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 35

Lineation M7 parallel to the ridge were identified in this area. These evidences
imply that the spreading ridge west of the Kashima Fracture Zone propagated in
mid-Cretaceous Quiet Period (84-125 Ma).

4.2 Topographic Expression of Bending-Related Structures

The two distinct types of development of bending-related topographic structures
were observed in the study area. The bending-related topographic structures
resulted from faulting by plate bending are parallel or subparallel to the trench axis
and their heights increase progressively trenchward. This type is typically observed
from 38°40¢N to 40°N in the northern part of the Japan Trench (Figs. 10 and 12 ).
Those originated in reactivation of oceanic spreading fabrics are oblique to the
trench axis and not always parallel to trench axis. Their height does not show
clearly gradual trenchward growth. This type is typically observed in the western
Kuril and southern Japan trenches. The bending-related topographic structures in
the Izu-Ogasawara Trench have a combined type.
The dominant topographic expression of the bending-related topographic struc-
tures resulted from newly formed faults by plate bending is dominantly a symmetric
graben (Table 3 ). That of the bending-related topographic structures originated in
reactivation of oceanic spreading fabrics is an asymmetric graben or a half graben.
In the northern Chile Trench, the similar result is obtained by Ranero et al. ( 2005 ).

5 Conclusions

The examination of the detailed bathymetrical maps and magnetic anomaly linea-
tions reveal the following evidences.

1. Most of bending-related topographic structures exist in the oceanward trench
slopes deeper than 5,600 m.
2. Bending-related topographic structures are developed parallel to the trench axis
or inherited seafloor spreading fabrics. Inherited seafloor spreading fabrics are
reactivated when they trend within ~30° from the trench axis.
3. The topographic expression of bending-related topographic structures is classi-
fied into two types according to whether new faults developed parallel to the
trench axis or inherited seafloor spreading fabrics reactivated.
4. Several curved Mesozoic magnetic anomaly lineations exist near the Japan Trench.
5. Discontinuity of magnetic anomaly lineations associated with a propagating
spreading ridge is documented around the Joban Seamounts.
6. A trough with elongated escarpments associated with the propagating ridge in
mid-Cretaceous Quiet Period was discovered near the trench-trench-trench triple
junction.