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

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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

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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-

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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

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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]. •

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3. The Kuril Trench inner slope and the KurilJapan scarp

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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

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SW

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5100

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5200

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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

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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.

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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.

References 1 J.P. Cadet et al, De la fosse du Japon h la fosse des Kouriles premiers rdsultats de la campagne oc6anographique franco-japonaise Kaiko (Leg III), C.R. Acad. Sci. Paris 301, 287-293, 1985 (in French with English abstract). 2 J.P. Cadet, K. Kobayashi, J. Aubouin, J. Boul6gue, C. Deplus, J. Dubois, R. von Huene, L. Jolivet, T. Kanazawa.

J. Kasahara, K. Koizumi, S. Lallemand, Y. Nakamura, G. Pautot, K. Suyehiro, S. Tani, H. Tokuyama and T. Yamazaki, The Japan Trench and its juncture with the Kuril Trench: cruise results of the Kaiko project, Leg 3, Earth Planet. Sci. Left. 83, 267-284, 1987 (this issue). 3 K. Kobayashi, J.P. Cadet, J. Aubouin, J. BoulOgne, J. Dubois, R. von Huene, L. Jolivet. T. Kanazawa, J. Kasahara, K. Koizumi, S. Lallemand, Y. Nakamura, G. Pautot, K. Suyehiro, S. Tani, H. Tokuyama and Y. Yamazaki, Normal faulting of the Daiichi-Kashima Seamount in the Japan Trench revealed by the Kaiko I cruise, Leg 3, Earth Planet. Sci. Lett., 83, 257-266, 1987 (this issue). 4 S. Lallemand, J.P. Cadet and L. Jolivet, M6canisme de tectog6n6se de la base du tour interne de la fosse du Japon (au large de Sanriku, Japon NE): rejeu de failles oc6aniques sous la marge, C.R. Acad Sci. Paris, S6r. I1 302, 319-324. 1986 (in French with English abstract). 5 S. Lallemand, J.P. Cadet and L. Jolivet, Geodynamic evolution of the Northeast Japan continental margins during Cenozoic times, new insights, Bull. Soc. G6ol. Fr. 8, IlI, 3, 31-41, 1987. 6 J. Aubouin et al., Initial Reports of the Deep Sea Drilling Project, 67, U.S. Government Printing Office, Washington, D.C., 1982. 7 J. Aubouin et al, Leg 84 of the Deep Sea Drilling Project, subduction without accretion: middle America trench off Guatemala, Nature 297, 458-460, 1982. 8 A. Mogl and K. Nishizawa, Breakdown of a seamount on the slope of the Japan trench, Proc. Jpn. Acad. 56, 257-259, 1980. 9 Hydrographic Department of Japan, Survey results of Daiiti-Kasima Seamount by narrow multi beam echo sounder and other geophysical instruments, Rep. Coord. Committee for Earthquakes Prediction (Japan) 32, 171-175. 1984 (in Japanese). 10 M. Nosaka, A. Nish~.awa, K. Suyehiro, H. Tokuyama and H. Kagami, Exploration of crustal structure off Kashimanada using OBS and airgun. Abstr. Spring Meet. Seismol. Soc. Jpn. 173, 1984 (in Japanese). 11 Y. Ueda, Analysis of geomagnetic anomalies on Daiiti Kasima and Katori Seamounts, Rep. Hydrogr. Res. 17. 13-30, 1982. 12 Y. Ueda, Geomagnetic study on seamounts Daiiti-Kasima and Katori with special reference to a subduction process of Daiiti-Kasima, J. Geomagn. Geoelectr. 37, 601-625, 1985. 13 Y. Tomoda. H. Fujimoto and Y. Ganeko. Interaction between trench and seamounts in case of Kashima seamounts, J. Seismol SOC.Jpn. 33, 493-499, 1980 (in Japanese). 14 Research Group for Daiichi-Kashima Seamount (Tokai University), Topography and geology of Daiichi-Kashima Seamount, off Inubo cape, southeastern Honshu. Japan, Chikyu Kagaku 30, 222-240, 1976 (in Japanese). 15 Research Group for Daiichi-Kashima Seamount (Tokai University), Daiichi-Kashima Seamount, Tokai University Press, 1985. 16 Research Group for Daiichi-Kashima Seamount (Tokai University), A note on the Daiichi-Kashima Seamount, J. Geol. Soc. Jpn. 89, 365-368, 1983. 17 N. Nasu et al., Muhichannel seismic reflection data across

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19

20

21

22

23

24

the Japan trench, Ocean Res. Inst.. Univ. Tokyo. IPOD Basic Data Ser. 3, 22 pp., 1979. R. von Huene, M. Langseth, N. Nasu and H. Okada., A summary of Cenozoic tectonic history along the IPOD Japan Trench transect, Geol. Soc. Am. Bull. 93, 829-846, 1982. 1,. Savostin, L. Zonenshain and B. Baranov, Geology and plate tectonics of the sea of Okhotsk, in: Geodynamics of the Western Pacific-Indonesian Regions, T.W.C. Hilde and S. Uyeda eds., Am. Geophys. Union, Geodyn. Ser. 11, 189-221, 1983. S. Lallemand and N. Chamot-Rooke, Sur la cause du d6crochement senestre entre les fosses du Japon et des Kouriles: Subduction-collision d'un ancien volcan sousmarin. C.R. Acad. Sci. Paris, in press, 1986 (in French with English abstract). S. Oshima, T. Ogino, T. Katsura, K. Ikeda, M. Uchida, M. Nagano, M. Hayashida, K. Muneda, S. Kasuga and S. Tani, Subduction of the Daiiti-Kasima Seamount into the landward slope of the Japan Trench, Rep. Hydrogr. Res., 20, 25-46, 1985. G. Pautot, K. Nakamura, P. Huchon, J. Angelier, J. Bourgois, K. Fujioka, K. Kanazawa, Y. Nakamura, Y. Ogawa, M. S4guret and A. Takeuchi, Deep-sea submersible survey in the Suruga, Sagami and Japan Trenches, preliminary results of the 1985 Kaiko cruise, Leg 2, Earth Planet. Sci. Lett. 83, 300-312, 1987 (this issue). J. Boul~gue, J.T. liyama, J.L. Charlou and J. Jedwab, Nankai Trough, Japan Trench and Kuril Trench: geochemistry of fluids samples by submersible "Nautile" Earth Planet. Sci. Left. 83, 363-375, 1987 (this issue). S. Ohta and L. Laubier, Deep biological communities in the subduction zone of Japan from bottom photographs taken

25

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28 29

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31

during "Nautile" dives in the Kaiko project, Earth Planetary Sci. Lett. 83, 329-342, 1987 (this issue). J. Boul6gue, X. l,e Pichon and J.T. liyama, Pr6vision des tremblements de terre dans la r6gion de Tokai (Japon), C.R. Acad. Sci. Paris 301, 1217-1219, 1985 (in French with English abstract). X. Le Pichon et al., La subduction du bassin de Shikoku et de ses marges le long du foss6 de Nartkai (Japon mdridional): r~sultats pr6liminaires du programme Kaiko (Leg 1). C.R. Acad. Sci. Paris 301, 273-279, 1985 (in French with English abstract). X. Le Pichon, T. liyama, H. Chamley, I. Charvet. M. Faure, H. Fujimoto, T. Furuta, Y. Ida, H. Kagami, S. Lallemant, J. Leggett, A. Muruta, 11. Okada, C. Rangin. V. Renard, A. Taira and H. Tokuyama, Nankai Trough and the fossil Shikoku Ridge: results of Box 6 Kaiko survey, Earth Planet. Sci. Lett. 83, 186-198, 1987 (this issue). L.D. Kulm et al., Oregon subduction zone venting, fauna and carbonates, Science 231,561-566, 1985. B. Carson and T.R. Bruns, Physical properties of sediments from the Japan Trench margin and outer trench slope: results from Deep Sea Drilling Project Legs 56 and 57, in: Initial Reports of the DSDP, 56/57. 1187-1199, U.S. Government Printing Office, Washington, D.C., 1980. M.A. McArthur, B. Carson and R. von lluene. Initial tectonic deformation of hemipelagic sediments at the leading edge of the Japan convergent margin, in: Initial Reports of the DSDP, 56/57, 569--586, U.S. Government Printing Office, Washington, D.C., 1980. B.H. Reck, Effect of water flow from dewatering subducted sediments on thermal gradients in the Northeast Japan accretionnary prism, Am. Geophys. Union Spring Meet., Abstr., EOS Trans. Am. Geophys. Union 67, 379, 1986.