Deep scientific dives in the Japan and Kuril Trenches

Earth and Planetary Science Letters, 83 (1987) 313-328
313
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
141
Deep scientific dives in the Japan and Kuril Trenches
J e a n P a u l C a d e t 1, K a z u o K o b a y a s h i 2, S e r g e L a l l e m a n d 3, L a u r e n t J o l i v e t 3, J e a n A u b o u i n 4,
J a c q u e s B o u l ~ g u e 5, J a c q u e s D u b o i s 6, H i r o s h i H o t t a 7, T e r u a k i I s h i i 2, K e n j i K o n i s h i 8,
Nobuaki
Niitsuma
9 and Hideki Shimamura
~0
i Laboratoire de G$ologie Dynamique ( C N R S UA 215), D~partement des Sciences de la Terre,
Uniuersitd d'Orlbans, B.P. 6759, 45067 Orldans Cddex 2 (France)
2 Ocean Research Institute, University of Tokyo, 1-15-1 Minami Dai. Nakano-ku, Tokyo 164 (Japan)
"~Ddpartement de G$ologie ( C N R S UA 215), Ecole Normale Sup~rieure. 24 rue Lhomond, 75231 Parts C~dex 05, (France)
4 Ddpartement de Gdotectonique ( C N R S UA 215) Universtt$ Pierre et Marie Curie, 4 place Jussteu.
75230 Paris Cddex 05 (France)
5 Laboratoire de Gdochirnie et M$tallog~nie ( C N R S UA 196). Unit, ersitd Pierre et Marie Curie. 4 place Jg~sieu.
75230 Paris C$dex 05 (France)
6 Laboratoire de Gdophysique ( C N R S UA 730), BStiment 509, Universitd Parts Sud, Orsay, (France)
7 Japan Marine Science and Technology Center, 2-15 Natsushima Cho, Yokosuka 237 (Japan)
8 Department of Geology, Kanazawa University, 1-1 Marunouchi, Kanazawa 920 (Japan)
9 Department of Earth Sciences, Shizuoka University, 836 Otani, Shizuoka 422 (Japan)
to Laborato~ for Ocean Bottom Seismolog)', Geophysical Instttute, Hokkaido University, Sapporo 060 (Japan)
Revised version accepted October 17. 1986
In the summer of 1985, during the French-Japanese Kaiko program, ten dives to depths of 6000 m in the Japan and
Kuril Trenches were made in the newly launched submersible "Nautile". The sites of the dives were selected on the
basis of surface geophysical surveys made during the preceding summer involving Seabeam mapping, geomagnetic and
gravimetric measurements, and single-channel seismic profiling. The results of the dives provide new constraints on the
geodynamics of these subduction zones. In the Japan and Kuril Trenches huge slump scars were observed on the
landward slopes of the trenches. Slumps produce a typical active erosional morphology with vertical or even
overhanging cliffs in poorly consolidated material. The slump scars allowed us to observe the internal structure of the
margin; the monoclinal structure on the northern Japan Trench margin deduced from the seismic profiles and DSDP
drilling was confirmed. Several dives on Kashima Seamount confirmed that this volcano has recently been split into
two parts by a normal fault system. Comparisons of lithology and paleontology on the two separated parts of the
seamount were made. Deep-sea clams colonies were observed from nearly 6000 m up to 5000 m on the landward slopes
of the trenches. It can be concluded that the whole margin is venting fluids from depths of 2-3 km which is consistent
with the indications of overpressure observed in drill sites on the Japan Trench margin. The fluids probably originate
by dewatering of the subducting sediments and then migrate to the seafloor.
1. Introduction
T h e P a c i f i c p l a t e s u b d u c t s n o r t h w e s t w a r d und e r the E u r a s i a n c o n t i n e n t a l p l a t e ; it p r o d u c e s ,
a m o n g o t h e r s , the J a p a n T r e n c h ( b e t w e e n 34 °
a n d 41 ° N). T h e s o u t h e r n e n d o f the J a p a n T r e n c h
is n e a r D a i i c h i - K a s h i m a S e a m o u n t a n d its n o r t h e r n e n d is m a r k e d b y E r i m o S e a m o u n t (Figs. 1, 2,
a n d 3). T h e J a p a n T r e n c h is c o n n e c t e d w i t h the
Izu-Bonin (Ogasawara) Trench south of DaiichiK a s h i m a S e a m o u n t , a n d w i t h the w e s t e r n p o r t i o n
o f the K u r i l T r e n c h n o r t h w e s t of the E r i m o
0012-821x/87/$03.50
~3 1987 Elsevier Science Publishers B.V.
S e a m o u n t . T h e J a p a n T r e n c h is c h a r a c t e r i z e d b y a
long and continuous deep-focus earthquake plane
s l o p i n g n e a r l y 40 o to a d e p t h o f 700 km. T h e rate
o f s u b d u c t i o n , e s t i m a t e d to b e a b o u t 9.5 c m / y r , is
o n e o f the fastest k n o w n . D u r i n g Leg 3 o f the
s e c o n d p h a s e of t h e K a i k o p r o j e c t we s t u d i e d the
J a p a n T r e n c h u s i n g the s u b m e r s i b l e " N a u t i l e " .
T h e o b j e c t i v e s w e r e to i n v e s t i g a t e the J a p a n a n d
W e s t e r n K u r i l T r e n c h e s as well as D a i i c h i Kashima and Erimo Seamounts. The newly
launched French deep-sea research submersible
" N a u t i l e " c a n d i v e to 6000 m w h i c h m a k e s the
314
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tops of the seamounts and the top of the mid-slope
area of the landward slope of the trenches accessible to direct visual observation.
The first phase of Kaiko [1-3] was accomplished in 1984 o n b o a r d R / V "Jean Charcot"
using the m u l t i - n a r r o w b e a m e c h o s o u n d e r
Seabeam as well as other geophysical instrument
systems such as proton precession magnetometer,
gravimeter and a seismic reflection system. During
Leg 3, a detailed survey of parts of the Japan and
western Kuril Trenches including the DaiichiKashima and Erimo Seamounts was made. One of
several important results obtained during the
survey was an indication of the erosional nature of
the landward slope of the Japan Trench. The
Japan Trench is a non-accretionary subduction
zone which is being tectonically eroded. The landward slope of the Japan Trench is cut by numerous normal faults trending sub-parallel to the
trench axis [4,5]. Along the landward slope is a 1
".
t
',
,
~
~
!
t
.....
.L,
~f~.,
Fig. 1. Geodynamic conte×t of Kaiko If, Leg 3 and the area
covered by Kaiko I, Leg 3.
Dive
Dive 52
53
~
H/DAKA
e
=. . . . .
Dive 49
Oive 50
KURIL TRENCH
48
\
\3
MID SLOPE
/
r~
./
/
////
k~
..s.,,..
Dive 55
P.c,,.c
PL..E
----~'
s.Mo~,,.
Fig. 2. Idealized morphology of the Japan and Kuril Trenches (from Seabeam mapping, seismic profiling, and diving) with the
location of the dives.
315
ward of the trench axis and this fault displacement
affects not only the subducting plate but also the
overlying one [4]. Two dives were made in the
northern Japan Trench.
The landward slope of the western Kuril Trench,
just northeast of the crest of Erimo Seamount is
very steep; the associated trench floor is flat and
covered by thick horizontal sediments. The landward slope is offset several tens of kilometers in a
left-lateral sense. We previously proposed that the
steepness of the scarp is due to a left-lateral
km high cliff that extends more than 100 km
formed by repeated landslides. This cliff is referred
to as the main scarp of the Japan Trench. The axis
of the Japan Trench is narrow and has a zig-zag
path due to offsets of the normal faults by short
obliquely trending strike-slip faults. The strike-slip
faults trend parallel to the strike of oceanic magnetic anomalies; thus the strike-slip faults probably correspond to reactivated ancient faults as
was shown for the Mid-America Trench [6,7]. The
reactivation of such faults is observed 25 km land-
(a)
E~
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loire_.,
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-
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20km
i
tFig. 3. (a) Structural map of the Japan and Kuril Trench area (Kaiko I, Leg 3, boxes 1 and 2) and the location of dives.
316
(b)
Fig. 3. (continued).
(b) Structural
map of the Daiichi-Kashima
transform fault [1,2]. One of the major objectives
of the present investigation
was to observe the
nature of the landward slope of the trench and to
test the hypothesis inferred from topographic
and
geophysical studies by making two dives (dives 51
and 52, see Figs. 2 and 3).
At the Daiichi-Kashima
Seamount,
the detail
morphology of the normal fault scarp splitting the
seamount was first revealed during Leg 3 of the
“Jean Charcot” survey, although this peculiar feature has been previously
discussed [g-13]. The
origin of the displaced
parts of the seamount,
whether formed by normal faulting or as a primary
structure, has been debated [14-161. Paleontological study of some dredged samples of the reefoidal
limestone capping the seamount showed that Barremian rocks were recovered from the lower part
and Albian rocks from the upper part. The authors
of the report interpreted
this evidence as supporting the idea that the Daiichi-Kashima
Seamount
had an originally stepped morphology and has not
been faulted. They argued that the two parts are
of different ages and that slow sea level changes
resulted in the limestone
caps of different ages.
One of the purposes of our dives was to study the
Seamount
area and location
of dives
slope of the lower part of the seamount
and to
compare it with the upper part which was studied
during Leg 2.
In the trench, the seamount displaces the morphological axis upward to 5400 m, a much shallower depth than on either side. This shallow
depth gave us the opportunity
to dive in the axial
zone which is generally
much deeper than the
6000 m depth limit of the “Nautile”.
The observations were made during Legs 2 and 3 of the
second phase of Kaiko. The scarp separating
the
parts of the seamount and the northwestern
margin
of the seamount
were investigated
during five
dives as part of Leg 2, whereas the southern slope
and southwestern
margin were investigated
during
three dives as part of Leg 3 (dives 55, 56, 57; Fig.
2).
The top of Erimo Seamount is situated 15 km
oceanward of the trench axis. Surface features of
the northern slope of the seamount were investigated during one dive (dive 49) to find a suitable
area for installing a natural laboratory
for geosciences. Two Ocean Bottom
Instant
Tiltmeters
(OBIT),
and one Ocean
Bottom
Installment
Seismometer
(OBIS)
were deployed
by the
317
strata of the margin. Two dives were devoted to
test this interpretation, one between 5969 and
5650 m followed by another one between 5679
and 5289 m. In this way a transect was made
across the scarp, except for the part below 6000 m
at its base. Features of particular significance were
slump scars and outcrops of Tertiary strata (Fig.
4, Table 1).
Sediment, consisting of uniform greenish or
pale grey rocks, diatomaceous mudstones (with
rare harder sandstones) of middle to late Miocene
age were observed. These rocks are similar to the
Neogene hemipelagic sedimentary section drilled
during DSDP Legs 56, 57, and 87A, and as such
they confirm the interpretation of the seismic profiles that such rocks extend across the margin.
Loose blocks of andesite and granodiorite (Table
1) were observed and sampled during the two
dives. They do not represent the Oligocene conglomerate recovered by drilling during DSDP Leg
57 at Site 439, because their age is 60 Ma rather
than 25 Ma (fide T. lshii); and thus are probably
ice rafted. Recent sediment is scarce (rare Pleisto-
" N a u t i l e " on a flat limestone terrace in the crestal
region (dive 50). Gravity measurements were made
on the bottom using a Lacoste Romberg portable
gravity meter with an accuracy better than 0.1
regal (dive 54) inside "Nautile".
To summarize: based on detailed observations
performed during the first phase of Kaiko, diving
sites were selected to test some hypotheses developed from those results such as the importance of
slumps on the landward slope of the Japan Trench,
the structure of the Kuril Trench, the contact
between the upper and lower plates of a subduction zone in the Kashima area. We also performed
geophysical experiments.
2. Japan Trench main scarp
The main scarp is a major feature of the
Seabeam map obtained during the first phase of
Kaiko (see plate IIA, map 4; Fig. 4) [1,2]. In
multichannel seismic records [1,17,18], the tectonic
features have been interpreted as belonging to a
denudation scarp or slump scar which has exposed
w
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Horizontal
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cross - section (m)
Fig. 4. Cross-section of main scarp of the Japan Trench inner wall drawn after re.analysis of video tapes during dives 48 and 53 (after
Lallemand et al., in preparation). These two dives were the first and the sixth of Leg 3, thus the new numeration is 3-1 and 3-6.
318
TABLE 1
Preliminary description of samples collected during the Leg 3
Dive
No.
Sample
No.
Sampling type
Japan Trench inner wall
48
1
rock
N A 3-1
2
rock
3
clams + mud
4
rock
5
rock + mud?
6
rock
Depth
(m)
Sample description
Age "
5969
pale greyish mudstone
5922
5899
5848
5653
5653
partly encrusted grey whitish mudstone
grey-whitish mudstone
andesite boulder
granodiorite boulder
marly limestone nodule
end of middle Miocene to
late Miocene (DF, DJ, RF)
late Miocene (RF, DJ)
1
rock
5677
entrusted white siltstone
2
rock
5538
entrusted white to brown siltstone
3
rock
5538
4
5
5"I"
rock
rock
mud
5496
5496
5375
quartz diorite boulder and entrusted siliceous
breccia with calcareous cement (?)
pyroxene andesite boulders
quartz porphyric boulder, leuco-microgranite
pale greyish argillous mud
6
rock
5226
coated siliceous siltstone
7
rock
5226
dacitic-andesite boulder
Kuril Trench inner wall
51
1
rock
NA 3-4
2
rock
3
rock
4
rock
4A
rock
5786
5774
5557
5402
?
grey-whitish tuffaceous mudstone
dark grey mudstone
grey-greenish sihstone
grey-whitish tuffaceous mudstone
greenish to brownish semi-soft mudstone
53
NA 3-6
52
NA 3-5
5
rock
5290
greenish to brownish clayey mud
6
6A
rock
rock
temperature
5290
5290
5129
HY15
HY16
7
water
water
rock
5129
5129
4981
1
rock
5912
2
rock
5624
white greyish tuffaceous mudstone
pale greenish argillous mud
1.111 o C outside of the
colony: 1.130 ° C inside the colony
within the colony
2 m off the colony
(4?) argillous pale
greyish mud
oxidized brown mudstone semi-indurated
volcanoclastic siliceous siltstone
rock
mud
rock
rock
rock + mud
4662
4535
4488
4452
3930
Erimo Seamount
49
1
NA 3-2
1
2
3
4
trachytic rock
grey-whitish silty sandstone
brecciated basalt
brecciated basalt
white-pinky fossiliferous limestone
mud? --* early Pliocene (D J)
end of late Miocene to
beginning of early Pliocene
(DF)
middle Miocene to beginning
of late Miocene (RF, DF, D J)
late Pliocene to Pleistocene
(DF, RF, DJ)
early Pleistocene (RF, DF,
D J)
late Pliocene to Pleistocene
(RF, DF, DJ)
middle Pleistocene (DF, D J)
middle Pleistocene with rehandled late Miocene (RF,
D J)
late Pliocene to Pleistocene
(DF, DJ)
Pleistocene (RF, DF, D J)
middle Pleistocene (RF)
Oligocene to middle Miocene
(Df)
lower Cretaceous (CF)
mud(?) ---*Pleistocene (D J)
319
TABLE 1 (continued)
Dive
No.
Sample
No.
Sampling type
Depth
(m)
rock
rock
rock
rock
rock
rock
rock
rock
3940~
3940
3940
3940
3940
3940
3940
3940
Sample description
Age a
Erimo Seamount
50
NA 3-3
1
2
3
4
5
6
7
8
brecciated alkaline basalt
Daiichi-Kashima S e a m o u n t area
55
NA 3-8
56
NA 3-9
1
2
3
5
6
7
8
ttY18
1
2
3
rock + mud
rock
rock
rock
rock
rock
rock
water
rock
rock
rock
5863
5832
5832
5832
5797
5775
5734
5611
5629
5617
5512
altered basalt
alkaline basalt
encrusted limestone nodule with inclusions
coquinoidal foraminiferal packstone
pumice of dacitic composition
alkaline basalt
whitish micritic limestone with tiny shell molds
4.TCI.
HY19
mud
5512
grey-olive green argillous mud and grey pale
siltstone
5
rock
4969
friable grey sandstone
6.TC2
mud
4969
pale grey siltstone and argilite
rock + mud
5773
bioclastic whitish limestone
rock
rock + mud
rock
rock
5747
5750
5590
5589
phosphatized limestone with calcareous cement
altered trachyte (?)
hawai'te or andesite
olive-green siltstone with metalliferous
mottled coating (DF, D J)
57
1
NA 3-10
2
3
4
5
a
very oxidized fossiliferous calcareous sandstone
bioclastic white shallow water limestone
dark grey argillous siltstone
1B -, Pleistocene (D J)
Gargasian (upper Aptian) (CF)
Gargasian (CF)
Gargasian (CF)
Gaxgasian (CF)
Gargasian (SF)
late Miocene to early
Pliocene (DF, N J)
less than 200000 years with
20% of rehandled middle to
late Miocene (RF, DF)
Pliocene (DF) to Pleistocene
(D J)
less than 210000 years with
10% rehandled late Miocene
(RF, DF, DJ)
Gargasian (CF)
mud (?) ~ Pleistocene (D J)
Gargasian (CF)
mud (?) --, Pleistocene (D J)
middle Pleistocene (DF, D J)
DF: diatoms (fide A.L. Monjanel); D J: diatoms (fide H. Maruyama); RF: radiolarians (fide J.P. Caulet); R.J: radiolarians (fide T.
Sakai); NF: planktonic nannofossils (fide C. Miiller); N J: planktonic nannofossils (fide H. Okada); CF: limestone fossils (fide A.
Pascal).
cene samples of mud) or nearly absent, probably
b e c a u s e o f the r a t h e r s t r o n g S W - N E c u r r e n t at
t h e s e d e p t h s . F r o m 6000 to 5600 m (Fig. 4, N A
3-1), the a v e r a g e i n c l i n a t i o n of the s l o p e is a b o u t
30 ° , b u t t w o m a i n t y p e s o f s l o p e s are dist i n g u i s h e d . O n e d i p s 2 0 ° is c o a t e d w i t h M n dio x i d e (Fig. 5a) a n d is g e n e r a l l y d e v o i d of r e c e n t
s e d i m e n t s . T h e t h i c k n e s s of the M n crust is v a r i a ble a n d c a n r e a c h 10 cm. T h e o t h e r t y p e of s l o p e
is v e r y i r r e g u l a r a n d c o n s i s t s o f a s u c c e s s i o n of
n e a r l y v e r t i c a l cliffs ( f r o m 5 to 10 m high), w i t h
o b v i o u s s l u m p scar m o r p h o l o g y (Fig. 5b) inters p e r s e d by b e n c h e s . T h e d e v e l o p m e n t of this uns t a b l e m o r p h o l o g y is r e c e n t b e c a u s e the s l u m p s
d e s t r o y s l o p e s w i t h the M n c o a t i n g (Fig. 5a).
Clam colonies covered by slump debris were obs e r v e d (Fig. 5c). T h e l a n d w a r d s l o p e o f the J a p a n
T r e n c h , b e t w e e n 6000 a n d 5600 m, has an o l d e r
p a r t m a r k e d by M n crusts w h i c h m a y c o r r e s p o n d
to a f o r m e r p r o f i l e o f e q u i l i b r i u m n o w cut a n d
320
and increase in number downslope. These observations are clearest when they affect sediment
with the Mn encrustation. The strike of these
fractures are random, but the most frequent are
the N30 ° and N80 ° trends. These two directions
are roughly parallel with major tectonic features.
N30 ° is approximately the Japan Trench direction, and NS0 ° is approximately the direction of
the oceanic magnetic anomalies. From 5795 to
5700 m several subvertical cliffs have a linear
base, which strike N30 °, N60 °. and N80 °, and
can locally be followed over more than 20 m.
These features can be interpreted as normal and
gravity or slump faults linked, expressing a fundamental tectonic process that affects the landward
slope of the Japan Trench (Fig. 3) [4].
•
•
,
,,~r~-~t~
,:~
:
•
,;.i.~: ~,
,
.
-:
3. The Kuril Trench inner slope and the KurilJapan scarp
4.,
Fig. 5. Photographs taken by the "Nautile" in the Japan
Trench. (a) Upper part of a slump scar showing grey mudstones covered with a mangartiferous crust which is cut by the
landslide. (b) Landslide on a steep slope, (c) Colony of (-'a.tvptogena sp. and associated animals.
eroded. This may result from the westward migration of the trench axis and subsidence of the
margin as described by von Huene et al. [18]
which results in an oversteepened lower slope. The
30 ° landward dip of sedimentary layers is constant. Fractures are ubiquitous in the mudstones,
Two dives (51 and 52) were made on the Kuril
Trench inner slope (between a depth of 5900 and
5000 m) (Figs• 2 and 3). Dive 51 (Fig. 6a) was on
the steepest section of the Kuril Trench inner
slope found during the "Jean Charcot" Seabeam
survey. Dive 52 was made to investigate the southwestern scarp of the Kuril Trench where it joins
the Japan Trench. During the first part of Kaiko
in 1984, this scarp was interpreted as a strike-slip
fault scarp. Direct observations on the seafloor
allowed detailed observations of the Kuril Trench
inner slope which is poorly known [19]. Morphological, lithologic (Table 1) and tectonic (Fig. 6a)
data were obtained.
On both scarps, the average dip of the slope,
which is up to 30 o, can be compared with those of
an Alpine mountain belt. Yet this dip gives a false
impression of the morphology which, in detail, is a
succession of vertical or even overhanging cliffs
and promontories interspersed with smooth slopes.
All but a few outcrops are very fresh, without Mn
coating except locally, and occur either at the base
of the cliffs or along gentler slopes. The cliffs are
in fact huge slump scars (Fig. 7a) that are similar
to the erosional features observed in the Japan
Trench. The coarse debris produced by landslides
accumulated at the base of the cliff to form a
smooth talus slope. The finer fractions go further
downslope and cover all flat area. The flat areas
also consist of bedrock covered by one or several
321
to)
NW
deFth (m)
~
5000
SE NE
7
SW
"nudsPones intercaLatated
• bwnth rwhite
o Puffaceuus
w n layers
[-'~
~
/T'. water
r ecen) sediments
,'ecen' sediments w,th pebbles
5100
yettow layers
[--'-~ou+crops withou* clear dipping
of strata
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Fig. 6. Cross-sections of main scarps of the Kuri] Trench inner wall drawn after reanalysis of video tapes during dives 51 and 52
(after Lallemand et al., in preparation). These two dives were the fourth and fifth of Leg 3, thus the new numeration is 3-1 and 3-6.
landslide debris deposits with local Mn coating. A
few slump scars have Mn coating attesting their
older age. Thus, gravity sliding has been a continuing process.
The talus slopes, which alternate with the cliffs,
are covered with recent thin resuspended sedi-
ments from the slumps. Ripple marks indicate a
current running parallel to the margin from the
northeast.
A very m o n o t o n o u s volcano-detritic sequence
was observed and sampled during the two dives.
White and grey tufts, mainly of volcanic glasses,
322
Fig. 7. Photographs taken by the "Nautile'" in the Kuril
Trench. (a) Vertical cliff and outcrop of mudstones and tufts
on the inner wall of the Kuril Trench (dive 51). (b) Vertical
"cleavage" in the mudstones observed during dive 52 on the
Kuril-Japan scarp.
alternate with brown hemipelagic mudstones containing volcanic glass, mineral grains, sponge spicules and diatoms. Because the sequence does not
contain any carbonates and is rich in siliceous
fossils, it has probably been deposited below the
calcium compensation depth (CCD). The sedimentation is irregular, the tuff beds often being
disrupted by syn-sedimentary normal faults. The
whole series is poorly consolidated and fails easily.
The apparent thickness is at least 800 m since we
could not observe the base, which is below 6000
m, or the upper part, and we could not appreciate
the effects of faults. This lithology is very different
from that of the Neogene sediment on Hokkaido
Island which is rich in coarse detritic deposits.
In spite of lithologic similarities between samples, the ages differ widely (Table 1). Samples
collected from the Kuril Trench (dive 51) are
upper Pliocene to Pleistocene, whereas the only
dated mudstone from the southwestern scarp of
the Kuril Trench (dive 52) contain abraided
Oligocene to middle Miocene diatoms which might
be reworked ( A i . Monjanel, written communication, 1986). If the sediments at equivalent depths
are indeed of different ages, vertical offsets may
affect this margin. During dive 52, the main structure observed is a monoclinal sequence, roughly
horizontal or dipping slightly towards the continent, with no folds or thrusts. Yet the whole
sequence is affected by faults (Fig. 6) with two
main directions measured along either the Kuril
Trench inner slope, or along the Kuril-Japan scarp.
The N330 ° direction is predominant on the
Kuril-Japan scarp; the N60 ° direction is predominant on the Kuril Trench inner slope. Thus, thc
Kuril inner slope and Kuril-Japan scarps are
parallel to fault systems. All the faults are nearvertical normal faults that cut the strata. The
strike-slip components of the faults were difficult
to observe because of the lack of slickensides and
a vertical reference structure. During dive 51, we
encountered the most intensely fractured areas
which are a succession of very closely spaced fault
planes (Fig. 7b). The fracture is sometimes not
easily distinguished from the bedding, but fortunately the strong color contrast between the
brown mudstones and the white tuffs clearly marks
the bedding planes, and at several stations the
horizontal bedding was cut by the vertical cleavage.
To summarize, the Kuril Trench inner slope
and Kuril-Japan scarps in both the 51 and 52 dive
areas are characterized, like the Japan Trench, by
a very steep slope which are inconsistent with the
poorly consolidated volcano-detritic sequence. The
slope morphology is controlled by fault planes
which parallel the strike of the scarps. Thus, structure and mass movement controls the morphology
of this continental margin. Continuing landslide
activity pr(~uces debris that are evacuated when
reaching the bottom of the trench, otherwise the
cliff would stabilize. Subduction is probably the
most efficient system to evacuate this material as
has been proposed for the Japan Trench. The two
scarps along the Kuril Trench inner slope an at
the junction between the Kuril and Japan Trenches
are characterized by slump and gravity faulting,
but their trends are controlled by a more basic
tectonic feature. Most normal faults have less than
323
vertical dips, whereas strike-slip faults commonly
have a near-vertical dip. Thus there may be indications from the vertical fault planes that both
normal and strike-slip faulting is involved in the
origin of the scarps. Also, since both scarps affect
Pleistocene sediment, they are young and probably active.
During the first phase of Kaiko, we proposed
that the Kuril Trench, as well as tile Japan Trench,
is non-accretionary [2]. Our direct observations of
active erosion due to landslides confirm this hypothesis. We proposed two alternate hypotheses to
account for the roughly 20 km re-entrant of the
landward slope at the junction between the Kuril
and Japan Trenches [2]: (1) the left-lateral offset
between both trenches along a N330 ° direction
could be the prolongation of a strike-slip fault
linked with an intra-continental plate boundary
between the Japanese microplate and the Okhotsk
plate; (2) the collision of a possible chain of
seamounts preceding Erimo Seamount. The latter
hypothesis was evoked to account for the sharp
curvature of the trench axes from N25 ° along the
Japan Trench to N55 ° along the Kuril Trench.
Recently, Lallemand and Chamot-Rooke [20]
demonstrated that a subducting volcano with a
volume two-thirds that of Erimo Seamount could
have produced this indentation, as well as the
uplift and major collapse of the margin [20]. Magnetism, seismic profiles, Seabeam cartography, and
observations from the submersible were used to
locate the summit of the seamount at the northwest corner of the re-entrant. The summit is probably buried under 2 km of sediments and one of
its flanks may crop out and form the lower slope
of the trench. Thus, the strike-slip fault system
may coincide with the slip lines created by the
indentation of the margin as the seamount collided with Japan in a N295 ° direction (convergent
vector between the Pacific plate and the Japanese
plate).
4. Daiichi-Kashima Seamount
Three dives (Figs. 2 and 3) were devoted to
investigate the down-faulted block of the DaiichiKahsima Seamount and its juncture with the inner
slope of the Japan Trench. Dive 55 was made on
the southern flank of the seamount to look at the
unconformity between the igneous base of the
seamount and its overlying limestone cover. Dives
56 and 57 started from the oceanic side of the
trench close to the axis, crossed, it, and climbed
up the base of the inner slope of the Japan Trench
in order to define the actual boundary between
the upper and lower plates. The principal questions posed were: does accretion occur, what are
the processes associated with subduction of Daiichi-Kashima Seamount, and how do these
processes compare with those in the ~northern portion of the juncture studied during Leg 2 of this
cruise?
Dive 55, along the southern slope of the downfaulted block, established the contact between
basaltic basement and the overlying shallow water
reefoidai limestone at a depth of 5835 m (Fig. 8a).
Basalt also occurs midslope (5775 m) either as an
inlier within the limestone or as a structural
juxtaposition associated with a transverse fault
perpendicular to the trench axis. The limestone is
partially phosphatized, encrusted with Mn dioxide, and contains upper Aptian (Gargasian) foraminifers. The limestone dips 10-15 ° southwestward and is probably unconformably overlain by
less consolidated chalk (hemipelagic calcareous
ooze) which is, in turn, veneered by siliceous sediments. The observed depth of the basalt-limestone
boundary and the lithologic affinity establishes
that the total thickness of the sedimentary caps on
the two blocks is similar. The elevation difference
between the basalt base of the two parts of the
seamount is 1300 m.
Dives 56 and 57 were devoted to the study of
the ocean-continent crustal boundary. Because of
the strong lithologic contrast between rocks of the
seamount (either reefoidal limestone or basahs)
and those of the landward slope of the trench
(probably hemipelagic mudstones), we expected a
clear contrast in rocks at the thrust contact. The
traverse of dive 56 was plotted across the contact
along a track perpendicular to the trench axis, that
of dive 57 to intercept the contact from the east,
and then trace it along strike (Figs. 3b, 8b, and
8c). Although the contact could not be clearly
pin-pointed during both dives, it was crossed by
the submersible and its position was plotted on a
Seabeam topographic map. An interesting observation was the occurrence of seamount material
(limestone) in the landward slope of the trench.
The limestone is 30 m higher in the landward
324
N
water
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ENE
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,,oo
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Fig. 8. Cross-sections of the Daiichi-Kashima Seamount area drawn after reanalysis of video tapes during dives 55, 56 and 57 (after
Lallemand et al., in preparation)• These three dives were the eighth, nineth and tenth of Leg 3, thus the new numeration is 3-8. 3-9
and 3-10.
325
• ) Accretion
;~-:~...
/
/
.
_ .
o f t h e upper p a r t
of
The straight trace of the normal fault bounding
the horst may explain the straight trace of the
subduction contact on the Seabeam map.
.--~
......
5. Geophysical experiments on the sea bottom
The first attempt to measure the crustal movements on the sea bottom were made during dive
50. In order to directly observe the subduction of
the oceanic plate, two sensitive Ocean Bottom
Implanting Tiltmeters (OBIT, Fig. 11) were installed on the top of the Erimo Seamount (3930 m,
Fig. lla). The sensitivity of the tiltmeter is 10 -8
radian. O B I T # 1 was installed on bare rock, and
O B I T # 2 on flat ooze. The sensor of O B I T # I
was cemented to bare basalt and the sensor of the
O B I T # 2 , about 115 m south of O B I T # 1 , was
forced into the ooze up to 3 / 5 of its height.
Although we do not know whether the condition
of observation of O B I T # 2 is as good as that of
O B I T # 1, we will have a comparison which could
be helpful for future deployments of the tiltmeters.
One Ocean Bottom Installed Seismograph
(OBIS, Fig. l l b ) was developed for the Kaiko
project, to record very-high-frequency signals from
nearby earthquakes. Since Erimo Seamount is
about to subside into the Japan Trench, numerous
Fig. 9. Two cross-sections of the Daiichi-Kashima Seamount
showing the two hypothesis concerning the nature of the
tectonic contact based on dives 56 and 57.
slope than on the down-faulted block of the
seamount and consisted of an accumulation of
large blocks. To account for this observation two
explanations were proposed (Figs. 9 and 10): the
first explanation is that the upper part of the
limestone is integrated within the innerslope by a
thrust fault that is part of a system of imbricate
thrusts, which is in agreement with the deformation observed during the dives of Leg 2 along the
northern part of the contact [22]. The second
explanation is that a horst of seamount material
has just been incorporated in the landward slope.
~,estevn
half
of
tl:e K.XSHLMA
SL,\%I(.)t,N[
Fig. 10. Tectonic diagram of the Daiichi-Kashima area.
326
Fig. 11. (a) Photograph of the OBIS resting on the top of the
Erirno Seamount, (b) Photograph of the OBIT installed on the
top of the Erimo Seamount during dive 50.
shallow earthquakes have been observed with
standard OBS instruments. The OBIS records frequencies up to 300 Hz, which allows observations
of ultramicro earthquakes, that are far smaller
than those usually observed. The sensitivity of the
OBIS, in the scale of the velocity of the ground
motion, is 10 9 m/s.
The OBIS has been installed on sediment 215
m southeast of O B I T ~ I . The sediment is not so
thick as to absorb the high-frequency seismic waves
from ultramicroearthquakes.
6. Summao' and conclusions
(1) The northern portion of Japan Trench and
the western portion of the Kuril Trench show no
surface expression of tectonic accretion as observed by the submersible (above 6000 m). The
landward slopes are steep and are associated with
slumps and landslides, indicative of active masswasting during subduction. The question then
arises whether the mass-wasting and disposal of
the debris by subduction is responsible for the
westward retreat of the landward slope of the
trench during the Tertiary [1,2,18]. Interestingly,
the Kuril Trench with abundant sediment fill in its
axis, as well as the Japan Trench with little or no
sediment in its axis, are non-accretionary. The
left-lateral strike-slip faults expected from the results of Kaiko I were not substantiated, although
strike-slip faults are compatible with the observed
structures.
(2) Clam colonies were found during almost all
the dives in both trenches and most remarkably in
the deepest accessible portion of the Japan Trench
close to the fault scarp. For detailed description of
the clam colonies and related species and the
geochemistry of geothermal water venting near the
colonies, see Boul~gue et al. [23], and Ohta and
Laubier [24]. The colonies were not observed seaward of the trench. Occurrence of the clam colonies indicates venting of pore water from deep
within the sediment [25--27]. In the Nankai Trough
the benthic colonies were related by Le Pichon et
al. [26], and Boulegue et al. [25] to hydrothermal
fluids migrating along the thrust planes of the
accretionary prism. A similar conclusion was
reached by Kulm et al. [28] in Oregon. In our case.
there are no thrust planes which could guide the
rising fluids, but horizontal layers along either the
Japan or Kuril Trenches could also provide conduits for fluids. An important concentration of
clams was observed along the Japan Trench main
scarp where the outcrops are continuously
refreshed by gravity slides. These fresh outcrops
without a sediment cover allow the expulsion of
fluids. Such migration of water from every part of
the inner slope clearly indicates a general overpressure in the underlying rocks. The results of
DSDP Legs 56 and 57 [29,30] had already shown
the probability of such overpressures along the
Japan Trench inner slope, linked with a fracture
of the rocks along the subduction zone. Our data
indicate expulsion of water everywhere along the
margin and not only along the thrust planes of the
accretionary prism as in the Nankai Trough. This
wide spread upward flow of water is consistent
with recent measurements of heat flow on the
margin which are not as low as expected, and have
327
b e e n a c c o u n t e d for by v e n t i n g o f w a t e r [31].
(3) T h e b o u n d a r y b e t w e e n t h e s u b d u c t i n g o c e a n i c a n d t h e l a n d w a r d p l a t e s w a s o b s e r v e d at the
s o u t h w e s t e r n f l a n k o f t h e D a i i c h i - K a s h i m a Seam o u n t . S o m e a c c r e t i o n m a y exist there. In a n y
case, t h e results o f b o t h K a i k o I a n d II i n d i c a t e
t h a t the s u b d u c t i o n o f this l a r g e s e a m o u n t o c c u r s
without distant transmission of strong compress i o n a l stress.
(4) O b s e r v a t i o n a l o n g the s e a w a r d s l o p e o f the
l o w e r p a r t o f the D a i i c h i - K a s h i m a S e a m o u n t reinforces the c o n c l u s i o n s f r o m d i v e s d u r i n g L e g 2,
p r e v i o u s d r e d g e s , a n d p r o f i l e r r e c o r d s , t h a t the
l o w e r a n d u p p e r p a r t s are p i e c e s o f a single
s e a m o u n t split in t w o by n o r m a l fault m o t i o n .
(5) A d e e p - s e a o b s e r v a t o r y was e s t a b l i s h e d o n
the n o r t h e r n crest o f the E r i m o S e a m o u n t w h e r e a
b o t t o m g r a v i t y m e a s u r e m e n t was s u c c e s s f u l l y
m a d e . T i l t i n g is n o w b e i n g m o n i t o r e d by use o f
O B I T s i n s t a l l e d b y the s u b m e r s i b l e t o g e t h e r w i t h
an O B I S . T h i s e x p e r i m e n t a t i o n s h o w s the t e c h n i cal c a p a b i l i t i e s o f the s u b m e r s i b l e a n d p r o v i d e s a
n e w tool for f u t u r e in situ g e o p h y s i c a l studies.
Acknowledgements
T h e K a i k o p r o g r a m has b e e n o r g a n i z e d by
I F R E M E R a n d C N R S o n the F r e n c h side, a n d
M o n b u s h o a n d O R I ( U n i v e r s i t y o f T o k y o ) o n the
J a p a n e s e side. T h e h e l p o f J. R o u x , C a p t a i n , pilots,
c r e w a n d t e c h n i c a l t e a m is w a r m l y a c k n o w l e d g e d .
We thank
J.P. C a u l e t ( M u s e u m
d'Histoire
N a t u r e l l e ) , H. M a r u y a m a ( T o h o k u U n i v e r s i t y ) ,
A . L . M o n j a n e l (Brest U n i v e r s i t y ) , C. Mialler
( p r i v a t e c o n s u l t a n t ) , H. O k a d a ( Y a n a g a t a U n i v e r sity), a n d T. S a k a i ( U t s u n o m i y a U n i v e r s i t y ) for
p r o v i d i n g us w i t h u n p u b l i s h e d age d e t e r m i n a t i o n s
and pre-prints of their manuscripts. The complete
results w e r e p r e s e n t e d d u r i n g the I n t e r n a t i o n a l
K a i k o C o n f e r e n c e o n S u b d u c t i o n Z o n e s , h e l d in
Tokyo and Shimizu (Japan) on November 10-15,
1986.
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25
26
27
28
29
30
31
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