Dark Bands in the Submarine Nankai Accretionary Prism – Comparisons with Miocene–Pliocene Onshore Examples from Boso Peninsula - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 243

All of the dark bands represent shear bands or slip planes that formed after
sedimentation and possibly during accretion, although the amount of dislocation
varies for the different types of band. The physical properties of the dark bands
(e.g., permeability and degree of consolidation) reflect the deformation processes
and the magnitude of stress during their formation. In the following sections, we use
the above classification system to categorize dark bands in the Nankai samples.

5.2 DB-1 in 6K#893 R-3

MS-1 and MS-2 are characterized by many radiolarian microfossils and by calcite
cementation (Fig. 8 ), but SS hardly included calcite. Thus, the boundary between
MS-1 and SS appears to post-date calcite cementation.
DB-1, the boundary between SS and MS-1, is divided into three patterns based
on the differences in grain alignment and grain size (Fig. 7b ). Pattern 1 abuts SS
with some shear zones in which Riedel shears are developed. This internal structure
is similar to type 2 of the Miura-Boso accretionary prism (Fig. 4 ). Flakes of smec-
tite are concentrated along and within the shear zones.
Pattern 2 is found at the boundary between DB-1 and MS-1, but this boundary
is not sharp in thin section. There is flow deformation along the boundary, and
the constituent grains are not strongly crushed and deformed (Fig. 7d ). This
deformation mode indicates independent particulate flow without shearing of
grains (Borradaile 1981 ; Rawling and Goodwin 2002 ), implying that DB-1 was
formed under unlithified or semi-lithified conditions.
Michiguchi ( 2008 ) proposed that the slide plane of the submarine landslide cor-
responds to the type 2 dark band because the internal structures are similar to those
of type 2. Cochonat et al. ( 2002 ) reported that submarine sliding occurred in the
hanging wall of the Tokai Thrust at the dive site of 6K#893. We suggest that DB-1
conforms to this theory because submarine landslides are common along Tenryu
Canyon. The sliding direction is from N to S, as determined from the Riedel shear
pattern, indicating that the submarine landslide occurred on the landward slope
(Fig. 1 ). These results suggest that gravitational collapses occurred progressively in
the hanging wall of the thrust.

5.3 DB-2 in 6K#893 R-3

DB-2 is characterized by Riedel shears (Fig. 6c ). Foliations along DB-2 are
dragged, and DB-2 cuts DB-1, indicating that DB-2 was formed after DB-1.
Internal structures within DB-2 are very similar to those in the type 3 bands of the
southern Boso Peninsula (Fig. 6 ). SEM images show that the constituent grains of
DB-2 are strongly deformed and obliquely aligned to the strain alignment in the