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 241

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

DB-2 in 6K#893 R-3 is developed between MS-1 and MS-2 (Fig. 7a ). The SEM
images show that clay-size minerals are also concentrated in DB-2 (Fig. 8d ). The align-
ment pattern of clay flakes in DB-2 indicates Riedel shear (Fig. 7c ). Furthermore,
foliation in MS-1 and MS-2 is dragged along the DB-2 boundaries. These results
clearly show that DB-2 is a shear band of right-lateral displacement at the band sur-
faces (Fig. 7c ). DB-1 has two or three shear planes within the dark band, but DB-2
represents a single shear plane. SEM images show that the constituent grains have
undergone ductile deformation, with the long axes oblique to the foliation of both
MS-1 and MS-2 (Fig. 8c, d ), suggesting Riedel shear.

4.3 6K #1056 R6-2

In thin sections, alignment of the constituent grains in this dark band is oblique to
that in the host rock, although clay flakes are arranged in a preferred orientation
subparallel to the bedding plane (Fig. 3d ). Under crossed nicols, clay flakes are
developed pervasively in the band. SEM images indicate that the constituent grains,
including smectite, along the boundary between the host rock and the dark band are
sheared. The above observation supports the interpretation that the dark band was
formed by dehydration or compression similar to that shown by DB-1 in 6K#893
R-3. CT images show that the density inside the dark band is higher than that of the
host rocks.

4.4 6K#1060 R2-1

In thin sections, the grain size inside the band of this sample is almost the same as
the host rock (Fig. 3f ). Various grains and radiolarian microfossils in the dark band
are undeformed. Clay flakes are strongly aligned parallel to the band boundaries.
SEM images show scaly flakes of clay similar to those in DB-1 in 6K#893 R-3.

4.5 6K#1060 R2-2

Thin section analysis under crossed nicols shows that pervasive clay flakes have
been newly formed in the dark band (Fig. 3h ). SEM images show that the constituent
grains along the band boundaries are sheared. Clay flakes (probably smectite) are
observed along the boundary.