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 184

the largest porosity (~65%). The porosity of the mudstone north of the boundary
fault (891R1, R2 outcrops) is relatively small (40-50%). The porosity becomes
maximum (54-57%) at the 891R4 and 891R6 outcrops in the rear part of the 5th
ridge. The porosity decreases southward from the 891R6 outcrop to the position of
the megasplay fault in the frontal part.
Uniaxial compressional strength shows good correlation with porosity: sedi-
ments with small porosity have high strength (Fig. 7c ). Microscopic observations
revealed that carbonate cementation (Fig. 7c inset microphotograph) is responsible
for a decrease of the porosity and an increase of strength of the sediments. The
distribution of such carbonate cementation was constrained between the megasplay
fault and the bifurcating faults.
Radiolarian microfossils were separated from mudstones/siltstones collected
from the megasplay fault zone (Anma et al. 2010 ). Reliable depositional ages
(Fig. 8 ) were obtained from sites 891R1 (1.98-1.65 Ma), 891R5 (4.3-1.98 Ma),
579R2 (1.03-0.43 Ma) and 890R5 (<0.43 Ma) from the north to the south. The
radiolarian biostratigraphy indicates that sediments with the oldest depositional age
(1.98-4.3 Ma) are distributed in the rear part of the 5th ridge (891R5 outcrop), and
the ages of deposition become younger in both directions (Fig. 7b ).
Sub-horizontal siltstones in the southernmost part of the transect (890R5 outcrop)
have the youngest depositional age (<0.43 Ma) and the highest porosity (~65%).
Considering the rate of plate motion (Fig. 1 ) and the young depositional age, we infer
that the sub-horizontal siltstone must have been deposited in a trench-slope basin.

Radiolarian Zone

Shionomisaki
Megasplay
FZ Samples

Radiolarian Event
Related to This Study

Time
(Ma)

Magnetic
Polarity

Epoch

Chron Middle-High Latitude

(NW Pacific)

Low Latitutde

0

B. invaginata
C. tuberosa
S. universus
Amphirhopalum
ypsilon
Anthocyrtidium
angulare

Late

B. aquilonaris
S. universus
(A. angelinum)

6K#890R5

n LO Stylatractus universus

Lamprocyrtis neoheteroporos
--> Lamprocyrtis nigriniae

C1

6K#579R2

1

LO Eucyrtidium matsuyamai

Lamprocyrtis heteroporos -->
Lamprocyrtis neoheteroporos

r

Eucyrtidium
matsuyamai

Theocorythium vetulum -->
Theocorythium trachelium trachelium
Phormostichoartus doliolum -->
Phormostichoartus corbula

6K# 891R1

n

2

FO Eucyrtidium matsuyamai
Haeckeloella inconstans -->
Haeckeloella sp. aff. inconstans

Pterocanium
prismatium

C2

r

FC Cycladophora davisiana

Cycladophora
sakaii

n

3

6K#891R5

C2A

Lychnodictyum
audex

LO Dictyophimus robustus

: Reliable depositional age
: Most likely depositional age
: Possible depositional age

(Dictyophimus
robustus)

r

4

P. doliolum

Fig. 8 Radiolarian biostratigraphy of the sediments of Shionomisaki submarine canyon
(Reproduced from Anma et al. 2010 ; ©Geological Society of Japan). Depositional age was deter-
mined from mudstones/siltstones at the sites 891R1, 891R5, 579R2 and 890R5. Magnetic polar-
ity: Gradstein et al. ( 2004 ), Middle to high latitude radiolarian zone: Motoyama and Maruyama
( 1998 ), Motoyama et al. ( 2004 ), and Kamikuri et al. ( 2004 ), Low latitude radiolarian zone:
Sanfilippo and Nigrini ( 1998 ). Radiolarian events used for estimation of the likely depositional
ages are listed in the right column . FO first occurrence, FC first consistent/common occurrence,
LO last occurrence, → transition