Download Outer slope faulting associated with the western Kuril and Japan

Geophys. J. Int. (1998) 134, 356–372
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. E-mail: [email protected].
2 Ocean Research Institute, University of T okyo, 1–15–1 Minamidai, Nakano-ku, T okyo 164 Japan
3 Institute of Geoscience, University of T sukuba, T sukuba 305 Japan
SU MM A RY
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 39°00∞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 39°00∞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.
I NT R O DU C TI O N
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
41°00∞N and 40°10∞N, N06°E between 40°10∞N and 38°10∞N,
and N30°E south of 38°10∞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
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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
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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.
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no major fracture zones exist in the surveyed area between
144°00∞E and 147°30∞E (Nakanishi 1993).
In this article we will examine detailed topography of the
deep-sea trenches based upon swath bathymetric data obtained
by three research cruises (Fig. 1). We attempt to analyse the
characteristics of tectonic fabrics in the trench slopes, paying
special attention to patterns of the fault structure in the outer
slopes of these trenches.
The occurrence of horst and graben fault structures in the
trench outer slopes has been treated by Jones et al. (1978) and
Hilde (1983), who concluded that these structures are formed
by extension as the plate bends downwards into subduction.
More recently, Masson (1991) summarized fault patterns of
world trenches using data available at the time and pointed
out that the orientation of normal faults in the trench outer
slopes is controlled by the relative angle of the trench axis
with respect to the magnetic lineations in the subducting
oceanic crust. He concluded that an angle of about 30°
discriminates two cases: if the angle is smaller than 30°, the
faults are parallel to the magnetic lineations, whereas the faults
are parallel to the trench axis if the angle is greater than 30°.
Our present results provide more quantitative information
regarding the control of relative orientations between the
trench axis and the inherited spreading fabrics on the fault
patterns in trench outer slopes. We have also documented
escarpment height and fault length on the outer slopes of these
trenches to compare them with the fault fabric at the East
Pacific Rise spreading centre (Cowie et al. 1994).
O U TL IN E O F TH E S UR VE Y
Bathymetric data for two of the three regions shown in Fig. 1
were obtained by Seabeam surveys with 100 per cent aerial
coverage during the cruises KH-90-1 and KH-92-3 of the
research vessel HAKUHO-Maru of the Ocean Research
Institute, University of Tokyo (Kobayashi 1991, 1993). The
ship’s positions were precisely fixed by GPS with an accuracy
of about 30 m. Swath bathymetric data obtained by adjacent
tracks are quite consistent without any corrections of position
(Figs 2a and b).
The third area, the northern Japan Trench from 39°30∞N to
the Erimo Seamount at the Kuril–Japan trench junction, was
surveyed by French research vessel Jean Charcot under the
French–Japanese cooperative KAIKO project (Le Pichon et al.
1987). Data from this survey were reprocessed to produce a
map with the same format as the others (Fig. 2c). The contour
interval is 20 m for all three maps. As accuracy of positioning
at this period was not as good as that of the later cruises,
because positions were mostly determined by Loran C and
only occasionally calibrated by GPS, the resulting contours in
the third area are slightly mismatched at swath boundaries,
causing artefacts trending parallel to the ship’s tracks. The
southern tip of the Japan Trench, close to the Daiichi–Kashima
Seamount, was also surveyed by the KAIKO project
(Kobayashi et al. 1987). The map is not reproduced here and
is only cited in discussion because only one large fault is
concentrated in the centre of the seamount.
The majority of our survey tracks in this study are aligned
in a direction roughly normal to the general trend of the
trench axis, that is NW–SE in the Kuril Trench area, and
E–W across the Japan Trench. The survey covers both outer
(oceanward) and inner (landward) slopes of the trenches, from
the crest of the outer swell to the mid-slope terrace of the
inner slope where water depths are less than 3000 m. In all of
these cruises, 3.5 kHz survey and/or single-channel seismic
reflection profiles together with magnetic and gravity anomalies were recorded. The oceanward margin of both trenches is
characterized by an outer swell, a gently uplifted topographic
feature trending parallel to the trench axis. The water depth
of the crest of the outer swell along the Kuril Trench is as
shallow as 5100 m, which is nearly 1000 m above the northwestern Pacific basin depth. The distance between the crest of the
outer swell and trench axis is approximately 70 km in the
Kuril Trench. The morphology of this swell is so prominent
that it is named the Hokkaido Rise. The outer swell of the
Japan Trench is slightly less clear than that of the Kuril
Trench. Its crest is deeper than 5200 m and is situated about
80 km east of the Japan Trench axis. The bottom topography
of the northwestern Pacific basin beyond these outer swells,
except for seamounts and knolls, is generally very smooth.
Seismic reflection profiles as shown in Fig. 3 (Cadet et al.
1987a) show that the acoustic basement of the Pacific basin is
covered by sediment roughly 600 m thick with a few horizontal
reflectors, and that both the basement and sediment section
are recently faulted on the trench slope. Highly solidified chert
was retrieved from subbottom depths of 380–397.5 m at
DSDP/IPOD site 436 drilled in the Pacific basin about 150 km
southeast of the trench junction ( Von Huene et al. 1980). The
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
is about 0.3° on the upper portion, 1.2° on the middle slope,
and rapidly increases to 2.6°–5° on the lower slopes. The
average outer slope of the Kuril Trench (roughly 2°) is slightly
steeper than that of the Japan Trench (#1.7°). The maximum
water depths of the trench axes are nearly the same (7200–
7400 m). Sediment cover of the axial deep appears to be thin
in both trenches except for a few fan deposits at the base of
deep-sea channels such as the Kushiro Canyon (shown in
Fig. 7).
The inner slope is covered by thicker sediment and is more
rugged and generally steeper than the outer slope. In the
western Kuril Trench the average dip angle of the inner slope
is roughly 5° in the lower part, about 3° in the middle slope
and less than 1° near the Hokkaido coast. In the Japan Trench,
the average dip of the inner slope is approximately 6° and can
be as great as 10°–27° in the lowest part of the slope. At
depths of 5300–5500 m there exists a flat mid-slope terrace
which traps sediments supplied from the land.
Two seamounts, the Erimo and Daiichi–Kashima seamounts
(denoted by ER and DK in Fig. 1) defining the north and
south tips of the Japan Trench, were investigated by Nautile
dives under the KAIKO project (Cadet et al. 1987b). Coral
limestone was found on the crests of both these seamounts,
indicating their tropical origin, great subsidence and longdistance drift to their present subarctic positions. Their present
depths are 3930 m for Erimo and as deep as 6000 m for the
western block of the Daiichi–Kashima Seamount. Both seamounts are dissected by normal faults. In particular, the
Daiichi–Kashima Seamount is cut by a large normal fault into
two blocks, with the western block being nearly vertically
offset from the eastern block by about 1600 m (Kobayashi
et al. 1987).
© 1998 RAS, GJI 134, 356–372
Figure 2. Swath bathymetric maps of (a) the western Kuril Trench (from KH-92-3) and ( b) the Japan Trench at latitudes between 37°50∞N and 39°40∞N (from KH-90-1). (c) Japan Trench at
latitudes between 39°38∞N and 40°50∞N (from KAIKO Cruise of Jean Charcot). Contour interval is 20 m.
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Figure 2. (Continued.)
In our bathymetric survey one seamount, called the
Takuyo–Daiichi Seamount (denoted by TD in Fig. 1), and two
knolls were precisely identified on the outer slope of the
western Kuril Trench. The knolls have diameters of a few
kilometres on their flank and heights smaller than 1000 m
above the surrounding floor. Their magnetic anomalies imply
a volcanic origin. One, provisionally named Kamuishu Knoll,
is situated close to the axial deep of the Kuril Trench. The
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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).
FA U LT ED ST R UCT UR E O F T HE T R EN CH
O U TE R S LO P E S
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.
© 1998 RAS, GJI 134, 356–372
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,
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Figure 3. Seismic reflection profile across the Japan Trench at 39°39∞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 37°50∞N and 39°40∞N and (c) the
Japan Trench at latitudes between 39°38∞N and 41°00∞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).
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Figure 4. (Continued.)
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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 39°20∞N (KH-90-1), (c) Japan Trench at
39°00∞N (KH-90-1), (d) Japan Trench at 39°39∞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
roughly equal, forming nearly symmetric horsts and grabens.
It can thus be concluded that the overall shape of the inclined
outer slope of the trench is attributable to the inward dipping
of subducted ocean floor approximated by the smoothed curves
in Fig. 5, and did not result from a step-down process due to
faults. This confirms a previous suggestion of Kasahara &
Kobayashi (1993) based on the KAIKO bathymetric data.
The research submersible Shinkai 6500 had several dives on
inward and outward escarpments in a N06°E-trending graben
(G in Fig. 5b) situated on the outer slope of the Japan Trench
located at 39°10∞–39°30∞N and 144°33∞–144°37∞E with water
depths of the bottom of the graben close to 6500 m (Hotta
et al. 1992). The length of the graben is about 30 km, with a
width of about 5 km. The bottom of the graben is gently tilted
westwards (inwards). The escarpments on either side are nearly
300–350 m high and divided by sequences of steep cliffs and
flat terraces. Each steep cliff is usually less than 80 m high and
is truncated by a gently westward-tilted terrace so that the
whole escarpment consists of a step-like profile. Outcrops
exposed on these escarpments observed by the submersible are
all composed of diatomaceous clay to silt with a few tephra
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layers. Neither basaltic rocks nor cherts were found on the
escarpments.
On the upper parts of both inward- and outward-dipping
escarpments, several small cracks were found. Most of these
cracks are elongated nearly parallel to the trench axis. The
occurrence of such cracks is consistent with extensional forces
on the superficial layer of bottom sediment. A more detailed
consideration of these cracks has been published elsewhere
(Ogawa et al. 1997). The outer slope of the Kuril Trench has
not yet been surveyed by submersibles.
Details of the characteristic topography of the western Kuril
and Japan trenches follow.
The western part of the Kuril Trench
As illustrated in a relief image (Fig. 6), escarpments on the
outer slope of the western Kuril Trench are quite linear and
parallel. Escarpments occasionally appear to be segmented
with small offsets roughly normal to the escarpments in a
similar manner to the non-transform offsets of the mid-oceanic
ridge. The dip angle of escarpments often exceeds 10° and
sometimes reaches 15°–18°. The vertical displacement of each
faulted escarpment is relatively small in the Kuril Trench and
rarely exceeds 150 m. Trends of these escarpments are heavily
concentrated in one direction, parallel to the magnetic anomalies (~N70°E), and are clearly distinguishable from the orientation of the trench axis (~N60°E), as seen in Fig. 7. The
concentration of trends of elongated escarpments is illustrated
in a rose diagram for a total of 92 elongated escarpments
(Fig. 8).
The linear alignment of the faulted escarpments seems likely
to be an expression of rejuvenated tectonic fabrics of the
ancient ocean floor formed at the spreading centre prior to
100 Ma (Kobayashi et al. 1995). Horizontal directional anisotropy in P-wave velocities, with the largest values along the
spreading centre, has been reported in the East Pacific (Raitt
et al. 1969), in the northwestern Pacific (Shimamura & Asada
1983) and in the Yamato Basin of the Sea of Japan (Okada
et al. 1978). This seems to indicate that there exist either linear
inherited tectonic fabrics or mechanical anisotropy that might
be attributable to a preferred orientation of crystals in the
ocean floor of spreading origin.
One depression was found during the Nautile dive on the
northern slope of the Erimo Seamount (Kobayashi et al. 1987).
It appears to be possible to correlate the depression to normal
faults, probably trending N60°E. Although the precise direction
of the depression has not been identified, it seems likely that
the faulting of the seamount body is parallel to the trench axis
rather than the basin lineaments. It might imply that the
faulting of the Erimo Seamount was formed mostly under
an extensional stress caused by the lithospheric bending,
irrespective of the tectonic fabrics of the surrounding ocean
floor.
The Takuyo–Daiichi Seamount and two knolls, Kamuishu
and Mashu, do not seem to have been much affected by the
predominant direction of escarpments identified on the outer
slope. The escarpments are found on the trench slope close to
the flank of the seamount and knolls but not on the features
themselves. The two knolls have round and conical shapes.
On the other hand, the shape of the seamount body appears
to be slightly elongated along the magnetic lineations. The
result suggests that the seamount and knolls were formed by
© 1998 RAS, GJI 134, 356–372
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post-spreading off-ridge volcanism which occurred along fissures in the ocean floor that can be correlated to the trend of
the spreading centre. As the bodies of seamount and knolls
are unaffected by faulting along inherited fabrics, it appears
that they are mechanically stronger than the surrounding floor.
Linear escarpments seem to influence the topography of the
trench axis. As recognized in Figs 2 and 6, the 4–5 km wide
axial valley with water depths ranging from 7000 to 7200 m is
segmented at more than seven sites where large horst-andgraben morphology on the outer slope intersects with the axis.
The trench segments, approximately 30–50 km in length, are
aligned in an en echelon feature at an angle of about 12°
clockwise to the general trench axis. Such segmentation of the
axis is formed as a result of subduction of the elongated horstand-graben structure.
The seabed and subsurface reflectors in the trench axis are
inclined inwards at a very shallow angle. This appears to be
due to the overall dip of the subducted outer slope underlying
the axial floor. Linear features parallel to those on the outer
slope are also seen on the lower part of the inner
slope, suggesting that the overlying wedge is affected by the
topography of the subducted outer crust.
In contrast, the floor of the trench axis west of 145°45∞E
contains the large fan-shaped flat basin extensively developed
around the mouth of the Kushiro Canyon, which has supplied
terrigenous clastics to the trench. No evidence of sediment
accretion on the inner slope has been obtained by either
topographic analysis or submersible observation (Cadet et al.
1987b).
The Japan Trench
The outer slope of the Japan Trench, particularly in its central
portion (Figs 2b,c and 4b,c) shows a sharp contrast with that
of the western Kuril Trench. Escarpment lengths can be up to
50 km and their heights often exceed 300 m, both being greater
than those of the Kuril Trench. The average dip of escarpments
amounts to 38°. In the northern part of the Japan Trench,
most of the faulted escarpments are parallel to the trench axis
(Figs 9a and b). A few elongated escarpments trending in a
direction parallel to the magnetic lineations similar to the
Kuril Trench were recognized north of 39°55∞N by the KAIKO
project (Cadet et al. 1987a). We presume that they were formed
under the influence of subduction in the western Kuril Trench,
since no escarpments with such an orientation are found south
of 39°55∞N.
Six rose diagrams (Figs 10a to f ) show latitudinal changes
in the orientation of escarpments. In latitudes from 39°40∞N
to 39°00∞N, most of the escarpments are parallel to the trench
axis trending N06°E. Escarpments trending N20°W are found
in addition to escarpments parallel to the trench axis. Such
escarpments appear to be nearly perpendicular to the magnetic
lineations. In the region at latitudes 39°00∞N~38°30∞N and
longitudes west of 144°30∞E, magnetic anomalies are disturbed.
Isochron M10B appears to be bent to N30°E. Escarpments
parallel to this orientation are identified in this region
(Fig. 10e). In the outer slope south of 38°30∞N (Fig. 10f ),
escarpments roughly perpendicular to the magnetic lineation
predominate over those parallel to the trench axis (N06°E
north of 38°10∞N and N30°E to the south).
These conjugate faults could be a consequence of disturbed
magnetic lineations between isochrons M10A and M10N. As
Figure 6. Relief image map of a selected zone of the Kuril Trench outer wall illustrating linear escarpments. Light is shot from 325°.
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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.
© 1998 RAS, GJI 134, 356–372
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
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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) 37°50∞N–39°40∞N (b) 39°39∞N–41°00∞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 144°00∞E, that traps
terrigenous clastics.
R E LAT IO N S HI P B ETW E EN H EI GH T A N D
L EN GT H O F TH E FA ULT E SC A R P M EN TS
O N TH E TR EN C H O U TE R S LO P E
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 6.4×10−3 for the western Kuril
Trench and 9.7×10−3 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 =1.3×10−2.
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 1.4×10−2 and L
=15 km. At
max
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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 =5.0×10−3 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.
T ECTO N I C S I GN I FIC AN CE O F FA U LT
E S CA R PM EN TS I N TH E TR E NCH ES :
D IS C U S S IO N OF TH E PR E S ENT R ES ULTS
I N CO M BI N ATI O N W I TH T HE I ZU –
O G A S AWA R A TR E NC H
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
© 1998 RAS, GJI 134, 356–372
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
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 39°30∞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 39°00∞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 32°20∞N and 27°30∞N
(Seta et al. 1991). The predominant orientations of elongated
escarpments in that region are N20°W to N35°W, which are
© 1998 RAS, GJI 134, 356–372
T rench outer slope faulting
only 5° to 15° oblique to the trends of the trench axis (N05°W
in the north and N20°W in the south). They are roughly
perpendicular to magnetic isochrons M10 to M18 of the
subducting Pacific crust. The escarpment orientations are
parallel to the Ogasawara and Kashima fracture zones,
although these fracture zones do not intersect the trench axis
in the surveyed area. The fault pattern in the Izu–Ogasawara
Trench seems to support our hypothesis that reactivation of
inherited non-transform offsets parallel to the stream lines of
seafloor spreading can be an origin of fault escarpments on
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
axis of the extensional stress and can be affected by it. No
conjugate faults are observed there.
We presume that one of the conjugate fault orientations in
the Japan Trench at latitudes south of 39°40∞N is roughly
parallel to the inherited non-transform offsets, although it is
so oblique to the trench axis that a new type of stress regime
may have to be taken into account to explain its origin. A
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
changes by as much as 24° from N06°E to the north of 38°10∞N
to N30°E in the south, causing overlapping subducted lithospheres around its hinge point, although as yet no earthquakes
with such a focal mechanism have been observed.
Our results on the maximum lengths and accordingly the
maximum heights of fault escarpments seem to provide a clue
to determining the strength of the upper crust along various
directions, because the size of faults depends on the relative
orientation chosen by the trend of the lithospheric bending
axis (i.e. the trench axis). In the western Kuril Trench, the
preferred orientation of faults is along the weakest line, so
numerous moderately small faults with quite concentrated
orientations were formed. In contrast, the trench axis of the
northern Japan Trench north of 39°00∞N is quite oblique to
any weak lines in the horizontal plane. Relatively large normal
faults parallel to the trench axis are generated under the
bending force. The fault cutting the Daiichi–Kashima
Seamount is an extreme case, since the seamount body has a
thicker and probably stronger basaltic layer than the normal
ocean floor. Extensional stress is concentrated at the fault line,
once generated, giving rise to a single large faulted escarpment.
Based upon the established plate kinematic model, providing
a rate of convergence of plates at the western Kuril and Japan
trenches of 8.6 cm yr−1, the time necessary to move from the
crest of the outer swell to the trench axis is approximately 1
Myr. Faulted escarpments now existing close to the trench
axis should thus be younger than 1 Ma. On the other
hand, submersible observations of escarpment exposure of
Cretaceous reef limestone on the Daiichi–Kashima Seamount,
situated close to the trench axis, revealed that faulting occurred
repeatedly but concentrated over a period older than 10 000 yr,
as inferred by partial Mn-oxide encrustation of limestone and
an overlying thin veneer of pelagic sediment (Konishi 1989).
In conclusion, our comprehensive analysis of trench outer
slopes has indicated that tectonic fabrics formed at the spreading centre some 120 million years ago are apparently rejuvenated in the subduction zones, if the relative orientation of the
old fabrics with bending axis is appropriate. Comparison of
faults among various circumstances has revealed that the crust
© 1998 RAS, GJI 134, 356–372
371
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. In any case, the
overall dip of the trench outer slope is determined by the longwavelength inclination of the oceanic crust rather than by
displacement along fault escarpments.
A CKN O W LE DG M ENT S
We are grateful to all the scientific members and crew who
participated in the cruises on which our data were collected.
The GMT software from Paul Wessel and Walter H. F. Smith
and the MB-System from David W. Caress and Dale N. Chayes
were used to make the figures in this article. We would like to
acknowledge them for their thoughtful help in providing their
programs.
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