Geology and Geomorphology of the Ogasawara Arc and the Sofugan Tectonic Line

Abstract

The Ogasawara Arc consists of four ridges; they are the Izu, Shichito (volcanic front), Ogasawara and Shinkurose Ridges. Geology and geomorphology of the arc are reviewed b sed on dredged submarine rocks and subaerial geology of islands on the arc.
On the basis of submarine topographic features the Shichito Ridge is divided into three parts (segments 1, 2 and 3). Topographic gaps are found between segments 1 and 2, and 3. The Shinkurose Ridge appears to be displaced at the gaps. The gaps are situated between Smith Rock (Sumisujima) and Torishima Is., and between Sofugan and Nishinoshima Is. The latter is correlated with the Sofugan Tectonic Line (YUASA 1983). Volcanic rock bulk chemistry of the volcanic front, and hypocenter distribution pattern are different between northern and southern parts of the Ogasawara Arc separated by the Sofugan Tectonic Line. Distribution of back-arc depressions and frontal arcs are also different between them.
In 1983, BANDY and HILDE proposed three right-lateral transverse faults on the Ogasawara Arc based on the displacement of outer ridge structure, and the existence of gaps on free-air gravity anomaly map. The right-lateral displacement proposed by them is apparently different from left-lateral displacement of the Sofugan Tectonic Line and dislocations within the Shichito Ridge.
The Sofugan Tectonic Line may not be the only transverse fault on the Ogasawara Arc. There seem to be other faults between segments 1 and 2 of the Shichito Ridge and elsewhere. The gap, however, affecting the geological and geophysical phenomena such as rock chemistry, and hypocenter distribution is only the Sofugan Tectonic Line.