Structural Profile and Development of the Accretionary Complex in the Nankai Trough, Southwest Japan: Results of Submersible Studies - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 185

With exception of the 890R5 outcrop, lateral variations of porosity (Fig. 7b )
indicate that the mudstones of the oldest part at the 891R4-891R6 outcrops have the
highest porosity (54-57%). The porosity decreases southward toward the megasplay
fault in the frontal part. Thus, the porosity decrease has a negative correlation with
the age of deposition of the sediments: i.e. older sediments have higher porosity.
Uniaxial compressional strength tests display a similar trend: younger sediments
are more consolidated and have high strength (Fig. 7c ), except for the 890R5 out-
crop. These observations are opposite to the general trend of normal consolidation
curve due to simple burial.
Microscopic observation of thin sections revealed that carbonate cementation is
responsible for a decrease of porosity and increase of strength of the sediments
(inset of Fig. 7c ). The carbonate cements have high oxygen and relatively low carbon
isotopic ratios (Fig. 9 ; Anma et al. 2010 ). The isotopic value is close to that of

10

Trend of progressive
contact metamorphism

San Andreas
host limestones
& diagenetic
calcite veins

Pelagic
cements

Eastern
Nankai
live clams

0

Pelagic
oozes

Tenryu
canyon

San Andreas
calcite veins

Recent
biogenic

Eastern
Nankai
shells

- 10

Shimanto
fault-related
calcite veins

Cascadia
chimney

- 20

- 30

Cascadia
hydrate
ridge

Authigenic, diagenetic &
biogenic carbonates
Cascadia hydrate ridge
Eastern Nankai
Fault-related calcite veins
Contact metamorphism
Tenryu submarine canyon
Shionomisaki submarine
canyon (890R4: cement)

- 40

Eastern
Nankai
cements

- 50

Methane-derived
cements

- 60

- 20 0

- 10 10

d 18 O(‰ PDB)

Fig. 9 Carbon and oxygen isotopic ratios of cements from the Shionomisaki submarine canyon
( solid star ) (Reproduced from Anma et al. 2010 ; ©Geological Society of Japan) (Data source:
Nojima faults: Arai et al. ( 2003 ), Shimanto fault-related calcite vein: Yamaguchi et al. ( 2004 ) and
Yamaguchi et al. ( 2007 ), San Andreas fault: Pili et al. ( 2002 ), Eastern Nankai: Sakai et al. ( 1992 ),
Cascadia: Kulm and Suess ( 1990 ) and Teichert et al. ( 2005 ), contact metamorphism: Baumgartner
and Valley ( 2001 ), veins and cements from the Tenryu submarine canyon: Kawamura et al. ( 2009 ),
cements from the Shionomisaki submarine canyon: Hirano et al. ( 2006 ))