The Japan Trench and its juncture with the Kuril Trench" cruise

Earth and Planetary Science Letters. 83 (1987) 267-284
267
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
[41
The Japan Trench and its juncture with the Kuril Trench"
cruise results of the Kaiko project, Leg 3
Jean-Paul Cadet ~, Kazuo Kobayashi 2, Jean Aubouin 3, Jacques Boul+gue 4 Christine Deplus 5,
Jacques Dubois 5, Roland von Huene 6 Laurent Jolivet 7, Toshihiko Kanazawa ~,
Junzo Kasahara 9, Kinichiro Koizumi 2, Serge Lallemand 7 Yasuo Nakamura ~0, Guy Pautot a~,
Kiyoshi Suyehiro ~2, Shin Tani ~3, Hidekazu Tokuyama 2 and Toshitsugu Yamazaki 14
l
l~aboratotrede G~ologie Dynamique (UA CNRS 215), D~partement des Sciences de la Terre. Unit)erstt~ d'Orl~ans,
45046 OrlOans C~dex (France)
2 Ocean Research Institute, Universio, of Tokyo, 1-15-1, Minamidai. Nakano-Ku. Tokyo 164 (Japan)
D~partement de G~otectonique (UA CNRS 215), Unwersit~ Pierre et Marie Curie. 4 place J~t~sieu, 75252 Paris C~dex 0_5 (France)
4 lxaboratoire de G~ochimie et M~tallog~nie (UA CNRS 196), Universit6 Pierre et Marie Curie, 4 place Jussieu.
75252 Paris C~dex 05 (France)
.s Laboratoire de G~ophyslque et de G~odynamique interne (UA CNRS 730), Unit;erstt~ Paris XI. 91405 Orsav (_'~dex (France)
Office of Marine Geology, U.S. Geological Surt,ev, Menlo Park, CA 94025 (U.S.A.)
7 Laboratoire de G~ologie (UA CNRS 215). Ecole Normale Sup~rieure. 24 rue Lhomond. 75005 Paris. (France)
Geophysical Institute, Unioersity of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113 (Japan)
Earthquake Research Institute, Unie,ersio" of Tol~vo, 1-1-1 Yayoi. Bunkyo-ku. Tokyo 113 (Japan)
io Department of Liberal Arts, Unieersi(v of Tokyo, 3-8-1 Komaba, Meguro-ku. Tokyo 153 (Japan)
i1 IFREMER, Centre Oc~anologtque de Bretagne. B.P. 337, 29273 Brest Cedex (France)
l: Department of Earth Sciences, Faculty of Science, Chiba University. 1-33 Yayoi-cho, Chiha 260 (Japan)
m ttydrographic Department, Maritime Safe O' Agenc:v, 5-3-1 Tsukqi, ('huo-Ku. Tokyo 104 (Japan)
m Geological Survey of Japan, 1-1-3 Yatabe-Htgashi, T~ukuha, lbaragi 305 (Japan)
Revised version accepted October 17, 1986
This paper presents the results of a detailed survey combining Seabeam mapping, gravity and geomagnetic
measurements as well as single-channel seismic reflection observations in the Japan Trench and the juncture with the
Kuril Trench during the French-Japanese Kaiko project (northern sector of the Leg 3) on the R / V "Jean Charcot".
The main data acquired during the cruise, such as the Seabeam maps, magnetic anomalies pattern, and preliminary
interpretations are discussed. These new data cover an area of 18,000 km 2 and provide for the first time a detailed
three-dimensional image of the Japan Trench. Combined with the previous results, the data indicate new structural
interpretations. A comparative study of Seabeam morphology, single-channel and reprocessed multichannel records
lead to the conclusion that along the northern Japan Trench there is little evidence of accretion but, instead, a tectonic
erosion of the overriding plate. The tectonic pattern on the oceanic side of the trench is controlled by the creation of
new normal faults parallel to the Japan Trench axis, which is a direct consequence of the downward flexure of the
Pacific plate. In addition to these new faults, ancient normal faults trending parallel to the N65 ° oceanic magnetic
anomalies and oblique to the Japan trench axis are reactivated, so that two directions of normal faulting are observed
seaward of the Japan Trench. Only one direction of faulting is observed seaward of the Kuril Trench because of the
parallelism between the trench axis and the magnetic anomalies. The convergent front of the Kuril Trench is off~t
left-laterally by 20 km relative to those of the Japan Trench. This transform fault and the lower slope of the
southernmost Kuril Trench are represented by very steep scarps more than 2 km high. Slightly south of the juncture,
the Erimo Seamount riding on the Pacific plate, is now entering the subduction zone. It has been preceded by at least
another seamount as revealed by magnetic anomalies across the landward slope of the trench. Deeper future studies
will be necessary to discriminate between the two following hypothesis about the origin of the curvature between both
trenches: Is it due to the collision of an already subducted chain of seamounts? or does it correspond to one of the
failure lines of the America/Eurasia plate boundary?
0012-821X/87/$03.50
.~ 1987 Elsevier Science Publishers B.V.
268
I. Introduction
During Leg 3 of the French-Japanese Kaiko
project using R / V "'Jean Charcot", we surveyed
the Japan Trench subduction zone [1] which is
associated with the modern seismicity of the
northern Honshu island. The detailed survey
covers about 18,000 square kilometers along the
trench with Seabeam bathymetry and simultaneous observations of the magnetic and gravity
fields. Single-channel seismic records were also
obtained. All these data were recorded continuously along the tracks. Navigation in a Loran-C
net provided continuous position information
nominally precise to about 100 m, and onboard
computer and plotting facilities enabled us to make
141°E
142%
143%
real-time interpretations. Here we summarize
mainly these on board interpretations and preliminary post cruise studies as well.
1.1. The objectives o f the surt, ev
The area we surveyed during the later part of
Kaiko Leg 3 (July 19-29, 1984) includes the
northern Japan Trench and its juncture with the
Kuril Trench at Erimo Seamount (Fig. 1). The
front of the landward slope of the Japan Trench,
once considered a typical convergent margin with
active accretion, was found from study of the
cores recovered during the Deep Sea Drilling Project (DSDP) Legs 56 and 57 to have an unexpected
history of massive subsidence and probably landward retreat of the slope. Accretion was limited to
144°E
145°E
146°E
43°N
H
i
i
42°N
i
i
/
41°N
Hachin(
NOR'
tic
40~
39°N
Fig. 1. Lcx:ation of the Kaiko survey (Leg 3, box 1 ) off northeast Japan in addition to the previous multichannel seismic lines: .INOC
1. J N O ( ' 2 (Japan National Oil Corporation, 1976). ORI 78-3. ORI 78-4 (Ocean Research Institute and Japan Petroleum Exploration
Company, 1978) and P-849 (Shell Oil Company, Beck et al. 1976). DSDP sites (I,egs 56-57 and 87) were also plotted on this map.
Isobaths are derived from the bathymetric chart of the adJacent seas of Nippon No. 6301 (Hydrographic Department. Maritime
Safety Agency, Japan. 1966).
269
a narrow zone on the lowermost part of the landward slope which was not well resolved even by
multichannel seismic records. This area of poor
resolution was considered accretionary by some
authors [2] but other authors argued for an origin
from mass movement [3]. A study of this problem
was one of our main objectives.
The second objective concerned the seaward
slope of the Japan Trench. It was here that the
horst and graben structure marking the flexure of
ocean crust was first described by Ludwig et al. [4]
and lwabuchi [5]. Linear magnetic anomalies trend
approximately 50 ° obliquely to the trench axis
and Honza [6] further proposed a trend of the
horst and graben nearly perpendicular to the magnetic anomalies (25 ° west of north). These trends
were debated especially in view of the tendency
for horst and graben along the Middle America
Trench to parallel magnetic anomalies [7].
The third major objective was to explore the
Japan and Kuril Trench juncture near Erimo
Seamount. This already complex juncture is further complicated by a southward projection of the
Central Tectonic Belt on the island of Hokkaido
that passes beyond Cape Erimo into the Pacific
(Fig. 1). The belt may have been a major plate
boundary either between the American and Eurasian plates or between the Okhotsk and Amurian
plates in Neogene time [8-10]. The role of Erimo
Seamount at this juncture is debated from several
points of view, because neither the bathymetry nor
the seismicity indicate a clear structural relation
with other major tectonic features. The junction of
the two trenches is also located at the mouth of
the so-called Hidaka Trough which is the southward continuation of the Sapporo-Tomakomai depression west of the Central Tectonic Belt in Hokkaido (Fig. 1). This depression is filled by a thick
accumulation of recent sediments [11] supplied by
rivers from the Hidaka Mountains. This strong
detrital supply should affect the morphology of
the trench in the area of the Erimo Seamount.
We focus our report of preliminary results on
these three problems and present some of the
interesting discoveries from the data acquired in
this initial stage of our study.
1.2. Background from the precious studies
The geophysical data across the Japan Trench
obtained in preparation for the drilling on Legs 56
and 57 included not only many single-channel
records but also multichannel seismic reflection
records which crossed the continental slope, and
part of the seaward slope of the trench [6,12] (Fig.
1). The accompanying seismic refraction measurements indicated a transition from continental to
oceanic crust well down the slope and generally
within 15-20 km landward of the trench axis
[13,14]. The combined seismic data show the subducted crust with an oceanic velocity structure
overlain by a crust with continental velocity structure (Fig. 2). Only a small wedge-shaped area at
the front of the margin has velocity in the range of
those found in accretionary complexes. The midslope terrace appears to mark a fundamental
tectonic juncture between oceanic and continental
elements as defined by refraction velocity. The
terrace also marks a transition in the character of
seismic reflection records from well organized
coherent reflections to less coherent faint reflections, or a short reflective sequence and increased
diffractions (Fig. 2). This mid-slope transition was
interpreted as the end of the coherent continental
section and the beginning of the deformation associated with subduction at the front of the margin
[15]. Within the continental reflective sequence is
a prominent unconformity that truncates landward dipping reflections of low amplitude and
refraction velocities between 3.6 and 4.8 k m / s .
The unconformity is in turn transgressed by the
overlying sequence of strata with velocities between 1.6 and 2.6 k m / s . The landward dipping
reflection below the unconformity in layers of
higher velocity, corresponds to the Cretaceous
rocks recovered by drilling on Leg 57 at Site 439,
whereas the transgressing subhorizontai reflections above the unconformity correspond to the
Neogene sequence [16]. Drill cores recovered lithologies and benthic microfossils that establish the
sub-aerial origin of the unconformity which can
now be seen in the seismic records at depths
greater than 5 km [15].
The Neogene tectonic history of the Japan
Trench landward slope is dominated by massive
subsidence occasionally associated with arc
volcanism and subduction until about 5 m.y. ago,
and one of uplift for the last 3 m.y. The rate of
vertical motion near shore is much less and perhaps negligible. This history is based on the lithologic and paleontological sequences recovered both
270
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Fig. 2. Composite seismic refraction data (modified by ,,'on Huene et al. [15] after Murauchi and Ludwig [13]) and typical topographic
profile corresponding to one survey line along 40 ° 07'N.
from drill holes near shore and also near the edge
of the deep-sea terrace. The temporal boundaries
located by mean of the drill holes were followed in
seismic records to show the seaward increase in
subsidence corresponding to the increase in depth
of the subaerial unconformity cutting the Cretaceous. The subsidence was like a large flap with a
quasi-stable hinge point near the present shoreline
and an edge that subsided most rapidly near the
trench. Subsidence resulted in the progressive seaward shift of the Neogene depocenter. Most of the
sediments sampled was a monotonous sequence of
silt and claystone of hemipelagic origin with rare
inter-beds of sand turbidites. Thus it is presumed
that despite Neogene subsidence, conditions of
sedimentation remained much the same except
near the trench where tilting and truncation of the
Pliocene and Pleistocene section is shown by
seismic and drilling results. A progressive microfracturing of the cored sediments whose origin
is thought to result from hydrofracturing as a
consequence of overpressured pore fluids increased progressively toward the trench [15,17].
The seaward slope of the trench is composed of
crust with a Cretaceous magnetic anomaly pattern
that crosses the trench obliquely [18]. Honza [6]
proposed a trend of the horst and graben perpendicular to the anomalies on the basis of a
series of single-channel seismic reflection records.
Off Mexico and Guatemala, however, horst and
graben follow the magnetic anomalies, that is to
say the original spreading structure of the oceanic
crust [7]. The topography conforming uniformly
thick (560-650 m) sediments on the ocean crust
consists of Cretaceous cherts disconformably overlain by Eocene to Miocene pelagic clay and then a
Neogene sequence of hemipelagic sediment. The
1933 Sanriku earthquake occurred on a normal
fault seaward of the axis of the trench [19].
Erimo Seamount, by its position at the Japan
and Kuril Trench juncture (Figs. 1 and 3), invokes
suggestion of a tectonic relation between the
seamount and the juncture [20]. On the other
hand, the seamount could also be a passenger of
the subducting oceanic plate. The range of speculation regarding the role of the seamount is broad
271
and can only be reasonably evoked after treatment
of the data because the juncture area shows a
complex topography not resolved by reconnaissance survey. Nor do the seismological data present a systematic pattern of dynamic parameters
except for a broad curvature in hypocentral depth
contours in the upper seismic plane [21]. The
sharpness of the change in trend of the subduction
zone at this juncture is a feature of large proportion and puzzlement.
2. The Japan Trench
2.1. Landward slope morphology, Seabeam data
The excellent conventional bathymetric maps
already published by Japanese institutions [22,23]
was detailed by our Seabeam survey. Although
many details of individual echosounder profiles
suggest various tectonic features, the two-dimensional Seabeam maps clearly define a morphology
produced by the modern Japan Trench subduction
zone. The general morphological subdivisions developed from seismic records and single bathymetric profiles are characterized in greater detail and
on the landward slope of the trench: an upper
slope, mid-slope terrace and lower slope are easily
distinguished. The lower slope is further characterized by an upper part, an escarpment, and a
peculiar base of slope area (Figs. 2, 3; Plate IIA,
map 4; Plate liB, diagram 4).
Above the mid-slope terrace is the relatively
smooth morphology of the upper slope. Its featureless character and average 3 ° dip is consistent with
the morphology in the conventional maps [22,23].
The transverse structure displays a well rounded
topography rather than sharp offsets. Transverse
features affect the mid-slope terrace causing an
apparent constriction or a down-slope step. Otherwise only small straight rills trending directly
down-dip cross the part of the slope that was
included in the Seabeam survey. A notable aspect
of the upper slope is the absence of a morphology
that reflects tectonic features. This is in stark
contrast with the disrupted morphology that
dominates the regime below the mid-slope terrace.
The mid-slope terrace is a nearly continuous feature across the Seabeam map at a general depth of
4400-4500 m. It is of variable width (Fig. 3; Plate
IIA, map 4; Plate liB, diagram 4). commonly
exhibiting ponded sediments, local ridges and
closed uplifts and depressions. Local constrictions
by lobate masses are possibly slumps. Slumps
were indicated in the core of site 440 in the
ponded unit of poorly sorted clayey sand and
gravel [24]. The age of the ponded sediments is
Holocene to Upper Pleistocene, maximum age
being 0.26 Ma. The mid-slope terrace has trapped
material transported down-slope and has probably
formed a resting place for local slumps. The terrace marks a fundamental change in the morphology of the slope from the smooth landforms
of the continental terrace to the first clearly linear
elements parallel to the trench.
The adjacent terrane immediate down-slope from the
terrace is the least disrupted part of the lower
slope. Here the Seabeam contours trend generally
north, show a 9 ° dip, and are locally deflected by
shallow re-entrants and small highs (Plate IIA.
map 4; Plate liB, diagram 4). Several short
"canyon-like" features trend down-dip, suggesting
down-slope transport of sediment. Notable is the
lack of a strong structural grain in comparison
with the adjacent areas.
A dominant morphological features of the
Seabeam map is a sinuous escarpment at about
6000 m depth that trends subparallel to the trench
axis (Fig. 3; Plate IIA; map 4; Plate liB, diagram
4). The strong continuity of the escarpment is
broken toward the southern part of the map and
although clearly defined in four of the five multichannel seismic records crossing the slope, the
principal record used to locate DSDP site, J N O C
1, is located on one of the gap (Fig. 1). The scarp
has a remarkably uniform width of about 2 km, an
average slope of 20 ° and a length of about 120
km. Locally it is 1 km high.
Between the scarp and the trench axis is an
area of generally disorganized topography (Plate
I1A, map 4; Plate liB, diagram 4). Closed depressions, hills and lobate bodies suggest slumps. A
locally strong but discontinuous linear topography
is developed near the trench axis and parallels it; a
steep front commonly bounds the axis. This morphology is about 10 km wide in the south of the
surveyed area and 38 km at its widest part in the
north (Fig. 3) as the escarpment and trench axis
diverge northward.
272
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Fig. 3. Tectonic map dra~n from the Kaiko results. See Fig. 1 for Zocation.
10
20kin
273
The trench axis trends increasingly east of north
toward Erimo Seamount (Fig. 3); its juncture with
the slope is sinuous on the scale of about 5 km.
Ellipsoidal closed depressions, rarely more than 3
km wide, are commonly separated by 5 - 2 0 km
narrow stretches along the trench floor. A profile
of depth along the axis shows a variability in
depth of about 150 m and a general southward
deepening of 600 m in about 3.5 ° latitude.
2.2. Landward slope geophysical data
Magnetic anomalies (Fig. 4) observed onboard
confirm the pattern reported previously [15,25]
and show in detail the N 7 0 ° E trending magnetic
anomalies M8, M9 and M10 of Cretaceous age.
The age of the anomalies increases southward
paralleling the increasing depth of the ocean floor
and trench axis. No lateral displacement along the
trench, inferred by Hilde et al. [18,26], is observed
in the present survey. The anomalies extend landward of the trench axis and decrease in amplitude
as reported previously [27] (Fig. 4). Gravity free
air anomalies show a north-south trending trough
just landward of the trench axis (Fig. 5). The
deepest negative anomaly at the northern end of
the Japan Trench does not correspond to a topographic feature and probably indicates crustal depression.
All but one of the 5 multichannel seismic records previously reported in this area [3,12,28] display the morphological elements detailed by
Seabeam. The sequence of eroded Cretaceous rocks
covered by Neogene slope sediments is disrupted
at the mid-slope terrace by structure not clearly
resolved in the best processed seismic records.
Generally few coherent reflections are resolved
from the mid-slope terrace to the trench axis, but
a recently reprocessed diplay of ORI 78-4 [3]
shows the structural configuration at the front of
the subduction zone [28]. A line drawing of the
seismic reflection record section (Fig. 6) shows the
escarpment to be a slump scar about 1 km high
truncating a nearly horizontal reflective sequence.
This reflective sequence is a part of coherent block
that comprises the lower landward slope of the
trench. In front of the escarpment is a thick pile of
sub-horizontal short reflections. The structure of
this pile is not that of the landward dipping reflections in an accretionary complex except at the
steep slope immediately adjacent to the trench
axis. The association of the pile. with the scarp
suggests that it consists of reworked slump debris
now being deformed at the trench axis. In support
of this interpretation is the lack of thick sediment
ponded in the trench axis to serve as material for
accretion.
At the base of the stratified sequences is a
section of subducting sediment which is not disrupted from the seaward slope of the trench for
about 40 km down the subduction zone. The
continuity of the subducted oceanic sequence at
the front of the margin indicates relatively low
friction along the subduction zone. From the
mid-slope terrace to the trench is an aseismic zone
based on observations using a local network of
ocean bottom seismometers [29,30]. This suggests
decoupling of the upper and lower plates in the
front of the subductioaa zone consistent with the
low compressive stress suggested by the massive
failure of the lower slope [31].
2.3. Seaward slope morphology and geophysics
The oceanic plate was surveyed from the trench
to 50 km eastward; in addition, four longer lines
(two bands, 8 km wide for each, between 39°40
and 39°50 of latitude) were surveyed 110 km
seaward of the trench axis in order to precisely
define the bending of the oceanic plate prior to
subduction beneath the continental plate.
The horst and graben structure. Faults were first
defined in single-channel seismic records recorded
simultaneously with the Seabeam data. These
faults were first plotted on topographic profiles
(given by the vertical beam) and subsequently on
the Seabeam map where they corresponded to well
defined scarps (Fig. 7). Shipboard maps indicated
an organized horst and graben structure and an
increase in the number of landward facing normal
faults toward the trench. Normal faults first occur
at the bulge which is well developed off the Japan
Trench [32]. No faults with large vertical offset
were recognized, during our survey in the ocean
basin, more than 50 km seaward of the trench
axis. They are subparallel to the Japan and Kuril
Trench axes respectively, and trend N 1 0 ° E (plus
or minus 20 o ) in the south, to N60 o E ( _+ 10 o ) in
the north, with an intermediate zone south of the
Erimo Seamount (Fig. 3). The most prominent
scarp is located south of 40 ° in latitude and
274
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rimo
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275
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276
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20-40 km east of the Japan "French axis. This
N I 0 ° W trending scarp corresponds to a normal
fault which shows a vertical displacement of 300
m on the basis of seismic data, creating a 12 °
slope facing the trench. It is more than 60 km long
(35 km are shown in Plate IIA, map 4 and its
southward extension was mapped during the return transit survey). The sediment blanket does
not appear to significantly mask the ocean floor
displacement above faults indicating recent displacements. We also observed an increase in displacement of normal faults toward the trench,
although the extent and vertical displacement in
the north are somewhat less than in the south.
The N65°E lineaments. Lineaments with a N65°E
strike are noted in stepwise offsets along ridges.
They are subparallel to the direction of the Lower
Cretaceous magnetic anomalies M8 to M10 (Figs.
3 and 4) and probably reflect the original grain of
the oceanic crust which is inherited from the
spreading axis. The direction of these lineaments
is very close to that of the Kuril trench so as to be
mingled with the horst and graben structure in the
north. That is to say, the Cretaceous tectonic
lineaments are probably rejuvenated near the
trenches inducing normal faults in the same direction as the magnetic lineations when the trench
axis strikes subparallel to them.
Basement morpholoyo,, lsobaths of the top of the
igneous oceanic crust illustrate well the undulations of basement in the directions of the magnetic
anomalies. They also show that the ocean crustal
flexure into the trench begins approximately 100
km seaward of the trench axis, reaches 3 or 4 ° at
the axis, and continues to steepen about 100 km
down the subduction zone where Murauchi and
277
KAIKO_LEG3 1984
V E R T I C A L E X A G E R A T I O N -~ d.
KURIL TRENCH
- - L ~..
.
.
.
.
.
.-
.-
..,
E R IM O S E A M O U N T
....
/
~~
100 k m
3APAN TRENCH
Fig. 7. Perspective diagrams of the surveyed area drawn using the Seabeam map and the seismic profiles.
Sedimentary cooer. The top of the Cretaceous chert
layer consists of diatom mud and the lower one is
made with biogenic oozes and clay [33]. The average thickness of both layers is about 600 m and
the variations are mainly associated with the horst
and graben morphology. That is to say, some
erosion of the upper layer occurs above the horsts
and the eroded material fills grabens. The thickness of trench fill sediment is generally less than
3 0 0 - 4 0 0 m in the elongated axial basins, but it
reaches 6 0 0 - 8 0 0 m in the Kuril Trench axis where
considerably more sediment is ponded (Fig. 8).
layer is well defined on the single channel seismic
profiles, but it is quite impossible to distinguish
the top of the oceanic igneous crust even in well
processed multichannel seismic data. The upper
Magnetism and grauity. The map of the magnetic
total field isoanomalies (Fig. 4) shows a very clear
W S W - E N E trend. The peak to peak amplitude is
Ludwig [13] showed a 8 - 9 ° angle (Fig. 2). In
detail, the average dip of the oceanic crust is 1.5 °
3 0 - 4 0 km seaward of the trench axis and varies
from 3 ° in the south to 5 ° in the northern Japan
Trench axis due to the presence of the Erimo
Seamount on the ocean crust. That is to say, the
flanks of the seamount, which are steeper than the
oceanic crust slope, can be followed under the
landward slope.
~ - ~
e--_~
'
Co,.
--_~ ~ ~ ,
~
~
i'----,\
..
I
~.
I
\
"
-
~
"
.-.~
~
~
O
."',
/
-zlo=
.° ~
"',2 °
~
-
"
.~
~
t
N.E. JAPAN
f
ESCARPMEN'F'_,x:--~=..
-"x
w~
~1
.... ' ~ " 1
,~r-
'
/
~
'
'
--
'
~
h
l'ig. g. Serial cross sections of box 1 interpreted fronl .,,ingle-channel ~,eismic profiles.
"*t~
0',~.."": "~'~--,.
\
VoJcani¢ ,ocks
I~^^1
MID - SLOPE
TERRAC6
...m., ,.o.,.
J]
>
--.~
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~
-
~
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"~
~
-
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"~--"~
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. . . . . . . . . . .
L20
/~.~--~.-----
PACIFIC
;
JAPAN
TRENCH
ERINO
SEANOUNT
KURIL
TRENCH
PLATE
S
OC
279
about 700 nT. Magnetic lineations M8 to M10
(about 130 Ma), caused by normal and reversed
polarity successions recorded in the remanent
magnetization of the Pacific plate, are recognized.
The plate being younger northward. No lateral
displacements along transform faults appear in
the surveyed area. The general trend of gravity
anomalies well conforms to the classical model of
free air anomalies over an oceanic subduction
zone: values near 0 on the oceanic lithosphere and
a minimum of - 139 mgal (in the south) to - 175
mgal (in the north) shifting landward of the trench
axis (from 4 km in the south to 19 km in the
north). As in the Daiichi Kashima area (see
Kobayashi et al. [43]), the absolute values of gravity observed are lower than predicted for a 130
Ma old subducting oceanic lithosphere [34]. On
the oceanic side the isoanomaly curves follow the
same eastward curvature as the isobaths, whereas
their orientation is approximately north-south on
the landward slope. We interpret this observation
in the following way: as the trend of the flow-line
of the Pacific plate diverges eastward away from a
north-south line of reference, its geometrical junction with the Eurasian plate deepens. This deepening can be observed on free-air anomaly minimum
trend (Fig. 5), but not on trench axis depth which
follows the oceanic isobaths.
3. Trench juncture, Erimo area
3.1. Morphologr', Seabeam data
The Seabeam map at the junction of the two
trenches shows drastic morphological differences.
We describe successively the trench axis, the landward slope and seaward slope.
Trench axis. Southwest of the Erimo Seamount the
trench is characterized by a sequence of small
elongated basins in a zig-zag pattern, in which the
trend of the faults parallel to the Japan Trench
and the trend of the faults parallel to the oceanic
magnetic anomalies alternate (Fig. 3). Basins narrow and disappear when a p p r o a c h i n g the
seamount, being squeezed between the seamount
and the landward slope. There the trench is restricted to a pass only 6250 m deep. Northeast of
the seamount the trench suddenly widens into a
very large and flat-floored basin (7100 m deep)
strongly contrasting with narrow basins of the
Japan Trench. It is bounded to the southwest by a
very steep scarp perpendicular to the direction of
the trenches suggesting that the structure of the
continental margin is here cut by a steep NW-SE
fault (Figs. 3 and 7, plate IIA, map 4).
Landward slope. The morphological pattern of the
landward slope changes abruptly at 40°40'N.
South of this latitude the north-south structural
trend characteristic of the Japan trench landward
slope is still clearly observed; north of this latitude
the north-south trend disappears giving way to a
complex morphology without any prominent directions. Generally, the mid-slope terrace which
was so obvious in the south is absent here.
Numerous erosional features are observed in the
Seabeam map such as canyon running on the
slope, or well rounded depressions. The disappearance of the Japan Trench character occurs at
the mouth of the Hidaka Trough which supplies
detrital material from the nearby Hidaka Mountains. It is thus likely that the structural trends of
the Japan Trench are hidden below detrital
material. This complex morphology ends abruptly
northeastward against the NW-SE fault scarp.
Only a small part of the Kuril Trench landward
slope has been mapped and the topography r e veals a very steep scarp parallel to the trench as it
was earlier emphasized by Savostin et al. [35]
based on single-channel seismic profiling.
Seaward slope. The Erimo Seamount occupies a
large area of the seaward slope. Although no large
faults were directly observed affecting the
seamount as for the Daiichi Kashima Seamount,
they may exist but they are less developed than in
the Kashima area. Around the seamount the oceanic crust is dissected by normal faults which
display two main directions. South of the seamount
the transition from faults parallel to the Japan
Trench to those parallel to the Kuril Trench is
observed. In the transition area, fault scarps with
zig-zag pattern control the outline of the trench
basins.
3. 2. Seismic data
Closely spaced single-channel reflection profiles records were good on the seaward slope and
in the trench (Fig. 8). Normal faults which correspond to scarps on the Seabeam map clearly offset
280
the sedimentary sequence. When approaching the
seamount, the thickness of the upper sedimentary
sequence decreases and finally disappears on the
seamount. The Kuril Trench is filled by an accumulation of highly reflective well stratified sediments, probably turbidites, resting unconformably
on the oceanic deposits. The thickness of the
sediments is 0.5 s (two-way travel time) or 500 m
assuming a velocity of 2 k m / s for the sediments.
3.3. Magnetic data
Fig. 4 shows the anomalies of the total geomagnetic field around the Erimo Seamount.
As in the southern part, oceanic anomalies
striking N 7 0 ° E pass below the landward slope.
The magnetic signature of the Erimo Seamount
consists of a pair of anomaly centers superimposed on the oceanic anomalies. The positive
center lies on the positive anomalies M8 (120 Ma)
and the negative center lies on the negative
anomaly north of M8. This observation suggests
that the Erimo Seamount and the oceanic plate
have the same magnetization. This is an indication
that the Erimo Seamount would have been emplaced close to the spreading center. Another pair
of anomaly centers is observed at the corner between both trenches suggesting the presence of an
already subducted seamount, as confirmed by
seismic records.
The three components of the magnetization of
the Erimo Seamount and a planar regional field
have been computed by inversion of the measured
magnetic anomalies. The tested model which best
fits the observed anomalies involves an identical
magnetization for the seamount and the supporting plate and a non-magnetic cap on the top of
the Erimo Seamount (Fig. 9). This non-magnetic
cap is due to the presence of a coral reef. The
depth of its base corresponds to the topographic
bench around 4600 m. The declination, inclination
and intensity of the magnetization are - 1 6 °, 32 °
and 1.1 × 10 -2 e m u / c m 3, respectively. The calculated position of VGP, 63°N, 5 0 ° W agrees with
an age of 120 Ma on the Pacific apparent wander
path [37,38] (Fig. 10). The northern anomalies can
be well explained by another seamount with nearly
the same magnetization as Erimo Seamount, which
was already subducted at the trench: its summit
would now be at the corner of the convergent
front north of the Erimo Seamount.
(O)
Calculated
/
/
4 1 ° 0 0 '-
40040 ,.
144°40 '
~
5'0'
"~o
145~00 '
1'0'
(cJ
Depth(m)
4000"
5000"
6000
7000.
Fig. 9. (a) Computed magnetic anomalies created by the model
of Erimo Seamount (magnetization: declination is - 1 6 °, inclination is 32 ° and intensity is 1.1 × 1 0 -2 emu/cm3). (b)
Residual anomalies. (c) Model.
3.4. Gravity data
The free air anomaly (FAA) map is shown in
Fig. 5. The minimum associated with the trench is
largely shifted (20 km) landward of the trench
axis. This is due to a large sedimentary infilling of
the trench in the junction area. As expected, the
281
A
- 70
\'\\
,\
\
~
...........
\
.30°~
. . . .
Fig. 10. Location of VGP of Erimo Seamount (63°N, 50°W) on the Pacific apparent polar wander path. The solid curve is the
apparent polar wander path of Cox and Gordon [38]: the dotted line is that Sager (1983) and the dashed line, that of Gordon (1983)
(from Sager and Keating [37]). The 95% confidence regions surrounding the mean paleomagnetic poles are shown as ellipses, and the
numbers within the ellipses arc the mean ages of the poles in Ma. Map projcction is polar equal area.
gravity a n o m a l y of the E r i m o S e a m o u n t is closely
c o r r e l a t e d with its t o p o g r a p h y . Isogals give an
i n d i c a t i o n of the s e a m o u n t m o r p h o l o g y in the
area where the s e a m o u n t is a l r e a d y u n d e r the
s e d i m e n t s of the inner slope.
G r a v i t y a n o m a l i e s have been i n t e r p r e t e d in
o r d e r to d e t e r m i n e the e m p l a c e m e n t age of the
s e a m o u n t [39]. O n c e it has been formed, a
s e a m o u n t acts as a load on an elastic plate. T h e
d e f o r m a t i o n of the plate is c o n t r o l l e d by its elastic
thickness Te at the time of l o a d i n g [40]. It has
been shown that the elastic thickness is related to
the age of the l i t h o s p h e r e [40].
T h e o r e t i c a l gravity a n o m a l i e s created b y the
t o p o g r a p h y of the s e a m o u n t a n d the flexure of the
l i t h o s p h e r e have been c o m p u t e d using a three-dim e n s i o n a l m e t h o d . Fig. 11 d i s p l a y s the c o m p u t e d
a n o m a l i e s along line 80 for different values of To.
O b s e r v e d gravity from which a regional field (line
B in Fig. 4) due to the s u b d u c t i o n zone has been
r e m o v e d is also shown in Fig. 11. T h e best fit is
o b t a i n e d for an elastic thickness of 2 km. This
implies that the Erimo S e a m o u n t has been emp l a c e d on a lithosphere whose age was between 0
a n d 3 Ma. T h e r e f o r e it a p p e a r s that the E r i m o
S e a m o u n t has been formed close to the s p r e a d i n g
center.
Results of K / A r d a t a t i o n give an age a b o u t 90
M a [36] which is in a p p a r e n t c o n t r a c t i o n with our
p r o p o s e d age of 120 Ma. But s o m e ambiguities in
the values o f this d a t a t i o n r e m a i n due to the
excess argon and p o s t - m a g m a t i c o x i d a t i o n effects.
T h u s this d a t a t i o n would be a lower value for this
age.
3.5. Discussion on the junction
T h e S e a b e a m t o p o g r a p h y shows clearly, first a
N W - S E left-lateral offset ( t r a n s f o r m fault?) by
a b o u t 20 k m of the convergent front in the Kuril
T r e n c h relative to the J a p a n Trench. Second, the
trench axis trends N 2 5 ° E south of the previous
tectonic b o u n d a r y a n d N 5 5 ° E n o r t h of it. Third,
the E r i m o S e a m o u n t is located i m m e d i a t e l y south
of this tectonic b o u n d a r y , but an a l r e a d y sub-
282
Mgal
FREE-AIR
NW
ANOMALY
SE
///~/
km
Te : 10 km
~ k .........
i ----n / -Te-: 53 krn
////
~x'~"Y~
100
Te : 20
Trench axis[
Te:
--
Te :
2krn
0
km
50
i ~
effect
0
LINE
80
*
Observed
Gravily
Anomaly
Fig. 11. Computed gravity anomalies created bv the topograph?, and the flexure of the lithosphere for different values of the elastic
thickness (7[.). compared with the observed anomalies. The location of line 80 is shown in Fig. 5.
ducted seamount (less important than Erimo) is
observed (on the basis of magnetic anomalies and
seismic profiles) at the upper corner of the trench
juncture. Are these results sufficient to say that
the sharp curvature of the trench axes is due to the
collision of a possible chain of seamounts preceding Erimo Seamount, according to the theory
of Vogt et al. [20]?
Keeping in mind that our survey includes only
the frontal zone of the continental plate and also
only the southernmost part of the Kuril Trench, it
seems very difficult to clarify this problem without
taking into account the adjacent areas, especially
northward and eastward of the present survey.
It seems that the oceanic plate is not concerned
by this curvature, because the normal faults are
parallel to both trenches with an intermediate
zone south of the Erimo Seamount, where we can
observe simultaneously both directions: N 1 0 ° E
and N60°E. Furthermore, if the collision is active
now, it has to be confirmed by the seismicity in
this area.
Anothcr possibility is to relate the "transform
fault" to a failure line of a present boundary
between the Japanese microplate and the Okhotsk
plate [41], but there is no evidence of intracontinental seismic activity in the assumed
boundary zone (between Erimo Cape and Erimo
Seamount). Concerning the curvature of trenches
near Erimo Seamount, it may be due to a different
tectonic behaviour of plates on either side of the
Ncogene plate boundary between America and
Eurasia [8].
4. Summa~ ~ and conclusions
The detailed survey made by the R / V "Jean
Charcot" in the Japan Trench and its junction
with the Kuril Trench during the French-Japanese
Kaiko project (northern part of Leg 3) brings
numerous new insights on one of the best known
trenches, especially about the interactions between
the continental margin and the oceanic crust.
(1) Sediment sliding in the trench: Mass sliding
occurs on the landward slope of the trench, fills
the axial zone of the trench and reduces it to a
succession of short and narrow basins which are
offset on the oceanic side by faults trending obliquely to the trench axis. Thus the topographic
Japan Trench is only the frontal boundary of the
sedimentary gravity sliding. These slide deposits
are then probably subducted together with oceanic
crust and its cover.
The Japan Trench could be a model for tectonic
erosion linked with subduction: the continental
slope is being destroyed by superficial collapse
and the resulting material may ultimately be carried down the subduction zone. The continental
plate has thus retreated. In addition, it is likely
that erosion of the continental margin from below
by the subducting oceanic plate occurs [42].
(2) Newly created faults and reactivated faults
283
on the oceanic side: T h e o c e a n i c c r u s t is c u t i n t o
horst and graben with normal faults parallel to the
t r e n c h . S o m e o f t h e f a u l t s a r e a s s o c i a t e d to l i n e a ments corresponding to the original structural
trend of the oceanic plate because they parallel the
magnetic anomalies oblique to the trench. Where
t h e o c e a n i c p l a t e s t r u c t u r a l t r e n d is h i g h l y o b l i q u e
t o t h e axis o f f l e x u r e , t h e n o r m a l f a u l t s b r e a k
across the structural grain of the oceanic plate and
t h e o l d e r f a u l t set r e m a i n s c r y p t i c . S u c h f a u l t s
seem to have long been truncated and buried
b e n e a t h t h e o c e a n i c s e d i m e n t s a n d t h e n w e r e rejuvenated when the oceanic crust underwent large
tectonic stress due to the bending of the plate at
the subduction zone.
(3) Trench junction: A t t h e t r a n s i t i o n w i t h K u r i l
Trench, a left-lateral transform fault offsets the
c o n t i n e n t a l s l o p e a n d l e a d s to a w i d e flat p l a i n in
the axial zone strongly contrasting with the small
basins of the northern Japan Trench. The Erimo
Seamount, located slightly south of the junction,
m a y b e l o n g to a c h a i n , a l r e a d y s u b d u c t e d , res p o n s i b l e to t h e t r e n c h c u r v a t u r e b y c o l l i s i o n . T h i s
t r e n c h t r a n s i t i o n m a y a l s o b e r e l a t e d to t h e i n tracontinental boundary between America and
Eurasia plates. Nevertheless, no definitive conclusion can be given at present without a deeper
study of focal mechanisms of earthquakes in this
a r e a a n d n o r t h o f it.
4
5
6
7
8
9
10
11
12
Acknowledgements
13
T h a n k s a r e d u e to t h e C a p t a i n , A l a i n G i r a r d ,
a n d t o t h e c r e w o f t h e R / V " J e a n C h a r c o t " for
their support. We also thank the reviewers for
their thoughtful comments. The figures were prepared by Annie Bourdeau and Jacques Brouillet.
14
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