Abstract A magnetic anomaly map of the northern part of the Philippine Sea plate shows two conspicuous north-south rows of long-wavelength anomalies over the Izu-Ogasawara (Bonin) Arc, which are slightly oblique to the present volcanic front. These anomalies are enhanced on reduced-to-pole and upward-continued anomaly maps. The east row is associated with frontal arc highs (the Shinkurose Ridge, SKR), and the west row is accompanied by the Nishi-Shichito Ridge (NSR). Another belt of long-wavelength anomalies very similar to the former two occurs over the Kyushu-Palau Ridge (KPR). To explain the similarity of the magnetic anomalies, we propose that after the spreading of the Shikoku Basin separated the Izu-Ogasawara Arc from the Kyushu-Palau Ridge, another rifting event occurred in Miocene time, which divided the Izu-Ogasawara Arc into the Nishi-Shichito and Shinkurose Ridges. The occurrence of Miocene rifting has also been suggested from the geology of the collision zone of the Izu-Ogasawara Arc against the Southwest Japan Arc: the Misaka terrain yields peculiar volcanic rocks suggesting backarc rifting at about 15 Ma. The magnetic anomaly belts over the Izu-Ogasawara Arc do not extend south beyond the Sofugan Tectonic Line (STL), suggesting difference in tectonic history between the northern and southern parts of the Izu-Ogasawara Arc.
We estimate that the Miocene extension was directed NE-SW, utilizing normal faults originally formed during Oligocene rifting. The direction is close to the final stage of the Shikoku Basin spreading. On a gravity anomaly relief map, NE-SW lineaments can be recognized in the Shikoku Basin as well as over the Nishi-Shichito Ridge. We thus consider that lines of structural weakness connected transform faults of the Shikoku Basin spreading system and the transfer faults of the Miocene Izu-Ogasawara Arc rifting. Volcanism on the Nishi-Shichito Ridge has continued along the lines of weakness, which could have caused the en echelon arrangement of the volcanoes.
Key words: Izu-Ogasawara (Bonin) Arc, Kyushu-Palau Ridge, rifting, magnetic anomaly
