W. Soh

Morphology and development of a deep-sea meandering canyon (Boso Canyon) on an active plate margin, Sagami Trough, Japan

Abstract The 100 km long Boso Canyon is located on a plate boundary in the Sagami Trough, off Boso Peninsula (central Japan). Large volumes of sediment from the Izu Collision Zone and adjacent volcanoes have been transported to the Sagami Trough and the Izu-Ogasawara Trench through the Boso Canyon. Boso Canyon displays a channel morphology ranging from low- (straight) to high-sinuosity (meandering) in the midstream. An abandoned channel is preserved on the bench along the downstream high-sinuosity channel. Quantitative analyses of the canyon morphology demonstrate that the Boso Canyon has definite similarities with meandering subaerial rivers, especially with entrenched meanders. Three morphological parameters, sinuosity, channel gradients and canyon gradients, suggest that the canyon slope appears to be compensated by an increase in sinuosity to maintain an optimum channel profile. Three morphological stages for the canyon development can be recognized, pre-abandonment, channel abandonment and post-abandonment. Assuming that the tectonic framework of the region, in transtension, has not changed significantly since 0.5 Ma, these stages would be formed corresponding to sea-level changes. During the preabandonment stage (low sea-level phase), the Boso Canyon was straight and large amounts of detritus flowed down as turbidity currents to the Katsuura Basin slope and the Mogi Trench Fan on the Izu-Ogasawara Trench floor. The sediment supply rates were sufficient to mask the tectonic modification in channel morphology. Sediment supply rates to the Boso Canyon decreased abruptly as sea level rose, and the channel sinuosity increased to maintain optimum channel slope gradient. The building of the small confined Boso submarine fan probably began at the mouth of Boso Canyon which masked the basin topography of the Katsuura Basin. At highstand, the channel became least active, and differential tectonic subsidence led to enhanced channel entrenchment. The Boso Canyon is a good example suggesting that tectonic and sea-level changes are important in the origin and development of deep-sea channels on an active plate margin.

A TRENCH FAN IN THE IZU-OGASAWARA TRENCH ON THE BOSO TRENCH TRIPLE JUNCTION, JAPAN

Soh, W., Taira, A. and Tokuyama, H., 1988. A trench fan in the Izu Ogasawara Trench on the Boso Trench triple junction, Japan. Mar. Geol., 82: 235-249. The Mogi Trench Fan, 18 km in diameter, is located in the 9.1 km deep Izu Ogasawara Trench. Analysis of the morphology and internal structure of the Mogi Fan was based on 3.5 kHz records, seismic reflection profiles and Seabeam bathymetry. The Mogi Fan was fed from a point source, and displays an even-shaped partial cone morphology which can be divided into upper, middle and lower fans. The upper fan is defined as an apparent topographic mound, having a large-scale single channel with well-defined levees that appear to be composed mainly of coarse-grained turbidites. The middle fan is characterized by divergent channels and lobes. The lower fan is a smooth mound with no channel features. It is postulated that the lower fan is constructed chiefly by turbidity currents that reflected back from the higher outer slope. Seismic reflection records across the fan show deformation resulting from plate subduction in the upper fan; this deformation can be traced laterally to the lower bulge of the inner slope neighbouring the Mogi Fan. Several characteristics of the morphology of the Mogi Fan indicate that it is presently inactive. The fan development is interpreted to have formed and prograded in the trench during a period of lowering sea level. During rising sea level, the sediment supply to the trench abruptly decreased, and the tectonic deformation of the fan morphology was correspondingly enhanced.