Abstract
The Northern Izu-Bonin Arc, one of the intraoceanic arc systems in Western Pacific, is marked by an along-arc arrangement of trench, volcanic front, and backarc rift, and across-arc NE-SW oriented seamount chains. Hachijojima Volcanic Group is located on the volcanic front of this arc, and consists of Nishiyama, Higashiyama, Hachijo-kojima Volcano, and one eroded submarine volcanic edifice. The structure and the evolution process of Hachijojima Volcanic Group are revealed in this topographic study.
The Hachijojima Volcanic Group comprises about ten stratovolcanoes. Eight of them were active during the past 140, 000 years. All these volcanoes were formed on the same submarine basement having a 30-35-kilometer bottom diameter. This might suggest that the volcanoes originated from the same mantle diapir. The volcanoes of the Hachijojima Volcanic Group are classified into two types: basalt-type and andesite-type. Andesitic or dacitic magmas or both sometimes erupted during the late-stage activity of the basalt-type volcanoes.
The NW-SE alignment of flank volcanoes of Nishiyama might indicate the direction of horizontal compressional stress, which is probably caused by a collision between the Izu-Bonin Arc and the Honshu (Japan) Arc in the Izu Peninsula. On the other hand, the instability of the eruption center and the low eruptive rate suggest that the Hachijojima Volcanic Group has been under the influence of a relatively tensional stress field. Crustal deformation caused by the compressional stress associated with the arc-arc collision is less remarkable in Hachijojima than around the Izu Peninsula-Oshima-Miyakejima region. Active backarc rifting associated with N-S-oriented normal faults and monogenetic volcanoes immediately west of the Hachijojima Volcanic Group supports this view. However the E-W tensional stress field in the backarc rift is differant from NW-SE compressional stress field in the volcanic front. The NE-SW alignment of volcanic edifices in Higashiyama is probably controlled by the NE-SW structure which continues from the across-arc seamount chain in the backarc region.
