PaleomagnetismoftheNijoGroupanditsimpliclltionfor ......
Transcript of PaleomagnetismoftheNijoGroupanditsimpliclltionfor ......
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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
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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
S
S
S
S
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
ツ
おoONRM
N
N
N
N
S
S
S
S
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
N
N
N
N
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
N
N
S
S
EDn
350`
、ヽ
岫
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
7
-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
0
0
0
11http://ww.//
○ノ0ノQ’。19
2
2
2
N
2
2
2
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
几
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
ん
52.8
34.7
436.8
一一-
833.1
233.0
4.2332.6
6.3114.4
DonzuruboFonmtion(10sites)
7
0
4
1
j
り
ー
{
Z
『
・
一
・
一
一
一
・
・
・
・
・
・
一
一
・
・
・
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
6
7
3
6
6
一
4
甲
咽
■
-
一
一
’
8
7
3
6
3
一
フ
フ
フ
フ
フ
6
/
り
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
E
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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