FAULT BELTS AND IGNEOUS ROCKS OF THE OKINAWA TROUGH AND ADJACENT AREAS:EVIDENCE FROM GRAVITY AND MAGNETIC DATA

The Okinawa Trough is an active tectonic area with widespread faults and igneous rock bodies. In order to study the characteristics of the fault belts and igneous rocks, we calculated the vertical and horizontal derivatives of gravity and magnetic anomalies on 1:1000000 scale. The results show that there are two groups of fault belts distributed in the Trough and adjacent areas, running parallel and perpendicular to the trough, respectively. The NW and NWW trending fault belts, which are perpendicular to the trough itself, are mainly strike-slip faults, formed in earlier time and gradually propagated southeastward together with the tectonic evolution of the East China Sea. Influenced by the tectonic stress fields in different periods, the strike of the fault belts changed from west to east, making the fault belts bulged northeastward. The igneous rocks are distributed in several belts. The belt located in the eastern side of the trough, which extends from Kyushu to Taiwan, is corresponding to the Ryukyu volcanic front. Another belt located on the eastern margin of the shelf, which corresponds to the Diaoyudao uplift, terminates to the south by one of the NW trending fault belts near 27°N. The Igneous rock belt located in the axial part of the southern Okinawa Trough is related to the rifting process. The tectonic nature of the Taibei-Diaoyudao volcanic belt is different from the Diaoyudao uplift to the north.