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 237

Cochonat et al. 2002 ; Mazzotti et al. 2002 ). It is characterized by multiple intraplate
shortenings and by recurrent earthquakes (Le Pichon et al. 1987 ). The Tokai Thrust
(Fig. 1 ) is one of the largest subhorizontal out-of-sequence thrusts within the
Nankai prism (Ashi et al. 2002 ; Mazzotti et al. 2002 ; Kawamura et al. 2009 ). In
addition, the eastern Nankai prism is characterized by a steeper slope than the
Kumano region which is just on the study area (Cochonat et al. 2002 ). In the eastern
Nankai prism, large-scale gravitational sliding is common, particularly between the
Kodaiba Fault and the Tokai Thrust (Cochonat et al. 2002 ; Kawamura et al. 2009 ,
2011 Chapter 10). The Tenryu Submarine Canyon cuts the geologic architecture
between the Kodaiba Fault and the Tokai Thrust (Figs. 1 and 2 ). Therefore, the side
walls of the Tenryu Canyon show the vertical and lateral changes in intraprism
geologic architecture (Kawamura et al. 2009 , 2011 Chapter 10).

3 Occurrence of Samples and Dark Bands

The samples described in this study were collected by the 6K from three sites in the
Tenryu Canyon during cruises YK05-08 Leg 2 and YK08-E04 (Fig. 1 ). Several
representative samples are described below.

3.1 6K#893 R-3

During cruise YK05-08 Leg 2, sample 6K#893 R-3 was collected from the Tokai
Thrust zone (Fig. 1 ). This sample includes dark bands in two zones: DB-1 and
DB-2 (Fig. 3a ). The sample is composed of sandstone (SS), mudstone-1 (MS-1),
and mudstone-2 (MS-2; Fig. 3b ). DB-1 is a boundary between SS and MS-1, and
DB-2 is between MS-1 and MS-2. MS-1 and MS-2 have similar lithofacies and
include similar fine elongate grains and radiolarian fossils (Fig. 3b ). However, the
attitudes of the DB-1 and DB-2 bedding planes are different.
DB-1 is about 10 mm wide, whereas DB-2 is only a few mm wide. DB-1 and
DB-2 are parallel to the SS and MS-2 bedding planes, respectively. Both bands are
oblique to the bedding plane of MS-1 (Fig. 3a, b ). The upper and lower surfaces of
DB-2 are sharply defined and straight, whereas DB-1 has an undulating upper sur-
face. Because DB-2 cuts DB-1, DB-1 was formed before DB-2.

3.2 6K#1056 R6-2, 6K#1060 R2-1 and R2-2

During YK08-E04, three samples including dark bands were collected: 6K# 1056
R6-2 (Fig. 3c ), 6K#1060 R2-1 (Fig. 3e ), and 6K#1060 R2-2 (Fig. 3g ). Dive
6K#1056 was on the eastern wall of the Tenryu Canyon, south of the Tokai Thrust
and west of the Yukie Ridge (Kawamura et al. 2008 ; Fig. 1 ). There is a broad