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231 Tectonophysics, 160 (1989) 231-241 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands Subduction of the Daiichi Kashima Seamount in the Japan Trench SERGE LALLEh4AND ’ Dbpartement de G&tectonique, ‘, RAY CULOITA 2 and ROLAND UnioersitC Pierre et Marre Curie, T26-El, 4 place Jussiey VON HUENE 2 75252 Paris Cbdex 5 (France) ’ Office of Pacific Marine Geology, U.S. Geological Suruey, 345 Middlefield Road, Menlo Park, CA 94025. (U.S.A.) (Received September 15,1987; revised December 17,198s) Abstract Lallemand, S., Culotta, R. and Von Huene, R., 1989 Subduction of the Daiichi Kashimi Seamount in the Japan Trench. In: J.P. Cadet and S. Uyeda (Editors), Subduction Zones: The Kaiko Project. Tectonophysics, In 1984-1985, 160: 231-247. the Kaiko consortium collected Seabeam, single-channel seismic and submersible sampling data in the vicinity of the Daiichi-Kashima seamount and the southern Japan trench. We performed a prestack migration of a Shell multichannel seismic profile, that crosses this area, and examined it in the light of this unusually diverse Kaiko dataset. Unlike the frontal structure of the northern Japan trench, where mass-wasting appears to be the dominant tectonic process, the margin in front of the Daiiclr-Kashima shows indentation, imbrication, uplift and erosion. Emplacement of the front one-third of the seamount beneath the margin front occurs without accretion. We conclude that the Daiichi-Kashima seamount exemplifies an intermediate stage between the initial collision and subduction of a seamount at a continental margin. Introduction lower body of the seamount. 24-channel seismic reflection The Daiichi-Kashima seamount is split into two bodies offset vertically by more than 1 km the margin (Mogi and 1987). The seamount) Nishizawa, lower body 1980; is partly subducted of the Japan trench. Kobayashi (western part et al., of in the southern An extensive the part survey (Fig. 1) was conducted during the first phase of the Kaiko program in 1984 (Leg 3, Kobayashi et al., 1987) using the R/V “Jean Charcot” equipped with Seabeam echosounder, single-channel seismic reflection, 3.5 kHz transducer, magnetometer and gravimeter. Eight Nautile dives to depths of 6000 m took place in 1985 from the R/V “Nadir” (Leg 2 and 3, Pautot et al., 1987; Cadet et al., 1987). One short 12-channel seismic reflection line was recorded by the Hydrographic Department of Japan (Ml on Figs. 1 and 2, Oshima et al., 1985) crossing the lowermost landward slope and the 0040-1951/89/$03.50 0 1989 Elsevier Science Publishers B.V. and the seamount Shell in 1972 (unpublished) 2). In September line by migration velocity stacks was recorded velocities picking part of this stacking from a “matrix” migrations (the method by (P844 on Fig. 1 and 1986, we reprocessed simultaneously prestack Also a 100 km long line crossing most of and constant is explained and Culotta, 1989, this vol.). The analyses of these data and of constant in: Von velocity Huene are the basis for proposing a model for the subduction of this seamount in the Japan trench and its effects on the landward slope. Previous studies and new insights The Seabeam morphology shows how the Daiichi-Kashima seamount is broken into two parts, 232 Y 350 20 110 50’ 135040’ pp. 233-236 231 Sedimentary basins Trench-fill l * . Seamount Man ./ sediment conro~rs thrust /Possible thrusts Oceamc normal d---@ landward slope Landward tllted \ Single-channel \ Canyons faults slump 0~ scars basins se,sm,c lines 8 0 I 9 I Fig. 3. New (compared single-channel I with those of Kobayashi seismic reflection I I et al., 1987) structural profiles. I map of the surveyed here referred to as the upper and lower bodies, separated by a major fault scarp that is slightly concave trenchward (Figs. 2 and 3). The margin in front of the seamount is indented about 7 km and uplifted several hundred meters close to the trench producing a trough parallel to the trench and 20 km landward of it. A revised structural map (Fig. 3) has been drawn on the basis of a further analysis of single channel seismic data recorded during the Kaiko I cruise (Lallemand et al., 1986) and also on the interpretation of multichannel seismic lines (Fig. 4). The lower body adjacent to the trench axis is larger than the upper body. By restoring the lower a I I I area. The big numbers The large open arrow shows the Pacific plate motion 1Okm 5 refer to the Kaiko relative to Japan. body to its pre-fault position and assuming a roughly symmetrical shape, we determined an original seamount approximately 60 km in diameter and 3.5 km in height. The fault scarp separating the two parts of the seamount is composed of two or three main faults with conjugates facing the trench. The total vertical offset along this fault system increases from 700 m to the south to 1.7 km in the middle of the seamount, based on the interpretation of single-channel lines (Fig. 5). Antithetic faults bound a 3 to 4 km wide graben between the two bodies. The upper body is highly fractured, as is the oceanic crust surrounding it, whereas the lower body appears less disrupted (the 238 239 WNW LANDWARD 10 0 t V.E. Fig. 5. Interpretative SLOPE 20 km I5 time line drawings of 8 of the 21 Kaiko exaggeration is 5 X The numbers single-channel are two-way seismic profiles (lines are located travel time in seconds. in Fig. 3). Vertical 240 offsets of faults are smaller Some single channel and on the lower body). seismic lines (II, 17 on Fig. 5) show a landward mal fault offsetting (profile 13). These normal the trench Two faults axis (Lallemand observations crust beneath suggest into the subduction depth to the seamount. mal faults seamount floor set separating the with of the crust is that by downward induced tation of the seismic and dive data (Kaiko its greatest Book, jn prep.). the norof the at the mid- beneath whereas the upper body is nearly problem has been considered during (Hilde et al.. this vol.) Pouclet find that ages of 100 to bodies are geochemically approximately oceanic crust and the Ohnenstetter lower distinct: II Data the dives on the radiometric 1976). in the our interpre- 120 Ma (Takigami et al., in press). 20 Ma younger than the surrounding (1989. the horizontal. rocks collected have yielded to landward by Ida (1986) Volcanic seamount also help to constrain if it is in a sense opposite flexure made by other investigators zone. First. parts The lower body is tilted 2.5’ studies and bio-lithostrati- project and only 700 m at the edges. horizontal, graphic petrologic Kaiko Second, two seumount Background The geochemical, the tilt of the top of the seamount. was originally seamount. achieves has a 1700 m displacement dle of the seamount However, are aligned depression trench of the Characteristics up to 800 m as the ocean the associated adjacent nor- et al., 1986). the seamount flexed downward dipping the lower body seismic line and dives M~tic~nel 13, 15. 16 and upper the samples coI- The lected on the lower body are composed mainly of mugearite, whereas those collected on the upper but body has yet to be resolved in a manner that explains all observations. Crustal response to subduction of are composed of basanite, mugearite and the seamount is observed and although not well understood, it should be considered in models of benmoreite. Also, the La/Yb ratios of the lavas of the two bodies indicate two different magmatic origins. The geochemical results in addition to the occurrence of tephra layers on the main scarp seamount Trench (Fig. 6) lead Pouclet and Ohnenstetter to propose that the seamount was originally composed of two subduction. fill is in contact the seamount only northward (see Fig. 3). Observations of active erosion suggest that arriving with the lower part of and southward made during of the lowermost sediment in the trench (Cadet soon basins on the lower landward coalescing According volcanoes separated to their hypothesis, slope from bending after with the margin et al., 1987; Pautot al., 1987). Some tens of square kilometer tilted the dives landward is subducted of it et landward slope of the of the subducting caused the limestone of a graben and depression According samples plate and collision the formation at the site of the suture the two volcanoes. by a depression. stresses resulting coliected to Konishi during between (1986), the dives trench appear to have formed by ponding of sediment behind a ridge uplifted during the collision indicate that the seamount was capped by an active reef during 10 Ma or more from Apto-Al- of the seamount. The preceding bian time. ‘Then an eustatic drowned the reef. The seamount conclusions were deduced mainly from analysis of the Seabeam map and the closely spaced single channel seismic lines (Fig. 5). Further information was obtained from the reprocessed Shell line P844 (Fig. 4) concerning the internal structure of the lower slope facing the seamount. The interpretation was constrained by observations made during dives NA 2-3 and NA 2-5 (Fig. 3) located on the main scarp exactly along P844’s track. The combined results were then incorporated into a revised structural map (Fig. 3). rise of sea-level then subsided as the cooling plate drifted from the equatorial Pacific to the Japan trench. Bio- and lithostratigraphic evidence (Konishi, 1986) suggests that identical limestones occur on both the upper and lower bodies of the seamount. The paleodepth of both bodies was the same during deposition of these limestones. Thus, even if there was a zone of weakness between the two bodies prior to the faulting, the vertical offset corresponds approximately to the depth blocks: 1500 m. difference between the two 241 Below the turbidites filling the graben between The P844 seismic line the upper and lower bodies are several irregular The acoustic character of the basement differs considerably between the two parts of the dipping reflectors, which we interpret to be chaotic seamount, from a very rough structure within the blocks sealed by recent sediments derived prim- upper body to a more layered structure inside the arily from the main scarp. The small terrace seen lower body (cf. Fig. 4). This contrast may be due at the base of the cross-section (Fig. 6) may corre- to a difference spond to an unburied block. in composition, the upper body with some explosive The northwestern flank of the seamount can be volcanics (A. Pouclet, oral cormnun., 1987), or to followed at least 30 km landward of the trench the more intense fracturing of the upper body. axis below the landward slope but it is difficult to being more heterogeneous The sediment discriminate caps on the upper and lower between straight reflectors corre- sponding to the cap of the seamount and layering of the basement as mentioned previously with bodies exhibit similar seismic character, being well stratified and nearly transparent like the hemipelagites covering the surrounding oceanic crust. regard to the lower body. The layered reflections However, the sediment cap on the lower body is twice as thick as that of the upper body. We could also correspond to continental offscraped and subducted. sediments attribute this difference to current erosion, as explained in a later section. Because the two dives took place on the main scar-p (Fig. 6) only the Geological cross section of the main scarp Figure 6 shows an interpretative cross-section thin cap of the upper body, representing a part of the total stratigraphic column, was entirely ex- made from analysis of the video tapes and de- amined. ples collected. Fresh and massive basalts outcrop scriptions recording during the dives, and the sam- WNW ESE. - 3800 - 3900 -4000 .4100 [tephra] * 4200 - 4300 - 4400 -4500 [altered -4600 and Joint Fault - 4700 directions - 4800 [w I\ * 4900 -5000 -5100 - 5200 - 5300 I - - -. D , -. - $00 El recent B yellow to brown argilite sediments .I’ ,,ooo =. . , . ,$+ . - *, * . . . $I@ 29 Ezl m sedimentary m chalky m interlayered welf stratified dark brown layer limestone shallow-water d - . - $P . ’ 3QQ limestone breccfa (ml Qv v volcanic rock m tephra layer Fig. 6. Geological cross-section of the main scarp separating the two bodies of the seamount drawn from the analyses of video tapes recorded during the Nautile dives: NA 2-3 (observer: Y. Nakamura) and NA 2-5 (observer: J. Bourgois). The description of samples (bold numbers) are issued from written communications of P. Pouclet (igneous rocks), A. Pascal and K. Konishi (limestones), J.P. Caulet, H. Charnley, S. Hasegawa, T. Maruyama, A.L. Monjanel, C. Miiller, M. Oda and Y. Takayanagi (other sedimentary rocks). The exact location of the dives can be seen on Fig. 3. There is no vertical exaggeration. 242 at depths of 5000 m and 4450 m along two maJor fault scarps. It is difficult to estimate the thickness seamount’s cap is sufficiently thrck. Thus. d~fferences in sediment thickness on the two parts .)f of the limestones the seamount frequent faults. tions, because of the According the limestone brown pled. This sequence of Mn-rich Miocene) layers are overlain limestones clay bv to the 15 m (Paleogene the upper to Cretaceous lower cherts recovered at DSDP site 436 (Leg 56, Langseth. Okada et al., 1982). One-hundred meters of argillites overlie these dark layers which may also correspond to the middle diatomaceous argilhtes by differential the upper layer of both bodies. increased locally with the recent erosiorr of Ilrosion may have fault acttvitv. Miocene C’haracteristrcs of the margin jucrng the seumolrnt alternat- which were not sam- may correspond brown overlying are caused that it may exceed 300 water limestones a few tens of meters of chalky ing with dark by observa- may be 120 m thick. whereas the seismic record indicates m. These shallow repetition to the dive to Quaternan, and silty clay recovered at Geologicul section across the truce o/‘ the suhduction zone and the lowermost lundward slope Five dives were made in the area of the seamount (see locations on Figs. 3 and 7). The most informative are dives NA 2-6 and Na 2-7 which make a transect north of the lower seamount body (Fig. 8). The base of the rcefoid cap of the seamount was observed on the oceanic side of the transect. No trench fill other than scattered blocks DSDP site 436. Two samples have been dated as early Pliocene (Monjanel et al., in press, see Fig. was found in the deepest part of the section. Subhorizontal layers of slope hreccias crop out 6) with reworked from just above the trench floor up to 5600 m. One sample of a breccia contaming diatoms (No. let, written Forty upper commun., normal Miocene sediments (Cau- 1987). faults or joints were observed along the dive transect. Most of them are subvertical. Three sets of directions are recognized (see Fig. 6). The first set strikes N30°, approximately parallel to the cliffs and the trench. This set of faults affects the lower Pliocene sediments and is probably still active. It may have originated 1 Ma ago when the seamount passed the oceanic bulge 100 km oceanward of the trench axis. The strikes of the other two sets, N-S and N140”, do not align with any other known regional features. Possibly they are related to the internal structure of the seamount basement. The N140” direction is exactly perpendicular to the trench and corresponds to major lineaments of the seamount (Fig. 2). at 2.5 to 3.2 Ma (Monjanel et al.. in press). The stepped morphology in lower part of the cross section was interpreted 3) has been dated the by the diver Fujioka) to sigmfy thrusting. The provides a conduit for nutrients In this area (Henry et al.. 1989. this vol.). The upper part of the transect shows small-scale folding with a N30” axis of Pleistocene (Monjanel et al.. in press) mudstone transverse layers and intense directions faulting (N115” ,md along mainly N145” ), but also along N20” and N50” (Fig. 8). The Nl IS” direction parallels both a pseudocleavage (very close vertical faults) observed in the mudstones. and the local vector of plate convergence. This faulting Geological cartography of the seamount’s cap As previously mentioned, the two parts of the seamount’s cap appear to differ in thickness on the seismic sections. The geological map (Fig. 7) is based on the seismic interpretation and results of the dives. We describe, on the basis of seismic profiles (Fig. 5), two different layers making the seamount’s cap. The lower layer is well stratified, whereas the upper layer is more transparent. Furthermore, the upper layer exists only when the (K. occurrence of living clams near sample No. 3 of dive NA 2-6 may indicate tectomc disruption that must be recent because Pleistocene sedi- ments are affected. Motion along the faults may be strike-slip. induced by the lateral displacement of material during subduction of the Daiichi Kashima seamount and/or tension gashes. Transverse faults or transverse trending tectonic features are commonly observed <jn the margin in front of subducting seamounts like the one at the junction of the Japan and Kurt1 trenches (Lallemand and Chamot-Rooke, 1986) or in front of the Bougainville guyot in the New Hebrides trench 243 E E142"30 ; lL2020' E 1~2~~0 lL2"50 I \ N 35"5( m recent inlillinp 2 Fig. 7. Distribution of the three different limestones with the lower layer showing correlate and with recent acoustic infilling for well stratified acoustic layers strong reflectors making the cap of the seamount reflectors, “possible” in depressions. layers below the inner slope. The main faults are simplified hemipelagites The dashed (see details lines correspond from the structural in the text). Shallow-water with the upper, more transparent to the possible contours map (Fig. 3). The locations layer of the of “Nautile” dives are plotted. (Daniel et al., 1986). Recent small-scale folds also affect Pleistocene sediments. They may be superfi- ond, during dive NA 3-9, a 20 m section of limestone talus accumulation was encountered on cial slump folds associated with the oversteepening of the slope. Alternatively they might be com- rived from incorporation pressive structures, but in this case we would expect to observe compressional folding at the base of the slope. Non-stratified breccias were observed at the base of the lower landward slope during dive NA 2-4 (Fig. 7). Dives NA 3-9 and NA 3-10 (Fig. 3 and 7; Cadet et al., 1987) also encountered breccias in the lower part of the slope but no clear the lowermost landward slope which may be deof the upper part of the limestone cap into the innerslope along a thrust fault (Cadet et al., 1987). Erosional channels and debris flows were observed during every dive. Analysis of seismic line P844 The reprocessing of seismic line P844 revealed deep structural information especially on the land- folding upslope. Two observations from those dives suggest minor accretion of the Daiichi- ward side of the trench (Fig. 4). The following are Kashima seamount. First, during dive NA 3-10, two isolated pieces of igneous rock were sampled among the landward slope breccias 30 m and 250 m vertically above the axis of the trench. These samples have petrological (alkali rocks) and chronological (115-120 Ma) affinities with the Daiicm-Kashima seamount (Ishii et al., 1986). Sec- (1) Two prominent thrusts that crop out 8 to 9 km landward of the trench axis separate an area of extensional deformation on the slope from a frontal wedge under compressive deformation. Their downward extension to the vicinity of the decollement remains speculative, but the upper two-thirds of the faults are clearly visible in the the main observations: 244 NW SE ,WSW F+iTq p-z+-----+ depth (ml / 5100 - ilO ENE pScu&JcleaVdgc ri-- volcanic ~~- / $%I Limestane I rl outcrop with0ut clear dipping of strata 5200. , 5300* 2 [mddl e Phstocene I.C so0 - . clav mud] recent j ’m ,.‘~tocc”e - rock ------ --.-. seamount cwer sediment slope breccta tren& nudstone] i_=? ssoo- fold alterations of mudstones and dark layers (ovrOctaaStit& ? 1 ..-_ I L-----_---_____--2.. li S600. Fig. 8. Geological cross-section of the trench axis area with the lowermost recorded during the Nautile (bold numbers) dives: NA 2-6 (observer: are issued from written K. Fujioka) communications and NA of J.P. Caulet, landward slope drawn 2-7 (observer: H. Charnley, from the analyses of video tapes P. Huchon). A.L. Monjanel The description of samples and H. Okada. The exact location of the dives can be seen on Fig. 3. There is no vertical exaggeration. multichannel record and can also be recognized in some single-channel records (Nos. 12-1.5. Figs. 4 A knoll located landward of the Daiichi-Kashima seamount (Fig. 1) is bounded by two pronounced and 5). headless (2) The frontal imbricated thrusting the cap appears faults. and folding. is deformed by Little or none of to have been accreted. (3) The middle list& wedge material The slope is disrupted shallow offset or the scarps cutting the seafloor indicate recent activity but the basal slip plane of the megaslump is cut by a landward thrust. (4) Some ambiguities in the reprocessed record are: (a) the poor continuity of reflections at depths of 5 to 10 km in the right part of the profile; (b) the complex transition zone below the seafloor between frontal thrusts and landward list& faults; and (c) the unexpected presence of well-developed listric faults which have not been commonly recognized elsewhere along the Japan trench margin. There are indications of other highs having preceded the Daiichi-Kashima seamount into the subduction zone (Lallemand and Le Pichon, 1987). perpendicular to the trench (Cyl and Cy2 on Fig. 1). One small canyon Fig. 1) cuts appears by normal and canyons itself across to predate (Figs. 2 and the front the development 3). thus (see Cy.I on of the knoll and of the knoll indicating a recent uplift. A swarm of small earthquakes associated with the lbaragi earthquake (m = 7.0, July 23. 1982, see location on Fig. 1) was related by Kikuchi and Sudo (1985) to the subduction The Kashima seamount is one of a seamount. of a chain of seamounts aligned along the direction of magnetic lineations, so that the earlier subduction of preceding seamounts is not unlikely. Also, magnetic anomalies across the slope are very disturbed in the area between 35 o N and 37 o N (Hydrographic Department, 1983). If we assume that the area of poor reflections (see (1) at the be@nning of the section) corresponds to a volcanic edifice, there is an ambiguity at the base of it because we can follow the d&ollement rather easily. All the above features might be 245 explained by underplating of a part of a seamount. ancient Subcrustal accretion could have produced the up- terpreted as parts of accreted seamounts and their lift of the knoll, the oversteepening sedimentary and the resulting listric of the slope, faults compensating a accretionary Sakakibara caps complexes (Naka, 1985; have been Ogawa, in- 1985; et al., 1986). The simplest models im- mass excess as proposed in the model of Platt ply that the seamount is sheared off at its base (1986). The line drawing of the time-section of the Ml and incorporated compressional seismic line (Oshima et al., 1985) has been dig- subduction zone. itized without modifications of interpretation and Evidence as a body with some horizontal deformation for partial at the front frontal accretion of the of the then converted into a depth section using the same Daiichi-Kashima velocity model as that used for the P844 seismic above the trench axis, of two isolated alkali rocks line. The velocity model used beneath the lower similar to those of the seamount’s basement (dive trench slope is based on the refraction NA 3-lo), and the observation of a 20 m limestone talus accumulation at the base of the land- data of Suyehiro et al. (1985). The depth section of Ml in Fig. 4, like P844, shows subduction of the front of the seamount beneath the inner slope and a trace seamount is the recovery, 250 m ward slope (dive NA 3-9). However, in seismic records the landward flank of the seamount ap- of the landward dipping thrust zone. pears to be subducted. The Ml seismic line (Fig. 4) and the single-channel line No. 14 (Fig. 5) show Discussion that even the sedimentary cap of the seamount is subducted without being deformed. As little trench and model when fill is observed despite the many erosional chan- dealing with the collision of a seamount is whether it is accreted or subducted. Some volcanic rocks in nels, debris flows, and fresh talus on the landward slope, such material must be subducted soon after One I fixed question commonly considered G-lOcm/year Fig. 9. Schematic model showing the effect of the subducted portion of the Daiichi-Kashima seamount on the margin without taking into account the possiblity of underplating. The drawing of the oceanic plate and the seamount has been voluntary simplified but the volumic proportion of the seamount compared with the frontal margin is real one. 246 arrival in the trench. Furthermore, the steep slope in front of the Kashima seamount has been oversteepened and appears to be collapsing into the The consequent oversteeping of the lower slope caused slope failure and erosion of the front of the margin. The absence of trench fill shows that trench axis. This collapsed material must also be collapsed subducted. The A possible way to explain the local incorporation of small quantities of material from material overall has been r;lpidly subduct,ed. process of collision the seamount into the lower slope is that some Huene (1986) turbidite containing America trench off Guatemala. Kashima seamount clasts transported was accreted just from the prior to The main process during collision of DaiichiKashima seamount has been subduction. The mass of the seamount is accommodated in three ways. First, the ocean crust beneath the seamount has subsided as shown by the increased vertical displacement of the normal faults that cross both a dentation based on studies of the Central would involve in- and collapse of the landward slope of the trench, and progressive subduction of the seamount (Fig. 9). between seamount and a plate margin. !nodelled by Van eventual subduction dismemberment or accretion and of parts of the seamount. An early stage in this process might be represented by Erimo seamount which is beginning to fracture as it approaches the northern Japan trench. A late stage might he observed at the junction of the Japan and Kuril trenches where the oceanic crust and the seamount. The vertical displacement across the seamount is 1 km greater a very large indentation than on the ocean crust. Depression of the crust is also indicated by the 100 m increase in depth of seamount has been consumed (Lallemand and Le Pichon, 1987). Our analysis of the Kaiko data and the trench axis. A second way to accommodate the the reprocessed line P&44 lead us to conclude that mass of the seamount is by uplift and thickening the present disposition of the sediment that comprises the lower slope of Kashima stage in the trench (Lallemand and Le Pichon,,l987). The lapse have occurred and massive slope col- as the trailing and structure seamount and a continental well with the position of the subducted front of the seamount. The narrow zone of deformation in Acknowkdgements pened slope and subduction of the debris from We of Daiichi seamount represents an intermediate the process of collision between a ridge associated with the subducted front of the seamount and its imbricate structure correspond front of this collision probably reflects a low strength of the material that comprise the lower slope. A third process, collapse of the overstee- flank of ti thank Shell margm. Internationale Petroleum Maatschappij B.V. for providing us the seismic record P844. The Kailco program was supported on the French side by C.N.R.S. and IFREMER masswasting, is suggested by the lack of fill in the and on the Japanese trench axis. A history of the subduction thank Prs. J.P. Cadet and X. Le. Pichon, Drs. P. Huchon and L. Jolivet for their encouragement of the seamount begins about 150,000 to 250,000 years ago (assuming a subduction rate of 10 cm/yr, Minster and Jordan, 1978), when the northwestern depression due to the load of the seamount was located at the trench axis and trapped sediment from the margin and slumps from the seamount itself. The tectonic style of the margin observed on the upper slope, existed at this time. The normal faults splitting the seamount existed also but probably with less vertical offset than at present. The lower landward slope was compressed by the colliding seamount, and was uplifted and folded, and some of the trench fill was incorporated into the inner slope. side by MONBU-SHO. during this work and Drs.W.T. We Coulbourn and T. Yamazaki for reviewing the manuscript. Drawings were prepared by A. Bourdeau. References Cadet, J.P., Kobayashi, K., LaUemand, S, Jolivet, L., Aubouin, J., Boul&gue, J., Dubois, J., Hotta, H., lshii, T., Konishi, K.. Niitsuma, N. and Shimamura, H., 1987. Deep scientific dives in the Japan and Kuril trenches. Earth Planet. Sci. Lett., 83: 313-328. Daniel, J., Collot, J.-Y., Monzier, M., Peiletier, B., Butscher, J., Deplus, C., Dubois, J., GQard, M., Maillet, P., Monjaret. M.C., Recy, J., Renard, V., Rigolot, P. and Temakon, S.J.. 247 1986. Subduction et collision le long de l’arc des Nouvelles- and Wories, H., 1978. Near the Japan Hebrides (Vanuatu): resultats pr&iminaires de la campagne begun. Geotimes, March 1978: 22-26. SEAPSO (Leg I). C.R. Acad. Sci. Paris, 303, Ser. II: 805-810 trench transects Minster, J.B. and Jordan, T.H., 1978. Present-day plate motion. J. Geophys. Res., 83: 5331-5354. (in French with English abstr.). Henry, P., LalIemant, S., Le Pichon, X. and Lallemand, S., Magi, A. and Nishizawa, K., 1980. Breakdown of a seamount 1989. Fluid venting along Japanese trenches: tectonic con- on the slope of the Japan trench. Proc. Jpn. Acad., 56: text and thermal modelling. 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