Tectonophysics, 160 (1989) 231-241 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 231 Subduction of the Daiichi Kashima Seamount in the Japan Trench SERGE LALLEh4AND ‘, RAY CULOITA 2 and ROLAND VON HUENE 2 ’ Dbpartement de G&tectonique, UnioersitC Pierre et Marre Curie, T26-El, 4 place Jussiey 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, 160: 231-247. In 1984-1985, 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 The Daiichi-Kashima seamount is split into two bodies offset vertically by more than 1 km (Mogi and Nishizawa, 1980; Kobayashi et al., 1987). The lower body (western part of the seamount) is partly subducted in the southern part of the Japan trench. An extensive 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 re- flection, 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 lower body of the seamount. Also a 100 km long 24-channel seismic reflection line crossing most of the margin and the seamount was recorded by Shell in 1972 (unpublished) (P844 on Fig. 1 and 2). In September 1986, we reprocessed part of this line by simultaneously picking stacking and migration velocities from a “matrix” of constant velocity prestack migrations and constant velocity stacks (the method is explained in: Von Huene and Culotta, 1989, this vol.). The analyses of these data 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 Dai- ichi-Kashima seamount is broken into two parts, 0040-1951/89/$03.50 0 1989 Elsevier Science Publishers B.V.
Transcript
Tectonophysics, 160 (1989) 231-241
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
231
Subduction of the Daiichi Kashima Seamount in the Japan Trench
SERGE LALLEh4AND ‘, RAY CULOITA 2 and ROLAND VON HUENE 2
’ Dbpartement de G&tectonique, UnioersitC Pierre et Marre Curie, T26-El, 4 place Jussiey 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, 160: 231-247.
In 1984-1985, 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
The Daiichi-Kashima seamount is split into
two bodies offset vertically by more than 1 km
(Mogi and Nishizawa, 1980; Kobayashi et al.,
1987). The lower body (western part of the
seamount) is partly subducted in the southern part
of the Japan trench. An extensive 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 re-
flection, 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
lower body of the seamount. Also a 100 km long
24-channel seismic reflection line crossing most of
the margin and the seamount was recorded by
Shell in 1972 (unpublished) (P844 on Fig. 1 and
2). In September 1986, we reprocessed part of this
line by simultaneously picking stacking and
migration velocities from a “matrix” of constant
velocity prestack migrations and constant velocity
d---- Oceamc normal faults 0~ landward slope slump scars
@ Landward tllted basins
\ Single-channel se,sm,c lines 8
\ 9 0 5 1Okm
Canyons I a I
I I I I I I I
Fig. 3. New (compared with those of Kobayashi et al., 1987) structural map of the surveyed area. The big numbers refer to the Kaiko
single-channel seismic reflection profiles. The large open arrow shows the Pacific plate motion relative to Japan.
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
body to its pre-fault position and assuming a roughly symmetrical shape, we determined an original seamount approximately 60 km in diame- ter and 3.5 km in height. The fault scarp separat- ing 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). An- tithetic 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 SLOPE
0 10 20 km t
V.E. I5
Fig. 5. Interpretative time line drawings of 8 of the 21 Kaiko single-channel seismic profiles (lines are located in Fig. 3). Vertical
exaggeration is 5 X The numbers are two-way travel time in seconds.
240
offsets of faults are smaller on the lower body).
Some single channel seismic lines (II, 13, 15. 16
and 17 on Fig. 5) show a landward dipping nor-
mal fault offsetting the lower body up to 800 m
(profile 13). These normal faults are aligned with
the trench axis (Lallemand et al., 1986).
Two observations suggest depression of the
crust beneath the seamount as the ocean crust is
flexed downward into the subduction zone. First.
the associated trench floor achieves its greatest
depth adjacent to the seamount. Second, the nor-
mal faults set separating the two parts of the
seamount has a 1700 m displacement at the mid-
dle of the seamount and only 700 m at the edges.
However, the tilt of the top of the seamount. if it
was originally horizontal, is in a sense opposite to
that induced by downward flexure beneath the
seamount. The lower body is tilted 2.5’ landward
whereas the upper body is nearly horizontal. The
problem has been considered by Ida (1986) but
has yet to be resolved in a manner that explains
all observations. Crustal response to subduction of
the seamount is observed and although not well
understood, it should be considered in models of
seamount subduction.
Trench fill is in contact with the lower part of
the seamount only northward and southward of it
(see Fig. 3). Observations made during the dives
of active erosion of the lowermost landward slope
suggest that sediment is subducted soon after
arriving in the trench (Cadet et al., 1987; Pautot et
al., 1987). Some tens of square kilometer landward
tilted basins on the lower landward slope of the
trench appear to have formed by ponding of sedi-
ment behind a ridge uplifted during the collision
of the seamount.
The preceding conclusions were deduced mainly
from analysis of the Seabeam map and the closely
spaced single channel seismic lines (Fig. 5). Fur-
ther information was obtained from the re-
processed 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).
M~tic~nel seismic line and dives
Characteristics of the seumount
Background
The geochemical, petrologic and bio-lithostrati-
graphic studies made by other investigators in the
Kaiko project also help to constrain our interpre-
tation of the seismic and dive data (Kaiko II Data
Book, jn prep.).
Volcanic rocks collected during the dives on the
seamount have yielded radiometric ages of 100 to
120 Ma (Takigami et al., in press). approximately
20 Ma younger than the surrounding oceanic crust
(Hilde et al.. 1976). Pouclet and Ohnenstetter
(1989. this vol.) find that the lower and upper
bodies are geochemically distinct: the samples coI-
lected on the lower body are composed mainly of
mugearite, whereas those collected on the upper
body are composed of basanite, mugearite and
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
(Fig. 6) lead Pouclet and Ohnenstetter to propose
that the seamount was originally composed of two
coalescing volcanoes separated by a depression.
According to their hypothesis, stresses resulting
from bending of the subducting plate and collision
with the margin caused the formation of a graben
at the site of the suture and depression between
the two volcanoes. According to Konishi (1986),
the limestone samples coliected during the dives
indicate that the seamount was capped by an
active reef during 10 Ma or more from Apto-Al-
bian time. ‘Then an eustatic rise of sea-level
drowned the reef. The seamount 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 approxi-
mately to the depth difference between the two
blocks: 1500 m.
The P844 seismic line
The acoustic character of the basement differs
considerably between the two parts of the seamount, from a very rough structure within the upper body to a more layered structure inside the lower body (cf. Fig. 4). This contrast may be due to a difference in composition, the upper body being more heterogeneous with some explosive volcanics (A. Pouclet, oral cormnun., 1987), or to the more intense fracturing of the upper body.
The sediment caps on the upper and lower bodies exhibit similar seismic character, being well stratified and nearly transparent like the hemi- pelagites covering the surrounding oceanic crust. However, the sediment cap on the lower body is twice as thick as that of the upper body. We attribute this difference to current erosion, as ex- plained in a later section. Because the two dives took place on the main scar-p (Fig. 6) only the
thin cap of the upper body, representing a part of the total stratigraphic column, was entirely ex- amined.
WNW
241
Below the turbidites filling the graben between the upper and lower bodies are several irregular
dipping reflectors, which we interpret to be chaotic
blocks sealed by recent sediments derived prim- arily from the main scarp. The small terrace seen at the base of the cross-section (Fig. 6) may corre- spond to an unburied block.
The northwestern flank of the seamount can be followed at least 30 km landward of the trench axis below the landward slope but it is difficult to discriminate between straight reflectors corre-
sponding to the cap of the seamount and layering of the basement as mentioned previously with regard to the lower body. The layered reflections could also correspond to continental sediments offscraped and subducted.
Geological cross section of the main scarp
Figure 6 shows an interpretative cross-section made from analysis of the video tapes and de- scriptions recording during the dives, and the sam- ples collected. Fresh and massive basalts outcrop
ESE.
Fault and Joint directions
[w I \
[tephra]
[altered
- 3800
- 3900
-4000
.4100
* 4200
- 4300
- 4400
-4500
-4600
- 4700
- 4800
* 4900
-5000
-5100
- 5200
- 5300
I - - -. , -. - .I’ =. . , . . - *, * . . .
D $00 ,,ooo ,$+ $I@ 29 d - . - . ’ (ml 3QQ $P El recent sediments m chalky limestone shallow-water limestone v v volcanic rock
B yellow to brown argilite Ezl
m interlayered welf stratified m sedimentary breccfa Q
dark brown layer 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
of the limestones because of the repetition by
frequent faults. According to the dive observa-
tions, the limestone may be 120 m thick. whereas
the seismic record indicates that it may exceed 300
m. These shallow water limestones are overlain bv
a few tens of meters of chalky limestones alternat-
ing with dark brown layers which were not sam-
pled. This sequence may correspond to the 15 m
of Mn-rich brown clay (Paleogene to lower
Miocene) overlying the upper Cretaceous cherts
recovered at DSDP site 436 (Leg 56, Langseth.
Okada et al., 1982). One-hundred meters of argil-
lites overlie these dark layers which may also
correspond to the middle Miocene to Quaternan,
diatomaceous argilhtes and silty clay recovered at
DSDP site 436. Two samples have been dated as
early Pliocene (Monjanel et al., in press, see Fig.
6) with reworked upper Miocene sediments (Cau-
let, written commun., 1987).
Forty normal faults or joints were observed
along the dive transect. Most of them are subverti-
cal. 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. Pos-
sibly they are related to the internal structure of
the seamount basement. The N140” direction is
exactly perpendicular to the trench and corre-
sponds to major lineaments of the seamount (Fig.
2).
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. Fur-
thermore, the upper layer exists only when the
seamount’s cap is sufficiently thrck. Thus. d~f-
ferences in sediment thickness on the two parts .)f
the seamount are caused by differential erosiorr of
the upper layer of both bodies. Ilrosion may have
increased locally with the recent fault acttvitv.
C’haracteristrcs of the margin jucrng the seumolrnt
Geologicul section across the truce o/‘ the suhduc-
tion 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
was found in the deepest part of the section.
Subhorizontal layers of slope hreccias crop out
from just above the trench floor up to 5600 m.
One sample of a breccia contaming diatoms (No.
3) has been dated at 2.5 to 3.2 Ma (Monjanel et
al.. in press). The stepped morphology in the
lower part of the cross section was interpreted by
the diver (K. Fujioka) to sigmfy thrusting. The
occurrence of living clams near sample No. 3 of
dive NA 2-6 may indicate tectomc disruption that
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 layers and intense faulting along mainly
transverse directions (N115” ,md 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 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. Trans-
verse faults or transverse trending tectonic fea-
tures are commonly observed <jn the margin in
front of subducting seamounts like the one at the
junction of the Japan and Kurt1 trenches (Lalle-
mand and Chamot-Rooke, 1986) or in front of the
Bougainville guyot in the New Hebrides trench
243
N 35"5(
; lL2020' E142"30 E 1~2~~0 E lL2"50
\ I
m recent inlillinp
2
Fig. 7. Distribution of the three different acoustic layers making the cap of the seamount (see details in the text). Shallow-water
limestones correlate with the lower layer showing strong reflectors, “possible” hemipelagites with the upper, more transparent layer
and with recent infilling for well stratified reflectors in depressions. The dashed lines correspond to the possible contours of the
acoustic layers below the inner slope. The main faults are simplified from the structural map (Fig. 3). The locations of “Nautile”
dives are plotted.
(Daniel et al., 1986). Recent small-scale folds also affect Pleistocene sediments. They may be superfi- cial slump folds associated with the oversteepen- ing of the slope. Alternatively they might be com- 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 brec- cias in the lower part of the slope but no clear folding upslope. Two observations from those dives suggest minor accretion of the Daiichi- 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 chro- nological (115-120 Ma) affinities with the Daii- cm-Kashima seamount (Ishii et al., 1986). Sec-
ond, during dive NA 3-9, a 20 m section of limestone talus accumulation was encountered on the lowermost landward slope which may be de- rived from incorporation of the upper part of the limestone cap into the innerslope along a thrust fault (Cadet et al., 1987).
Erosional channels and debris flows were ob- served during every dive.
Analysis of seismic line P844
The reprocessing of seismic line P844 revealed deep structural information especially on the land- ward side of the trench (Fig. 4). The following are the main observations:
(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
244
depth (ml
5100 -
5200.
5300 *
so0 -
ssoo-
S600.
NW SE ,WSW ENE
p-z+-----+ F+iTq
ri-- ~~- - ------ --.-. volcanic rock
/
ilO pScu&JcleaVdgc
/ $%I Limestane seamount cwer
I rl outcrop with0ut clear dipping of strata
2 [mddl e Phstocene clav mud] j recent sediment ,
I.C . ,.‘~tocc”e ’ m slope breccta nudstone] tren&
i_=? alterations of mudstones and dark layers
li I (ovrOctaaStit& ? 1 L-----_---_____--2.. ..-_ fold
Fig. 8. Geological cross-section of the trench axis area with the lowermost landward slope drawn from the analyses of video tapes
recorded during the Nautile dives: NA 2-6 (observer: K. Fujioka) and NA 2-7 (observer: P. Huchon). The description of samples
(bold numbers) are issued from written communications of J.P. Caulet, H. Charnley, A.L. Monjanel 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
and 5).
(2) The frontal wedge material is deformed by
imbricated thrusting and folding. Little or none of
the cap appears to have been accreted.
(3) The middle slope is disrupted by normal or
list& faults. The shallow offset and 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).
A knoll located landward of the Daiichi-Kashima
seamount (Fig. 1) is bounded by two pronounced
headless canyons perpendicular to the trench (Cyl
and Cy2 on Fig. 1). One small canyon (see Cy.I on
Fig. 1) cuts across the front of the knoll and
appears to predate the development of the knoll
itself (Figs. 2 and 3). thus 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 of a seamount.
The Kashima seamount is one of a chain of
seamounts aligned along the direction of magnetic
lineations, so that the earlier subduction of pre-
ceding 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) corre-
sponds to a volcanic edifice, there is an ambiguity
at the base of it because we can follow the d&olle-
ment rather easily. All the above features might be
explained by underplating of a part of a seamount. Subcrustal accretion could have produced the up-
lift of the knoll, the oversteepening of the slope, and the resulting listric faults compensating a mass excess as proposed in the model of Platt
(1986). The line drawing of the time-section of the Ml
seismic line (Oshima et al., 1985) has been dig- itized without modifications of interpretation and then converted into a depth section using the same velocity model as that used for the P844 seismic line. The velocity model used beneath the lower trench slope is based on the refraction 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 of the landward dipping thrust zone.
Discussion and model
One question commonly considered when dealing with the collision of a seamount is whether it is accreted or subducted. Some volcanic rocks in
245
ancient accretionary complexes have been in- terpreted as parts of accreted seamounts and their
sedimentary caps (Naka, 1985; Ogawa, 1985; Sakakibara et al., 1986). The simplest models im- ply that the seamount is sheared off at its base and incorporated as a body with some horizontal compressional deformation at the front of the subduction zone.
Evidence for partial frontal accretion of the Daiichi-Kashima seamount is the recovery, 250 m above the trench axis, of two isolated alkali rocks similar to those of the seamount’s basement (dive
NA 3-lo), and the observation of a 20 m lime- stone talus accumulation at the base of the land- ward slope (dive NA 3-9). However, in seismic records the landward flank of the seamount ap- pears to be subducted. The Ml seismic line (Fig. 4) and the single-channel line No. 14 (Fig. 5) show that even the sedimentary cap of the seamount is subducted without being deformed. As little trench fill is observed despite the many erosional chan-
nels, debris flows, and fresh talus on the landward slope, such material must be subducted soon after
I fixed 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 over- steepened and appears to be collapsing into the trench axis. This collapsed material must also be subducted. A possible way to explain the local incorporation of small quantities of material from
the seamount into the lower slope is that some turbidite containing clasts transported from the Kashima seamount was accreted just prior to subduction of the seamount (Fig. 9).
The main process during collision of Daiichi- Kashima 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 the oceanic crust and the seamount. The vertical displacement across the seamount is 1 km greater than on the ocean crust. Depression of the crust is also indicated by the 100 m increase in depth of the trench axis. A second way to accommodate the mass of the seamount is by uplift and thickening of the sediment that comprises the lower slope of the trench (Lallemand and Le Pichon,,l987). The ridge associated with the subducted front of the seamount and its imbricate structure correspond well with the position of the subducted front of the seamount. The narrow zone of deformation in front of this collision probably reflects a low strength of the material that comprise the lower slope. A third process, collapse of the overstee- pened slope and subduction of the debris from masswasting, is suggested by the lack of fill in the trench axis.
A history of the subduction of the seamount begins about 150,000 to 250,000 years ago (assum- ing 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 verti- cal 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.
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 collapsed material has been r;lpidly subduct,ed.
The overall process of collision between a seamount and a plate margin. !nodelled by Van
Huene (1986) based on studies of the Central America trench off Guatemala. would involve in- dentation and collapse of the landward slope of the trench, and progressive dismemberment and eventual subduction or accretion of parts of the seamount. An early stage in this process might be represented by Erimo seamount which is begin- ning 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 a very large indentation and massive slope col- lapse have occurred as the trailing flank of ti seamount has been consumed (Lallemand and Le Pichon, 1987). Our analysis of the Kaiko data and the reprocessed line P&44 lead us to conclude that the present disposition and structure of Daiichi Kashima seamount represents an intermediate stage in the process of collision between a seamount and a continental margm.
Acknowkdgements
We thank Shell 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 and on the Japanese side by MONBU-SHO. We thank Prs. J.P. Cadet and X. Le. Pichon, Drs. P. Huchon and L. Jolivet for their encouragement during this work and Drs.W.T. 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,
