Earth and Planetary Science Letters. 83 (1987) 267-284 267 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

[41

The Japan Trench and its juncture with the Kuril Trench" cruise results of the Kaiko project, Leg 3

Jean-Paul Cadet ~, Kazuo Kobayashi 2, Jean Aubouin 3, Jacques Boul+gue 4 Christine Deplus 5, Jacques Dubois 5, Roland von Huene 6 Laurent Jolivet 7, Toshihiko Kanazawa ~, Junzo Kasahara 9, Kinichiro Koizumi 2, Serge Lallemand 7 Yasuo Nakamura ~0, Guy Pautot a~, Kiyoshi Suyehiro ~2, Shin Tani ~3, Hidekazu Tokuyama 2 and Toshitsugu Yamazaki 14

l l~aboratotre de G~ologie Dynamique (UA CNRS 215), D~partement des Sciences de la Terre. Unit)erstt~ d'Orl~ans, 45046 OrlOans C~dex (France) 2 Ocean Research Institute, Universio, of Tokyo, 1-15-1, Minamidai. Nakano-Ku. Tokyo 164 (Japan) D~partement de G~otectonique (UA CNRS 215), Unwersit~ Pierre et Marie Curie. 4 place J~t~sieu, 75252 Paris C~dex 0_5 (France) 4 lxaboratoire de G~ochimie et M~tallog~nie (UA CNRS 196), Universit6 Pierre et Marie Curie, 4 place Jussieu. 75252 Paris C~dex 05 (France) .s Laboratoire de G~ophyslque et de G~odynamique interne (UA CNRS 730), Unit;erstt~ Paris XI. 91405 Orsav (_'~dex (France) Office of Marine Geology, U.S. Geological Surt, ev, Menlo Park, CA 94025 (U.S.A.) 7 Laboratoire de G~ologie (UA CNRS 215). Ecole Normale Sup~rieure. 24 rue Lhomond. 75005 Paris. (France) Geophysical Institute, Unioersity of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113 (Japan) Research Institute, Unie,ersio" of Tol~vo, 1-1-1 Yayoi. Bunkyo-ku. Tokyo 113 (Japan) io Department of Liberal Arts, Unieersi(v of Tokyo, 3-8-1 Komaba, Meguro-ku. Tokyo 153 (Japan) i1 IFREMER, Centre Oc~anologtque de Bretagne. B.P. 337, 29273 Brest Cedex (France) l: Department of Earth Sciences, Faculty of Science, Chiba University. 1-33 Yayoi-cho, Chiha 260 (Japan) m ttydrographic Department, Maritime Safe O' Agenc:v, 5-3-1 Tsukqi, ('huo-Ku. Tokyo 104 (Japan) m Geological Survey of Japan, 1-1-3 Yatabe-Htgashi, T~ukuha, lbaragi 305 (Japan)

Revised version accepted October 17, 1986

This paper presents the results of a detailed survey combining Seabeam mapping, gravity and geomagnetic measurements as well as single-channel seismic reflection observations in the Japan Trench and the juncture with the Kuril Trench during the French-Japanese Kaiko project (northern sector of the Leg 3) on the R/V "Jean Charcot". The main data acquired during the cruise, such as the Seabeam maps, magnetic anomalies pattern, and preliminary interpretations are discussed. These new data cover an area of 18,000 km 2 and provide for the first time a detailed three-dimensional image of the Japan Trench. Combined with the previous results, the data indicate new structural interpretations. A comparative study of Seabeam morphology, single-channel and reprocessed multichannel records lead to the conclusion that along the northern Japan Trench there is little evidence of accretion but, instead, a tectonic erosion of the overriding plate. The tectonic pattern on the oceanic side of the trench is controlled by the creation of new normal faults parallel to the Japan Trench axis, which is a direct consequence of the downward flexure of the . In addition to these new faults, ancient normal faults trending parallel to the N65 ° oceanic magnetic anomalies and oblique to the Japan trench axis are reactivated, so that two directions of normal faulting are observed seaward of the Japan Trench. Only one direction of faulting is observed seaward of the Kuril Trench because of the parallelism between the trench axis and the magnetic anomalies. The convergent front of the Kuril Trench is off~t left-laterally by 20 km relative to those of the Japan Trench. This transform and the lower slope of the southernmost Kuril Trench are represented by very steep scarps more than 2 km high. Slightly south of the juncture, the Erimo Seamount riding on the Pacific plate, is now entering the zone. It has been preceded by at least another seamount as revealed by magnetic anomalies across the landward slope of the trench. Deeper future studies will be necessary to discriminate between the two following hypothesis about the origin of the curvature between both trenches: Is it due to the collision of an already subducted chain of seamounts? or does it correspond to one of the failure lines of the America/Eurasia plate boundary?

0012-821X/87/$03.50 .~ 1987 Elsevier Science Publishers B.V. 268

I. Introduction real-time interpretations. Here we summarize mainly these on board interpretations and pre- During Leg 3 of the French-Japanese Kaiko liminary post cruise studies as well. project using R/V "'Jean Charcot", we surveyed the Japan Trench subduction zone [1] which is 1.1. The objectives of the surt, ev associated with the modern seismicity of the The area we surveyed during the later part of northern Honshu island. The detailed survey Kaiko Leg 3 (July 19-29, 1984) includes the covers about 18,000 square kilometers along the northern Japan Trench and its juncture with the trench with Seabeam bathymetry and simulta- Kuril Trench at Erimo Seamount (Fig. 1). The neous observations of the magnetic and gravity front of the landward slope of the Japan Trench, fields. Single-channel seismic records were also once considered a typical convergent margin with obtained. All these data were recorded continu- active accretion, was found from study of the ously along the tracks. Navigation in a Loran-C cores recovered during the Deep Sea Drilling Proj- net provided continuous position information ect (DSDP) Legs 56 and 57 to have an unexpected nominally precise to about 100 m, and onboard history of massive subsidence and probably land- computer and plotting facilities enabled us to make ward retreat of the slope. Accretion was limited to

141°E 142% 143% 144°E 145°E 146°E 43°N H

i i i i 42°N

/

41°N

Hachin(

40~ NOR' tic

39°N

Fig. 1. Lcx:ation of the Kaiko survey (Leg 3, box 1 ) off northeast Japan in addition to the previous multichannel seismic lines: .INOC 1. JNO('2 (Japan National Oil Corporation, 1976). ORI 78-3. ORI 78-4 (Ocean Research Institute and Japan Petroleum Exploration Company, 1978) and P-849 (Shell Oil Company, Beck et al. 1976). DSDP sites (I,egs 56-57 and 87) were also plotted on this map. Isobaths are derived from the bathymetric chart of the adJacent seas of Nippon No. 6301 (Hydrographic Department. Maritime Safety Agency, Japan. 1966). 269

a narrow zone on the lowermost part of the land- and 57 included not only many single-channel ward slope which was not well resolved even by records but also multichannel seismic reflection multichannel seismic records. This area of poor records which crossed the continental slope, and resolution was considered accretionary by some part of the seaward slope of the trench [6,12] (Fig. authors [2] but other authors argued for an origin 1). The accompanying seismic refraction measure- from mass movement [3]. A study of this problem ments indicated a transition from continental to was one of our main objectives. oceanic crust well down the slope and generally The second objective concerned the seaward within 15-20 km landward of the trench axis slope of the Japan Trench. It was here that the [13,14]. The combined seismic data show the sub- horst and graben structure marking the flexure of ducted crust with an oceanic velocity structure ocean crust was first described by Ludwig et al. [4] overlain by a crust with continental velocity struc- and lwabuchi [5]. Linear magnetic anomalies trend ture (Fig. 2). Only a small wedge-shaped area at approximately 50 ° obliquely to the trench axis the front of the margin has velocity in the range of and Honza [6] further proposed a trend of the those found in accretionary complexes. The mid- horst and graben nearly perpendicular to the mag- slope terrace appears to mark a fundamental netic anomalies (25 ° west of north). These trends tectonic juncture between oceanic and continental were debated especially in view of the tendency elements as defined by refraction velocity. The for horst and graben along the Middle America terrace also marks a transition in the character of Trench to parallel magnetic anomalies [7]. seismic reflection records from well organized The third major objective was to explore the coherent reflections to less coherent faint reflec- Japan and Kuril Trench juncture near Erimo tions, or a short reflective sequence and increased Seamount. This already complex juncture is fur- diffractions (Fig. 2). This mid-slope transition was ther complicated by a southward projection of the interpreted as the end of the coherent continental Central Tectonic Belt on the island of Hokkaido section and the beginning of the deformation asso- that passes beyond Cape Erimo into the Pacific ciated with subduction at the front of the margin (Fig. 1). The belt may have been a major plate [15]. Within the continental reflective sequence is boundary either between the American and Eura- a prominent unconformity that truncates land- sian plates or between the Okhotsk and Amurian ward dipping reflections of low amplitude and plates in Neogene time [8-10]. The role of Erimo refraction velocities between 3.6 and 4.8 km/s. Seamount at this juncture is debated from several The unconformity is in turn transgressed by the points of view, because neither the bathymetry nor overlying sequence of strata with velocities be- the seismicity indicate a clear structural relation tween 1.6 and 2.6 km/s. The landward dipping with other major tectonic features. The junction of reflection below the unconformity in layers of the two trenches is also located at the mouth of higher velocity, corresponds to the Cretaceous the so-called Hidaka Trough which is the south- rocks recovered by drilling on Leg 57 at Site 439, ward continuation of the Sapporo-Tomakomai de- whereas the transgressing subhorizontai reflec- pression west of the Central Tectonic Belt in Hok- tions above the unconformity correspond to the kaido (Fig. 1). This depression is filled by a thick Neogene sequence [16]. Drill cores recovered lith- accumulation of recent sediments [11] supplied by ologies and benthic microfossils that establish the rivers from the Hidaka Mountains. This strong sub-aerial origin of the unconformity which can detrital supply should affect the morphology of now be seen in the seismic records at depths the trench in the area of the Erimo Seamount. greater than 5 km [15]. We focus our report of preliminary results on The Neogene tectonic history of the Japan these three problems and present some of the Trench landward slope is dominated by massive interesting discoveries from the data acquired in subsidence occasionally associated with arc this initial stage of our study. volcanism and subduction until about 5 m.y. ago, and one of uplift for the last 3 m.y. The rate of 1.2. Background from the precious studies vertical motion near shore is much less and per- The geophysical data across the Japan Trench haps negligible. This history is based on the litho- obtained in preparation for the drilling on Legs 56 logic and paleontological sequences recovered both 270

,~ tondword slope ~< sectword stope

TYPIEAL v/ / ~,d-slopo torroco If TOPOGRAPHIC ~,1, zowe~ s~ope I PROFILE ~ ~+ REEORDED \ T ---~...._ ~0~p,~,~t DURINGKAIKO-ERUISE ~ ~/ VE=/+X ~ 'SechOS of OR/ zs- ~

~ :,<~.~ ..... _ -'-"-~2_"- J ~ ~ - ~ . ~ I v q) £E 10 Ill I-- LU o _J ~. 2o

moho 8.0 f,.~8.6 ° VE -- 4.25X 30

Fig. 2. Composite seismic refraction data (modified by ,,'on Huene et al. [15] after Murauchi and Ludwig [13]) and typical topographic profile corresponding to one survey line along 40 ° 07'N.

from drill holes near shore and also near the edge The seaward slope of the trench is composed of of the deep-sea terrace. The temporal boundaries crust with a Cretaceous magnetic anomaly pattern located by mean of the drill holes were followed in that crosses the trench obliquely [18]. Honza [6] seismic records to show the seaward increase in proposed a trend of the horst and graben per- subsidence corresponding to the increase in depth pendicular to the anomalies on the basis of a of the subaerial unconformity cutting the Creta- series of single-channel seismic reflection records. ceous. The subsidence was like a large flap with a Off Mexico and Guatemala, however, horst and quasi-stable hinge point near the present shoreline graben follow the magnetic anomalies, that is to and an edge that subsided most rapidly near the say the original spreading structure of the . Subsidence resulted in the progressive sea- crust [7]. The topography conforming uniformly ward shift of the Neogene depocenter. Most of the thick (560-650 m) sediments on the ocean crust sediments sampled was a monotonous sequence of consists of Cretaceous cherts disconformably over- silt and claystone of hemipelagic origin with rare lain by Eocene to Miocene pelagic clay and then a inter-beds of sand turbidites. Thus it is presumed Neogene sequence of hemipelagic sediment. The that despite Neogene subsidence, conditions of 1933 Sanriku earthquake occurred on a normal sedimentation remained much the same except fault seaward of the axis of the trench [19]. near the trench where tilting and truncation of the Erimo Seamount, by its position at the Japan Pliocene and Pleistocene section is shown by and Kuril Trench juncture (Figs. 1 and 3), invokes seismic and drilling results. A progressive mi- suggestion of a tectonic relation between the crofracturing of the cored sediments whose origin seamount and the juncture [20]. On the other is thought to result from hydrofracturing as a hand, the seamount could also be a passenger of consequence of overpressured pore fluids in- the subducting oceanic plate. The range of specu- creased progressively toward the trench [15,17]. lation regarding the role of the seamount is broad 271 and can only be reasonably evoked after treatment IIA, map 4; Plate liB, diagram 4). commonly of the data because the juncture area shows a exhibiting ponded sediments, local ridges and complex topography not resolved by reconnais- closed uplifts and depressions. Local constrictions sance survey. Nor do the seismological data pre- by lobate masses are possibly slumps. Slumps sent a systematic pattern of dynamic parameters were indicated in the core of site 440 in the except for a broad curvature in hypocentral depth ponded unit of poorly sorted clayey sand and contours in the upper seismic plane [21]. The gravel [24]. The age of the ponded sediments is sharpness of the change in trend of the subduction Holocene to Upper Pleistocene, maximum age zone at this juncture is a feature of large propor- being 0.26 Ma. The mid-slope terrace has trapped tion and puzzlement. material transported down-slope and has probably formed a resting place for local slumps. The ter- 2. The Japan Trench race marks a fundamental change in the mor- phology of the slope from the smooth landforms 2.1. Landward slope morphology, Seabeam data of the continental terrace to the first clearly linear The excellent conventional bathymetric maps elements parallel to the trench. already published by Japanese institutions [22,23] was detailed by our Seabeam survey. Although The adjacent terrane immediate down-slope from the many details of individual echosounder profiles terrace is the least disrupted part of the lower suggest various tectonic features, the two-dimen- slope. Here the Seabeam contours trend generally sional Seabeam maps clearly define a morphology north, show a 9 ° dip, and are locally deflected by produced by the modern Japan Trench subduction shallow re-entrants and small highs (Plate IIA. zone. The general morphological subdivisions de- map 4; Plate liB, diagram 4). Several short veloped from seismic records and single bathymet- "canyon-like" features trend down-dip, suggesting ric profiles are characterized in greater detail and down-slope transport of sediment. Notable is the on the landward slope of the trench: an upper lack of a strong structural grain in comparison slope, mid-slope terrace and lower slope are easily with the adjacent areas. distinguished. The lower slope is further char- A dominant morphological features of the acterized by an upper part, an escarpment, and a Seabeam map is a sinuous escarpment at about peculiar base of slope area (Figs. 2, 3; Plate IIA, 6000 m depth that trends subparallel to the trench map 4; Plate liB, diagram 4). axis (Fig. 3; Plate IIA; map 4; Plate liB, diagram 4). The strong continuity of the escarpment is Above the mid-slope terrace is the relatively broken toward the southern part of the map and smooth morphology of the upper slope. Its feature- although clearly defined in four of the five multi- less character and average 3 ° dip is consistent with channel seismic records crossing the slope, the the morphology in the conventional maps [22,23]. principal record used to locate DSDP site, JNOC The transverse structure displays a well rounded 1, is located on one of the gap (Fig. 1). The scarp topography rather than sharp offsets. Transverse has a remarkably uniform width of about 2 km, an features affect the mid-slope terrace causing an average slope of 20 ° and a length of about 120 apparent constriction or a down-slope step. Other- km. Locally it is 1 km high. wise only small straight rills trending directly Between the scarp and the trench axis is an down-dip cross the part of the slope that was area of generally disorganized topography (Plate included in the Seabeam survey. A notable aspect I1A, map 4; Plate liB, diagram 4). Closed depres- of the upper slope is the absence of a morphology sions, hills and lobate bodies suggest slumps. A that reflects tectonic features. This is in stark locally strong but discontinuous linear topography contrast with the disrupted morphology that is developed near the trench axis and parallels it; a dominates the regime below the mid-slope terrace. steep front commonly bounds the axis. This mor- phology is about 10 km wide in the south of the The mid-slope terrace is a nearly continuous fea- surveyed area and 38 km at its widest part in the ture across the Seabeam map at a general depth of north (Fig. 3) as the escarpment and trench axis 4400-4500 m. It is of variable width (Fig. 3; Plate diverge northward. 272

EI~ °

E~

N41 o '1~ I a p CONTINENT - _L~ j. _l PLATE ~RG N

, / //

Legend

J main thrust 7" major escarpments /7 normal faults J strike slip or subverbcal faults

N40 ° seamount contours Ufilled basins ~i?& • ~slumps

'i 0 10 20kin f ,U mm

Fig. 3. Tectonic map dra~n from the Kaiko results. See Fig. 1 for Zocation. 273

The trench axis trends increasingly east of north axis. The association of the pile. with the scarp toward Erimo Seamount (Fig. 3); its juncture with suggests that it consists of reworked slump debris the slope is sinuous on the scale of about 5 km. now being deformed at the trench axis. In support Ellipsoidal closed depressions, rarely more than 3 of this interpretation is the lack of thick sediment km wide, are commonly separated by 5-20 km ponded in the trench axis to serve as material for narrow stretches along the trench floor. A profile accretion. of depth along the axis shows a variability in At the base of the stratified sequences is a depth of about 150 m and a general southward section of subducting sediment which is not dis- deepening of 600 m in about 3.5 ° latitude. rupted from the seaward slope of the trench for about 40 km down the subduction zone. The 2.2. Landward slope geophysical data continuity of the subducted oceanic sequence at Magnetic anomalies (Fig. 4) observed onboard the front of the margin indicates relatively low confirm the pattern reported previously [15,25] friction along the subduction zone. From the and show in detail the N70°E trending magnetic mid-slope terrace to the trench is an aseismic zone anomalies M8, M9 and M10 of Cretaceous age. based on observations using a local network of The age of the anomalies increases southward ocean bottom seismometers [29,30]. This suggests paralleling the increasing depth of the ocean floor decoupling of the upper and lower plates in the and trench axis. No lateral displacement along the front of the subductioaa zone consistent with the trench, inferred by Hilde et al. [18,26], is observed low compressive stress suggested by the massive in the present survey. The anomalies extend land- failure of the lower slope [31]. ward of the trench axis and decrease in amplitude as reported previously [27] (Fig. 4). Gravity free 2.3. Seaward slope morphology and geophysics air anomalies show a north-south trending trough The oceanic plate was surveyed from the trench just landward of the trench axis (Fig. 5). The to 50 km eastward; in addition, four longer lines deepest negative anomaly at the northern end of (two bands, 8 km wide for each, between 39°40 the Japan Trench does not correspond to a topo- and 39°50 of latitude) were surveyed 110 km graphic feature and probably indicates crustal de- seaward of the trench axis in order to precisely pression. define the bending of the oceanic plate prior to All but one of the 5 multichannel seismic rec- subduction beneath the continental plate. ords previously reported in this area [3,12,28] dis- play the morphological elements detailed by The horst and graben structure. Faults were first Seabeam. The sequence of eroded Cretaceous rocks defined in single-channel seismic records recorded covered by Neogene slope sediments is disrupted simultaneously with the Seabeam data. These at the mid-slope terrace by structure not clearly faults were first plotted on topographic profiles resolved in the best processed seismic records. (given by the vertical beam) and subsequently on Generally few coherent reflections are resolved the Seabeam map where they corresponded to well from the mid-slope terrace to the trench axis, but defined scarps (Fig. 7). Shipboard maps indicated a recently reprocessed diplay of ORI 78-4 [3] an organized horst and graben structure and an shows the structural configuration at the front of increase in the number of landward facing normal the subduction zone [28]. A line drawing of the faults toward the trench. Normal faults first occur seismic reflection record section (Fig. 6) shows the at the bulge which is well developed off the Japan escarpment to be a slump scar about 1 km high Trench [32]. No faults with large vertical offset truncating a nearly horizontal reflective sequence. were recognized, during our survey in the ocean This reflective sequence is a part of coherent block basin, more than 50 km seaward of the trench that comprises the lower landward slope of the axis. They are subparallel to the Japan and Kuril trench. In front of the escarpment is a thick pile of Trench axes respectively, and trend N10°E (plus sub-horizontal short reflections. The structure of or minus 20 o ) in the south, to N60 o E ( _+ 10 o ) in this pile is not that of the landward dipping reflec- the north, with an intermediate zone south of the tions in an accretionary complex except at the Erimo Seamount (Fig. 3). The most prominent steep slope immediately adjacent to the trench scarp is located south of 40 ° in latitude and 274

Ei44" E144°30 ' : :ii ~. E145"

Main ,,,-,,-,5~ eh,~u,~

I

• N41 ° ~---- LINE~ rimo Seamount I

~777.~.2,j52.52,2,:::::::::::::::::::::::::::::::::::::::::::::::: 2 5?52 52"552.?52 I+2+ .• '..- • 2'2 ~';•;.'.

. I00

. • •....•••. • •.•...•.•.

N ~30'-- -- .5~'2" M8" Legemd

~ Negative values

~ Er~mo seamount ~- col3tours 6~lmlP Main thrust

J Normal faults

f Isogammes i::i::i:iiiiii::ii

N 40° .....

300

~0 ..?.'

I:ig. 4. Total magnelic field isoanomaly map, u~it.s in nanole~,las. I.ocalion of line B, referred to in section 3..-1. i~, indicalcd. 275

E144" E~'30' ;-'.~.:.'~EI45"

N41"

t t

.:.-.: LINE B0

I. o, Legend

~ FAA >-50 mgal

\ ~FAA < -IS0 n~al

~+ Erimo seamount contours f Main thrust

,; / Normal faults

f Isogals

• .+, ++ +~° ', J ~r a ~ ...% • ++. "I,

Fig. 5. Free air anomaly map, units in milligals. Location of line 80 (see Fig. 10) is indicated. 276

6 - W , 1 , , ...... , , ~ , E___L

~ • - -~. --. __ .... MIGRATED SECTION - TIME

--~ - - VE 2.4

8 -- scar.p......

.Oa°,reno. . ~;" axis . -

1 0 " - " - " '-~"~

12 s-e-c subsurface / ~ ~ -~-..=-;--~'~-'--~ continuation ""-"-"-"-"-'J~;'*"~--~---~"-~-_--~-'----- of scarp . frontal 6 -- ~,~-/~ ~--- ~---- "- / Slump debris thrust ~"--- --~ V.-~.,*~ ~ faults

"Usi~liimUe

oecoliemenl ---~_~. -- -"ri-ii~/'Si~_l ~ ,

subd,cted ,.-~~~__~" / ' d IGRATED SECTION - DEPTH

sediment --12 ~~ ~ faults • km thrust fault ?

Fig. 6. Reprocessed seismic record section ORI 78-4 (location is shown in Figs. 1 and 2: from [28]).

20-40 km east of the Japan "French axis. This 3 and 4) and probably reflect the original grain of NI0°W trending scarp corresponds to a normal the oceanic crust which is inherited from the fault which shows a vertical displacement of 300 spreading axis. The direction of these lineaments m on the basis of seismic data, creating a 12 ° is very close to that of the Kuril trench so as to be slope facing the trench. It is more than 60 km long mingled with the horst and graben structure in the (35 km are shown in Plate IIA, map 4 and its north. That is to say, the Cretaceous tectonic southward extension was mapped during the re- lineaments are probably rejuvenated near the turn transit survey). The sediment blanket does trenches inducing normal faults in the same direc- not appear to significantly mask the ocean floor tion as the magnetic lineations when the trench displacement above faults indicating recent dis- axis strikes subparallel to them. placements. We also observed an increase in dis- placement of normal faults toward the trench, Basement morpholoyo,, lsobaths of the top of the although the extent and vertical displacement in igneous oceanic crust illustrate well the undula- the north are somewhat less than in the south. tions of basement in the directions of the magnetic anomalies. They also show that the ocean crustal The N65°E lineaments. Lineaments with a N65°E flexure into the trench begins approximately 100 strike are noted in stepwise offsets along ridges. km seaward of the trench axis, reaches 3 or 4 ° at They are subparallel to the direction of the Lower the axis, and continues to steepen about 100 km Cretaceous magnetic anomalies M8 to M10 (Figs. down the subduction zone where Murauchi and 277

KAIKO_ LEG 3 1984 VERTICAL EXAGERATION -~ d. KURIL TRENCH ERIMO SEAMOUNT

--L ~...... - .- .., .... / ~~

100 km

3APAN TRENCH Fig. 7. Perspective diagrams of the surveyed area drawn using the Seabeam map and the seismic profiles.

Ludwig [13] showed a 8-9 ° angle (Fig. 2). In layer consists of diatom mud and the lower one is detail, the average dip of the oceanic crust is 1.5 ° made with biogenic oozes and clay [33]. The aver- 30-40 km seaward of the trench axis and varies age thickness of both layers is about 600 m and from 3 ° in the south to 5 ° in the northern Japan the variations are mainly associated with the horst Trench axis due to the presence of the Erimo and graben morphology. That is to say, some Seamount on the ocean crust. That is to say, the erosion of the upper layer occurs above the horsts flanks of the seamount, which are steeper than the and the eroded material fills grabens. The thick- oceanic crust slope, can be followed under the ness of trench fill sediment is generally less than landward slope. 300-400 m in the elongated axial basins, but it reaches 600-800 m in the Kuril Trench axis where Sedimentary cooer. The top of the Cretaceous chert considerably more sediment is ponded (Fig. 8). layer is well defined on the single channel seismic profiles, but it is quite impossible to distinguish Magnetism and grauity. The map of the magnetic the top of the oceanic igneous crust even in well total field isoanomalies (Fig. 4) shows a very clear processed multichannel seismic data. The upper WSW-ENE trend. The peak to peak amplitude is OC W S

> ,~r-

J] .... '~"1 h "~ / ~1 .. f /

w~ N.E. JAPAN PACIFIC PL ATE

--.~ ...m., ,.o.,. >%\

I~^^1 VoJcani¢ ,ocks ~ -- ~ "~--"~ /~.~--~.-----

KURIL TRENCH

MID - SLOPE -"x ."', [] L20 ERINO TERRAC6 ~ / SEANOUNT ' i'----,\ " 0',~.."": "~'~--,. \ ESCARPMEN'F'_,x:--~=.. O .~ t/'~~- ~ "*t~ e--_~ ~ I "',2 ° "~~-"~'~' ~-~ --_~ ~ ~, I .° ~~ ' JAPAN Co,. ~. \ - -zlo= - .,' ~ ,' ...... ; TRENCH

.-.~

l'ig. g. Serial cross sections of box 1 interpreted fronl .,,ingle-channel ~,eismic profiles. 279 about 700 nT. Magnetic lineations M8 to M10 Japan Trench. It is bounded to the southwest by a (about 130 Ma), caused by normal and reversed very steep scarp perpendicular to the direction of polarity successions recorded in the remanent the trenches suggesting that the structure of the magnetization of the Pacific plate, are recognized. continental margin is here cut by a steep NW-SE The plate being younger northward. No lateral fault (Figs. 3 and 7, plate IIA, map 4). displacements along transform faults appear in the surveyed area. The general trend of gravity Landward slope. The morphological pattern of the anomalies well conforms to the classical model of landward slope changes abruptly at 40°40'N. free air anomalies over an oceanic subduction South of this latitude the north-south structural zone: values near 0 on the oceanic lithosphere and trend characteristic of the Japan trench landward a minimum of - 139 mgal (in the south) to - 175 slope is still clearly observed; north of this latitude mgal (in the north) shifting landward of the trench the north-south trend disappears giving way to a axis (from 4 km in the south to 19 km in the complex morphology without any prominent di- north). As in the Daiichi Kashima area (see rections. Generally, the mid-slope terrace which Kobayashi et al. [43]), the absolute values of grav- was so obvious in the south is absent here. ity observed are lower than predicted for a 130 Numerous erosional features are observed in the Ma old subducting oceanic lithosphere [34]. On Seabeam map such as canyon running on the the oceanic side the isoanomaly curves follow the slope, or well rounded depressions. The disap- same eastward curvature as the isobaths, whereas pearance of the Japan Trench character occurs at their orientation is approximately north-south on the mouth of the Hidaka Trough which supplies the landward slope. We interpret this observation detrital material from the nearby Hidaka Moun- in the following way: as the trend of the flow-line tains. It is thus likely that the structural trends of of the Pacific plate diverges eastward away from a the Japan Trench are hidden below detrital north-south line of reference, its geometrical junc- material. This complex morphology ends abruptly tion with the deepens. This deepen- northeastward against the NW-SE fault scarp. ing can be observed on free-air anomaly minimum Only a small part of the Kuril Trench landward trend (Fig. 5), but not on trench axis depth which slope has been mapped and the topography re- follows the oceanic isobaths. veals a very steep scarp parallel to the trench as it was earlier emphasized by Savostin et al. [35] 3. Trench juncture, Erimo area based on single-channel seismic profiling.

3.1. Morphologr', Seabeam data Seaward slope. The Erimo Seamount occupies a The Seabeam map at the junction of the two large area of the seaward slope. Although no large trenches shows drastic morphological differences. faults were directly observed affecting the We describe successively the trench axis, the land- seamount as for the Daiichi Kashima Seamount, ward slope and seaward slope. they may exist but they are less developed than in the Kashima area. Around the seamount the oc- Trench axis. Southwest of the Erimo Seamount the eanic crust is dissected by normal faults which trench is characterized by a sequence of small display two main directions. South of the seamount elongated basins in a zig-zag pattern, in which the the transition from faults parallel to the Japan trend of the faults parallel to the Japan Trench Trench to those parallel to the Kuril Trench is and the trend of the faults parallel to the oceanic observed. In the transition area, fault scarps with magnetic anomalies alternate (Fig. 3). Basins nar- zig-zag pattern control the outline of the trench row and disappear when approaching the basins. seamount, being squeezed between the seamount and the landward slope. There the trench is re- 3. 2. Seismic data stricted to a pass only 6250 m deep. Northeast of Closely spaced single-channel reflection pro- the seamount the trench suddenly widens into a files records were good on the seaward slope and very large and flat-floored basin (7100 m deep) in the trench (Fig. 8). Normal faults which corre- strongly contrasting with narrow basins of the spond to scarps on the Seabeam map clearly offset 280

the sedimentary sequence. When approaching the (O) Calculated // seamount, the thickness of the upper sedimentary sequence decreases and finally disappears on the seamount. The Kuril Trench is filled by an ac- 41°00 '- cumulation of highly reflective well stratified sedi- ments, probably turbidites, resting unconformably on the oceanic deposits. The thickness of the sediments is 0.5 s (two-way travel time) or 500 m assuming a velocity of 2 km/s for the sediments.

3.3. Magnetic data Fig. 4 shows the anomalies of the total geomag- netic field around the Erimo Seamount.

As in the southern part, oceanic anomalies 40040 ,. ~ "~o striking N70°E pass below the landward slope. 144°40 ' 5'0' 145~00 ' 1'0' The magnetic signature of the Erimo Seamount consists of a pair of anomaly centers superim- posed on the oceanic anomalies. The positive center lies on the positive anomalies M8 (120 Ma) and the negative center lies on the negative anomaly north of M8. This observation suggests that the Erimo Seamount and the oceanic plate have the same magnetization. This is an indication that the Erimo Seamount would have been em- placed close to the spreading center. Another pair of anomaly centers is observed at the corner be- tween both trenches suggesting the presence of an already subducted seamount, as confirmed by seismic records. The three components of the magnetization of the Erimo Seamount and a planar regional field have been computed by inversion of the measured (cJ Depth(m) magnetic anomalies. The tested model which best 4000" fits the observed anomalies involves an identical magnetization for the seamount and the support- 5000" ing plate and a non-magnetic cap on the top of 6000 the Erimo Seamount (Fig. 9). This non-magnetic cap is due to the presence of a coral reef. The 7000. depth of its base corresponds to the topographic Fig. 9. (a) Computed magnetic anomalies created by the model bench around 4600 m. The declination, inclination of Erimo Seamount (magnetization: declination is -16 °, in- and intensity of the magnetization are -16 °, 32 ° clination is 32 ° and intensity is 1.1 ×10 -2 emu/cm3). (b) and 1.1 × 10 -2 emu/cm 3, respectively. The calcu- Residual anomalies. (c) Model. lated position of VGP, 63°N, 50°W agrees with an age of 120 Ma on the Pacific apparent wander path [37,38] (Fig. 10). The northern anomalies can 3.4. Gravity data be well explained by another seamount with nearly The free air anomaly (FAA) map is shown in the same magnetization as Erimo Seamount, which Fig. 5. The minimum associated with the trench is was already subducted at the trench: its summit largely shifted (20 km) landward of the trench would now be at the corner of the convergent axis. This is due to a large sedimentary infilling of front north of the Erimo Seamount. the trench in the junction area. As expected, the 281

A

- 70

\'\\ ,\ \ \ ~ .30°~

......

Fig. 10. Location of VGP of Erimo Seamount (63°N, 50°W) on the Pacific apparent polar wander path. The solid curve is the apparent polar wander path of Cox and Gordon [38]: the dotted line is that Sager (1983) and the dashed line, that of Gordon (1983) (from Sager and Keating [37]). The 95% confidence regions surrounding the mean paleomagnetic poles are shown as ellipses, and the numbers within the ellipses arc the mean ages of the poles in Ma. Map projcction is polar equal area.

gravity anomaly of the Erimo Seamount is closely implies that the Erimo Seamount has been em- correlated with its topography. Isogals give an placed on a lithosphere whose age was between 0 indication of the seamount morphology in the and 3 Ma. Therefore it appears that the Erimo area where the seamount is already under the Seamount has been formed close to the spreading sediments of the inner slope. center. Gravity anomalies have been interpreted in Results of K/Ar datation give an age about 90 order to determine the emplacement age of the Ma [36] which is in apparent contraction with our seamount [39]. Once it has been formed, a proposed age of 120 Ma. But some ambiguities in seamount acts as a load on an elastic plate. The the values of this datation remain due to the deformation of the plate is controlled by its elastic excess argon and post-magmatic oxidation effects. thickness Te at the time of loading [40]. It has Thus this datation would be a lower value for this been shown that the elastic thickness is related to age. the age of the lithosphere [40]. Theoretical gravity anomalies created by the 3.5. Discussion on the junction topography of the seamount and the flexure of the The Seabeam topography shows clearly, first a lithosphere have been computed using a three-di- NW-SE left-lateral offset (transform fault?) by mensional method. Fig. 11 displays the computed about 20 km of the convergent front in the Kuril anomalies along line 80 for different values of To. Trench relative to the Japan Trench. Second, the Observed gravity from which a regional field (line trench axis trends N25°E south of the previous B in Fig. 4) due to the subduction zone has been tectonic boundary and N55°E north of it. Third, removed is also shown in Fig. 11. The best fit is the Erimo Seamount is located immediately south obtained for an elastic thickness of 2 km. This of this tectonic boundary, but an already sub- 282

FREE-AIR ANOMALY Mgal NW SE

100 Te : 20 km Te : 10 km Trench axis[ ///~/ ~ kin/--...... ----- Te : 35 krn

//// ~x'~"Y~ Te: 2krn -- Te : 0 km 50

effect i~

0

LINE 80 * Observed Gravily Anomaly

Fig. 11. Computed gravity anomalies created bv the topograph?, and the flexure of the lithosphere for different values of the elastic thickness (7[.). compared with the observed anomalies. The location of line 80 is shown in Fig. 5.

ducted seamount (less important than Erimo) is Ncogene plate boundary between America and observed (on the basis of magnetic anomalies and Eurasia [8]. seismic profiles) at the upper corner of the trench juncture. Are these results sufficient to say that 4. Summa~ ~ and conclusions the sharp curvature of the trench axes is due to the collision of a possible chain of seamounts pre- The detailed survey made by the R/V "Jean ceding Erimo Seamount, according to the theory Charcot" in the Japan Trench and its junction of Vogt et al. [20]? with the Kuril Trench during the French-Japanese Keeping in mind that our survey includes only Kaiko project (northern part of Leg 3) brings the frontal zone of the continental plate and also numerous new insights on one of the best known only the southernmost part of the Kuril Trench, it trenches, especially about the interactions between seems very difficult to clarify this problem without the continental margin and the oceanic crust. taking into account the adjacent areas, especially (1) Sediment sliding in the trench: Mass sliding northward and eastward of the present survey. occurs on the landward slope of the trench, fills It seems that the oceanic plate is not concerned the axial zone of the trench and reduces it to a by this curvature, because the normal faults are succession of short and narrow basins which are parallel to both trenches with an intermediate offset on the oceanic side by faults trending ob- zone south of the Erimo Seamount, where we can liquely to the trench axis. Thus the topographic observe simultaneously both directions: N10°E Japan Trench is only the frontal boundary of the and N60°E. Furthermore, if the collision is active sedimentary gravity sliding. These slide deposits now, it has to be confirmed by the seismicity in are then probably subducted together with oceanic this area. crust and its cover. Anothcr possibility is to relate the "transform The Japan Trench could be a model for tectonic fault" to a failure line of a present boundary erosion linked with subduction: the continental between the Japanese microplate and the Okhotsk slope is being destroyed by superficial collapse plate [41], but there is no evidence of in- and the resulting material may ultimately be car- tracontinental seismic activity in the assumed ried down the subduction zone. The continental boundary zone (between Erimo Cape and Erimo plate has thus retreated. In addition, it is likely Seamount). Concerning the curvature of trenches that erosion of the continental margin from below near Erimo Seamount, it may be due to a different by the subducting oceanic plate occurs [42]. tectonic behaviour of plates on either side of the (2) Newly created faults and reactivated faults 283 on the oceanic side: The oceanic crust is cut into reflection data, Legs 56 and 57, Japan Trench transect, horst and graben with normal faults parallel to the DSDP, in: Initial Reports of the DSDP, 56-57, Part 1, pp. 489-504, U.S. Government Printing Office, Washington, trench. Some of the faults are associated to linea- D.C., 1980. ments corresponding to the original structural 4 W.G. Ludwig, J.l. Ewing, M. Ewing, S. Murauchi, N. Den, trend of the oceanic plate because they parallel the S. Asano, H. Hotta, M. Hayakawa, T. Asanuma, K. magnetic anomalies oblique to the trench. Where Ichikawa and I. Noguchi, Sediments and structure of the the oceanic plate structural trend is highly oblique Japan trench, J. Geophys. Res. 71, 2121-2137, 1966. 5 Y. Iwabuchi, Topography of trenches east of the Japanese to the axis of flexure, the normal faults break islands, J. Geol. Soc. Jpn. 74, 37-46, 1968 (in Japanese with across the structural grain of the oceanic plate and English abstract). the older fault set remains cryptic. Such faults 6 E. Honza, Pre-site survey of the Japan trench transect, seem to have long been truncated and buried DSDP, in: Initial Reports of the DSDP, Legs 56-57, Part 1, beneath the oceanic sediments and then were re- pp. 449-458, U.S. Government Printing Office, Washing- ton, D.C., 1980. juvenated when the oceanic crust underwent large 7 J. Aubouin, R. von Huene et al., A summary of DSDP Leg tectonic stress due to the bending of the plate at 67, Shipboard results from the mid-America Trench tran- the subduction zone. sect off Guatemala, 26th Int. Geol. Congr., 7-17 July 1980, (3) Trench junction: At the transition with Kuril Paris, Oceanol. Acta, Spec. Issue 4, 225-232, 1981. Trench, a left-lateral transform fault offsets the 8 M.E. Chapman and S.C. Solomon, North American-Eura- continental slope and leads to a wide flat plain in sian plate boundary in northeast Asia, J. Geophys. Res. 81, 921-930, 1976. the axial zone strongly contrasting with the small 9 G. KJmura and K. Tamaki, Collision, rotation and back-arc basins of the northern Japan Trench. The Erimo spreading: the case of the Okhotsk and Japan Seas, Tecton- Seamount, located slightly south of the junction, ics 5, 389-401, 1986. may belong to a chain, already subducted, re- 10 L. Jolivet and J.P. Cadet, Mouvements d~crochants, sponsible to the trench curvature by collision. This structuration et limite de plaques dans rile d'Hokkaido (Japon septentrional), Ann. Soc. Grol. Nord CIII, 345-352, trench transition may also be related to the in- 1984. tracontinental boundary between America and 11 Y. Ishiwada, Petroleum geology of the continental terraces Eurasia plates. Nevertheless, no definitive conclu- surrounding the Japanese islands, J. Jpn. Pet. Inst. 18, sion can be given at present without a deeper 460-465, 1975 (in Japanese). 12 N. Nasu, Y. Tomoda, K. Kobayashi, H. Kagami, S. Uyeda, study of focal mechanisms of in this S. Nagumo, I. Kushiro, M. Ozima, K. Nakamura, H. Okada, area and north of it. S. Murauchi, Y. Ishiwada and Y. Ishii, Multichannel seismic reflection data across the Japan Trench, Ocean Res. Inst., Acknowledgements Univ. Tokyo, IPOD Basic Data Ser. 3, 22 pp., 1979. 13 S. Murauchi and W.J. Ludwig, Crustal structure of the Thanks are due to the Captain, Alain Girard, Japan trench: the effect of subduction of the ocean crust, in: Initial Reports of the DSDP, Legs 56-57. Part 1, pp. and to the crew of the R/V "Jean Charcot" for 463-470, U.S. Government Printing Office, Washington, their support. We also thank the reviewers for D.C., 1980. their thoughtful comments. The figures were pre- 14 S. Nagumo, J. Kasahara and S. Koresawa, OBS airgun pared by Annie Bourdeau and Jacques Brouillet. seismic refraction survey near sites 441 and 434 (J-IA), 438 and 439 0-12), and proposed site J-2B; Legs 56 and 57, DSDP, in: Initial Reports of the DSDP, Legs 56-57, Part 1, References pp. 459-462, U.S. Government Printing Office, Washing- ton, D.C., 1980. 1 J.P. Cadet, K. Kobayashi, J. Aubouin, J. Boul~gue, J. 15 R. von Huene, M. l.angseth, N. Nasu and H. Okada, A Dubois, R. yon Huene, L. Jolivet, T. Kanazawa, J. Kasahara, summary of Cenozoic tectonic history along the IPOD K. Koizumi, S. Lallemand, Y. Nakamura, G. Pautot, K. Japan trench transect, Geol. Soc. Am. Bull. 93, 829-846, Suyehiro, S. Tani, H. Tokuyama and T. Yamazaki, De la 1982. fosse du Japon b. la fosse des Kouriles: premiers rrsultats 16 Shipboard Scientific Party, Sites 438 and 439, Japan deep de la campagne oc~anographique franco-japonaise Ka'iko sea terrace, Leg 57, in: Initial Reports of the DSDP, Legs (Leg III), C.R. Acad. Sci. Paris, S~r. 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