Earth and Planetary Science Letters, 83 (1987) 243-256 243 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

[41

Trench triple junction off Central Japan--preliminary results of French-Japanese 1984 Kaiko cruise, Leg 2

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i IFREMER, Centre Ocbanologique de Bretagne, B.P. 337, Brest (France) 2 Earthquake Research Institute, University of Tokyo, Bunkyo-ku, Tokyo 113 (Japan)

3 D~partement de G~otectonique, Unwersit~ Pierre et Marie Curie, 4 place Jtt~sieu, 75230 Paris (France) 4 Laboratoire de G~ophystque, Universit~ de Paris Sud, 91405 Orsay (France)

_s Qcean Research Institute, University of Tokyo, 1-15-1 Minamtdai, Nakano-ku, Tokyo 164 (Japan) D~partement de G~ologie, Ecole Normale Sup~rieure, 24 rue Lhomond, 75231 Paris (France) z Department of Earth Sciences, Chiba University, 1-33 Yayoi-cho, Chiba 260 (Japan)

Laboratmre de G~ologie structurale, Universtt~ des Sciences et Techniques du Languedoc, 34060 Montpellier (France) 9 Department of Geology, Kyushu University, llakozaki, l-'ukuoka 812 (Japan)

to International Institute of Seismology and Earthquake Engineering, Building Research Institute, Ministr)' of Construction, Tsukuba, Ibaraki 305 (Japan)

H Department of Earth Sciences, Toyama Unit, erstty, Gofuku 3194, Toyama 930 (Japan) t2 Marine Geology Dicision, Geological Survey of Japan, Tsukuba, Ibaraki 305 (Japan)

J~ Laboratoire de Tectonophysique, Universit~ de Nantes', 44072 Nantes (France)

Revised version accepted October 17, 1986

Leg 2 of the French-Japanese 1984 Kaiko cruise has surveyed the trench triple junction off central Japan, where the Japan, Izu-Bonin and Sagami Trenches intersect. The Izu-Bonin Trench is deeper than the Japan Trench and filled by a thick turbiditic series. Its anomalous depth is explained by the westward retreat of the edge of the northwestward moving Philippine Sea plate. On the contrary to what happens in the Japan Trench. horst and graben structures of the Pacific plate obliquely enters the Izu-Bonin Trench, suggesting that the actual bounda~" between these two trenches is located to the north of the triple junction. The inner wall of the Izu-Bonin Trench is characterized in the triple junction area by a series of slope basins whose.occurrence is related to the dynamics of this area. The northernmost basin is overthrust by the edge of the fore-arc area of the Northeast Japan plate. The plate bounda~" is hardly discernible further east, which makes it impossible to locate precisely the triple junction itself. These features suggest that large intra-plate deformation occurs there due to the interaction of the plates involved in the triple junction and the weak mechanical strength of the wedge-shaped margin of the overriding plates.

1. Introduction

Leg 2 of the F rench-Japanese 1984 Ka iko cruise, on boa rd R / V " Jean Charcot" , has surveyed about 30,000 km 2 of sea floor off Cent ra l J apan (Fig. 1). The two first targets, the Suruga and Sagami Troughs , which are the eas tward con t inua t ion of the Nanka i Trough, are descr ibed in a c o m p a n i o n pape r [1]. The third target was to under s t and the

* Co-chief scientists.

0012-821x/87/$03.50 ~., 1987 Elsevier Science Publishers B.V

structure, dynamics and evolut ion of the tr iple j unc t ion off Cent ra l Japan (named Off-Boso tr iple j unc t ion by the Hydrog raph ic Depa r tmen t of the Mar i t ime Safety Agency of Japan) , which is the only known example of a TTT(a) - type tr iple junc - t ion [2]. We descr ibe herein the main results ob- ta ined dur ing the survey of the tr iple junc t ion area and p ropose p re l iminary in terpre ta t ions .

The Sagami Trough, which is the b o u n d a r y be tween the Phi l ippine Sea (PHS hereafter) and the Nor theas t J apan (NEJ hereafter) plates, jo ins

244

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the Izu-Bonin and Japan Trenches to form the trench triple junction. The Pacific (PAC hereafter) plate subducts beneath the NEJ and the PHS plates toward west-northwest and west, respec- tively [3,4] (Fig. 2). The direction of relative mo- tion between the NEJ and the PHS plates near the junction depends on whether the NEJ plate be- longs to the Eurasian or to the North American plate or is an independent microplate [5,6]. In the former case, the direction of convergence is west- northwest [4] which is almost parallel or highly oblique to the trend of the Sagarni Trough. In the second case, it is northwest, which implies a larger rate of convergence across the trough.

2. Previous data available in the surveyed area

A bathymetric map at 1:1,000,000 scale has been published by the Hydrographic Department

of the Maritime Safety Agency [7], compiling con- ventional sounding data. A Seabeam bathymetric map of the Sagami Trough and of the central part of the triple junction became available from the Hydrographic Department just before our cruise. Compiled magnetic anomalies, free-air and Bouguer gravity anomaly maps, and a 1 : 3,(,~)0,000 geological map were also available [8,9]. Some seismic reflection profiles and a few dredges and piston cores were obtained in and around the surveyed area by the Geological Survey of Japan []01.

3. General Seabeam morphology and pattern of clastic .sediment transport and deposition

Plate IIA, map 2 and Plate liB, diagram 2 present the general morphology of the surveyed area. Major morphological units are, from the east

245

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to the west, the outer trench s lope on the PAC side, the t rench f loor and the inner trench s lope on the PHS and NEJ side. A series of basins develops on the nor theas te rn corner of the PHS plate. Canyons dra in clast ic sediments from central Honshu and nor theas te rn Izu-Bonin is land arc in to these basins and the Izu-Bonin Trench.

The trench f loor of the Izu-Bonin Trench is flat, 5 -16 km wide, at abou t 9000 m water dep th (Pla te IIA, map 2). Near 3 3 ° 5 0 ' N , it nar rows to

less than 1 km wide, due to reliefs of the oceanic p la te which emerge from the t rench fill and divide the trench floor in two parts. The southern part , the southward con t inua t ion of which has not been surveyed, is only 5 - 7 km wide and seems to na r row southwards . The nor thern par t is much wider. At its nor thern extremity, it narrows again and is reduced to a V-shaped depress ion, where it passes to the nor th to the Japan Trench. Near 3 4 ° 3 0 ' N , the t rench displays an en-6chelon pat-

246

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tern due to the horst and graben morphology of the oceanic plate which obliquely enters the trench. This southern part of the Japan Trench is also less deep than the northern Izu-Bonin Trench. More- over, north of 34°35'N, the deepest point does not correspond to the trench floor, but to the floor of a graben-like depression on the oceanic plate (pro- file 95, Fig. 3).

On the outerwall of the trench, a striking mor- phological and structural grain trending NNW- SSE is recognized. We show below that this grain corresponds to normal faulting of the oceanic plate which bends before subduction. Another trend, although it is less obvious, also appears on the map: it is represented mainly by the NE-SW trending reliefs which narrow the trench floor near 33°50'N. Near 34°50 'N, a few lineaments and volcanic reliefs with the same direction are present (Fig. 3). In this area, a seamount that we named "Ka iko 1 Seamount" has been discovered.

The inner wall of the trench is morphologically much more complicated. In the northwestern corner of our Seabeam map (Plate IIA, map 2), we can recognize the lower course and the mouth of the Boso Canyon, which is described elsewhere [1]. Close to 141°30'E, a complex system of north- south trending basins develops on the northeast- ern corner of the PHS plate. This system of basins extends about 100 km southwards. The northern basin is about 30 km x 30 km wide at about 7000 m water depth. This basin, which is named the Northern Basin hereafter (NB in Fig. 3), is bounded to the north by a series of nearly east-west scarps. These scarps are the eastward continuation of the Boso Escarpment, which corresponds to the boundary between the PHS and the NEJ plates [1]. To the south, a terrace that lies 400 m higher than the Northern Basin floor separates it from the Southern Basins: a Northern Sub-Basin (NSB in Fig. 3) deepens from 7000 to 8000 m and enters a closed circular depression (Central Depression, CD in Fig. 3) 8200 m deep at 33°50 'N; south of this Central Depression, another basin (Southern Sub-Basin, SSB in Fig. 3), fed by a canyon which is visible just at the southwestern corner of our Seabeam map, symmetrically deepens northwards from about 7000 to 8000 m where it enters the Central Depression. The Central Depression (and thus the whole Southern Basins) has no connec- tion with the trench floor, while the Northern

247

Basin is drained through a narrow canyon cutting the lower slope at about 34°10'N.

These basins are filled by sediments derived from two drainage systems, the Awa-Mikura sys- tem, which drains the northeastern Izu-Bonin Ridge and the Boso system, which drains eastern central Honshu. The former system includes five linear east-west trending canyons but only the lower part of the Awa and Mikura Canyons have been mapped. The Awa Canyon enters the North- ern Basin whereas the other ones end into the Southern Basins. The three main canyons of the Boso system, which are the Boso, Katsuura and Katakai Canyons from west to east, all enter the Northern Basin. The Katsuura and Katakai Canyons drain the large deltaic shelf where clastic sediments are supplied by the Tone river, east off Boso Peninsula. This river collects sediments eroded from the high mountains of central Honshu. The Boso Canyon, the most important one in this area, closely follows the subduction boundary [1] between the PHS and the NEJ plates and collects sediments from areas of high present tectonic uplift rate (Tanzawa Mountains and Boso Peninsula).

4. Tectonics and sedimentation of the Pacific plate in the triple junction area

The outer wall of the Izu-Bonin Trench has been mapped using twenty-two east-west profiles. We thus obtained a nearly continuous image of the sea-floor topography within a quadrangle of 50 km east-west and 175 km north-south (Plate IIA, map 2). Moving seawards from the trench axis, the water depth decreases from 9000 m to about 6000 m over a distance of 50 km, corre- sponding to an average slope of 3.5 ° . The trench outer wall exhibits a complex morphology of gen- tle west-facing slopes and steep east and west-fac- ing scarps that bound graben-like depressions (Fig. 3). These scarps present rectilinear segments with a N160-180°E major trend and N20-40° E minor one.

The seismic profiles (Fig. 4a) show that the wide gentle slopes of the trench outer wall corre- spond either to the top of the acoustic basement or to the top of an almost acoustically transparent layer, generally about 0.5 s two-way travel time thick, probably corresponding to pelagic to

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hemipelagic sediments covering the oceanic crust (Fig. 4a). This sedimentary cover is eroded in some places and is probably the main source of the sedimentary fill in the depressions.

In the southern half of the map, a striking feature is one of the graben-like depressions that continuously parallel the trench. This depression has a flat surface at a depth of about 8800 m, thus only 200-300 m above the trench floor. It is about 4 -6 km wide and, like in the trench, seismic reflection profiles reveal a thick (0.3 two-way travel time) turbiditic fill. A few similar, but much shorter and narrower closed basins scatter on the outer- wall. Their average trend is N 160-170 o E but they are locally offset by short scarps trending N20-40°E , resulting in a more or less rhombic shape (Fig. 3). In the northern half of the map, the main N160-170°E trend is still present but north-south trending scarps progressively domi- nate (Fig. 3).

On the seismic profiles (Fig. 4a), most of the N 160-180 ° E trending scarps, both east-facing and west-facing, clearly present aspect of normal faults. In some cases, however, it is difficult to determine the actual nature (normal or reverse) of the faults: this occurs at some of the east-facing walls of the depressions located in the southern half of the surveyed area (profiles 63 and 73, Fig. 4a). We acknowledge that a difficulty is still present in the interpretation of these faults in seismic profiles and we conclude that, although NNW-SSE trend- ing normal faults have been clearly identified, the existence of possible reverse faults in the outer wall close to the trench cannot be ruled out.

Further north, in the area mapped during Leg 3 of the Kaiko cruise [11] around the Daiichi Kashima Seamount (Fig. 1), the strike of the normal faults on the PAC plate is N20-30°E. These normal faults are mechanically related to the bending of the PAC plate before subduction. The progressive change from N160°E to N-S, then to N20°E of the strike of the normal faults when going northwards probably reflects the pro- gressive change in direction of the flexure of the PAC plate. Although this change is gradual, it is clearly located to the north of the triple junction itself, south of the Daiichi Kashima Seamount. This location of the curvature point may be con- sidered as the actual "mechanical junction" be-

249

tween the Japan Trench and the Izu-Bonin Trench [12]. This interpretation is also independently sup- ported by the distribution of the isobaths of the top of the PAC plate beneath Honshu [12-14].

Another point to notice is that the dip of the acoustic basement is greater in the southern part of the surveyed area than in the northern part (Fig. 4a). The acoustic basement appears to be nearly horizontal to the north and the trenchward increase in depth is accounted for mostly by nor- mal faulting. To the south on the contrary, the basement itself dips trenchward. The greater bend- ing of the oceanic plate to the south is therefore accommodated by both normal faulting and tilt- ing of the basement.

The N 2 0 - 4 0 ° E trending faults are without doubt normal faults. They parallel both the mag- netic anomalies and the alignment of volcanic reliefs located in the central part of the map and that of the Kaiko 1 Seamount (Plate IIA, map 2 and Fig. 3). We consequently conclude that this trend corresponds to the initial structural grain of the ocean floor. The analysis of the Seabeam map shows that these faults are generally cut by the N160-180°E ones. However, some grabens in the southern half of the map seem to be offset by N 2 0 - 4 0 ° E faults, suggesting a possible reactiva- tion of these inherited faults when the oceanic plate bends before subduction.

Another interesting feature of the outer wall is Kaiko 1 Seamount. In spite of its smaller size, it shows a similarity with Daiichi Kash ima Seamount. Kaiko 1 Seamount has a diameter of 12 km at its base and 3.5 km at the crater rim. It lies on a flat surface and is about 1.5 km high. Its conical shape is modified by two large N160- 170°E trending normal fault scarps. The eastern one faces the west and is 0.6-1 km high. On the uplifted (eastern) block, there is another crater whose rim is higher than that of the main volcano. The scarp on the western side of the volcano faces east, so that the central part of the Kaiko 1 Seamount lies on the floor of a 5-7 km wide, NNW-SSE trending graben. Further west, another fault scarp, 1 km high, faces west and bounds the main trench. The normal faulting which affects the Kaiko 1 Seamount consequently differs from that affecting the Daiichi Kashima seamount, that is a large trench-facing normal fault [11].

250

5. Tectonics and sedimentation of the lzu-Bonin Trench

On the seismic reflection records, the trench fill has a parallel and continuous configuration with high-amplitude reflectors, interpreted as turbi- dites. These turbidites onlap the oceanic basement of the Pacific plate as well as its hemipelagic sedimentary cover along the outer wall border, progressively burying the horst and graben mor- phology of the outer wall. The turbiditic sequence shows a slight westward divergent configuration due to the progressive bending of the subducting plate. Along the base of the inner trench wall, the turbidites are gently folded in an anticline trend- ing parallel to the subduction front (Fig. 3 and profile 79, Fig. 4a). Because of the limited power of our water guns, especially at these large water depths, the trench fill could not, however, be traced beneath the inner wall. A terrace 200 m higher than the trench floor may represent an accreted trench fill unit, but may also be interpre- ted as slid masses at the foot of the inner wall. The folding, however, suggests a possible occur- rence of limited accretion.

The turbiditic currents by-passing the drainage system of the inner wall through the Northern Basin join the trench at about 9000 m water depth through a narrow canyon trending NW-SE. A large and gently convex fan develops at the canyon mouth. Because this is the only fan in the trench, the main part of the trench fill, the surface of which is slightly lower than the fan, is thought to have been supplied through this canyon.

6. Tectonics and sedimentation on the innerwall

The morphology of the inner wall, as described earlier is very complex and the seismic profiles generally give little information about the sub- surface structure, except in the sedimentary basins. For this reason, we emphasize the sedimentation processes and the tectonic deformations recorded in the basins developed on the northeastern corner of the PHS plate.

In the Northern Basin, a large valley, 60 km long, follows the NW-SE diagonal of the basin from the mouth of the Boso Canyon at 6700 m water depth to the head of the lower canyon at 7300 m (Fig. 3). In the central part of the basin,

the valley is divided into two branches, the eastern one being the larger, up to 10 km wide. The valley has a flat bottom and is flanked by wide flat terraces. The northeastern terrace is 200 m higher than the valley floor, the southeastern one 400 m higher. Seismic profiles (Fig. 4a) show that the valley is an erosional feature, which implies that the basin infill is an earlier sedimentary accumula- tion and that this area is now by-passed by turbi- ditic currents which join the trench through the lower canyon. The Awa Canyon has its mouth in a sink-hole 6 x 6 km wide and 100 m deep devel- oped in the southwestern terrace (Fig. 3). Other minor sink-holes, up to 80 m deep, also occur in the lower course of the Awa Canyon itself. Seismic profiles of the Northern Basin show a parallel configuration with rather good continuity and high-amplitude reflectors. Locally, oblique reflec- tors and discontinuous configurations occur in the central part of the basin. This seismic sequence is interpreted as locally channelized turbidites. The acoustic basement can be recognized only in the southern part of the basin and dips northward (profiles 61-62, Fig. 4b). In the north, right south of the east-west plate boundary, sediment thick- ness is greater then 2 s two-way travel time. Sedi- ments show a northward divergent configuration and also a slight westward divergence from the eastern scarp to the center of the basin (Fig. 4a).

The Southern Basins system is narrower, about 10 km wide, and more irregular in shape. This is due to several WNW-ESE trending fault scarps along its eastern side, between the basins and the trench (Fig. 3). The southern terrace of the North- ern Basin extends southwards to 34°08 'N , north of the Kita-Mikura Canyon. In the Northern Sub-Basin, three canyons, an unnamed one, the Kita-Mikura and the Mikura Canyons have their mouths in sink-holes, 8 x 10 km wide and 60 m deep, 2 × 2 km wide and 60 m deep, 2 z 1 km wide and 40 m deep, respectively. The Southern Sub-Basin appears to be controlled on its eastern side by a NW-SE trending fault.

Sink-holes located at the mouth of canyons probably represent erosional/depositional fea- tures created by turbiditic currents as the result of hydro-dynamic processes induced by the change in slope between the steep canyon thalweg and the flat-bottom basin at its mouth [15]. In this inter- pretation, most of the turbiditic currents by-pass

these depressions to join lower parts of the basin, that is the Northern Basin valley in the case of Awa Canyon, and the Central Depression of the Southern Basins in the case of Kita-Mikura and Mikura Canyons. On the other hand, the larger sink-hole located at the mouth of the unnamed canyon north of Kita-Mikura Canyon, as well as the Central Depression appear to be tectonically controlled subsiding basins, bounded by E-W and WNW-ESE trending faults (Fig. 3). The latter is also controlled by the westward tilting of the inner wall above the subduction contact. The occurrence of these sink-holes consequently results from tectonic processes as well as from low sedimentary supply, which is insufficient to fill them. In this context, the Northern Basin is understood as the one where the sediment supply rate surpassed the tectonic subsidence.

As for the deformation, the northward tilting of the basement of the Northern Basin, as well as the northward divergent configuration of the sedimen- tary fill (profiles 61-62, Fig. 4b) clearly suggests underthrusting of the Izu-Bonin Arc beneath the NEJ plate. In this interpretation, the staircase morphology of the NF_,J side slope can be interpre- ted as the result of reverse faulting associated with convergence (Fig. 4a). East-west seismic profiles across the Northern Basin show a gentle north- south trending syn-sedimentary synclinal structure (profile 87, Fig. 4a). On the western side, the basin fill onlaps the Izu-Bonin arc sedimentary cover. The eastern side of the basin rather seems to be fault controlled. Near the foot of the scarp bound- ing the basin to the east, a synsedimentary fold located beneath the northeastern terrace (profile 85, Fig. 4a) suggests the occurrence of a reverse fault in the basement. The northeastern terrace is also slightly westwards tilted (profile 87, Fig. 4a). However, the evidence of a general east-west com- pression is not overwhelming, since the fold is seen only on profile 85. In other profiles, the sedimentary layers simply tilt and diverge toward the center of the basin and the nature of the eastern border cannot be evidenced from our data. Note that the Southern Sub-Basin seems to have a similar structure and that the Northern Sub-Basin and the Central Depression show a westward tilt- ing of the basement of their eastern border.

Outside of the basins, the structure is much more difficult to decipher. The slope of the Izu-

251

Bonin Ridge above the basins is generally covered by a thick (1 two-way travel, time) acoustically transparent unit which appears to be eroded in places and affected by a few normal faults (Fig. 4a). The base of the trench inner wall, on both the NEJ and the PHS plates, is characterized by a staircase morphology. This suggests a structure controlled by sinuous north-south trending faults. Some profiles (profiles 81, 85 and 95, Fig. 4a) show a westward tilted transparent sedimentary cover; tilting is probably due to faulting. It is, however, difficult with our data to distinguish surficial slump scars from normal faults or im- bricated thrust faults. Is the extensive convergent margin model [161 proposed for the Japan Trench valid near the junction? Or, on the contrary, does this area correspond to a compressive model? Al- though it is impossible to answer definitely, it is obvious that a large accretionary prism similar to that of the Nankai Trough [17] is not developed here. One should notice that a structure on profile 73 (Fig. 4a) near 33°45 'N can be reasonably interpreted as a slump scar. The inner trench wall also shows numerous transverse rectilinear WNW- ESE fault scarps; the lower canyon that joins the Northern Basin with the Izu-Bonin Trench is ap- parently controlled by such a transverse fault, the meaning of which remains obscure, however.

7. Geophysical data

On the free air anomaly map (Fig. 5), the most striking features are a sharp negative anomaly near 34°N, 142°E, corresponding to the deepest part of the trench and a NW-SE trending branch over the Northern Basin. The location of these anomalies is due to the fact that the free air anomaly at sea mainly reflects the topography. A preliminary analysis of the gravity along a NW-SE line crossing the Northern Basis shows that the 40 mgal difference between the deepest part of the trench, at 9000 m water depth, and the Northern Basin, at 7000 m water depth, is simply explained by the depressed topography and the deficit of mass in the Northern Basin due to the large depth of the basement.

The magnetic anomaly map (Fig. 6) has been obtained without correction of diurnal, monthly and storm-type variations, but by correcting the

252

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'" i ; : .~/. ~ . /

-) " ': ~ , ~ ~i~>O O - , ¢,i" / / ... , ~ . : !~-~.~",,~:..,~

' I . ~ - - - : , .'~"

141 1 4 2

40 nT. Sagam; Trough area is included [I}. Plate boundaries

values at track crossings by hand. This would probably introduce minor errors but certainly does not distort the general pattern of magnetic anomalies since cross checking errors were less than 10 nT while the anomaly is 240 nT peak to peak. On the eastern side of the trench axis, east of 142 ° E, clear magnetic lineations trending NE- SW can be observed. They are the western exten- sion of the Mesozoic magnetic lineations in the northwestern Pacific and are tentatively correlated to M15-M16 series (135-137 Ma, Neocomian). The magnetic anomaly pattern can be followed more than 50 km landward crossing the trench axis with a gradual decay. This decay reflects the deepening of the magnetic source layer as well as the decrease of magnetization due to heating while the oceanic lithosphere subducts.

A progressive change in trend of this oceanic magnetic anomaly pattern occurs between the northern part of the surveyed area, where it strikes N60°E, and the southern part where it strikes N35°E. This change may either reflects an origi- nal feature of the oceanic crust or is due to rotational block movements. The larger dip of the outer wall to the south compared to the north is insufficient to explain the observed change in strike of the anomalies.

Another major feature is the occurrence of a large NW-SE trending negative anomaly amount- ing - 8 0 nT near 33°30'N, 141°40'E. This anomaly abruptly interrupts the general NE-SW pattern of lineations, resulting in a 160 nT dif- ference with the + 80 nT positive anomaly of the NE-SW oceanic lineations. Simple two-dimen- sional direct models of this anomaly show that it may be due to a NW-SE striking fault in the subducted part of the PAC plate, displacing the southwestern block by a few kilometers down- ward. As the negative anomaly does not extend east of the trench axis, this fracture would only affect the part of the PAC plate which has already subducted. This is consistent with the southward increasing dip of the PAC slab on a more regional scale [12-14]. A similar feature occurs near 34°20 'N, 141°20'E, where a 160 nT positive anomaly is cut by a NW-SE trending, 0 nT anomaly. We suggest that the same mechanism as the previous one can be applicable to this anomaly.

It is noteworthy that a similar mechanism of deformation of the subducting PAC plate has

253

been deduced from focal mechanism studies of earthquakes near the triple junction. The large (M~=7.9) earthquake of November 25, 1953, whose epicenter is located at 33°52'N, 142°E and aftershocks distributed to the northwest up to 70 km from the epicenter, has a normal fault-type mechanism with one of the nodal planes striking N128°E and dipping 72 ° southwest with the southwestern block downdropping [18]. The earth- quake of September 18, 1984 (M.~ = 6.7), which is located at 34°N, 141°40'E, has nearly the same mechanism [19]. This earthquake has a depth of 30 km and is thus likely to have occurred in the PAC plate. Further south, near 33°40'N, 141°20'E, at the northwestern extremity of the large NW-SE negative magnetic anomaly, two smaller (m b = 5.7 and 5.9) events have a similar type of fault plane solution [20]. The NW-SE trending normal fault- ing, as revealed by the fault plane solutions and the magnetic anomaly cutting the NE-SW trend- ing oceanic lineations, is therefore the predomi- nant mode of faulting that affects the PAC plate beneath the PHS plate in the triple junction area.

8. Discussion and conclusions

Using the data collected during Leg 2 of the Kaiko cruise, as well as additional geophysical data, we have been able to clarify the overall structure and the dynamics of the triple junction area. The first problem to discuss is that of accu- rately identifying plate boundaries. One easily identifies the subduction boundary between the PAC plate and the PHS and NEJ plates, at the foot of the inner wall of the Izu-Bonin and Japan Trenches (Fig. 7, deformation front). The identi- fication of the boundary between the PHS and the NFA plates near the triple junction is more prob- lematic. In the Sagami Trough, the active boundary follows the Boso Escarpment [1 ]. To the southeast, it seems to follow the major scarps which bound the Northern Basin to the north (Fig. 7). We propose this location because the northward dip- ping basement of the Northern Basin belongs very likely to the PHS plate. On the other hand, we cannot clearly depict the boundary between the PHS and the NEJ plates in the area between the Northern Basin and the Izu-Bonin Trench. This difficulty arises partly because of the limited power and resolution of our seismic profiling system.

254

00y 0 0 ~ ', " ~ t

PAC 3crn yr

oH S • ~ PAC ,!

'~\\ D e t o r m a t l o t

I T r e n c h f i l l

B q $ i n f i l l

--I 3 4 ° 5 0 ' N

~ 3 4 °

± _L -~ 33°10' 141°E 142 ° 14 2 °40 '

Fig. 7. Schematic map of main features of the triple junction lhe delormation front is drav, n dotted where we could not surely trace it. NB. SO: Northern and Southern Basins. Inset: velocity diagram of the Philippine Sea ( P/ IS ). Northeast Japan ( NE.I ) and Pacific ( P A C ) plates. The westward motion of the PHS plate produces ([) the westward retreat of the Izu-Bonin Trench, and (2) the formation of a series of active basins (stippled). The shaded area indicates a pos~,ible zone of tectonic erosion which makes the trench axis to shift westward north of the triple junction.

However. the difficulty in identifying the plate boundary may also be closely related to the dy- namics taking place in the junct ion area.

The two most striking features of the triple junct ion area are (1) the occurrence of a series of active basins on the inner trench slope of the PHS plate (Fig. 7), and (2) the larger depth of the Izu-Bonin Trench compared with the Japan Trench. Any model of the structure and evolution of the triple junct ion should at least explain these two facts, on the basis of the interactions of the three plates involved.

The motion between the PHS and the N EJ plates has been west-northwest for the past few million years at most [4,21]; this theoretically im- plies a westward retreat of the PHS plate (arrow in inset of Fig. 7). This retreat may result either in an east-west extension of the PHS plate border or in a westward migration of the lzu-Bonin Trench. Both may have occurred since the former hypothe- sis would explain the formation of the inner slope basins while the latter would explain the greater depth of the Izu-Bonin Trench compared to that of the Japan Trench: if the PAC plate cannot

entirely follow thc westward motion of the PHS plate, because of the barrier objected by the NE3 plate to the north, or if local isostatic readjust- ment does not take place, the lzu-Bonin "French would migrate westwards and downwards along the surface of the subducting PAC plate. This mechanism would thus produce an increase in the depth of the trench. However, part of the observed larger trench depth to the south might also be accounted for by the larger dip of the PAC slab beneath the PHS plate compared with that be- neath the NEJ plate. The large depth of the lzu- Bonin Trench also explains why the landward and arcward derived turbidites are deposited there while the Japan Trench has almost no turbiditic infiil.

But such a trench migration would result in an offset of the Izu-Bonin Trench at the triple junc- tion (inset of Fig. 7); since no offset is actually observed, the trench axis has migrated not only south of the triple junction, but also north of it. How could this migration north of the triple junc- tion have been accommodated? One may invoke tectonic erosion at the base of the NEJ plate [22],

but we have no indication of such a phenomenon at present. Another explanation may be either plate consumption at the west of this margin [23] or compressional deformation of the NEJ margin [121.

An alternative to the extensional model for the formation of the inner slope basins is to consider that compressional deformation occurs on the northeastern corner of the PHS plate [24]. This may be due to an interaction at depth of the sinking PHS and PAC plate [12,24,25] as the PHS plate is not free to sink because of the restricted free space between the overriding NEJ plate and the underlying PAC plate.

In any case, the slope basins show that intra- plate deformation occurs in the triple junction area, which also may explain why it is difficult to define the plate boundary north of the Northern Basin and thus why it is difficult to precisely locate the triple junction itself. Such an intra-plate deformation would be made easier by the weak mechanical strength of the toes of the PHS plate and the NEJ plate.

The last point we briefly discuss concerns the past motion of the PHS plate. On-land studies [26,27], as well as geophysical considerations [21] and marine geological studies [1], suggest that the P H S / N E J relative motion was more northerly 1 or 2 Ma ago than at present. It is noteworthy that moving back the lzu-Bonin Trench at its location 2 Ma ago, if we accept the migration hypothesis, would eliminate the present larger depth com- pared with that of the Japan Trench.

As discussed above, interpretation of the struc- ture and evolution of the triple junction is not easy. Leg 2 of French-Japanese 1984 Kaiko cruise brought numerous new data, but many more is needed, including multichannel seismic profiling, deep-sea drilling and also more advanced models based on regional geophysical data and on theo- retical considerations.

Acknowledgements

This cruise was part of the phase I (1984) of the French-Japanese cooperative program Kaiko, con- ducted under the auspices of C N R S and I F R E M E R in France and Monbusho and ORI in Japan. The Captain of the R / V "Jean Charcot" and all the crew members are greatly acknowl-

255

edged for their cooperative work during the cruise. We are grateful to all engineers and electronic and computer technicians for their quick and reliable maintenance of the hardware devices and quick responses to our requisites. We thank three anony- mous reviewers for their comments.

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