Hahajima Seamount: An enigmatic tectonic block at the junction between the Izu–Bonin and Mariana Trenches
Article first published online: 21 DEC 2005
Volume 14, Issue 4, pages 616–622, December 2005
How to Cite
Fujioka, K., Tokunaga, W., Yokose, H., Kasahara, J., Sato, T., Miura, R. and Ishii, T. (2005), Hahajima Seamount: An enigmatic tectonic block at the junction between the Izu–Bonin and Mariana Trenches. Island Arc, 14: 616–622. doi: 10.1111/j.1440-1738.2005.00488.x
- Issue published online: 21 DEC 2005
- Article first published online: 21 DEC 2005
- Received 12 May 2005; accepted for publication 6 September 2005.
- Hahajima Seamount;
- Izu–Bonin Arc;
- Parece Vela Basin;
- serpentine seamount;
- transform fault
Abstract The Hahajima Seamount, located at the junction between the Izu–Bonin and Mariana forearc slopes, is a notable rectangular shape and consists of various kinds of rocks. An elaborated bathymetric swath mapping with geophysical measurements and dredge hauls showed the Hahajima Seamount is cut by two predominating lineaments, northeast–southwest and northwest–southeast. These lineaments are of faults based on the topographic cross-sections and a 3-D view (whale's eye view). The former lineament is parallel to the transform faults of the Parece Vela Basin, whereas the latter is parallel to the nearby transform fault on the subducting Pacific Plate. The rocks constituting the seamount are ultramafic rocks (mostly harzburgite), boninite, basalt, andesite, gabbro, breccia and sedimentary rocks, which characterize an island arc and an ocean basin. Gravity measurement and seismic reflection survey offer neither a definite gravity anomaly at the seamount nor definite internal structures beneath the seamount. A northwest–southeast-trending fault and small-scale serpentine flows were observed during submersible dives at the Hahajima Seamount. The rectangular shape, size of the seamount, various kinds of rocks and geophysical measurements strongly suggest that the Hahajima Seamount is not a simple serpentine seamount controlled by various tectonic movements, as previously believed, but a tectonic block.