Outer slope faulting associated with the western Kuril and Japan trenches

Transcription

1 Geophys. J. Int. (1998) 134, Outer slope faulting associated with the western Kuril and Japan trenches Kazuo Kobayashi,1 Masao Nakanishi,2 Kensaku Tamaki2 and Yujiro Ogawa3 1 Japan Marine Science and T echnology Center (JAMST EC), 2 15 Natsushima-cho, Yokosuka 237 Japan. kobayashik@jamstec.go.jp. 2 Ocean Research Institute, University of T okyo, Minamidai, Nakano-ku, T okyo 164 Japan 3 Institute of Geoscience, University of T sukuba, T sukuba 305 Japan INTRODUCTION The northwestern margin of the Pacific plate is now being subducted under the northern Japanese Islands, Hokkaido and Honshu, in a direction of N62 W at a rate of 8.6 cm yr 1 (DeMets et al. 1994). The western Kuril Trench, bordering Hokkaido, trends in a direction of N60 E, whereas the Japan Trench is elongated in a direction of N20 E between latitudes N and N, N06 E between N and N, and N30 E south of N (Fig. 1). The difference in orientation of the trench axis between the western Kuril and the Japan trenches exceeds 50. Erimo Seamount is located at the junction of the Kuril and the Japan trenches. In the study area, the Pacific plate has a series of parallel SUMMARY Elongated fault escarpments on the outer slopes of the western Kuril and Japan trenches have been investigated through detailed swath bathymetric mapping. Numerous horsts and grabens formed by these escarpments were identified. Distinct N70 E linear alignment of the escarpments, parallel to the magnetic anomaly lineations, was revealed on the outer slope of the western Kuril Trench. In the Japan Trench north of N, most of the escarpments are parallel to the trench axis and oblique to the magnetic lineations. A zig-zag pattern of faulting exists south of N. Each topographic profile was decomposed by computer analysis into two curves representing (1) the smoothed long-wavelength slope of the subducting ocean-crust surface and (2) the short-wavelength (<10 km) roughness of plateaus and valleys edged by outwardand inward-facing fault escarpments. Throughout the surveyed areas, escarpment heights increase from the crest of the trench outer swell down to a depth of about 6000 m on the slope of the outer trench wall, but with no distinct increase below that depth. No significant difference is recognized in fault throws towards and away from the trench. It can be concluded that these elongated escarpments originate from normal faults on the upper layer of the oceanic crust under extensional stress in a direction perpendicular to the trench axis, which is caused by downward bending of the subducting lithosphere. The relationship of escarpment height to escarpment length is similar to that obtained from normal fault escarpments in the East Pacific Rise crest. The maximum length and height of escarpments are small in the Kuril Trench compared with those in the Japan Trench, implying a difference in mechanical strength depending on the fault orientation. The crust is weakest along the inherited spreading fabric, second weakest probably along the non-transform offset direction and strongest in directions very oblique to these orientations. Seamounts appear to be more rigid than normal ocean crust, with no particular weak orientations, resulting in fewer but larger faults along the axis of plate bending, as most clearly represented in the subducting Daiichi Kashima Seamount. Key words: Japan Trench, Kuril Trench, normal faulting. magnetic anomalies (Japanese Lineations) trending N70 E. One lineation crossing the axis of the Kuril Trench at its western tip is identified as isochron M7, which was formed at 129 Ma. Isochron M6 is situated 27.5 km north of M7 (Nakanishi et al. 1989). The age of the basin increases in a southerly direction. It has thus been concluded that the past spreading centre for this area, trending parallel to the magnetic lineations, was located to the north of these anomalies and was lost by subduction a long time ago. Reconstruction of the past plate configuration shows that the half-spreading rate of this part of the North Pacific at 130 Ma was about 6 cm yr 1, which is roughly the same as the present half-rate of opening at the East Pacific Rise. It has been confirmed by analysis of magnetic lineations, together with seismic reflection data, that RAS

2 T rench outer slope faulting 357 Figure 1. Index map showing locations of the trench axis (depths greater than 7000 m are darkly shaded) and outer swell (Hokkaido Rise shallower than 5400 m is lightly shaded) together with magnetic isochrons M5~17 (dotted lines) and fracture zones (FZ) in the northwestern Pacific margin. Rectangles indicate regions for which swath bathymetric maps are given in Fig. 2. The direction of plate convergence is denoted by a thick arrow. TD: Takuyo Daiichi Seamount, ER: Erimo Seamount, DK: Daiichi Kashima Seamount, SK: Mashu Knoll, KK: Kamuishu Knoll.

3 358 K. Kobayashi et al. no major fracture zones exist in the surveyed area between (oceanward) and inner (landward) slopes of the trenches, from E and E (Nakanishi 1993). the crest of the outer swell to the mid-slope terrace of the In this article we will examine detailed topography of the inner slope where water depths are less than 3000 m. In all of deep-sea trenches based upon swath bathymetric data obtained these cruises, 3.5 khz survey and/or single-channel seismic by three research cruises (Fig. 1). We attempt to analyse the reflection profiles together with magnetic and gravity anomalies characteristics of tectonic fabrics in the trench slopes, paying were recorded. The oceanward margin of both trenches is special attention to patterns of the fault structure in the outer characterized by an outer swell, a gently uplifted topographic slopes of these trenches. feature trending parallel to the trench axis. The water depth The occurrence of horst and graben fault structures in the of the crest of the outer swell along the Kuril Trench is as trench outer slopes has been treated by Jones et al. (1978) and shallow as 5100 m, which is nearly 1000 m above the northwest- Hilde (1983), who concluded that these structures are formed ern Pacific basin depth. The distance between the crest of the by extension as the plate bends downwards into subduction. outer swell and trench axis is approximately 70 km in the More recently, Masson (1991) summarized fault patterns of Kuril Trench. The morphology of this swell is so prominent world trenches using data available at the time and pointed that it is named the Hokkaido Rise. The outer swell of the out that the orientation of normal faults in the trench outer Japan Trench is slightly less clear than that of the Kuril slopes is controlled by the relative angle of the trench axis Trench. Its crest is deeper than 5200 m and is situated about with respect to the magnetic lineations in the subducting 80 km east of the Japan Trench axis. The bottom topography oceanic crust. He concluded that an angle of about 30 of the northwestern Pacific basin beyond these outer swells, discriminates two cases: if the angle is smaller than 30, the except for seamounts and knolls, is generally very smooth. faults are parallel to the magnetic lineations, whereas the faults Seismic reflection profiles as shown in Fig. 3 (Cadet et al. are parallel to the trench axis if the angle is greater than a) show that the acoustic basement of the Pacific basin is Our present results provide more quantitative information covered by sediment roughly 600 m thick with a few horizontal regarding the control of relative orientations between the reflectors, and that both the basement and sediment section trench axis and the inherited spreading fabrics on the fault are recently faulted on the trench slope. Highly solidified chert patterns in trench outer slopes. We have also documented was retrieved from subbottom depths of m at escarpment height and fault length on the outer slopes of these DSDP/IPOD site 436 drilled in the Pacific basin about 150 km trenches to compare them with the fault fabric at the East southeast of the trench junction (Von Huene et al. 1980). The Pacific Rise spreading centre (Cowie et al. 1994). chert layer is overlain by relatively soft pelagic sediment containing several tephra layers in its upper strata. The overall angle of dip of the outer slope in both trenches OUTLINE OF THE SURVEY is about 0.3 on the upper portion, 1.2 on the middle slope, and rapidly increases to on the lower slopes. The Bathymetric data for two of the three regions shown in Fig. 1 average outer slope of the Kuril Trench (roughly 2 ) is slightly were obtained by Seabeam surveys with 100 per cent aerial steeper than that of the Japan Trench (#1.7 ). The maximum coverage during the cruises KH-90-1 and KH-92-3 of the water depths of the trench axes are nearly the same (7200 research vessel HAKUHO-Maru of the Ocean Research 7400 m). Sediment cover of the axial deep appears to be thin Institute, University of Tokyo (Kobayashi 1991, 1993). The in both trenches except for a few fan deposits at the base of ship s positions were precisely fixed by GPS with an accuracy deep-sea channels such as the Kushiro Canyon (shown in of about 30 m. Swath bathymetric data obtained by adjacent Fig. 7). tracks are quite consistent without any corrections of position The inner slope is covered by thicker sediment and is more (Figs 2a and b). rugged and generally steeper than the outer slope. In the The third area, the northern Japan Trench from N to western Kuril Trench the average dip angle of the inner slope the Erimo Seamount at the Kuril Japan trench junction, was is roughly 5 in the lower part, about 3 in the middle slope surveyed by French research vessel Jean Charcot under the and less than 1 near the Hokkaido coast. In the Japan Trench, French Japanese cooperative KAIKO project (Le Pichon et al. the average dip of the inner slope is approximately 6 and can 1987). Data from this survey were reprocessed to produce a be as great as in the lowest part of the slope. At map with the same format as the others (Fig. 2c). The contour depths of m there exists a flat mid-slope terrace interval is 20 m for all three maps. As accuracy of positioning which traps sediments supplied from the land. at this period was not as good as that of the later cruises, Two seamounts, the Erimo and Daiichi Kashima seamounts because positions were mostly determined by Loran C and (denoted by ER and DK in Fig. 1) defining the north and only occasionally calibrated by GPS, the resulting contours in south tips of the Japan Trench, were investigated by Nautile the third area are slightly mismatched at swath boundaries, dives under the KAIKO project (Cadet et al. 1987b). Coral causing artefacts trending parallel to the ship s tracks. The limestone was found on the crests of both these seamounts, southern tip of the Japan Trench, close to the Daiichi Kashima indicating their tropical origin, great subsidence and long- Seamount, was also surveyed by the KAIKO project distance drift to their present subarctic positions. Their present (Kobayashi et al. 1987). The map is not reproduced here and depths are 3930 m for Erimo and as deep as 6000 m for the is only cited in discussion because only one large fault is western block of the Daiichi Kashima Seamount. Both seaconcentrated in the centre of the seamount. mounts are dissected by normal faults. In particular, the The majority of our survey tracks in this study are aligned Daiichi Kashima Seamount is cut by a large normal fault into in a direction roughly normal to the general trend of the two blocks, with the western block being nearly vertically trench axis, that is NW SE in the Kuril Trench area, and offset from the eastern block by about 1600 m (Kobayashi E W across the Japan Trench. The survey covers both outer et al. 1987).

4 T rench outer slope faulting 359 Figure 2. Swath bathymetric maps of (a) the western Kuril Trench (from KH-92-3) and ( b) the Japan Trench at latitudes between N and N (from KH-90-1). (c) Japan Trench at latitudes between N and N (from KAIKO Cruise of Jean Charcot). Contour interval is 20 m.

5 360 K. Kobayashi et al. Figure 2. (Continued.) In our bathymetric survey one seamount, called the kilometres on their flank and heights smaller than 1000 m Takuyo Daiichi Seamount (denoted by TD in Fig. 1), and two above the surrounding floor. Their magnetic anomalies imply knolls were precisely identified on the outer slope of the a volcanic origin. One, provisionally named Kamuishu Knoll, western Kuril Trench. The knolls have diameters of a few is situated close to the axial deep of the Kuril Trench. The

6 T rench outer slope faulting 361 Figure 2. (Continued.) other, called Mashu Knoll, is located about 30 km south of the trench axis (denoted by KK and SK, respectively, in Fig. 1). FAULTED STRUCTURE OF THE TRENCH OUTER SLOPES Swath bathymetric maps for the western Kuril and northern Japan trenches reveal that the outer slopes of trenches are dissected by a great number of elongated escarpments dipping both outwards (facing the Pacific Ocean) and inwards (facing the trench axis and island arc) to form the horst and graben structure. These structures are more clearly recognized in stacked profiles of water depths (Figs 4a,b,c). The escarpments apparently originate from normal faulting caused by the extensional stress associated with downward bending of the subducting lithosphere (Hilde 1983). The lengths of several escarpments are as great as 40 km, but most are approximately 10 km long. The spacing of adjacent escarpments is irregular but generally about a few kilometres throughout the slope. Escarpments are recognized near the crest of the outer swell but not in the abyssal plain to the southeast. Fig. 5(a to d) shows examples of topographic profiles. From them we have calculated smoothed curves describing the regional slope. Residuals of the original minus smoothed values provide local topography correlated to the faulted escarpments. In this set of figures, escarpments are relatively small in the upper slope shallower than 5500 m and attain their maximum heights at about 6000 m in water depth. No gradual increase in escarpment heights is observed at water depths greater than 6000 m. This indicates that most of escarpments are formed near the crest of the trench outer swell but do not substantially increase on the lower part of the outer slope. It must be noted that the maximum height of the inwarddipping and outward-dipping escarpments (in other words,

7 362 K. Kobayashi et al. Figure 3. Seismic reflection profile across the Japan Trench at N (the same track as Fig. 5d). The record was obtained on Jean Charcot using a water-gun for the acoustic signal source (Cadet et al. 1987a). The vertical scale is two-way traveltime in seconds. Figure 4. Profiles of water depths across (a) the western Kuril Trench, (b) the Japan Trench for latitudes between N and N and (c) the Japan Trench at latitudes between N and N. Areas shallower than 5500 m are shaded. The thick arrow denotes profile KH-92-3 Line 59 in (a), KH-90-1 Line 33 in (b) and KAIKO Line 2 in (d) of Fig. 5. The white arrow marks KH-90-1 Line 25 in Fig. 5(c).

8 T rench outer slope faulting 363 Figure 4. (Continued.)

9 364 K. Kobayashi et al. Figure 5. Selected examples of topographic profiles nearly normal to the trench axis of the western Kuril and Japan trenches. Water depth curves with smoothed slope topography ( below) and escarpments (above). Profiles of escarpments were calculated by extracting smoothed values from the original water depths. (a) Eastern part of the western Kuril Trench (KH-92-3), ( b) Japan Trench at N (KH-90-1), (c) Japan Trench at N (KH-90-1), (d) Japan Trench at N (KAIKO). G in (b) marks the position of a graben where a detailed submersible survey was conducted (see text). throws away from and towards the trench, respectively) are located at N and E with water roughly equal, forming nearly symmetric horsts and grabens. depths of the bottom of the graben close to 6500 m (Hotta It can thus be concluded that the overall shape of the inclined et al. 1992). The length of the graben is about 30 km, with a outer slope of the trench is attributable to the inward dipping width of about 5 km. The bottom of the graben is gently tilted of subducted ocean floor approximated by the smoothed curves westwards (inwards). The escarpments on either side are nearly in Fig. 5, and did not result from a step-down process due to m high and divided by sequences of steep cliffs and faults. This confirms a previous suggestion of Kasahara & flat terraces. Each steep cliff is usually less than 80 m high and Kobayashi (1993) based on the KAIKO bathymetric data. is truncated by a gently westward-tilted terrace so that the The research submersible Shinkai 6500 had several dives on whole escarpment consists of a step-like profile. Outcrops inward and outward escarpments in a N06 E-trending graben exposed on these escarpments observed by the submersible are (G in Fig. 5b) situated on the outer slope of the Japan Trench all composed of diatomaceous clay to silt with a few tephra

10 T rench outer slope faulting 365 layers. Neither basaltic rocks nor cherts were found on the post-spreading off-ridge volcanism which occurred along fissures escarpments. in the ocean floor that can be correlated to the trend of On the upper parts of both inward- and outward-dipping the spreading centre. As the bodies of seamount and knolls escarpments, several small cracks were found. Most of these are unaffected by faulting along inherited fabrics, it appears cracks are elongated nearly parallel to the trench axis. The that they are mechanically stronger than the surrounding floor. occurrence of such cracks is consistent with extensional forces Linear escarpments seem to influence the topography of the on the superficial layer of bottom sediment. A more detailed trench axis. As recognized in Figs 2 and 6, the 4 5 km wide consideration of these cracks has been published elsewhere axial valley with water depths ranging from 7000 to 7200 m is (Ogawa et al. 1997). The outer slope of the Kuril Trench has segmented at more than seven sites where large horst-andgraben not yet been surveyed by submersibles. morphology on the outer slope intersects with the axis. Details of the characteristic topography of the western Kuril The trench segments, approximately km in length, are and Japan trenches follow. aligned in an en echelon feature at an angle of about 12 clockwise to the general trench axis. Such segmentation of the The western part of the Kuril Trench axis is formed as a result of subduction of the elongated horstand-graben structure. As illustrated in a relief image (Fig. 6), escarpments on the The seabed and subsurface reflectors in the trench axis are outer slope of the western Kuril Trench are quite linear and inclined inwards at a very shallow angle. This appears to be parallel. Escarpments occasionally appear to be segmented due to the overall dip of the subducted outer slope underlying with small offsets roughly normal to the escarpments in a the axial floor. Linear features parallel to those on the outer similar manner to the non-transform offsets of the mid-oceanic slope are also seen on the lower part of the inner ridge. The dip angle of escarpments often exceeds 10 and slope, suggesting that the overlying wedge is affected by the sometimes reaches The vertical displacement of each topography of the subducted outer crust. faulted escarpment is relatively small in the Kuril Trench and In contrast, the floor of the trench axis west of E rarely exceeds 150 m. Trends of these escarpments are heavily contains the large fan-shaped flat basin extensively developed concentrated in one direction, parallel to the magnetic anomal- around the mouth of the Kushiro Canyon, which has supplied ies (~N70 E), and are clearly distinguishable from the orientation terrigenous clastics to the trench. No evidence of sediment of the trench axis (~N60 E), as seen in Fig. 7. The accretion on the inner slope has been obtained by either concentration of trends of elongated escarpments is illustrated topographic analysis or submersible observation (Cadet et al. in a rose diagram for a total of 92 elongated escarpments 1987b). (Fig. 8). The linear alignment of the faulted escarpments seems likely to be an expression of rejuvenated tectonic fabrics of the The Japan Trench ancient ocean floor formed at the spreading centre prior to The outer slope of the Japan Trench, particularly in its central 100 Ma (Kobayashi et al. 1995). Horizontal directional anisotropy portion (Figs 2b,c and 4b,c) shows a sharp contrast with that in P-wave velocities, with the largest values along the of the western Kuril Trench. Escarpment lengths can be up to spreading centre, has been reported in the East Pacific (Raitt 50 km and their heights often exceed 300 m, both being greater et al. 1969), in the northwestern Pacific (Shimamura & Asada than those of the Kuril Trench. The average dip of escarpments 1983) and in the Yamato Basin of the Sea of Japan (Okada amounts to 38. In the northern part of the Japan Trench, et al. 1978). This seems to indicate that there exist either linear most of the faulted escarpments are parallel to the trench axis inherited tectonic fabrics or mechanical anisotropy that might (Figs 9a and b). A few elongated escarpments trending in a be attributable to a preferred orientation of crystals in the direction parallel to the magnetic lineations similar to the ocean floor of spreading origin. Kuril Trench were recognized north of N by the KAIKO One depression was found during the Nautile dive on the project (Cadet et al. 1987a). We presume that they were formed northern slope of the Erimo Seamount (Kobayashi et al. 1987). under the influence of subduction in the western Kuril Trench, It appears to be possible to correlate the depression to normal since no escarpments with such an orientation are found south faults, probably trending N60 E. Although the precise direction of N. of the depression has not been identified, it seems likely that Six rose diagrams (Figs 10a to f ) show latitudinal changes the faulting of the seamount body is parallel to the trench axis in the orientation of escarpments. In latitudes from N rather than the basin lineaments. It might imply that the to N, most of the escarpments are parallel to the trench faulting of the Erimo Seamount was formed mostly under axis trending N06 E. Escarpments trending N20 W are found an extensional stress caused by the lithospheric bending, in addition to escarpments parallel to the trench axis. Such irrespective of the tectonic fabrics of the surrounding ocean escarpments appear to be nearly perpendicular to the magnetic floor. lineations. In the region at latitudes N~38 30 N and The Takuyo Daiichi Seamount and two knolls, Kamuishu longitudes west of E, magnetic anomalies are disturbed. and Mashu, do not seem to have been much affected by the Isochron M10B appears to be bent to N30 E. Escarpments predominant direction of escarpments identified on the outer parallel to this orientation are identified in this region slope. The escarpments are found on the trench slope close to (Fig. 10e). In the outer slope south of N (Fig. 10f ), the flank of the seamount and knolls but not on the features escarpments roughly perpendicular to the magnetic lineation themselves. The two knolls have round and conical shapes. predominate over those parallel to the trench axis (N06 E On the other hand, the shape of the seamount body appears north of N and N30 E to the south). to be slightly elongated along the magnetic lineations. The These conjugate faults could be a consequence of disturbed result suggests that the seamount and knolls were formed by magnetic lineations between isochrons M10A and M10N. As

11 366 K. Kobayashi et al. Figure 6. Relief image map of a selected zone of the Kuril Trench outer wall illustrating linear escarpments. Light is shot from 325.

12 T rench outer slope faulting 367 Figure 7. Distribution of faulted escarpments in the western Kuril Trench. Magnetic isochrons (after Nakanishi et al. 1989) are shown by broken curves. Figure 8. Rose diagram showing the orientation of 92 escarpments on the outer wall of the western Kuril Trench. T: trend of the trench axis; PL: direction of plate convergence; Mag: orientation of magnetic lineations. the seabed was formed from an unstable spreading centre during this period, zig-zag patterns of fault escarpments are likely to occur. The predominant direction of faults, N15 W, can be correlated to non-transform offsets in this part of the subducting slab. Although no fracture zones are found in this region, non-transform offsets aligned in a direction subnormal to the past spreading centre probably play a role as the second weakest line. This will be discussed later in this article, in combination with results from the Izu Ogasawara (Bonin) Trench. In the southern part of the Japan Trench close to the Daiichi Kashima Seamount, the faulted escarpments are parallel to the trench axis trending N30 E. Most remarkable here is the existence of a single large escarpment dividing the Daiichi Kashima Seamount into two halves. The dip of the fault plane is about 35 at the crestal zone of the seamount. The vertical displacement of the seamount body along the fault amounts to 1600 m, although submersible observation by Nautile indicated that it was formed by several repeated faulting motions rather than by one continuous movement (Kobayashi et al. 1987). The horizontal length of this fault exceeds 100 km, extending beyond both the northern and southern flanks of the seamount toward the Japan Trench axis. Its northern extension coincides with the deepest portion of the Japan Trench axis (D=7938 m), although vertical displacement gradually decreases toward the tips of the fault. Faulted topography appears to extend towards the landward wedge of the Japan Trench in a similar manner to in the

13 368 K. Kobayashi et al. Figure 9. Distribution of faulted escarpments in the outer slope of the Japan Trench. Magnetic isochrons (after Nakanishi et al. 1989) are shown by broken curves. (a) N N (b) N N. A horse-shoe-shaped slumping topography on the inner slope is also shown in (a). western Kuril Trench. No fan-shaped basin is found in the Japan Trench, in contrast to the westernmost Kuril Trench. This characteristic of the Japan Trench is due to the existence of a large mid-slope ridge (basement high), elongated roughly in a N S direction at a longitude of about E, that traps terrigenous clastics. RELATIONSHIP BETWEEN HEIGHT AND LENGTH OF THE FAULT ESCARPMENTS ON THE TRENCH OUTER SLOPE Fig. 11 shows the relationship of maximum fault escarpment height h versus fault length L for the western Kuril and Japan trenches. Only escarpments longer than 3 km were measured. Escarpments of the trench axial deep are excluded from this estimation, since they are influenced by other factors such as bottom current erosion and sedimentation. Values measured from both the maps and profiles are scattered. Nevertheless, linear correlation between h and L is recognizable. The h/l values are approximately for the western Kuril Trench and for the Japan Trench. The h/l ratio is greater for the Japan Trench than for the Kuril Trench. A distinct contrast in absolute values of h and L seems to max max occur between the two trenches. In the Kuril, L is 35 km max or less, whereas L in the Japan Trench amounts to 50 km. max As mentioned in the previous section, a normal fault dissecting the Daiichi Kashima Seamount has h=1600 m and L = 120 km, giving h/l = Cowie et al. (1994) provided a linear relationship between the height h and length L of normal fault escarpments on the East Pacific Rise. At 12 N, with a similar half-spreading rate of 5.5 cm yr 1, h/l is about and L =15 km. At max

14 T rench outer slope faulting 369 Figure 10. Rose diagrams showing the orientations of escarpments on the outer slope of the Japan Trench for six latitudinal segment divisions (a f ). T: trend of the trench axis; PL: direction of plate convergence; Mag: orientation of adjacent major magnetic lineations; N: number of samples. (T ) and (Mag) indicate orientations of nearby trench and magnetic lineations. 3.5 S, h/l = and L =55 km. These values are max roughly equal to those obtained in the trench outer slopes. This similarity in two different tectonic settings, convergent and divergent zones, appears to imply that normal faulting may be caused by a relatively simple mechanism under extensional forces exerted on the upper part of the oceanic crust (probably Layers 1 and 2) which is kept basically unchanged in the processes of the plate motion. TECTONIC SIGNIFICANCE OF FAULT ESCARPMENTS IN THE TRENCHES: DISCUSSION OF THE PRESENT RESULTS IN COMBINATION WITH THE IZU OGASAWARA TRENCH Analyses of orientation and size of faulted escarpments on the outer slopes of the western Kuril and Japan trenches have clearly indicated that distinct linear patterns of both outwardand inward-dipping escarpments are formed on the upper outer slopes shallower than 6000 m within 50 km from the crest of the outer swell. The generation of such escarpments is associated with extensional stress perpendicular to the trench axis under the influence of downward bending of the oceanic lithosphere. Theoretical calculations (e.g. Ida 1984) have shown that the observed topography of the outer swells can be explained by an assumption of a viscoelastic lithosphere. In an appropriate case, the upper surface of the oceanic lithosphere is under extension normal to the downward-bending axis. As the superficial layer of the oceanic lithosphere is brittle, normal faults can be formed on its upper surface. The prevalence of extensional stress on the upper zone of subducting lithosphere from the crest of the outer swell to about 80 km landwards from the trench axis is revealed by focal mechanism solutions of earthquakes (Utsu 1971; Yoshii 1979; Christensen & Raff 1988). Kanamori (1971) showed that the mechanism of a gigantic earthquake in 1933 causing destructive tsunami on the Sanriku coast of the northeastern

15 370 K. Kobayashi et al. Figure 11. Correlation of maximum fault escarpment height h with fault length L. Note that a predominant occurrence of fault escarpments larger than L >25 km and h>200 m is observed only in the Japan Trench. (a) Western Kuril Trench, (b) Japan Trench north of N. Honshu, Japan, was a high-angle (45 ) normal fault cutting the oceanic lithosphere at the Japan Trench outer slope perhaps close to the crack sites mentioned above (marked G in Fig. 5b), although the position of its epicentre is rather inaccurate due to a lack of sufficient ocean-bottom seismograph networks at that time. Such earthquakes may be associated with motion on a normal fault escarpment in the upper zone of the trench outer slope, triggering tsunami by large offset of the ocean floor. The swarm of conjugate faults found on the Japan Trench outer slope south of N seems to be a unique discovery. We postulate that the two predominant directions of faults represent the weakest and the second weakest lines of the Pacific crust reactivated by the lithospheric bending. Faulted escarpments trending nearly normal to the magnetic lineations were first found in the outer slope of the Izu Ogasawara (Bonin) Trench at latitudes between N and N (Seta et al. 1991). The predominant orientations of elongated escarpments in that region are N20 W ton35 W, which are

16 T rench outer slope faulting 371 only 5 to 15 oblique to the trends of the trench axis (N05 W is weakest along the inherited spreading fabric, second weakest in the north and N20 W in the south). They are roughly probably along the non-transform offset direction, and strong- perpendicular to magnetic isochrons M10 to M18 of the est in directions very oblique to these orientations. Seamounts subducting Pacific crust. The escarpment orientations are appear to be more rigid than normal ocean crust with no parallel to the Ogasawara and Kashima fracture zones, particular weak orientations, resulting in fewer but larger faults although these fracture zones do not intersect the trench axis along the axis of plate bending, as most clearly represented in in the surveyed area. The fault pattern in the Izu Ogasawara the subducting Daiichi Kashima Seamount. In any case, the Trench seems to support our hypothesis that reactivation of overall dip of the trench outer slope is determined by the longinherited non-transform offsets parallel to the stream lines of wavelength inclination of the oceanic crust rather than by seafloor spreading can be an origin of fault escarpments on displacement along fault escarpments. the trench outer slope, if the ambient stress condition is optimum for them. In the Izu Ogasawara Trench, the nontransform direction is orientated within 15 of the principal ACKNOWLEDGMENTS axis of the extensional stress and can be affected by it. No We are grateful to all the scientific members and crew who conjugate faults are observed there. participated in the cruises on which our data were collected. We presume that one of the conjugate fault orientations in The GMT software from Paul Wessel and Walter H. F. Smith the Japan Trench at latitudes south of N is roughly and the MB-System from David W. Caress and Dale N. Chayes parallel to the inherited non-transform offsets, although it is were used to make the figures in this article. We would like to so oblique to the trench axis that a new type of stress regime acknowledge them for their thoughtful help in providing their may have to be taken into account to explain its origin. A programs. plausible explanation of conjugate fault occurrence seems to be the existence of compressional stress in a direction roughly parallel to the trench axis, as the trend of the trench axis REFERENCES changes by as much as 24 from N06 E to the north of N Cadet, J.-P. et al., 1987a. The Japan Trench and its juncture with the to N30 E in the south, causing overlapping subducted lithoplanet. Kuril Trench; cruise results of the Kaiko project, Leg 3, Earth Sci. L ett., 83, spheres around its hinge point, although as yet no earthquakes Cadet, J.-P., Kobayashi, K., Lallemand, S., Jolivet, L., Aubouin, J., with such a focal mechanism have been observed. 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