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 245

6 Summary and Conclusions

We investigated dark bands in fine-grained sedimentary rocks in the Nankai
accretionary prism and compared them to similar bands in rocks from the Boso
Peninsula. Dark bands found on the Boso Peninsula had previously been classified
into four types: 1-1 (independent particulate flow), 1-2 (flexural-slip faults),
2 (slide planes of submarine landslides), and 3 (thrust faults). We confirmed that
types 1-1, 1-2, 2, and 3 are also present in the Nankai accretionary prism. Before
this study, the dark bands from the Nankai accretionary prism were attributed only
to the tectonic effects of accretionary processes. However, the results presented
here clearly show that some of those dark bands were formed by gravitational
forces, such as submarine landslides (Fig. 9 ).
Several deformation structures in the Miura-Boso and Nankai accretionary
prisms were found to have similar permeabilities and degrees of consolidation, and
to have been subjected to stresses of similar magnitudes during their formation.
Our detailed study of the outcrops and textures in rocks from these two accre-
tionary prisms has improved our understanding of the geologic forces that have
formed the two regions.

Acknowledgments We thank Dr. Kiichiro Kawamura, chief scientist on board submersible vessel
Shinkai 6500 during cruises YK05-08 Leg 2 and YK08-E04, for his help and support. Sincere grati-
tude is also extended to Captains Sadao Ishida (YK05-08 Leg 2) and Shinya Ryono (YK08-E04)
and their crews. Reviews by Drs. Kohtaro Ujiie and Ryo Anma are greatly appreciated.

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