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 2

reactivation of the faults producing earthquakes up to several hundred kilometers
deep into the subduction zones. The percolation of water into the overlying astheno-
sphere reduces the melting temperature of the mantle rock and thus inherently
affects arc magmatism (Rüpke et al. 2002 ). Faults developed by bending the incom-
ing oceanic plate facilitate the infiltration of seawater into the oceanic plate (Ranero
et al. 2003 ). Seismic reflection data support this hypothesis because bending-related
faults cut at least 6 km into the uppermost mantle (Grevemeyer et al. 2005 ). The
strike of bending-related faults may give us the useful information about the inter-
mediate-depth seismicity because the seismicity may occur by reactivation of
bending-related faults (Ranero et al. 2005 ). These evidences indicate bending-
related faults play an important role in tectonic activity at subduction zones.
Bending-related faults of the oceanward slope are ubiquitous structures of oce-
anic plates incoming to trenches. In general, the faults are thought to be formed
parallel or subparallel to the bending axis of the incoming plate, namely the trench
axis, in most trenches. Oceanward slopes of several trenches have bending-related
structure with a strike different from the trench axes (Masson 1991 ; Kobayashi
et al. 1995, 1998 ; Ranero et al. 2003 ). In these areas, abyssal hill fabrics made
parallel to spreading centers by activity of normal faults were reactivated instead of
the creation of new faulting parallel to the trench axes.
The Mesozoic Pacific plate is subducting along the Kuril, Japan, Izu-Ogasawara,
and Mariana trenches (Nakanishi et al. 1992 ). Kobayashi et al. ( 1995, 1998 ) inves-
tigated the bending-related structures of the oceanward trench slope of the western
Kuril and northern Japan trenches using the bathymetric data obtained by the mul-
tibeam echo-sounder, SeaBeam. They concluded that the abyssal hill fabrics are
revalidated when abyssal hill fabrics trend within 30° of trench axes. This conclu-
sion is consistent with the previous work by Masson ( 1991 ). Billen et al. ( 2007 )
indicated the critical angle for the reactivation of abyssal hill fabrics is 25°, which
is smaller than that in the previous works. Renard et al. ( 1987 ), Kato ( 1991 ), and
Seta et al. ( 1991 ) described bending-related structures of the oceanward trench
slope only near the trench axis of the Izu-Ogasawara Trench. Bending-related struc-
tures from the southern Japan and Izu-Ogasawara trenches are not well known.
To examine controlling factors for strikes of bending-related structures, it is
indispensable to describe oceanic spreading fabrics and to identify magnetic anom-
aly lineations. The oceanic spreading fabrics consist of inherited abyssal hill fabrics
and other preexisting weak zones related to seafloor spreading process, which are
fracture zones, non-transform offsets, and so on. Magnetic anomaly lineations on
the Pacific plate incoming to trenches east of Japan were identified by Nakanishi
et al. ( 1989, 1999 ). The curved lineation was identified at the Japan Trench near
38°N (Nakanishi et al. 1989, 1991 ), but was not assigned an age. No lineations
were identified very near the trench axis of the Izu-Ogasawara Trench (Nakanishi
et al. 1989 ). In these areas, it is difficult to examine controlling factors for strikes
of bending-related structures.
In this article, detailed topography of the outer slopes of the trenches near the
Japan Islands will be examined based upon an abundance of multibeam bathymetric
data (Fig. 1 ) to analyze the characteristics of tectonic fabrics in the outer slopes of the