Topographic Development of the Area from the Eastern
Margin of the Sagami Trough to the Boso Triple Junction
Soh et al. ( 1990 ) suggested that tectonic instability of the Boso triple junction
caused rapid downwarping in the area of the junction in recent times, and that sub-
sidence of the drainage system within the Boso submarine canyon led to significant
changes to the meandering drainage system there. Seno et al. ( 1989 ) suggested that
the tectonic instability of the junction is associated with the 0.5 Ma jump of the
NAM plate boundary to the eastern margin of the Japan Sea. Ogawa et al. ( 1989 )
and Ogawa et al. ( 2008 ) built upon the work of Seno et al. ( 1989 ) and suggested
that a wide area of the NAM to the immediate northwest of the triple junction has
become very unstable due to differential downwarping in response to gravitational
instability of the area since the PHS began to move northwestward.
Lallemant et al. ( 1996 ) considered the active forearc sliver faults on the Miura
and Boso peninsulas and their extensions into the Pacific on the northeast side of
the Sagami trough to be a response to oblique subduction with a transform compo-
nent. They noted that the forearc sliver faults, as exemplified by the presence of the
Mineoka belt and a fault along Katsuura Canyon might represent right-lateral dis-
location, which may have caused three or more linear NW-SE displacements of the
bathymetry off the Boso Peninsula near the Katakai, Katsuura, and Boso canyons
(Figs. 2 and 3 ).
Katsuura Basin and Mogi Submarine Fan in the Bando
The Katsuura and Bando basins are two large, well-defined basins within the Boso
triple junction area (Fig. 3 ). The Katsuura Basin is a lens-shaped depression imme-
diately northwest of the triple junction and bordered by a north-south trending
ridge on the landward side of the Izu-Bonin trench (the Taito Spur of Iwabuchi et al.
1990 ; Fig. 4 ) (Ogawa et al. 1989 ; Seno et al. 1989 ). The Bando Basin (Bando deep-
sea basin of Iwabuchi et al. 1990 ) directly overlies the Boso triple junction and
hosts the Mogi Fan (Iwabuchi et al. 1990 ). The North and South Basins of Ogawa
et al. ( 1989 ) (the northern part of the former coincides with the Katsuura Basin)
form a north-south oriented rectangular basin that includes two terraces, one at
6 km water depth and the other at 7 km depth (Fig. 4 ).
The present axial channel within the Sagami trough cuts through the Katsuura
Basin, running through a gorge crossing the Taito Spur between the Katsuura Basin
and the trench floor. The channel then crosses the Mogi Fan and flows into the
Bando Basin (Ogawa et al. 1989 ) (Figs. 2 - 4 ). Thus, the Katsuura Basin developed
as a ponded basin containing a thickness of least 2 km of sediments as a result of
damming by the ridge. A small, 20 km-long accretionary prism can be recognized
from seismic profiles (Figs. 3 and 4 ).