The Oki-Daito Ridge, which is located at thenorthern end of the West Philippine Basin, has unique topographic character. The ridge extends over 500 km in a straight line from the Ryukyu Trench in northwest to the Kyusyu-Palau Ridge in southeast. The most interesting feature is the narrow trough along the ridge axis, which divides the ridge into two parts, the northern plateaus and the southern sharp ridge. Why does it have such unique form? And what on earth is the Oki-Daito Ridge?
The Oki-Daito Ridge is one of the oldest elements constructing the Philippine Sea Plate, so the origin and the history of this region must be the key to the evolution of the Philippine Sea before Oligocene. In this paper, we make a careful study of topography, geology, gravity and geomagnetism in the Oki-Daito Ridge region, The data used here were obtained in the Continental Shelf Survey Program, Our data indicate that the Oki-Daito Ridge, including the Minami-Daito Basin, is a remnant arc and was formed near the equator in Late Cretaceous to Paleocene, The narrow trough developing along the ridge axis is considered as a left-lateral fault, which might be caused by the oblique subduction. The results of the geomagnetic analyses show that the Oki-Daido arc was undergone anticlockwise rotation after its formation and that this region had moved northward, Ad, the observed data support the idea that the West Philippine Basin had opened as the back arc basin behind the Oki-Daito arc system.