Bending-Related Topographic Structures of the Subducting Plate in the Northwestern Pacific Ocean - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 13

Izu-Ogasawara Trench Japan Trench Kuril Trench

1200 1000 800 600 400 200 0

5500 5500

K1

6000 6000

ER

6500 6500

7000 7000

7500 7500

8000 8000

8500 8500

MG

9000 9000

9500 9500

1200 1000 800 600 400 200 0

Distance (km)

Fig. 5 Depth along the trench axis. ER Erimo Seamount, K1 Daiichi-Kashima Seamount, MG
Mogi Seamount

3.3 Bending-Related Structures of the Oceanward Trench Slopes

The bathymetric map (Fig. 4 ) displays pervasive elongated structures, escarpments
and ridges, that dissect the oceanward trench slopes (Fig. 6 ). Increased multibeam
bathymetric data made it possible to expose more elongated structures than previ-
ous works. Elongate escarpments consist of oceanward- and trenchward-dipping
escarpments. Many pairs of oceanward- and trenchward-dipping escarpments form
horst and graben structures. Identification of trenchward-dipping escarpments is
less certain than that of oceanward-dipping escarpments because of the difficulty in
separating these slopes from those made by plate bending and trenchward-dipping
escarpments. Moreover, several segments of trenchward dipping escarpments are
collapsed owing to subduction of the Pacific plate. Thus, we identified bending-
related heights with crests of less than several kilometers width as ridges.
Elongated escarpments and ridges of the oceanward slope of the Japan and Izu-
Ogasawara trenches are almost restricted to areas deeper than 5,600 m (Fig. 6 )
except for around 38°N, where the strike of the Japan Trench changes. The 5,600 m
contours are 70 km away from trench axes. The crests of the outer swells along the
trenches are situated far away from the trench axes, implying that elongated topo-
graphic structures are not formed at the crest of the outer swell.

3.3.1 Kuril Trench

The strike of the Kuril Trench is N65°E. The age of subducting Pacific plate is
about 130 Ma (Nakanishi et al. 1989 and see the Sect. 3.4 ). The bathymetric map
(Fig. 7 ) shows that the distinctive bending-related topographic structures in the
oceanward slope are elongated escarpments and ridges. The strike of the bending-
related topographic structures range N60-80°E. The length of many escarpments