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called the Sanriku Escarpment (Fig.
4
), forms the boundary between the lowermost
toe and the upper trench wall. This escarpment has been interpreted as the scarp
of a large thrust fault (e.g., von Huene and Culotta,
1989
). Taira and Ogawa
(
1991
), however, showed that the upper part of the escarpment is characterized by
large-scale submarine sliding caused by slope instability, which is later to be
known further controlled by thrust faulting (Iwabuchi et al.,
1996
; Sasaki,
2004
)
as shown below.
Following Iwabuchi et al. (
1996
) and Ogawa et al. (
1996
), Sasaki (
2004
) sum-
marized the mechanisms generating the topographical features of the Japan trench
landward slope, attributing them to erosional subduction zone tectonics. According
to Sasaki (
2004
), the occurrence of many thrust faults within both the upper and
lower slope has resulted in the formation of a steep, unstable slope prone to land-
sliding. The rocks from the slope are fairly old. The traces of most of these thrust
faults along the slope are characterized by abundant chemosynthetic biocommuni-
ties, dominated by
Calyptogena
, supported by methane-bearing fluids (Ogawa
et al.
1996
).
The submersible
SHINKAI 6500
(JAMSTEC) made several dives at the foot
of the Sanriku Escarpment and further upslope (Fig.
4
). The assemblages of
fossil diatoms in rock and sediment samples collected by the submersible from
the landward slope, identified by Itaru Koizumi, were of middle Miocene to
Fig. 4
Multi-narrow-beam swath map of the area around the Sanriku escarpment, off Miyako at
40°N (contour interval 20 m) (Adapted from Ogawa et al. (
1996
))
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