19
1lu】lethl()ftheGc2心)gica155ul・veyofJEtl)an,V()1.45(8/9),p.477-495,1994 MiocenelateralbendingofcentraIJapan -lntra-arcdeformationatarc-arcc()Ilisionzone一 MasakiTAX,XHAsHI* ’rAliAllAslllMasakH1994)IX/Hocene】ateralbending〔〕fcentra】Japan clefol-mationatarc-arccollisionzone-.茄,乱Gひ屁Sg,孔ノ砂の7, 1λ477-495,H)figs.,1table. -Iはra-arc vol.45(8/9), Abstract:Recentpa】eonlagnetics5tudiesintheKanto?,・1〔〕u】ltai】ls,(:entra)Japan, haverevealedtheMioceneclo(虫wiserotationoftheeast-wingofthesharpbentstl’ucture ofpre-Neogenegeologic化rl-anes(KantoSyntaxis).Morethan9『cl皿kwiserotation oftheKantoMountainstTookplacebetween15and6Ma.Aboutahalfofitsoverall rotationprobablyoccurredataround15Mainassociationwiththel-apidclo(2kwise rotationofSo吐hwestJapan.Therestrotationoccurredti116Ma,1-c?su】tingthesharp bentstl’uctureoftheKant()Syntaxis.Thelzu-BoninarchasbeencoHidingwithcent- 1-aにlapansinceMiddle5び匝)cene,whichmaybethecauseofthisilltra-arcdeformation. 1.1ntroduction Thejf2lpanesels12mdsares;ituatedonthe (?2lstemnlarginoftheEljlrasianPlatewhereboth thePacificandPhilippineSeaPlatesare subducting.0neofthemostdlarElderistic ge()logicalf(j21turesofthe,1211)an(?sels121ndsisthe zonald沁tributionofpre,Neogenet(?ITanesas showninFig.1.Thiszonecol-11prisestlll‘eebelts (Sanbagawa,ChichibuandShimantoBelts)and canbetracedfromKyushutoc(2ntralJapan. Whilethetrendofthiszonalstrudureisparallel totheaxisoftheislandarcinSouthwestJapan, thiszoneissharplybentatcentralJapan,which iscalledtheKantoSyntaxis.Detailedtimingof thef()1・mationofthisbendhasbeencontr()ver- 551211sincethelastc(?ntury。 TheSouthFossaMagnaregion紬locatedat (:entra1Japanwherethelzu-Boninarconthe northwestwardmigratingPhilippineSeaPlateis (mHiding(Fig.2).Recently,fielclinvestigations l’(?vealedthattwoornloreexot弛blocksofthe lzu-BoninarchadcollidedsinceMiddleMiocene *FuelRc?sourcesDeμ1rtment -477 (Niil:s;umaandM21tsuda,1985;Koyama,1991; Aman0,1991).Thickcong吊merateov(?rlying abyssalm1.1dstonedistributedbetweencollided blocksseemstol’el〕resentthetimingofthe collision.Thecollisionofthelzu-Boninarcwith centralJapanmaybethecauseofthenorth- wal・d-convexstructureofpre-Neogenetel‘ranes。 Thispaperpl‘(?sentsthepaleonlagnetic l’esultsoftheKantoMountainsandadjacent areastoshowtheblockrotationwithinthe islandarc.The(N)111si()ntectonicsbetweenthe lzu-Boninarcandccmtra1JapanattheSouth FossaMagnaregionisalsoreviewed.The illterrdationsamongtherotationsofSoutllwest andNol’theastJapanarcs,coHisi()ntectonicsin centr211jlapanan(nateralbodingoftheKanto Syntaxisarealsodis5cussed.Thesynthesisof NeogenetedonicdevelopmentoftheJapanese lslandsisproposedbasedonmanykindsof geologicaldata21ndthegeophysicalc()nstl-aints. Keyw()1‘ds:tcctonicl’otation,iは1‘a-arcd(2函rmation,paleo, malJnetism,Miocene,(こ()IHsiontec2tonics,K2!ntoSyntaxis, (2e?ntraIJapan

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Page 1: MiocenelateralbendingofcentraIJapan -lntra ... (1994b).pdf · TElnzawaBlockcollidedatF5一一3MaandthelzuBlockco回nencedtocol】ideat1Ma(Aman0,1991).

1lu】lethl()ftheGc2心)gica155ul・veyofJEtl)an,V()1.45(8/9),p.477-495,1994

MiocenelateralbendingofcentraIJapan-lntra-arcdeformationatarc-arcc()Ilisionzone一

MasakiTAX,XHAsHI*

’rAliAllAslllMasakH1994)IX/Hocene】ateralbending〔〕fcentra】Japan

clefol-mationatarc-arccollisionzone-.茄,乱Gひ屁Sg,孔ノ砂の7,

1λ477-495,H)figs.,1table.

-Iはra-arc

vol.45(8/9),

Abstract:Recentpa】eonlagnetics5tudiesintheKanto?,・1〔〕u】ltai】ls,(:entra)Japan,

haverevealedtheMioceneclo(虫wiserotationoftheeast-wingofthesharpbentstl’ucture

ofpre-Neogenegeologic化rl-anes(KantoSyntaxis).Morethan9『cl皿kwiserotation

oftheKantoMountainstTookplacebetween15and6Ma.Aboutahalfofitsoverall

rotationprobablyoccurredataround15Mainassociationwiththel-apidclo(2kwise

rotationofSo吐hwestJapan.Therestrotationoccurredti116Ma,1-c?su】tingthesharp

bentstl’uctureoftheKant()Syntaxis.Thelzu-BoninarchasbeencoHidingwithcent-

1-aにlapansinceMiddle5び匝)cene,whichmaybethecauseofthisilltra-arcdeformation.

1.1ntroduction

Thejf2lpanesels12mdsares;ituatedonthe

(?2lstemnlarginoftheEljlrasianPlatewhereboththePacificandPhilippineSeaPlatesare

subducting.0neofthemostdlarElderistic

ge()logicalf(j21turesofthe,1211)an(?sels121ndsisthezonald沁tributionofpre,Neogenet(?ITanesas

showninFig.1.Thiszonecol-11prisestlll‘eebelts

(Sanbagawa,ChichibuandShimantoBelts)andcanbetracedfromKyushutoc(2ntralJapan.Whilethetrendofthiszonalstrudureisparallel

totheaxisoftheislandarcinSouthwestJapan,

thiszoneissharplybentatcentralJapan,which

iscalledtheKantoSyntaxis.Detailedtimingof

thef()1・mationofthisbendhasbeencontr()ver-

551211sincethelastc(?ntury。TheSouthFossaMagnaregion紬locatedat

(:entra1Japanwherethelzu-Boninarconthe

northwestwardmigratingPhilippineSeaPlateis(mHiding(Fig.2).Recently,fielclinvestigations

l’(?vealedthattwoornloreexot弛blocksofthe

lzu-BoninarchadcollidedsinceMiddleMiocene

*FuelRc?sourcesDeμ1rtment

-477

(Niil:s;umaandM21tsuda,1985;Koyama,1991;Aman0,1991).Thickcong吊merateov(?rlying

abyssalm1.1dstonedistributedbetweencollided

blocksseemstol’el〕resentthetimingofthe

collision.Thecollisionofthelzu-Boninarcwith

centralJapanmaybethecauseofthenorth-

wal・d-convexstructureofpre-Neogenetel‘ranes。Thispaperpl‘(?sentsthepaleonlagnetic

l’esultsoftheKantoMountainsandadjacent

areastoshowtheblockrotationwithinthe

islandarc.The(N)111si()ntectonicsbetweenthelzu-Boninarcandccmtra1JapanattheSouth

FossaMagnaregionisalsoreviewed.The

illterrdationsamongtherotationsofSoutllwest

andNol’theastJapanarcs,coHisi()ntectonicsin

centr211jlapanan(nateralbodingoftheKanto

Syntaxisarealsodis5cussed.Thesynthesisof

NeogenetedonicdevelopmentoftheJapaneselslandsisproposedbasedonmanykindsof

geologicaldata21ndthegeophysicalc()nstl-aints.

Keyw()1‘ds:tcctonicl’otation,iは1‘a-arcd(2函rmation,paleo,

malJnetism,Miocene,(こ()IHsiontec2tonics,K2!ntoSyntaxis,

(2e?ntraIJapan

Page 2: MiocenelateralbendingofcentraIJapan -lntra ... (1994b).pdf · TElnzawaBlockcollidedatF5一一3MaandthelzuBlockco回nencedtocol】ideat1Ma(Aman0,1991).

LEGEND

召❹信加げ/加Gc)んずg/&4c・J,Q八/砂o,ド❹.45yW.ダリ

136°E

Phi㈲

-- ■

-

W ,

|||

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111111111111111111111 11 il111111111111111 illl 11101111 liijllll

凛見

モ.こ

二回neSeaPlate X

j4oEI

く≫

しJOj5cm/y「-

\XXX

PacificPlate

一 一

Fig.1Thepl‘esenttedonicsketdloftheJapanesehlands.Thesolidal-rows5indicatethepl‘esentplaten!otions

relativetotheEumsianPlate(MinisterandJordan,1978,1979).The,1211)anesel引an面havebeenunder

conlpr(jssionaltectonicl‘egimesince3Ma(SatoandAnlan0,1991).Thelzu-Boninarccollideswith

(ごentr211Japanxvherezonalcl心tributionofthepre-Nc?ogenegeologict(ll’ranesiss5harplybent。

- 4 7 8 -

|戸ヅ,ザ

(y比

42・N]ノ尹

j叩郎

, 01 こ

1320E

4c❹y「l

1340E

S臼

ぷ1皿(

掻6oE11

360N

自 万 言 靴 言=鸞二言言で?苔三ニニss̈「el゙ eツ幽乱レ。一JynormalfaUlt/strike-s;lipfault。ダ・゛‾thrust∧ノMTL:MedianTectonic LjneISTL:ltoigawaihizuoka`I’e(;tonicm1KantoMountains

ノ’咽

Page 3: MiocenelateralbendingofcentraIJapan -lntra ... (1994b).pdf · TElnzawaBlockcollidedatF5一一3MaandthelzuBlockco回nencedtocol】ideat1Ma(Aman0,1991).

j石z)とヽ9e/α/o所占c、❹泌ぜげにがg7/知力四回かs訟/刀zxyz/47、s/岬

(瓦atemaryロコlsedimentElfyrocksrンぐ司lvolcarli(;lplr()|(juctsF4eogene

「∇曰匠コr万円

(沁taceous町m`FFTlsedimentaryrocks[し:KjsJraniticrocks

sedimantaryrocks-1:ChichibuBasin9lcanjcrocks-2:しJchiyamaarea

F)re`Neogeneaccretionary,complexes

[≡三IShimantoBeltFyで7回clni(がbuBeltに二)Mir10,AshioBeltsMetamorptlicrocks

睡旦回S21nbagawaBelt[S五hlraniticr()cks-3:ChihibUQuartzDiorile謳 鞘 ⊇ 分 子 り ] 言 言 | , ・ で 猟 | 万 | ド 回 | 白

7:lzuBlockずElnticlineFig.2Detai】edgeologica】mapofcelltr21】JapallmodifiedfromGo]ogicalSurveyouapall(1992).COHisionsof

theKuslligatayamaandIX/lisakaBlo(21ぐsl)1・obablytookplaceat12Maand9-7Ma,1-(≧s;pectively.The

TElnzawaBlockcollidedatF5一一3MaandthelzuBlockco回nencedtocol】ideat1Ma(Aman0,1991).

- 4 7 9 -

Page 4: MiocenelateralbendingofcentraIJapan -lntra ... (1994b).pdf · TElnzawaBlockcollidedatF5一一3MaandthelzuBlockco回nencedtocol】ideat1Ma(Aman0,1991).

召❹か//j7ゾ/加G67c7/rヅa7/&宍仰,げノ司

2.TectonicrotationoftheKantoMountains

Pjeomagneticinvestigationsareindispensa-

bleforstudyingislandarctectonicsofJapan,

xx7herc?rotationalnlotionsarecornrnonaswellas

x乙erticalmov(?丁lents.Thezonaldistributionof

thepre-Neogenet(?I’ranesintheKanto

Mountainshasad迂ferenttrend行ominSouth-

westJapan,suggestinganoccurrenceofad汀-・硲rencialrotationasapartoftheintra-arc

d(2fommtion.AstheKantoMountainsarelocatedontheeast-wingoftheKantoSyntaxis,

the(jfferenceofthepaleonlagneticdirections

betweentheKantoMountainsandthe21djacent

areasrepl‘(そsentsad(2f()rmationh沁toryoftheKanto5E;y7ntaxis.Atfirst,thel⊃211eorllagnetic

(:1而ctionswithagesofthreeareas(C111chibu

Basin,UchiyamaareaandtheChichibu(jual‘tz

Lyc)r江e)intheKantoMol.mtainsarereviewedin

detailtoshowaclo(こkwiserotationoftheeast-

wingoftheKantoSyntaxis(Tablel).2.1ChichibuBasin

TheMiocene(:;hichibuBasinis51()catedata

centralpartoftheKantoMotjmtains(Fig.2),xv1-1eremorethan5000m-thickse(:lhnentswere

depositedinspiteofitssmaHarealextension

(油out1(50km2).SedimentaryrocksintheChichibuBasinconsistofcongl()merate,

sandstoneandssmstonewiththintufflayers.

HyodoandNiits;uma(1986)m(・2lsuredthe

paleonla!Jnetismofthe55edimentaryrocksintheCh祐hibuBasininanefforttoclarifythe

timingofthebendingoftheKantoSyntaxis.A川loughtheycolleded1:)aleomagnetics5anlples

from133sitesinthebasin,only58specirnens

of↓1sitesshowedstablepaleomagnetic

directionsbythestepwise21卜ernatil堪field(AF)

21ndtherma1(Th)dem21gneliizations.Fig.3柚dicateseachtilt-coITectedpale-

omagneticdiredionofllsitesintheChichibu

Basin.Eachsolidarrowisforthenorma1

1:)olal-ityandtheopenarrowshowsl‘(?vc?rsed

polaritywithoppositedil-ection.Anlean

l⊃211eomagneticdiredionoftheChichibuBasinisD=93.7°andl=52.7°withtheradiusof95%c()nfi(jencecircle(α95)of8.3°,afterthe

inversionofthereversedpolarityclirectionstothenormalpolarity.べvhilethenlean

inclinationisnotsignificantly(jjヲferentfrom

thatofsfc・ocentricaxialdipolefield(56°),the

meand(?clinationis(jeflectedeasterlyfrom

north.Asthesellsitescompriseboth

normalandl‘eversedpo121ritieswhichrel:)l-esent

asufficientlylongperiodtoaveragethe

geomagneticsecularvajation,HyodoandNHtsuma(1986)concludedthatthec2astward

paleonlagneticdeclinationisprobablycausedbytheclockwiserotationoftheKantoMountains。

ThegeologicageoftheChichibuBasinis

zoneN8(Blow,1969)basedonthep121111Uonic

mi(;l’ofossils(Takahashi❼ご皿,1989;Tak21hasl福1992).Thel’eforethepaleol!lagnetic叩1arity

s5c?quenceoftheCh沁hibuBasincanbecol-relat-

edwithChronC5B(B(ll‘ggrenば屁,1985).The

dul・ationofthezoneN8iscaHll)ratedtobe16.6-15.2MaafterBerggren❼砿(1985).

Consequently,morethan90°clockwisel・()tationoftheKantoMountainstookplace

since15.2Ma.2.2UchiyamaArea

Fig.4showsthegeologicmapofthe

uchiyamaareawithpaleomagneticclata

11’le・asl.lredbyT21kahashiandWatanbe(1992).

lntheuchiyanlaarea,northwesterncomer

oftheKantoMo1.mtains(Fig.2),1;(jver211facies

ofvolcanicrocksandintrusionsaredistl-ibuted.AmongaH,theYaekuboForn!ationisagood

targetforpaleonlagneticstudy,considering

intercalatingnlanyfreshandesitelava.The

paleomagnetismofthesnTlallintrusion(Takono-minePorphyrite)wasalsomeasured.TheK-Ar

agesofthe2111(jesitelava(11.9±0.9Ma,12.2±0.4Ma,12.4±0.4Ma)andoftheT21旨momine

P()1^ r)11yrite(12.5±0.4Ma)slj1託estthe(hiserela-

t硲nsbetweenthehltrusionoftheporphyriteand

t:heeruptionoftheandesitelava(Nomuraand

Kosaka,1987;NomuraandEbihara,1988)。Paleomagneticsamplingwascarriedoutat9

sitesfortheandesitelavaoftheupperYa(2kubo

F()mmtionandat2sitesfortheTakonominePorphyrite.Thetotalnumberofs;amplesand

spe(jnlensare47and271,resp(jctively。Bythe

(221refult:reatmentoftheThand/orAF

denlaSrnetizations,reHablepaleomagneticdi-I’ectionswereobtainedfron12111samples。

EacharrowintheFig.4showss5江e-nlean

diredionwh叱hareca1(:ulatedfromthe

- 4 8 0 -

Page 5: MiocenelateralbendingofcentraIJapan -lntra ... (1994b).pdf · TElnzawaBlockcollidedatF5一一3MaandthelzuBlockco回nencedtocol】ideat1Ma(Aman0,1991).

訂,7,c・ot加如❹&,・❹加μQノ″clg・/,❹力加・・り1勣g肩71直z/。ヽ/岬

Table!Paleomagneticdirectionsol)tain(jdfromLheChichibuBasi11,UchiyamaareaandtheChichibuQ皿rtz

Dk)rite.Someim凹八antplanktonicmicl-ofossilsandrad而nletricdataare削5solisted.yandj7arethe

lulmbel‘ofthesanlF)lesalldspecimens,l‘espe(凭ively;力?is5intensityofnaturalremanentmaμnetization

(NRM)(×103elllu/c❹り;Demag.isdemailnetizationlevel;Dec.andlnc.al・ed(?clinationand

inclinationofl-(2111anentmagn(jtizationafter21卜ematingdemaylne?tizati()n;Dec.*andlnc.*are

de(jinationandinclinationaftertilt-ccjrrection;α95isra(1iusof95%confi(lencelimitin心grees;んis

l)I‘(・cisionpar21nleter;Lat.(N)andL()ng.(E)arelatitudeand岫噌itudeofvirtualge(mlagneticpole

(VGP)。

S i t e Ⅳ n J n D e m a g . D e c . l n c .

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Page 6: MiocenelateralbendingofcentraIJapan -lntra ... (1994b).pdf · TElnzawaBlockcollidedatF5一一3MaandthelzuBlockco回nencedtocol】ideat1Ma(Aman0,1991).

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-

-

2 2 ‘

482

- -

- -

-

-

-

-

-

-

--

宍]呂呂ぐ弐・・’●・・・

-

一 一- - 一

一 こ Ξ i て ・ ÷ ‾ ‾ ‾ ‾ し 一

-

-

-

Fig.3Pa】e(jmagnetjcresu)toftheChichibuBasin.Each2lrrowr{?I}resentstilt-c()rrecteds;江e-meandiredion

(HyodoandNMsuma,1986).G(2010gicalnlapandstrJltigraphicdivisionareafterTakahasM(1992).The(2匹terlydeflecteddirectionssug卯sttheclo(2kwiserotationoftheChichibuBas5inafter15.2Ma.

-

-

20-

- - 一一 -

一 -

- 一

一 - -

NA

V試

- ㎜

-

1 〃

=谷34

32

‾ / 1

- 4

〃 ●

-

一= j

-

-

-

YokozeFormationNaguraFormationoganoFormationShirokuFormationushi1(ul)ito9eFormation

-

-

ヱムこと三==三二二言吊二三てこ=E

三 垂 士 彗 言 三 三一 一 一 - - ヽ - 一 一

-

-- 一 一 一 一 - -

] - ざ £ フ i j 万 二 二 三 二 三 二 三 二 三 二一 一 一 - ス ー 一 一 -

良 二 一 一 ‾ 一 匹 = = 二 = y T T - 一 一 ニ

皿 縦 ⊇ 言 三 三 芝 三 垂一 一 - - 一 一 - - - - - - - - -

-

-

-

-

-

IEqualAreaNet

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j4石肖・。丿α/c,❹/5a力。gがcg❹・❹μ?り乙z7回?1をsd,・乃加/7α功り

UclliyamaArea(12.5J.9Ma)

lnclination

6 0 ・ →5 0 ・ →4 0 ・ →3 0 ・ →

LEGEND

[二回MizuochikannonLava

回三]ShigaWeldedTuff‘

Polarityopenarrox/1/:reversedW

(oppositedir6・ction)・solidarrow:nonal

巴二こ]Hiraovolcanics[と11⊇]KosakaFOrmationに]EIJTakonominePorphyrite

i¢aekuboFOrmation

Dz34,701z51.20a95ニ4.40

Num.9

じln]YaekuboF°ormatior巾pper)匠回`・’・leklj1)()Formatioil(lower)犀71KoillagJomeF()rmation

Fig.4PalecjmagneticresultoftheuchiyamaareamodifiedfromT21kahashiandW21tanabe(1993).The

(?21stwarddeflecteddirectionsoftheTakonominePorl3hyrite吊ornlalpolarity)areinconcordantwith

thoseoftheYaekuboFol・mation(reversedone).Baseontheradk)nletricagesoftheseigneousrocks,

about3y(210ckwiserotationoccurredafterabout12Ma.

s521nlple-meandirectio】ls.XVhilethean-owsof

theandesitelavaaretilt-corrected,the

paleomagneticdil・ectionsoftheporphyritearein-s5itudiredions.Asthelavabedsoftenhave

aninitialdip,thesel⊃211eomagneticdiredions

were出t-c(汀rectedbasedonthebeddingplane

ofhltercalateds;andstoneortufflayers.The

formatioltmeandiredion,D=-145.3≒I=ニー51.2°,α95=4.4°,wasthengivenbytheti卜

(2()ITededs沁e-meandiredionsofthegsites。Despitetherearenom沁ropaleontological

datafortheYaekuboFol-mation,theradiomet,

ricagesofboththelowestandtheupperande

sitelavaareapproximately12MawjthsmaH

erl・or.Basedontheserad沁metricdataandthe

paleonlagneticpolarity,theupperYaekuboFormationcanbecorrelatedwiththeChronC5

AofBerggren❼皿(1985)。

Thep211eomagneticclirectionoftheYaeku-

boFoT‘mationshowseastwardden(jctionafterinversionofthel-(jversedpolaritydirectiontoa

normalone,whichs;tronglysuggeststheclock-

wisel’otationofthestudyareasince12Ma.

TheTakonominePcj叩hyrite(12.5Ma)alsohas

aeastwardd(?necteddeclinationwithnorma1

1:)olarity,whichisconcordantwiththatofthe

4 8 3 -

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召・jか//・叩/丿加G回/cy沁7/乱4砂げ面力四づ/❹//5ぶり.ダリ

YaekuboFormation.Consequently,itcanbe

(mllcludedthatabout35°eastwarddeneded

clil・edionisprobablyduetotheposレ12Maclo(ごkwiserotationoftheKantoMountains.

2.3ChichibuquartzDiorite

TheChichibu(λ1artzDiorite柚trudedinto

thepre-Neogenerod(softheChichibuBeltat

thec凹tra1F)artoftheKantoMountaills(Fig.2).

Thisis;11eousrodiisastockofmediuln-grained

lllasivequartz(lioritewithsmaHdikesof

(luartzporphyryandporphyrite.Paleonlagnetic(lil-ectionoトtheChichibu(QuartzDioritewas

lTle21suredbyTakahashiandNonTlura(1989).

Totalnumberofol‘ientedhandsanTlplesand

spe〔jmensare51and304,resl〕edively.Bythe

(mrefultl’eatmentofAFdemagnetization,reli,

ablep211eomagneticdiredionswereobtainedfrom21s5amplesasshowninFig.5。

Paleonlas否netismofthenorthenlpartofthe

Ch沁h什)u〔⊇uartzDioriteshowsl‘c・vc・rsed

l〕olarity.0ntheotherhand,itssouthwestembodyi11(こ1udesboth叩rmala面r(五?V(?rsedpolari-

ties.Thelitcjfaciesofthe(:lhichibu(こ!uartz

L:)ioriteisnotsigllificantlyheteroSji(?neous,im-

plyingamonogenicorigin.Thefactthatthereversedpolarityisrecognizedonlynearthe

lllarginofthedioritebodysuggestsachangein

geomaSlnetisnlduringtheperiodofcoolingofthe(luartzdiorite。

MeanpaleomagneticdiredionoftheChichi-

buQuartzDiorite(D=3.5°,I=64.4≒α95=5ユ゜)isc211culatedafterinversionofthereversed

l〕olaritydiredionstoan(jrmalone.Pala)mag-neticdirectionsofreliable21sanlplesshowboth

normalandreversedpo121ritiesandare(:1沁tribut-

edinantipodaldiredions,whichs5汝nifiesthat

thenyasureddirectionsl‘epresentmagnetic’fi(?ldsofasufficientlylongperiodtoaverage

geomagnetics5(2cularvariations.TheobtainedclirectionoftheChidibuQuartzL:)ioritecanbe

21s5sumedtorel⊃l‘esentanaveragedipolefield。

Thepaleonlagneticd(?clinationshowsno

s5ignificantd(?flectionfromthc?northasshowninFig.5.訊咀ileeacharrowin(licatesanin-situ

clirectionbecauseoftheintrusiveorigin,no

clc?fl(?cteddiredions53tronglysuggestthatthe

l-()1:ation211motionoftheKantoMo1ム1ntainsafter

theilltrusionofthisdiorit(?isneijigiL)1ysma11.Theradi()1万netricagesofthisdiorite(6.6±0.3

Ma,5.9±0.4Ma:Uenoand5511n:)ata,1986)may

representthelatestllimingoftheceaseofthe

rotation.

3.Diflヲerentialrotationwithinislandarcs

Fig.6showss5tratigraphicpositionsofthe

NeogenerocksoftheCh弛F吐)uBasin,

UchiyamaareaandtheChichibuQuartz

l:)iorite.Also5;hownarethetimeconstraints

basedontheradiometricagesorplanktonic

micl‘ofossilsaswellaspaleonlasrneticdirec,

tions.AsthelateTGtiarypaleomailneticpoles

ofEul-asiaalmostcoincidewiththel:)14esent

ge()Sil‘aphicpole(lrvingandII-ving,1982),the(1(?fledionsindeclinationoftheCMchibuBa-

sin,UchiyamaareaandtheChichibuQuartz

Dioriterepl^ esentthesumofclockwiserotation

anglesoftheKantoMoljlntainsafter15,12and6Ma,respectively.Therefore,theKantoMoun-

tainsrotatedabout60°clockwiselyl-elativeto

thegeomagneticnorthbetween15and12Ma,

|:11(ごnmosto員herestangle(31°)isEltt:ril:)utedto

theperiodbetween12and6Ma,b(?causethel’otationsince6MaisnelJligiblys5maH.

Fig.7indicatesthepaleomagneticdeclina-

tionvaluesduringtheT(2rtiaryobtainedfrom

SouthwestJapan,Nohiarea,NortheastJapan

andtheKantoMo1.1ntains.Thedlangesinpa-

1eonTlagneticd汁ectionoftheseareasre1:)resent

eachhistoryoftectonicrotationl-elativetothemagneticnorth,sothatwecanknowthediffer-

entialrol:ationbytheconlparison21mongthem.

ltbecameapparentinthelastclecadethat

SouthwestJapanexperiencedalargeclockwise

rotationconclJrrentwiththeJapanSeaopening.

(OtofujiandMatsuda,1983,1984,1987;C)tofujid❹.,1985a,b,c;Hayashidaandltoh,1984;

Hayashida,1986;Hayashidad❹.,1991).Basedonadatasetobtained斤omSouthwest

Japans函ce1982,0tofuji❼屁(1985a)(?stimatedthe2lnlountofclo(2kwiserotationas47≒They(j⊃tainedthetimingofthisrotation

usingb(jstfittingcurvesfolJotationontheplots

ofdeclinationagainstagedata;theclimaxof

l‘otationisat14.9Ma.Recently,0tofuji❼混

(1991)m(・as1.1redK-AragesasweHas;paleomag-11etismofMiocenevolcanicrocksinSouthwest

Japanto(,hcidatethedurationofthel・oliation.

484

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な1,いぽ7,.g加的7z/加i❹jヌ笞がm狛❹ノ砂αJ?fjl趾sd,・7滋r,/s的

(池ichibuQuartzDiorite(6.61.91Vla)

NA

|「lclination

6 0 ・ →5 0 ・ →4 0 ・ →3 0 ・ →

r)olarjty

openarrovv:reversed師positE・dire(lion)

soliarrovxl:normalN

Dz3.40

1z64.40

a95=5,10

Num,21

Hemisphere●lower

oupper

EEqualAreaNet

Fig.5Paleomagneticl-esultoftheCljchibuQuartzL)ioritepartlynldifi(司fromTakahashiandNomura

(1989).Althoughthesepaleolllagneticclirectionsarein-situones,211nipodald脳tl-ibutionwithN-S

trendsignifiesthatthemtationoftheKantoTNyl()untainsafterilnrusionofthisbodyisneSjigibly

smal1.

Theirdata・5howthatmol-ethan80%(;39°)ofthe

()verallntation(47°)ofSouthwestJapan(犯(2urredbetweenappl‘oxil-nately16and14Ma,

whichrepl-(?sentstherapidmtationofSouthwest

Jal:)21n(2()拍T1.yr.).1’hereare,however,somepaleomagnetic

datasuggestingnTluchs5mallerrotationa121ngles

〔j〕tainedfromNohi21re21,1ocatedonthec?astem

partofS5outhwest.Japan(ltoh,1986,1988).SincetheNohiarea硲1ocatedonthewest-wing

oftheKantoSyntaxis(Fig.1),thepaleonlag-neticl-c?sultsshouldl-(う1:)resental’otationofthe

wc・st-wingoftheKanto55yntaxis.ltoh(1988)

showedthepaleomasぶneticpro1:)ertiesofLate

Cret21(2eousandMio(2enc?1-ocksintheNohiarea,which柚dicatedasHghtd(j4Flecteddeclination

4 8 5 -

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フミ?・z/か加丿/加Gむj乙7/昭たw/極々 砂丿置φ凹,ド❹.丿了yvむ・.召/リ

召≪

O-Cr二

晦○ -

駒[工二≪工 QO(/つ○山○

MAGNETIC

POLARITY

PLANKTON

ZONES

白奥 ZOQ二工○

OZZ≪Z §

2Q、

--=

ト ニ---

2 二ー--

3一二ー---

4 で---

5 ----

6.二ー--

7 こ---

8一二ー---

9 ----

10一二ー--

1 1 二ー--

12.二ー---

13で--

14一二ー---

1 5 ----

16二ー--

17こ---

18こ

11-=2-

12A・3

13A■4-■4A一一

1一一

:5A

iS5B

1:-

BLU. ヱ 旦否 A

CN13こ

ヨ 五Q A

CN11Ξ

9 !S LA

N23

MAT.N22

山トー

牡Q_>-

N21

GAU.

N19

二N18二

N17

GIL.

バZ白2

肖-2

-

>-i

5B-

CN9A

N16

二N15二N14

二N13こ

N12

89

CN8-10

CN7

レC5

CN6

CN5 -

C5A

C5AAC5AB’-C5AC

N11

N10

二N9二

N8

C5AD

CN4C5B

C5CCN3

N7CN2C5D N6

ふぶ二Q.心・一口)-収5よ=C/)○-ΦにCICU2Chichibu

OuartzDiorite

リ(士づこニご

Paleoma9neticDiect10ns

5.9±0.4Ma

6.6±0.3Ma

しJchiyamaArea(YaektJt)()Forrnation&Tak(匹)minePorphyrjte)

ジ andesitelava4V01canicbreccia

ノ片耳tUgbreccia

?lporphyrjlej____tuffaceoussandstone

12.2±0.3Ma12.4±0.4Ma11.9±0.9Ma

12.5±0.4Ma

(;FlichibuBasinf・r(り/,肥rs!7/,mmsaJ/・詐心---ごー.匹りtalusbrecciaR().岫η畝間・a

し賑二慧E皆・1ヽ・,介巳万万回sac1ノrva

j;.忽諾li安祐厖急窟

レ縦

D=34.7°

|=51.20

E3erg9「en!?ぱ(1985)

Fig.6Summaryofther)aleomagneti(2resultsintheKantoMotlntains.Thes5{!(Tluentialsh匝ofthepaleomagnetic

cl(ごclinationrc?1)resentstheclockwiserotationoftheKantoMoulltains.

(13-17°).Thesep;lkom2lgneticdatainferthatthewest-wingoftheKantoSyntaxisrotatedmorethan30°coljmter-clockwiselyrelativeto

thenlainpartofSouthwestJapanbetween15-12Ma.Thenorthwarclbendingofthezonal

s5trudureattheNohiareaisint(?1’pretedtohave

beenfol-medbytheleft:1ateralplasticd(?4Fc)rma-

tionate21stenlendofthel’otatingSouthwest

Japan,basedonthegeologicfeaturesofcentral

Japanaswellaspaleomagneticdata.Therehavebeenonlyfewpaleomagnetic

worksonthelateT(?rtiaryrocksinNortheast

Japan(OI:ofujD/・❹.,1985;ToshaandHamano,

486

1986,1麓斗徳N圓711taniandTanoue,1988,

Yamazaki,1989).0tofujid砿(1985(:)l’eported

paleonlagneticdatafromawideareaofNorth-eastJapanandconcludedthatNortheastJapan

hadrotatedcourlter-clockwisethrough47°be-

tween2land11Ma.Theyattributedthediffer-

entia1圃tationsbetweenSouthwestandNorth-eastJapanarcstotheback-arcopeningofthe

JapanSeawithadouble-doormode.ToshaandHanlano(1986,198tgい-evealedthatNol’出east

Japanwass5nニuatedalongtheeastcoastoftheAsiancontinentsinceEarlyCr(?t21ceousand

rotatedbetween22and15Ma.Theesl:hTlated

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100090 80706050403020100 009

(゜)ヱ○一にg]OS

ME o 111111111jl711

-100

訂・・c・し回z,yかz/じ7❹加μ7方昭・げじ四/心/知力四M責

105

一 一 一 一 一

゛ ね t

487

TheSouthFossaMagnaisthoughttobeabeneathboththeEt11-asianandPhilippineSea

-WEST-一一・

NeogenecoHisionzonebetweenthelzu-Bonin

arconthePhilippineSeaPlateandc(?ntral

JapanontheElju‘asianPlate(Figs.1,2).AfterSugin!ura(1972)r)ointedoutthattheboundary

betweenthePhilippineSeaandE1ム汀asianPlates

waslocatedintheSouthFossaMagna,ithas

beenbdievedthatthecollisionofthelzu-Bonin

archadcausedthecuspstructureofpre-Neogenet(うrranes(Matsuda,1978,1984;

N血s3uma,1982,1989).lnl-ecentyears,

detailed柚vestigationsonthegeologyand

geophysicsoftheSouthFossaMagnarevealedthetedonichistoryofthisarc-arc(2()msionzone.Sincesuchcollisionisexpectedtohave

causedastrongcleformationinand21roundthe

(20111dingblock,briefreviewsofthecollision

l:edonicsatcentra1Japanareshownhere。ThetectonicsofcentralJapanarer(?1atedto

theEtム1rasian,PhilippineSeaandPacificPlates

asshowninFig.1.ThePacificPlatesubduds

anlountoftherotationwasson!ehows3maH

(l‘oughly20°)thanther)1^ evious(?s;timation

(Otofujid混,1985c).l`ヽJ沁hitaniandTanoue(1988)also16・terminedthepaleomaSrnc・tismofNol-出eastJapantoreconstructthepaleo-

positionandtoelucidatethetinlingofthel‘otati()naln!otion.Theirpale()nlagneticl-esults

showedthatthecoun12er-clockwiserotationtook

placebetween23and13Ma,whichr(21:)resentsalongd1.1rationofrotationl-(う1ativetoSouthwest

Japan.Recently,Yan!azaki(1989)performeda

l)aleomagneticstudyoftheNeogell(2rocksalongthePacificside.HeshowedthattherotationofNollheastJapanhadceasedl:)efore16Ma,

whichsuggeststhedほerenceintimeof

l‘()tationsbetweenSouthwestandNol’theast

Japanarcs。

4,COIlisiontectonicsatcentraIJapan

Fig.7De,clinatjonversusagefc31-theKantoMcjaltajns(HyodoandN阻suma,1986;T21)121hashiandNc3mura,

1989;TakahashiandWM21nabe,1993),Southw6tJapan((:)tofuji❼❹.,1985a),Nohhrea(ltoh,1986,

15)88)andNOI’theasuapan(Otofujidごz伝!985c;1`ヽJ沁hitaniandTこmoue,1988,Yamazaki,198S);Hoshi

d混,1992)partlymodifi(・dfr()mTahhashi(1994).Ea(lh(!rr()rb211-isth(21-adius005%corlfidencecircle

(α95).

1 5 2 0AGE(Ma)

□□/V()肋θasりapar7

謡三:ニ 昂 l g エ ヨ 。 ,

し … … ・ - - 一 一 ギ 。 - - … … … … N O R T H 一 一 -

C b : C h i c h i b u B a s i n ’ “ ” ” ’ ‘ ’ ” ’ ‾ ‾ ‾ ‾ ’ “ ’

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励・/忽加げ/加Gt7と7/昭俵z/乱,7砂丿五

Plates.ThenthePh雨ppineSeaPlatealsohasa

s3tlbdudionboundarywiththeE1.u・asianPlate,whileano辻herntipofitcollideswithcentra1

JapanontheIE:ur21sianPlate(Fig.2).ThesubductionboundarybdweentheEurasianand

PhilippineSeaPlatescanbetracedonshoreat

theSouthFossaMagnawheretheboundary

naturechangestoacollisionalone.The

comsionofthelzu-BoninarcwithccltralJapan

maybeattributedtothebuoyantnatureofthe

activeislandarc。R(2cently,Amano(1991)i(肪ntifiedfourexot-

icblocksintheSouthFossaMagna(Fig.2);

Ku血igatayama(Kg),Misaka(Ms),Tanzawa

(Tz)andlzu(lz)1310dis.Theseblocksarecomposedofthickpilesof551.1bmarinevolcanics

andpyroclasticswhichareexceedinglydeform-edand21比ered.Basedonthelithological

chal-aders,thesefourblocksareregardedas

thetectonicsegn!entsofthelzu-Boninarc,

whichcoHidedednorthwardwithcc?ntralJapan.

TheagesoftheirvolcanicactivitiesareEarlytoMiddleM沁cene.Thethickpilesofd(?12ritus

coveringthesefourexoticseSでmentsare(jistrib-

utedalongthemarginsbetweentheblod(s.’Γhesesedimentsshowsljr(2nerallyupward-

coarseningnatureandisl:)l‘obablyrc?1:)resenting

thecollision-relatedsequences.Forexample,theJikeMljldstone,disliributedalongthc・north-

emmarginoftheTanzawaBlock,isthought

tohavebeendepositedonthetroughslopeor

onthetroughbottom,infl-ontofthenorth-

wardnligratingT21nzawaBlockasshownin

Fig.8(NiitsumaandAkiba,1985).TheOchiai

Conglonlerate,ovc?rlyingtheJikeMuclst:one,is

(刀1Tnposedofco&11-5seぶraineddctt2ritusoffan,del,tasderivedfromtheKantoMoulltains,which

fmedupadeeptroughrapidly.Thebuoyant

llatureofcomdingTanzawaBlockwasthe

(こauseofabruptupliftingoftheKantoMoun-tainswhichsuppliedtheco211一色5e,grainedclastics

tothedeeptroughrapidly.Thusthe1ム1pward

coal‘seningsequenceisthoughttobeagoodin-

(Ecatorwhentheblockcollided.Th(?l-efore,

theTanzawaBlockhlitiatedtocollidewith

(:entra1Japanat5Ma.Basedonthebio-andlithostl’atigraphyoftheconSJlomeratedomi-

natedhorizons,Amano(1991)concludedthe

t211-ningsofthecollisionoftheKush汝atayama,

Misaka,TanzawaandlzuBlod{sat12Ma,9-7

Ma,5-3Maand1Ma,respectively.Theseco1-

Hsionsaree)cpectedtocausestrongdefor-

rl121tionsinanc121roundtheSouthFossaMagna。

5.LateralbendingofcentraIJapan

Basedontheac(:ulllulatedresultsofpaleo-

magneticl-es(?arch,whichcandetectdil‘ectlya

componentoflargelatitudinaldriftaswellas

l’otation,idealizedl-ecollstructionsofthetec-

tonichistoryoftheJapaneselslandsareshowninFigぎ.9and10.

TheJapaneselslandswerel()catedalongthe

(?astemmarginoftheAsiancontinentbeforethe

JapanSeaorj(jning.ltisdearthatthezonals5tl’tlctureofthepre-Neogenegeologicl:(jITanes

wasalmostinlineal-trendinEarlyMiocenetime

asl‘(?constructingbythepaleonlalmeticdata.Thisimpliesthatthesharpbentstructuredid

notexistbeforetheJapanSeaopening.Back-

arcs51⊃readingoftheShikokuBasinstartedat25

Ma(Kol:)21yashiandNakada,1978),andits

activespreadingcenterandthenewlyf()rmedoceaniclithosphereonthebothsidesalso

subcluctedb(2neaththejlal)aneselslands.Aboutz17°clockwisel・otationofSouthwestJapantook

placeat2tround15MaandtheKantoMountains

(east-wingoftheKantoSyntaxis)probablyrotatedclo(;kwise,whilethewestwingofthe

KantoSyntaxis(Nohiarea)migratedwithonly

s1泌htl・otation,resultingthebendings;tructure

oftheKantoSyntaxis.

I)LlringthisF)el‘iod(lateEarlyMioceneto

(?arliestMiddleMio(2ene),agreatnumberofhalfsll-abenswere4万〇rmedespedaHyintheJapan

SeasicleofNol-theastJapan(Yanlaji,1989,1990),whichin(IH(Jatc・sthatNol’1111easl:釦panwas

531.1bjectedtoextensionaltectonics(YamajiandSat0,1989).Basedonthes;tratigraphiccon-

s3tl-aints,itcanbe(;()ncludedthattheintervalof

thisextensionaldc?formationcontinuedfron118

to15Ma.Thefornlationofmanyhalfslrabens

and2lbrupt53ubsidencearethoughtasaresponsetothestretchingandl・汀tingofthelil:hossphereof

thearc(TakeshitaandYamaji,1990).Akhough

the(letailedtimingoftherotationofN()11heast

Japanisstillobscureowingtos3carcenessof

pal(2omagneticdata,itisreasonabletoEls3sume

488

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ylyl‘,g・り鎚/α的7z/加,涵・昭がcり27j❹力φ皿r71仇sd,’毀ルrz/,μ,sがリ

EASTERNMARGINOFTHETANZAWAMTS,

STRATIGRAPHICCOLUMN

OchiaiConglomerate

JB§|§ヨク㎎1(xls3胎

言 ご

TanzawaGroup

v(Mcanicroく;ks

basaltlava

CollisionoftheTanzawaBlock

、 ・ yl z u B 1 0 c k 匹

匹V V V V V V V V V V V VV V V V V V V V V V V

十 十 十

++※; 琵

ゝ△,乙 △

’り’v’vlv’v’v’v’v’V’v’vlvlvンyU12£coarse

v v v v v v v v v v v v v v ° c l a s t i c s“ V V V V V V V V 十 十

ゝ 5 M a C O L L I S 1 0 N 足 谷 于 ≒

卜 し v v v v v v v v v v 6 ム . - p ’ F I A P I D ‾ ィ + 与vvvvvvv゛゛゛゛゛9`隔心゛UPLIFTING十V リ V V V X / X / り V V V V V I ゛ + † 十 十 そ 十PHILIPPINESEAPLATEvvv、V-≒+j+≒九にV V V V V V V V V V V V V V ゛ 十 + 十 +̈ v v v v 四 斗 十 十

= ぅ 丁 、 - 一 一 、 j ツ t ° E ゛ . . サ 言 七 羊ゾ v ゛ ゛ ゛ ゛ ゛ ゛ ゛ ゜ “ ゜ d e e p s e a m u d 十 十 十 十v v v v v v △ 彿 十 + ÷ 十 十 十 十 十 十 十

V ツ ド サ j 宍 で 1 1 才 で ↓ 1 1V V V V V V V V V V V V V V V

/ づ j v v v v v v v v

乙 △ 乙A A A A A

V・,TanzawaB10cFvvv

匹 白 v v v v v v ヤ .V V V V V V V V V V V V V V ’ み 1

ト V V V V V Y V v v v v y v V jv v v v v v v v v v v v v v v

十十十・

÷ 十

てー ●I●・-■■・WF

JMountains十 十 十 十 十千 十 十 十 十 一

十 十 十 十 十十 十 十 十 十柚心i;赤居㎜-■¶■■W●■●■●・

J7√+・PLATE・十V V V V V V V V V ゛ / V V V V V V

十 + +

Fig.8S5(lh(2maticdiagram55hwi11gfourstagesduringtheMioce】leandPH(jcellec(jl)isjona]historyofthe

TanzawaBlockwiththeKalnoMotlntains(Takahashi,1994).TheJikep,/ludstonecl(2positedinthe

deeptroughhtween山eK皿toM皿ntainsandtheT21nzawaBlock,andoverlyingcoarseclastics

(OchiaiConsllomerate)abl・uptlyfilledupthetrough,whichmaybe21ssociatedwiththecollision

andaccretionoftheTanzawank)ck.

thatthecount(?r-cl()diwisel-otatio11()(工urredcluringthattime,thatwasl⊃1-obablycausedby

theopeningoftheJapanSea。

I)rasticeventswass1.Kldenlys5toppedatthe

endofthel-otationofSouthwej,;tJapan(ca.14

Ma).Sincethen,theJapanarcgotintoa

tectonicallyqui(?tsl:ateexceptforc(jntra1Japan.NortheastJapansJ:)sidedgentlyalldintra-arc

cl(2fol-n121tionwassnlallsince14Ma.Thes5tress

fieldd(2ducedfron!dikes,veinsandminorfaults

s1!owsthatNol‘theastJapanxvere1.mderweakcmTlpressionalorneutralregimeti11EarlyPlio-

cene.0ntheotherhand,theconl叫essional

deformationtookplaceinc(mtralJapanat14Ma

(OishiandTakahashi,1989),sugl岬stingtllatthecoHisionofthelzu-Boninarcinitiatedatthat

time.Theabout30°additionalrotationoftheKantoMoljmtainsoccurredbetween12and6Ma,

1’(2sultingthecuspshape就r1.1(:tureoftheKanto

489

Syntaxis.ltislikelythatthisrotationwascausedbytheco川sionsoftheKushig21t21yama

andMisakaBlocksagainsttheKantoMoun-

tains。

ThetectonicregimeoftheJapaneselslands

wasneutralorweakcompl‘essionalinLateMiocen(j(Aman0,1980;Satod混,1982;Sat0,

1986;(:)tsuki,1990b).Many(;211deras;tl-uctures

l-elatedtofelsics51.1barialvolcanismswere

jテ()rmedinN()rtheastJapan.Althoughthe

TanzawaBlockcollidedat5Ma,thel-()tationoftheKantoMol.mtainsafter6Mawasnotmore

than珀゜(Fig.5),whichs;吊nifiesthatthe

(こ()HisionoftheTanzawaBlodididnotplayan

importantroleinformationoftheKanto

Syntaxis.ThecollidingTanzawaBlocks3eemstohavedeformedon】yNeogenestratainthe

SouthFossaMagnaarea。

S;inceca.3Ma,the,J2lpaneselslandshave

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Z?・r//e/加げ/加G凹/昭ja/乱々砂丿ノ砂匹,ド❼.石ya.汝タ

Fig.9Pakogec3graphicmapofthe,1211)anese臨landsinthe】atestEarlyMicjcenetoearlyMiddleMioc。e.

The,s;tressaxesofびII-maxinfe,rredfronldikes(Yamamot0,1S)91)andveinl;(()tsuki,1989,1990b)arealso

shown.Thereconstrudedplateboundaries,aswellaso琵shor(2area,arepartlyn!odifiedfromotsuki

(1990a).Thezon211stru(ごtul・eofpre-N(2ogenet(21’raneswas211nlostinline21rtrendbeforetheJapanSea

()r)(jning.Thispc?riodischal・aderizedbytheextensionalte(ごtonicsa吋oceanw21❹driftingwithrotation

oftheJal)2lnes(きlslands.

1ミ)eens3ufferingthecollll)ressionaldef()I・mationasisalnlostnormaltothetrenchinNor-theast

showninFig.1.Manybotlndarynorm211jFaultsJapan,whiletheび1trendinSouthwestJapan

oftheEarlyMiocenehalfSII‘abenshavereadedisdiagonaltotherelativenlotionofthe

asl:hrustswherethickNeogenestrataarePhilippineSeaPlate.Thusthes;tressfieldin

stronglyfolded.Therecon,3truded血一essfieldSouthwestJapanmayhavebeensubjectedto

suggeststhatthenlaximumprincipa1就ressisboththexvestxへ7ardrnigrationofthePacificPlate

()I・ientedhol-izontally.The(治-ectionoftheび1andnorthwestwardnlotionofthePhilippineSea

490

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訂7・し7むヽ碍・肥かz/りw//7四r/,’所丿rc❹mノルノ)のリガ四廊花/Jび/,心/岬

co‖isionalsegmenlsoftllelzu-Boninarc

lz:lzuB10ckTz:TanzawaB10ck

Ms:MisakaB10ckKg:KUshりatayamaBlock

LEGEND

日∃ShimantoBelt

[:二](DFlichibuBelt㎜S3ε1nbagawaBelt

[:IEΞ]inactive!;prEladingcenter&youn9oceanjccrust

.ょvolcanicfront/mean(消{?ntationofdikes,ystressaxisof(yH.maJnferredfromveins

9()calderaswarm奮熊KantoMOUntains

Fig.10Paleogeographjcmapsofcentra】JapaninMiddleix・liocelletoEarlyP】iocenetime.T】lenlajor

l’otRtionsofSouthwestandNortlleastJapanarcsceasedat!.4Ma,whiletheKanto?,仙untains

rotatedtil1LateMiocene.TheKushigatayama,MisakaandTanzawaBlockshavecollideda】ld

2tc(ごretedattheSouthFossaMaμnaregioninsuccc?ssion.The2lclclitionalrotationoftheKanto

Mountains,probablycausedbythecollisionofthelzu-Boninarc,1-(?sultedintheshal・pbent

s;tructureoftheKantoSyn幽xis.

491

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7j・・・/か/加丿/加Gcy・7/偕加z/乱川砂ヅルカ凹,じ❹.・y5A気猷夕

Plate.Asforthe(:entr21にlapa11,thelzuBlock

initiatedtocollideat1Ma(Amanoぱ混,1986)

andtherec()nstructedr)rincipleslEressaxesare

almostequaltotherelativemotionofthel)hilippineSeaPlate(Aman0,1991).Thetec-

tonicsofcentra1Japanhavebeendominatedby

thecollisionofthelzu-BoninarcsinceMiddleMiocene.

6.Conclusions

1.Thezonald沁tril:)utionofthepre-

Neogenetel’l・aneswasalmostinlineartrendl)eforetheJapanSeaopening,thatis,the

(:Llspatestl‘1jlctureoftheKantoSyntaxisat

(氾ntr❹Japandidnotexistthen。

2.ThelateralbendingoftheKanto

Syntaxisfirstlyoccurredataround15Maby

thecl汀ferentialrotationsatcentralJapaninas-s5()ciationwiththerapidclockwiserotationof

SouthwestJapan。

3.TheKantoMo1.1ntains,onthe(221st-wing

oftheKantoSyntaxis,rotatedtmLate

M函cene,1-(?5sultingasharpbendatc(2ntral

ムlapan.Probablecauseofthisadditionalrota-tionwasthecollisionofthelzu-Boninarconthenorthwestwardmigl・atingPhHippineSea

Plate.

Acknowledgements:Manythanksaredueto

Drs.K沁£汁)uroKodanlaandYoshioWatanabe

oftheGeologica1SurvayofJapanforl-ading

themanl.1scriptandofferingvaluablesugges-

tions。

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MANUSCRⓣTRECEIVEDNOVEMBER24,1993

MANUSCRⓣTACCEPTEDMARCH7,1994

中新世における中部日本の折れ曲がり一島弧一島弧衝突域における弧内変形 -

高橋雅紀

要 旨

典型的島弧である日本列島の中部域は,複数の昌弧(東北日本弧・西南日本弧一伊豆-小笠原弧)が会合し,複雑な地史を経てきたと推察される.実際,九州から西南日本外帯に連続して追跡される先中新統の帯状配列は,中部日本において著しく北西方向に湾曲し,

関東対曲構造(KantoSyntaxis)として前世紀よりその形成時期および成因が議論されてきた.このような島弧規模の変形過程を把握するために,地質体の回転量を定量的に表わす古

地磁気データを時間軸にそってブロヽンク毎にまとめ,さらに屈曲構造形成に大きく関与したと考えられる伊豆一小笠原弧の衝突と関連させて,中部日本の変形史を考察した.その゛

結果,前期中新世の日本海の拡大にともない,時計回りに回転した西南日本の東端において湾曲構造が形成されたことが強く示唆された.そしてその後のフィリピン海プレートの

北西方への沈み込みにより,伊豆一小笠原弧が南部フォッサマグマにおいて衝突し,湾曲構造の東翼である関東山地がさらに回転した結果,より屈曲した構造が6Maまでに形成されたことが判明した.

495