Download Subduction of the Daiichi Kashima Seamount in the Japan Trench

231
Tectonophysics, 160 (1989) 231-241
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Subduction of the Daiichi Kashima Seamount
in the Japan Trench
SERGE
LALLEh4AND
’ Dbpartement de G&tectonique,
‘, RAY CULOITA
2 and ROLAND
UnioersitC Pierre et Marre Curie, T26-El, 4 place Jussiey
VON HUENE
2
75252 Paris Cbdex 5 (France)
’ Office of Pacific Marine Geology, U.S. Geological Suruey, 345 Middlefield Road, Menlo Park, CA 94025. (U.S.A.)
(Received September 15,1987;
revised December 17,198s)
Abstract
Lallemand, S., Culotta, R. and Von Huene, R., 1989 Subduction of the Daiichi Kashimi Seamount in the Japan Trench.
In: J.P. Cadet and S. Uyeda (Editors), Subduction Zones: The Kaiko Project. Tectonophysics,
In 1984-1985,
160: 231-247.
the Kaiko consortium collected Seabeam, single-channel seismic and submersible sampling data in
the vicinity of the Daiichi-Kashima
seamount and the southern Japan trench. We performed a prestack migration of a
Shell multichannel seismic profile, that crosses this area, and examined it in the light of this unusually diverse Kaiko
dataset. Unlike the frontal structure of the northern Japan trench, where mass-wasting appears to be the dominant
tectonic process, the margin in front of the Daiiclr-Kashima
shows indentation, imbrication, uplift and erosion.
Emplacement of the front one-third of the seamount beneath the margin front occurs without accretion. We conclude
that the Daiichi-Kashima
seamount exemplifies an intermediate stage between the initial collision and subduction of a
seamount at a continental margin.
Introduction
lower body of the seamount.
24-channel seismic reflection
The Daiichi-Kashima
seamount
is split into
two bodies offset vertically by more than 1 km
the margin
(Mogi
and
1987).
The
seamount)
Nishizawa,
lower
body
1980;
is partly subducted
of the Japan
trench.
Kobayashi
(western
part
et al.,
of
in the southern
An extensive
the
part
survey (Fig. 1)
was conducted during the first phase of the Kaiko
program in 1984 (Leg 3, Kobayashi
et al., 1987)
using the R/V “Jean Charcot”
equipped
with
Seabeam echosounder,
single-channel
seismic reflection, 3.5 kHz transducer,
magnetometer
and
gravimeter.
Eight Nautile dives to depths of 6000
m took place in 1985 from the R/V “Nadir” (Leg
2 and 3, Pautot et al., 1987; Cadet et al., 1987).
One short 12-channel
seismic reflection line was
recorded
by the Hydrographic
Department
of
Japan (Ml on Figs. 1 and 2, Oshima et al., 1985)
crossing the lowermost
landward
slope and the
0040-1951/89/$03.50
0 1989 Elsevier Science Publishers B.V.
and
the seamount
Shell in 1972 (unpublished)
2). In September
line
by
migration
velocity
stacks
was recorded
velocities
picking
part of this
stacking
from a “matrix”
migrations
(the method
by
(P844 on Fig. 1 and
1986, we reprocessed
simultaneously
prestack
Also a 100 km long
line crossing most of
and constant
is explained
and Culotta, 1989, this vol.).
The analyses of these data
and
of constant
in: Von
velocity
Huene
are the basis
for
proposing
a model for the subduction
of this
seamount in the Japan trench and its effects on
the landward slope.
Previous studies and new insights
The Seabeam morphology
shows how the Daiichi-Kashima
seamount is broken into two parts,
232
Y 350
20
110 50’
135040’
pp.
233-236
231
Sedimentary
basins
Trench-fill
l *
.
Seamount
Man
./
sediment
conro~rs
thrust
/Possible
thrusts
Oceamc
normal
d---@
landward
slope
Landward
tllted
\
Single-channel
\
Canyons
faults
slump
0~
scars
basins
se,sm,c
lines
8
0
I
9
I
Fig. 3. New (compared
single-channel
I
with those of Kobayashi
seismic reflection
I
I
et al., 1987) structural
profiles.
I
map of the surveyed
here referred to as the upper and lower bodies,
separated by a major fault scarp that is slightly
concave trenchward (Figs. 2 and 3). The margin in
front of the seamount is indented about 7 km and
uplifted several hundred meters close to the trench
producing a trough parallel to the trench and 20
km landward of it.
A revised structural map (Fig. 3) has been
drawn on the basis of a further analysis of single
channel seismic data recorded during the Kaiko I
cruise (Lallemand et al., 1986) and also on the
interpretation of multichannel seismic lines (Fig.
4). The lower body adjacent to the trench axis is
larger than the upper body. By restoring the lower
a
I
I
I
area. The big numbers
The large open arrow shows the Pacific plate motion
1Okm
5
refer to the Kaiko
relative to Japan.
body to its pre-fault position and assuming a
roughly symmetrical shape, we determined an
original seamount approximately 60 km in diameter and 3.5 km in height. The fault scarp separating the two parts of the seamount is composed of
two or three main faults with conjugates facing
the trench. The total vertical offset along this fault
system increases from 700 m to the south to 1.7
km in the middle of the seamount, based on the
interpretation of single-channel lines (Fig. 5). Antithetic faults bound a 3 to 4 km wide graben
between the two bodies. The upper body is highly
fractured, as is the oceanic crust surrounding it,
whereas the lower body appears less disrupted (the
238
239
WNW
LANDWARD
10
0
t
V.E.
Fig. 5. Interpretative
SLOPE
20 km
I5
time line drawings
of 8 of the 21 Kaiko
exaggeration
is 5 X The numbers
single-channel
are two-way
seismic profiles
(lines are located
travel time in seconds.
in Fig. 3). Vertical
240
offsets
of faults
are smaller
Some single channel
and
on the lower body).
seismic
lines (II,
17 on Fig. 5) show a landward
mal fault offsetting
(profile
13). These normal
the trench
Two
faults
axis (Lallemand
observations
crust beneath
suggest
into the subduction
depth
to the seamount.
mal
faults
seamount
floor
set separating
the
with
of
the
crust is
that
by downward
induced
tation
of the seismic and dive data (Kaiko
its greatest
Book, jn prep.).
the norof the
at the mid-
beneath
whereas
the upper
body is nearly
problem
has been
considered
during
(Hilde
et al..
this vol.)
Pouclet
find
that
ages of 100 to
bodies are geochemically
approximately
oceanic crust
and
the
Ohnenstetter
lower
distinct:
II Data
the dives on the
radiometric
1976).
in the
our interpre-
120 Ma (Takigami et al., in press).
20 Ma younger than the surrounding
(1989.
the
horizontal.
rocks collected
have yielded
to
landward
by Ida (1986)
Volcanic
seamount
also help to constrain
if it
is in a sense opposite
flexure
made by other investigators
zone. First.
parts
The lower body is tilted 2.5’
studies
and bio-lithostrati-
project
and only 700 m at the edges.
horizontal,
graphic
petrologic
Kaiko
Second,
two
seumount
Background
The geochemical,
the tilt of the top of the seamount.
was originally
seamount.
achieves
has a 1700 m displacement
dle of the seamount
However,
are aligned
depression
trench
of the
Characteristics
up to 800 m
as the ocean
the associated
adjacent
nor-
et al., 1986).
the seamount
flexed downward
dipping
the lower body
seismic line and dives
M~tic~nel
13, 15. 16
and
upper
the samples
coI-
The
lected on the lower body are composed mainly of
mugearite,
whereas those collected on the upper
but
body
has yet to be resolved in a manner that explains
all observations.
Crustal response to subduction
of
are composed
of basanite,
mugearite
and
the seamount
is observed and although not well
understood,
it should be considered
in models of
benmoreite.
Also, the La/Yb ratios of the lavas of
the two bodies indicate two different
magmatic
origins. The geochemical results in addition to the
occurrence
of tephra layers on the main scarp
seamount
Trench
(Fig. 6) lead Pouclet and Ohnenstetter
to propose
that the seamount was originally composed of two
subduction.
fill is in contact
the seamount
only northward
(see Fig. 3). Observations
of active erosion
suggest
that
arriving
with the lower part of
and southward
made during
of the lowermost
sediment
in the trench (Cadet
soon
basins
on the lower landward
coalescing
According
volcanoes
separated
to their hypothesis,
slope
from bending
after
with the margin
et al., 1987; Pautot
al., 1987). Some tens of square kilometer
tilted
the dives
landward
is subducted
of it
et
landward
slope of the
of the subducting
caused
the limestone
of a graben
and depression
According
samples
plate and collision
the formation
at the site of the suture
the two volcanoes.
by a depression.
stresses resulting
coliected
to Konishi
during
between
(1986),
the dives
trench appear to have formed by ponding of sediment behind a ridge uplifted during the collision
indicate
that the seamount
was capped
by an
active reef during 10 Ma or more from Apto-Al-
of the seamount.
The preceding
bian time. ‘Then an eustatic
drowned the reef. The seamount
conclusions
were deduced
mainly
from analysis of the Seabeam map and the closely
spaced single channel seismic lines (Fig. 5). Further information
was obtained
from the reprocessed Shell line P844 (Fig. 4) concerning
the
internal
structure
of the lower slope facing the
seamount.
The interpretation
was constrained
by
observations
made during dives NA 2-3 and NA
2-5 (Fig. 3) located on the main scarp exactly
along P844’s track. The combined
results were
then incorporated
into a revised structural
map
(Fig. 3).
rise of sea-level
then subsided as
the cooling plate drifted from the equatorial Pacific
to the Japan trench. Bio- and lithostratigraphic
evidence (Konishi,
1986) suggests that identical
limestones
occur on both the upper and lower
bodies of the seamount.
The paleodepth
of both
bodies was the same during deposition
of these
limestones.
Thus, even if there was a zone of
weakness
between the two bodies prior to the
faulting, the vertical offset corresponds
approximately
to the depth
blocks:
1500 m.
difference
between
the two
241
Below the turbidites filling the graben between
The P844 seismic line
the upper and lower bodies are several irregular
The acoustic character of the basement differs
considerably
between
the
two
parts
of
the
dipping reflectors, which we interpret to be chaotic
seamount, from a very rough structure within the
blocks sealed by recent sediments derived prim-
upper body to a more layered structure inside the
arily from the main scarp. The small terrace seen
lower body (cf. Fig. 4). This contrast may be due
at the base of the cross-section (Fig. 6) may corre-
to a difference
spond to an unburied block.
in composition,
the upper body
with some explosive
The northwestern flank of the seamount can be
volcanics (A. Pouclet, oral cormnun., 1987), or to
followed at least 30 km landward of the trench
the more intense fracturing of the upper body.
axis below the landward slope but it is difficult to
being more heterogeneous
The sediment
discriminate
caps on the upper and lower
between
straight
reflectors
corre-
sponding to the cap of the seamount and layering
of the basement as mentioned previously with
bodies exhibit similar seismic character, being well
stratified and nearly transparent like the hemipelagites covering the surrounding oceanic crust.
regard to the lower body. The layered reflections
However, the sediment cap on the lower body is
twice as thick as that of the upper body. We
could also correspond to continental
offscraped and subducted.
sediments
attribute this difference to current erosion, as explained in a later section. Because the two dives
took place on the main scar-p (Fig. 6) only the
Geological cross section of the main scarp
Figure 6 shows an interpretative
cross-section
thin cap of the upper body, representing a part of
the total stratigraphic column, was entirely ex-
made from analysis of the video tapes and de-
amined.
ples collected. Fresh and massive basalts outcrop
scriptions recording during the dives, and the sam-
WNW
ESE.
- 3800
- 3900
-4000
.4100
[tephra]
* 4200
- 4300
- 4400
-4500
[altered
-4600
and Joint
Fault
- 4700
directions
- 4800
[w I\
* 4900
-5000
-5100
- 5200
- 5300
I
-
-
-.
D
,
-.
-
$00
El
recent
B
yellow to brown argilite
sediments
.I’
,,ooo
=.
.
,
.
,$+
.
-
*,
*
.
.
.
$I@
29
Ezl
m
sedimentary
m
chalky
m
interlayered welf stratified
dark brown layer
limestone
shallow-water
d - . - $P
. ’
3QQ
limestone
breccfa
(ml
Qv v volcanic rock
m
tephra
layer
Fig. 6. Geological cross-section of the main scarp separating the two bodies of the seamount drawn from the analyses of video tapes
recorded during the Nautile dives: NA 2-3 (observer: Y. Nakamura) and NA 2-5 (observer: J. Bourgois). The description of samples
(bold numbers) are issued from written communications of P. Pouclet (igneous rocks), A. Pascal and K. Konishi (limestones), J.P.
Caulet, H. Charnley, S. Hasegawa, T. Maruyama, A.L. Monjanel, C. Miiller, M. Oda and Y. Takayanagi (other sedimentary rocks).
The exact location of the dives can be seen on Fig. 3. There is no vertical exaggeration.
242
at depths of 5000 m and 4450 m along two maJor
fault scarps. It is difficult to estimate the thickness
seamount’s
cap is sufficiently
thrck. Thus. d~fferences in sediment thickness on the two parts .)f
of the
limestones
the seamount
frequent
faults.
tions,
because
of the
According
the limestone
brown
pled. This sequence
of
Mn-rich
Miocene)
layers
are overlain
limestones
clay
bv
to the 15 m
(Paleogene
the upper
to
Cretaceous
lower
cherts
recovered at DSDP site 436 (Leg 56, Langseth.
Okada et al., 1982). One-hundred
meters of argillites overlie these dark layers which may also
correspond
to the middle
diatomaceous
argilhtes
by differential
the upper
layer of both bodies.
increased
locally with the recent
erosiorr of
Ilrosion
may have
fault acttvitv.
Miocene
C’haracteristrcs of the margin jucrng the seumolrnt
alternat-
which were not sam-
may correspond
brown
overlying
are caused
that it may exceed 300
water limestones
a few tens of meters of chalky
ing with dark
by
observa-
may be 120 m thick. whereas
the seismic record indicates
m. These shallow
repetition
to the dive
to Quaternan,
and silty clay recovered
at
Geologicul section across the truce o/‘ the suhduction zone and the lowermost lundward slope
Five
dives
were
made
in
the
area
of
the
seamount
(see locations on Figs. 3 and 7). The
most informative
are dives NA 2-6 and Na 2-7
which make a transect north of the lower seamount
body (Fig. 8). The base of the rcefoid cap of the
seamount was observed on the oceanic side of the
transect.
No trench
fill other than scattered
blocks
DSDP site 436. Two samples have been dated as
early Pliocene (Monjanel
et al., in press, see Fig.
was found in the deepest part of the section.
Subhorizontal
layers of slope hreccias crop out
6) with reworked
from just above the trench floor up to 5600 m.
One sample of a breccia contaming
diatoms (No.
let, written
Forty
upper
commun.,
normal
Miocene
sediments
(Cau-
1987).
faults
or joints
were
observed
along the dive transect. Most of them are subvertical. Three sets of directions
are recognized
(see
Fig. 6). The first set strikes N30°, approximately
parallel to the cliffs and the trench. This set of
faults
affects
the lower Pliocene
sediments
and is
probably still active. It may have originated
1 Ma
ago when the seamount passed the oceanic bulge
100 km oceanward of the trench axis. The strikes
of the other two sets, N-S and N140”, do not
align with any other known regional features. Possibly they are related to the internal structure of
the seamount
basement.
The N140” direction
is
exactly perpendicular
to the trench and corresponds
to major lineaments
of the seamount
(Fig.
2).
at 2.5 to 3.2 Ma (Monjanel
et
al.. in press). The stepped
morphology
in
lower part of the cross section was interpreted
3) has been dated
the
by
the diver
Fujioka)
to sigmfy
thrusting.
The
provides a conduit for nutrients In this area (Henry
et al.. 1989. this vol.). The upper part of the
transect shows small-scale
folding with a N30”
axis of Pleistocene
(Monjanel
et al.. in press)
mudstone
transverse
layers and intense
directions
faulting
(N115”
,md
along mainly
N145” ), but
also along N20” and N50” (Fig. 8). The Nl IS”
direction
parallels
both a pseudocleavage
(very
close vertical faults) observed in the mudstones.
and the local vector of plate convergence.
This
faulting
Geological cartography of the seamount’s cap
As previously mentioned,
the two parts of the
seamount’s
cap appear to differ in thickness on
the seismic sections. The geological map (Fig. 7) is
based on the seismic interpretation and results of
the dives. We describe, on the basis of seismic
profiles (Fig. 5), two different layers making the
seamount’s cap. The lower layer is well stratified,
whereas the upper layer is more transparent. Furthermore, the upper layer exists only when the
(K.
occurrence
of living clams near sample No. 3 of
dive NA 2-6 may indicate tectomc disruption
that
must
be recent
because
Pleistocene
sedi-
ments are affected. Motion along the faults may
be strike-slip. induced by the lateral displacement
of material
during subduction
of the Daiichi
Kashima seamount and/or
tension gashes. Transverse faults or transverse
trending
tectonic
features are commonly
observed <jn the margin in
front of subducting
seamounts
like the one at the
junction
of the Japan and Kurt1 trenches (Lallemand and Chamot-Rooke,
1986) or in front of the
Bougainville
guyot in the New Hebrides
trench
243
E
E142"30
; lL2020'
E
1~2~~0
lL2"50
I
\
N 35"5(
m
recent
inlillinp
2
Fig. 7. Distribution
of the three different
limestones
with the lower layer showing
correlate
and with recent
acoustic
infilling
for well stratified
acoustic
layers
strong
reflectors
making
the cap of the seamount
reflectors,
“possible”
in depressions.
layers below the inner slope. The main faults are simplified
hemipelagites
The dashed
(see details
lines correspond
from the structural
in the text). Shallow-water
with the upper,
more transparent
to the possible
contours
map (Fig. 3). The locations
layer
of the
of “Nautile”
dives are plotted.
(Daniel et al., 1986). Recent small-scale folds also
affect Pleistocene sediments. They may be superfi-
ond, during dive NA 3-9, a 20 m section of
limestone talus accumulation was encountered on
cial slump folds associated with the oversteepening of the slope. Alternatively they might be com-
rived from incorporation
pressive structures, but in this case we would
expect to observe compressional folding at the
base of the slope.
Non-stratified breccias were observed at the
base of the lower landward slope during dive NA
2-4 (Fig. 7). Dives NA 3-9 and NA 3-10 (Fig. 3
and 7; Cadet et al., 1987) also encountered breccias in the lower part of the slope but no clear
the lowermost landward slope which may be deof the upper part of the
limestone cap into the innerslope
along a thrust
fault (Cadet et al., 1987).
Erosional channels and debris flows were observed during every dive.
Analysis of seismic line P844
The reprocessing of seismic line P844 revealed
deep structural information especially on the land-
folding upslope. Two observations from those
dives suggest minor accretion of the Daiichi-
ward side of the trench (Fig. 4). The following are
Kashima seamount. First, during dive NA 3-10,
two isolated pieces of igneous rock were sampled
among the landward slope breccias 30 m and 250
m vertically above the axis of the trench. These
samples have petrological (alkali rocks) and chronological (115-120
Ma) affinities with the Daiicm-Kashima
seamount (Ishii et al., 1986). Sec-
(1) Two prominent thrusts that crop out 8 to 9
km landward of the trench axis separate an area
of extensional deformation on the slope from a
frontal wedge under compressive deformation.
Their downward extension to the vicinity of the
decollement remains speculative, but the upper
two-thirds of the faults are clearly visible in the
the main observations:
244
NW
SE ,WSW
F+iTq
p-z+-----+
depth (ml
/
5100 -
ilO
ENE
pScu&JcleaVdgc
ri--
volcanic
~~-
/ $%I
Limestane
I rl
outcrop with0ut clear
dipping of strata
5200.
,
5300*
2
[mddl e
Phstocene
I.C
so0 -
.
clav mud]
recent
j
’m
,.‘~tocc”e
- rock ------
--.-.
seamount
cwer
sediment
slope
breccta
tren&
nudstone]
i_=?
ssoo-
fold
alterations
of mudstones
and dark layers
(ovrOctaaStit& ? 1 ..-_
I
L-----_---_____--2..
li
S600.
Fig. 8. Geological
cross-section of the trench axis area with the lowermost
recorded during the Nautile
(bold
numbers)
dives: NA
2-6 (observer:
are issued from written
K. Fujioka)
communications
and NA
of J.P. Caulet,
landward
slope drawn
2-7 (observer:
H. Charnley,
from the analyses of video tapes
P. Huchon).
A.L.
Monjanel
The description
of samples
and H. Okada.
The exact
location of the dives can be seen on Fig. 3. There is no vertical exaggeration.
multichannel
record and can also be recognized in
some single-channel
records (Nos. 12-1.5. Figs. 4
A knoll located landward of the Daiichi-Kashima
seamount (Fig. 1) is bounded by two pronounced
and 5).
headless
(2) The frontal
imbricated
thrusting
the cap appears
faults.
and folding.
is deformed
by
Little or none of
to have been accreted.
(3) The middle
list&
wedge material
The
slope is disrupted
shallow
offset
or
the scarps
cutting the seafloor indicate recent activity but the
basal slip plane of the megaslump
is cut by a
landward thrust.
(4) Some ambiguities
in the reprocessed record
are: (a) the poor continuity
of reflections
at
depths of 5 to 10 km in the right part of the
profile; (b) the complex transition
zone below the
seafloor between
frontal
thrusts and landward
list& faults; and (c) the unexpected
presence of
well-developed
listric faults which have not been
commonly
recognized elsewhere along the Japan
trench margin.
There are indications
of other highs having
preceded the Daiichi-Kashima
seamount into the
subduction
zone (Lallemand
and Le Pichon,
1987).
perpendicular
to the trench (Cyl
and Cy2 on Fig. 1). One small canyon
Fig.
1) cuts
appears
by normal
and
canyons
itself
across
to predate
(Figs.
2 and
the
front
the development
3). thus
(see Cy.I on
of the knoll
and
of the knoll
indicating
a recent
uplift. A swarm of small earthquakes
associated
with the lbaragi
earthquake
(m = 7.0, July 23.
1982, see location on Fig. 1) was related by Kikuchi
and Sudo (1985) to the subduction
The Kashima
seamount
is one
of a seamount.
of a chain of
seamounts aligned along the direction of magnetic
lineations,
so that the earlier subduction
of preceding seamounts
is not unlikely. Also, magnetic
anomalies
across the slope are very disturbed
in
the area between 35 o N and 37 o N (Hydrographic
Department,
1983).
If we assume that the area of poor reflections
(see (1) at the be@nning
of the section) corresponds to a volcanic edifice, there is an ambiguity
at the base of it because we can follow the d&ollement rather easily. All the above features might be
245
explained by underplating of a part of a seamount.
ancient
Subcrustal accretion could have produced the up-
terpreted as parts of accreted seamounts and their
lift of the knoll, the oversteepening
sedimentary
and the resulting
listric
of the slope,
faults compensating
a
accretionary
Sakakibara
caps
complexes
(Naka,
1985;
have
been
Ogawa,
in-
1985;
et al., 1986). The simplest models im-
mass excess as proposed in the model of Platt
ply that the seamount is sheared off at its base
(1986).
The line drawing of the time-section of the Ml
and incorporated
compressional
seismic line (Oshima et al., 1985) has been dig-
subduction zone.
itized without modifications
of interpretation
and
Evidence
as a body with some horizontal
deformation
for partial
at the front
frontal
accretion
of the
of the
then converted into a depth section using the same
Daiichi-Kashima
velocity model as that used for the P844 seismic
above the trench axis, of two isolated alkali rocks
line. The velocity model used beneath the lower
similar to those of the seamount’s basement (dive
trench slope is based on the refraction
NA 3-lo), and the observation of a 20 m limestone talus accumulation at the base of the land-
data of
Suyehiro et al. (1985). The depth section of Ml in
Fig. 4, like P844, shows subduction of the front of
the seamount beneath the inner slope and a trace
seamount is the recovery, 250 m
ward slope (dive NA 3-9). However, in seismic
records the landward flank of the seamount ap-
of the landward dipping thrust zone.
pears to be subducted. The Ml seismic line (Fig.
4) and the single-channel line No. 14 (Fig. 5) show
Discussion
that even the sedimentary cap of the seamount is
subducted without being deformed. As little trench
and model
when
fill is observed despite the many erosional chan-
dealing with the collision of a seamount is whether
it is accreted or subducted. Some volcanic rocks in
nels, debris flows, and fresh talus on the landward
slope, such material must be subducted soon after
One
I
fixed
question
commonly
considered
G-lOcm/year
Fig. 9. Schematic model showing the effect of the subducted portion of the Daiichi-Kashima
seamount on the margin without taking
into account the possiblity of underplating. The drawing of the oceanic plate and the seamount has been voluntary simplified but the
volumic proportion of the seamount compared with the frontal margin is real one.
246
arrival in the trench. Furthermore, the steep slope
in front of the Kashima seamount has been oversteepened and appears to be collapsing
into the
The consequent oversteeping of the lower slope
caused slope failure and erosion of the front of the
margin. The absence of trench fill shows that
trench axis. This collapsed material must also be
collapsed
subducted.
The
A possible way to explain the local
incorporation
of small quantities of material from
material
overall
has been r;lpidly subduct,ed.
process
of
collision
the seamount into the lower slope is that some
Huene (1986)
turbidite
containing
America trench off Guatemala.
Kashima
seamount
clasts transported
was accreted
just
from the
prior
to
The main process during collision of DaiichiKashima seamount has been subduction. The mass
of the seamount is accommodated in three ways.
First, the ocean crust beneath the seamount has
subsided as shown by the increased vertical
displacement
of the normal faults that cross both
a
dentation
based on studies of the Central
would involve in-
and collapse of the landward slope of
the trench, and progressive
subduction of the seamount (Fig. 9).
between
seamount and a plate margin. !nodelled by Van
eventual subduction
dismemberment
or accretion
and
of parts of the
seamount. An early stage in this process might be
represented by Erimo seamount which is beginning to fracture as it approaches the northern
Japan trench. A late stage might he observed at
the junction of the Japan and Kuril trenches where
the oceanic crust and the seamount. The vertical
displacement across the seamount is 1 km greater
a very large indentation
than on the ocean crust. Depression of the crust is
also indicated by the 100 m increase in depth of
seamount has been consumed (Lallemand and Le
Pichon, 1987). Our analysis of the Kaiko data and
the trench axis. A second way to accommodate the
the reprocessed line P&44 lead us to conclude that
mass of the seamount is by uplift and thickening
the present disposition
of the sediment that comprises the lower slope of
Kashima
stage in
the trench (Lallemand
and Le Pichon,,l987).
The
lapse have occurred
and massive slope col-
as the trailing
and structure
seamount and a continental
well with the position of the subducted front of
the seamount. The narrow zone of deformation in
Acknowkdgements
pened slope and subduction
of the debris from
We
of Daiichi
seamount represents an intermediate
the process of collision between a
ridge associated with the subducted front of the
seamount and its imbricate structure correspond
front of this collision probably reflects a low
strength of the material that comprise the lower
slope. A third process, collapse of the overstee-
flank of ti
thank
Shell
margm.
Internationale
Petroleum
Maatschappij B.V. for providing us the seismic
record P844. The Kailco program was supported
on the French side by C.N.R.S. and IFREMER
masswasting, is suggested by the lack of fill in the
and on the Japanese
trench axis.
A history of the subduction
thank Prs. J.P. Cadet and X. Le. Pichon, Drs. P.
Huchon and L. Jolivet for their encouragement
of the seamount
begins about 150,000 to 250,000 years ago (assuming a subduction rate of 10 cm/yr, Minster and
Jordan,
1978), when the northwestern
depression
due to the load of the seamount was located at the
trench axis and trapped sediment from the margin
and slumps from the seamount itself. The tectonic
style of the margin observed on the upper slope,
existed at this time. The normal faults splitting the
seamount existed also but probably with less vertical offset than at present. The lower landward
slope was compressed by the colliding seamount,
and was uplifted and folded, and some of the
trench fill was incorporated into the inner slope.
side by MONBU-SHO.
during this work and Drs.W.T.
We
Coulbourn and T.
Yamazaki for reviewing the manuscript. Drawings
were prepared by A. Bourdeau.
References
Cadet, J.P., Kobayashi, K., LaUemand, S, Jolivet, L., Aubouin,
J., Boul&gue, J., Dubois, J., Hotta, H., lshii, T., Konishi, K..
Niitsuma, N. and Shimamura, H., 1987. Deep scientific
dives in the Japan and Kuril trenches. Earth Planet. Sci.
Lett., 83: 313-328.
Daniel, J., Collot, J.-Y., Monzier, M., Peiletier, B., Butscher, J.,
Deplus, C., Dubois, J., GQard, M., Maillet, P., Monjaret.
M.C., Recy, J., Renard, V., Rigolot, P. and Temakon, S.J..
247
1986. Subduction et collision le long de l’arc des Nouvelles-
and Wories, H., 1978. Near the Japan
Hebrides (Vanuatu): resultats pr&iminaires de la campagne
begun. Geotimes, March 1978: 22-26.
SEAPSO (Leg I). C.R. Acad. Sci. Paris, 303, Ser. II: 805-810
trench transects
Minster, J.B. and Jordan, T.H., 1978. Present-day plate motion. J. Geophys. Res., 83: 5331-5354.
(in French with English abstr.).
Henry, P., LalIemant, S., Le Pichon, X. and Lallemand, S.,
Magi, A. and Nishizawa, K., 1980. Breakdown of a seamount
1989. Fluid venting along Japanese trenches: tectonic con-
on the slope of the Japan trench. Proc. Jpn. Acad., 56:
text and thermal modelling. Tectonophysics, 160: 277-291
Hilde, T., Isezaki, N. and Wageman, J.M.,
seafloor spreading in the North-Pacific.
1976. Mesozoic
In: G.H. Sutton,
M.H. Manghnani and R. Moberly (Editors),
The Geo-
physics of the Pacific Ocean Basin and Its Margin. Geo-
257-259.
Monjanel, A.L., Caulet, J.P. and Mtller, C., in press. Micropaleontological
analysis
of
diatoms,
radiolarians
and
calcareous nanofossils of the Late Neogene sedimentary
Kaiko samples. In: Ifremer (Editor), The Japanese Trenches.
Naka, J., 1985. Broken seamount fragments in the Setogawa
phys. Monogr., Am. Geophys. Union, 19: 205-226.
Hydrographic Department M.S.A., Japan, 1983. Geomagnetic
total intensity anomaly chart of the adjacent sea of Nippon,
subduction complex. In: N. Nasu et al. (Editor), Formation
of Active Ocean Margins. Terrapub., Tokyo, pp. 747-773.
Ogawa, Y., 1985. Variety of subduction and accretion processes
1: 3000,000.
Ida, Y., 1986. Mechanical analysis of subducting seamounts.
Abstr. Int. Kaiko Conf. on Subduction Zones, 1986, Tokyo
in Cretaceous to recent plate boundaries around southwest
and Central Japan. Tectonophysics, 112: 493-518.
O&ma,
and Shimizu, p. 113.
I&ii, T., Nakamura, Y., Haramura, H, Togashi, K., Minai, Y.,
S., Ogino, T., Katsura, T., Ikeda, K., Uchida, M.,
Nagano, M., Hayashida, M., Muneda, K., Kasuga, S. and
Tominaga, T., Yoshida, T. and Aoki, K., 1986. The igneous
Tani, S., 1985. Subduction of Daiiti-Kasima
rocks collected during Kaiko 1985. Abstracts of the Inter-
the landward slope of the Japan trench. Rep. Hydrogr.
national Kaiko Conference on Subduction zones, 1986,
(in Japanese with English abstr.).
Pautot, G., Nakamura, K., Huchon, P., Angelier, J., Bourgois,
Tokyo and Shimizu, pp. 97-98.
K&u&i,
Res., 20: 25-46
seamount into
M. and Sudo, K., 1985. Fault process of Ibaragi
earthquake, July 23, 1982-Subduction
of seamount and
J., Fujioka, K., Kanazawa, T., Nakamura, Y., Ogawa, Y.,
Seguret, M. and Takeuchi, A., 1987. Deep sea submersible
survey in the Suruga, Sagami and Japan trenches: pre-
asperity. Earth Mon., 7 (2): 72-78 (in Japanese).
Kobayashi, K., Cadet, J.P., Aubouin, J., Boulegue, J., Dubois,
J., Von Huene, R., Jolivet, L., Kanazawa, T., Kasahara, J.,
liminary results of the 1985 Kaiko cruise, Leg 2. Earth
Planet. Sci. Lett., 83: 300-312.
Koizumi, K., Lallemand, S., Nakamura, Y., Pautot, G.,
Platt, J.P., 1986. Dynamics of erogenic wedges and the uplift
Suyehiro, K.,Tani, S., Tokuyama, H. and Yamazaki, T.,
of high-pressure metamorphic rocks. Geol. Sot. Am. Bull.,
1987. Normal faulting of Daiichi Kashima seamount in the
Japan trench revealed by the Kaiko 1 cruise, Leg 3. Earth
Planet. Sci. Lett., 83: 257-266.
chemistry of volcanic rocks from the Kaiko diving cruise
Konishi, K., 1986. Fate of Cretaceous Guyots; fact and speculation from Kaiko dives. Abstr. Int. Kaiko Conf. Subduction Zones, 1986, Tokyo and Shim&u, pp. 102-103.
Lallemand, S. and Chamot-Rooke,
decrochement
Kouriles:
dun
ancien volcan sous-
marin. C.R. Acad. Sci. Paris, 303, II, 16: 1443-1448
(in
French with English abstr.).
model applied to the subduction of seamounts in the Japan
trench. Geology, IS: 1065-1069.
S., Boulegue, J., Bourgois, J., Huchon,
Tajika, J., Katoh, T., Yoshida, A. and Research Group of
the Tokoro belt, 1986. Nature and tectonic history of the
Tokoro belt.Monogr. Assoc. Geol. Collab. Jpn., 31: 173-187
(in Japanese with English abstr.).
H., 1985. Crustal structure beneath the inner trench slope
of the Japan trench. Tectonophysics, 112: 155-191.
Takigami, Y., Kaneoka, I., I&ii, T. and Nakamura,
P. and
Seguret, M., 1986. Modalities of the “subduction-collision”
of the Daiichi Kashima seamount in the Japan trench.
Abstr. Int. Kaiko Conf. on Subduction Zones, 1986, Tokyo
and Shim+
aspects. Int. Kaiko Conf.,
1986, Tokyo-Shimizu.
Suyehiro, K., Kanazawa, T., Nishizawa, A. and Shimamura,
Lallemand, S. and Le Pichon, X., 1987. The Coulomb wedge
Lallemand,
(Japan, 1985)-Geodynamical
Sakakibara, M., Niida, K., Toda, H., Kite, N., Kimura, G.,
N., 1986. Sur la cause du
senestre entre les fosses du Japon et des
Subduction-collision
97: 1037-1053.
Pouclet, A. and Ohnenstetter, M., 1989. Petrography and Geo-
pp. 104-105.
Langseth, M., Okada, H., Adelseck, C., Bruns, T., Harper Jr.,
H.E., Kurnosov, V., Muller, G., Murdmaa, I., Pisciotto,
K.A., Robinson, P., Sakai, T., Thomson, P.R., Whelan, J.
press. 40Ar/39Ar
and K/Ar
Y., in
age data on igneous rocks
collected during Kaiko. In: Kaiko II Data Book.
Von Huene, R., 1986. To accrete or not accrete, that is the
question. Geol. Rundsch., 75 (1): l-15.
Von Huene, R. and Culotta, R., 1989. Tectonic erosion at the
front of the Japan trench convergent margin. Tectonophysits, 160: 75-90.