PaleomagnetismoftheNijoGroupanditsimpliclltionfor ......

13
あz//77❹げ訂加砂ど7石齢T7/剛ど/八ヤθ加加Γα/又治7c佩レθ加哨?9j,/7曜yj?θj-2/j,2θθθ PaleomagnetismoftheNijoGroupanditsimpliclltionfor thetimingofclockwiserotationofsouthwestJapan HiroyukiHosHI*,DaisukeTANAKA*,MasakiTAKAHAsHI** andToshiyukiYosHIKハwA*** *Z)印α・‘陶le/jlげ£丿幼り/1,Jleμ/α/£α,・訪Sc1?,ICa/1か/1/ひがye,ざめノザ £ど/z/c詞必/7,//7み・併剛ぼ7,瓦ど7/りひ,j趾加44Q **A/ダy,7ど/77/ど7,❹凡❹/?び(フzμで6Z:)印αだ/77e砿G四かがcど7/ぶ/z・P砂げjど7ρど7/?, 7でyl涜面ど7,治山口肩j∂j-&沁乙ゐ/7ど7/7 ***Geぴかり・/J)ど/7山ソ/7肥/?/,(λ?θ/θがcα/,SにP砂 ザjど7ρど7/7, 7で叉/ん❼7ど7,治ど7,77肩jθj々?励Zjど7/7ど7/・ Ar〕ale(:)magneticstudywasconductedfol・weH-datedゝ/olcani(2rockst71・omtheMiddleMioce)leNijoGroup intheMt.NijoaE1、southwest(SWリapan.5;1lmpleswerecollectedn・om33sites.Detailed5;tc?pwise de111agnetizationwithbothahemating-lieldandthermaltecllniqu(?sl・ev❼edwel卜(let(?rnlined5;ite一mean directions斤om29sites.’I’Htc()rrectionwasappli(;ableto13sitcj5;、yieldinganoveraHnlean(lil-ectionoFZ)= 352ご、/=51.7≒and匈5=7.fandapalc?()1Tlagneticpoleat84.3°N、35.5sE、and玉5=8.5へThebedcling-tilt testpassedwith95%probability.Althoughther{ごN/ersaltestwa5;11egativejhep14esenceoFd皿lpolと11-itiesand astudyoFangularstandarddeviationorvil・tualgeomagneticpolesinlplythattheovel・allnleandil-ection 11N/eragesout、toa(;ertainextentバhepastgeomagneticsaularx/m・iation.Magneticpo】arity白・atigraphy cl(うtel’nline・sthe昭eoFtheNijoGI’oup;the5/oLlngeragelimitis14.6Ma(toporpolaritygbchronC5ADr) andtheoMeral;elimitis15.2Ma(base()1`polaritysubdlronCj5Bn.2n).ComparisonoFthelニ)aleolmgnetic polewiththoseFromtheAsian(;olltinent5;howsnol-otationintheMt.NiJoareasince14.6MaJuggesting thatSWJapanhasnotc?xpc?l`iencedl・otationsincethattime.We(2011cludethatthedock、、・isり・otationoF SW.lapanバhoughttohaveo(;curl・edinMiocenetimeinl・ejationtothe017)eningoFtheJapan5;ea、had llh・eadyendedby14.6Ma.Twoclo(iwise-(jeflecteddirectionsonheMiddleMiocenerocks(Muro l:)yro(21asticFlowDepositandKunlanoAcidicRocks)intheKiiP(?ninsulaal・eprobablyinst21ntaneous recol・dingsoFageonmgneticneld(2)(cursionortransition. lntroduction The、JapanArc(?onsistspjmarilyoftwol・iaedlithos- r)hericf71-と11gments、nowthebasementofsouthwest(SW) andn()】-theast(NE)Japan、withaba(うk-arcseとlnamed theJapanSea(Fig.I).Usingpale()lmlgneticdata、 Kawaieta1.(1961)firstd泌ussedthef()mlationorthe bow-shapedJapanArcandconsideredtha丿tresLIlted 斤omthebendingoFaninitiallystraightlithospllel・ic s;liverinMesozoicorearlyTertiarytime.5Subscう(luently、 11bLmdantpaleonmgneticstlJdies、e?5;IDeciaHy斤omCeno- }-LHoshi,星博幸,hoshi@aueccJichi-edu.ac.JpCorre- sr)olldil瑶aLlthor D.’I≒1naka,田中大介,to一(klisuke(??j`I)op2Lo(ln.ne.jp M.T11kallashi,高橋雅紀,rllasaki@:gsJ・goJp T.Yosikawa,吉川敏之,}/o,;hikilw@=1;sj・go・Jp z()k】・ocksorSW,lapanjlave11111 aveledthedefomlatio】1 1)1`()(jessesonheJapanArc(Oto朗i,1996).Basedon (mshoreandotlミshoredataJ(2veralrevisedmodc?hhave beenp)-oposedtorthea)rmationoFtheJapanArc- JapanSeい;ystem(Otot1リletaL,1985;Hayashidaetal., 1991リolivetetal。1995)。 Dc?(;linations斤omSWJapanare5;ulnmarizedin F2igurel.Aneal・lyconふtentclo(;kwisedc?11ection marksLate(:フ1・etaceoustoPale,ogenedecHnations(100to 24Ma).TheclockwiserotationoFSWhpanafler F)Jc・ogenetimecausedthisddection(OtoFuJiand M£1tsuda,1987).Coevalde(jinationsneal-theTsu- shima-GotoTe?ctonicLine(TGTL)a】-enoはh❼yand north-northw鋤匹芒ly,whichnlighthavebeen(;ausedby plasticorcluasi-1ニ)lastic(2rustaldeformationduringrota- tionatthewestern111al・ginoFSWJapan(lsllikawa, 1997).lncjontl’astバhen刈orityoFMiddleMiocene

Transcript of PaleomagnetismoftheNijoGroupanditsimpliclltionfor ......

あz//77❹げ訂加砂ど7石齢T7/剛ど/八ヤθ加加Γα/又治7c佩レθ加哨?9j,/7曜yj?θj-2/j,2θθθ

PaleomagnetismoftheNijoGroupanditsimpliclltionfor

thetimingofclockwiserotationofsouthwestJapan

HiroyukiHosHI*,DaisukeTANAKA*,MasakiTAKAHAsHI**

andToshiyukiYosHIKハwA***

*Z)印α・‘陶le/jlげ£丿幼り/1,Jleμ/α/£α,・訪Sc1?,ICa/1か/1/ひがye,ざめノザ

£ど/z/c詞必/7,//7み・併剛ぼ7,瓦ど7/りひ,j趾加44Q**A/ダy,7ど/77/ど7,❹凡❹/?び(フzμで6Z:)印αだ/77e砿G四かがcど7/ぶ/z・P砂げjど7ρど7/?,

7でyl涜面ど7,治山口肩j∂j-&沁乙ゐ/7ど7/7***Geぴかり・/J)ど/7山ソ/7肥/?/,(λ?θ/θがcα/,SにP砂’ザjど7ρど7/7,

7で叉/ん❼7ど7,治ど7,77肩jθj々 ?励Zjど7/7ど7/・

Ar〕ale(:)magneticstudywasconductedfol・weH-datedゝ/olcani(2rockst71・omtheMiddleMioce)leNijoGroup

intheMt.NijoaE1、southwest(SWリapan.5;1lmpleswerecollectedn・om33sites.Detailed5;tc?pwise

de111agnetizationwithbothahemating-lieldandthermaltecllniqu(?sl・ev❼edwel卜(let(?rnlined5;ite一mean

directions斤om29sites.’I’Htc()rrectionwasappli(;ableto13sitcj5;、yieldinganoveraHnlean(lil-ectionoFZ)=

352ご、/=51.7≒and匈5=7.fandapalc?()1Tlagneticpoleat84.3°N、35.5sE、and玉5=8.5へThebedcling-tilt

testpassedwith95%probability.Althoughther{ごN/ersaltestwa5;11egativejhep14esenceoFd皿lpolと11-itiesand

astudyoFangularstandarddeviationorvil・tualgeomagneticpolesinlplythattheovel・allnleandil-ection

11N/eragesout、toa(;ertainextentバhepastgeomagneticsaularx/m・iation.Magneticpo】arity白・atigraphy

cl(うtel’nline・sthe昭eoFtheNijoGI’oup;the5/oLlngeragelimitis14.6Ma(toporpolaritygbchronC5ADr)

andtheoMeral;elimitis15.2Ma(base()1`polaritysubdlronCj5Bn.2n).ComparisonoFthelニ)aleolmgnetic

polewiththoseFromtheAsian(;olltinent5;howsnol-otationintheMt.NiJoareasince14.6MaJuggestingthatSWJapanhasnotc?xpc?l`iencedl・otationsincethattime.We(2011cludethatthedock、、・isり・otationoF

SW.lapanバhoughttohaveo(;curl・edinMiocenetimeinl・ejationtothe017)eningoFtheJapan5;ea、had

llh・eadyendedby14.6Ma.Twoclo(iwise-(jeflecteddirectionsonheMiddleMiocenerocks(Muro

l:)yro(21asticFlowDepositandKunlanoAcidicRocks)intheKiiP(?ninsulaal・eprobablyinst21ntaneous

recol・dingsoFageonmgneticneld(2)(cursionortransition.

lntroduction

The、JapanArc(?onsistspjmarilyoftwol・iaedlithos-

r)hericf71-と11gments、nowthebasementofsouthwest(SW)andn()】-theast(NE)Japan、withaba(うk-arcseとlnamed

theJapanSea(Fig.I).Usingpale()lmlgneticdata、

Kawaieta1.(1961)firstd泌ussedthef()mlationorthe

bow-shapedJapanArcandconsideredtha丿tresLIlted

斤omthebendingoFaninitiallystraightlithospllel・ic

s;liverinMesozoicorearlyTertiarytime.5Subscう(luently、

11bLmdantpaleonmgneticstlJdies、e?5;IDeciaHy斤omCeno-

}-LHoshi,星博幸,hoshi@aueccJichi-edu.ac.JpCorre-

sr)olldil瑶aLlthor

D.’I≒1naka,田中大介,to一(klisuke(??j` I)op2Lo(ln.ne.jp

M.T11kallashi,高橋雅紀,rllasaki@:gsJ・goJp

T.Yosikawa,吉川敏之,}/o,;hikilw@=1;sj・go・Jp

z()k】・ocksorSW,lapanjlave11111’aveledthedefomlatio】1

1)1` ()(jessesonheJapanArc(Oto朗i,1996).Basedon(mshoreandotlミshoredataJ(2veralrevisedmodc?hhave

beenp)-oposedtorthea)rmationoFtheJapanArc-

JapanSeい;ystem(Otot1リletaL,1985;Hayashidaetal.,

1991リolivetetal。1995)。

Dc?(;linations斤omSWJapanare5;ulnmarizedin

F2igurel.Aneal・lyconふtentclo(;kwisedc?11ection

marksLate(:フ1・etaceoustoPale,ogenedecHnations(100to

24Ma).TheclockwiserotationoFSWhpanafler

F)Jc・ogenetimecausedthisddection(OtoFuJiand

M£1tsuda,1987).Coevalde(jinationsneal-theTsu-

shima-GotoTe?ctonicLine(TGTL)a】-enoはh❼yand

north-northw鋤匹芒ly,whichnlighthavebeen(;ausedby

plasticorcluasi-1ニ)lastic(2rustaldeformationduringrota-tionatthewestern111al・ginoFSWJapan(lsllikawa,

1997).lncjontl’astバhen刈orityoFMiddleMiocene

WenTleasuredthepaleoma!111etisnTlofvo]canicrocks

n・omtheMiddleMioceneNuoGroupintheMt.NiJO

areaatthefootortheKiiPe面lsulバFig.l).TheNりo

GroupissuitableR)rexaminingthetimingoFtheclock-

wisel・otation,t)ei(ごause,nrstly,ithasasequence()1` many

volcanicrockunits,andsecondlyjthasbeenwelldated

throughK-Arand函sion-track(FT)methods.Sev-

(?1’と111・ese£1rdlershavestudiedthepaleonlaS;netismorthe

group(Katoetal.,1971;To哺丿983;Nakamuraeta1。1998).Torii(19S33)reportedsev(・nsite・-meandirections

chと11-acterizedbylittledeUecteddeclinationwithdua1

1:)()larities.Katoetal.(1971)andNakanluraetal.

【】998)conductedpaleomagnetic5;tudieswithvo1-canologica1柚terestinordertoexamineemplacement

conditionsofPyrodasticnows.’Fheser)14eviousstudies

suggestlittlelTotationintheMt.Nりoarea,butthe

5scarcityoFdataledustor’e?examinethegroupf7rom

204

40°N。,・

j へ X L

; y 「

30°N

1 3 0 ° E

/ ́ ン、..// ́

|-liroyukiHoshi、DaisukeTanaka、MasakiTi(ahashiandToshiyukiYoshikawa

: X

140cE、・

ダ/、/、.

l y ‘ I

l y‘ ’ 1 1 1

. - ノ / ゛ ’ J

. / ” ‘ ’ ‘

PノX[]μニ゜/CO眉脳彗Late(jet23ceous十二▽\yく十‾/toPと3k?o9ene

declinations(16to日Ma)arer)11raHelor・;ubp11ra】lelto

themel・idia111Radiometl・icandstratigraphicmethods

datenlostoftheselongitudinaldil・ectionsataboutl5

Maoryounger.TheMuroPyroclasticFlowDeposit

(MuroPFD)andKumanoAcidicRocks(KARs),bothintheKiiPe?ninsula,al・econspicuous化)rtheir(うasterly

directions.Theyhavelongbeenconsideredtohave

resulted斤omtherapidclockwiserotationoFSWJapan

lltllboutl5Ma(Taganli,1982;TOI’H,1983).Re・cently,

however,Pota∬ium-Argon(K-Ar)and‘loAr/39Ar

studiesgaver)1’ecisedatesorabout14.3MatotheMuro

PFDandKARs(Utoeta!.,1996,1997;yII-niieta1.

1998).13c?causethee?2lst❼ydirectionss;e?emedtohave

r)(jst(latedthe1()rlgittldinalones,Utoetlll.(班皿)andS;1lrlliietaL(19913)claim(2dthatthedockwisel・otationof

SNVJapantookplaceaaer15Majeavingmanylongitu-

dinaldirectionsoutoFconsideration.

34°N

PEβ,///り台む/tj・;エヤc`s=。\

△ト。上上1。万エエ万万F)aleorllagjrleticDiection

\\∧>ニダニ:。j:j/yMiddleMiocene

Figurel.Maporso1Jth,vesljapa】1函owillgl()cmi〔〕northestudyarea(Mt.刈joarea).AITow哨11dicatepaleo111agneticde(jinalionsor

LateCma(ごeoustoPal(2ogenealldMiddleMioceneages.DataFI-omozimaet111.(1968),Tagal11ロ1982),Tc,削(1983),TOI・iiand

lsllikawl1(1986),ToriiandTsurudome(1987),OtofiりiandMatsu(la(1987),OtoruJietal.(199し1999)jshikiawaet11L(1989),lshikawa

andTagallli(1991tlshika,va(1997),KodamE1(1990),alldNど1k11JimaetaL(1990).R・・vel` sed-r,olal・itydil・ec6onshavebeenhlverted

tothenormalpolal・ity.Mul・oPFD,MuroPyroclastic日owDeposit.’I’11eFOIlowil瑶11121Joralultsal・eshown:ATL,AkaishiTc2ctonic

Line;BTL,ButsuzoTectonicLine;ISTL,ltoigawa-ShizuokaTectonicLine;MTL,MedianTect()nicLine;TGTL,TRlshilm1-Goto

TectonicLine.

Paleom£11;netismonheNりoGI・oup

t{2ctonicandmaglleto!;tl‘atigraphicpointsorview

Geologicsetting

SouthwestJapanhaslongbeenaplacewhereigneous

actlvltyoccursml4elatlontocontlne】lta卜oceanlcplate

convel` gence.AtthebelginningoFtheMiddleMiocene

(aboutl6-15Ma)バ1volcanic斤ont吋gratedl’apidlytrenchward,producingigneousrocksinthepFesent帛re-

arcl’egion.Thel・c?sultantMiddleMiocenevolcanic

rocks,c?sl)eciallyinandaroundtheinlandshallowsea

betweenHonshuandShikoku(Fil;.|),are(Jcjllectively

calledtheSetouchivolcanicRocks.Theyarechar-

と1(2terizedbytheoccuITence()fhigh-Mg皿desite(Ta-

tsumi,1983),aswellasbyaITcllativelyshortperiodor

activityin16-13Ma(Yo涵ikawa,1997),volcanic

l4ocksintheMt.NijoareaaretyPicaloFtheSeitouchi

volcanicRocks(M()rimotoetaL,1953).

ThegeologyoFthestudyareaconlr)risesvarious

x/olcanicandclasticl・ocksoFMiddleMiocene昭e,which

・29

\j?55”、\

2∂ /’TERAYAMA▲

24

725°26

\几

2 0

〃 - _

J ・

〃 r r - 一 j - ‥ 〃 r 「

205

cropoutextellivelyandarenamedtheNりoGroup

(M(3rimotoeta1.,1953).Therocksareunconfi:)l-l-nablyoverlyil瑶orinfaultcontactwiththepre-Miocene

basement(F7ig.2).ThebasementrocksconsistoF

!;1’anitoidsandh汝h-tempel’aturemaarnorphicrocksoFtheRyokeBelt.Thegroup沁intumcoveredbypoorly

consolidatedclasticdepositsoFtheupperPlioceneto

PleistoceneosakaGroup.

lo」o忿z

xmjl

UIレレ↑ト1 一

113403

/Syncline

Fault

Flexure

Samplingsite

…………………………IJogashiroFormation

丿……………ム。。.HarakawaFormation・ヽ=・Aphyricandesite

圖|-(sanukitoid)

宍]]]JDonzuruboFormation

二]]Pre-TertiaryBasement………2oごIS12Mstrikeofweldin9

・2oDipandstrjkeofstrata

Terrace,backmarsh,

&landslidedeposits

OsakaGroup

EXPLANATION

Lithostratigraphy

TheNijoGroupiscomposedoflllreefbl-n!ations,the

Donzurubo,H211-akawa,andJopsl柚’oFomlationsうn

as(relldingorder(Fig.3).Morimotoeta1.(1953),

I` JヽiJosanReseど1rchGroup(1986),andMiyachieta1.

(195)8)havedo(Jnlentedthecletailedstratigl・aphy。TheDonzuruboFormation,morethan750mthick

atMt.Nijo,(;()nsistsmai❹yoFllsictohltermediate

lavasandl)yr()(;1asticrocks.0(XLll・renceofgamet

pheno(2rystsch211・acterizesthisfol-nlation.Thelowest

………□………ゾルルプ回|

□………-J………・二 卜 半 三 矢 冊 卜

\TAISHI

参宮才171gure2.G・1ogicm叩ortheMt.NiJo訂(la,simpliliedandparllynlo(lifiedl`1・omMiyachietaL(1998).Lo㎝tionsofpaleomagnelic

sl1111plil瑶sitesareshownbydots.

206 Hil-oyukiHoshi、DaisukeTanaka.MasakiTakahashia】ldToshiyukiYosllikawa

Lithostrati9raphy≒cAge(Ma)&geomagneticpolaritytimescale

1 0 1 1 1 2 1 3 1 4 1 5 1 6

こ C 5 C 5 A C 5 A C C 5 A D l n 2 n C 5 B C 5 C-

EU-

2●W

ICて10月

Biotjterhyolitetodacitetuff

(TamateyamaTuff)- - - 一 一 - 一 一 一 - 一 一 一 一 - - - - - - -

Con9lomerate一 一 一 一 一 - - ~ - - - - 一 一 - - 一 一 - 一 一

Biotiteorthopyroxenedacite

げerayamavolcanics)

l a n よ ( 。 H M -

1 4 . 8 士 1 . 0 M a ゛ 2 匹

(mean:14.9士0.6Ma)::? ワ

→ 一一 : : 1 5 1 + 0 3 M a ゛ 3

CO●〃4一一J(て5

EL_OL1.

(X53(て3ヱ応

(て3工

0livineclinopyroxenebasalt

_ 宍 i 恍 叩 五 守 ( 悦 芒 良 ‥ ‥ ‥ _ _(⊃livjneorthopyroxeneclinopyroxene

一リり凱(g)際り5)戸|(?2りし‥一一,

にIMけodakevolcanics)

jlラU` l(Kashiwaminevolc・)

IS老I(lshjmakurivojcj

l言言にKasugayamavolc.)

_ _ ‥ ‥ _ E _ _ ] 芒 ( 惣 ご ? ヤ I 竺

Con9lomerate,sElndstone&mudstone

| 大. 8 ± 0 , 3 M a ’ 3

†1.9±0.2Ma’3

15.8士1.4Ma゙ 1

1 3 . 0 士 o 7 こ て ‾ ● ‾ ‾ ‾ ‘ →

゛ ‾ ● 7 . 8 ± 0 . 2 M a ’ 3 1 3 . 8 ± 1 . 6 M a ゛ 1

L,

EL」_○jココ⊃N⊂○□

IGametbiotiterhyolite右lvc)lcarliclasticrocks

万 言 参 席 し 二 二

]宍笑竺片端!yFやうク!一白.

硝 石 j j 白 玉 ] - - - … -1㈲hikiribavolcanics)

干ご二二偏二二こ心‾よ‾‾

|」 ●K-Arage

■Fission-trackage

*IHoshielal、(1999)

421wanoandDanhara(1999)*3Yoshikawa(1997〉*4Tatsumietal・(1980)*5Kawanoandueda(1964)

*6Nakamuraetal.(1998)

→ -15.4±0.3Ma゛3

1 6 M y 5 ●

15.5±1.0Ma゛1ふ

Figure3.5;tlnlma】’yorth(!litllostl・atil-叩hyandr2ldi(3111etl・icdatesortheNijo(3roup.Thelithostl・atigrapllyisbased011Mc)rimotoet

11L(1953),NiJosanRe,seardlGI・our,(1986)バ1ndMiyachietaL(1998).Ageerl‘Orsal’eshowllat2び1eve1.Theweightedmeandate

oFtheTam趾防amaTu汀(14.9±0.6Ma)hast〕elencjculatedbytheauthorsbasedc311theいvol・c,porteddates.Thegeom昭netic叩lal-ity

timescakisaaerCandeandKe,ntり995).

pall,|^ lamedtheLowerDonzuruboF(:)11Tlation[Mo-rimotoet乱]953),comprisesd(?nselyweldedrhyolitic

lapiHituぼ,inwhicht?utaxitictextul・eisde雨nitely

ident雨ed.Thetc?xtureshowsasteepnc)】-thwal’ddip

n圃rthecontactwiththebおenlent.Ur)sectionうnteIT-

mediatevolcanicrocksdonlinatethemiddlepartoFthis

f1)1・nlation(MiddleDonzuruboFc,m!ε1tion).Thevo1-

(ごanicsincludegametbiotitendesite(ls1111ribavo1-

(2anics)andき1met-bearinghornblendeorthopyroxene

lllldesitenows(Hatavokallics).Theupperpart

(UpperDonzuruboFomlation)consistsnlainlyoFgar-

netbiotiterhyolite(Medakevoklnics)andf)yrodastic

nowdeposits。

TheトlarakawaFomlatjonisnlorethan200m

thickatMyojn’yam4conforlTlablyov❼iestheDonzu-

ruboF(:)rmation,andconsistsnlainlyof` hltel゙ mediateto

mancvolcanicrocks.val-iousthic1くnessesoFclastic

rockssuchascongl()lnerate,ls21ndstone,andmudstone

occupyitsbasalpa】tFossildiatonls臨(licatetheir

depositioninafTI゛eshwaterenvil・onment(Nりosan

lle・!;earch(3roLlp,1986).0ccurrenceoFaphyricandesite

nows{MyoJill’yama,KasuS;ε1}/alylla,andlshi111akurivo1-

(Jlics)andintl-usives(OdakeandKashiwaminevo1-

(Jlics,andpartoFtheMyojitl’yamavokmics)charac-

tel-izethisf(J)1・1‘11ation.Thea❹esitesare!5anulくitoids,

witha硲wphen()crystsinaglassygroundmassand

shoxvingconchoidalf1’acture5;ul-alces.Theupperpart

oFthisfom!ationiscomposedoFbasalt】avas

[ShibayamaandDorokorovo]canics).TheJO匹shiroFormation,moretl珀n200mthick

atTel・ayilma,cont1:)I’mably(2ove?rstheHarakawaFOITna-

tion.ltconsistsoFporl)hyriticbiotiteorthopyroxene

dacite(Te・l` 2lyamavokanics),biotitel・hyolitetodacite

tuぼげitnlat(2yamaTu印andcoarsedasticrocks.The

Pale()maglletismoFtheNi鎖Group

Tama犯yamaTu汀showsnoc?videnceofwddi】lg,but

hightempc?rし1turec?lnplacementis5suS;gestedn・omgas

・;(・!gregationstl-uctures(M卜21chietal・,1998).Consistent

(消entationofgas5;(l!;regationpipesisobservedat

Takaidl1Jromwhichwecanrecognizeagentlestratal

diptowardthesoutheast.

Radiometricdates

SincetheNりoGroupisnonmarilleandc()ntainsno

昭e-diagnostic化)ssils,theagehasbeend沁cussedbasedonrad硲nletricdates(Kawanoandueda,1964;Tatsumi

eta1。1980;Yoshikawa,1997;Hoshietal.,1999;lwano

andDanharE1,1999).l)ublishedmdiometricdate5irange

fronlabout16to10Ma(Fig.3).WhlerockK-A「

dates斤omthesanukitoidsaresignincantlyyoungerthan

datesn’omtheoverlyingJogashiroFormationaswellas

theund(jl’lyingDonzuruboFormation.Herewe

shouldrecallthat5;anukitoidsarechlll・acterizedbya

glassygroundmassandthatthepl` esenceordeivitrined

glass!g(2neraHyl-esultsinaloxx/eringofK-Ardates.Yoshikawa(1997)attributedtheyotJngerK-AI・datesto

poorArretentivityorglassyand(?sites.0ntheotherhand,FTdatesoftheKasu!;ayamaandhhimakuri

volcanicsareconcordantwithK-ArandFTddtesn・om

theov(・rlyingandし1n(j(jrlyingt()rmations(Hoshietal。

1999),mostlike!yrepl-c2sentingsolidmcationagesoF

these5;allukitoids.Exceptfi)rtheK-ArdatesfTI・omthe

sanuk迂oids,K-ArandFTdates斤omaHthreea)rlm1-

tionsdusteraround15Maandareindistinguishableat

the2び1evel.Thisdenlo11乱rates,asYoshikawa(1997)

sug留sted,thatvolcanicactivityinthisareaisl・estl・icted

towithilla函ortertimespanthancanbel・{2solvedbythe

K-ArandFTtechniquesataboutl5Ma.

Geologicstructure

The刈oGrouphasbeenfoldedintoagentles;}zllcline

withaNE-Sへvfoldaxisrunningapproximately斤om

TerayamatothesouthoFMyoiJ❹yanla(Fig.2).0n

thesoutheastlimb,thedistributionandattitudesor

clasticsedimentarybedsshowthatthestratalge?nerally

diptowal・dthenorthtononhwestbyabout30二From

Donzurubotothenonheastけiltismoreenll)hasizedby

llnintrafbldlocalnexure.Eutaxitic硲1iationstructures

inweldedtu什soFtheLowerDonzuruboF()rmation

showapparentlysteeptiltingoF40°ornlore.

Paleotopographycouldhaver)alllyinnuencedth沁struc-

ture,t)e(2とlusethisrockunitcompl・isesthebasalpartoF

thegrouplyinguponanunconf1)rmitysul尨ce.0nthe

()tllerhand,thenorthw5tlimbhass;lj隈うredonlygentle

tiltingtothesoutheast.Fol-(?xample,asnlentioned

207

above、theco回istentorientationol゙ hldinedgass(i11rega-

tionpipesinthepoorlylithifiedTamateyamaTu什

indicatesasoutheastward5;trataldipat8へ

Amacl・oscopicNE一一5;ヘヘ/EaultandaNへV-SE

nexurehavedeFomledthes;yllclinalstrtl(2ture.The

formerisnamedtheYamatogawaFaultandthelatter

theKalllino-’raishiFlexure(Morimotoetaに1953).

Alongthem、s;(jdimentsoFtheupperPliocenetoPleis-

toceneosakaGroupandtheunderlyingNijoGroup

havebeenlultedorsteeplytilted.

Samplingandlaboratorymethods

We(2alTiedoutabriefgeologicsurveyinthefjeldbenE)re

s;とtmplingwiththeaidoFthegeologicmapoFMりachiet

21L(1998).jべttentionwasconc(jntratedonobtaining

l’l’1(ごsoscopicgeologicstruc?山res斤omoutcrops.We

aimedtousethesedatafortilt(ごolTectionof

pale()nlagneticN/(ごctors.Atotalor203sampleswere(2〔〕11ectedn・om33sitesn-om10di什erentrockunits,as

shownonthegeologicmapinFig.2:theLowerDon-

zuruboFomlationosites),|・;hikiribavolcanics(2),

Hatavolcanics(3),lsllimakurivolcanics(I),Kashi-

waminevol(?anics(1),Kasugayamavokanics(3),

MyoJill’yamavolcanics(6),0dakevolcanics(4),

S;111bltyamavo臨tnics(I),andTe・1` ayamavolcitllics(フ),

Ateachsiteレnvetoseven5;と1mpleswerecolTedwitha

igasoline-powereddrillandorientedwithamagneticcompass.0necylindricalsp(?(うimen,22mnT11ong,was

preparedn’omeachcores;anlple。Remanentl‘l‘lagnetizationswereme?こlsuredwitha

NMsuharaSSM-85spinnel` I‘rla!;net()meterattheGeo-

1ogical5;し1rveyoFJapanandaSchonstedtSSM-2A

5;pinne】’nlalgnet()111eteratAichiuniversityoFEducation.

Asapilotstudy,twospe(;inlenspersitewel’edlosenfor

detailedstepwisedemal;netizationbytherma】andalter-

nating-neldtechniques.ProS;ressivealtemating-tield

(1(2111agne,tization(PAFD)was(larriedout柚sevento14

stepsupto100mT.PITol;1゙ essivethermald{?mとtgnetiza-

tion(PThD)was(;arriedoutinairinm(:)1・ethanlosteps

upto690°C.Denlalgnetizationl` 6ultswereanalyzed

usingvectorcomponentd柚gramsandequ❹べu-eapro-

,je・ctions.1ニ)I柚cipalcomponentllnalysis(Kil・schvink,1980)wasappliedtolitmagneticvc?(ごtorendpointswith

aline.

Paleomagneticdil・ection

BothPAFDandPThDemcientlydc?tectedstable

m昭netizationcomponentsfbr29jtes(about90%oFthe

5;iteslln❹yzed).lnpilot(・xp(,rimentscjFtllese,;ites,the

(lil-ectiononhestableco)11ponentt111・oughPAFDwas

208 Hil-oyukiHoshi、DaisukeTallaka、MasakiTakahashiandTc〕shiyukiYoshikawa

silTnilartotllatthroughPThD.T】leremaillillgspeci-

mensweretllerefiニ)redema!gnetizedonlybyPAFDto

obtainsite-nleandirections.Typicalc?)ぐanlplesare

showninFig.4,wherepilotl-esultsoFsitesNJ13,NJ22,

NJ24,N」30,andNJ33arer)1-esented.lnsiteNJ24,the

linearcomponentabove30mTthatledtotheo】’igin,

【出aracterizedbyasoutherlydirectionwith】・(うversed

polarity,was5;illlilartotheHneal・conlponentabove490°C.Thetl‘ど1jectorybelow30mTalsohada(;onsid-

erableHnearitybutdidnotleadtothe()rigin,indicating

asecondarycomponent.0ntheotherhand,thepilot

(2xp(21’imentsoFfbEsites「N」04,NJ21,NJ28,andNJ31)

showedinconsistencyindil’ectionbetweenPAFDand

PThD,andsite?-meancalcu】ationwusimpossible.For

c・)(む1mpleい゙ esultsn-omsiterヽ ,JJ31(Fig.4)showedthata

stableHneal・componentobtainedabove30mThada

nol-thw6terlydeclinationwhilethermaltrc2atmentabove

350°Cgaveanestel・1ycomponelltleadingtotheorigin;

tllecauseo臼hediscrepancyisunclear。

Tweうnty-ninesite-meandirectionswere(2と11culated

usingthe・;tandardstatisticsorl7ish(2r(j953)aslistedin

Tablel.Almostallthesitesprovidedl:)recisenlean

directions;両5va!ueswere5;maHerthan10°。

Thirteensite-meandirectionsxverecor】-ected化)r

tilt.E3(3(;ausetheNE-SW5;ynclineisgelltle,strLI(;tural

(201Tectionisnot(うs;5;entialR)rs;e2veralsitesnearthe

s;}/nclinalaxis.Fieldobs;ervationssuglg5tthatsiteNJ27

0nthesoし1tllwestemendoFtheaxishasbee?nallectedby

theactiveKamino-TaishiFkxure.The5;ae-rnean

(lil-ectionwa5;hencel-(?storedbasedonastructuralatti-

tudeoFthenearbyosakaGroup.0nthenollhwest

limb,thes;itl-meand沁ctionoFsiteNJ33(Shibayama

vokanics)wasc田7ected帛「tiltbasedon畑・ucturaldata

fTron‘lhlclinedgassegrc名ationpipesintheTamatc?yama

Tu什atTalくaida.0nthecountel・11mb,foLlrsitesoFthe

Kasul;£1yと1maandMyoLJhl’yamavohlnicswerec(ニ)rrected

fi)rtilt.Wewel-enotabletodefinequ征ぷ冬匹tivelythe

ltmountonilt化)rtheother16sitesbecausethel・ewere

too硲wbedding-tiltdata斤omnormal5;edimentary

stl゙ata。

Astratl!graphlcchangelnm昭11etlz21tlondII・ectlonls

showninl:7igure5.TheTe?I・ayamavolcanicsinthe

Jo即shiroFOI-mationhaveQversed-Polaritydirections.

Asouth-south(?21sterlyde(?linationandasomewhatshal-

lowin(jination(jlaracterizesite-meandirections斤om

theT(?rayamavol(2al-lics.0ntheotherhand,thesites

oFtheDonzul・uboandHarakawaForlllationsal-ea11

n()rmallymagnetized。

Iニ’igul・e6plotsallthesite-1Tleandirections.ln

geographic(山t-uncユc)rrected)co()1・dinates,thehlclina-tionishighlyvariable,r・anging斤onlhorizontalto

へ/tう「tical.Thedecljnationisalsovと1riable,butnortherly

andgutherlydirectionsaredominant.lnlstratigraphic

(tilt-colTected)cool・dinates,norma卜polaritys;而-nlean(lil-ections(2luster111-oundf)=0.7°and/=59.3°.For

theMyoji11’yamaandTerayamavokanics,within-unit

(;lus;tel・ingisbette51‘aaertiltcolTectionthaningeolgraphic

co()1-(:linates(Tablel).lnvc,rtingthel-e,,zersed-Iニ)olarity

s;itしnleandil・ectionstothenomlalpolarity,thetilt-

(JITectedoverallmeandil-ectionbecomes£)=352.2≒

/=51.7へandα95ニフすforthe13sites.

Fieldtests

lmprovcMTlentin(jLlsteringoFsite-meandil’ectionsis

evidentbymaking・;tl’ucturalcorrection(Fig.6).TO

testthis,weappliedthebedding-tilttestoFMCEHlinny

【】964)tothe13sit(3-means.Theratio臨/ん。=2.59

(心=32玉んu=12.4),u/here札andんuarether〕1・ecision

parllmete】` soFtheti1トco汀ectedandtilt-unc()ITected(iil・ections,respectively.Theratioexceededthe/7-ratjo

value(1.98)fbrthe5%,;i!;11雨cantleveLindicatingthat

thebedding-tilttestiss汝n消cantwith95%probability.

Thus,thesittyllleandirectionswe】・eacquiredbefore

stratilltilting.Accol・dingtoMiyachietaL0998),the

upper円iocenetoPleistoceneOsakaGrouphasnot

beendeformedbytheNE-SXvfolding.Thecjil-e(2tions

obtainedwe)’ethereforeacquil-edbefi)】-etheLate

lニ゙ H()cene,probablyatthetimeorfi)rmationofthe

MiddleMioceneNりoGroup。

Be(こausetheNりoGrouphadtwogroupsoFpolar-

ity,weconductedthel-(?V(lrsaltestoFMCFaddenand

MCElhinny(1990).Aroughantipodeofthepolarity

gl4oupsisshowninFigure6.Thetest,however,showedanegative?result;thet匝-(;orrectednleandirec-

tiononhenorm❹-polaritygroup(£)=0.7≒/=59.y,

両5ニ8.3≒A/ニフ)andthatofthere,z,1・sed-polarity

!group(Z)=165.4≒/=-42.y,偽5=8.5べA/=6)isnotantipodalwith95%probability。

Se?veralpo5;sibilitiesn!aycauseanegativel・e?へ/ersa】

test.Iニ7il・st,iFnleandirectionsarebiasedbyasecondary

rnal;11etizationJlnegativel-esultrnaybel1(;hieved.0ur

directions,however,havebeenc❹culatedFrom5;table

componentsdetcll・minedthrough(a11-eFulstepwise

(畑11agnetizationfig.4),sothisl:)()・;sibilityshouldbe

(2)((;luded.Second,inappropriatetiltc()ITectionmay

l’(2sultinallqativetest.Thiswoulds;ometimesbethe

(Mseinanとlncientvotcanicfieldsu(jlastheNりoGroup・

However,b(2causeweuseonlys沁-meandataoFst)-uc-

tural】yweiH-dennedoutcropsfoにhetest,thispos;乱)ility

canalsobed沁carded.LastoFall,ourpre弛】Tedprob-

ablecauseisthata四gativetestmayoccuriF111ean

directionsdonot匹el・昭eoutthel;c2()magnetic5;aular

N/21riationthoroughly.Thenomla卜polaritydirection

WUp

/\二言詮げθΓa押ma妁/c]ar7/と剛

WUp210

140佞RM350て≒)--Q490

討 宍 r 4 2 0 ` ヽJO°0.16A/m

WUp

E乱ツ徊NRM

/\三三びQs叩a押maW/(7an/とか

WUp

NJ22・3A

PThDJO゙0.23A/m

530

EDn

111 01111 111311 11 11 011 1111

おoONRM

PaleonlllgnelismoFtheNiJOGroup

WUp

100mT

EDn

NJ33・3A

PAFDJO=1.04A/m

10‰NRM

/\七三β肋ε1押ma妁/t7an/圃

WUp

N SWUp

‾EDn

/\毒心

砂7y可❹yamaW/c;即/・c剛

WUp

209

650

EDn

70,610

へX0

530≒ゝゝゝ

WUp

100

EDn

NJ33-6A

PThDJOニ0.56A/m

3`50420cヒン9yjuhRM

60

b5omTNJ13-5A

PAFDJO=0.08A/m

4oqQ35

39‰ミ

/\三三r/s77//(/Wbax/o/(7∂n/恂

EDn

6104Tc

,ズヅ尹

EDn

350`

、ヽ

WUp

EDn

NJ30-3A

PThDJO°0,18Aym

7o・c` t)、

。 〃

ぶ丁50叉

`ONRM

NJ31-2A

PAFDJOニO、04A/m

NRM

/\/J3ブ仰yq/❹押ma妁/[]a改削

EDn

Figure4.Examp]esorslepwisedem昭11(ltjzalionl’esults.Solidandopendl・clesarei)1’oJectionsornlaglletizalionvectorsontothe

llorizontaにmdverticaIN-Splaneyl-espe(;tively、inthe!gcl(場raphicco()I・clilmtes.Jo、huensityof` naturalremanent111agnetization

(NRM);PAFD、r)I‘()gl’essive辿em川ng-tjelddemllgn(2tization;PThD、F31・ogressivejthermald・nagnetization.

obtainedri・omthreedjl穏rentrockunitsisindistinguish-

able斤onlthe§;c?()centricaxialdipolenelddil・ection

(£)=O°,/=5ギ),whilethel°(・V(・rsed-POlaritydil-ection

(;211GIlated斤0111site-meansoron)yasingleへ/olcallicunit

(Te?I・ayamavokanics)isnot.Asi】lgleへ/()lcanicnlassoftheT(?1` ayamavol(?anicshasP)-obublyl-c?c()rdedan

210

Site

Tablel.

ηA/ηT/y

Terayamavolcanics

NJ01NJ24NJ25NJ26NJ27NJ28NJ29

6/1/フ6/1/76/1/フ6/1/75/l/6-一一

5/1/6Me圃

111royukiHoshi、DaisukeTanaka、MasakiT液ahashiandToshiyukiYo!;hikawa

Paleomagneticdatarl・omtheMidd】eMioceneNりoGroup,Mt.Nijoarea,s(川thwestJapan

£)

174.0

170.8

169.4163.9

02.5-一一

156.7168.3

-42.3

-43.2

,37.1

-57.0

-0.1

---

-42.2

-37.5

S/D

msllu

msltu

insitulnSltu

134/55

msltulnsltu

(α95=167)

Shibaya】1!avolcanicsN J 3 3 5 / 1 / 6 3 5 7 . 9 5 5 . 2 0 4 5 / 0 8

0dakevolcanics(9nukitoid)

N J 1 4 6 / O / 6 3 5 0 . 8 3 4 . 2 N D

N J 1 5 5 / 1 / 6 3 3 4 . 8 1 1 . 4 N DN J 1 6 5 / 1 / 6 3 3 2 . 6 2 9 . 5 N D

N J 1 7 5 / 1 / 6 3 3 6 . 1 2 3 J N DMean338.324.7φ95=14.00)

M7)μn’yamavolcanics(salltlkitoid)

NJ04NJ05NJ06NJ30NJ31NJ32

O/5/5

4/1/5

5/1/6一一-

5/I/6Mean

12.968.6

320.160.6

4.256、8

250/10250/10250/101nsitumsltu

351.757.Smsltu351.562.4(cz95=13.7°)

Kasug;ayamavolcanics(sanukitoid)

NJ21NJ22NJ23

5/1/65/1/6Mean

-一一

53.6

61.6

58.0

66.1

60.8

63.5

KashiwanlinevolcanicsGanukitoid)NJ18

33.732.7

Hatavolcanics

N J 0 7 6 / 1 / 7 3 3 0 . 1 7 7 . 5

N J 0 8 6 / 1 / 7 2 3 8 . 5 8 9 . 0

N J 0 9 6 / 1 / 7 1 3 . 2 7 6 . 7Mean350.182、1

1shikiribavolcanics

N J 1 2 6 / 1 / 7 3 4 7 、 0 5 1 . 5N J 1 3 6 / 1 / 7 3 4 7 . 2 5 8 . 0

Mean347.154.8

11http://ww.//

○ノ0ノQ’。19

皿皿皿

(cz95=13.2°)

ND

ND

R11yoljticweldedtufl` (LowerDonzuruboFo回ation)N J 0 3 6 / 1 / 7 3 5 6 . 3 4 7 . 5 N D

N J 1 0 5 / 1 / 6 3 4 2 . 7 4 9 . 8 N D

N J H 5 / 1 / 6 3 4 1 . 7 4 6 . 7 N D

N J 1 9 6 / 1 / 7 3 5 8 . 4 4 3 . 8 N DN J 2 0 5 / 1 / 6 3 5 5 . 5 4 6 . 1 N D

M e a n 3 5 0 4 7 . 0 ( α 9 5 = 5 , 6 ° )

- 一 一

- - 一

- 一 一

フ.460.6

HarakawaFomlation(16sjtes)

158.4

一一-

156、7

165、4

170.8-43.2

169.4-37.1163.9-57.0

JogashiroFonllation(フsites)

174.0-42.32.2752.7

£)「

一一-

3,2

324.6

4.2

~一一

351.7

349.9

-S-

14.1

26.4

20.4

-一一

一 一 -

一 一 一

一 一 一

一 一 一

一 一 一

-42.3(α95=8.5°)

-30.7

一一一

一42、2

一一一

59.7

51.1

56.8

一一←

57.8

57.5

-一一

60,5

58.9

59.8

---

一 一 -

3.3410.7

(α95=12.7°)

一一-

10.6

13.2

3.2

---

4.9

5.0

4.2

3.2

4.3238.7

7.8

41

8.6

9.8

一一←

7.6

α95

-一一

2.3

4、4

151.0

179.0

260.3

426.1

61.3

221.8

50.2

47.3

一一-

79.5

52.8

34.7

436.8

一一-

833.1

233.0

4.2332.6

6.3114.4

DonzuruboFonmtion(10sites)

一 一 一

一 一 一

一 一 -

- 一 一

- 一 一

一 一 -

- - -

- - 一

一 一 一

一 一 -

1.8

1.3

O″`UりjQyl

1J1(Z7jりー

264.9

400.9

639.2

H04.6

2280.8

403.51831.6

816.8

238、9466、0

-

NorthvGP

8 0 . 9

一 一 -

一 一 -

一 一 -

-一一

83.5

60.4

85.6

-一一

82.3

-一一

76.9

68.4

一 一 一

一 一 一

- - -

一 一 一

一 一 一

一 一 一

一 一 一

- - -

- - -

一 一 一

- - -

- - -

135°391281’135°38’38’「

135°38’37̈

135°38140̈135°38’16̈

135038117”

135°38’08’I

344.5

358.5

352.563.6

8.2---

25.3

34033’03̈

34032’20̈

34032114̈

34°32’071’34°32’08’1

34032135̈

34032’25̈

173.33403410811135°39116’I

34033’26̈

34033’29̈

34°33125「’

34033’38̈

:340JM’33̈34034’27̈

13504P251112S5()41119”

135°41’29’1

135°40107̈

135040123̈135°40’08̈

- 一 一

一 一 一

一 一 一

- 一 一

-一一

157.8

50.4

185.2

-一一

73.3

189、4

204.1

- 一 一

- - -

- 一 一

一 一 -

一 一 一

一 一 一

- - -

- - -

- - -

- - -

- - -

34031’34̈34031’401’

3403114311

34031144̈

34032’08”

34°32’08”

34°32’Oプ

135041102̈135°41’04̈

13j5°4110P

135°40155「I

135°40’59”

135°40159’1

135041’06̈

135039’07̈

135°39105̈

135°39100”

34°3112211135°39’4311

34°31’05”135039’32’`

34031146̈

34°31147̈

34°311521’

34°31’51̈

3403P47̈

34031’26̈34°30155̈

34030’5811

34°31124̈34°31122̈

135°40’10̈

135°401141’

135°39’57̈135°401231’

135040’32”

135°39’47̈135040’02̈

j?,,,numberorsampl(!い|-eatedbyi)|’()gressivealterllatillg-tjelddem2!!lnetization;心。number〔丿slmple?slreatedbyr〕|’()11ressivetherma】

demagnaiJtion;/V,totalnLlmbel’ot` 5;amplesusedfbl・de旧・nliningsite-meannmgnclUzation;/)and/,tilt-Llnc()I‘rectedde(ご1inationand

inclination;S/D,strike/dipdatausedrortiltcご()ITection,shownbythcn-ight-handrule(ND,notdcごtermined);Z)calld/c,tilt-coITected

declin川onandilxlination;じと99radiusorconeor95%col俑c!ence;ん,r)re(jsiollpal゙ し1nleter;VGP,vil・tujgeomagneticpole.

10.447.3

lshimakurivolcanics(sarlukitoid)

NJ02

4/1/5Dlcan:5/1/6

/V

Paleoma!gnetismortheNiJOGroup

/\/佃(:ョ?/l)叩

LOとj二(刀応DO‾つ Terayamavolc.

|モLL宋1工

Shibayamavolc.

’OF㎜召色目(/)

Odakevolc

Kashiwaminevolc

rshjmakurivolc

Kasugayamavolc.

Myoj❹yamavolc.

こ○・-←・司

OLL

O£コL。コNCOQ

L_①aO.⊃

2て⊃召瓦

Hatavolc.

lshikiribavolc.

LowerDonzuruboFm.

Hemisphere●Lower(tilt-c(mected)sLower(tilt-uncorrected)oupper(tilt-(;()rrected)

Figure5.Suitesor!;ile-n!eand而ctionsr〔〕rin(jiヽ ・i〔hlalrock

unitsoFtheNijoGrouP.Equal-areal〕I‘oJections.

instantorthesaularvarialion、whichwouldbethe

caLlseo臼henegativel’c2vel’saltest、

Magnet()stratigraphicdating

Wediscussmagneticpolaritystl・atigl-aphybasednot

onlyontilt-cot-recteds;ite-nleandatabutalso()nuncor-

recteddata。

TheNりoGroupl’e?(;ordsa(jlangeinpohlrity,as

showninF7咆ures3and5.Apreviousstudy(2011firmed

thepolarityoFpyroclasticnoNvdepositsoFtheupper

DonzuruboFormationasnormal(Nakamuraeta1.,

1998).SincetheHarakawa/DonzuruboR)】Tmation

t)oLmdaryiscon帛I‘mable(Miyachietal。囚98)バhe

normaHymagnetizedDonzuruboandHa】-akawaFor-

nlationswereformedwithinasinglenomla卜polarity

interva1.The5;tratigraphicpositionoFthepolal-ity

boundarycorresPondstothelithostratilgl・aPhicbound-

211

arybetweelllheトlarakawaandJogshiroF()rmations.

K-Ardatesfy()mthe5E;111bayanlavokanics(14.8±

0.3Ma)andTc・I’ayamavo臨1nics(15j±0.3Ma)叩ec蒔

the昭eoFthepolarityboundaryatabout15Ma.’rhel・e

aretwoN-to-Rpolaritymversalsatabout15Maonthe

ct」rrentlywe1卜adoptedgeonlagneticpolaritytimescale

oFCandeandK(2nt(199j5バFig.3).TheobservedN-

to-Rboundaryiscorrelatedtoeitherthesubchron

t)otlndaryC5Bn,Ir/C5Bn.2n(15.0Ma)orC5ADr/

C5Bn.ln(14.8Ma).Comidering2びuncertainu逗_soF

K-Ardates,nodennite(201Telationispossible,butat

leastwecancondしldethatthe昭eo臼heNりoG】-ouPis

oldertllanthetopoFSL11:)chronC5ADr(14.6Ma)and

}/oungerthanthebaseorSubchronC5Bn.2n(15.2Ma).

Discussion

Theoverallnleandirection(£)=352.2べ/=51.7≒両5=

フ.4゜)isc❹culatedn・oml3site-meansofbothnormal

andl・c5vel・sedpoklrities.Theinclinationisnotdiscord-

antwiththeexpectedone斤omthelg(jocel‘1tricaxial

dipoleneld.Ther)llssageo臼hebeddhlg一tilttestsug-

gestsal:)rimaryoriginforthenlagnetization.Addition-2111y,thel3sitescox/c?14plul’alstratigrapllichorizonsinthe

NりoGroup(Fig.5).Weconsidc・rthatjnspiteorthe

negativerclへ/ersaltestけheova-aHmeandirectionav(ぅrages

out,toacellalnextent,thepastsecularvarlatlon.

5;tudyingtheangular・;tandarddeviation(ASD)of

/ヽirtualge(:)mとtgneticpoles(VGPs)spr)ortsthis.The

13VGPsc❹cL11atedforthel3tilt-corl-ectedsitespro-

duceap❹eomagneticpoleat84.3゜N,35.yE,and玉5=

8.5≒FOHowingCox(1969),the13VGPsyieldan

ASDof16.4≒withlowerandupper95%conndence

!jlllits(Sland瓦)(涯110≒lnd22.2°,1’(・spectively.This

ASDvaluecouldhavepotentialcirrorsderived斤om

undet(2ctabletiltowingtovolcanicl・ocks.Still,the

valueisindistinguishablen’omtheO-5Maglobaltl・end

(14ダフ9≒SI=13.96ぺ瓦=15.72°)obtainedf〔〕rth(うlatitude

range25.0-J34Jn’omaworldwidedat21base(MCElhin-

nyandMCFadden,1997).Thus,theoverallnlean

givenaboveis(2onsideredtobethet面e?-avel゛aged(jil-ectionintheMt.Nijoal・eaataboutl5Ma.

lnthelateCenozoic,thepositionofthetime-

ごtveraged!gc?omagneticpolewa5;nearlyidenticalwiththe

Earth’sspinaxis(BesseandCourtiHot,1991).Hoshi

(加冬)conlpiledTmiaryr)aleomagneticpolesfフl・omtheAsiancontinentandobtainedanleanpaleomagnetic

poleat89・2°N,226.0°E,and京百八=5.y.The

r)aleomagneticpolen)「theNりoGroup(84.3°N,35.yE,/195二8.y)isnotdistinguishable斤omthepresentgeo-

graphicpole,nor斤onltheTertiarynleanpolefortheAsi皿continent.ThiscomparisonoFPoleshldicates

212 FliroyukiHoshi、DaisukeTallaka、MasakiTal(2lhashiandToshiyukiYojlikawa

Figure6.E哨1laトarear〕rojectionso目a)ti】t-uncol・rectedand(b)lilt-(Jrl・ectedsite-meandirections.Thedirectionsonthelower

(uppcぅr)hem肺hel・(2thatcanbecolTectedらrtiltal-eshownbysoli(j(opell)symbolsパ1ndthoseonthelowcM・hemisPheretllaにannot

be(rorrecteda)rtiltarere,pl・esentedin(a)bygl・aysymbols.0VEllsal・ound山eclil・eclionsdejnotethe9沁co叩dell(;elimit.

norotationintheMt.NiJoarea.T11eupper】imitR〕r

theageoFtheNりoGI・oupdated斤om111昭neticpolarity

stl゛atigraphyis14.6Ma.Thus,weconcludethatno

rotationhaso(2culTedintheMt.NiJoa)’easince14.6Ma。

Thisiscosistentwithdetailedpaleomagnetic

resultsFI・omSan’in,Fukui,andYatsuo(ltoh,1988;

N冰EIJimaetal。1990;OtonJjietal.,1991),sL111gesting

ternlinationoFthedockwisel` otationofSWJapanby15

Ma.(::)tot7Lリieに11.(1991卜tudied山er)111eomaglletism

oFMiocenerocksinSan’in.Theyrepol・tedl8site-

n‘1eanclirectionsand15K-Ardatesl-anging斤om~16

to~13Ma斤omtheMiddleMioceneonloriForma-

tion,y泌ldingapaleomagneticpoleat86.ド,214.0°E,and

j95=10.5°.Thispoleisindistinguis;hab】en-omthe

paleomagneticpolea)rtheNiJOGrouP.NakaJimaet211.(1990)d(・heatedatemporalchangeilldeclination

斤onlawelトdatedMiocenesequenceinFukui.Time-

llvemgedmeandeclinationsbeforelフMal11・emarkedby

adockwisedenectionaslargeas40-50≒whereasthose

posterio】TtolフMaseen71toapproachinsequencetoO°.NakaJimaetal.(1990)concludedthatt11(・maJ()rpartof

thedockwiserotationoFSXVJapanoccurredbetween17

andl5Ma.lnYatsuo,ltoh(1988)de・tecteda・;maH

paleon・lagneticrotationina17-15Macontinuoussequenceandshowedthatthem司orpallortherotation

()ccし1rreddしH’ingChronC5Br(16.0-15.2Ma).The

sm❹1amountoFrotationisexplainedbytheintra-arc

ditkrentialrotationduringthel-otationoF5;砥/Japan

(h()h,】988;ltohandk0,1989)。’rhesepale(:)i‘nagneticl-esultsareconcol-dantwitheadlotherandhavebeen

|’(?portedn-omawideareaofSWJ叩1111(Fig.1).’rhus,

we(2()11cludethatSWJapanhasnotu❹(2rgoneロ’otation

since14.6Ma.hlotherwc)1’dsバheclockwise】・otation

oFSWJal:)anrelatedtotheopeningoFtheJapanSeahad

alreadyendedbythistime.

Asmaltionedintheintroduction,theMul・oPFD

andKARsintheKiiPcうnillsula,bothdatedatabout

l4.3Ma(Utoeta1.,1996,1997;Sumiieta1。1!)98),have

aclo(:1(wise-clc・nected(jirection【Fig.】).TheMuro

PFD,about400mthick,consistsnlai❹yoFdensely

weldedrhyolitictapiHitu汀(Nishiokaetal.,1998).

FieldobservationssuggestthatthesecluenceconsistsoF

5;(lveraトnowunits.Howeverけheserocksconlpriseone

(J()olingullit(NishiokaetaL,1998).Additionally,the

sitei-】Tleandirections斤omtheMuroPFD(justerweH

al`ound£)=22z10,6°and/=-66T(T(:)山丿983),andthe

偽5be,comesalowvalue()f` 3.7°(::!lsites).’I’kinclina-tionis;sigl’1ificant1}/lal・ge?rthane?xpectdinthe!ge?(xentric

axialdipoleneld.Basedonthesegeologicand

r)£11(2omagneticobs(jrvations,theclockwise-de?nected(lil-ectionoFtheMuroPFDismostlikelyanin5;tと1ntane-

ousrecordingoFthepastS;tうomagneticQldandhence

doe5;notsignifyaclockwisete?ctonicl-otation.Si11111ar-

1y,theclockwise-(1(?necteddil・ectionoFtheKARsmust

l’(lr)1’esentanolltectonicde,nection.’I’agami(1982)re-

portedameandilTectionoF£)=63.9°and/=48.4≒(2a!ctllated斤onll4site-means.How匹elツheんand的5

x/aluesR)rthisnleanwerenotpl゙ tうsented.Thesite-

111eanshavebothnormalandl’eix/el・sedpolaritiesrnaking

anearlyantipodaldistribution,whichhavebeentaken

I)211eolllagnetisllloFtheNiJOGroup

as(,,/idenceofadockwisetectonicl・otationげagami。

1982;Sumiieta1.,1998).However,thesite-means

wereobtained斤omonlya硲wrockunits,theKonogi

RhyoHteけu爪andSgranitep()rphyrydefinedbyAramaki

andHada(1965).TheKonogiRI!yolitellasnormal

pola】-ityandthetu什andlgraniteporphyryshowl’(?vel’sed

polarity.Radiome?tljcaHy,theseunjtsarenotdistin-

lguishllble(Sumiieta1。1998).Rapidcoolingiぐntど-redrortheKARsbasedonastudyoFFTthermo-

(;hronology(F‘lasebeetal。1993).We(JIlsiderthatthe

KARsxvel・efomledduringanexcLII・sionoratl・と1nsition

afierthetectonicrotationofSWJapan.’Fhisshouldbe

testedbyadetailedpaleonlagneticstudy化)rtheKARs.

Conclusions

ApaleonlagneticstudyoFvolcanicrocks斤omthe

MiddleMioceneNりoGroup、datedra(1柚metricaHyat

aboutl5Ma、ledtothefoHowingprincipalconclusions:

1.Thegroupl・(ごcordsanN-to-Rpolaritybound-

arythatcanbecolTelatedeithertothesubchronbound-

aryC5Bn.lr/C5Bn.2n(15.0Ma)ortoC5ADr/C5Bn.1n

(14.8Ma)onthegeoll’lagneticpolaritytimescaleoF(ごandeandKent(1995).Thel)olarityboundary

appl’()ximatestothelit11()strati§;raphicf()1・mationbound-

aITybetweentheHarakawaand、lo呂shiro}Jommtions.

The(j(?po5;itionalageorthegroupiso旧erthantktop

oFS;油chronC5ADr(14.6Ma)andyoungerthanthe

base(汀S;ubchronC5Bn.2n「」5.2Ma).

2.The1101!herlyoveraHnleandil・ectiononhe

NりoGroup田=352.2≒/=51.7≒偽5=フ.4°)、detel’-

mined斤om13site-I-11ean5いndi(ごatesnorotationinthe

studyareasince14.6Ma.Alongwithpaleomllgnetic

(latafyomoth(?1・areasけhisdil・ectione皿tblishesthatSW

Japanhasnotundergonetectonicrotationsincethat

time.The昭e14.6Maddel-mined介omthe111agneto-

stl’atigraphicapproachplacesco115straintontheupper

agelimittotheclo(2kxviserotationoFSWJapan.

3.Thedockwise-denecteddirectionsoFtheMid-

dleMioceneMuroPFDandKARsintheKiiPenin-

55ula、whichhavebeentakenasshowil噌thedockwise

l4otationofSWJapaninMiddleMiocenetime、were

probablyacquireddul・inga!gc?()ma!gneticneldexcLlrsion(瓦transitionafierthe(?1()ckwiserotationoFSWJapan.

Acknol,s・kdgments

WethankS.rりishida(Narauniv.oFEduc而c)n),K.

lshii(Osakauniv.oFEdtJcation),andKJugano

(Osakauniv.oFEducation),whol:)rovideda・;sistanceorN/2lriouskhldsintheneld.D沁cussionswithT.Yoshida

(Tohokuuniv.)andT,Danhara(KyotoFission-Track

213

Co.Ltd)wereofgreatbenent.Wewholehc・artedly

thanklニ4.1wano(KyotoF7沁sion-TrackCo.Ltd)for

(ニ1沁cussionandforcriticallyreadingtheea1㈲milntjscript

andmakingmanyvaluablesugestions.Y.0toFuji

(Kobeuniv.)andA.F-l砂ashida(Doshishauniv.)are

alsoUlankedfor(21・iticaHyl’eadingtheearlymanus(2ript,

ReviewsbyK.Kodama(Kochiuniv.)andl-tShibuya

(Kmllamotouniv.)e削lancedthemanuscript.Support

toH.H.xvasprovidedbytheDaikoFoundation.

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