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strike are not so much as those with an NNW-SSE strike. One long escarpment with
an N-S strike is situated along 142°50¢E between 33°20¢N and 34°N (PF in Fig. 18 ).
There are rarely escarpments or ridges east of the PF escarpment. The bending-related
topographic structure near the trench axis south of 33°35¢N is a ridge about 50 km
long. A large escarpment with an NE-SW strike exists between 32°20¢N and 32°40¢N
southeast of the Mogi Seamount. The height of the escarpment is more than 500 m and
increases trenchward, which is larger than other escarpments with NE-SW strikes.
There is a curved trough with an NE-SW strike from 33°50¢N to 35°10¢N near
the trench triple junction. The trough is bounded by curved NE-SW trending escarp-
ments of an adjacent elongated ridge. The maximum depth of the trough is 6,000 m.
It narrows northeastward. The shape of the trough bears a close resemblance to that
of a tip of a propagating spreading ridge observed in Pacific-Antarctic Ridge around
64°S (Briais et al. 2002 ). The elongated ridge in the trough resembles a neo-volcanic
ridge in a propagating spreading ridge (e.g., Kleinrock and Hey 1989 ; Blais et al.
2002 ). We conclude that the trough is an extinct propagating spreading ridge and that
is not directly related to plate bending. Curved escarpments east of the trough are
interpreted as traces of pseudofaults associated with propagation of ridge. The south-
west extension of the trough is a fragmented ridge on the outer slope of the Izu-
Ogasawara Trough discovered by Kobayashi ( 1991 ) and named as the KAIREI
Ridge by Ogawa ( 1999 ). Basaltic rocks were recovered from the KAIREI Ridge
near the trench axis (Ogawa 1999 ). The continuity between the KAIREI Ridge and
the extinct of the propagating spreading ridge indicates that the KAIREI Ridge is a
part of the sidewall of the extinct propagating spreading ridge. The KAIREI Ridge
is wider than the extinct propagating spreading ridge. The KAIREI Ridge seems to
have grown up by off-ridge volcanism along the escarpment.
Magnetic Anomalies
Amplitudes of magnetic anomalies in the outer side of the trenches are generally
more than 300 nT, which are larger than those of other contemporary areas in the
western Pacific Ocean (Fig. 3 ). These have northward increases in amplitude from
300 to 500 nT.
Figure 19 illustrates magnetic anomaly lineations identified in this study. Most of
magnetic anomaly lineations younger than M10 are not changed from Nakanishi
et al. ( 1989 ). Results of the identification in this study is similar to that in previous
works (Nakanishi et al. 1989, 1999 ) except for the areas around the Joban Seamounts,
and between the Kashima Fracture Zone and the Izu-Ogasawara Trench.
Figure 20 shows correlations of magnetic anomalies among selected cruise
tracks and comparison with a synthetic profile calculated from the geomagnetic
reversal timescale of Gradstein et al. ( 2004 ). The skewness parameter is −230°,
which is the same as those for the Japanese lineation set (Nakanishi et al. 1989,
1999 ). Resemblance of shapes of magnetic anomaly profiles makes it easy to identify
magnetic anomaly lineations.
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