購

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，andtheOgasawaraPlateauanditsneighboringseamounts along 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．

35。

1． 亙鵬ro岨胆cti樋 〆’．F讐〉 Pacific Ocean Weconductedmagneticsurvey overtheIzu－ H Ogasawara（Bonin）Arc and the Mariana 幅、 F’伊2、 ・’勾 齢① Trough from1984to1989as a part of the ∫¢ ゆ research program “Submarine Hydrothermal 驚三 ∫ 杢”：罵 o Activity in the Izu－Ogasawara Arc” to G o understand better the geological structure in セ S h i k o k u 還 蟻 6 this region．Survey areas include the whole 。 ．9 30 Ba s i n ロしじロニ 6 1？F’9・3轟 0 3 1zu－Ogasawara Arc from Hachijojima Is．to 一・臼・；ミ｝ 0 1121・甲 0 Minami－lwojima Is．，the Ogasawara Trough， ～7・ 0 ‘・｛ 0 05 も the Ogasawara Plateau and neighboring 0 急 ← 喜、一 seamomts，the Mariana Ridge and the Mar－ iana Trough north of203N，and along the 、”嘉N辱鷺∫ spreading center of the Mariana Trough 僻1峯ぎ 懸 around18。N．The spacings of the survey Iines 霧・・甲 ．瓢 were2to4nautical miles（3．7to7．4km〉in §1 叢 general，and they were as close as l mile in 25。 09白sawara PIateau several detailed sur▽ey areas． Such high density of ship tracks made us possible to F79．7 draw magnetic anomaly contour maps of high Parece Veia Basin accuracy． The purpose of this report is to present

magnetic anomaly distributions of above Backarc rifts mentioned areas，and discuss the geological －Spreading center stmcture deduced from the magnetic anom－ ▲ Quaternary vQlcanoes alies． 20 嘱40。 145。

Fig．1 Stmctural framework of the Izu－Ogasawara－ 2．B蹴kgro腿認 Mariana Arc．Dotted lines represent1000m and 3000m iso－depths．S．T．L．：Sofugan Tectonic Line（modified from Yuasa（1985））．H：Hachi－ 2．1 亘zu－Og総aw躍a Arc jojima Island，N：Nishinoshima Island． The Izu－Ogasawara（Bonin）Arc lies on the eastem rim of the Philippine Sea plate sub－ are known to be compositionally bimodal ducted by the Pacific plate （Fig．1）． It is （Tsuya，1937；Kuno，1962）．Occurrence of about1200km long extending from the Izu acidic rocks are abundant in the northern part Peninsula to Minami－lwojima Is．It con－ of the Shichito Ridge，but little in the southern tinues southward to the Mariana Arc。 part（Yuasa，in prep．）． The Shichito Ridge is occupied by a line of A row of small grabens occurs just behind Quatemary arc volcanoes，which forms a the volcanic front in the northern part of the recent volcanic front of the Izu－Ogasawara Izu－Ogasawara Arc，from Hachijojima Is．to Arc．Volcanic rocks from the Shichito Ridge 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 other topographic lows Iying between the grabens． They have been considered to be Shichito Ridge and the Nishi－Shichito Ridge young back－arc rifts，or incipient back－arc （Izu Ridge），but in this report does not． basins（Karig and Moore，1975；Honza and The Ogasawara Ridge is a forearc basement Tamaki，1985）．Recent ODP（Ocean Drilling high of Eocene age（Tsunakawa，1983）．The Program）drillings in the Sumisu Rift，one of Ogasawara Trough lies between the these back－arc rifts，proved the young age of Ogasawara Ridge and the Shichito Ridge．A the rift．The age of the sediments in the multi－chamel seismic profile revealed that the Sumisu Rift is not older than1．1Ma（Leg126 Ogasawara Trough was formed by rifting Scientific Drilling Party， 1989）． Signs of （Abe 6渉 α1．，1989）．The rifting of the hydrothermal activity were reported from the Ogasawara Trough may be simultaneous with Sumisu Rift（Taylor6渉σ1．，19901Urabe and the rifting of the Nishinoshima Trough in the Kusakabe，1990）and the Aogashima Rift Oligocene（Yamazaki6臨1．，1988a）．The two （Yamazaki6厩1．，1991）．In the southem part troughs may have been a part of one paleo－ of the Izu－Ogasawara Arc between Nishino－ rift，and later growth of the present Shichito shimaIs．and Minami－lwojima Is．，onthe other Ridge may have separated them． hand，no backarc rifts are recognized． In the Izu－Ogasawara Arc，magnetic sur－ In the back－arc of the Izu－Ogasawara Arc， veys have been conducted by several institu－ geological structures being oblique to the tions for two decades but rather sporadically． trend of the arc are remarkable（Karig and Here we present a magnetic anomaly map of Moore，1975；Honza and Tamaki，1985）．In the whole area for the first time，which is the north of about30。N，several rows of based on systematic survey with dense ship’s basement highs，on whi¢h relatively large tracks． Previously the Geological Survey of seamomts are located in line，run in NE－SW Japan conducted a recomaissance magnetic li or ENE－WSW direction（so－called Nishi－ survey along lines at intervals of15miles（28 匠1 ル ま ミ Shichito Ridge or Izu Ridge）．Examples are km）in1979，and the results were presented as き i1 the Enpo and Genroku Seamount Chains in magnetic anomaly profiles（Miyazaki6渉召1．， 難 Fig．2（a）． In the south of30。N，on the other 1981〉．The Hydrographic’Department，Mar－ ド 藍 ずヨド hand，troughs of NNE－SSW strike are con－ itime Safety Agency Japan has published a ぞ spicuous．The most prominent among them is series of bathymetric maps of1／200，000scale the Sofugan Tectonic Line（Fig．1），a steep over the northem part of the Izu－Ogasawara cliff of probably normal fault origin mming Arc，a part of which includes a magnetic total－ linearly south－southwestward from the south force map of the same scale． of Sofugan Island． Yuasa （1985）considered 欝 that the Izu－Ogasawara Arc can be structur－ 2。2遡翻躍aTro髄9㎞ 灘 ally divided into the southern and northern The Mariana Trough is known as one of part by this fault．The Nishinoshima Trough active back－arc basins in the world（Karig， ／ lies on the east of the Sofugan Tectonic Line． 1971）．Recently active hydrothermal vents It is bordered on the east by the Shichito were discovered by submersible ALVIN at18。 ／ Ridge． Yuasa （1991a） proposed that the N（Craig6緬1．，19871MooreandStakes，1990）．

Nishinoshima Trough was formed as a north－ Based on the results of DSDP （Deep Sea 聚 繋 ern tip of the incipient opening of the Parece Drilling Project）drillings，Hussong and Uyeda 灘 （1982）considered that the rifting of the Mar・ Vela Basin in Oligocene time（Mrozowski and li Hayes，1979）．The Sofugan Tectonic Line iana Trough at18。N would have begun in the would consist a westem wall of the paleo－rift． latest Miocene，about6Ma，with a half The “Nishinoshima Trough” of Honza and spreading rate of2．15cm／year．Bibeeασ1． Tamaki（1985）and Yuasa（1985）includes （198g）correlated several magnetic anomaly

一657一 βz61」6あ％oゾ云h6 G601091641S％7％ly6ゾノ4ゆα％，『Vlol．42，一No，12

N ゲ 33。N （a》 」 C） 7 5 ♂識。勧 Q

O oO悔麟乳 ♂ O b 〈）o d Φ ．凝 ミ ⑥ 咲 ［ ） 黙o o AQgashimaRift も 〃 ’ I／ £ 〆 ◎M ojinKnoII 320 ミ o 巴、 σ 一rの ①o℃周噛曙S ミ MyolinshoCaldera 診塵 ． r∠、G i熊葱．， ロ、 ］ ◎睾 ぺ umisuCaldera 2 D 0 ・呼づ ・餅〆 〔つDaisan一SumisuKnoIl “ へ む の ノO 丘 伊po G ＃ 31。 ミ o 0 いa、“ q σ づ ■隔 ◎ G 亀 ・・（7評笹 0 2 のも戴熱励 ◎〆稽 お ∈ 〜 ＠ 0 9 0 ◎ ． の ＠◎♂00 0 ⑦ ／ 獄 30。 139。 140。 141。E

Fig．2 （a）Outline of topography in the northem Izu－Ogasawara Arc from30。to330N．Contours are at500m intervals．Unit ofthefiguresis1000m．（b）Magnetic anomaliesinthesame area．Contoursαre at100 nT intervals．Positive anomalies are shaded．（c）Survey lines of the magnetic anomalies． profiles between17。307to18。N and estimated 1ines are closely spaced（Karig，1971；Ander－ a half spreading rate of approximately l。65 son，19751Karig6！α1．，19781Fryer and Hus－ cm／year within50km of the spreading axis。 song，1982〉．One reason is that amplitudes of The mamer of spreading in the Mariana the magnetic anomalies are generally very low Trough，however，has not been understood because the trough is close to the magnetic we1L Survey lines of magnetic anomalies are equator and the strike of the spreading axis is still scarce except for the area near the close to north－south，Another reason would spreading axis between17。30’to18。N．It is be that the spreading mamer of the Mariana very difficult to correlate magnetic anomaly Trough is not simple and many discontinuities profiles in the Mariana Trough mless these 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 （b）

3200 ○ 毒． 灘難 繊 o 鱗縷 ．ぺ灘 蔓灘 鱗蒙 爵醸 昏驚講．熱 黙 ’8、穐蝋

㊥ くン リ （〉。Q⊃ ○ Q 榛綴 3100 醸 翻 oΩ ノ 麟雛 講歎α撫黛 羅難i ． 癬o＜ 1薦熱

⊃ 3000 138 139 140 141 00 00 00 00

Fig．2 continued

1977） and overlapPing spreading centers proton precession magnetometer（GeoMetrics （Lonsdale，19831Macdonald and Fox，1983） G801）towed about250m behind the R／V may exist． Hakurei－maru．Data were logged on a mag－ We have identified for the first time mag－ netic tape every lO seconds，and finally edited netic lineations in the northem Mariana as a series of records on every minute，which Trough between20。to22。N based on dense contain ship’s position，speed and geophysical magnetic profiles，and discuss the mamer of data（magnetics，gravity and depth〉．Resid－ seafloor spreading there． ual total magnetic intensity field，that is， magnetic anomaly，was obtained by subtract－ 3．Bataae麟s宜t亘樋腿姐re幽腿etion ing the1985version ofthe IGRF（lnternational Geomagnetic Reference Field） （IAGA Magnetic total force was measured using a Division 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 《c》

、 1

一 欝

、 ， 革

‘』 一・ ～

難． 嗣 ～鴇・ 3200 一穏1

膠【■旧 繋￥ 1、 1 ハ

一

’i羅li難 、

9

自｝ il 碍ノ ” 鷺寄 》 、 刈

』 1 ㌧ 、欝・ ＼

一＿一一撒 3100 ＼ f簾r 1 鰹膣 ■欄 ■ 一虚 懇璽… 穣

、I

［

一 、｝一噂コ 一 一 一 畿鰹

【． 二二＝【二 ヨ王＝’一 3000 3 138 139 140 141 00 00 00 00 Fig．2 continued

observed data．Diumal correction was not spiky noise，the data were interpolated onto a applied．Shipシs positions were determined by gridρf l km spacing．The gridding was done a satellite－Loran C integrated navigation sys－ by a weighted averaging method using a tem（Magnavox model200）．This system commercially available program package generally has an accuracy of about O．1nautical UNIRAS．The data collected during the mile（180m）．After1988，the GPS （Global previous cmises of the Geological Survey of Positioning System）was also used for naviga－ Japan in1979（Miyazakiαα1．，1981）were tion． also included． After editing out anomalous data such as

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

Back－arc rifts，which lie just behind the 4． 亙薦e聯reta色亘o聡 volcanic front，show no remarkable magnetic anomalies except for a chain of volcanoes 4．1N⑪r騒ern亘z眼一〇9薦題w麗題Arc： crossing obliquely the Sumisu Rift（Fig．2〉． 臨e恥ijoJi勲亘s．toT罐s臆ma亘s． The basalts taken from the intra－rift vo1－ Quaternary volcanoes stand in line along the canoes in the Sumisu Rift have chemical com－ Shichito Ridge，the volcanic front of the Izu－ position of back－arc basin basalts （lkeda and Ogasawara Arc （Fig．2（a））．They are the Yuasa，19891Fryer切α1．，1990）．However， Aogashima，Myojinsho，Sumisu and Tori－ the fact that no strong magnetic signals like shima Volcanoes，and the Daisan－Sumisu and the magnetic lineations of mid－oceanic ridges Myojin Knolls．All but the Myojin Knoll were observed suggests that these basalts show strong and short wave－1ength magnetic would be volumetrically small．It is thus anomalies （Fig．2（b）〉．The complicated considered that seafloor spreading has not anomaly pattems of these volcanoes suggest started yet and the basement of the back－arc that strongly and weakly magnetized parts are rifts would consists dominantly of island－arc mixed，which may reflect the bimodal volcan－ crust． ism of this area．The islands emerged from The Nishi－Shichito Ridge lies to the west of the sea are known to be composed of basalts the back－arc rifts，which consists of topo－ （Kmo，1962〉，but dacites are dredged from graphic highs trending obliquely to the Izu－ submarine calderas（Yuasa，in prep．〉． Ogasawara Arc．The ridges （the Enpo A typical example is the magnetic anomaly Seamomt Chain and the Genroku Seamomt distribution over the Torishima volcano． Chain） show relatively simple magnetic Strong dipole－type anomalies were observed anomaly distribution＝negative anomalies oc－ in the close vicinity ofTorishima Island．The cur north of topographic highs，and positive area of shallow water depth extending north south （Fig．2）．This pattem indicates the of the island，however，shows little anomalies magnetization of normal polarity．The ages （Fig．2（b）〉．The island is composed of basalts oftheridgesarenotclear．AK－Arageof2． （Kuno， 1962），which would have strong 2±1．1Ma was reported from the Enpo remanent magnetization．The shallow area Seamomt（Yuasa，1985）．On the other hand， north of it would be comprised by weakly historical volcanic activity around the Genro－ magnetized rocks such as dacites．This imp－ ku Seamomt was documented（Kmo，1962）． 1ies bimodal volcanism of the Torishima vo1－ The normal polarity of the magnetization cano．This suggestion is consistent with might indicate yomg age of the ridges Okamura and othersン（1991）consideration （younger than the Bmnhes－Matuyama that thick volcaniclastic sediments in the bomdaryatO．73Ma），butthereisapossibility Sumisu Rift，which are composed mainly of that induced magnetization or viscous dacitic pumice（Nishimura6厩1．，1991），were magnetization of the present geomagnetic derived from the Torishima volcano or its field direction have overcome the primary vicinity． remanent magnetization～Magnetic proP・ The Myojin knoll accompanies little mag－ erties of the rocks which comprise these ridges netic anomalies although the volume of the have not yet been known． volcano is not much smaller than that of oth－ ers．It is suggested that the Myojin：Knoll is 4。2 M童翻且e亘z眼一〇gas＆w麗我A麗：Sof腿9樋 composed of weakly magnetized rocks such as 亘s．馳Nis踊齢曲量m箆亙s． dacites．This inference has recently been The Sofugan and Nishinoshima volcanoes， confirmed by the observation using the sub－ the crests of which are emerged from the sea， mersible Shinkai2000（Yuasa，1991b）． 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

沙 （a》

q例 ／ ！ ／ 』 心 し／ k － 30◎N の ＼ 1 ／ 。 〃 1 ／ ノ〆～ノ ノ、／－、 、、 ／ ／一フ ＼＿＿一一r l （ ＼ ＼ 魁 o 、／ 、 ）「 ！ ＼ 、＾、’、一、 ノ 》 ～ ！ ／、 1 し ／ 、 ～ ＼ ！ 、 ＼一へ 蓼 o 2．韓M慧顯 k ’ρ一、啄 、 、 ＼ 1 ／ ゾ k〔へ、、ノ ／ 、〕 、一、 ／ 墾滑 10machiSmtつ・さ一、 ） v ▽ 、 ～v〆匹⇒♪

o 噂 さ D 観 、 入、T一げ ！、 ソ1／ ノ ！ 29。 ㌧＼ 1 も ◎も ざQ ト）ヤ 1、 獣階、 呂／、い 隷 久 ＿ ！’ρ 、 箆ノ・継 3b ノ＼ノ、ぞ7 oり “ 、“N ひ ㎝ Y へ＼ 一善錨枷 て∫＼號 ト 。 1 0 事 1 ／ 曳 ／ 曳 ！、 ソ イ ノ 魅 280 ほo＠t伽蹴ミ鱗添 ⑩ ／f川川 G o ざ o 監 …川1’ 歳 1”11バ 。 〈／ 論副 o へμ1 r の 川1 〃。o嗜 ‘aノ． 嶋ら ヒDoyo Smt． 鳴 尽㌧♪ N、 遥 、＼ 、、“、 ’◎ 、・く O へ、 t o ＼ ◎ い 、、1 夢・慧 ＼ 入 川〃々、llllI1﹇、月 、 、◎ 、 君 μ ＼ 、く 111’ ＼ イ 11ノ 3 ◎εぷ 〉 1μ1’ ！’1 ◎ ⑨ ’u lノ刀！ 範 騨野縛 【ハ

27。 細T 、 ll感［ r￥） 貯 ノ＼fl“ 』 enpoSmt． 〔 140。 14唱。 唱42◎E

Fig．3 （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．

volcanic front．They have dipole－type mag－ The Shichiyo Seamount Chain and neigh－ netic anomalies of normal polarity（Fig．3（b））． boring seamounts can be divided into fo皿 There is no subaerial volcano between the groups based on their magnetic anomalies two，but seven seamomts，recently named as （Fig．4）． The first group（group l）has strong the Shichiyo Seamount Chain，are located on dipole－type anomalies ofnormal polarity（Fig． the Shichito Ridge in a row（from the Nichiyo 3（b））．The Kayo Seamount，the Kinyo Seamomt to the Doyo Searhount）（Fig．3（a））． Seamomt 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

（b》 30 c 。 o 柔懸籍G つ 懸馨醗 ＼蹴 回瀦．Q・・蒙響ぐ ．灘 塗騒 29 なだ 霧雛 く⊃ ら瀬 o 。趨暴欝蕪 轟10 艶

（』） ノ 駄 28 晃 「 廷 d 隈 ＼

ノ 〉繭／麟

①⊂＞ 、 （ Q 一 ○ 騰 ㌶灘、熟。． ○ 爺、） 27 139 140 141 142

Fig．3 continued

ed in this group．These seamomts are con－ tion would have been taken place under the sidered to be Quaternary arc volcanoes like extensional regime which formed the Ni－ the Sofugan and Nishinoshima volcanoes． shinoshima Trough． The second group （group 2 in Fig．4） The third group（group3in Fig，4）which accompanies weak anomalies compared with includes the Suiyo Seamomt and the Mokuyo the volumes of their edifices（Fig．3（b））．The Seamount is characterized by an unique examples are the Nichiyo Seamomt，the Sawa anomaly distribution which cannot be ex－ Seamomt and an umamed seamount at28。 plained by the magnetization of the present 30’N， 140。15’E， It is considered that these geomagnetic field direction（Fig．3（b））．The seamounts are not young volcanoes but blocks Suiyo Seamomt has a positive，stronger of old island－arc crust。 Seismic reflection anomalies above its westem flank and a nega－ profiles revealed that these seamomts are tive，weaker anomalies above the eastem tilted blocks cut by faults and covered with flank（Fig。3（b））．This anomaly pattem can sedimentary layers（Fig．5（a））．The deforma一 be interpreted as the magnetization of east一

一663一 Bz61」6ガ銘（ゾ孟h6 G60JogJo81S％7z尼ッ（ゾノ吻4％，Viol．42，No。12

3000 （c》 ］

～

1 n

’1 1 、

2900 ノ 1 II

1lI ［

’ 1 」 聖 覧

… し 、

犀 ！ 1 ‘切 1 1

2800 ξ ＼ 1 「 ； 1

し ‘aノ

1 冒 1

陰

＼ 6

l I 1 2700 一 1 138 139 140 ！41 142 00 00 00 00 00

Fig．3 continued

ward declination and shallow inclination（Fig． and after that it was rotated with the 麟 6）．The direction of the magnetization is PhilipPine Sea plate． fairly close to the paleomagnetic direction of The anomaly distribution of the Mokuyo ／ Ogasawara（Bonin）Islands on the Ogasawara Seamomt is even more complicated（Fig．3 Ridge（Kodama6渉σ1．，1983）。The east de－ （b））．A pair of positive’and negative anom－ ／ nected declinations have been reported not alies of north－south direction occurs on the 灘 only from the Ogasawara Islands but also seamomt，but its spread is restricted just 難支 from the Mariana Islands（McCabe and above its crest． Its vicinity， on the other Uyeda，1983），the Palau Islands（Hastonασ1．， hand，shows the anomaly pattem of eastward 1988），and Guam and Saipan（Haston and magnetization similar to that of the Suiyo Fuller，1991），and they can be explained by the Seamount．It is thus estimated that the edi・ clockwise rotation of the Philippine Sea plate fice of the Mokuyo Seamount would have since the Oligocene （Louden，19771Seno and formed mainly at about the same time as the Mamyama，19841Haston6厩1。，1988，Haston Suiyo Seamomt，but recently rejuvenated and Fuller，1991）。The magnetization direc・ volcanic activity has taken place at the crest． tion thus suggests that the Suiyo Seamomt There are some other observations which would have emplaced soon after the formation support this inference．Seismic reflection of the Nishinoshima Trough in the Oligocene， profiles across the Mokuyo Seamount（Fig．5

一664一 ル望2zg銘6あo ご〃zo窺41づ6s oゾ’孟h6 12κ一ノ耀～z7乞ごz％ごz1470s ピT． y2znz6z2ごzhづ （話ごzl．、）

圏眉 qφ “ 魅 300 睡2 σ 〃 b ＿＿ 麗劃3 魅 0 麗講4 25 0 、騨鵬 Smt． 梅鑓 鞭螺。 ▽・騨 machi Smt．

2goo 倉 1灘罰t 二 ゆQ離・動・s

じ ， ◎ も

28◎ 0 9〃。q。 。1、館 o》 S，羅鎌 9

Tenp◎Smt． 〔 1400E 1410

Fig．4 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 fairly large，more than500nT．The broad－ seamomt has well stratified reflectors，which ness of the anomalies and their rather poor suggests that the seamomt is covered with correspondence to the topography suggest that fine sediments rather than volcaniclastic the major cause of these anomalies would not debris．It is thus inferred that the seamount be the magnetization of the edifices of the would not be young except for the top． seamomts but somewhat deeper magnetized Recent SeaBeam mapping revealed that the bodies．On the Omachi Seamount，K－Ar ages crest has caldera structure （Nagaoka 6渉 α1．， of32to34Ma and occurrence of late Eocene 1991）．In the caldera，hydrothermal activity microfossils were reported （Yuasa α α1．， was discovered by dives of the submersible 1988），A sedimentary rock having microfos－ Shinkai 2000 （Nagaoka，1990），indicating a sil age of the early to middle Miocene was recent volcanic activity there． taken from the top of the Tenpo Seamount Around the Tenpo Seamount and the Oma－ （lkari and Nishimura，1991）．It is thus con－ chi Seamomt（group4in Fig．4），broad posi－ sidered that these seamounts are not yomg tive magnetic anomalies are conspicuous（Fig． volcanoes，but are composed of old island－arc 3（b））．The intensities of these anomalies are cmsts which formed before the Izu一

一665一 閃 ひq ハ ’『wo隅way t『avel tlme in seconds τWO爾Way t『aVel time in SeCOndS 9 ㎝ σ （ 》 翻 轟 9 時 面幽 ） 轟 o 時 幽 →盟 甲■ ・”練 ウ o げ昌 ’猟 鋤lr Φ磯 O I一Φ O o 織 讐げ い1騨ヤ繍・ δ’き ’気 bu 塁ま ミ o 一 ミ 転 賄OD ◎蹄題紫 ミ の も◎ 9あ’ ミ9 窃目． 1・ 一閣1轟 趣 難蕪懇一「一 署 勲 ’ ㌃・『 P 聖 ． ド ロ ト 甑 ぺ や 鋤 8亀 ◎ ド じロ ペ 導 旨9 ㌦亀φ鱒罐転←年 碗① ，難 鳴 甲 ’ 』 ■ 紛 転 ’ 1 ド ド 窃鼠 し ㊤ 餌9 鋤§o隠 驚撫 鋤 o レ1 レ L 5 』一 s 80 叢 り § ご 鴇9 鴨 難 偽 ◎ 89 睾号 麟 麹 〇7零0 匙 號 諭 し・1 ピ距 6 鵠 50R甥 陣 の 『鮪“ ミ ま 一監6ρ～・嗣“ ’ 這 略 ずr一’旨 ① 鵯6 、．瀞 趣 ω箒 M頃ヒ5b 旨卜Lし 1．，‘お，1熱灘製 宙（ノ） 勲 ）四 ”ミ P四 繍 …廓 ミ α ‘跳礁紬s鼠 （（ノ） レr 卜 ；・・二‘ 搬 繋鱒 一噌 § gs 4τ＝σ ヨ ＿＿＿盤、 巨 2 ．．副，，べ継融潤 ぎ 筥 ひ1 一 』 ・；。二 ．・㌔…購爵蝋騨 ト 貧 N叙 四 … ＝㌧h勤鳩 昌 4 ■ ro 1 α さ 竃 け 峰》 滋 『 ・．2、避榊 』 ・κ縮 貼 置 無’！＝■・』覧’

トも Σ5 ＿一 1、．．・鐘贈 身 ｛昭 一o宍ヨ の 昌 ．、ご1調溶 o げ、鱗魏〆 ズ o ヨ ω 鴇L 8 麟 ヨ 灘継一 8 ，i韻，戯 旨 ’、 ，’が勲i、1ず蹄， ∂ 瑠48・冗顔6伽o窺α1∫εs oゾ孟hε盈％辺毎π1α％α14γcs‘T y伽zα9盈σ召渉α1．ノ

騰i

￥ 』 』 唱 』 一100 』 P 』 自 』 礁嘆瀬、，、 翻 勢e

，ダ 欝 10km 、、『』享 一 享 、灘

Fig，6 Magnetic anomaliesproducedby apyramid－shaped magnetizedbody．Thedeclination oftheremanent magnetization 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－ attributable to deeper magnetized bodies such ge were separated by the creation of the Shi・ as intmsives．The elongating direction of the koku and Parece Vela Basin。 anomalies，WNW－ESE，isperpendiculartothe The broad positive anomalies similar to structural trend of this area，represented by those observed over the Omachi and Tenpo the Sofugan Tectonic Line． Seamomts occur sporadically in the fore－arc region in the northern part of the Izu－ 4．3 SOuthem Iz既一〇9我s我wara Arc＝t恥e Ogasawara Arc．They are at southeast of K我ikaねSeamo聡醜ttoM亘醜我mi－lwoj亘漁夏s． Torishima Is．and east of Sumisu Is。shown in Volcanoes on the Shichito Ridge（Fig．7（a）） Fig．2（b），and the Shinkurose Bank（northeast show strong，short wave－1ength magnetic of Hachijojima Is．），presented in Ishihara6砲1． anomalies（Fig．7（b）〉reflecting their Iarge （1981〉。 These regions correspond to the volume and shallow water depths．The basic uplifted portions of acoustic basement on pattem of magnetic anomaly distribution over seismic profiles，called the Shinkurose Ridge the Kaikata Seamomt，the：Kaitoku Seamomt （Honza and Tamaki，1985）．The basement and Kita－lwojima Is．is of normal－polarity highs are interpreted to be composed of magnetization：the anomalies over the north－ Paleogene and Miocene sedimentary rocks on em flank of the volcanoes are dominantly the geological map of Honza6砂1．（1982）．It negative，and those over the southern flank are is thus considered that the broad positive positive．Being superimposed on this pattem， magnetic anomalies would characterize dipole－type anomalies of shorセer wave－1ength basement highs of the old Izu－Ogasawara Arc． are distributed．Some of the①correspond to In the west of the Nishinoshima Trough， topographic highs，but otherS do not．The magnetic anomalies having relatively long latter would be due to highly variable magnet－ wave－1ength are distributed （Fig．3（b））． ization，reflecting thermal and compositional Topography of this region is fairly smooth inhomogeneity of the volcanoes． （Fig．3（a））．These anomalies are probably The Iwojima and the、Minami－lwojima Vol一

一667一 Bz61」6訪z6ゾ云h6 G60Jo4αzl Sz67∂召：y6ゾ忽）ごz％，yiol．・42，．〈ろo．12

《観》

！’ノ朴！ 頑 q葱 制 o該 糖 、 、 27。N ） 11 r、 Q ヲ＼ ノノいl ◎ ’ ノ Y ㊤ o ll、t Q Kalka竜aS醜い1 ’目、＼1＼ ＼き＼バノ黛 q心 騨湊 o 陶軸 鵬 り 物亀バ 〃 紬 ノ 柊 o 一 1 ＼ ～ ＼ 、 、 ゾ u冷 脳 o 働 ノ ノ1 デ 殉 ’、 唱 D Kal量◎k幽s朧量． ＼￥ ○ ご◎￥ o 聰転圃馨 ノ 、 26。 ⑨3 で＾ノ、

／ 1 D し 1 o o 憶 、 l ◎ 9噛 l ’ 軸 圃 1、 ■巴曜駒 （7 鵡o覧自回鵡鵠 凱 ∫’tlS 鰍 らG ◎

1《1量a－lwol辮a瀕 ．／ ¢◎ ～か ￥ ／ く 、 ／一、 4 ）

◎ 25。o ∠） 働9 4／ 殉 o㊥1脇麟 鞠 眺 o ＞D O 賄齢翻一lwoll朧als、 o 3。 ◎ 24。o

嘱40。 14調。 142。E

Fig．7 （a）Outline of topography in the southem Izu－Ogasawara Arc from24。to27。N。 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。 canoes are characterized by extremely com－ spacings of survey lines are still too large plicated magnetic anomaly distribution（Fig．7 compared with the observed short wave－ （b））．The anomalies have very high amplitude length of the anomalies（Fig．7（c）），and the （up to 3000nT） and short wave－1ength． accuracy of positioning is low aromd Iwojima Their complexity is more than that of the Is．，sometimesworse than500m，because oftoo three volcanoes mentioned above。This short distance from a Loran－C station being would reflects higher inhomogeneity of these located on the island． In the vicinity of the volcanoes，suggesting longer history of the Iwojima Is．，the Hydrographic Department， volcanic activity．The magnetic anomaly Maritime Safety Agency Japan has already contour map of these two volcanoes，however， conducted not only tota1－intensity ⑳easure・ has some roomfor furtherrevision because the ment by a ship but also three－component

一668一 翻

惚n痂oα％o脚1づ6s oゾ孟h6彦％一妬z7勿％z4κsピT．y4彫召㌶hJ切召θ

（b》 2700 縷 G 6籔， ，欝 Q ．、命 の籐δ G Q 穣鐵 馨磐 碁 』 総渦 1 ＼念㌶羅 し、奪 0雌灘鰹『 ズ 曳 。 饗。織灘翻餐灘 2600 ） 醗＼ o 》 津慧1㈱ ∂ 置 oハ）簸

2500 糠’〉 膿拶／。。 欝灘響 、隷 塵 懸、難．饗・ ∂な愚 〆譲蕪欝購糞駿り雛礁難

2400 艶、繕藤叢 139 140 141 142 00 00 00 00 Fig．7 continued

measurement by an airplane（Oshimaαα1．， The origin of these anomaly belts is not clear， 1982；1985）．They estimated using some mod－ but they cannot be explained by the effect of eling technique that the center of the Iwojima surface topography because the topography of Volcano from surface to2km in depth is non・ this area is rather smooth（Fig．7（a））．The magnetic because of high temperature．This anomalies may reflect a relief of the basement explains why dipole－type anomalies of rever・ covered with thick sedimentary layers derived sed direction （north side is positive and vice from the arc volcanoes．Another possibility versa）occur on the center of the volcano（Fig． is that the basement of this area may be 7（b））． norしheastern extension of the oceanic crust in In the back－arc area west of the Shichito the Parece Vela Basin having magnetic linea－ Ridge，northwest trending belts of positive and tions．This hypothesis implies that the pres－ negative anomalies altemate like the magnetic ent Izu－Ogasawara Arc of this region has Iineations of mid－oceanic ridges（Fig．7（b））． grown on the oceanic crust．

一669一 Bπll6あ多z（ゾ 云h60601092αzl Sz躍％ly6ゾノ4ゆα多z，Viol．42．No．12

（c） 2700 1 i 『， π 1 ＼ 『 li・H／ゴ 1 監 賦 隔聖〆、 ’∠ 。 レ7寧 ・ 瓢曝’ 1！巨 ノ 1 」＿ ～

・ 鱒 ・バ 輪1 ノ ξ ．謡、一

『 ／／ し1 馨 肛！ 1 畠『一←、＼ 1 ／ ・嚢褻 一蓑， 〆 」、一 ・！嘉L『』一’1 P ￥、 1 1 li ㌔ ／ … ’ 聖 ’ 、 聖1 ！ 『 ヤ 1 1’…，1 ｝ 覧コ 2600 i 一h ／ ｛ 1 』 1 、＼ ／ 1 ＼

Ii 1げ ～ ＼ 91 1 i Ii ＿1 ！ 1 Jl 、

旨 、 1

1｛ し 『 i 2500 〕 イ 一 1 ノ ＼ 1 、曹 ／ ユ 1 ＼

！ ～ ＼ 〔 ＼ユ．霧蓬憲議臥． 一一 2400 一 139 140 141 ！42 00 eo 00 00

Fig．7 continued

The Ogasawara Trough（Figs．3（a）and7（a）） 4．4，0gasawara Plateau an、《I neighboring shows slight negative magnetic anomalies，and seamounts no lineations can be recognized（Figs．3（b）and The Ogasawara Plateau and three other 7（b））．This is consistent with the considera－ seamomts east of it（the Yabe Seamomt，the tion that the basement of the Ogasawara Hanzawa Seamomt and the Katayama Trough is rifted island－arc cmst（Abe6渉α1．， Seamomt） which we studied （Fig．8（a）） 1989）． On the other hand， Isezaki 6渉 α1． occupy the westem end of the Marcus－Wake （1989） carried out three－component mea－ seamount chain （the Michelson Ridge of surements of the geomagnetic field，and found Smoot（1983））．The Ogasawara Plateau has lineations of north－south strike on the east and collided against the Izu－Ogasawara Arc at the vertical components in the Ogasawara junction between the Izu－Ogasawara and Trough． But the origin of the lineations is not Mariana Trenches． clear． 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 sible explanation for this discrepancy is that Ogasawara Plateau（Fig．8（b））。The ampli－ induced magnetization and／or viscous tude of the anomalies of the Yabe Seamomt magnetization of the present field direction was about－700nT，and that of the Hanzawa have overcome the primary remanent magnet－ Seamomt and the Katayama Seamount ex－ ization of the seamomts which would have ceeded －1000nT． The fact that negative been acquired in low latitudes。Another pos－ magnetic anomahes occur above the bodies of sibility is that these seamounts were the seamomts indicate that their magnetiza－ remagnetized by rejuvenated volcanic activ－ tions are normal polarity an（1have shallow ities of relatively young age，that is，after the inclinations，while too scarce survey lines in directional change of the movement of the the northern and southern vicinity of those Pacific plate from NNW to WNW，which is seamounts（Fig，8（c））prevent from calculating known to have occurred at about43Ma their magnetization more quantitatively by an （Clague and Jarrard，1973）．Nemotoαα1． inversion method．The ages of these （1986）suggested the existence of Eocene vo1－ seamounts are estimated to be the Cretaceous canic activities at the Higashi Seamomt． from the microfossil age of limestone covering The stripes of the positive and negative the top of the Yabe Seamomt（Shiba，1979）． anomalies over the Ogasawara Plateau seems It is thus considered that these seamounts were to continue landward across the trench emplaced in an equatorial region in the although their amplitudes decrease．This Cretaceous，and have been moved to the pres－ observation suggests that the westem flank of ent position by the northward shift of the the Ogasawara Plateau has been already sub－ Pacific plate since then （e．g． Clague and ducting undemeath the forearc wedge of the Jarrard， 1973）． In the vicinity of these Izu－Ogasawara Arc． seamounts，Vacquier and Uyeda （1967） reported another seamount（No．Z－4－3at27。 4．5N艦her醜M麟a舩Troug恥 N，148。407E）having the magnetization of The Mariana Trough is initiated at about shallow inclination． 24。N，just south of Minami－Iwolima Is。（Fig．9 The Ogasawara Plateau has several peaks （a））．It develops from a rifting stage to a on it（Fig．8（a））．The Kita Seamomt on the spreading stage at about22。N（Yamazaki6孟 Ogasawara Plateau has dominantly negative α1．，1988b；Murakamiαα1．，1989）．The anomalies（Fig．8（b）），which is similar to the trough widens southward from80to120km at seamomts mentioned above．The Higashi this boundary at22。N，and deepens more than Seamomt（the Broken－Top Guyot of Smoot 500m．Small ridges and grabens trending （1983）），the Nishi Seamomt and the Minami NNW－SSE become conspicuous in the south Seamount have，on the other hand，a pair of of the boundary． positive and negative anomalies （positive The characteristics of magnetic anomaly anomalies are on the southem flanks of the distribution also vary significantly at this bodies，and vice versa）（Fig．8（b））． This type boundary （Fig．9（b））． In the north of22。N， of anomaly distribution suggests that the dipole－type anomalies or short wave－1ength magnetization directions of the seamounts are anomalies are remarkable，which are of close to that of the present geomagnetic field seamount or intmsive origin．No magnetic there．This implies that these seamounts lineations are observed． The seafloor of this were magnetized at a place near the present regionwould consist ofstretched and sundered latitude．However，fossil evidence shows that island－arc crust． In the south of22。N，on the the Higashi Seamount was formed in the other hand，NNW trending magnetic llnea－ Cretaceous in an equatorial region like the tions can be recognized，which would be Yabe Seamomt（Konishi，1985）。One pos一 caused by magnetization of oceanic crust．

一671一 一z乙9一 紬 ㊨霧

紬 ロ 睨 罵 0

ぬ 轟 鯵 ㎡ ．、 ζ o ・gasaw疲◇

問 ひq oo

きヨε 熊9 O δo〈50巴R菖 Q ω巴Φ gD 帥∫ρo ¢ 吋 胤 の ④ 魯ε8け ＆匡畠 1乞フ，舞 ρ 軌 貸讐8のゆい o ＿9菖 o ε800 訣 （ノ）餌噛 菱＼1◎ 醤 但昌9 恥 還§ 輪 § 爲 講 韓 陣 貯箒帥9貯舞 箒ω国 ゆ ゆ ぬ o 甜Φ翌ヨヨ8 轟 諾巽鋤 魎 く §串且 ◎ ④ 9｝ 昌 ぴ ⇒（臼）㊤． Φ のの ヨ旨浮 ◎D 箔2自 i…i 霧鵠日● 9》磯 酋 器紹 跨目 ゆ 一2 口幹 8旨 → 霊 一．O 9 肖昌 P O 瓢 r o Φ 6圏 N 薗 r 〈仁 卿 一ω ∈ 曽 ω 即 勺 H Φ ω 貯 ○帥 諄。一 一。o 讐評 〈o Φ O 曾

ス 鋤 oむ ゆ 《 轟 9D ⑳ ロ 置 囲 o ω ヨ 《麟 槽 》

8τ’02V‘昂フ0レ1物朔ρ孤3磁％Sノ聯8070∂θ∂解ρ％3ヌ∂1ノ％9 2700 ｝． 3． 賑装鱗oハ 嚢s轟 （b）

辱・。 鞭胸／？ （ 辻グ・ヤ、〆ノ と 2600 復 蓼 ㎜ ⑨ § ら ま 還薫藤事 麹ぐ蕊へ ’ 獲 影篠撰終 § § 一 鴉 § の 蒲論 亀 誉 2500 医難 143 1器 1壽 1器 11『 11『 ぎ 1 0D 零 § もQ ミ・ 2700 ミ （c） 専 8 爲 § § 翁 26GO 蕊 魁 “ 卜 ㌧

2500 143 1器 145 146 11『 11『 00 00 00 Fig．8 continued ■ lI

Bz61」6あ麗（ゾ玩6G601040ごzl Sz67∂6：yのr／4ρα％，Vbム42〈砂．ヱ2

麟噺．臨 （a》

◎

240 ／，1 q d 駕 翁 ◎ 、 ぺ 過 Minaml－Hiyoshi Smt． 姉 9 d 態 響◎ 難 飼 Nikko Smt． ▽9 ㈱ 23。 ⑨ ’『akasu Smt． の ロ 141。 藁 ．懸 駕 ％ 翁 網 ◎ oo ℃ 。D 巾 黙 D考 。 鰭覧 Fukujin Smt． も 22。 s 参 ◎ “ 咳 も φ 傘 Kasuga Smt． 2 1 噛 識 θ も q q o Inaml－Kasuga SmI． 怒 142。 4 塔気 偽 ◎ 噌φ 2傷 210 3 靱 繋、 檎

1。。km孫 嘔 、 O 鴇 B 20。N 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 遷響驚〆 （b） ．，』1ξ藩㌧つ 鱗載、驚郵） 2300 ・響繋、 〃『o 響・lil o ・爵簿裁・：ご7 2200 9 誉縷抄膓 寒繁、 恥◎ 蓼塙＼画 奪識，9．灘簿 鵯誤 2100 1” 辛，黙 （灘 纂、 2000 1 、 14！ 142 143 144 145 00 00 00 00 00 Fig．9 continued

Correlation is obvious on the magnetic anom－ between21。and22。N．We adopted the aly profiles along the survey lines（Fig，10（a））， skewness parameter of－58。，which is derived particularly between21。and22。N． on the assumption that two－dimensional 4．5．1耽％痂脇加げ窺α9襯」61吻読oηs blocks having N20。W strike were magnetized To assign ages to the magnetic lineations， at the present latitude．We thus considered we performed forward modeling by a two－ that the formation of oceanic cmst between22。 dimensional block modeL The model shown and21。N would have started at3Ma or a little in Fig．11fits well the anomaly profiles 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 N 《c》

一 、

黛

2300 ＼ 、 N 1

、

、 、 峯、

t ＼ li 220G

I I 嵩

馨．

きら き 『 で 2100 き

凸

2000 141 142 ！43 ！44 ！45 00 00 00 00 00

Fig．9 continued

cm／year．隅The rate would have decreased to which suggests that spreading rate becomes about l cm／year at present．The present higher northward（Macdonald，1986）。 spreading rate at the northern end of the The correlation between the model and the spreading axis close to22。N is estimated to be obser▽ation on the eastern flank of the a little faster，about2cm／year。The topog・ spreading center is not good because the raphy of the spreading axis supPorts this magnetic lineations are disturbed by the interpretation． The rift valley on the spread－ anomalies accompanied by seamounts（Figs．9 ing axis becomes less retnarkable northward （b） and 10（a））．They are the Fukujin on the topographic profiles shown in Fig．10（b）， Seamount and a chain of submarine volcanoes

一676一 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）

騰駐 22。N 課 Q （a）

G 剛 ノ．

2GO“て 縞。。

21。

．．◎ 擁 副 冊 蒙 型縦織グ

ノ 1 ｝添 フ

訳嗣 ◎

20。 象 142。 143。 144。 145。E

220N （b》

講

f5．．，①

麟

21。

縛 翻 憾麟

200 142。 143。 144。 145。E

Fig．10 （a）MagneticanomalyprofilesalongthesurveylinesofENE－SWSdirectionintheMarianaTroughfrom 20。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 欝 羅 婁

嚢’

B％ll6あnρゾ 渉h6 G60104αzl Sκ7∂6ッのF／41）ごz％，Viol．42，No．ヱ2

Moqle’ Skewness－58 o

400nT l。

4 2 0 2Ma Polarity ／i 2、5cm／year i 1．5 11・011・01 1・5 ［2・5 Rate 10km 〔Thickness3km，lntensity5A／m） 離 一 嚢 Fig．11 Magnetic anomalies produced by a two－dimensional block mode1．Spreading rates were adjusted so 蒙 that the theoretical anomaly profile fits the observed ones around21。30’N in the Mariana Trough。 馨

Solid blocks represent the normal polarity and open ones are reversed．The magnetic polarity time 鐘 萎 scale of Harland6オα1．（1982）was used， 蒙 馨 藝 馨 increases toward the OSCs at21。N（Fig．12）． 1ineated southwestward from the Kasuga 嚢 Seamomt（Cross－chain seamountsl Hussong This feature issimilar to the OSCs on the EPR， 馨．三 嚢 and Fryer，1983）（Fig．9（a）），which were em－ and explained by that the amomt of magma 嚢事 li placed after the formation of oceanic crust。 supply is small at OSCs（Macdonaldαα1．， 熱 4．5．20∂67妙卿gs卿α伽go6漉zs認 1988）。 21。ノV We estimate from seismic reflection profiles Our interpretation of magnetic anomaly that the spreading at、the westem axis would profiles leads to the conclusion that two have been subsiding now，and in the east of it， spreading centers overlap from20。3αto21。N a new spreading system is instead under （Fig．10（a））．The characteristic anomaly pat－ developing with destroying older oceanic ／ tem which has been recognized on the profiles cmsts．Aseismicreflectionprofileshownin ／、1 north of22。N continues southward to about20。 Fig．13shows that the westem spreading axis 30’N，althoughthe amplitudes ofthe anomalies accompanies a typical rift valley．The east－ become sma11，an order of about lOO nT．To em axis is，on the other hand，more compli－ the east of it，on the（）ther hand，another iden－ cated in geological stmcture．Many faulted tical anomaly pattem mns parallels to the blocks exist there，but no clear rift valley． former from20。3奪to21。N。The latter con－ The OSCs fomd at21。N in the Mariana i tinues southward to2αN，where axi謡valleys Trough is anomalous in the following points oftheMid－Atlantic Ridge（MAR）type canbe compared with already documented OSCs． ／ seen on topographic profiles （Fig． 10（b〉）． First，the size is large． The OSCs at21。N 嚢 甕、自 Between the two spreading axes，an offset about30km and overlap each other by 難

ヤ anomalously deep graben occurs（Figs．9（a） abo興t50km．It is known，however，the lat－ 蒙． and10（b））．The depth reaches5300m at max－ eral offsets of the OSCs on the EPR are less 鑓 imum．This situation，that is，two spreading than15km（Macdonald and Fox，1983，Mac－ 態欝 axes and an intervening graben，is similar to donald 6！召1．，1988）．Second，OSCs were 麟 the Overlapping Spreading Centers （OSCs） commonly observed on fast－spreading ridges which has been observed commonly on the such as the EPR，but never on slow－spreading East Pacific Rise（EPR）as a form of non－ ridges like the MAR（Macdonald and Fox， transform offsets of mid－oceanic ridges 1983）．The spreading rate of the Mariana （Macdonald and Fox，1983；Lonsdale，1983）． Trough is nearly as small as that of the MAR．

毒 The graben at21。N would correspond to the A spreading center－transform fault pattem 襲 Overlap Basin，which is a basic constituent of prevails in the case that an offset of ridges is 饗 OSCs（Macdonald6齢1．，1988）．The depth of large or a spreading rate is smal1．This is 鑑 羅 the spreading centers in the Mariana Trough because spreading axes are in contact with 霧

一678一 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

0 50km 一一＿＿＿一一一一一 2ゴN 220

∈ ど 一3 5静 －4

Fig．12Axial depth profile along spreading centers in the northem Mariana Trough．

82 3 OverlapPing Spreading CentersOverla P等 の 1 ∈ 3 … 轡 5 F㌃撮 二，馨既 聖■『i 詰4 礼“仁 『「暇げ阿「『「『配「四「「 ’噂鰭 一陀 護’一豫 1『 轟1 競 聯細 ＞ 、 邸 5 市 芒毒 恐》卜 ｝ 葦 1 ◎ 綴機 饗、謬ミ ． 轟虐燕 ’ 、，彰 シ 6 響卿 ヂ仁濫■， 轟」 陣 一 ・毒，，1 ，、 …懸灘 2藤、 3魯ト 巳r 占←珪8 『臼 ｝ ・耀

1

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 volcanoes on the Mariana Ridge．Some of small offset or a fast spreading rate，and thus them have simple dipole－type anomalies，but the cmsts would not be weak enough to allow othershave complicated anomaly distributions non－rigid deformation like the OSCs。It is which do not correspond well to the topogra－ hence suggested that the cmst ofthe Mariana phy of the seamounts（Fig．9（b））。The former Trough spreading center may be weaker examples are the Fukujin Seamount and the （thinner and／or hotter）than those of major－ Minami－Kasuga Seamount，and the latter the ocean spreading centers． The third point is Nikko Seamomt and the Minami－Hiyoshi that the magnetic anomalies of the Mariana Seamomt。This difference would reflect dif－ Trough at21。N are smaller than those of ference in the ages of these seamomts．The neighboring areas while the OSCs in the EPR latter would have had longer volcanic activity have stronger magnetic anomalies（Lonsdale， than the former，resulting in highly in－ 19831Sempereαα1．，1984）．It is considered homogeneous magnetization。Magnetization that high magnetization intensity which is of normal and reversed polarities may be carried by highly fractionated basalts enriched mixed．Topography of these seamomt sup－ in iron is responsible for the strong magnetic ports this inferencelthe former has a simple anomalies of the OSCs on the EPR（Sempere conical shape，but the latter shows complex 6砲1．，198411988）．The weak anomalies at21。 topography having two or more peaks．On N in the Mariana Trough suggests that the the Fukujin Seamount，the spread of the iron enrichment may not have occurred there． anomalies is narrow compared with the size of 4．5．3惚7加α．1～惣6魏4脆s渉〃47加α the seamomt body．It is thus suggested that ノ～惣6 the seamomt would be covered with thick Next we discuss the magnetic anomalies volcaniclastic debris，and highly magnetized accompanied by a row of Quatemary arc 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， are vague on an anomaly contour map（Fig．14 does not have strong magnetic anomalies in （b））due to the low amplitude of the anomalies general（Fig．9（a））．The Takasu Seamount，in （200nT or less），anomaly profiles along main particular，accompanies little anomalies survey lines can be correlated each other（Fig． although it has a flat top of very shallow 15）．Spreading rate estimated from compar、 depth，about50m．This suggests that the ingthe profileswiththe model in Fig．11varies Takasu Seamount would not be a volcano． somewhat from place to place，but it is an An mnamed small peak at about21。20’N order of2cm／year．This agrees with the showed the anomaly pattem which indicates previous estimationsby Bibee6厩1．（1980）and reversed magnetization． Hussong and Uyeda （1982）．Discontinuities in magnetic lineations can be recognized at17。 4。6 S騨e翻直盟g ax量s漉18。N亘聡tぬe M鍛董＆舩 40’N，18。N and 18。20’N （Fig．15）． They Tr側9璽 roughly coincide with topographic and struc－ The Mariana T罫ough is widest at about18。 tural discontinuities，that is，the Pagan frac－ N，and the occurrence of active spreading ture zone between17。30’and17。40’N，the17．9。 ridges is well documented （e．g．Fryer and N offset and the18．5。N offset of Volpe6！σ1． Hussong，1981〉． Thus the existence of the （1990），respectively．The trend of the mag－ magnetic anomalies of seafloor－spreading netic lineation between18。and18。20’N is close originisexpected．Whilemagneticlineations to north－south（Fig．15）although the strike of

眉8030’閥 《繍》 嘉 ℃嬬 飯0

0 賎、 、 噂魁 o 鱗 璽 ド 簸簗徽舞 σ Q 勲 越 灘． 爲b q 掘 島 ◎ 羅騨 ’塾 匝 qq 醗講Qり d馨 〜廼黙漁 0 も 、 望 暗 o d 180009 、o“ 、

“ ￥、 陵 撚

鷺 」 x

噂 φ㎎ o の 、 陰 り ． q も o 職

ノ？ ん 聯

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．

一680一 羅 撰 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 1830 ㈲ ら （c》 賊 0茜 一

鮮＿

襲 輝． r暦 一 一ヤ1 1800 （ 1800 き

o コ

Q 1730 「 ， 1730 144 11計 1壽 144 1器 1壽 00 00

Fig．14 continued

ユ8 30

｛

ユ8 00 締．芒1「

簸灘「

1200nT

i7 30 i44 i44 ユ45 ユ45 00 30 00 30

Fig。15 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．

the rift valley is NNW（Fig．14（a））．This Ogasawara Arc and the northem Mariana suggests that a small change in spreading Trough along dense ship tracks． Geological direction and accompanying reorganization of interpretation of the resulting magnetic normal faults would have occurred recently， anomaly distribution are summarized as fol－ say an order of100k，y．before． 10ws． （1） Volcanoes on the Shichito Ridge，the 5． Co館c且腿s亘o醜s volcanic front of the IzurOgasawara Arc，have usually strong magnetization probably owned We conducted magnetic survey on the Izu一 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

き Nishinoshima Volcano）has rather simple （7）Four peaks on the Ogasawara Plateau dipole－type anomalies，but others have very and three other seamomts in the east of it can complicated anomaly distribution （e．g。the be divided into two groups from magnetic Iwolima Volcano），which may reflect the dif－ anomalies．The first group having dominant－ ference in the length of the volcanic activity． 1y negative anomalies was magnetized in an （2） On the northern part of the Shichito equatorial region probably in the Cretaceous．

きド Ridge，some volcanoes accompanies only The second group shows the magnetization weak magnetic anomalies（e．g．the Myojin close to the present geomagnetic field direc－ ／

Knol1），which suggeststheir compositionbeing tion， suggesting induced and／or viscous 灘 該 acidic rocks such as dacite． This reflects the magnetization or remagnetization by re－ 熱 bimodal volcanism in this region． juvenated volcanism． 蒙 （3） Back－arc rifts in the northem Izu－ （8）We show for the first time that mag－ 嚢 Ogasawara Arc show no remarkable magnetic netic anomaly profiles inthe Mariana Trough 嚢l l i anomaliesexcept for cross－chain seamounts in southof22。Ncanbecorrelatedclearly．A 灘 the Sumisu Rift．Seafloor spreading has not forward modeling indicates that the seafloor started yet． spreading aromd21。307N started at3Ma or a （4）The Shichiyo Seamomt Chain and little earlier at a half spreading rate of2．5cm／ ／ neighboring seamounts in the middle Izu－ year．The rate has decreased to about l cm／ ／ Ogasawara Arc can be divided into three year． groups based on their magnetic anomalies． （9〉Spreading axes overlap about50km in The first group is Quaternary volcanoes， theMarianaTroughbetween20。30’and21。N． which have strong dipole－type anomalies of Previously the existence of OSCs was not the normal polarity．The second is faulted known on slowly spreading ridges．Further－ blocks of old island－arc crusts，accompanying more，the offset of the two axes，about30km， little anomalies．The third is old volcanoes of is much larger than that of the already 藝 possibly Oligocene age，which have the documented OSCs on the East Pacific Rise． magnetization of eastward declination and The cmst at theOSCs inthe Mariana Trough shallow inclination． may be hotter and／or thinner than that of the （5〉Fore－arc basement highs of Paleogene majorocean． or Miocene age（the Shinkurose Ridge）are （10）Some seamounts on the northem Mar－ characterized by broad positive magnetic iana Ridge have simple dipole－type anomalies 1． anomalies．Similar anomalies are observed （e．g．the Fukujin Seamount），but others show on the Tenpo Seamount，an old basement high complicated anomaly pattems which poorly lying in the back－arc region west of the correspond to their topography （e．9．the Nik－ Sofugan Tectonic Line． ko Seamomt）．This would reflect difference （6〉 In the back－arc region north of 30。N， in the length of the・volcanic activity．The topographic highs striking obliquely to the arc northem part of theWest Mariana Ridge，the （the Nishi－Shichito Ridge） have relatively remnant arc，accompanieS no strong anom－ simple magnetic anomalies of normal polarity． alies in genera1（e．g．the Takasu Seamount）． Topographically smooth area around28。N has （11） The spreading rat6at頓e ridges at18。N relatively large anomalies caused by deeper in the Mariana Trough is about2．cm／year at source．In the back－arc between24。to27。N， present．A small change in spreading direc－ alternating belts of positive and negative tion from E－W to ENE－WSW would have anomalies run in NNE－SSW direction．This occurred recently． may renect a relief ofthe basement，or suggest the possible existence of oceanic crust under－ Acknow且e岨gements：We wish to thank M． neath thick sedimentary cover兜 Yuasa and A．Nishimura for valuable discus一

一682一 覇

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． data from the Philippine Sea Plate and Saito and K．Watanabe for providing topo－ their tectonic significance． ノ1． 060カh夕s． graphic maps used in this report，E，Kikawa， ノ～6s．，vol．96，p．6073－6098． Y．Okamura and M．Satoh for helping the ， and Schmidtke，E．（1988） Paleomagnetic results from Palau，West measurement，and Y．Okubo for helpful com－ Carolinelslands：Aconstrainton ments on the manuscript．The magnetic sur－ Philippine Sea plate motion．060Jo劇， vey was performed with cooperation of all on vo1．16，p．654－657． －board scientists，officers and crew of the Izu Hey，R（1977） A new class of “pseudofaults” －Ogasawara－Mariana cmises of the R／V Ha－ and their bearing on plate tectonics： a kurei－maru（chief scientists，M。Yuasa，A． propagating rift modeL E召7孟h j％彫孟． Usui and M．Nohara）． S6づ、L6泥．，vo1．37，P．321－325． Honza，E．and Tamaki，K．（1985） The Bonin Arc． References InTh60磁nδ嬉％s伽4彫召顧ns，∂ol．Z4， edited by A．E．M。Naim6渉σ1．，Plenum Abe，S．，Tokuyama，H。，Kuramoto，S．，Suyehiro， Press，New York，p．459－502． K．，Nishizawa，A．and Kinoshita，H。 ， ，Yuasa，M．，Tanahashi，M．and （1989） Report on DELP1987cruises in Nishimura，A．（1982） Geological map of the Ogasawara area Part II：seismic the northem Ogasawara Arc．Marine reflection studies in the Ogasawara Geology Map ser．17，Geological Survey of Trough． B麗ll。Eαπ勿．ノ～6s．ノi㌶s渉．，vol．64， Japan． p．133－147． Hussong，D．M．and Fryer，P．（1983） Back－arc Anderson，R．N．（1975）HeatflowintheMariana seamounts and the SeaMARC II seafloor Trough．∫．080ρh夕s．1～6s．，vol．80，p．4043－ mapping system．EOS Tz朋s．，vo1．64，p． 4048． 627－632． Bibee，L．D．，Shor Jr．，G．G．and Lu，R．S．（1980） and Uyeda，S．（1982） Tectonic proces－ Inter－arc spreading in the Mariana ses and the history of the Mariana Arc：a Trough．ル勉7．G601．，voL35，p．183－197． synthesis of the results of deep－sea dri1－ Clague，D．A．and Jarrard，R．D．（1973） Tertiary ling project Leg60．1％Hussong，D．M．， Paciflc plate motion deduced from the Uyeda，S．6孟召1．，動髭．R勿お．五）SZ）P，voL60， Hawaiian －Emperor chain．G601．So6． Washington（U．S．Govt．Printing Office）， ノ1錫z6飢Bz611．，vo1．84，p．1135－1154． P．909－929． Craig，H．，Horibe，Y．，Farley，K。A．，Welhan，J．A．， IAGA Division I Working Group1（1985）Inter－ Kim，K．一R．and Hey，R．N． （1987） national Geomagnetic Reference Field Hydrothermal vents in the Mariana revision1985． ∫．G60万zごzg．G606166云67．，vol． Trough：results of the first A玉vin dives． 37，p．1157－1163． EOS T名ヒz％s．，vo1．68，P．1531。 Ikari，K and Nishimura，A．（1991） Geologic his－ Fryer，P．and Hussong，D．M．（1982）Seafloor tory of the Tenpo Seamomt of the Nishi－ spreading in the Mariana Trough：results Shichito Ridge，the Izu－Bonin Arc． β％ll． of Leg60drilling site selection surveys．五％ G601． S％7∂．ノ41）ご〃z，vol．42，P．19－41 （in Hussong，D．M．，Uyeda，S．6渉召」．，動露． Japanese with English abstract）． 1～6郷．．OSZ）P，vo1．60，Washington （U．S． Ikeda，Y。and Yuasa，M．（1989）Volcanism in Govt．Printing Office），p．71－74． nascent back－arc basins behind the Shi－ ，Taylor，B．，Langmuir，C．H．and Hoch－ chito Ridge and adjacent areas in the Izu－ staedter，A．G． （1990） Petrology and Ogasawara arc，northwest Pacific：evi－ geochemistry of lavas from the Sumisu dence for mixing between E－type MORM and Torishima backarc rifts。勘πh and island arc magmas at the initiation of P旋z7z6ムS6♂．五6あ．，vo1．100，p．161－178． back－arc rifting． Con渉フ4δ．ノ砿7％6名召1．P召か Harland，W．B．，Cox，A．V。，Llewellyn，P．G．，Pick－ 701．，vol．101，P．377－393． ton，C．A。G．，Smith，A。G．and Walters，R． Isezaki，N。，Inokuchi，H．，Ishikawa，H．，Ichikawa， （1982） 、4G60Jogづ6孟珈6so漉，Cambridge H．， Takahashi， K．， Inoue， Y． and Univ。Press，131p． Sugimoto，K．（1989） Report on DELP Haston，R．and Fuller，M．（1991）Paleomagnetic 1987cruises in the Ogasawara area Part

一683一 Bz61」6渉勿z6ゾ孟h6 060108」αzl Sz67∂6ッ6ゾノ功）α％，Vbl．42，No．12

V．Measurement of three components 302，P．55－58． and total intensity of the geomagnetic ，Fox，P．J．，Perram，L．」．，Eisen，M．F．， field in the Ogasawara Trough． B％ll． Haymon，R．M．，Miller，S．P．，Carbotte，S． Eごzπh（1．ノ～6s．乃zs渉，，vol．64，P．179－222， M．，Cormier，M．一H．and Shor，A．N．（1988） Ishihara，T．，Okamura，Y．and Nakamura，K． A new view of the mid－ocean ridge from （1981） Geomagnetic anomalies north－ behaviour of ridge－axis discontinuities． east of Hachijojima Island．G601．Sκ7∂， 皿z云％名6，vo1．335，P．217－225． ノ纏）ごz7z C鰯乞s6ノ～67）渉．，no．16，P．25－26． McCabe，R．and Uyeda，S．（1983）Hypothetical Karig，D．E。 （1971） Structural history of the modelforthebendingoftheMarianaArc． Mariana Island Arc system．（｝601。So6． InThの鰯o競研4960Jo9比6∂o嬬加げ Soz諾h6硲！∠しsづ（z多z s6召sζz％4寛んz％4s．●Rz7云2， ノ1ηz6名Bz611．，vol．82，P．323－344． and Moore，G．F．（1975）Tectonic com－ edited by Hayes， D．E．， Geophysical plexities in the Bonin Arc system． Monograph27，American Geophysical 7166ホo多zo1）h』ソsづos，vol．27，P．97－118． Union，p．281－293． ，Anderson，R．N．and Bibee，L．D．（1978） Miyazaki，T．，Murakami，F．，Nishimura，K．and Characteristics of back arc spreading in Ishihara，T．（1981） Geomagnetic survey． the Mariana Trough．∫．G60少h夕s．R6s．， 0601．Sz67∂．ノ41）召7z Cn薇s6ノ～4瑳．，no．14，P．79 vol．83，P．1213－1226． －82． Kodama，K．，Keating，B．H．and Helsley，C．E． Moore，W．S．andStakes，D．（1990〉Agesofbarite （1983） Paleomagnetism of the Bonin －sulfide chimney from the Mariana Islands and its tectonic significance． Trough．Eσπh P伽％6云．S6づ．L6甜．，vo1．100， 7セo孟oη01》h夕sづos，vo1．95，P．25－42． p．265－274， Konishi， K． （1985） Cretaceous reefal fossils Mrozowski，C．L．and Hayes，D．E．（1979） The dredged from seamounts of the Ogas－ evolution of the Parece Vela Basin，east－ awara Plateau．In P名召12彫初αη7のoπ6ゾ ern PhilipPine Sea． Eσπh P彪％6云． S6づ．

孟h6 研zたz6ho ノ吻7％ on掘s6 畑4－1，edited ．乙6材．，vol．46，P．49－67． ．，1 by K．Kobayashi，Ocean Research Insti－ Murakami，F．，Yamazaki，T．，Saito，E．and Yuasa， tute，University of Tokyo，p．169－180． M。（1989）Rifting and spreading proces－ Kuno，H．（1962） Catalogue of the active vo1－ ses at the northem Mariana Trough． canoes of the world including solfatara EOS T名とz％s．，vol．70，p．1313． field，part XI（Japan，Taiwan and Mar－ Nagaoka，S．（1990） Volcanic geology of the ianas）． Intemational Association of Mokuyo and Doyo Seamounts in the Shi－ Volcanology，Roma，332p． chiyo Seamount Chain，Izu－Ogasawara Leg126Scientific Drilling Pa枕y（1989） ODP Leg Arc．且6s渉瓶o孟s（ゾヵの6鴬，孟h67孟h sッ彫ρo－ 126drills the Izu－Bonin arc． G60あ解6s， sJz6％z o％ 名召sαzz6h6s z偲ぢ％8 孟h6 sz6δ盟z6欝狛16 vo1．34，P．36－38， Sh初た厩2000，p．74－77（in Japanese）． Lonsdale，P．（1983） Overlapping rift zones at the ，Okino， K． and Kato，S． （1991） Land－ 5．5。SoffsetoftheEastPacificRise．ノ1． forms of submarine volcanoes in central G60φh夕s．ノ～6s．，vo1．88，P．9393－9406． part of the Izu－Ogasawara Arc by multi－ Louden，K．E．（1977） Paleomagnetism of DSDP beam sounding system．ノ～砂オ．鈎み09名8－ sediments，phase shifting of magnetic 1）hあ ノ～6s．，vo1。27，p．145－172 （in Japanese anomalies，and rotation of the West with English abstract）． Philippine Basin． 1．G60ρh夕s．R6s．，vol． Nemoto，K．，Shiba，M．，Ogawa，H．，Ito，H．，Izu，S．， 82，p．2989－3002。 Sato，T．and Shibasaki，T．（1986） Sub－ Macdonald，K．C．（1986）The crest of the Mid－ marine topography and geology of the Atlantic Ridge：Models for crustal gener－ Ogasawara Sea－plateau．In z4 s甥ψo－ ation processes and tectonics． In－Th6 s勿∫盟z o多z” ∠）61）名召ssづoアzα％4z4）1猛” ，edited 960Jo8：ソ （ゾ〈b7孟h／1ηz67彦αz， V「ol．ノ砿 edited by Fijita，Y．6∫ごzl．，Preparation office for by Vogt，P．R．and Tucholke，B．E．，Geo－ “Research center of geoscience，” P．131－ logical Society of America，p，51－68． 151（in Japanese with English abstract）， and Fox， P．J． （1983） OverlapPing Nishimura，A．，Rodolfo，K．，Koizumi，A．，Gill，J． spreading centres：new accretion geome－ and Fujioka，K，（1991） Episodic deposi－ try on the East Pacific Rise．皿z渉z6名召，vo1． tion of Pliocene－Pleistocene pumice

一684一

菱 ル五zgn6云」6 召7zo澱zlづ6s α〆 孟h6 12z6一ノ倣z7乞ごz％6z／176s ピT． Hz％祝z2ζzたづ 6！ごzl．ノ

deposits of Izu－Bonin Arc，Leg．126。 the geology and petrology of Idu and the 0∫）P S6づ（ηz渉弱’o石～6sz61孟s，vol．126，in press． southern Is1’and． Bz61乙 Eαπh（1．ノ～6s．乃zsよ Okamura，Y．，Murakami，F．，Nishimura，A．and To勿o∫物）．U勉∂．，vo1．15，p．215－357． Hiscott，R．（1991） Late Quaternary sed－ Urabe，T．and Kusakabe，M．（1990）Barite silica imentation and faulting in the Sumisu chimneys from the Sumisu Rift，Izu－Bonin Rift。 ノ：．（3609名とz1）hグ，vo1．100，P．464－474，（in Arcl possible analog to hematitic chert Japanese with English abstract）． associated with Kuroko deposits．Eα7孟h Oshima，S．，Kaneko，T．，Onodera，K．，Nakagawa， P彪％α．S6乞．，乙α渉．，vo1．100，P．283－290． H．，Tozaki，T。，Mishina，M．and Ossaka，J． Vacquier， V． and Uyeda， S． （1967） （1982） Results of geomagnetic survey Palaeomagnetism of nine seamomts in and magnetic properties of dredged rocks the westem Pacific and of three volcanoes at sea aromd Iwo－zima．ノ～の孟．功み〇一 in Japan。 Bz611．Eごzπhg．ノ～（3s，Z箆s渉．，vo1，45， 9名とz1）hあ ノ～6s．， vol． 17， P． 289－315 （in p．815－848． Japanese with English abstract）． Volpe，A．M。，Macdouga11，J．D．，Lungmuir，G。W．，

ン り ヲ ヲ ン Hawkins，J．W．and Lonsdale，P，（1990） and （1985） Fine－scale isotopic variation in Mariana Geomagnetic anomaly distribution of Iwo Trough basalts： evidence for heter・ －zima。∫．G608’観ゆhy，voL94，p．100－113 ogeneity and a recycled component in （in Japanese with English abstract）， backarc basin mantle．勘7云h PJ碗6云．S6づ． Sempere，」．一C．，Macdonald，K．C．and Miller，S，P． L6泥．，vo1．100，p．251－264． （1984） Overlapping spreading centers：3－ Yamazaki，T．，Murakami，F．，Yuasa，M．and D inversion of the magnetic field at9。03’N Iizasa，K． （1991） Volcanism and on the East Pacific Rise． G60ρh：ys．∫．R． hydrothermal activity in the Aogashima 薦渉名Soo．，vol．79，P。799－811． Rift， northem Izu－Ogasawara Arc． ，Meshkov，A．，Thommeret，M．and Mac－ JAMSTECTR Deep Sea Research（Pro－ donald，K．（1988）Magnetic properties of ceeding ofthe7th symposium on deep－sea some young basalts from the East Pacific research using the submersible”SHIN－ Rise． ノ協z7．G601）h』ソs．ノ～6s．，vo1．9，P，131－ KAI2000”system），vo1．7，p．105－114（in 146． Japanese with English abstract）， Seno，T．and Maruyama，S．（1984） Paleogeogra－ ， ， and Nishimura，A． phic reconstruction and origin of the （1988a） Geological stmcture，heat flow PhilipPine Sea． T66云o％oρhysガ6s，vo1．102， and magnetic anomalies around the p．53－84． Sofugan Tectonic Line，Izu－Ogasawara Shiba，M。（1979）Geological history of the Yabe Arc－Is the Ogasawara Trough Paleo－ guyot to the east of the Ogasawara rift ？ ∠4わs伽とzo云s（ゾ孟h61）ζψ6zs（ゾ孟h61988 Islands．1．G601．So6．ノ41）ごzn，vo1．85，P。209－ s1）万％80％z66あ％86ゾ診h6S6ゑsηzolo9づαzl So6ガ（3砂 220（in Japanese with English abstract）． ρブノ4μz％，p．146 （in Japanese）． Smoot，N．C．（1983） Ogasawara Plateau：multi－ ， ，Saito，E．and Yuasa，M．（1988 beam sonar bathymetry and possible b）Opening of the northem Mariana tectonic implications，1．G60Jo劇，voL91， Trough． In T66ホonづos （ゾ Eαs渉6規 ∠4s毎 p．591－598． 礁4恥s渉67n勘6舜OoCo励n6吻1物顧％ Taylor，B．，Brown，G．，Fryer，P。，Gi11，」．B．，Hoch－ （Abstracts of the papers of the l988DELP staedter，A．G．，Hotta，H．，Langmuir，C．H．， Tokyo Intemational Symposium），p．25－ Leinen，M．，Nishimura，A．and Urabe，T． 26． （1990） ALVIN－SeaBeam studies of the Yuasa，M．（1985）Sofugan Tectonic Line，a new Sumisu Rift，Izu－Bonin arc。Eα7渉h tectonic boundary separating northern P々z多z（3∫．S6づ．L6だ．，vol．100，P．127－147． and southem parts of the Ogasawara Tsunakawa，H．（1983） K－Ar dating on volcanic （Bonin） Arc， Northwest Pacific． In rocks in the Bonin Islands and its tectonic Fo7彫ごzガo％（ゾ’ごzoガ∂6006αアz窺ごz堰卸zs，edited implication。T60渉onoρh卿os，voL95，p．221 by N。Nasuασ」．，TERRAPUB，Tokyo，p． －232． 483－496． Tsuya，H．（1937） On the volcanism of the Huzi （1991a） The reason for the differences volcanic zone，with special reference to in geologic phenomena between northem

一685一 羅

Bz6〃6ガ％（ゾオh6G60Jo9づαzl Sz6フ・zノβy6ゾノ4ρ召勉，Vマ）乙422Vo．12

and southem parts of the Izu－Ogasawara Shinkai2000． ノ1δsケとz6孟s（ゾ孟h6ヵ6z1）6鴬 （ゾ Arc．∫G60g名砂h夕，vol．100，p。458－463．（in 云h6 8孟h Sh初hごzづ s：y刀勿osづz6窺，p．69－70 （in Japanese with English abstract）． Japanese）． Yuasa，M．（in prep．）Origin of the along arc ，Uchiumi，S．，Nishimura，A．and Shibata， variations of geologic phenomena on the K． （1988） K－Ar age of a forearc volcanic front，Izu－Ogasawara Arc． seamount adjacent to the volcan圭c front of B％1よG60乙S％7∂．血釦％． the Izu－Ogasawara Arc．B％1乙7016催o乙 （1991b） Submarine caldera in the So（乙 ノ41）召銘， vo1． 33， p． 352－353 （in northern part of the Shichito Ridge： Japanese）． Observation of the Myojin Knoll Using

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）

一686一