Structural architecture and active deformation of the Nankai

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Geological Society of America Bulletin
Structural architecture and active deformation of the Nankai Accretionary
Prism, Japan: Submersible survey results from the Tenryu Submarine Canyon
Kiichiro Kawamura, Yujiro Ogawa, Ryo Anma, Shunji Yokoyama, Shunsuke Kawakami, Yildrim Dilek,
Gregory F. Moore, Satoshi Hirano, Asuka Yamaguchi, Tomoyuki Sasaki, YK05-08 Leg 2 and
YK06-02 Shipboard Scientific Parties
Geological Society of America Bulletin published online 28 August 2009;
doi: 10.1130/B26219.1
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Copyright © 2009 Geological Society of America
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PAP
Structural architecture and active deformation of the
Nankai Accretionary Prism, Japan: Submersible survey
results from the Tenryu Submarine Canyon
Kiichiro Kawamura1,†, Yujiro Ogawa2,§, Ryo Anma2, Shunji Yokoyama3, Shunsuke Kawakami4,#, Yildirim Dilek5,
Gregory F. Moore6, Satoshi Hirano7,*, Asuka Yamaguchi8, Tomoyuki Sasaki8, and YK05-08 Leg 29 and
YK06-02 Shipboard Scientific Parties10
1
Fukada Geological Institute, 2-13-12 Honkomagome, Bunkyo, Tokyo 113-0021, Japan
Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8572, Japan
3
Kochi University, 2-5-1 Akebonocho, Kochi 780-8520, Japan
4
Geological Survey of Japan, Tsukuba Central 7, 1-1, Higashi 1-Chome, Tsukuba 305-8567, Japan
5
Miami University, 116 Shideler Hall, Oxford, Ohio 45056, USA
6
School of Ocean and Earth Science and Technology (SOEST), University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA
7
IFREE4, Japan Marine-Earth Science and Technology Center, 2-15, Natsushimacho, Yokosuka, 237-0061, Japan
8
University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
9
Ken-ichiro Hayashi (University of Tsukuba), Yuji Yagi (University of Tsukuba), Tomohiro Toki (Ryukyu University), Ryota Endo
(Inpex Corporation), Teppei Ota (University of Tsukuba), and Satoru Sano (Nippon Marine Enterprises)
10
Akira Nakamura (University of Tsukuba), Hidetoshi Hara (Geological Survey of Japan), Hiroyuki Mizumoto (Agency for Fishery
Research Center), Driss Elouai (Kyoto University), Yoko Michiguchi (University of Tsukuba), Hisatoshi Sato (University of Tsukuba),
Ai Togami (University of Tsukuba), Satoru Muraoka (University of Tsukuba), Satoshi Okada (Nippon Marine Enterprises), and
Hideki Mukoyoshi (Marine Works Japan) (onshore)
2
ABSTRACT
Two research cruises that deployed submersible surveys were undertaken along the
Tenryu Submarine Canyon to directly observe the structural architecture of the eastern Nankai Accretionary Prism off the coast
of southern Japan. The surveys have demonstrated that the accreted sediments are
strongly deformed turbidite sequences that
occur in repeated thrust-anticline structures. From the leading edge of the prism
near the trench toward the arc, the following deformation zones have been identified within the prism: Frontal Thrust zone,
Prism Toe zone, Imbricate Thrust zone, and
Tokai Thrust zone (or out-of-sequence thrust
or OOST zone). The Frontal Thrust zone is
†
E-mail: [email protected]
E-mail: [email protected]
#
Present address: Earth Appraisal Company, F&F
Royal Building 4F, 2-4-6 Kanda-awajicho, Chiyoda,
Tokyo, 101-0063, Japan
*Present address: Marine Works Japan, c/o Kochi
Core Center, B200, Monobe, Nangoku 783-8502,
Kochi, Japan
§
characterized by debris deposits within the
hanging wall that have an age of 0–0.43 Ma,
as determined from radiolarian biostratigraphy. The Prism Toe zone is characterized
by unconsolidated turbidite sequences that
are 1.98–3.4 Ma; these sequences are cut
by normal and thrust faults. The Imbricate
Thrust zone includes consolidated muddy
layers and unconsolidated sandy layers that
contain numerous fracture cleavages. The
OOST zone consists of highly deformed consolidated sediments, ranging in age from 0.18
to 1.03 Ma. From the Prism Toe zone to the
Imbricate Thrust zone, the uniaxial compressive strength increases gradually from 0.5–
3.0 to 1.0–6.0 MPa, while the anisotropy of
magnetic susceptibility changes from oblate
to prolate shapes, and porosity decreases
from 40%–50% to 30%–50%. These data
indicate that the eastern Nankai Accretionary Prism appears to have been deformed
toward the Imbricate Thrust zone just south
of the OOST. Stable isotope analyses of calcite veins and calcite cement of the sandstone
samples from the Tokai Thrust zone have
shown that fluid temperatures for calcite
precipitation were 24–63 °C in the OOST
zone. The occurrence of highly deformed and
consolidated rocks within the Nankai Accretionary Prism likely resulted from tectonic
transportation of deeply buried rocks along
major out-of-sequence thrust faults, such
as the Tokai OOST. We infer therefore that
out-of-sequence thrust faults play a major
role in transporting deeply buried, deformed
rocks in accretionary prisms to the shallower
depths and even to the seafloor during ongoing subduction.
INTRODUCTION
The Nankai Accretionary Prism, situated off
southwest Japan, is one of the most intensively
studied modern accretionary prisms among all
of the world’s convergent margins (Le Pichon
et al., 1987a, 1987b, 1992, 1996; Taira et al.,
1992; Moore et al., 2001). Seismic data suggest
that the Nankai Accretionary Prism is composed
of a stack of thrust sheets of turbidites deposited within the Nankai Trough and neighboring
Shikoku Basin, and that off-scraping has been a
significant process in its formation (Aoki et al.,
1982; Kato et al., 1983; Leggett et al., 1985;
Moore et al., 1990, 2001; Shipboard Scientific
GSA Bulletin; November/December 2009; v. 121; no. 11/12; p. 1629–1646; doi: 10.1130/B26219.1; 17 figures; 4 tables.
For permission to copy, contact [email protected]
© 2009 Geological Society of America
1629
Kawamura et al.
Party, 1991; Ashi and Taira, 1992; Taira et al.,
1992; Kuramoto et al., 2000; Moore et al., 2005).
Two-dimensional and three-dimensional (3-D)
images of the Nankai Prism show that macrostructures range in scale from 100 m to 1 km.
Studies of the Ocean Drilling Program (ODP)
core samples have provided information about
varying microstructures of ~1 mm to ~1 m scale
(Maltman et al., 1993; Morgan and Karig, 1993;
Morgan, 1997; Moore et al., 2001; Ujiie et al.,
2003, 2004; Morgan and Ask, 2004). Detailed
3-D observation of the prism itself is much
needed to understand the ongoing accretionary processes. Direct observations by deep-sea
submersible of the geologic structures exposed
along the walls of submarine canyons are among
the best ways to achieve this goal.
A large amount of information on the mesoscopic geology of the Nankai Accretionary
Prism was collected during three international
(France-Japan) projects: the KAIKO, KAIKONankai, and KAIKO-Tokai projects. Submersible dives executed during these projects
were conducted along slopes rather than canyons, documenting various aspects of the surface geology, geophysics, and geochemistry
(Le Pichon et al., 1987a, 1987b, 1992, 1996;
Kobayashi, 2002). Except for these KAIKO
projects, only a few submersible dives were organized in a systematic way (Kawamura et al.,
1999; Anma et al., 2002).
Kawamura et al. (1999) were the first to report the structural architecture of the Nankai
Accretionary Prism, obtained by submersible
observations along a submarine canyon. These
researchers conducted three dives by the remotely operated vehicle (ROV) KAIKO (an
unmanned ROV, herein referred to as 10K) to
a depth of 4 km, and documented the deformational structures at the mouth of the Tenryu
Submarine Canyon (Fig. 1). Subsequently,
Anma et al. (2002) reported prism structures
from the depths of 4 km along the Shionomisaki
Submarine Canyon (Fig. 1) using the manned
submersible Shinkai 6500 (herein referred to as
6K) and successfully obtained radiolarian ages
of the accretionary prism rocks.
This study is the first attempt at direct observation of accretionary prism exposures along
submarine canyon walls and at sampling the full
extent of a transect by deep-sea submersible,
and complements the previous KAIKO and
KAIKO-Nankai projects (Le Pichon et al.,
1987a, 1987b; Kobayashi, 2002). The dives
were carried out along the Tenryu Submarine
Canyon in the northeastern part of the Nankai
Trough. During the course of three dives using
10K and nine dives using 6K, vertical and lateral changes in the internal architecture of the
accretionary prism were observed on the side
1630
walls of part (off Omaezaki Cape) of the Tenryu
Submarine Canyon. Canyon walls provide the
best exposures along both vertical and lateral
profiles, mostly resulting from slope collapse
and erosion associated with strong periodic bottom currents along the canyon floor.
In this paper, we describe the mesoscopic
structural architecture of the accretionary prism
as well as microscopic deformational structures
of the rock samples recovered during the dives.
We also document the lateral variations in age,
physical, magnetic, and mechanical properties
of the prism rocks, and the zonal arrangement
of the rocks and their structures. The raw data
of this study are available on the website of the
Japan Agency for Marine Science and Technology (JAMSTEC; http://www.jamstec.go.jp/)
(YK05-08 Leg 2, YK06-02; Shipboard Scientific Party, 2005, 2006).
TOPOGRAPHIC FEATURES
OF THE EASTERN NANKAI
ACCRETIONARY PRISM
A bathymetric survey of the study area was
conducted using the SeaBeam 2012 System
aboard research vessel (R/V) Yokosuka (the
mother ship of the 6K). The resulting map
was drawn at a scale of 1:25,000 with 10-m
contour intervals (Fig. 2). We have subdivided
the eastern Nankai Trough into four structural
zones that show distinct topographic expressions (Fig. 2). These zones include: the Frontal
Thrust, the Prism Toe, the Imbricate Thrust, and
the Tokai Thrust zones, and the boundaries between them are generally thrust faults dipping
toward the island arc.
We show the dive locations, porosity values,
and cold seep locations on this map (Fig. 2).
The bathymetry of the study area is characterized by nearly NE-SW–oriented, asymmetric
ridges and valleys parallel to the boundary
thrusts, and by steeper seaward and gentler
landward slopes (Fig. 2). We interpret these
topographic features as thrust-cored anticlines.
The same features were identified in seismic
profiles obtained offshore from Shikoku (Ashi
and Taira, 1992). The thrust faults appear to be
located at the base of the seaward slopes of the
asymmetrical ridges (Fig. 2).
The close spatial correlation between the
topography and the geologic structures, thought
to be formed by ongoing accretionary processes,
suggests that the thrust anticlines of several
kilometers scale are actively growing structures. This observation is an important clue
for understanding the tectonic evolution of the
Nankai Accretionary Prism. Three major faults,
the Kodaiba Fault, Tokai Thrust, and Tenryu
Frontal Thrust (Figs. 1 and 2), are all active as
determined by direct observations during submersible surveys (Ashi et al., 2002). Among
these, the Tokai Thrust has an extremely low
dip and appears to be an out-of-sequence thrust
(OOST) (Ashi et al., 2002), whereas the other
two are gently inclined toward arc (Fig. 3;
Le Pichon et al., 1992).
Structures generally trend north-south in the
southwestern part of the study area but east-west
in the western part (Fig. 2). This abrupt change
of structural trend is a result of the impingement
of the northwestward advancing Zenisu Ridge,
part of a seamount chain on the western flank of
the Izu-Bonin-Mariana (IBM) island arc system
(Fig. 1). These structures also provide evidence
for the earlier subduction of the paleo-Zenisu
Ridge at the Nankai Trough (Le Pichon et al.,
1996), as discussed in a later section.
STRUCTURAL ARCHITECTURE OF
THE EASTERN NANKAI PRISM
Three 10K dives (10K#42, #43, and #52)
were conducted at a location where the Frontal Thrust is curved around a topographic
ridge (Fig. 2). Submarine exposures were directly observed during cruises KR97-05 and
KR97-06 (Fig. 4). The 6K dives were conducted
along the Tenryu Canyon as follows: four dives
to the Tenryu Canyon mouth on the prism slope
of the Nankai Trough (6K#755 during cruise
YK03-03, dive scientist: K. Kawamura;
6K#888 and #894 during cruise YK05-08, dive
scientists: S. Kawakami and Y. Ogawa, respectively; and 6K#939 during cruise YK06-02,
dive scientist: K. Kawamura) (Figs. 4–7). Three
other dives were conducted along the Imbricate
Thrust zone (6K#885, #886, and #887 during
cruise YK05-08, dive scientists: K. Kawamura,
S. Yokoyama, and Y. Dilek, respectively) (Figs.
8 and 9), and two more in the Tokai Thrust
(OOST) zone (6K#892 and #893 during cruise
YK05-08, dive scientists: K. Kawamura and
R. Anma, respectively) (Table 1) (Figs. 10
and 11). The internal architecture of the four
structural zones within the Nankai Accretionary
Prism is described below from the most oceanward Frontal Thrust zone to the innermost Tokai
Thrust zone (Fig. 2).
Frontal Thrust Zone
The trenchward, southern margin of this zone
is defined by a frontal thrust at the foot of the
landward slope, but the northern boundary is unclear. Mesoscopically, this zone is characterized
by debris and/or slump deposits, cold seepages,
and strongly deformed turbidite layers. Cold
seepages are present within ~100 m upslope
of the cliff at the depth of 3750 m (Fig. 4).
Geological Society of America Bulletin, November/December 2009
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Figure 1. Index maps of the eastern Nankai Accretionary Prism off SW Japan and the location of the study area. Abbreviations: IODP—
Integrated Ocean Drilling Program; OOST— out-of-sequence thrust; SEIZE—Nankai Seismogenic Zone Experiments.
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1631
Kawamura et al.
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Porosity
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Figure 2. Structural elements of the study area. E-W ridges comprise shallow landward slopes and steep seaward slopes. Narrow topographic depressions occur commonly in front of these ridges, which formed by the thrust-related folding of strata. A thrust-anticline model,
as shown in the inset, explains the asymmetrical slope gradient and related topography.
1632
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Figure 3. Interpretation of a seismic profile from the eastern Nankai Accretionary Prism
(after Le Pichon et al., 1992), showing major faults and the “to-be-subducted” Zenisu Ridge.
The location of the survey line is shown in Figure 1.
Discontinuous, steeply dipping turbidite layers (Fig. 5) are exposed on the flanks of small
mounds and on small ridges several meters high
(Fig. 4). Both ductile and brittle shear structures (Fig. 5) together with numerous deformation bands containing aligned clay minerals
(Fig. 12A) are observed in this area, suggesting
that the layers were repeatedly deformed and
disrupted by thrust faulting. These structures
are similar to those documented from the eastern margin of the Tenryu Submarine Canyon,
during dives to the foot of the landward slope
conducted as a part of the joint Japan-French
KAIKO project (Chamot-Rooke et al., 1992).
The locations of active faults are marked by
chemosynthetic biocommunities of bacterial
mats and Calyptogena and/or clams that occur
within small topographic depressions in the
Frontal Thrust zone. Such biocommunities are
known to be confined to cold-seepage areas located along hidden active faults or open cracks
beneath surface sediments within the slopes of
the Nankai Trough (Le Pichon et al., 1987a,
1987b, 1992) as well as in the Japan Trench
(Ogawa et al., 1996).
Prism Toe Zone
The Frontal Thrust zone is bounded to the north
(landward) by the Prism Toe zone (Fig. 6). It is
~3 km in width and comprises subhorizontal or
gently dipping (Fig. 6) and highly microfractured
(Figs. 7A and 7B) turbidite layers. These layers
vary in thickness and lithology from <100-cmthick, unconsolidated and sand-dominated layers
to <30-cm-thick, semiconsolidated, and muddominated (Fig. 6). The sequence is repeated
due to duplication associated with layer-parallel
thrusts (Fig. 7C). Liquefaction-related textures
are common, indicating ductile deformation
(Fig. 12C) and folding (Fig. 7D). In thinner parts
of these layers, the mudstone is cut by clay-filled
veins (Fig. 12B), which have been thought to
represent dewatering in the prism toe (Maltman
et al., 1993; Ujiie et al., 2004). In addition to
thrusts, normal faults are also observed here, suggesting the instability of the steep seaward slope.
These normal faults are associated with visible drag structures in unconsolidated sediments
(Fig. 7E) and are interpreted to have originated
from submarine landslides.
Imbricate Thrust Zone
The Imbricate Thrust zone is characterized
by weakly folded and inclined turbidite beds of
fractured mudstone and deformed sand layers
(Figs. 8 and 9). Mudstone layers vary in thickness from several centimeters to several meters
and contain thin lamina, burrows, and minor
thrust faults. Two or possibly three cleavage
systems are observed near fold axes (Fig. 9A).
In contrast, the sand layers contain white
mineral veins (probably carbonate) oriented at
low angles to layering (Fig. 9B). Deformational
textures within the mudstone and sand layers
were probably formed by off-scraping accompanied by folding (due to lateral compression) at
the time of accretion of the prism toe. The compression might have led to formation of fracture
cleavages within mudstone layers. Folding of
turbidite sequences comprising semiconsolidated mudstone layers and unconsolidated sand
layers caused layer-parallel slip, mostly within
sand layers. Mineral veins have been developed
obliquely cutting the layers.
The Tokai Thrust is one of the largest, subhorizontal, out-of-sequence thrusts within the
Nankai Prism (Ashi et al., 2002). It is ~200 m
wide and is characterized by abundant chemosynthetic biocommunities (bacterial mats,
tubeworms, and Calyptogena and/or other
clams) and highly fractured sedimentary rocks
(Fig. 10). Strata within the thrust zone commonly dip steeply, and some of the layers form
fault gouge zones (Fig. 11A) composed of subangular sedimentary rock fragments embedded
in a clay matrix. Numerous calcite veins are
oriented oblique to bedding planes (Fig. 11A).
The occurrence of extremely hard rock composed of numerous calcite veins (Figs. 12E,
12G, and 12H) indicates that calcite was actively precipitated within the Tokai Thrust
zone. Samples of these rocks were collected
from the central part of the Tokai Thrust zone
during dive 6K#893 (Table 1).
Other conspicuous rocks in this zone are sandstones collected from the middle slope of the
sidewall of the Tenryu Submarine Canyon (Figs.
11B and 12D) and highly deformed low-grade
metamorphic rocks collected from the base of
the Tokai Thrust zone during dive 6K#892 (Figs.
11C and 12F) (Table 1). The sandstones have
many planar black fault zones (Fig. 12D), which
crosscut each other due to repeated faulting.
Analyses of these highly deformed rocks suggest that the rocks were deformed at depths more
than several km within the prism, and that they
were subsequently transported upward to the
seafloor by movement on the Tokai Thrust zone
(Kawamura et al., 2006; Ogawa et al., 2006).
PHYSICAL, MECHANICAL,
AND MAGNETIC PROPERTIES
OF PRISM ROCKS
We successfully recovered ~120 rock samples from 33 sampling sites along the Tenryu
Submarine Canyon (Fig. 2; Table 1). Physical,
mechanical, and magnetic properties of these
samples are described below.
Porosity
We calculated the porosity of the rocks using their water content, grain density, and wet
bulk density measured via techniques described
by The Japanese Geotechnical Society (2000).
Measurements of grain densities were done with
different tools; for samples 6K#885–#894, we
used a pentapycnometer produced by Quantachrome Company, and for samples 6K#755 and
6K#939, we used a pycnometer and an AccuPyc
1330 produced by Micrometrics Company.
1633
Kawamura et al.
33° 37′N
Porosity 40–60%
UCS 0.1–1 MPa
Porosity 40–60%
UCS 1–10 MPa
3500
00
37
Thrust
Bedding plane
Cold seep
Fig. 7A 6K#755
Fig. 12B R-004
Strike and dip
of strata
0.47 T value
Prism Toe zone
(lower)
0.18
36
A
~1 m
Left
Down
Small mound and ridge
00
B
Huge block
Gutter
Muddy turbidite (<50 cm thick)
6K#755
Bedding plane
37
00
~1 m
10K#43
Fig. 5A
Figure 5. Outcrop photographs of the Frontal Thrust zone taken during the Shinkai
6500 and KAIKO dives. (A) Brittle deformation of turbidite layers (see Fig. 12A)
(10K#43). (B) Brittle deformation of turbidite layers (for sample #755 R-002, see
Fig. 12A) (6K#755).
Tenryu Frontal Thrust zone
–0.46 6K#755
R-003
6K#755 Fig. 5B
R-002
Fig. 12A
–0.88
10K#52
Highly deformed muddy turbidite
(<50 cm thick)
Nankai Trough floor
380
0
33° 36′N
10K#42
Proto thrust?
Undeformed strata
6K#755
The rock porosities vary in different zones
(Table 1). Porosities of samples in the lower
part of the Prism Toe and Frontal Thrust zones
(6K#755, #888, and #939) fall mostly in the
range of 40%–50%, whereas values for samples
from the higher part of the hanging wall of the
Frontal Thrust (6K#894) are generally 30%–
40% (Fig. 13). Porosities of the rocks within the
Imbricate Thrust zone (6K#885) are 30%–50%,
which are similar to those of the Prism Toe zone.
Those of the rocks from the Tokai Thrust zone
(6K#887, #892, and #893) are much lower,
~10%–50% (Fig. 13). In contrast, porosities of
samples from slopes in the Tokai Thrust zone
(samples 6K#892 R-005 and 6K#886) are as
high as ~60% (Table 1).
Uniaxial Compressive Strength (UCS)
137° 32′E
137° 33′E
Figure 4. Dive routes, photo and sample localities, geologic structures, and anisotropy of
magnetic susceptibility (AMS) parameters for samples 10K#42, #43, and #52, and 6K#755
at the Frontal Thrust and Prism Toe zones observed at the mouth of the Tenryu Submarine
Canyon. Contour interval is 10 m. Abbreviation: UCS—uniaxial compressive strength.
1634
Samples collected during dive 6K#755 were
cut into cylinders 4 cm long and 2 cm in diameter with the long axis perpendicular to the bedding plane. Uniaxial compressive strength and
deformation behavior were measured using
the unconsolidated compression testing system
(Seiken, Japan). The strain rate of the test was
set to 1.0% per minute.
00
−25
−3000
−3050
−3
−2
80
0
0
15
−3
50
0
55
0
00
−37
50
−31
0
0
75
−3
−3
80 750
0
−3
0
00
−365
0
−3600
−34
50
−35
00
−35
50
−34
00
0
0
70
−3750
−3850
−3
35
0
75
0
−3
70
−3
−3
65
0
−380
0
−3750
0
70
−3
−3800
0
−330
−390
75
0
−3
−370
0
−31
00
−3650
0
−3
35
00
−36
50
−3
−
−3400 3450
0
20
−3
50
−35
−3050
−3150
33°31′N
0
50
−36
−3
70
0
50
−31
00
−3150
−3
−30
00
Prism Toe zone (lower)
Muddy turbidite (< 30 cm thick)
50
−3
0
20
−3
−32
−3
−39
−
0.95 6K#939 R-002
00
−33
00
0
6K#939 R-001
00
−3
65
0
−3
35
0
34
−
−32 3200
50
−305
0
00
00
−33
00
−37
6K#939 R-003
−39
00
−3050
−300
0
6K#888R-003
6K#888R-001&-002
−3
6K#939
−39
00
6K#888
00
−32
50
−33
−3
05
0
0
90
−3
Fig. 12C
6K#888R-004 0.78
00
6K#939 R-004
0
00
0
6K#939 R-005-1&-2 0.74
−3850
−31
00
−3
2
0
−360
0
−355
0
50
Fig. 7E
0
00
−3500
50
−34 0
40
50
−3 −33
−31
−310
−3
33°32′N
−3
40
−38
50
00
−39
0
−3250
−3100
33°33′N
−39
−3900
−3
30
0 0
−3050
−3900
50
−38
50
−38
−32
00
−3
50
0
0
45
−375
0
−375
0
−350
−3650
00
−38
−3650
−325
0
−320
−3300
0
−3850
0
0
50
−3
0
15
−38
−3800
00
80
−3
0
31
50
hrust
al T
Front
− 7
−3800
−3600
50
00
00
−35
4
−3
0
45
35
0
−35
−3
−3
−3750
50
0
60
−3
33°34′N
−3
00
−37
−36
−3450
0
55
−3
−3300
50
−32
−3200
−330
0
80
−3
55
u
Tenry
−3
−3
45
0
5
0
70 −37
−3
50
70
−3
−3
20
25
0
0
−3
30
0
−33
50
0
60
−3
−36
0
75
Fig. 7C
−3100
−3150
−3200
−3250
0
10
−3
−3
0
55
−3
−3
−3500
−3700
−3
6
−30
9
−2
0
−3500
−3
6K#894
50
−3600
0
45
−3600
0
−3
0
45
−3
0
−3400
50
50
3
−3
50
−33
0
−35
0
40
−3
−3100
50
0
−3
75
0
−2
Fig. 7B
−30
−295
900
−2
00
Fig. 7D
00
−3600
−3450
R-005 −
R-004
−3350
R-003 0.79
−3
0.72
R-002
15
0
R-001 0.55
50
−2850
0
35
−3
−3
−3150
−27
−3
−3400
0
20
0
25
−3
0
10
−3
Prism Toe zone (upper)
Sandy turbidite (< 100 cm thick)
−2800
0
30
50
−31
−3
00
0
05
0
−2
60
0
−2
650
−3
0
−2750
00
70
0
0
−29
50
−2
65
0
−2
5
−27
−2
70
−2
33°35′N
0
−3
Strike and dip
T value
−31
5
00
0
−32 −325
Cold seep
0.47
10
0
0
10
−3
70
0
−2650
Thrust
0
−300
0
−305
−34
00
50
85
0
−2
7
−2
−29
00
65
0
50
−29
−2
−2
−3500
UCS (MPa)
<0.1 MPa
−2
70
0
0.1–1
MPa
1–10 MPa
>10 MPa
−26
00
33°36′N
0
0
85
55
−2
−3
0
−3
80
−2
45
0
50
−27
00
0
−3050
−2650
70
−3
10
−310
50
−3
200
−32
5
−33 0
0
−3 0
35
0
−2
0
65
−2
0
60
−2
50
−25
0
0
55
−2
0
−30
−
5
25
900
33°37′N
450
−2
−2 9
50
0
−25
>60%
40–60%
40–20%
<20%
−2
Porosity
0
−3700
0
−3800
−325
0
50
−38
0
70
−3
−3750
−3750
−3900
0
70
−3
33°30′N
137°23′E
137°24′E
137°25′E
137°26′E
137°27′E
137°28′E
(km)
0
1
2
(m)
3
−5000
−4500
−4000
−3500
−3000
−2500
−2000
−1500
Figure 6. Dive routes, photo and sample localities, geologic structures, and anisotropy of magnetic susceptibility (AMS) parameters for
samples 6K#888, #894, and #939 from the Prism Toe zone west of the mouth of the Tenryu Submarine Canyon. Contour interval is 10 m.
1635
Kawamura et al.
A
The uniaxial compressive strength of all other
rock samples was measured using a needle penetrometer SH-70 (Maruto Testing Machine
Company). Penetration pressure for each depth
is proportional to the hardness of the rock (e.g.,
Japanese Society of Civil Engineers, 1991).
Value of hardness was converted to uniaxial
compressive strength (UCS) by the following
conversion formula (Japanese Society of Civil
Engineers, 1991; The Japanese Geotechnical
Society, 2004):
B
Mud
stone
al
rm
No
~1 m
fa
Th
rus Sandstone
t
Th
rus
t
Mudstone
LogY = 0.978 × LogX + 2.621
t
ul
C
Y = Uniaxial compressive strength (kN/m2)
X = Penetration pressure (N)/
Penetration length (mm).
Siltstone
st
ru
Th
~1 m
The values obtained are listed in Table 1.
They increase gradually from the Prism Toe to
the Tokai Thrust zone (Fig. 13). Those in the
lower part of the Prism Toe zone and Frontal
Thrust zone (6K#755, #888, and #939) fall in
the range from 0.5 to ~1.0 MPa. Those in the
upper part of the Prism Toe zone (6K#894)
range from 2.0 to ~3.0 MPa (Fig. 13). Those
in the Imbricate Thrust zone (6K#885) are
1.0–6.0 MPa, whereas those in the Tokai Thrust
zone (6K#887, #892, and #893) are extremely
high, ~20 MPa (Fig. 13). In contrast, UCS values of bedrock samples from slopes in the Tokai
Thrust zone (6K#892 R-005 and 6K#886 R-1)
are as low as ~0.1 MPa (Fig. 13).
t
D
rus
Th
Anticline
Thrust
Thrust
one
Siltst
ion
fact
e
Liqu
Mudstone
Sandstone
Anisotropy of Magnetic Susceptibility
No
rm
al f
au
lt
~1 m
E
No
rm
e
ton
al
nds
Sa
Normal fault
~1 m
fau
lt
e
ton
ds
Mu
Norm
al fau
lt
~1 m
Figure 7. Photographs of outcrops of the Prism Toe zone, taken during the Shinkai 6500 dives.
(A) Horizontal turbidite layers (6K#755; locality shown in Fig. 4). (B) Turbidite layers (see
Fig. 12C) (6K#894). These layers are cut by faults that record normal displacements of several
tens of centimeters. (C) Turbidite layers cut by thrust faults (6K#894). White layers are siltstone. (D) Meter-scale tight folds associated with a thrust anticline with SE vergence (6K#894).
Note pinch-and-swell structures within sandstone layers. (E) Synthetic normal faults.
1636
As an indicator of the deformational fabric,
we measured the anisotropy of magnetic susceptibility (AMS). Test specimens were encased
in plastic cubes of 7 cm3 in volume. AMS was
measured using the AGICO KLY-3 anisotropy
magnetic susceptometer set at 0.04 mT (low
magnetic fields [LF]). Obtained AMS values
are represented by magnetic ellipsoids of which
the maximum, intermediate, and minimum axes
represent Kmax > Kint > Kmin. In general, the
magnetic ellipsoid is determined by the degree
of alignment of magnetic particles in sediments
(Tarling and Hrouda, 1993).
We use AMS parameters to show the ellipsoid shape by the following formula of Jelinek
(1981):
P′ (anisotropy degree) =
exp √{2[n1 − nm]2 + (n2 − nm)2 + (n3 − nm)2}
T (shape parameter) = (2n2 − n1 − n3)/(n1 − n3),
where n1 = lnKmax, n2 = lnKint,
n3 = lnKmin, and nm = 3√n1 × n2 × n3.
rust
00
−2
3
−2
30
0
−2
50
0
Toka
50
−27
−2700
UCS (MPa)
<0.1 MPa
−2300
0.1–1 MPa
1–10 MPa
>10 MPa
−2750
−2
40
0
−2
55
0
−2
7
−2350
−240
0
−245
0
−25
− 2 00
550
−2 6
00
−2 6
50
−2650
−2700
50
00
−2
60
0
50
−24
0
25
−2
−23
−2250
0
75
−2
00
−22
>60%
40–60%
40–20%
<20%
0
40
−2
−23
33°45′N
7
−2
−2
55
0
i Th
Porosity
00
0
25
−2
00
0
0
−2
30
50
0
−2
−2250
−2 2
50
0
−2300
−2
50
0
−
245
−2
55
0
0
−2
80
0
40
−2
00
−21
40
−2
0
−2
0
15
15
0
0
0
450
−2
0
−
650
−2
0
−28
00
0
85
95
0
−3
00
0
−2
−2
90
−2
−2150
10
0
−275
−2800
Imbricate Thrust zone
0
−23
50
−2
65
−3
−30
00
00
0
−30
50
0
95
−2
−2
75 −2
0
0
00
−27
00
−21
−29
00
−2
80
0
−3150
−3100
st
20
0
−3
65
0
−28
50
−2900
Prism Toe zone
−
0
Ten
r
137°30′E
137°31′E
0
0
20
−3
50
−35
−2
137°29′E
0
00
−34
0
90
−2
45
−2750
−2800
0
50
−27
hru
tal T
−3
40
−034
50
−3
50
0
−35
50
−3
−3500
yu F
ron
−3
40
0
−3
50
−25
−2600
0
−265
−2700
−2
−30 −3000 950
50
0
30 0
−3
35
−3
−310
0
−3150
−320
0
−3
55
0
−
−2500
05
−3
137°32′E
137°33′E
137°34′E
(km)
0
1
2
−2
95
−31
−3
0
−3 00
05
0
0
0
−2400
−2450
−3
00
0
−29
00
−255
0
35
0
0
−24
50
40
−2
−2
50
−350
0
0
36
−2350
0
85
−2 00
9
−2
−2
85
0
0
0
65
0
−3
10
25
−2
400
0
00
0
35 −2
45 25
−2
−
−315
0
0
−2250
−2300
−25
50
−26
00
−265
0
−270
0
−2750
−280
0
0
85
−2 900
−2
35
−3
00
7
−2
0
−3600
0
90
−2
950 0
00 050
3
− 3
−
−2
0
95
0
−3
45
0
75
−2
−2800
−3
30
−3
0
−285
−3400
0
33°39′N
20
−3
700
−2
−2
0
30
0
0
65
−2
−2500
−2550
600
−2
80
−2
4
−2
50
−24
00
0
−3250
−3350
0
05
−3 0
0
−30
950
−2
50
−23
10
−2200
−2
−3
00
−32
0
10
−3
−
−3050 300
0
−3
−2150
00
0
−2900
−2600
137°28′E
90
00
−33 0
5
−32 0
−320
0
15
−3
−2
70
−2550
−2750
−2
−3
15
0
0
50
−2
50
00
−2800
−30 −2950
00
−3050
0
−2850
−2650
0
−24
50
95
80
−2
−2600
−2
7
−28
−2
0
0
33°40′N
−3
25
0
0
−215
50
−27 0
80
−2
−275
00
−32
−2850
−2700
−310
0
−285
0
−265
−250
0
50
00
5 0 −23
00
−22 −22
−2
6
50
0
0 50 −25 2600 0
5
−
45 −2
−26 700
−2
−2
−25
50
−2
0
40
−2
0
−2
65
0
50
0
75
−2 00
−27
00
29
−2850
0
90
−2
−2
35
0
−2
6
45
−2
0
65
90
0
00
−27
−275
0
050
00
60
−2
0
55
−2
0
35
−2
40
R-004
−3
0.40
00
−3
−2700
0
65
−2 0
70
−2
0
75
−2
−2200
with joint sets
−
−2450
00
−25
−
R-003
−26
0
−260
0
0
70
−2
−2
0
65
00
−25
30
−2
0
−255
40
Fig. 9B
−2
0.47
00
4
−2
0
−2
65
0
90
−2
−2950
−2
0
−235
0
R-001
R-002
0.68
−2450
−2400
−2350
−2300
−2500
00
23
−2
−2750
−2800
−280
0
−2
6
−2550 00
−2450
−22
6K#885
−2
85
0
75
0
50
0
−21
50
0
85
−28
−245
33°41′N
−28
50
−2300
−2
−2
−2700
−2600
−2550
−2500
−2200
−28
00
0
Fractured mudstone
−2700
−2650
−2
−275
50
−26
50
33°42′N
−2800
T value
0.47
−2
4
−2 50
5
−200
55
0
−2
60
−27
−235
0
00
0
−240
R-003
R-002
0.44 R-001
−2
600
−24
0
60
−2 0
65
−2
0
0
−27
0
33°43′N
−2
Fractured mudstone
Fractured sandstone
−26
with gentle folds
0.54
0.48
0
30
−2
Strike and dip
0
55
75
−2
Fig. 12E
Cold seep −2250
0
60 650
−2 −2 2700
−
R-004
00
−24
Thrust
−2300
50
−23
–0.27
6K#887
0
R-005
−2
70
0
−2300
−23
0.15
−2750
−2550
0
−24
R-006
50
33°44′N
−2
60
0
26
50
Tokai Thrust zone
50
−22
(m)
3
−5000
−4500
−4000
−3500
−3000
−2500
−2000
−1500
Figure 8. Dive routes, locations of photographs and samples, geologic structures, and anisotropy of magnetic susceptibility (AMS)
parameters for sample 6K#885 from the Imbricate Thrust zone, and sample 6K#887 from the Tokai Thrust zone in the lower Tenryu Submarine Canyon. Contour interval is 10 m.
1637
Kawamura et al.
B
A
Vein
Vein
~1 m
N-S
~1 m
Fracture cleavage
E-W
Figure 9. Photographs of outcrops from the Imbricate zone, taken during the Shinkai 6500
dives. (A) Turbidite layers within the Imbricate Thrust zone (6K#885). Gray areas are sandy
layers, which contain conjugate sets of white-colored mineral veins. (B) Turbidite layers
within the Imbricate Thrust zone showing E-W and N-S joint sets (6K#885).
Because the contribution of different minerals to the AMS varies considerably, it is important to determine which minerals are the
most important contributor. To do this, we
used MicroMag AGM Model 2900 (Princeton
Measurements Corporation, UK) to measure the
high-field magnetic susceptibility of minerals
included in a small (few mm3) sample. Magnetic
susceptibility under high magnetic fields (HF) of
500–1000 mT is related mainly to paramagnetic
minerals rather than ferrimagnetic minerals
(Housen and Sato, 1995; Housen, 1997). In
contrast, LF susceptibility can be attributed to
both ferrimagnetic and paramagnetic minerals
(Housen and Sato, 1995; Housen, 1997). The
ratio HF/LF is inversely proportional to the relative contribution of ferrimagnetic minerals to
LF (Housen and Sato, 1995; Housen, 1997).
HF/LF values of the analyzed rock samples
are listed in Table 2. Most of the magnetic
susceptibility and its anisotropy are carried by >50% of the paramagnetic fraction,
which corresponds mainly to the magnetic
susceptibility of clay minerals within muddy
turbidites.
A plot of P′ versus T is shown in Figure 14
(see also Table 3). Although P′ values of samples collected from the Frontal Thrust zone
to the Tokai Thrust zone are in the range of
1.02–1.10, T values can be classified into the
following three groups: the Frontal Thrust zone
(–0.9 to –0.4) and Prism Toe zone (0.1–1.0),
the Imbricate Thrust zone (–0.1–0.7), and the
Tokai Thrust zone (–0.3–0.8). High-T values
approaching +1.0 indicate that the magnetic
ellipsoids are oblate in shape, whereas low-T
values approaching –1.0 represent prolate
ellipsoids (Jelinek, 1981; Tarling and Hrouda,
1993). Triaxial shapes (neutral ellipsoids)
plot close to T = 0.0 (Jelinek, 1981; Tarling
and Hrouda, 1993). The magnetic ellipsoids
obtained for the Prism Toe zone are oblate in
shape, whereas those for the Frontal Thrust, the
Imbricate Thrust, and the Tokai Thrust zones
range from weakly prolate to oblate.
TABLE 1. PHYSICAL AND MECHANICAL PROPERTIES OF ROCK SAMPLES COLLECTED FROM THE EASTERN NANKAI ACCRETIONARY PRISM
Water Depth
WC
Sample no.
Locality
(m)
Zone
Lithology
(wt%)
gd (g/cc)
gdry (g/cc)
PO (%)
VR
UCS (MPa)
gsat (g/cc)
32.46
2.72
1.87
1.41
47.92
0.92
1.10
755R-002
33°36.2900′N
137°32.2200′E
3827
TFT
Mudstone
755R-003
3827
TFT
Mudstone
41.96
2.75
2.24
1.57
42.91
0.75
0.50
33°36.2900′N
137°32.2200′E
27.09
2.72
1.93
1.52
44.11
0.97
1.19
755R-004
33°36.7600′N
137°32.2200′E
3691
LPT
Mudstone
25.49
2.66
1.99
1.58
40.61
0.68
5.63
885R-001
33°42.6059′N
137°31.9790′E
2778
IBT
Mudstone
885R-002
2742
IBT
Sandstone
21.14
2.70
2.08
1.72
36.55
0.58
5.63
33°42.3985′N
137°31.7658′E
36.11
2.76
1.88
1.38
50.03
1.00
1.22
885R-003
33°42.1904′N
137°31.5932′E
2707
IBT
Mudstone
34.11
2.65
1.86
1.39
47.60
0.91
3.58
885R-004
33°41.9163′N
137°31.6277′E
2660
IBT
Mudstone
58.33
2.76
1.67
1.05
61.82
1.62
<0.1
886R-001
33°43.0652′N
137°23.1066′E
2584
TOT
Mudstone
41.67
2.71
1.80
1.27
53.15
1.13
<0.1
886R-002
33°43.0958′N
137°23.1014′E
2566
TOT
Mudstone
25.57
2.68
1.98
1.58
41.23
0.70
1.22
887R-001
33°43.0342′N
137°28.6258′E
2696
TOT
Mudstone
28.87
2.80
1.99
1.54
44.83
0.81
0.50
887R-002
33°43.1352′N
137°28.6275′E
2700
TOT
Mudstone
28.02
2.69
1.96
1.53
43.13
0.76
1.54
887R-003
33°43.1965′N
137°28.6444′E
2690
TOT
Mudstone
18.48
2.85
2.21
1.86
34.69
0.53
1.22
887R-005
33°43.6008′N
137°28.6793′E
2454
TOT
Sandstone
34.10
2.68
1.88
1.40
47.93
0.92
1.14
887R-006
33°43.7306′N
137°28.7315′E
2394
TOT
Mudstone
33.23
2.70
1.89
1.42
47.44
0.90
0.90
3690
LPT
Mudstone
888R-002
33°32.6498′N
137°26.2066′E
31.00
2.76
1.95
1.48
46.29
0.86
1.14
888R-003
33°32.7057′N
137°26.1088′E
3649
LPT
Mudstone
37.58
2.69
1.84
1.33
50.41
1.02
0.82
888R-004
33°32.7234′N
137°26.0658′E
3610
LPT
Mudstone
892R-002
33°50.8677′N
137°33.6037′E
2194
TOT
Mudstone
12.09
2.73
2.14
1.91
30.18
0.43
892R-003
33°50.5828′N
137°33.6858′E
2149
TOT
Mudstone
12.01
2.73
2.30
2.05
24.94
0.33
4.94
892R-004
33°50.4194′N
137°33.8669′E
2001
TOT
Mudstone
33.52
2.70
1.89
1.41
47.64
0.91
0.82
892R-005
1852
TOT
Mudstone
60.28
2.71
1.65
1.03
62.13
1.64
<0.1
33°50.3664′N
137°34.0864′E
13.08
2.66
2.22
1.97
26.02
0.35
2.33
893R-001
33°46.2981′N
137°31.5386′E
2643
TOT
Mudstone
16.83
2.41
1.99
1.70
29.31
0.41
19.17
893R-002
33°46.3591′N
137°31.5131′E
2619
TOT
Sandstone
893R-003
2538
TOT
Mud
33°46.5791′N
137°31.4786′E
19.17
893R-004
33°47.0912′N
137°31.4332′E
2434
TOT
Mudstone
11.39
2.62
2.26
2.04
22.46
0.30
>20
893R-004
33°47.0912′N
137°31.4332′E
2401
TOT
Sandstone
893R-005
2401
TOT
Sandstone
4.47
2.73
2.54
2.43
11.07
0.13
33°47.1392′N
137°31.4025′E
24.28
2.67
2.01
1.61
39.46
0.65
<0.1
3287
UPT
Mudstone
894R-001
33°34.7275′N
137°24.9569′E
23.33
2.77
2.11
1.74
36.59
0.58
<0.1
894R-002
33°34.7488′N
137°24.8302′E
3202
UPT
Mudstone
26.38
2.73
2.00
1.58
42.06
0.73
2.41
894R-003
33°34.9652′N
137°24.7078′E
3131
UPT
Mudstone
25.25
2.74
2.03
1.63
40.41
0.68
2.80
894R-004
33°35.0485′N
137°24.6608′E
3065
UPT
Mudstone
22.02
2.71
2.07
1.69
37.53
0.60
2.21
894R-004
33°35.0485′N
137°24.6608′E
3065
UPT
Sandstone
19.79
2.68
2.09
1.75
34.85
0.54
3.19
894R-005
33°35.0759′N
137°24.6526′E
3074
UPT
Sandstone
41.53
2.82
1.80
1.29
52.72
1.11
0.42
939R-001
33°32.6209′N
137°26.4478′E
3778
LPT
Mudstone
28.34
2.83
1.97
1.58
43.59
0.77
0.90
939R-002
33°32.6231′N
137°26.2247′E
3700
LPT
Mudstone
35.62
2.70
1.87
1.39
49.13
0.97
0.66
939R-003
33°32.6231′N
137°26.2247′E
3700
LPT
Mudstone
26.02
2.74
2.01
1.62
41.47
0.71
0.86
939R-004
33°32.7919′N
137°25.8745′E
3526
LPT
Mudstone
31.23
2.76
1.74
1.35
41.33
0.70
0.86
939R-005
33°33.0913′N
137°25.6630′E
3416
LPT
Mudstone
Note: WC—water content, gd—grain density, gsat—wet bulk density, gdry—dry bulk density, PO—porosity, VR—void ratio, UCS—uniaxial compression strength, TFT—
Tenryu Frontal Thrust zone, LPT—lower Prism Toe zone, UPT—upper Prism Toe zone, IBT—Imbricate Thrust zone, TOT—Tokai Thrust zone.
1638
15
−2
−1
95
0
−2
−1
80
0
−1750
R-005
Brown clay
−2
3
−23 00
50
00
−19
−18
00
8
−1
−1900
00
0
25
−2
0
05
−2
0
3
−2
25
−2
2
−2
−2000
−2050
00
0
15
−2150
−2
20
0
0
00
−2
10
0
−1850
−240
0
0
−1900
00
−2
4
−
−2 225
30
0 0
−21
00
−2
15
0
0
−2
0
−235
0
−240
−2450
−205
0
−22
50
−22
00
−2
15
0
−22
50
−230
0
−24
00
−23
00
−2450
−23
−22
50
00
00
0
−2050
−2000
−2
10
0
50
−21
hrust
T
Tokai
−2100
−2
−25
50
0
−240
−2150
0
0
−235
−2400
−2450
00
−25
0
55
−2
−24
50
−2
50
−235
0
0
−2200
50
−19
0
10
−2000
0
−205
−2100
−2300
10
0
−225
0
100
−2
0
−220
−2
−210
0
−2
20
0
−2
0
−2100
−2100
−2100
0
−2
15
10
0
0
20
−2 0
−225
−2600
−255
−2 0
−2 500
45
0
0
10
−2
Sheared blocks
50 R-002
0.01
0
−2
−245
Breccia
0.40 R-001
−2550
Fig. 12G
−2
Mudstone
−215
0
600
00
−25
5
−2
40
0
−2
25
0
−2600
−2
3
0
−2100
0
15
−2
−2550
−2500
−2450
−2400
−2350
0
60
−2
t
hrus
00
50 23
0
40 −23 −
00
−22
−2
−2650
50
65
0
1
−2
−2150
0
5
−21
−2
2
−2 50
20
0
−2
iT
00
0
−19
R-003
0.37
00
−25
−2000
100
−2
Sandstones (< 10 cm thick)
50
−2400
00
−25
00
0
R-004 & 5
−21
−2350
−1950
−2050
6K#893
Fig. 11B
Fig. 12F
00
−22
95
0
−2100
−1850
−2050
−22
−1950
00
−20
−1
−21
−2150
−1
85
50
0
85
25
−2
0
−200
−20
−1
0
950
−1
00
0
45
−19
0
30
−2
50
−23
50
−23
0
20
−2
90
−1
−19
5
−1900
−1850
50
−2100
−2
−1
95
0
−18
00
33°49′N
50
6
−2
−2
00
0
0
−22
−22
−1950
25
−2100
0
15
−2
−2
10
0
0
00
−185
0
R-003 Foliated mudstone (< 10 cm thick)
R-004 Turbidite
−2
0
a
Tok
−1700
0
−2100
− 20
00
−20
50
0
50
−1
90
−1
8
−2
00
0
−2
05
0
25
−2
−2
−2250
0
20
−2
−2250
−2200
0
25
−2
00
−24
0
45 00
−2
50
−22
00
−23
300
−2
−2250
−2200
25
0
0
−2
35
0
−230
0
70
−2
−26
−26 50
−2 00
−2550
50
0
−27
33°45′N
30
0
−220
0
−2
−2
0
75
−2700
−2
33°46′N
−1650
00
0
T value
50
00
−2300
33°50′N
33°47′N
−19
0
−2200
−15
−16
R-002 Foliated mudstone 0.71
Fig. 11C
Fig. 12H
−220
Strike and dip
−2100
10
0
−185
Cold seep
0
95
−1
−1
6K#892
0
15
−2
−
0
33°48′N
0
85
−2
5
−21
−2150
850
−1
0
−1500
0
5
19
Thrust
0.47
00
− 16
85
0
−155
−1
−18
00
50
0
−190
−1
85
0
− 17
85
0
UCS (MPa)
<0.1 MPa
0.1–1 MPa
1–10 MPa
−18
50
>10 MPa
33°51′N
00
−1
80
0
50
−20
−18
00−
1
0
90
−1
>60%
40–60%
40–20%
<20%
−1850
0
50
−19
Porosity
−17
−1
90
33°52′N
50
−2
−27
30
0
0
30
−2
137°30′E
137°31′E
137°32′E
137°33′E
137°34′E
137°35′E
137°36′E
(km)
0
1
2
(m)
3
−5000
−4500
−4000
−3500
−3000
−2500
−2000
−1500
Figure 10. Dive routes, locations of photographs and samples, geologic structures, and anisotropy of magnetic susceptibility (AMS)
parameters for samples 6K#892 and #893 from the out-of-sequence thrust (OOST) (Tokai Thrust) zone in the middle of the Tenryu Submarine Canyon. Contour interval is 10 m.
1639
Kawamura et al.
A
B
A
~1 m
500 µm
5 mm
B
D
C
~1 m
C
5 cm
5 cm
F
E
~1 m
Figure 11. Photographs of outcrops within
the out-of-sequence thrust (OOST) zone,
as taken during the Shinkai 6500 dives.
(A) Fault gouge zone within the Tokai
Thrust zone (6K#887). (B) Sandstone within
the Tokai Thrust zone (sample #893 R-004;
see Fig. 12F) (6K#893). (C) Slate outcrop
(sample #892 R-002; see Fig. 12F) within the
Tokai Thrust zone (6K#892).
5 cm
5 cm
G
H
RADIOLARIAN BIOSTRATIGRAPHY
For shipboard examination, rock samples
were disaggregated following a standard
process: (1) break ~5 g of a sample into 0.5-cm
pieces and place them in a beaker; (2) rinse
and cover with a mixed acid of 5% hydrogen
peroxide solution and 5% hydrofluoric acid by
boiling on a hot plate until the reaction is complete. We then sieved the samples and retained
the fraction that passed through #36 and #200.
We dipped the extracted particles into <1%
hydrofluoric acid for several seconds. We repeated extraction until a radiolarian population
large enough for age determination was obtained before mounting them in Canada balsam
for observation using an optical microscope.
1640
1 mm
1 mm
Figure 12. Photographs of microstructures within retrieved rock samples. (A) Photomicrograph of a black shear band indicating the sense of thrusting. The sample was collected from
the Frontal Thrust zone (#755R-002), cross-polarized light. Clay flakes and quartz grains
are aligned parallel to the band. (B) Photomicrograph of a muddy layer within the turbidite
shown in (C), cross-polarized light. Vein structures that are characterized by aligned clay
minerals are light in color. (C) Muddy turbidite collected from the Prism Toe (#888R-004).
(D) Sandstone with black shear bands collected from the Tokai Thrust zone (#893R-004).
(E) Mudstone with numerous calcite veins collected from the Tokai Thrust zone (#893R-001).
(F) Slate collected from the Tokai Thrust zone (#892 R-002). (G) Photomicrograph of calcite
cementation within sandstone shown in (D), cross-polarized light. (H) Photomicrograph of
calcite veins within mudstone shown in (E), cross-polarized light.
70
25
60
Porosity (%)
Figure 13. Longitude versus
porosity and uniaxial compressive strength. The porosity decreases slightly to the north,
while the uniaxial compressive
strength increases gradually
within the Imbricate Thrust
zone from 0 to 6 MPa.
Tokai Thrust
zone
20
50
15
40
10
30
5
20
10
33°30′N
Estimated ages of deposition for ten samples collected from the eastern Nankai Accretionary Prism are shown in Figure 15 and
Table 4. A sample from the Frontal Thrust
zone (6K#755 R-002) was collected from a
block-in-matrix zone that we interpret to have
formed as debris and/or slump deposits associated with the collapse of the hanging wall
during slip on the Frontal Thrust. The 6K#755
R-002 sample contains an assemblage that
includes Stylatractus universus (Axoprunum
angelinum: Sanfilippo and Nigrini, 1998),
marking the last occurrence (LO) age of
0.43 Ma (Kamikuri et al., 2004; Motoyama
et al., 2004), and Buccinosphaera invaginata, marking the first occurrence (FO) age of
0.18 Ma (Sanfilippo and Nigrini, 1998). This
finding suggests a mixed assemblage of different ages. We consider that sample 6K#755
R-002 was collected from matrix constituting
debris and/or slump deposits. The depositional
age was estimated to be Late Pleistocene to
Holocene (Botoryostrobus aquilonaris zone).
Sample 6K#755 R-004, from the lower prism
toe, contains an assemblage characteristic in
Cycladophora sakii zone with absence of Dictyophimus robustus (LO: 3.4 Ma; Motoyama
and Maruyama, 1998) and Eucyridum matuyamai (FO: 1.98 Ma; Kamikuri et al., 2004;
Motoyama et al., 2004). Radiolarian assemblage from the upper prism toe (6K#888 R-004)
is slightly younger (uppermost Cycladophora
sakii zone) than those in the lower prism. These
microfossils are characterized by the presence
of Cyladophora sakaii (LO: 2.2 Ma to 2.4 Ma;
Kamikuri et al., 2004) and the absence of Lamprocyrtis heteroporos (LO: 1.5 Ma to 1.9 Ma;
Kamikuri et al., 2004). Thus, the age of deposition of samples from the prism toe is constrained
as the late Pliocene (1.98–3.4 Ma).
Samples 6K#886 R-002, 6K#892 R-002,
6K#892 R-005, 6K#893 R-001, and 6K#893
R-003 were collected from the Tokai Thrust
zone. Sample 6K#892 R-002 contains Col-
0
33°40′N
Latitude
33°50′N
Uniaxial compressive strength (MPa)
Frontal Thrust and
Imbricate Thrust
Prism Toe zones
zone
Porosity
Uniaxial compressive strength
losphaera tuberosa (FO: 0.47 Ma to 0.61 Ma;
Sanfilippo and Nigrini, 1998). With absence of
Stylatractus universus and Buccinosphaera invaginata (FO: 0.18 Ma; Sanfilippo and Nigrini,
1998), the deposition age of the sample 6K#892
R-002 was estimated to Collosphaera tuberosa
zone (0.42 Ma to ~0.18 Ma; Sanfilippo and
Nigrini, 1998) that corresponds to the lower
part of the Botoryostrobus aquilonaris zone
(Motoyama et al., 2004). Samples 6K#893
R-001 and R-003 have similar radiolarian assemblages to 6K#892 R-002 sample, but Collosphaera tuberosa is absent in these samples,
indicating slightly older depositional age. Sample 6K#892 R-005 has assemblages commonly
seen in Stylatracyus universus zone (1.03 Ma
to 0.43 Ma; Motoyama et al., 2004). The lower
limit could be younger because of the absence of
Lamprocyrtis neoheteroporos (Fig. 15). 6K#886
R-002 has also an assemblage commonly seen
in Stylatractus universus zone, but the upper
limit is older than 6K#892 R-005 because of
the presence of Lamprocyrtis neoheteroporos
(Fig. 15). These data indicate that rocks from
the Tokai Thrust zone range in age from Early
to Middle Pleistocene.
STABLE ISOTOPE ANALYSIS
We analyzed δ13C and δ18O of four samples of
calcite veins and calcite cements of sandstones
collected from the Tokai Thrust zone (6K#887
R-006 and 6K#893 R-001 for calcite veins and
6K#893 R-004 and R-005 for calcite cements).
TABLE 2. HIGH-FIELD (HF) MAGNETIC SUSCEPTIBILITY
COMPARED WITH LOW-FIELD (LF) MAGNETIC SUSCEPTIBILITY
HF (SI)
LF (SI)
HF/LF (%)
755R-002
5.73E-07
1.41E-06
40.68
755R-003
9.42E-07
7.85E-06
12.00
755R-004
7.91E-07
8.12E-06
9.74
885R-001
2.83E-07
3.45E-07
81.84
885R-002
3.20E-07
8.28E-06
3.86
885R-003
2.53E-07
2.39E-06
10.56
885R-004
4.59E-07
4.36E-06
10.53
886R-001
4.10E-07
6.99E-07
58.74
886R-002
4.92E-07
6.40E-07
76.85
887R-001
1.08E-06
3.15E-05
3.42
887R-002
1.10E-06
3.47E-05
3.16
887R-003
1.33E-06
1.75E-05
7.58
887R-004
5.79E-07
2.17E-05
2.67
887R-005
3.58E-07
1.65E-05
2.17
887R-006
1.30E-06
3.61E-06
36.03
888R-001
4.94E-07
8.16E-07
60.52
888R-002
2.39E-07
1.58E-06
15.15
888R-003
7.25E-07
1.17E-06
62.14
888R-004
1.97E-07
3.25E-06
6.07
892R-002
8.38E-07
8.87E-07
94.53
892R-003
1.26E-06
1.33E-06
94.53
892R-004
6.92E-07
8.88E-07
77.89
892R-005
3.56E-07
3.13E-06
11.37
893R-001
9.97E-07
1.07E-06
93.35
893R-002
1.99E-07
3.32E-07
59.98
893R-004-2
9.76E-08
1.66E-07
58.87
893R-004
3.00E-07
4.57E-07
65.62
893R-005
3.40E-07
1.04E-06
32.58
894R-001
3.49E-07
5.46E-07
63.83
894R-002
3.43E-07
4.65E-07
73.62
894R-004-1
4.78E-07
8.70E-07
54.94
894R-004
3.98E-07
2.03E-06
19.55
894R-005
5.71E-07
7.64E-07
74.77
939R-001
5.29E-07
5.48E-06
9.65
939R-002
6.76E-07
9.32E-07
72.47
939R-003
4.81E-07
3.82E-06
12.59
939R-004
5.38E-07
7.45E-07
72.30
939R-005
5.53E-07
5.35E-06
10.34
(600~700 mT)
(0.01~0.05 mT)
Note: The ratio HF/LF indicates the paramagnetic fraction with low-field
magnetic susceptibility, as shown in Table 3. LF value is for samples with
different volume (usually a few mm3) and therefore does not correspond to
the Km value in Table 3.
1641
Kawamura et al.
1
Prism Toe zone
zon
Oblateness
0.5
T
0
AMS carried by <50%
paramagnetic minerals
AMS carried by >50%
paramagnetic minerals
Tokai Thrust zone
–0.5
1
1.00
6K#755
6K#885
6K#886 Prolateness
6K#887
6K#888
6K#892
6K#893
6K#894 1.12
1.10
6K#939
Frontal Thrust zone
1.02
1.04
1.06
1.08
P′
1642
0
Magnetic
Polarity
Radiolarian
Zone
Buccinosphaera
aquilonaris
Middle
Pleistocene
Late
C1
Stylatractus
universus
Samples
6K#755 R-002
6K#755 R-003
6K#886 R-001
6K#892 R-002
6K#893 R-001
6K#893 R-003
Prospective
Radiolarian Event (Ma)
F.O. = Buccinosphaera invaginata (0.18)
L.O. = Stylatractus universus (0.42)
F.O. = Collosphaera tuberosa (0.47-0.61)
6K#892 R-005
6K#886 R-002
Early
1
Chron
Time
(Ma)
Lamprocrytis neoheteroporos
→Lamprocyrtis nigriniae
L.O. = Spumellaria genn. et. sp. indet.*
Lamprocyrtis heleroporos
→Lamprocyrtis neoheteroporos
Theocorythium vetulum
→Theocorythium trachelium trachelium
Phomostichoatus doliolum
→Phomostichoatus corbula
Eucyrtidium
matuyamai
Haecheliella inconstans
→Haeckeliella sp. aff. inconstans
2
6K#888 R-004
C2
Pliocene
TABLE 3. ANISOTROPY OF MAGNETIC
SUSCEPTIBILITY (AMS) RESULTS FOR ROCK
SAMPLES COLLECTED FROM THE
EASTERN NANKAI ACCRETIONARY PRISM
Km (SI)
P′
T
755R-002
7.46E-04
1.03
–0.878
755R-003
1.44E-03
1.029
–0.461
755R-004
7.86E-04
1.046
0.184
885R-001
9.06E-05
1.019
–0.09
885R-003
2.30E-03
1.074
0.683
885R-004
1.26E-03
1.127
0.474
886R-001
1.48E-04
1.02
0.4
886R-002
1.73E-04
1.056
0.596
887R-001
4.27E-03
1.101
0.444
887R-001
5.34E-03
1.057
0.765
887R-002
2.62E-03
1.097
0.483
887R-003
1.14E-03
1.127
0.538
887R-004
6.94E-03
1.023
–0.271
887R-005
4.34E-03
1.05
0.151
888R-002
1.35E-03
1.067
0.379
888R-003
3.39E-04
1.022
0.752
888R-004
4.80E-04
1.059
0.866
892R-002
1.30E-04
1.063
0.711
892R-003
2.05E-04
1.077
0.778
893R-001
1.49E-04
1.012
0.395
893R-002
1.04E-04
1.026
0.08
893R-002
1.25E-04
1.024
0.012
893R-004
7.30E-05
1.055
0.365
894R-001
1.14E-04
1.041
0.546
894R-002
4.68E-04
1.023
0.72
894R-003
3.00E-04
1.038
0.794
894R-004
6.04E-04
1.081
0.701
894R-005
9.98E-04
1.101
0.813
939R-001
1.14E-03
1.052
0.113
939R-002
1.51E-04
1.038
0.95
939R-003
1.01E-03
1.064
0.865
939R-004
5.59E-04
1.077
0.339
939R-005
9.48E-04
1.07
0.743
Note: Km (SI)—mean magnetic susceptibility
normalized to 10 cc volume sample, P′—degree of
AMS, and T—shape factor of the magnetic ellipsoid.
Epoch
Figure 14. Plot of P′ (degree of anisotropy of magnetic susceptibility) against T (shape factor
of the magnetic ellipsoid) (after Jelinek, 1981). Oblate shapes (disks) have positive T values
approaching +1, whereas prolate shapes (rods) plot as negative values approaching –1.
Triaxial shapes (neutral ellipsoids) plot close to T = 0.0 (Tarling and Hrouda, 1993).
Sample 6K#887 R-006 is a brecciated, semiconsolidated mudstone in which 1-mm-thick
rare calcite veins occur in between these clasts.
Sample 6K#93 R-001 is characterized by brecciated well-consolidated mudstone with many
calcite veins that are <0.5 mm thick (Figs.
12E and 12H). Samples 6K#893 R-004 and
R-005 are well-consolidated, fractured, fine- to
medium-grained sandstone with calcite cementation (Figs. 12D and 12G). All these samples
are from the Tokai Thrust zone.
Calcite was separated from the specimens
using a small drill and was analyzed by X-ray
diffraction to confirm that the samples contained
calcite. CO2 was extracted by dipping the specimen into phosphoric acid. Stable isotope ratios
of CO2 were then measured using a Mass Spectrometer IsoPrime (GV Instruments Company).
δ13C and δ18O values are described in normal
delta notation relative to the standard by the
Peedee belemnite (PDB).
The δ13C and δ18O values of the calcite veins
are –2.0 to −1.0 per mil (‰) PDB and –8.9 to
−7.2‰ PDB, respectively, and those of the calcite cements are –3.2 to –3.3‰ PDB and –6.8 to
–6.7‰ PDB, respectively (Fig. 16). These values are not consistent with those of carbonates
collected from the Nankai or Oregon Prism
slopes (Kulm and Suess, 1990; Sakai et al.,
3
Late
Cycladophora sakaii
→Cycladophora davisiana
Cycladophora
sakaii
6K#755 R-004
C2A
→ = evolutionary occurrence
F.O. = first occurence
L.O. = last occurrence
Spumellaria genn. et. sp. indet.*
: in Anma et al. (2002) Fig. 12 (16).
Figure 15. Radiolarian biostratigraphy of the eastern Nankai Accretionary Prism. The
lengths of the bars represent possible depositional ages. Radiolarian biostratigraphy is after
Kamikuri et al. (2004); magnetic polarity is after Cande and Kent (1995).
1992), which formed as a result of seawater
circulation at shallow depths. They are comparable rather to those of carbonates from the
San Andreas Fault, the Nojima Active Fault of
the Kobe earthquake, and the Frontal Thrust of the
Oregon Prism, which were all precipitated from
fluid seepage from deeper horizons.
The observed isotopic signatures of the calcite veins and cements in the Tokai Thrust zone
may indicate that the oxygen isotopes are controlled by elevated fluid temperatures during
precipitation. According to Sample et al. (1993),
the fractionation of δ18O into water and calcite
decreases with increasing temperature, following the equation below.
1000 lnACaCO3 − H2O = 2.78 × (106 × T−2) − 3.39
(O’Neil et al., 1969;
Friedman and O’Neil, 1977),
where ACaCO3 − H2O =
(1 + δ18OCaCO3/1000)/(4 + δ18OH2O /1000),
and generally
1000 lnACaCO3 − H2O ≈δ18OCaCO3 − δ18OH2O.
Assuming δ18OH2O to be SMOW (standard
mean ocean water), a fluid temperature of 24–
63 °C during precipitation of the calcite could
account for the largest shift in δ18O values.
EFFECT OF THE PALEO-ZENISU
RIDGE SUBDUCTION ON THE
ACCRETIONARY PRISM
The Zenisu Ridge is currently approaching
the eastern Nankai Prism in this study area due
to the subduction of the Philippine Sea plate.
Several lines of evidence indicate that a number of Zenisu-like ridges (paleo-Zenisu Ridges)
have already been consumed at the eastern
Nankai subduction zone (Kodaira et al., 2004).
Seismic data reveal that in the past the prism toe
around the study area was deformed by ridge
subduction (Le Pichon et al., 1996). Bathymetric maps of the eastern Nankai Trough area
show that the present prism toe is significantly
bent northward (Fig. 1); this is the topographic
expression of deformation of the prism structure
due to seamount subduction. Thus, we know
that the subduction of seamounts has played a
major role in the tectonic evolution of the eastern Nankai prism.
Seismic surveys undertaken in the study
area reveal that at least three ridges (similar
to the Zenisu Ridge) have been subducted beneath the eastern Nankai Accretionary Prism
(Kodaira et al., 2000; Park et al., 2003). Twodimensional analog experiments replicating seamount subduction have shown that uplifting and
associated structures develop in an accretionary
prism as the seamount descends (Dominguez
TABLE 4. RADIOLARIAN FOSSILS EXTRACTED FROM ROCK
SAMPLES COLLECTED FROM THE EASTERN NANKAI ACCRETIONARY PRISM
6K#755 R-002 (Late Pleistocene: Botoryostrobus aquilonaris Zone)
Radiolarians: abundant, moderate preservation
Anthocyrtridium michelinae, Stylatractus universus, Cycladophora davisiana davisiana, Didymocyrtis
tetrathalamus tetrathalamus, Eucyrtidium calvertense, Haekeliella inconstans, Spongaster tetras tetras,
Stichocorys peregrina, Thecosphaera dedoensis, Theocorythium trachelium trachelium
Remarks: Buccinosphaera invaginata (modern form, but only one specimen)
6K#755 R-003 (Middle Pleistocene: Botoryostrobus aquilonaris Zone)
Radiolarians: common-abundant, moderate preservation
Acrosphaera lappacea, Cycladophora davisiana davisiana, Didymocyrtis tetrathalamus tetrathalamus,
Lamprocyrtis neoheteroporos, Lamprocyrtis nigriniae, Pterocorys hertwigii, Spongaster tetras tetras,
Theocorythium trachelium trachelium, Thecosphaera dedoensis
6K#755 R-004 (Early~Late Pliocene: Cycladophora sakaii Zone)
Stylatractus universus, Cycladophora davisiana davisiana, Didymocyrtis tetrathalamus
tetrathalamus ,Eucyrtidium calvertense, Sphaelopyle langii, Spongaster tetras tetras
6K#886 R-001 (Late Pleistocene: Botoryostrobus aquilonaris Zone)
Amphirhopalum ypsilon, Didymocyrtis tetrathalamus, Spongaster tetras irregularis, Cycladophora
davisiana davisiana, Lamprocyrtis nigriniae, Theocorythium trachelium trachelium, Haeckeliella
inconstans
Remarks: Some reworked skeletons were mixed in this assemblage, e.g., Haeckeliella inconstans.
6K#886 R-002 (Early Pleistocene: Stylatractus universus Zone)
Thecosphaera dedoensis, Stylatractus universus, Didymocyrtis tetrathalamus, Spongaster tetras tetras,
Cycladophora davisiana davisiana, Cycladophora sp. aff. sakaii, Lamprocyrtis neoheteroporos,
Theocorythium trachelium trachelium
6K#888 R-004 (Late Pliocene: Uppermost part of Cycladophora sakaii Zone)
Radiolarians: rare, moderate preservation
Porodiscus macroporos, Cycladophora davisiana davisiana, Cycladophora sakaii, Botoryostrobus aquilonaris
Remarks: The age was determined by the co-occurrence of Cycladophora davisiana davisiana and
Cycladophora sakaii, and the absence of Lamprocyrtis heteroporos and Lamprocyrtis neoheteroporos.
6K#892 R-002 (Middle Pleistocene: Botoryostrobus aquilonaris Zone)
Radiolarians: rare, moderate preservation
Didymocyrtis tetrathalamus, Cycladophora davisiana davisiana, Theocorythium trachelium trachelium,
Botryostrobus aquilonaris, Amphirhopalum ypsilon, Thecosphaera dedoensis, Collosphaera tuberosa
6K#892 R-005 (Early~Middle Pleistocene: Stylatractus universus Zone)
Haeckeliella inconstans, Haeckeliella sp. aff. inconstans, Sphaeropyle langii, Axoprunum angelinum,
Amphirhopalum ypsilon, Didymocyrtis tetrathalamus, Spongaster tetras tetras, Spongaster tetras irregularis,
Cycladophora davisiana davisiana, Theocorythium trachelium trachelium, Botryostrobus aquilonaris
Remarks:
(1) All of the Haeckeliella inconstans were broken skeleton, and so, this typical Pliocene radiolarian
was assumed reworking. Haeckeliella sp. aff. inconstans has fewer spine blades than the typical H.
inconstans. Both of them may have a phylogenetic relationship.
(2) Bifurcation patterns of the Amphirhopalum ypsilon might show stratigraphic changes. In early
Pliocene, their forked shape was unclear, but Pleistocene type had a clear bifurcation, and recent type
shows decorated bifurcation.
6K#893 R-001 (Middle Pleistocene: Botryostrobus aquilonaris Zone)
Didymocyrtis tetrathalamus, Spongaster tetras tetras, Cycladophora davisiana davisiana, Eucyrtidium
calvertense, Lamprocyrtis nigriniae,Theocorythium trachelium trachelium, Botryostrobus aquilonaris,
Phormostichoartus corbula
6K#893 R-003 (Middle Pleistocene: Botryostrobus aquilonaris Zone)
Radiolarians: common, moderate preservation
Didymocyrtis tetrathalamus, Spongaster tetras tetras, Eucyrtidium calvertense, Botryostrobus
aquilonaris
et al., 2000). Based on the relief gradient along
the Tenryu Submarine Canyon, the topography
shows that the Nankai Trough floor at the mouth
of the Tenryu Canyon has subsided by at least
800 m (Soh and Tokuyama, 2002).
Model experiments (Dominguez et al., 2000)
based on seismic profiles across the eastern
Nankai Prism suggest that the prism toe in
this area was rejuvenated by subduction of the
paleo-Zenisu Ridges. Kodaira et al. (2004),
Le Pichon et al. (1992), and Mazzotti et al.
(2002) reported rejuvenated thrust sheets south
of Daini Atsumi Knoll (Figs. 1 and 2). Complex
seismic structures have been observed north
of this site. Based on analyses of seismic images, Le Pichon et al. (1996) suggested that the
deeper paleo-Zenisu Ridge might be located
beneath the area between the Tokai Thrust and
the Kodaiba Fault (Fig. 1). Kodaira et al. (2004)
interpreted the presence of a paleo-Zenisu
Ridge beneath the Yukie Ridge (Fig. 1), based
on seismic reflection data. Dominguez et al.
(2000) reported that the prism was rejuvenated
immediately after subduction of the seamount.
These previously published studies strongly
support the view that, following subduction of
the paleo-Zenisu Ridges, the accretionary prism
migrated to the south from Daini-Atsumi Knoll
to resume normal accretionary processes in the
eastern Nankai Trough (Figs. 1 and 2). We conclude that the eastern Nankai Prism along the
Tenryu Canyon to the south of Tokai Thrust was
formed by normal accretionary processes without being affected by the paleo-Zenisu Ridges.
1643
Kawamura et al.
10
0
–10
δ13C
–20
–30
–40
Tenryu Canyon (Vein)
6K#887 R-006, 6K#893 R-001
Tenryu Canyon (Cement)
–50
–60
–20
6K#893R-004, R-005
Nojima Active Fault (Arai et al., 2003)
San Andreas Fault (Pili et al., 2002)
Oregon prism (Klum and Suess, 1990)
Oregon prism (frontal thrust) (Sample et al., 1993)
Nankai Trough (Sakai et al., 1992)
–10
0
10
δ18O
Figure 16. Plots of δ13C versus δ18O.
DISCUSSION AND CONCLUSIONS
ODP studies have identified vertical changes
in deformational structures, in physical and
mechanical properties, and in age of presentday accretionary prisms such as Nankai (Taira
et al., 1992), Barbados (Behrmann et al., 1988),
and Cascadia (Shipley et al., 1982; Morgan
and Karig, 1993). For instance, porosities
determined from ODP cores in the western
Nankai Prism demonstrate that the porosity of
prism sediments depends on the vertical effective stress (Morgan and Ask, 2004), and that
porosity decrease is caused by the mechanical
rearrangement of grains during burial and compaction processes (Ujiie et al., 2003).
Our study shows that the development of lateral zonation in the structural architecture of the
Nankai Accretionary Prism is highly dynamic.
Seismic images (e.g., Moore et al., 2005), analog model experiments (e.g., Dominguez et al.,
2000), and computer simulations (e.g., Yamada
et al., 2005) have documented lateral variability in prism development processes. Our dive
results clearly show significant variations in deformational structures of sediments, and in the
age, physical, mechanical, and magnetic properties of the constituent prism rocks in accordance
with the successive evolution of the structural
zones (Fig. 17). These progressive changes include systematic spatial variation of porosity,
rock strength, magnetic fabrics, and accreted
rock ages, as discussed below.
1644
(1) Porosity of the constituent rocks decreases
from ~40% to ~30% from the Prism Toe
zone toward the Imbricate Thrust zone
(Fig. 13).
(2) Prism rocks become progressively stronger
from the Frontal Thrust and Prism Toe
zones (0.5–3.0 MPa) to the Imbricate
Thrust zone (1.0–6.0 MPa), as indicated by
uniaxial compressive strength (UCS) data
(Fig. 13).
Tokai Thrust zone
Rads age 0.18–1.03 Ma
Porosity 10–50%
UCS >20 MPa
T value –0.3–0.8
(3) In the Tokai Thrust zone, particularly in sample 6K#893, the porosity is remarkably low
(10%–50%), but UCS is remarkably high
(~20 MPa) (Fig. 13).
(4) AMS data reveal that magnetic fabrics
change from oblate ellipsoid in the Prism
Toe zone to prolate ellipsoid in the Frontal
Thrust and Tokai Thrust zones. Magnetic
fabrics in the Imbricate Thrust zone are intermediate between the two (Fig. 14).
(5) Radiolarian biostratigraphy ages indicate
that the Frontal Thrust zone rocks are between 0 and 0.43 Ma, Prism Toe zone rocks
are between 1.98 and 3.4 Ma, and the Tokai Thrust zone rocks are between 0.18 and
1.03 Ma. Note that the rocks of the Frontal
Thrust zone were probably transported to
their present site by debris flows, as suggested by deformational structures and inconsistent ages (Fig. 15).
(6) Stable isotope analyses indicate that the
calcite veins and cements in the samples
collected from the Tokai Thrust zone are
equivalent to those found in active faults,
such as the San Andreas and Nojima Faults
(Kobe earthquake). The estimated temperature at the time of calcite precipitation was
24–63 °C.
On the basis of analytical results of ODP
cores and numerical calculations, Morgan
and Karig (1993) suggested that sediments in
the western Nankai Prism were progressively
consolidated by lateral compression, which
resulted also in formation of fabric anisotropy
and reduction in porosity (several percent).
In the present study, we have demonstrated in
Rads age ?
Porosity 30–50%
UCS ca. 0.1–6 MPa
T value –0.1–0.7
Prism Toe zone
Rads age 1.98–3.4 Ma
Porosity 30–50%
UCS 0.5–3 MPa
T value 0.1–1.0
Frontal Thrust zone (Debris)
Rads age 0–0.43 Ma
Porosity 40–50%
UCS ca. 1 MPa
T value –0.9––0.4
Fracture cleavage
t
i
ka
To
rus
Th
Vein
tal
ron
t
rus
Th
Nankai Trough
F
ryu
n
Te
Figure 17. Schematic profile through the eastern Nankai Accretionary Prism, showing:
(1) the internal structure; (2) the physical, mechanical, and magnetic properties; and (3) the
radiolarian biostratigraphy. Abbreviation: UCS—uniaxial compressive strength.
detail the gradual strengthening of prism rocks
from the Frontal Thrust zone to the Imbricate
Thrust zone, as well as the reduction of porosity and the fabric changes. We agree with Morgan and Karig (1993) that the above-mentioned
changes of the physical properties of rocks resulted from tectonic lateral compression of turbiditic sediments.
Anisotropy of magnetic susceptibility of the
rocks changes likewise. In the eastern Nankai
Prism, the AMS changes gradually from the
Frontal Thrust zone to the Imbricate Thrust
zone. Although range of P′ (1.01–1.10) is constant throughout the prism, T values change
from 0.1 to 1.0 (corresponding to roughly oblate
ellipsoid) in the Prism Toe zone to –0.1 to 0.7
(slightly shifting to prolate ellipsoids) in the Imbricate Thrust zone. These data indicate that the
sedimentary grains have been rearranged from
primary planar orientation to secondary linear
orientations.
During our submersible dives, we observed
the development of fracture cleavages within
mudstone layers and mineral veins within sand
layers of the Imbricate Thrust zone (Fig. 9, but
not in the Prism Toe zone [Fig. 7]). This observation suggests that the fracture cleavages and
mineral veins may have developed selectively
in the deformed rock. In these rocks, the UCS
attains 6.0 MPa, porosity 30%, and T values
–0.1. It is clear that lateral compression leads
to hardening of the accretionary prism rocks,
accompanied by reduction of porosity and
changes of fabrics.
It should be noted that lateral compression in
the Nankai Accretionary Prism is not dependent
on the ages of the rocks. Here, the age means
the time elapsed since the time of deposition.
Time elapsed during the in situ consolidation
should be treated separately from the time of
secondary lateral compression during accretion. Although the Frontal Thrust zone is clearly
older than the Tokai Thrust zone, rocks of the
latter have undergone actually more lateral compression than those of the Frontal Thrust zone,
as explained earlier. This difference is due to the
variations in the length of time during which
the rocks underwent deformation. The rocks of
the Frontal Thrust zone suffered lateral compression for a much shorter time interval than
those of the Tokai Thrust zone. During the past
1.03 Ma, lateral compression has occurred in
the area between the Frontal Thrust and Tokai
Thrust zones (or Imbricate Thrust zone), only
under brittle conditions.
Rocks of the Tokai Thrust zone collected
from the dive site of 6K#893 exhibit remarkably
low porosity (~20%) and high UCS (~20 MPa)
(Fig. 13), suggesting their once deep burial
within the accretionary prism. Samples 6K#892
R-002 and R-003 in the Tokai Thrust zone consist of highly deformed, low-grade metamorphic rocks. Comparing the physical properties
of these samples to the porosity change curve
of the samples from ODP Site 808 (Taira et al.,
1992), we infer that our samples are likely to
have come from depths of several kilometers
into the Nankai Prism. According to sandbox experiments by Dominguez et al. (2000), subduction of seamounts leads to rapid exhumation of
consolidated and highly deformed rocks along
an OOST that acts as a path along the boundary between pre- and post-seamount subduction
prisms. Similarly, we suggest that the rocks in
our study could also have been brought to the
seafloor by tectonic transport along an OOST
from the deeper part of the prism (Kawamura
et al., 2006; Ogawa et al., 2006). Out-ofsequence thrust faults are, therefore, important
in exhuming deeply buried, deformed rocks in
accretionary prisms and in transporting them to
the shallower depths and to the seafloor during
ongoing subduction.
ACKNOWLEDGMENTS
We would like to acknowledge the assistance of Dr.
Mutsuo Hattori (Japan Agency for Marine-Earth Science and Technology), the Chief Scientist for cruise
KR97-06, Dr. Juichiro Ashi (University of Tokyo),
the Chief Scientist for cruise YK03-03, and the captain, crew, and operation teams of ROV KAIKO and
the submersible Shinkai 6500 during cruises KR9705/06, YK03-03, YK05-08, and YK06-02. We greatly
appreciate the work of Dr. Toshitsugu Yamazaki
(Geological Survey of Japan) and Dr. Toshiya
Kanamatsu (Japan Agency for Marine-Earth Science
and Technology), who measured the magnetic properties of our samples. Mr. Takashi Yoshida and Mr.
Osamu Kyono (OYO Corporation) are thanked for
data sampling and correcting the uniaxial compression tests, and Dr. Saburo Sakai (Japan Agency for
Marine-Earth Science and Technology) for measurement of stable isotopes. An early draft was critically
reviewed and revised by Dr. Roy H. Wilkens and Dr.
Tadashi Sato, to whom we are very grateful. Part of
this study was supported by a Grant-in-Aid from the
Ministry of Education, Culture, Sports, Science, and
Technology of Japan (B-14740295).
REFERENCES CITED
Anma, R., Yamamoto, Y., and Kawakami, S., 2002, Structural
profile of the Nankai accretionary prism and Calyptogena colonies along the Shionomisaki submarine canyon: Results of “SHINKAI” 6K#522 and #579 dives:
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MANUSCRIPT RECEIVED 10 MARCH 2007
REVISED MANUSCRIPT RECEIVED 12 FEBRUARY 2008
MANUSCRIPT ACCEPTED 13 MARCH 2008
Printed in the USA

Structural architecture and active deformation of the Nankai

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