200-500nT and their wave lengths range from 20 to 30km.
(3) The magnetic anomaly lineations were concentrated within the width of 20km from the central rift trending ENE-WSW.
(4) Two dimensional block model based upon the geomagnetic, reversal time scale seems to well explain the magnetization pattern in the middle Okinawa Trough. Calculated result suggests that the major spreading of the Okinawa Trough had occurred between around 1.8Ma and 0.5Ma.
(5) However, another geological evidence never shows oceanic condition in this area because the axial area of the middle Okinawa Trough has continental crust, and arc volcanisms have been occurred.
