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Fracture Zone, east of the Izu-Ogasawara Trench, is identified by Nakanishi et al.
( 1989 ) from magnetic anomaly lineations and examination of single-beam bathymet-
ric data. The Kashima Fracture Zone is characterized by complex trough morphology
(Nakanishi 1993 ). The 30 km-wide trough is bounded by ridges. The expression of
the fracture zone is diminished by closing seamounts near 34°N. The western side of
the fracture zone is shallower than the eastern side. Vertical relief of the seafloor
ranges from 400 to 1,000 m. There is no systematic change of the vertical relief with
distance from the axis of the Izu-Ogasawara Trench, implying that the Kashima
Fracture Zones has not reactivated by recent subduction-related tectonism of the Izu-
Ogasawara Trench.
The bathymetric features of the Joban Seamounts, just east of the Japan Trench,
are described in Kobayashi ( 1991, 1993 ). The trend of the seamounts is not aligned
with any supposed hotspot tracks on the Pacific plate. Daiichi-Kashima and Erimo
seamounts are subducting at the Japan and Kuril trenches, respectively. The sea-
mounts were investigated by Nautile dives under the KAIKO project (Cadet et al.
1987 ). Both seamounts are dissected by normal faults. Daiichi-Kashima Seamount is
cut by a large normal fault into two blocks, the western half of which is nearly
vertically offset from the eastern block by ~1,600 m (Kobayashi et al. 1987 ). Mogi
Seamount, also cut by normal faults, is subducting at the Izu-Ogasawara Trench at
Bottom topography of the northwestern Pacific basin beyond these outer swells
is generally very smooth except for seamounts and knolls. Numerous knolls are
situated around 38°N. Hirano et al. ( 2008 ) indicated that the knolls are formed by
recent volcanism, not related with any plate boundaries.
Abyssal hill fabrics exist on the seafloor east of the Kashima Fracture Zone and
Joban Seamounts. Between 38°N and 41°N, there are abyssal hill fabrics east of
145°E. However, the fabrics are not resolvable on the outer swell along the Kuril
Trench. Most of the abyssal hill fabrics trend N70°E, parallel to that of magnetic
anomaly lineations, with heights of less than 100 m.
Topographic Expression of the Trenches
The trench axes were determined where direction of trench slopes changes from
oceanward to landward. The depth of the trench axis in the Kuril Trench shallows
westward from 7,300 to 7,100 m (Fig. 5 ). The Japan Trench deepens southward
from 7,500 to 9,000 m except for the portion of the collision of the Daiichi-Kashima
Seamount around 37°N. The trench axis in the Izu-Ogasawara Trench deepens
southward from 9,000 to 9,500 m. The seafloor of the trench axis of the western
Kuril Trench is smooth at a depth of 7,200 m. The smoothness is due to sediment
flow from the Kushiro Submarine Canyon. The seafloor of the trench axis of the
northern Izu-Ogasawara Trench around the triple junction between Izu-Ogasawara
Trench and Sagami Trough is smooth at a depth of 9,250 m, owing to sediment flow
in the Sagami Trough and other deep-sea channels (Ogawa et al. 1989 ).
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