Abstract
Philippine Sea has been thought to be formed by repeated back-arc spreading. The Kyusyu-Palau Ridge is thought to be remnant arc and is divided by the opening of the Shikoku and Parece Vela basins from 30 to 15 Ma. Its eastern half is presumed to be the West Mariana Ridge and the Nishi-Shichito Ridge. The Nishi-Shichito Ridge is topographically divided into two parts (northern and southern parts). The northern part consists of several echelon mountain ridges. The southern part consists of N-S trending seamounts array. The age data so far reported from this Ridge are Pliocene or younger. However, recently, the Miocene data have been obtained from the Tenpo Seamount based on the foraminiferal assemblages of the calcareous sediments covering the top of the seamount.The Tenpo Seamount, being located on the southern part of the Nishi-Shichito Ridge, was formed before 15 Ma, by the time of cessation of the Shikoku Basin opening. This fact suggests that this seamount is the remnant arc and the Kyusyu-Palau Ridge is equivalent counterpart. The magnetic anomaly of the Tenpo Seamount is large positive and it is not recognized on any other seamounts in the Nishi-Shichito Ridge. The Ohmachi Seamount, situated on the Shichito-Iwojima Ridge, has similar magnetic anomaly and the K-Ar age of the volcanic rocks from this seamount is Oligocene. These facts probably indicate that a part of the remnant arc will extend further south to this seamount.The seamounts on the Nishi-Shichito Ridge except the Tenpo Seamount were formed as the present arc after the spreading of the Shikoku Basin ceased. The seamounts of the northern part of the Nishi-Shichito Ridge have obviously dipole anomaly corresponding with each seamount. On the other hand, magnetic anomalies of the seamounts in the south part of it are obscure and faint. There is difference in tectonics and age of formation between the northern and the southern part. Based on the above mentioned observations, the Nishi-Shichito Ridge is thought to be a complex arc.
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