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numerous calcite veins, and cold seeps inhabited by chemosynthetic biocommunities
(Moore et al.
2007
; Kawamura et al.
2008, 2009
). Cochonat et al. (
2002
) and Kawamura
et al. (
2009
) suggested that the Tokai margin of the Eurasian plate was formed by slope
failure and mass movements, followed by tectonic accumulation to form the accretionary
prism. Thus, it is thought that both tectonic forces and gravitational instability have
influenced this region.
During cruises YK05-08 Leg 2 and YK08-E04 of the manned submersible vessel
Shinkai 6500
(hereafter 6K) of JAMSTEC (Japan Agency for Marine Science and
Technology), rock specimens were collected from the accretionary prism area
along the Tenryu Submarine Canyon, which dissects the eastern Nankai Trough
(Figs.
1
and
2
). The rock specimens contain dark planar bands, 1 to 10 mm in
width, that are oblique or subparallel to bedding planes (Fig.
3
). Similar structures
have been reported from cores obtained from accretionary prisms during cruises of
deepsea drilling, where they were called shear bands or deformation bands. In the
Nankai accretionary prism, Lundberg and Moore (
1986
) described these bands as
kink bands. Maltman (
1993
) reported that they may reflect an early response to
bulk prism shortening. Ujiie et al. (
2004
) interpreted them as compactive shear
bands developed under plate convergence stresses.
In the unmetamorphosed Miocene-Pliocene accretionary prism of the southern
Boso Peninsula, central Japan (the Miura-Boso accretionary prism), dark bands
have been interpreted as healed faults (Ishimaru and Miyata
1991
) or as layer-
parallel faults formed during accretion (Hanamura and Ogawa
1993
; Yamamoto
et al.
2000
). Michiguchi and Ogawa (
2011
) classified various dark bands developed
in the Miura-Boso accretionary prism into four major types on the basis of distribu-
tion, crosscutting relations, and internal texture. As is discussed in more detail later,
they suggested that the bands were formed at various stages and in different settings
(Figs.
4
and
5
); some are of pre-accretion gravitational origin and some formed
during accretion (Fig.
6
). Such dark bands have been reported only at plate conver-
gent margins and are thought to be tectonic structures, but the mechanism by which
the dark bands are formed remains poorly understood.
During the dive survey of 6K, many rock samples containing dark bands of various
origins were collected. These structures are common, and it is important to under-
stand the roles they play in the deformational development of the accretionary
prism. Aiming to understand their origin, we described the various dark bands from
the Nankai prism in detail and compared them with similar onshore structures
described by Michiguchi and Ogawa (
2011
). Based on our classification, we con-
sidered possible formation mechanisms and their implications for each dark band
sampled from the eastern Nankai accretionary prism.
2
Geological Setting of the Study Area
The Tenryu Submarine Canyon is in the eastern part of the Nankai accretionary
prism off central Japan (Fig.
1
). The eastern part of the Nankai prism is strongly
affected by the Izu-Bonin and Honshu arc-arc collision (Le Pichon et al.
1987
;
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