The Izu-Bonin (Or Izu-Ogasawara) and Mariana Arcs Are Major Topo- Graphic Features in the Circum-Pacific Belt, Extending Curvili

Home , Japan Trench, Nankai Trough, Ryukyu Trench, Sagami Trough, Suruga Trough

J. Phys. Earth, 24, 275-311, 1976

GRAVITY IN THE JUNCTION BETWEEN THE JAPANESE AND THE IZU-BONIN ISLANDS

Jiro SEGAWA* and Carl BOWIN**

*Ocean Research Institute, Universityof Tokyo, Tokyo, Japan **WoodsHole OceanographicInstitution , WoodsHole, Mass., U.S.A. (Received July 22, 1975;Revised June 21, 1976)

Gravity values from the central Honshu to the northern part of the Izu-Bonin (Izu-Ogasawara) Arc have been compiled, and the free air and Bouguer gravity anomaly maps as well as gravity tables have been given. The broad gravity low over the triple junction between the Pacific plate, the Eurasia plate and the Philippine-Sea plate has been explained by consider- ing the possible behavior of the Pacific plate descending from the junction between the Japan and the Izu-Bonin Trenches: It may be that the slab descending at the Japan Trench and the slab descending at the Izu-Bonin Trench geometrically have converging velocity components under the junction area to cause subduction of the Philippine-Seaplate beneath the Eurasia plate, and this subduction gives rise to the negative gravity anomaly. Ex- planation of some other gravity lows observed over the Japanese Islands has also been made tentatively.

1. Introduction The Izu-Bonin (or Izu-Ogasawara) and Mariana Arcs are major topographic features in the circum-Pacific belt, extending curvilinearly through about 3,000km, and separating the Philippine Basin from the western Pacific Basin, This paper discusses the gravity in and around the northern portion of the Izu-Bonin Arc north of 30°N. This region constitutes a T-T-T triple junction between the Pacific plate, the Eurasia plate and the Philippine-Sea plate. These plates are bordered by the Japan Trench (Pacific vs. Eurasia), the Izu-Bonin Trench (Pacific vs. Philippine) and the Nankai Trough plus the Sagami Trough (SUGIMURA,1972) (Eurasia vs. Philippine). Gravity anomaly maps in and near the Japanese Islands were published by TOMODA(1973), who combined the land and marine data. Although the maps cover the northern end of the Izu-Bonin Arc, the data are not sufficient. In this paper, more data have been added, so that the gravity field can be better understood. The authors are of the opinion that a reduced contoured map of gravity anomalies would not always be satisfactory if one wants to use the gravity

275 276 J. SEGAWA and C. BOWIN anomalies for his own purposes. This is the reason why this paper includes gravity tables, although it might appear to some as a mere waste of paper. There are three major junctions between oceanic trenches near Japan: They are, from northeast to southwest, the junction of the Kurile Trench with the Japan Trench (Kurile-Japan junction), the junction of the Japan Trench with the Izu-Bonin Trench (Japan-Bonin junction), and the junction of the Nankai Trough with the Ryukyu Trench (Nankai-Ryukyu junction). The regions at these three junctions are associated with broad gravity lows, where active seismicity and crustal movements have been observed (MIYA- MURA,1972; TSUBOKAWA,1972). In this paper the authors first discuss the relationship between the gravity low and the tectonic movement in the area of the Japan-Bonin junction, and then similar discussions are expanded to the other junctions between two adjoining trenches.

2. Gravity Data

Gravity data from fourteen sources have been used; thirteen for the sea region and one for the Japanese Islands. In the following descriptions the LaCoste and Romberg land gravity meter, the LaCoste and Romberg air-sea gravity meter and the Tokyo surface ship gravity meter are denoted by L & R G-meter, L & R S-meter and TSSG, respectively. 1) L & R G-meter data covering the Japanese Islands obtained by the GEOGRAPHICALSURVEY INSTITUTE of JAPAN(1955, 1957, 1964, 1965, 1966 and 1969). 2) Data from the Vening Meinesz pendulum (KUMAGAI,1953; WORZEL, 1965). 3) L & R S-meter measurements of the Hakuho-maru cruise KH72-1 of the Ocean Research Institute between May and August 1972 (unpublished). 4) L & R S-meter measurements of the Hakuho-maru cruise KH72-2 between October and December 1972 (unpublished). 5) TSSG measurements of the Takuyo cruise of the Hydrographic Department, Maritime Safety Agency, Japan, in April 1966 (unpublished). 6) TSSG measurements of the Meiyo cruise of the Hydrographic Department in June 1968 (HYDROGRAPHICDEPARTMENT, 1969). 7) TSSG measurements of the Umitaka-maru cruise of the Tokyo Uni- versity of Fisheries from July to August 1966 (TOMODAand SEGAWA,1967). 8) TSSG measurements of the Umitaka-maru cruise from July to August 1967 (SEGAWA,1968). 9) TSSG measurements of the Hakuho-maru cruise from July to August 1967 (SEGAWA,1970). 10) TSSG measurements of the Umitaka-maru cruise from November Gravity in the Junction between the Japanese and the Izu-Bonin Islands 277

1967 to February 1968 (unpublished). 11) TSSG measurements of the Hakuho-maru cruise KH68-3 from July to August 1968 (SEGAWA, 1970). 12) TSSG measurements of the Hakuho-maru cruise KH69-1 in April 1969 (unpublished). 13) TSSG measurements of the Hakuho-maru cruise KH69-2 from April to June 1969 (unpublished). 14) Graf-Askania Gss2 meter measurements of the Lamont-Doherty Geological Observatory (Only plotted sheets of free air anomalies (WATTS and TALWANI, 1974) are available to the authors). The data sources Nos. 1, 2, 6, 7, 9 and 11 are documented in detail in the references where gravity tables are also given. Therefore, only the data sources Nos. 3, 4, 5, 8, 10, 12 and 13 are handled here, and the gravity logs during the measurements will be summarized as follows. The gravity data from these measurements are given in Tables 1 to 9.

2.1 Data source No. 3 KH72-1 cruise (OCEANRESEARCH INSTITUTE, 1975). Data numbers 1 to 99 in Table 1. The L & R S-meter (S-32) used for this measurement was operated on a gyro-stabilized platform. The meter was calibrated at each port of call by means of gravity ties with the known gravity stations on land. Unnegligible amount of haphazard drift of the meter was observed during the survey, as follows:

Tables 1-9. Gravity tables.

NO: Data number. JST: Japanese Standard Time. GMT: Greenwich Mean Time. LAT: Latitude. LON: Longitude. Depth: Water depth in meter. R: Normal gravity in gal, according to the International Gravity Formula of 1930. G: Observed gravity in gal. dGf: Free air gravity anomaly in mgal. dGb: Simple Bouguer gravity anomaly in mgal.

Note: Employment of a new absolute gravity at Potsdam and a new gravity formula (1967) would give rise to the change of gravity anomalies of the areas concerned by the amount of less than 1.5 mgal. 278 J. SEGAWA and C. BOWIN

Tab le 1 Gravity in the Junction between the Japanese and the Izu-Bonin Islands 279 280 J. SEGAWA and C. BOWIN

Tab l e 2 Gravity in the Junction between the Japanese and the Izu-Bonin Islands 281 282 J. SEGAWA and C. BOWIN

Table 3 Gravity in the Junction between the Japanese and the Izu-Bonin Islands 283 284 J. SEGAWA and C. BOWIN

4 Table Gravity in the Junction between the Japanese and the Izu-Bonin Islands 285 286 J. SEGAWA and C. BOWIN

Table 5 Gravity in the Junction between the Japanese and the Izu-Bonin Islands 287 288 J. SEGAWA and C. BOWIN

Table 6 Gravity in the Junction between the Japanese and the Izu-Bonin Islands 289 290 J. SEGAWA and C. BOWIN

l e Tab7 Gravity in the Junction between the Japanese and the Izu-Bonin Islands 291 292 J. SEGAWA and C. BOWIN

Table 8 Gravity in the Junction between the Japanese and the Izu-Bonin Islands 293 294 J. SEGAWA and C. BOWIN

Table 9 Gravity in the Junction between the Japanese and the Izu-Bonin Islands 295

where Gg and Gs denote the gravity values measured at ports by the land meter and the sea meter respectively. The differences, Gs-Gg, indicate the sea meter drifts in the interval between two successive ports of call. The drift of the sea meter used was positive, with the magnitude of 3.5 mgal or less during the period of 10 to 20 days. These drifts have been corrected by assuming that the amount of the drift was proportional to the time lapse. Because of the malfunctioning of the sea meter, the authors could not make the calibra- tion at the last station (Tokyo) in this case. Ship's positions and speeds were measured by using not only the celestial and radio navigation systems but also continuous records of the ship's speed and heading. Although the satellite navigation equipment was not used in this case, the continuous records of the ship's speed and heading were very useful for the estimation of the Eotvos corrections. So far as the data used for this study are concerned, the errors of measurement resulted from the meter drift may be less than 1 mgal, whereas those resulted from inaccurate ship's speeds may be a few milligals. The systematic errors of positioning may be usually one nautical mile or less, but in some adverse case it is suspected to be more.

2.2 Data source No. 4 KH72-2 cruise (OCEAN RESEARCH INSTITUTE, 1975). Data numbers 100 to 676 in Tables 1 to 3. The same L & R S-meter was used as during the cruise KH72-1. A Mag- navox satellite navigation equipment was installed on the Hakuho-maru. Cal- ibration of the sea meter by comparing with the land meter carried out at each port of call has resulted as follows:

The differences, Gs-Gg, are not systematic. It was found that the land meter (G-124) had been subjected to a drift as large as -1.95 mgal during the cruise from Tokyo back to Tokyo. This drift has been corrected by assuming that it was proportional to the time lapse. The amount of the sea meter drift 296 J. SEGAWA and C. BOWIN during this cruise appeared similar to that during the previous cruise (KH72- 1), indicating that approximately 3 mgal drift occurred during the period of 10 to 20 days.

2.3 Data source No. 5 Takuyo cruise 1966. Data numbers 677 to 933 in Tables 4 to 5. A TSSG was used for the measurement. Although the recent version of the TSSG is very stable, as was verified by the comparison measurement with an L & R meter (SEGAWAand BOWIN, 1975), the version used for this measurement did not function properly from time to time, because of the electronic defects. The largest problem having arisen during this measurement was that the data at both the ports of departure and arrival were completely lost, because the meter, unfortunately, did not work at either of them. The data in the midway between the ports were obtained, though they were not calibrated. As the survey area of this cruise is very important for the present study, the authors have tried to correct the data against other measurements at the crossings of the tracks of survey. This appears to be successful. Although it is difficult to determine the accuracy of these data, it is probably better than

±20 mgal, if estimated from the discrepancy of the data at the crossings of tracks.

2.4 Data source No. 8 Umitaka-maru cruise 1967 (SEGAWA,1968). Data numbers 934 to 1116 in Table 5. The TSSG was used for this measurement. Because of the imperfection of the mini-computer used for this measurement (SEGAWA,1970) the results are suspected to involve some amount of the non-linear rectification errors (TALWANI,1970) that are inherently associated with, but should be removed from the TSSG. The result of comparison with the L & R S-meter at the crossings of the ship's tracks (SEGAWAand BOWIN,1975) suggests that the errors are less than 10 mgal.

2.5 Data source No. 10 Umitaka-maru cruise 1967 to 1968 (SEGAWA,1970). Data numbers 1117 to 1502 in Tables 6 to 7. The TSSG equipped with a mini-computer was used. Conditions of the measurement were similar to the case of the measurement No. 8.

2.6 Data source No. 12

Hakuho-maru cruise KH69-1. Data numbers 1503 to 1801 in Tables 7 to 8. Gravity in the Junction between the Japanese and the Izu-Bonin Islands 297

Gravity was measured by the TSSG connected to a high speed , middle size electronic computer. Differing from the previous measurement , the corrections for the non-linear rectification errors were made almost satisfactorily in this measurement. The ship's position and speed were determined by a conventional method without the control of satellite navigation , so that some amount of errors in positioning and consequent errors in the Eotvos corrections have to be taken into consideration. Instrumental errors may be less than 5 mgal.

2.7 Data source No. 13 Hakuho-maru cruise KH69-2 (OCEANRESEARCH INSTITUTE, 1971). Data numbers 1802 to 1902 in Table 9. The system of the TSSG used for this measurement was exactly the same as was used for the previous measurement (cruise KH69-1), except that in this case two sets of the TSSG were operated at the same time but independently (SEGAWA,1970). Comparison between the two sets of the meters showed that the errors involved in this measurement were about ±3 mgal on an average. Figure 2 shows the tracks of gravity measurements. In the land area are indicated the routes of the land gravity surveys. The gravity data in Tables 1 to 9 include data numbers, time of measurements (Greenwich Mean Time, GMT, or Japanese Standard Time, JST), positions in latitude and longitude, water depths in meters, normal gravity after the International Gravity For- mula of 1930 (HEISKANENand VENINGMEINESZ, 1958), observed gravity, free air gravity anomaly, and simple Bouguer anomaly calculated by assuming that the sea water can be replaced by an infinitely large flat plate having the same thickness with the water depth at each position and with a density 2.67g/cm3.

3. Interpretation 3.1 Characteristics of gravity anomaly In Fig. 1 the bathymetric features of the area in question are shown. In Fig. 3 the free air gravity anomaly is contoured with the intervals of 20 mgal. In Fig. 4 the simple Bouguer gravity anomaly is contoured also with the intervals of 20 mgal. Figure 5 shows the geographic names cited in the following descriptions for convenience. Like most of the island arcs, the Izu-Bonin Arc shows remarkable negative free air anomalies over the trench and positive free air anomalies over the ridge. The most conspicuous gravity low appears at the Japan-Bonin junction, where the anomaly reaches the minimum of -320 mgal. The trough of free air anomaly over the trench tends to become shallow southwards up to about -210 mgal along the Izu-Bonin Trench north of 30°N. The zone of 298 J. SEGAWA and C. BOWIN

Fig. 1. Topographic features of the area concerned. Bathymetric contours are drawn every 1,000m, except for the case of 200m depth contour. Gravity anomaly maps of Figs. 3 and 4 are drawn for the area encircled by the thick solid line. these negative free air anomalies branches northwestwards at the Japan-Bonin junction along the Sagami Trough. The free air anomalies over the Izu-Bonin Ridge show their maxima where there are active volcanoes constituting the Izu-Bonin volcanic front (SUGIMURAet al., 1963). The area of comparatively lower free air anomaly between the volcanic front and the Izu-Bonin Trench axis (south of 32°N in Figs. 1 and 3) seems to be a subsiding zone character- istic of the arc-trench gap (DICKINSON,1973). A broad area of high free air anomaly (>100 mgal) can be recognized around the Hachijo-shima Island

(north of 32°N in Figs. 1 and 3). Broadening of the zone of gravity high in this region may be due to the lack of the topographic low observed generally in the arc-trench gap. The belt of positive free air anomalies east of the Izu- Bonin Trench is named Outer Gravity High (WATTSand TALWANI,1974) and correlated with the Outer Topographic Rise. Gravity in the Junction between the Japanese and the Izu-Bonin Islands 299

Fig. 2. Tracks of gravity measurements (thick solid line). Small open circles connected by broken lines show the sites of gravity measurements by use of the Vening Meinesz Pendulums. Each survey is denoted by the cruise names. On land areas are shown the routes of land gravity surveys. The tracks denoted by A-B, C-D, ... K-L are the locations of the profiles shown in Fig. 6.

Other notable negative free air anomalies are observed from the Bay of Tokyo eastwards to the continental shelf across the neck of the Boso Peninsula, and in the northern Shikoku Basin. A negative zone reaching the minimum of -50 mgal runs SW to NE from the Nankai Trough to the Suruga Trough, continuing northwards onto the land, though a short break of the trend is seen in the contours of Fig. 3. Another negative zone of northern Shikoku Basin is recognized south of this negative zone, trending SWS to NEN with a minimum of about -20 mgal. This elongated negative zone extends south- westwards to about 32°N, but it is uncertain whether the anomaly is correlated with the Nankai Trough. Its northward extension seems to join the northern negative zone at the Suruga Bay. Figure 6 shows east-west profiles of free air anomaly and bathymetry across the Izu-Bonin Trench, and Fig. 7 shows similar profiles across the Japan 300 J. SEGAWA and C. BOWIN

Fig. 3. Free air anomaly contours with the intervals of 20 mgals. Solid lines show zero or positive anomalies, and dashed lines show negative anomalies.

Trench (SEGAWA,1970) for comparison. These profiles indicate that gravity decreases at the continental slopes as well as the landward slopes of the trenches. These features can not be explained by the topographic effect alone, suggesting the presence of thick sedimentary layers, as can most easily be in- ferred from the profile C-D of Fig. 6. Figure 8 shows a simplified free air anomaly map in and near Japan, compiled using Tomoda's map and the present data. Among the negative anomaly zones, three broad zones are found at the Kurile-Japan , the Japan- Bonin and the Nankai-Ryukyu junctions. Therefore, broadening of negative zones seems to originate from the trench tectonics and to be inherently associated with the junctions of two adjoining oceanic trenches. Although less remarkable than the above explained ones, there are other negative anomaly zones on the coastal and continental shelf areas of Japan: They are related geographically to bays or straits of the island arc (the Bay of Tokyo, the Bay of Sagami, the Bay of Suruga, the Bay of Ise, and the Strait of Kii and the Gravity in the Junction between the Japanese and the Izu-Bonin Islands 301

Fig. 4. Simple Bouguer anomaly contours with the intervals of 20 mgals. Solid lines show zero or positive anomalies, and dashed lines show negative anomalies.

Strait of Bungo in the outer side of Honshu; the Bay of Toyama and the Bay of Wakasa in the inner side). The elongated zones of gravity low running parallel to the trench, which are generally observed at the continental shelf, can be understood as the result of the descent of the oceanic plates from trenches: Repetition of vertical movements at the tip of the continental plate due to the descent of the plate on the oceanic side causes an extensional zone to appear in the crust on the continental side (This should correspond to a 'hinge line of movement.' See the next. section), resulting in the subsidence of the crust, which gives rise to mass deficiency and negative anomalies. Broadening of negative free air anomaly zones associated with the junctions of two adjoining trenches, however, has not so far been well explained, although the fact has long been no- ticed (HESS,1948). In the following section, the authors attempt to explain the phenomena in the trench or arc junction from the plate tectonic stand- point. 302 J. SEGAWA and C. BOWIN

Fig. 5. The geographic names cited in this paper.

3.2 Relationshipbetween gravity anomaliesand tectonicsof arc junctions MCKENZIEand MORGAN(1969) pointed out the existence of a T-T-T(a) type triple junction ('unstable' triple junction) at the intersection of the Japan Trench, the Izu-Bonin Trench and the Nankai Trough. But, since the connec- tion of the Nankai Trough with the other two plate boundaries was not properly explained, SUGIMURA(1972) proposed a more realistic boundary between the Eurasia and the Philippine-Sea plates by incorporating the Sagami and the Suruga Troughs into the Nankai Trough. The negative free air anomaly zones over both the Japan-Bonin and the Kurile-Japan junctions show a configura- tion of three branches, the anomalies in the latter junction being more significant than in the former (see Fig. 8). As to the junction of the Nankai Trough with the Ryukyu Trench (Nankai-Ryukyu junction), the negative zone appears somewhat different from the above two cases, presumably due to the anomalous nature of the Nankai Trough as a trench. Therefore, the authors will not refer to this zone any more in this paper. AOKI(1974) proposed a probable shape of the Pacific plate which has descended from the Japan-Bonin junction (Fig. 9). When a plate descends at a plate boundary including a junction of trenches, it is subjected to strong deformation at the boundary between the plate descending from one trench Gravity in the Junction between the Japanese and the Izu-Bonin Islands 303

Fig. 6. Profiles of free air anomaly (solid lines) and bathymetry (shaded lines) across the Izu-Bonin Arc along tracks A-B, C-D, ..., K-L in Fig. 2. Water depths are in kilometers. Latitudes attached to the profiles are approximate latitudes of the tracks. The free air anomaly and bathymetry outside the arrow marks in each profile are those estimated from contoured maps, hence may be less exact. 304 J. SEGAWA and C. BOWIN

Fig. 7. Profiles of free air anomaly (solid lines) and bathymetry (shaded lines) across the Northeast Honshu Arc along the parallels at every one-degree latitude. The axis of the Japan Trench and the line of free air minimum are offset in this case (Reproduced from SEGAWA(1970)). and that descending from the other trench. Aoki's estimation about the sub- ducted Pacific plate is based on the assumption that the total surface area of the plate is conserved during the descent until disruptions of the plate occur along the boundary between the slab descending from the Japan Trench (North Honshu slab, after Aoki) and that descending from the Izu-Bonin Trench (Izu-Bonin slab, after Aoki). Figure 9 shows contours of the depth to the proposed upper surface of the descending Pacific plate. Aoki has con- firmed that this model is consistent with seismic travel time anomalies, hy- pocentric distributions, dispositions of volcanoes and mechanism of the deep- focus earthquakes in the area concerned. The boundary between the North Honshu slab and the Izu-Bonin slab lies beneath the line connecting the Boso Gravity in the Junction between the Japanese and the Izu-Bonin Islands 305

Fig. 8. A simplified free air gravity anomaly map in and near Japan. and the Noto Peninsula, which approximates the trend of the Pacific plate movement but obliquely intersects the trend of the Fossa Magna. Aoki's model will become more significant to the tectonics of plate boundary if the movement of the deformed slab is taken into account. If it is assumed that the flat Pacific plate bends considerably as it descends from a junction of two trenches, while it is deformed as shown by the model, there occurs a new movement in the down-going Pacific slab. This movement is different from the movement of the slab descending from a straight trench. In Fig. 10 is shown schematically a top view of the streamlines on which the points assumed fixed on the surface of a descending slab is supposed to move. Two points on the descending slab, one on the North Honshu slab and the other on the Izu-Bonin slab, draw the streamlines that converge about the boundary between the two slabs. These streamlines show horizontal and convergent velocity components (that is, the components perpendicular to the boundary of the two slabs) with the magnitude of approximately one twentieth of the velocity of the Pacific plate (～10cm/yr). The vertical and downward components of the velocity (though not shown in Fig. 10) tend to be weakened where horizontal convergences occur, because the total velocity of the descending slab is assumed to be conserved. Thus, the Pacific plate descending from the Japan-Bonin junction pushes the overlying continental plate, i.e., 306 J. SEGAWA and C. BOWIN

Fig. 9. Aoki's best-fit model of the descending lithosphere from the junction between the Japan Trench and the Izu-Bonin Trench. The contours show the depth to the upper surface of the descending slab. Solid triangles show the locations of Quaternary Volcanoes. The broken line (AO) shows the boundary between the North Honshu slab and the Izu-Bonin slab. In the area OBC, two slabs have folded over due to disruption. part of the Eurasia plate, not only downwards but horizontally, in such a way that the overlying lithosphere moves convergently about the boundary between the North Honshu slab and the Izu-Bonin slab. The convergent crustal movement in the area near the Sagami Trough has been supported by the geomorphological studies: The late Cenozoic coastal terraces show that the Boso and Miura Peninsulas have tilted towards N45°E at the rate of 4.0×10-8 radian/yr, and that the Izu Peninsula, on the other hand, has tilted towards N10°W at the rate of 0.3×10-8 radian/yr (SUGIMURAand NARUSE,1954; YOSHIKAWA, 1970). The significant tilting on the side of the Boso and Miura Peninsulas can be explained by assuming the existence of a reverse fault beneath the Sagami Trough, where the crust (or lithosphere) west of the Sagami Trough is underthrusting beneath the crust (or lithosphere) east of it, that is to say, the plate convergence is occurring at the Sagami Trough. The crustal movement associated with the great Kanto earthquake of Gravity in the Junction between the Japanese and the Izu-Bonin Islands 307

Fig. 10. A top view of the schematically represented streamlines (solid lines) on which the points (solid circles) assumed fixed on the descending Pacific plate are supposed to move. Convergent movement occurs about the boundary (thick dashed line) between the North Honshu slab and the Izu-Bonin slab. Thin dashed lines are extensions of the equally spaced parallels (solid lines) at the Pacific lithosphere, drawn for comparison. 1923 presents an additional piece of evidencewhich has verified a convergent force occurring beneath the Sagami Trough. ANDO(1971) explained the observed crustal movement in the case of the great Kanto earthquake on the assumption that the crust moved on a fault plane dipping from the Sagami Trough with an angle of 45° towards north-east. A best-fit model of the crustal movement obtained by Ando shows that the crust above the fault plane, i.e., the crust on the side of the Boso and Miura Peninsulas, moved southeastwards by 6m (strike-slip component) and thrust towards southwest by 3m (dip-slip component), relative to the crust below the fault plane. The presence of the overthrusting component in the movement of the crust (or lithosphere) east of the Sagami Trough implies that the trough is a zone of plate convergence, although it may be small in scale as compared with the convergence taking place at the ordinary oceanic trenches. Thus, from the 308 J. SEGAWA and C. BOWIN evidence so far mentioned, the Sagami Trough is regarded as a zone where a kind of plate convergence is taking place, hence as a zone well worthy of the name of plate subduction zone. The accretion of low density deposit and the appearance of gravity lows may reasonably be attributed to the behavior of the Sagami Trough as a subduction zone. FITCH(1972) speculated the origin of the Sagami Trough differently. He argued that the Sagami Trough is a transform fault and has resulted from the evolution of the triple junction: The Sagami Trough is merely a trace on which the triple point of the Philippine-Sea plate moved northwestwards, when the separation of the Philippine-Sea plate from the Pacific plate as well as the appearance of a new oceanic plate followed (Fitch thought the Izu- Bonin Ridge is a new oceanic plate). The authors think, however, that the speculation of Fitch is not favored by such observed geophysical features as gravity anomalies and crustal movements at the triple junction. As to why the horizontal compression results in negative gravity anomalies, the downbuckling theory of crust or lithosphere (HEISKANENand VENING MEINESZ,1958) may be plausible: Horizontal compression causes a horizontal lithospheric plate to thicken as well as to downbuckle due to gravity. In some cases, dip-faults obliquely cut in the downbuckled plate appear, the plate of one side underthrusting beneath the plate of the other side. Such is thought to be the case with the Sagami Trough. Underthrusting of a plate is generally associated with local topographic depression as well as deposition of low density material, hence causing local mass deficiency and decrease of gravity. If the occurrence of horizontal and compressional forces as in the case of the Sagami Trough were a common feature associated with the oceanic plate descending at a junction of two adjoining trenches, then it would be very likely

Fig. 11. Elongated zones of negative free air anomalies (thick solid lines) in the Japanese Islands. Crosses show negative anomaly zones at the continental shelves, which are parallel to the trenches. KT, JT, IT, RT and NT mean the Kurile, Japan, Izu-Bonin and Ryukyu Trenches, and the Nankai Trough, respectively. Gravity in the Junction between the Japanese and the Izu-Bonin Islands 309 that a junction of the two trenches develops into a kind of T-T-T triple junction. This seems to be the reason why a bathymetric trough and/or a wide- spread gravity low is usually observed over the continental shelf near a junction of two trenches. In order to test the hypothesis mentioned above, zonings of gravity lows in the Japanese Islands are shown tentatively in Fig. 11, where the negative area at Wakkanai, northern end of Hokkaido, has been connected to the negative zone south of Hokkaido, and the negative area at the Toyama Trough has been connected to those of the Sagami and Suruga Troughs across the central Honshu. The negative zone at the Bay of Tsuruga can be connected either to the Bay of Ise or to the Strait of Kii by way of the Lake Biwa. From this figure it is seen that the trend of the negative anomaly zones branching from the Kurile-Japan junction and from the Japan-Bonin junction appear roughly parallel. A remarkable feature about these branching gravity lows is that each of them is coupled with another belt of gravity low on one side. This may be the result from the vertical movement of crusts on one side and the accompanied subsidence of crusts at the 'hinge line of the crustal movement' on the other side (YOSHIKAWA,1968): The northern zone of gravity low at the Japan-Bonin junction might have resulted from a hinge movement due to the vertical motion of the southern zone, and the northeastern zone at the Kurile-Japan junction similarly from a hinge movement due to the vertical motion of the southwestern zone. If this is true, the coupled belts of gravity low in Hokkaido may suggest the presence there of a buried trough similar to the Sagami Trough: Unusually thick sedimentary deposits (～10km) recently recognized both on land and at sea by seismic re- fraction surveys and multichannel seismic profilings (Ogawa, 1975, personal communication) may be a favourable evidence for this inference. The nature of the other elongated zones of gravity low which are smaller in scale has not so far been well explained. Since the zones of these gravity lows cross the trend of Honshu, they may be correlated with some kind of tectonic faults which cut the landmass crosswise. But, it is uncertain whether or not these gravity lows are also due to some kind of plate convergence. Among them, the negative zones at the east and west sides of the Kii Peninsula appear particularly interesting, because the area aligns with a possible spread- ing center of the Shikoku Basin (WATTSand WEISSEL,1975; TOMODAet al., 1975), and the zones of gravity low may possibly be discussed in relation to the interaction between the southwestern Honshu and the Shikoku Basin. In order to correlate gravity anomalies with past tectonics, it is important to date the major geological settings responsiblefor the gravity anomalies. In the southwestern Honshu the major topographic features are thought to have been determined by the crustal movement during the recent hundreds of thou- 310 J. SEGAWA and C. BOWIN sand of years (HUZITAet al., 1973). Since gravity is strongly affected by topo- graphy, the greater part of observed gravity anomalies may be regarded as having been generated at the time as old as the topographic structures, and the rest of the anomalies as relicts reflecting the buried and older origins: If some part of the gravity anomalies should survive the age as old as 10 my or more, they could be discussed in relation to the past tectonics, say, in the Tertiary, in which the marginal sea tectonics around Japan is generally supposed to have commenced.

The authors thank the officers and crew of the R/V Hakuho-maru and Umitaka-maru, who helped them during the gravity measurements at sea. Some of gravity data used were supplied from the Lamont-Doherty Geological Observatory, which the authors acknowledge. The authors also express their sincere thanks to Professors Y. Tomoda, A. Sugimura and S. Uyeda for critically reading the manuscript.

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