Geoscience Reference
In-Depth Information
4
Discussion
4.1
Strike of Bending-Related Topographic Structures
There is a strong likelihood that the strike of bending-related topographic structures
should be parallel to the trench axis. Previous authors (Masson 1991 ; Kobayashi
et al. 1995, 1998 ; Billen et al 2007 ) suggested that pre-existing abyssal hill fabrics
are also one of the factors controlling the strikes of bending-related topographic
structures. Hilde ( 1983 ) questioned the occurrence of any consistent control of this
process. To examine the relationship between strikes of bending-related topo-
graphic structures and oceanic spreading fabrics, rose diagrams were made in refer-
ence to the strike of the strike of trench axes (Fig. 23 ). Most elongated topographic
structures are not straight. To determine the dominant orientation of topographic
structures, one topographic structure is divided into several straight segments which
length is from several to tens kilometers. After the division, strikes of segments
were separately calculated per each segment.
Table 3 shows a summary of bending-related topographic structures in ocean-
ward trench slopes and possible controlling factors. It is obvious that strikes of
bending-related topographic structures are controlled by strikes of trench axis or by
the oceanic spreading fabrics. This result supports the conclusions by Masson
( 1991 ) and Kobayashi et al. ( 1998 ). Masson ( 1991 ) showed that convergence direc-
tion and age of the subducting plate have no obvious influences for strikes of
bending-related topographic structures like elongated escarpments. The conver-
gence direction of the Pacific plate is almost the same in the study area, N65-67°W
(Gripp and Gordon 2002 ). The Pacific plate in the study area ranges from 130 to
140 Ma in age. The homogeneity of the convergent direction and the narrow age
range support the conclusion by Masson ( 1991 ).
In the oceanward slope of the western Kuril Trench, bending-related topographic
structures have a mode of strikes, N65-70°E, which is the same direction of mag-
netic anomaly lineations (Fig. 23a ). The topographic expression is a half graben or
ridge and is similar to that of abyssal hill fabrics. These observations support the
conclusion of Kobayashi et al. ( 1995, 1998 ) that the bending-related topographic
structures of the oceanward slope of the western Kuril Trench is formed by reacti-
vation of abyssal hill fabrics associated with subduction of the Pacific plate.
The graben structure is predominant in the oceanward slope of the Japan Trench
north 38°N (Table 3 , Fig. 23b, c ). Most escarpments have a strike parallel to the
trench axis, around north-south. Several escarpments south of 39°30¢N have a dif-
ferent strike, N20°W. Kobayashi et al. ( 1998 ) suggested that the predominant direc-
tion of escarpments is due to reactivation of non-transform offsets. The magnetic
anomaly lineations in this area elucidate a northeastward propagating spreading
ridge before chron M10 (Fig. 19 ). The seafloor in this area was formed near the
inner pseudofault related with southwestward propagating spreading ridge. The
elongated escarpments with an NW-SE strike seem to be developed in the transform
zone between propagating doomed ridges.
Search WWH ::




Custom Search