Erosional Subduction Zone in the Northern Japan Trench: Review of Submersible Dive Reports - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 40

Moreover, a large-scale submarine landslide has developed elsewhere on the
landward slope, the orientation of which is controlled by the lineaments of normal
faults in the subducting Pacific slab (Kobayashi 1991 ; Ogawa and Kobayashi 1994 ;
Sasaki 2004 ; Nakanishi et al. 2011 ).
The Nankai trough and the Japan trench margins differ primarily in that the former
is characterized by a sediment-supplied trench floor without a normal-faulted slab,
and the latter by a sediment-starved trench and a normal-faulted slab. In the Nankai
trough, the frontal thrust propagates into trench turbidites, whereas in the Japan
trench, the slope rocks slide into grabens in the trench (Taira and Ogawa 1991 ).
Thus, the Japan trench is a typical erosional subduction boundary characterized not
by sediment accretion but by erosion. The landward slope of the trench has prob-
ably been retreating since at least the Miocene (e.g., DSDP Legs 56, 57, and 87B;
von Huene et al. 1980 ; von Huene and Lallemand 1990 ; Lallemand and Le Pichon
1987 ; Clift and Vannucchi 2004 ).
However, the geology of the Japan trench has not been fully described except in
the areas of the French-Japanese KAIKO project, such as its junction with the Kuril
trench at its northern end (Erimo area; Cadet et al. 1987 ) and the Daiichi-Kashima
Seamount area at its southern end (Kobayashi et al. 1987, 1998 ).
Since the KAIKO Project, Hakuho-Maru cruises operated by the University of
Tokyo have completed mapping the rest of the Japan trench and have identified the
Miyako area of the northern trench to be suitable for characterization (Ogawa and
Kobayashi 1994 ; Kobayashi et al. 1998 ) (Fig. 1 ). These studies reported the topo-
graphical and geological similarities and differences between the oceanward and
landward toes and confirmed that erosion of material into the grabens in the subduct-
ing slab causes the landward advance of the trench, as suggested by Hilde ( 1983 ).
The dive results for the oceanward slope have been described by Ogawa et al.
( 1991, 1997 ) and Ogawa ( 1994 ), and those for the landward slope by Ogawa
et al. ( 1991, 1996 ), Iwabuchi et al. ( 1996 ), and Kuwano et al. ( 1997 ). However,
the significance of the trench topography with regard to its tectonic features has
not yet been discussed, although the erosional character of the trench may be
greatly affected by the geology of the oceanward slope.

2 Oceanward Slope Topography and Geology off Miyako

Ogawa and Kobayashi ( 1994 ) briefly described the topography of the toe of the
oceanward slope of the northern Japan trench off Miyako, and Kobayashi et al.
( 1998 ) discussed its tectonic significance. The slope is characterized by the sys-
tematic development of horst and graben structures (Fig. 2 ), each of which is
some kilometers in width with steep walls up to several 100 m in height. The
angle of the oceanward slope varies from 10° to 20° (average, 17°), as estimated
from a topographic map with a 10-m contour interval. However, direct observa-
tion from the submersible showed that the slope is composed of a series of small
steps (Fig. 3 ).