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 244

host rock (Fig. 8 ). Based on the tectonostratigraphic relationships, we interpret that
DB-2 formed as a result of compressive stresses, such as those associated with
thrusting.

5.4 6K#1056 R6-2

The clay flakes in this dark band are dominantly in two preferred orientations.
Grains and clay flakes are oblique to the bedding place trace. Thus, in addition to
the SEM observation that the constituent grains along the boundary of the dark
band were sheared, it is suggested that this boundary is attributable to faulting.
Additionally, CT images show that the density of the dark band is higher than that
of the host rocks. These properties correspond to the type 1-2 and 3 bands from the
southern Boso Peninsula.
In the area of dive 6K #1056 there is a broad syncline, whose formation was
attributed by Kawamura et al. ( 2009 , 2011 ) to the combined effects of accretionary
processes and seamount subduction or collision. Thus, the dark band in 6K#1056
R6-2 may be related to the formation of this syncline and it may have been
subjected to similar flexural-slip movement to that of the type 1-2 band (Fig. 6 ).

5.5 6K#1060 R2-1

For this sample: (1) the grain size in the dark band is almost the same as in the host
rock, (2) the constituent grains have a preferred sub-horizontal orientation, (3) the
dark bands include many undeformed grains and radiolarians, and (4) clay flakes
are concentrated near the boundaries of the bands (Fig. 3f ). These dark bands result
from independent particulate flow. Therefore, the characteristics of the dark bands
in 6K#1060 R2-1 are similar to those of the type 1-1 band of the Boso Peninsula
(Fig. 6 ) and are interpreted to be of gravity origin.

5.6 6K#1060 R2-2

The dark band in 6K#1060 R2-2 is characterized by sheared grains and the align-
ment of clay flakes (Fig. 3h ). It is similar to the type 1-2 and 3 bands of the Boso
Peninsula.
Although this sample was collected from a location close to 6K#1060 R2-1 (for
which the bands were formed by gravitational processes), the dark band in 6K#1060
R2-2 was formed by tectonic processes. Thus, the sediments in the accretionary prism
in the region of this site have been affected by both gravitational and tectonic forces.