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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
LEGEND
召❹信加げ/加Gc)んずg/&4c・J,Q八/砂o,ド❹.45yW.ダリ
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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 -
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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
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 -
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 -
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
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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.
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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 -
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
W
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
E
Dz3.40
1z64.40
a95=5,10
Num,21
Hemisphere●lower
oupper
EEqualAreaNet
示
S
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 -
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
抑
牡Q_>-
生
N21
GAU.
N19
二N18二
N17
GIL.
W
バZ白2
肖-2
-
>-i
5B-
CN9A
6
7
N16
二N15二N14
二N13こ
N12
89
CN8-10
CN7
レC5
CN6
B
CN5 -
A
C5A
C5AAC5AB’-C5AC
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二N9二
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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
100090 80706050403020100 009
(゜)ヱ○一にg]OS
ME o 111111111jl711
-100
0
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 ’ “ ” ” ’ ‘ ’ ” ’ ‾ ‾ ‾ ‾ ’ “ ’
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
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
S
、 ・ 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 △
N
’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
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
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
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
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