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Bulletin of the Geological Survey of Japan,vol,42(12),p.655-686,1991
:M:我g館et且c題齢朧,我旦亘es ove貿t恥e夏z聡一〇解s我w我聡(:B艦量麟A鷲,
M蹴餓訟A罵蹴盛M鑓訟餓丁騰聰9恥
Toshitsugu YAMAzAKI*,Takemi IsHIHARA*
and Fumitoshi MuRAKAMI*
YAMAzAKI,Toshitsugu,IsHIHARA,Takemi and MuRAKAMI,Fumitoshi(1991) Magnetic
anomalies over the Izu-Ogasawara(Bonin)Arc,Mariana Arc and Mariana Trough.
Bzfll,060乙Sz67∂.ノ41)召%,vol.42(12),P.655-686,15fig.
Abstract:We performed magnetic anomaly measurement over the Izu-Ogasawara(Bonin)
Arc,theMarianaArcandTrough,andtheOgasawaraPlateauanditsneighboringseamountsalong dense ship tracks,the intervals of which are2to4miles in generaL
Most submarine volcanoes on the Shichito Ridge and the Mariana Ridge(the volcanic
front of the Izu-Ogasawara and Mariana Arcs)accompany large magnetic anomalies.
Some volcanoes have fairly simple dipole-type anomalies(e,g.the Nishinoshima Volcano and
Fukujin Seamount),and others have complicated anomaly distributions(e.g,the Iwojima
Volcano)and/or anomalies poorly correspond to the shape of volcanoes(e.g.the Torishima
Volcano and Nikko Seamount).The differences in the magnetic inhomogeneity of these
volcanoes may reflect differences in the length of the volcanic activity. For volcanoes in the
northem part of the Shichito Ridge,the magnetic inhomogeneity may reflect the bimodal
volcanism.The back-arc rifts in the northem part of the Izu-Ogasawara Arc show no
remarkable anomalies,which suggests that the seafloor spreading has not started yet.
Magnetic anomalies are useful for estimating the origin of seamounts。Seamomts
distributed between the Sofugan and Nishinoshima Islands in the middle Izu-Ogasawara Arc
(the Shichiyo Seamount Chain,the Omachi,Sawa and Tenpo Seamounts,etc.)are classified
into four groups from magnetic anomaly pattems.The first group is Quatemary volcanoes
having strong(1ipole-type anomalies.The second is faulted blocks of old island-arc cmsts,
accompanying little anomalies,The third is old volcanoes of possibly Oligocene age,which
have the magnetization ofeastward declination and shallow inclination,The fourth is broad
positive anomalies over the Omachi and Tenpo Seamomts.These seamounts are inferred to
be old basement highs because the similar anomalies are observed on forearc basement highs
in the northem Izu-Ogasawara Arc(the Shinkurose Ridge)having Paleogene or Miocene
age.
We have found for the first time clear magnetic lineations in the Mariana Trough
between20D and22。N.A forward mo(1eling showed that the seafloor spreading at21。307N
has started at3Ma or a little earlier at a half spreading rate of2,5cm/year.The rate has
decreased to about l cm/year at present。Spreading axes probably overlap from20。30シto
21。N.Their offset is about30km.The occurrence of the large-offset overlapping spread・
ing centers is unusual for ridges of slow spreading rates,which suggests that the crust here
would be weaker(hotter an(i/or thimer〉than that of major-ocean spreading centers.
Four peaks on the Ogasawara Plateau and three other seamounts in the east of it,
which are thought to have been formed in the Cretaceous,can be divided into two groups from
*Marine Geology Department Keywords=magnetic anomaly,Izu-Ogasawara(Bonin)Arc,
Mariana Trough,Ogasawara Plateau,seamomt,seafloor
spreading,backarc
一655一
βz61」6ガn6ゾ孟h6 060109づαzl Sz67∂6y(ゾノ功りごz多z,VbJ.42,No.12
magnetic anomalies.The first group having negative anomalies over the seamount bodies
was magnetized in an equatorial region in the Cretaceous.The second group has the
magnetization close to the present geomagnetic field direction,suggesting induced and/or
viscous magnetization or remagnetization by rejuvenated volcanism.
1. 亙鵬ro岨胆cti樋
35。
Weconductedmagneticsurvey overtheIzu-Ogasawara(Bonin)Arc and the Mariana
Trough from1984to1989as a part of theresearch program “Submarine Hydrothermal
Activity in the Izu-Ogasawara Arc” to
understand better the geological structure in
this region.Survey areas include the whole 。 30
1zu-Ogasawara Arc from Hachijojima Is.to
Minami-lwojima Is.,the Ogasawara Trough,
the Ogasawara Plateau and neighboringseamomts,the Mariana Ridge and the Mar-
iana Trough north of203N,and along the
spreading center of the Mariana Trougharound18。N.The spacings of the survey Iines
were2to4nautical miles(3.7to7.4km〉ingeneral,and they were as close as l mile in 25。
several detailed sur▽ey areas. Such high
density of ship tracks made us possible to
draw magnetic anomaly contour maps of high
accuracy. The purpose of this report is to present
magnetic anomaly distributions of above
mentioned areas,and discuss the geological
stmcture deduced from the magnetic anom-alies. 20
〆’.F讐〉
H
S h i k o k u
Ba s i n
幅、 F’伊2、・’勾 齢①∫¢ ゆ
驚三 杢”:罵
還 蟻 .9
ロしじロニ
1?F’9・3轟一・臼・;ミ}
1121・甲
~7・ ‘・{
050←
、”嘉N辱鷺∫
僻1峯ぎ
叢
F79.7
Pacific Ocean
∫ o G o
セ 6 6 3 0 0 0 0 0 も 急
喜、一
懸霧・・甲
§1
.瓢09白sawara
Parece Veia Basin
PIateau
Backarc rifts
-Spreading center ▲ Quaternary vQlcanoes
2.B蹴kgro腿認
2.1 亘zu-Og総aw躍a Arc
The Izu-Ogasawara(Bonin)Arc lies on the
eastem rim of the Philippine Sea plate sub-
ducted by the Pacific plate (Fig.1). It is
about1200km long extending from the Izu
Peninsula to Minami-lwojima Is.It con-tinues southward to the Mariana Arc。
The Shichito Ridge is occupied by a line of
Quatemary arc volcanoes,which forms arecent volcanic front of the Izu-Ogasawara
Arc.Volcanic rocks from the Shichito Ridge
Fig.1
嘱40。 145。
Stmctural framework of the Izu-Ogasawara-
Mariana Arc.Dotted lines represent1000m and
3000m iso-depths.S.T.L.:Sofugan Tectonic
Line(modified from Yuasa(1985)).H:Hachi-
jojima Island,N:Nishinoshima Island.
are known to be compositionally bimodal
(Tsuya,1937;Kuno,1962).Occurrence of
acidic rocks are abundant in the northern part
of the Shichito Ridge,but little in the southern
part(Yuasa,in prep.).
A row of small grabens occurs just behind
the volcanic front in the northern part of the
Izu-Ogasawara Arc,from Hachijojima Is.to
Nishinoshima Is.The Aogashima and
一656一
痂9彫渉づ6αno〃観1づ6s oゾ云h6及%一磁z甥召鰯z4κsピT.y4窺α㌶hJ6渉σ∂
Sumisu Rifts in Fig.2(a)are examples of such
grabens. They have been considered to be
young back-arc rifts,or incipient back-arc
basins(Karig and Moore,1975;Honza and
Tamaki,1985).Recent ODP(Ocean Drilling
Program)drillings in the Sumisu Rift,one of
these back-arc rifts,proved the young age of
the rift.The age of the sediments in the
Sumisu Rift is not older than1.1Ma(Leg126
Scientific Drilling Party, 1989). Signs of
hydrothermal activity were reported from the
Sumisu Rift(Taylor6渉σ1.,19901Urabe and
Kusakabe,1990)and the Aogashima Rift(Yamazaki6厩1.,1991).In the southem part
of the Izu-Ogasawara Arc between Nishino-
shimaIs.and Minami-lwojima Is.,onthe other
hand,no backarc rifts are recognized.
In the back-arc of the Izu-Ogasawara Arc,
geological structures being oblique to the
trend of the arc are remarkable(Karig and
Moore,1975;Honza and Tamaki,1985).In
the north of about30。N,several rows of
basement highs,on whi¢h relatively large
seamomts are located in line,run in NE-SW
or ENE-WSW direction(so-called Nishi-
Shichito Ridge or Izu Ridge).Examples are
the Enpo and Genroku Seamount Chains inFig.2(a). In the south of30。N,on the other
hand,troughs of NNE-SSW strike are con-
spicuous.The most prominent among them is
the Sofugan Tectonic Line(Fig.1),a steep
cliff of probably normal fault origin mming
linearly south-southwestward from the south
of Sofugan Island. Yuasa (1985)considered
that the Izu-Ogasawara Arc can be structur-
ally divided into the southern and northern
part by this fault.The Nishinoshima Trough
lies on the east of the Sofugan Tectonic Line.
It is bordered on the east by the Shichito
Ridge. Yuasa (1991a) proposed that the
Nishinoshima Trough was formed as a north-
ern tip of the incipient opening of the Parece
Vela Basin in Oligocene time(Mrozowski and
Hayes,1979).The Sofugan Tectonic Linewould consist a westem wall of the paleo-rift.
The “Nishinoshima Trough” of Honza and
Tamaki(1985)and Yuasa(1985)includes
other topographic lows Iying between theShichito Ridge and the Nishi-Shichito Ridge
(Izu Ridge),but in this report does not.
The Ogasawara Ridge is a forearc basement
high of Eocene age(Tsunakawa,1983).The
Ogasawara Trough lies between theOgasawara Ridge and the Shichito Ridge.A
multi-chamel seismic profile revealed that the
Ogasawara Trough was formed by rifting(Abe 6渉 α1.,1989).The rifting of the
Ogasawara Trough may be simultaneous with
the rifting of the Nishinoshima Trough in the
Oligocene(Yamazaki6臨1.,1988a).The two
troughs may have been a part of one paleo-
rift,and later growth of the present Shichito
Ridge may have separated them.
In the Izu-Ogasawara Arc,magnetic sur-
veys have been conducted by several institu-
tions for two decades but rather sporadically.
Here we present a magnetic anomaly map of
the whole area for the first time,which is
based on systematic survey with dense ship’s
tracks. Previously the Geological Survey of
Japan conducted a recomaissance magneticsurvey along lines at intervals of15miles(28
km)in1979,and the results were presented as
magnetic anomaly profiles(Miyazaki6渉召1.,
1981〉.The Hydrographic’Department,Mar-
itime Safety Agency Japan has published a
series of bathymetric maps of1/200,000scale
over the northem part of the Izu-Ogasawara
Arc,a part of which includes a magnetic total-
force map of the same scale.
2。2遡翻躍aTro髄9㎞
The Mariana Trough is known as one of
active back-arc basins in the world(Karig,
1971).Recently active hydrothermal vents
were discovered by submersible ALVIN at18。
N(Craig6緬1.,19871MooreandStakes,1990).
Based on the results of DSDP (Deep Sea
Drilling Project)drillings,Hussong and Uyeda
(1982)considered that the rifting of the Mar・
iana Trough at18。N would have begun in the
latest Miocene,about6Ma,with a halfspreading rate of2.15cm/year.Bibeeασ1.
(198g)correlated several magnetic anomaly
一657一
li
匠1ル
ま
ミき
i1
難
藍 ド ずヨド
ぞ
欝灘
//
聚繋灘
li
βz61」6あ%oゾ云h6 G601091641S%7%ly6ゾノ4ゆα%,『Vlol.42,一No,12
Fig.2
33。N
320
31。
30。
N ゲ
(a》 」
5
♂
d Φ b 〈)o
ミ ⑥ 咲
も £ ミ
C) 7
勧 Q ♂識。
O o OO
悔麟乳 .凝
黙o 〃 ’ 〆 ◎M
[ )o AQgashimaRift I/ ojinKnoII
o 巴、 ミ 診塵
] ロ、 ぺ◎睾
伊po
σ 一rの MyolinshoCaldera
. r∠、Gi熊葱., 0 2 D・餅〆“ へ む
丘 ノO G
①o℃周噛曙SumisuCaldera
・呼づ〔つDaisan一SumisuKnoIl
の
# ミ
0 ◎ G ・・(7評笹◎〆稽 0 0◎ .
@◎♂00
いa、“ q σ づ
0 2 ∈ お 9~ @ 0 ⑦
/
o ■隔 亀 の
も戴熱励
の
獄
139。 140。 141。E
(a)Outline of topography in the northem Izu-Ogasawara Arc from30。to330N.Contours are at500m
intervals.Unit ofthefiguresis1000m.(b)Magnetic anomaliesinthesame area.Contoursαre at100nT intervals.Positive anomalies are shaded.(c)Survey lines of the magnetic anomalies.
profiles between17。307to18。N and estimated
a half spreading rate of approximately l。65
cm/year within50km of the spreading axis。
The mamer of spreading in the Mariana
Trough,however,has not been understood
we1L Survey lines of magnetic anomalies are
still scarce except for the area near the
spreading axis between17。30’to18。N.It is
very difficult to correlate magnetic anomaly
profiles in the Mariana Trough mless these
1ines are closely spaced(Karig,1971;Ander-
son,19751Karig6!α1.,19781Fryer and Hus-
song,1982〉.One reason is that amplitudes of
the magnetic anomalies are generally very low
because the trough is close to the magnetic
equator and the strike of the spreading axis is
close to north-south,Another reason would
be that the spreading mamer of the Mariana
Trough is not simple and many discontinuities
such as fracture zones,propagating rifts(Hey,
一658一
物9魁露朋o㎜伽oゾ云h8伽一〃47加α∠470s‘T.y4耀撤昭副
3300
3200
3100
3000
(b)
毒.
.ぺ灘
oΩ 麟雛
癬o<
1薦熱
○
蔓灘
鱗蒙
㊥
Q
o
黙
灘難
鱗縷爵醸’8、穐蝋
くン リ
(〉。Q⊃ ○ 榛綴 醸 翻
繊
⊃
ノ
昏驚講.熱
講歎α撫黛羅難i .
13800
13900
14000
14100
Fig.2 continued
1977) and overlapPing spreading centers
(Lonsdale,19831Macdonald and Fox,1983)
may exist.
We have identified for the first time mag-
netic lineations in the northem Mariana
Trough between20。to22。N based on dense
magnetic profiles,and discuss the mamer of
seafloor spreading there.
3.Bataae麟s宜t亘樋腿姐re幽腿etion
Magnetic total force was measured using a
proton precession magnetometer(GeoMetrics
G801)towed about250m behind the R/VHakurei-maru.Data were logged on a mag-
netic tape every lO seconds,and finally edited
as a series of records on every minute,which
contain ship’s position,speed and geophysical
data(magnetics,gravity and depth〉.Resid-
ual total magnetic intensity field,that is,
magnetic anomaly,was obtained by subtract-
ing the1985version ofthe IGRF(lnternational
Geomagnetic Reference Field) (IAGADivision I Working Group1,1985)from the
一659一
Bz61彪あ%(ゾ孟h6 G60Jo4αzl Sz67z76ッ6ゾノ4ψ召%,Vbl.42,No.12
3300
3200
3100
3000 138 00
《c》
~
、 1
一
欝
、 , 革
‘』 一・
嗣難.
~鴇・
一穏1
膠【■旧
繋¥ 11、 ハ
一
’i羅li難9
、
自}
il 碍ノ》” 鷺寄
、
刈
、欝・』 1
\㌧
\ f簾r一_一一撒
鰹膣1
■欄 ■ 一虚
懇璽…穣
、I
[
コ一 、}一噂
一 畿鰹一 一
【. 二二=【二 ヨ王=’一3
139 00
140 00
141 00
Fig.2 continued
observed data.Diumal correction was not
applied.Shipシs positions were determined by
a satellite-Loran C integrated navigation sys-
tem(Magnavox model200).This systemgenerally has an accuracy of about O.1nautical
mile(180m).After1988,the GPS (GlobalPositioning System)was also used for naviga-
tion.
After editing out anomalous data such as
spiky noise,the data were interpolated onto a
gridρf l km spacing.The gridding was done
by a weighted averaging method using a
commercially available program packageUNIRAS.The data collected during theprevious cmises of the Geological Survey of
Japan in1979(Miyazakiαα1.,1981)werealso included.
一660一
物9吻ぎ6α%o耀1づ6sげ云h6Zゑ%一惚7伽召∠4κsピTy碗αz襯α副
4. 亙薦e聯reta色亘o聡
4.1N⑪r騒ern亘z眼一〇9薦題w麗題Arc:臨e恥ijoJi勲亘s.toT罐s臆ma亘s. Quaternary volcanoes stand in line along the
Shichito Ridge,the volcanic front of the Izu-
Ogasawara Arc (Fig.2(a)).They are the
Aogashima,Myojinsho,Sumisu and Tori-shima Volcanoes,and the Daisan-Sumisu and
Myojin Knolls.All but the Myojin Knoll
show strong and short wave-1ength magnetic
anomalies (Fig.2(b)〉.The complicated
anomaly pattems of these volcanoes suggest
that strongly and weakly magnetized parts are
mixed,which may reflect the bimodal volcan-
ism of this area.The islands emerged from
the sea are known to be composed of basalts
(Kmo,1962〉,but dacites are dredged from
submarine calderas(Yuasa,in prep.〉.
A typical example is the magnetic anomaly
distribution over the Torishima volcano.
Strong dipole-type anomalies were observed
in the close vicinity ofTorishima Island.The
area of shallow water depth extending north
of the island,however,shows little anomalies
(Fig.2(b)〉.The island is composed of basalts
(Kuno, 1962),which would have strong
remanent magnetization.The shallow area
north of it would be comprised by weakly
magnetized rocks such as dacites.This imp-
1ies bimodal volcanism of the Torishima vo1-
cano.This suggestion is consistent withOkamura and othersン(1991)consideration
that thick volcaniclastic sediments in the
Sumisu Rift,which are composed mainly of
dacitic pumice(Nishimura6厩1.,1991),were
derived from the Torishima volcano or its
vicinity.
The Myojin knoll accompanies little mag-
netic anomalies although the volume of the
volcano is not much smaller than that of oth-
ers.It is suggested that the Myojin:Knoll is
composed of weakly magnetized rocks such as
dacites.This inference has recently beenconfirmed by the observation using the sub-
mersible Shinkai2000(Yuasa,1991b).
Back-arc rifts,which lie just behind the
volcanic front,show no remarkable magnetic
anomalies except for a chain of volcanoes
crossing obliquely the Sumisu Rift(Fig.2〉.
The basalts taken from the intra-rift vo1-
canoes in the Sumisu Rift have chemical com-
position of back-arc basin basalts (lkeda and
Yuasa,19891Fryer切α1.,1990).However,
the fact that no strong magnetic signals like
the magnetic lineations of mid-oceanic ridges
were observed suggests that these basalts
would be volumetrically small.It is thus
considered that seafloor spreading has not
started yet and the basement of the back-arc
rifts would consists dominantly of island-arc
crust.
The Nishi-Shichito Ridge lies to the west of
the back-arc rifts,which consists of topo-
graphic highs trending obliquely to the Izu-
Ogasawara Arc.The ridges (the EnpoSeamomt Chain and the Genroku SeamomtChain) show relatively simple magnetic
anomaly distribution=negative anomalies oc-
cur north of topographic highs,and positive
south (Fig.2).This pattem indicates the
magnetization of normal polarity.The ages
oftheridgesarenotclear.AK-Arageof2.2±1.1Ma was reported from the EnpoSeamomt(Yuasa,1985).On the other hand,
historical volcanic activity around the Genro-
ku Seamomt was documented(Kmo,1962).
The normal polarity of the magnetization
might indicate yomg age of the ridges
(younger than the Bmnhes-MatuyamabomdaryatO.73Ma),butthereisapossibility
that induced magnetization or viscousmagnetization of the present geomagnetic
field direction have overcome the primary
remanent magnetization~Magnetic proP・erties of the rocks which comprise these ridges
have not yet been known.
4。2 M童翻且e亘z眼一〇gas&w麗我A麗:Sof腿9樋
亘s.馳Nis踊齢曲量m箆亙s.
The Sofugan and Nishinoshima volcanoes,
the crests of which are emerged from the sea,
are Quaternary arc volcanoes located on the
一661一
多
βz61」6渉づ%oゾ孟h6 G60JogJαzl S%7z761y(1ゾノ4かz7z,’Vb乙42,No.12
(a》
30◎N
Fig.3
29。
280
27。
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⑩ /f川川 歳 1”11バ 監 …川1’ o へμ1 の 川1ヒDoyo Smt. 鳴 尽㌧♪ 遥 、\ 、・く O へ、 t
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140。 14唱。 唱42◎E
(a)Outline of topography in the middle Izu-Ogasawara Arc from27。to30。N,Contours are at500m
intervals.Unit of the figures is1000m.Broken lines indicate the location of seismic profiles shown in
Fig.5.(b)Magnetic anomalies inthe same area.Contoursare at100nT intervals,Positive anom-
alies are shaded.(c)Survey lines ofthe magnetic anomalies.Bold lines indicate the location of seismic
profiles shown in Fig,5.
蓼
volcanic front.They have dipole-type mag-
netic anomalies of normal polarity(Fig.3(b)).
There is no subaerial volcano between the
two,but seven seamomts,recently named as
the Shichiyo Seamount Chain,are located on
the Shichito Ridge in a row(from the Nichiyo
Seamomt to the Doyo Searhount)(Fig.3(a)).
The Shichiyo Seamount Chain and neigh-
boring seamounts can be divided into fo皿
groups based on their magnetic anomalies(Fig.4). The first group(group l)has strong
dipole-type anomalies ofnormal polarity(Fig.
3(b)).The Kayo Seamount,the KinyoSeamomt and the Doyo Seamount are includ一
一662一
ル勉g%漉卿oη昭1乞6s oゾ孟h6左%一ル勉7宛槻、4κsピT。y砺z薦盈づ6渉召0
30
29
28
27
(b》
\蹴.灘
なだ
霧雛
艶
「
(』)
駄廷隈
(
騰
o c 。
柔懸籍Gつ 懸馨醗
蒙響ぐ回瀦.Q・・
瀬 o く⊃ ら
轟10
d
塗騒
。趨暴欝蕪
Q①⊂>
\
ノ
ノ
晃
㌶灘、熟。.一爺、)
、○○
〉繭/麟
139 140Fig.3 continued
ed in this group.These seamomts are con-
sidered to be Quaternary arc volcanoes like
the Sofugan and Nishinoshima volcanoes.
The second group (group 2 in Fig.4)
accompanies weak anomalies compared withthe volumes of their edifices(Fig.3(b)).The
examples are the Nichiyo Seamomt,the Sawa
Seamomt and an umamed seamount at28。30’N, 140。15’E, It is considered that these
seamounts are not young volcanoes but blocks
of old island-arc crust。 Seismic reflection
profiles revealed that these seamomts are
tilted blocks cut by faults and covered with
sedimentary layers(Fig.5(a)).The deforma一
141 142
tion would have been taken place under the
extensional regime which formed the Ni-
shinoshima Trough.
The third group(group3in Fig,4)which
includes the Suiyo Seamomt and the Mokuyo
Seamount is characterized by an unique
anomaly distribution which cannot be ex-
plained by the magnetization of the present
geomagnetic field direction(Fig.3(b)).The
Suiyo Seamomt has a positive,strongeranomalies above its westem flank and a nega-
tive,weaker anomalies above the eastemflank(Fig。3(b)).This anomaly pattem can
be interpreted as the magnetization of east一
一663一
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3000
2900
2800
2700 138
00
(c》]
~
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’1 1 、
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1 II
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Fig.3 continued
ward declination and shallow inclination(Fig.
6).The direction of the magnetization is
fairly close to the paleomagnetic direction of
Ogasawara(Bonin)Islands on the Ogasawara
Ridge(Kodama6渉σ1.,1983)。The east de-
nected declinations have been reported not
only from the Ogasawara Islands but also
from the Mariana Islands(McCabe andUyeda,1983),the Palau Islands(Hastonασ1.,
1988),and Guam and Saipan(Haston andFuller,1991),and they can be explained by the
clockwise rotation of the Philippine Sea plate
since the Oligocene (Louden,19771Seno and
Mamyama,19841Haston6厩1。,1988,Hastonand Fuller,1991)。The magnetization direc・
tion thus suggests that the Suiyo Seamomt
would have emplaced soon after the formation
of the Nishinoshima Trough in the Oligocene,
and after that it was rotated with thePhilipPine Sea plate.
The anomaly distribution of the Mokuyo
Seamomt is even more complicated(Fig.3(b)).A pair of positive’and negative anom-
alies of north-south direction occurs on the
seamomt,but its spread is restricted just
above its crest. Its vicinity, on the other
hand,shows the anomaly pattem of eastward
magnetization similar to that of the Suiyo
Seamount.It is thus estimated that the edi・
fice of the Mokuyo Seamount would haveformed mainly at about the same time as the
Suiyo Seamomt,but recently rejuvenatedvolcanic activity has taken place at the crest.
There are some other observations which
support this inference.Seismic reflectionprofiles across the Mokuyo Seamount(Fig.5
麟
//灘難支
一664一
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Fig.4
300
2go
28◎
qφ “ 魅
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魅 025 0
梅鑓o
b __
、騨鵬鞭螺。
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9
。1、館
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圏眉
睡2
麗劃3
麗講4Smt.
machi Smt.
Tenp◎Smt. 〔
1400E 1410
Classification ofthe Shichiyo Seamount Chain and neighboring seamomts from their magnetic anomaly
pattems。(1)Quatemary volcanoes having strong dipole-type anomalies of the normal polarity.(2)
Faulted blocks of old island-arc crust accompanying weak magnetic anomalies。(3)Old volcanoes
having east-deflected magnetization direction.(4)01d basement highs characterized by broad positive
magnetic anomalies.
(b)) show that except for the crest the
seamomt has well stratified reflectors,which
suggests that the seamomt is covered with
fine sediments rather than volcaniclasticdebris.It is thus inferred that the seamount
would not be young except for the top.
Recent SeaBeam mapping revealed that thecrest has caldera structure (Nagaoka 6渉 α1.,
1991).In the caldera,hydrothermal activity
was discovered by dives of the submersible
Shinkai 2000 (Nagaoka,1990),indicating a
recent volcanic activity there.
Around the Tenpo Seamount and the Oma-
chi Seamomt(group4in Fig.4),broad posi-
tive magnetic anomalies are conspicuous(Fig.
3(b)).The intensities of these anomalies are
fairly large,more than500nT.The broad-
ness of the anomalies and their rather poor
correspondence to the topography suggest that
the major cause of these anomalies would not
be the magnetization of the edifices of the
seamomts but somewhat deeper magnetized
bodies.On the Omachi Seamount,K-Ar ages
of32to34Ma and occurrence of late Eocenemicrofossils were reported (Yuasa α α1.,
1988),A sedimentary rock having microfos-
sil age of the early to middle Miocene was
taken from the top of the Tenpo Seamount
(lkari and Nishimura,1991).It is thus con-
sidered that these seamounts are not yomg
volcanoes,but are composed of old island-arc
cmsts which formed before the Izu一
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』 』 自 』 』 P
礁嘆瀬、,、
翻勢e
,ダ
欝 10km、、『』享 一 享、灘
Fig,6 Magnetic anomaliesproducedby apyramid-shaped magnetizedbody.Thedeclination oftheremanentmagnetization of the pyramid is90。E,inclination20。and intensity7A/m.The declination and
inclination of the ambient field are-4。and38。,respectively(value of the1985version of the IGRF at
28。N,140。E).DepthLof the top of the pyramid is l km,and the bottom4km.Contours are at100nT
intervals.Positive anomalies are shaded.
Ogasawara Ridge and the Kyushu-Palau Rid-
ge were separated by the creation of the Shi・
koku and Parece Vela Basin。
The broad positive anomalies similar to
those observed over the Omachi and Tenpo
Seamomts occur sporadically in the fore-arc
region in the northern part of the Izu-
Ogasawara Arc.They are at southeast ofTorishima Is.and east of Sumisu Is。shown in
Fig.2(b),and the Shinkurose Bank(northeast
of Hachijojima Is.),presented in Ishihara6砲1.
(1981〉。 These regions correspond to the
uplifted portions of acoustic basement on
seismic profiles,called the Shinkurose Ridge
(Honza and Tamaki,1985).The basement
highs are interpreted to be composed of
Paleogene and Miocene sedimentary rocks on
the geological map of Honza6砂1.(1982).It
is thus considered that the broad positive
magnetic anomalies would characterizebasement highs of the old Izu-Ogasawara Arc.
In the west of the Nishinoshima Trough,
magnetic anomalies having relatively longwave-1ength are distributed (Fig.3(b)).
Topography of this region is fairly smooth
(Fig.3(a)).These anomalies are probably
attributable to deeper magnetized bodies such
as intmsives.The elongating direction of the
anomalies,WNW-ESE,isperpendiculartothestructural trend of this area,represented by
the Sofugan Tectonic Line.
4.3 SOuthem Iz既一〇9我s我wara Arc=t恥e
K我ikaねSeamo聡醜ttoM亘醜我mi-lwoj亘漁夏s. Volcanoes on the Shichito Ridge(Fig.7(a))
show strong,short wave-1ength magneticanomalies(Fig.7(b)〉reflecting their Iarge
volume and shallow water depths.The basic
pattem of magnetic anomaly distribution over
the Kaikata Seamomt,the:Kaitoku Seamomt
and Kita-lwojima Is.is of normal-polarity
magnetization:the anomalies over the north-
em flank of the volcanoes are dominantly
negative,and those over the southern flank are
positive.Being superimposed on this pattem,
dipole-type anomalies of shorセer wave-1ength
are distributed.Some of the①correspond to
topographic highs,but otherS do not.Thelatter would be due to highly variable magnet-
ization,reflecting thermal and compositional
inhomogeneity of the volcanoes.
The Iwojima and the、Minami-lwojima Vol一
一667一
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Fig.7
26。
25。
24。
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嘱40。 14調。
(a)Outline of topography in the southem Izu-Ogasawara Arc from24。to27。N。
intervals.Unit ofthe figures is1000m.(b)Magnetic anomalies inthe same area
nT intervals.Positive anomalies are shaded.
canoes are characterized by extremely com-
plicated magnetic anomaly distribution(Fig.7
(b)).The anomalies have very high amplitude
(up to 3000nT) and short wave-1ength.
Their complexity is more than that of the
three volcanoes mentioned above。Thiswould reflects higher inhomogeneity of these
volcanoes,suggesting longer history of the
volcanic activity.The magnetic anomalycontour map of these two volcanoes,however,
has some roomfor furtherrevision because the
142。E
Contours are at500m
Contours are at100
(c)Survey lines of the magnetic anomalies。
spacings of survey lines are still too large
compared with the observed short wave-length of the anomalies(Fig.7(c)),and the
accuracy of positioning is low aromd Iwojima
Is.,sometimesworse than500m,because oftoo
short distance from a Loran-C station being
located on the island. In the vicinity of the
Iwojima Is.,the Hydrographic Department,
Maritime Safety Agency Japan has already conducted not only tota1-intensity ⑳easure・
ment by a ship but also three-component
一668一
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(b》
2700
2600
2500
2400
139 00
,欝
碁し、奪 曳 。
縷 GQ .、命
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6籔,馨磐
1 \
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) 醗\ o 》
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饗。織灘翻餐灘
糠’〉
膿拶/。。
∂な愚〆譲蕪欝購糞駿り
艶、繕藤叢
欝灘響、隷 塵懸、難.饗・
雛礁難140
00
14100
Fig.7 continued
measurement by an airplane(Oshimaαα1.,
1982;1985).They estimated using some mod-
eling technique that the center of the Iwojima
Volcano from surface to2km in depth is non・
magnetic because of high temperature.This
explains why dipole-type anomalies of rever・
sed direction (north side is positive and vice
versa)occur on the center of the volcano(Fig.
7(b)).
In the back-arc area west of the Shichito
Ridge,northwest trending belts of positive and
negative anomalies altemate like the magnetic
Iineations of mid-oceanic ridges(Fig.7(b)).
14200
The origin of these anomaly belts is not clear,
but they cannot be explained by the effect of
surface topography because the topography of
this area is rather smooth(Fig.7(a)).The
anomalies may reflect a relief of the basement
covered with thick sedimentary layers derived
from the arc volcanoes.Another possibility
is that the basement of this area may be
norしheastern extension of the oceanic crust in
the Parece Vela Basin having magnetic linea-
tions.This hypothesis implies that the pres-
ent Izu-Ogasawara Arc of this region has
grown on the oceanic crust.
一669一
翻
Bπll6あ多z(ゾ 云h60601092αzl Sz躍%ly6ゾノ4ゆα多z,Viol.42.No.12
(c)
2700
2600
2500
2400 139 00
1 i 『, li・H/ゴ 1π
1 \ 『。 レ7寧
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1
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/
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140 eo
Fig.7
The Ogasawara Trough(Figs.3(a)and7(a))
shows slight negative magnetic anomalies,and
no lineations can be recognized(Figs.3(b)and
7(b)).This is consistent with the considera-
tion that the basement of the OgasawaraTrough is rifted island-arc cmst(Abe6渉α1.,
1989). On the other hand, Isezaki 6渉 α1.
(1989) carried out three-component mea-
surements of the geomagnetic field,and found
lineations of north-south strike on the east and
vertical components in the OgasawaraTrough. But the origin of the lineations is not
clear.
141
00
continued
!42
00
4.4,0gasawara Plateau an、《I neighboring
seamounts The Ogasawara Plateau and three otherseamomts east of it(the Yabe Seamomt,the
Hanzawa Seamomt and the KatayamaSeamomt) which we studied (Fig.8(a))
occupy the westem end of the Marcus-Wake
seamount chain (the Michelson Ridge of
Smoot(1983)).The Ogasawara Plateau has
collided against the Izu-Ogasawara Arc at the
junction between the Izu-Ogasawara and
Mariana Trenches.
A belt of negative magnetic anomalies was
一670一
ル死zg%6海6 ご〃zo㎜zl♂6s 6ゾ 孟h6 1≧z6一ノ協z7乞ごz銘ごz/170s ピT. y4nzαzごzhづ 6渉ごzl.ノ
observed over the three seamounts east of the
Ogasawara Plateau(Fig.8(b))。The ampli-
tude of the anomalies of the Yabe Seamomt
was about-700nT,and that of the Hanzawa
Seamomt and the Katayama Seamount ex-
ceeded -1000nT. The fact that negative
magnetic anomahes occur above the bodies of
the seamomts indicate that their magnetiza-
tions are normal polarity an(1have shallow
inclinations,while too scarce survey lines in
the northern and southern vicinity of those
seamounts(Fig,8(c))prevent from calculating
their magnetization more quantitatively by an
inversion method.The ages of theseseamounts are estimated to be the Cretaceous
from the microfossil age of limestone covering
the top of the Yabe Seamomt(Shiba,1979).
It is thus considered that these seamounts were
emplaced in an equatorial region in theCretaceous,and have been moved to the pres-
ent position by the northward shift of the
Pacific plate since then (e.g. Clague and
Jarrard, 1973). In the vicinity of these
seamounts,Vacquier and Uyeda (1967)reported another seamount(No.Z-4-3at27。
N,148。407E)having the magnetization of
shallow inclination.
The Ogasawara Plateau has several peaks
on it(Fig.8(a)).The Kita Seamomt on the
Ogasawara Plateau has dominantly negativeanomalies(Fig.8(b)),which is similar to the
seamomts mentioned above.The HigashiSeamomt(the Broken-Top Guyot of Smoot
(1983)),the Nishi Seamomt and the Minami
Seamount have,on the other hand,a pair of
positive and negative anomalies (positive
anomalies are on the southem flanks of the
bodies,and vice versa)(Fig.8(b)). This type
of anomaly distribution suggests that the
magnetization directions of the seamounts are
close to that of the present geomagnetic field
there.This implies that these seamountswere magnetized at a place near the present
latitude.However,fossil evidence shows that
the Higashi Seamount was formed in the
Cretaceous in an equatorial region like the
Yabe Seamomt(Konishi,1985)。One pos一
sible explanation for this discrepancy is that
induced magnetization and/or viscousmagnetization of the present field direction
have overcome the primary remanent magnet-
ization of the seamomts which would have
been acquired in low latitudes。Another pos-
sibility is that these seamounts wereremagnetized by rejuvenated volcanic activ-
ities of relatively young age,that is,after the
directional change of the movement of the
Pacific plate from NNW to WNW,which is
known to have occurred at about43Ma(Clague and Jarrard,1973).Nemotoαα1.(1986)suggested the existence of Eocene vo1-
canic activities at the Higashi Seamomt.
The stripes of the positive and negative
anomalies over the Ogasawara Plateau seems
to continue landward across the trench
although their amplitudes decrease.Thisobservation suggests that the westem flank of
the Ogasawara Plateau has been already sub-
ducting undemeath the forearc wedge of the
Izu-Ogasawara Arc.
4.5N艦her醜M麟a舩Troug恥 The Mariana Trough is initiated at about
24。N,just south of Minami-Iwolima Is。(Fig.9
(a)).It develops from a rifting stage to a
spreading stage at about22。N(Yamazaki6孟
α1.,1988b;Murakamiαα1.,1989).The
trough widens southward from80to120km atthis boundary at22。N,and deepens more than
500m.Small ridges and grabens trending
NNW-SSE become conspicuous in the southof the boundary.
The characteristics of magnetic anomaly
distribution also vary significantly at this
boundary (Fig.9(b)). In the north of22。N,
dipole-type anomalies or short wave-1ength
anomalies are remarkable,which are of
seamount or intmsive origin.No magneticlineations are observed. The seafloor of this
regionwould consist ofstretched and sundered
island-arc crust. In the south of22。N,on the
other hand,NNW trending magnetic llnea-
tions can be recognized,which would be
caused by magnetization of oceanic crust.
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Fig.8 continued
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卜㌧
■ lI
Bz61」6あ麗(ゾ玩6G601040ごzl Sz67∂6:yのr/4ρα%,Vbム42〈砂.ヱ2
240
23。
麟噺.臨 ◎
q 駕
(a》
d
翁 ◎
姉響◎ ⑨
過 9
d
▽9
141。’『akasu Smt.
D考
22。
参
咳
噛
も
、 ぺ
Minaml-Hiyoshi Smt.
飼 Nikko Smt.㈱
の ロ
.懸
翁 網 ◎
。D 巾
。 鰭覧も
O
も
識142。
210
φ
◎
繋、1。。km孫
鴇
20。N
B
s
駕 %
oo
℃
黙
Fukujin Smt.
/,1
態
難
藁
傘q q
o 怒
4
◎
2 “
Kasuga Smt. 1
θ
Inaml-Kasuga SmI.
噌φ 2傷
塔気 偽
檎
3
、
靱
嘔
1430E 144。
Fig.9 (a)Outline of topography in the northem Mariana Trough from20。to240N.Contours are at500m
intervals.Unit ofthe figures is1000m.(b)Magnetic anomalies inthe same area.Contours are at100
nT intervals.Positive anomalies are shaded.(c)Survey lines of the magnetic anomalies.
一674一
難
舩zg多z6あ6 ごz%o万別zlJ6s (ゾ 孟h6 /2z6一ノ晩z7乞4%σノ170s ピT。 y~z盟z召2召左づ 6オ召乙ノ
2400
2300
2200
2100
2000 14! 00
遷響驚〆.,』1ξ藩㌧つ
鱗載、驚郵)
(b)
〃『olil o
:ご7
・響繋、 響・
・爵簿裁・9
寒繁、蓼塙\画奪識,
誉縷抄膓
9.灘簿1”
恥◎ 鵯誤
辛,黙
(灘 纂、
1 、142
00143
00144
00145
00
Fig.9 continued
Correlation is obvious on the magnetic anom-
aly profiles along the survey lines(Fig,10(a)),
particularly between21。and22。N.
4.5.1耽%痂脇加げ窺α9襯」61吻読oηs To assign ages to the magnetic lineations,
we performed forward modeling by a two-
dimensional block modeL The model shown
in Fig.11fits well the anomaly profiles
between21。and22。N.We adopted theskewness parameter of-58。,which is derived
on the assumption that two-dimensional
blocks having N20。W strike were magnetized
at the present latitude.We thus considered
that the formation of oceanic cmst between22。
and21。N would have started at3Ma or a little
earlier with a half-spreading rate of about2.5
一675一
Bz61」6痂z6ゾ云h6 G60Jo4侃l Sz67z76ッ(ゾ忽)召%,Vbl.42,〈石o.12
2400
2300
220G
2100
2000 141 142 00 00
Fig.9
cm/year.隅The rate would have decreased to
about l cm/year at present.The present
spreading rate at the northern end of the
spreading axis close to22。N is estimated to be
a little faster,about2cm/year。The topog・
raphy of the spreading axis supPorts this
interpretation. The rift valley on the spread-
ing axis becomes less retnarkable northward
on the topographic profiles shown in Fig.10(b),
N 《c》
一 、 黛
\、
1
N
、
、
、
峯、t
\
凸
!43 !44 !45
00 00 00
continued
which suggests that spreading rate becomes
higher northward(Macdonald,1986)。
The correlation between the model and the
obser▽ation on the eastern flank of the
spreading center is not good because the
magnetic lineations are disturbed by the
anomalies accompanied by seamounts(Figs.9
(b) and 10(a)).They are the Fukujin
Seamount and a chain of submarine volcanoes
一676一
li
I
I嵩
馨.
きら
き 『
で き
ノレ勉9%6〃o 召%o㎜」∫(器 6ゾ●云h612z6一ノ晩z甥召多祝z/170s ピT. y4郷ごz2召たガ 6渉ごz乙ノ)
22。N
21。
20。
142。
駐
ノ.
Q
G 剛
課(a)
2GO“て
縞。。
..◎
冊 蒙 型縦織グ
ノ
擁 副
1 }添
フ
訳嗣
◎
象
騰
143。 144。 145。E
220N
21。
200 142。
Fig.10
麟
憾麟
143。
翻
144。
講
縛
(b》
f5..,①
145。E
(a)MagneticanomalyprofilesalongthesurveylinesofENE-SWSdirectionintheMarianaTroughfrom20。to22。N.Solids are positive anomalies and opens are negative.Interpretation is superimposed:
shaded belts represent the magnetization ofthe normal polarity.B:the Brunhes Epoch(0-0.73Mal
Harland6麺1.(1982)),0:the Olduvai Event(1.67-1.87Ma),G:the Gauss Epoch(2.48-3.40Ma).(b)
Topographic profiles along the same lines.Dots indicate the location of spreading centers estimated
from the magnetic lineations.Line A is the location ofthe seismic reflection profile shown in Fig、13.
一677一
羅
B%ll6あnρゾ 渉h6 G60104αzl Sκ7∂6ッのF/41)ごz%,Viol.42,No.ヱ2
Fig.11
Moqle’ oSkewness-58
400nT l。
4 2 0 2MaPolarity
10km
一
2、5cm/year i 1.5 11・011・01 1・5 [2・5 Rate
〔Thickness3km,lntensity5A/m)
Magnetic anomalies produced by a two-dimensional block mode1.Spreading rates were adjusted so
that the theoretical anomaly profile fits the observed ones around21。30’N in the Mariana Trough。
Solid blocks represent the normal polarity and open ones are reversed.The magnetic polarity time
scale of Harland6オα1.(1982)was used,
1ineated southwestward from the KasugaSeamomt(Cross-chain seamountsl Hussongand Fryer,1983)(Fig.9(a)),which were em-
placed after the formation of oceanic crust。
4.5.20∂67妙卿gs卿α伽go6漉zs認21。ノV
Our interpretation of magnetic anomaly
profiles leads to the conclusion that two
spreading centers overlap from20。3αto21。N
(Fig.10(a)).The characteristic anomaly pat-
tem which has been recognized on the profiles
north of22。N continues southward to about20。
30’N,althoughthe amplitudes ofthe anomalies
become sma11,an order of about lOO nT.To
the east of it,on the()ther hand,another iden-
tical anomaly pattem mns parallels to the
former from20。3奪to21。N。The latter con-
tinues southward to2αN,where axi謡valleys
oftheMid-Atlantic Ridge(MAR)type canbeseen on topographic profiles (Fig. 10(b〉).
Between the two spreading axes,ananomalously deep graben occurs(Figs.9(a)
and10(b)).The depth reaches5300m at max-
imum.This situation,that is,two spreading
axes and an intervening graben,is similar to
the Overlapping Spreading Centers (OSCs)
which has been observed commonly on theEast Pacific Rise(EPR)as a form of non-
transform offsets of mid-oceanic ridges(Macdonald and Fox,1983;Lonsdale,1983).
The graben at21。N would correspond to the
Overlap Basin,which is a basic constituent of
OSCs(Macdonald6齢1.,1988).The depth of
the spreading centers in the Mariana Trough
increases toward the OSCs at21。N(Fig.12).
This feature issimilar to the OSCs on the EPR,
and explained by that the amomt of magma
supply is small at OSCs(Macdonaldαα1.,
1988)。
We estimate from seismic reflection profiles
that the spreading at、the westem axis would
have been subsiding now,and in the east of it,
a new spreading system is instead under
developing with destroying older oceanic
cmsts.AseismicreflectionprofileshowninFig.13shows that the westem spreading axis
accompanies a typical rift valley.The east-
em axis is,on the other hand,more compli-
cated in geological stmcture.Many faulted
blocks exist there,but no clear rift valley.
The OSCs fomd at21。N in the Mariana
Trough is anomalous in the following points
compared with already documented OSCs.
First,the size is large. The OSCs at21。N
offset about30km and overlap each other by
abo興t50km.It is known,however,the lat-
eral offsets of the OSCs on the EPR are less
than15km(Macdonald and Fox,1983,Mac-donald 6!召1.,1988).Second,OSCs were
commonly observed on fast-spreading ridges
such as the EPR,but never on slow-spreading
ridges like the MAR(Macdonald and Fox,
1983).The spreading rate of the Mariana
Trough is nearly as small as that of the MAR.
A spreading center-transform fault pattemprevails in the case that an offset of ridges is
large or a spreading rate is smal1.This is
because spreading axes are in contact with
一678一
欝婁
嚢’
/i
離嚢蒙馨
鐘
萎蒙
馨
藝
馨
嚢馨.三
嚢嚢事
li
熱
//、1
i
/嚢甕、自
難
ヤ
蒙.
鑓
態欝
麟
毒襲
饗
鑑羅
霧
物9耀づoαηo窺α1づ召soμh6伽一伽吻瓢・470s‘T.}物郷α~襯6砲1。ノ
∈
ど一35静-4
0 50km一一___一一一一一 2ゴN 220
Fig.12Axial depth profile along spreading centers in the northem Mariana Trough.
823の∈ 3
5詰4
>邸 5}1◎
シ 6一
1 OverlaP等
…
二,馨既
F㌃撮
市
礼“仁
聖■『i
’
’噂鰭
、
巳r
一陀
護’一豫
芒毒 葦
轟1
綴機轟虐燕、,彰
・毒,,1
2藤、
珪8
『「暇げ阿「『「『配「四「「
1『
恐》卜
占←
『臼 }
響卿
3魯ト
饗、謬
ヂ仁濫■,
1
競 聯細
ミ .
轟」 陣
,、 …懸灘
・耀
OverlapPing Spreading Centers
轡
Fig.13 Single-channel seismic profile across the overlapping spreading centers in the Mariana Trough.
Positions of the spreading centers indicated by arrows were estimated from magnetic lineations.
Location of this profile is shown in Fig.10。
cooler and thicker crusts than in the case of a
small offset or a fast spreading rate,and thus
the cmsts would not be weak enough to allow
non-rigid deformation like the OSCs。It is
hence suggested that the cmst ofthe Mariana
Trough spreading center may be weaker(thinner and/or hotter)than those of major-
ocean spreading centers. The third point is
that the magnetic anomalies of the Mariana
Trough at21。N are smaller than those of
neighboring areas while the OSCs in the EPR
have stronger magnetic anomalies(Lonsdale,
19831Sempereαα1.,1984).It is considered
that high magnetization intensity which is
carried by highly fractionated basalts enriched
in iron is responsible for the strong magnetic
anomalies of the OSCs on the EPR(Sempere
6砲1.,198411988).The weak anomalies at21。
N in the Mariana Trough suggests that the
iron enrichment may not have occurred there.
4.5.3惚7加α.1~惣6魏4脆s渉〃47加αノ~惣6
Next we discuss the magnetic anomalies
accompanied by a row of Quatemary arc
volcanoes on the Mariana Ridge.Some of
them have simple dipole-type anomalies,but
othershave complicated anomaly distributions
which do not correspond well to the topogra-
phy of the seamounts(Fig.9(b))。The former
examples are the Fukujin Seamount and the
Minami-Kasuga Seamount,and the latter the
Nikko Seamomt and the Minami-HiyoshiSeamomt。This difference would reflect dif-
ference in the ages of these seamomts.The
latter would have had longer volcanic activity
than the former,resulting in highly in-
homogeneous magnetization。Magnetizationof normal and reversed polarities may be
mixed.Topography of these seamomt sup-ports this inferencelthe former has a simple
conical shape,but the latter shows complex
topography having two or more peaks.On
the Fukujin Seamount,the spread of theanomalies is narrow compared with the size of
the seamomt body.It is thus suggested that
the seamomt would be covered with thick
volcaniclastic debris,and highly magnetized
part is relatively sma11in volume.
一679一
麟
Bz61」6あ多z oゾ孟h6 G60Jo9ガαzl Sz67z尼y6ゾノ4ゆごz%,Vわ1.42,.〈々).12
The West Mariana Ridge,the remnant arc,
does not have strong magnetic anomalies in
general(Fig.9(a)).The Takasu Seamount,in
particular,accompanies little anomaliesalthough it has a flat top of very shallow
depth,about50m.This suggests that theTakasu Seamount would not be a volcano.
An mnamed small peak at about21。20’N
showed the anomaly pattem which indicates
reversed magnetization.
4。6 S騨e翻直盟g ax量s漉18。N亘聡tぬe M鍛董&舩
Tr側9璽
The Mariana T罫ough is widest at about18。
N,and the occurrence of active spreading
ridges is well documented (e.g.Fryer and
Hussong,1981〉. Thus the existence of the
magnetic anomalies of seafloor-spreading
originisexpected.Whilemagneticlineations
are vague on an anomaly contour map(Fig.14
(b))due to the low amplitude of the anomalies
(200nT or less),anomaly profiles along main
survey lines can be correlated each other(Fig.
15).Spreading rate estimated from compar、
ingthe profileswiththe model in Fig.11varies
somewhat from place to place,but it is an
order of2cm/year.This agrees with the
previous estimationsby Bibee6厩1.(1980)and
Hussong and Uyeda (1982).Discontinuities
in magnetic lineations can be recognized at17。
40’N,18。N and 18。20’N (Fig.15). They
roughly coincide with topographic and struc-
tural discontinuities,that is,the Pagan frac-
ture zone between17。30’and17。40’N,the17.9。
N offset and the18.5。N offset of Volpe6!σ1.
(1990),respectively.The trend of the mag-
netic lineation between18。and18。20’N is close
to north-south(Fig.15)although the strike of
眉8030’閥
180009
調7030’ 唱44000, 喰44030, 眉45000,匿
Fig.14(a)Bathymetric contour map aromd a spreading axis(shaded)at18。N in the Mariana Trough.
Contours are at200m intervals.(b)Magnetic anomalies in the same area.Contours are at lOO nT
intervals.Positive anomalies are shaded.(c)Survey lines of the magnetic anomalies.
《繍》
噂魁
o℃嬬
嘉
璽 鱗 ド 簸
飯0
0 賎、 、
σ Q
q
勲 簗徽舞灘.
羅騨
越爲b 掘
望0
匝島 ◎
暗
o
d馨醗講
~廼黙
Qり
漁
’塾
も
d
、
“
、
陵
、o“
撚 ¥、
噂
q
φ㎎
も
o の り .
o
鷺 」 x
職、 陰
ノ?
ん聯
一680一
撰 羅傘
ル五zg%6ガo ごzno㎜lJ6s oゾ’孟h6乃%一ノ協z7麦znごz/170s ピT. Kz〃安z2召hづ 6!召よノ
1830
1800
1730 144
00
賊㈲ ら 0
茜 (
o
,「Q
11計 1壽
1830
1800
1730 144
00
(c》
一
鮮_
襲r暦 一 一ヤ1
輝.
き
コ
1器 1壽
Fig.14 continued
Fig。15
ユ8 30
ユ8 00
i7 30
i44 i44 ユ45 ユ45 00 30 00 30
Magnetic anomaly profiles along main survey lines around a spreading axis at l8。N in the Mariana
Trough.Solids are positive anomalies and opens are negative.
締.芒1「
簸灘「
1200nT
{
the rift valley is NNW(Fig.14(a)).This
suggests that a small change in spreading
direction and accompanying reorganization of
normal faults would have occurred recently,
say an order of100k,y.before.
5. Co館c且腿s亘o醜s
We conducted magnetic survey on the Izu一
Ogasawara Arc and the northem MarianaTrough along dense ship tracks. Geological
interpretation of the resulting magnetic
anomaly distribution are summarized as fol-
10ws.
(1) Volcanoes on the Shichito Ridge,the
volcanic front of the IzurOgasawara Arc,have
usually strong magnetization probably owned
by basaltic rocks.Some volcanoes(e.g.the
一681一
βz61」6ガ%6ゾ孟h6 G60Jo4αzl Sz67∂61y6ゾノ4望)ごz%,Vわ」。42.〈わ.ヱ2
Nishinoshima Volcano)has rather simpledipole-type anomalies,but others have very
complicated anomaly distribution (e.g。the
Iwolima Volcano),which may reflect the dif-
ference in the length of the volcanic activity.
(2) On the northern part of the Shichito
Ridge,some volcanoes accompanies onlyweak magnetic anomalies(e.g.the Myojin
Knol1),which suggeststheir compositionbeing
acidic rocks such as dacite. This reflects the
bimodal volcanism in this region.
(3) Back-arc rifts in the northem Izu-
Ogasawara Arc show no remarkable magnetic
anomaliesexcept for cross-chain seamounts in
the Sumisu Rift.Seafloor spreading has not
started yet.
(4)The Shichiyo Seamomt Chain andneighboring seamounts in the middle Izu-
Ogasawara Arc can be divided into three
groups based on their magnetic anomalies.
The first group is Quaternary volcanoes,
which have strong dipole-type anomalies of
the normal polarity.The second is faulted
blocks of old island-arc crusts,accompanying
little anomalies.The third is old volcanoes of
possibly Oligocene age,which have themagnetization of eastward declination andshallow inclination.
(5〉Fore-arc basement highs of Paleogene
or Miocene age(the Shinkurose Ridge)are
characterized by broad positive magnetic
anomalies.Similar anomalies are observed
on the Tenpo Seamount,an old basement high
lying in the back-arc region west of the
Sofugan Tectonic Line.
(6〉 In the back-arc region north of 30。N,
topographic highs striking obliquely to the arc
(the Nishi-Shichito Ridge) have relatively
simple magnetic anomalies of normal polarity.
Topographically smooth area around28。N has
relatively large anomalies caused by deeper
source.In the back-arc between24。to27。N,
alternating belts of positive and negative
anomalies run in NNE-SSW direction.Thismay renect a relief ofthe basement,or suggest
the possible existence of oceanic crust under-
neath thick sedimentary cover兜
(7)Four peaks on the Ogasawara Plateau
and three other seamomts in the east of it can
be divided into two groups from magnetic
anomalies.The first group having dominant-
1y negative anomalies was magnetized in an
equatorial region probably in the Cretaceous.
The second group shows the magnetizationclose to the present geomagnetic field direc-
tion, suggesting induced and/or viscous
magnetization or remagnetization by re-
juvenated volcanism.
(8)We show for the first time that mag-
netic anomaly profiles inthe Mariana Trough
southof22。Ncanbecorrelatedclearly.Aforward modeling indicates that the seafloor
spreading aromd21。307N started at3Ma or a
little earlier at a half spreading rate of2.5cm/
year.The rate has decreased to about l cm/
year.
(9〉Spreading axes overlap about50km in
theMarianaTroughbetween20。30’and21。N.Previously the existence of OSCs was not
known on slowly spreading ridges.Further-
more,the offset of the two axes,about30km,
is much larger than that of the already
documented OSCs on the East Pacific Rise.
The cmst at theOSCs inthe Mariana Troughmay be hotter and/or thinner than that of the
majorocean. (10)Some seamounts on the northem Mar-
iana Ridge have simple dipole-type anomalies
(e.g.the Fukujin Seamount),but others show
complicated anomaly pattems which poorlycorrespond to their topography (e.9.the Nik-
ko Seamomt).This would reflect difference
in the length of the・volcanic activity.The
northem part of theWest Mariana Ridge,the
remnant arc,accompanieS no strong anom-alies in genera1(e.g.the Takasu Seamount).
(11) The spreading rat6at頓e ridges at18。N
in the Mariana Trough is about2.cm/year at
present.A small change in spreading direc-
tion from E-W to ENE-WSW would haveoccurred recently.
Acknow且e岨gements:We wish to thank M.
Yuasa and A.Nishimura for valuable discus一
一682一
き
きド
/灘該
熱蒙
嚢嚢l
l i
灘
//
藝
1.
ル勉g麗批伽o〃昭IJ6s6ゾ孟h6乃%一〃47毎%。4κsピT.y4吻召紹hガαα1.ノ
sion throughout this work.We also thank E.
Saito and K.Watanabe for providing topo-
graphic maps used in this report,E,Kikawa,
Y.Okamura and M.Satoh for helping the
measurement,and Y.Okubo for helpful com-
ments on the manuscript.The magnetic sur-
vey was performed with cooperation of all on
-board scientists,officers and crew of the Izu
-Ogasawara-Mariana cmises of the R/V Ha-
kurei-maru(chief scientists,M。Yuasa,A.
Usui and M.Nohara).
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(1989) Report on DELP1987cruises in
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伊豆・小笠原弧,マリアナ弧及びマリアナトラフの磁気異常
山崎俊嗣・石原丈実・村上文敏
要 旨
伊豆・小笠原弧,マリアナ弧,マリアナトラフ及び小笠原海台とその付近の海山について,磁気異常
の測定を行った.測線間隔は一般に2-4マイルと密である.
七島海嶺及びマリアナ海嶺(伊豆・小笠原弧,マリアナ弧の火山フロント)上の海底火山の大部分は強
い磁気異常を示す.このうちいくつかの火山は比較的単純なダイポール型の異常を示すが(例えば,西之
島火山や福神海山),他方では,非常に複雑な異常分布を示すもの(例,硫黄島火山)や,火山体の地形と
異常との対応の悪いもの(例,日光海山,鳥島火山)が見られる.このような磁気的な均質性の差は,火
山活動の長さの差を反映していると推定される.さらに,七島海嶺北部の火山に見られる不均質な磁化
は,バイモーダルな火山活動が影響していると考えられる.
磁気異常は海山の起源を推定するのに役立つ.伊豆・小笠原弧中部の嬬婦岩と西之島の間に分布する
海山(七曜海山列,大町海山,沢海山,天保海山など)は,磁気異常から次の4つのグループに分類でき
る.第一は,強いダイポール型の異常を持つ第四紀火山,第二は,ほとんど磁気異常を伴わない,断層
で切られた島弧地殼の断片,第三は,東向きの偏角と浅い伏角の帯磁を持つ,漸新世の年代と推定され
る古い火山である.第四のタイプは大町海山と天保海山で見られる,やや長波長の正異常であり,同様
の異常が,伊豆小笠原弧北部の前弧に発達する,古第三紀あるいは中新世の年代と考えられる基盤の高
まり(いわゆる新黒瀬海嶺)に対応して分布することから,これらの海山は古い基盤の高まりと推定され
る.
マリアナトラフの北緯20度から22度にかけて,海洋底拡大に伴う明瞭な磁気縞模様が存在することを
初めて明らかにした.北緯21度30分付近では,海洋底拡大は3Maあるいはそれより少し早い時期に,片
側拡大速度約2.5cm/年で開始した.現在では拡大速度は1cm/年に減少している.北緯20度30分から21
度にかけて,拡大軸が約30kmの間隔をもって重複している.このような大規模な重複拡大軸の存在は拡
大速度の遅い海底拡大系では異例であり,ここの地殼の強度が通常の海底拡大系のそれより小さい,す
なわち厚さが薄い,または高温であることを示唆している.
白亜紀に形成されたと考えられている小笠原海台上の4つのピーク及びその東の3つの海山依,磁気
異常より2つに区分できる.第一のグループは山体上に負の異常を伴い,白亜紀に赤道域で磁化したと
推定される.第二のグループは現在の地球磁場の方向に近い帯磁を持ち,誘導磁化あるいは粘性磁化が
卓越している,又は比較的新しい時期に再び起こった火山活動により再磁化したと推定される.
(受付:1991年6月18日;受理:1991年8月14日)
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