大三啦-大地構(gòu)造05plate tectonicsthe divergent boundary

大地構(gòu)造學

（Tectonics）5.DivergentboundariesPLATEBOUNDARIES板塊邊界是指兩個板塊之間的接觸帶。地球表面最重要構(gòu)造活動帶?；鹕健⒌卣饛娏?，占總數(shù)85%。從板塊間相對運動方式來說，可以將板塊邊界分為三種類型：離散型板塊邊界(divergentboundaries)聚斂型板塊邊界(convergentboundaries)轉(zhuǎn)換斷層邊界(transformfault)Theearth’sfracturedsurfaceUsuallystartwithincontinents—growstoeoceanbasin5.1DIVERGENTBOUNDARIES

—fromcontinentalriftstomid-oceanridges1.Themid-oceanicriftsystemEastPacificrise:東太平洋洋隆Galapagos:加拉帕戈斯CarlsbergRidge:卡爾斯伯格海嶺Bouvetridge:布維島海嶺ScotiaArc:斯科舍島弧Azores:亞述爾Gibraltar:直布羅陀REYKJANESRIDGE:雷克雅內(nèi)斯海嶺SevernayaZemlya:北地群島ThemainriftsystemThemid-oceanicriftsystem東太平洋洋隆太平洋-南極洲洋脊澳大利亞-南極洲洋脊東南印度洋洋脊西南印度洋洋脊大西洋洋脊EastPacificrise:東太平洋洋隆Galapagos:加拉帕戈斯CarlsbergRidge:卡爾斯伯格海嶺Bouvetridge:布維島海嶺ScotiaArc:斯科舍島弧Azores:亞述爾Gibraltar:直布羅陀REYKJANESRIDGE:雷克雅內(nèi)斯海嶺SevernayaZemlya:北地群島（2）SubsidiaryriftsystemGalapagosridgeSouthChileriseCarlsbergridgeRedSeariftAzoresrift勞·哈佛(LauHavre)海槽CaymanRidge:開曼海嶺SouthScotiaridgeCaymanRidge:開曼海嶺(3)SmallridgesegmentsEastPacificrise:東太平洋洋隆Galapagos:加拉帕戈斯CarlsbergRidge:卡爾斯伯格海嶺Bouvetridge:布維島海嶺ScotiaArc:斯科舍島弧Azores:亞述爾Gibraltar:直布羅陀REYKJANESRIDGE:雷克雅內(nèi)斯海嶺SevernayaZemlya:北地群島5.1DIVERGENTBOUNDARIES

—fromcontinentalriftstomid-oceanridges2.continentalriftzones:Landmasssplitsintotwoormoresegments(1)BasinandrangeprovincesofNorthAmerica(2)AfricaandtheEastAfricanRiftThreemodesofrifting,Buck,1991Gueydanetal.,2008Buck,1991(1)BasinandrangeprovincesofNorthAmerica(1)BasinandrangeprovincesofNorthAmerica(1)BasinandrangeprovincesofNorthAmerica(1)BasinandrangeprovincesofNorthAmerica(1)BasinandrangeprovincesofNorthAmericaExtensionrateincreasesfromeasttowestacrosstheSierraNevadaScience:3/12/99Science:11/30/07Strainrates(1)BasinandrangeprovincesofNorthAmerica(1)BasinandrangeprovincesofNorthAmerica(1)(2)(3)(4)(1)BasinandrangeprovincesofNorthAmericaModels(2)AfricaandtheEastAfricanRiftContinentallithosphereisstretchedhorizontallyandthinnedverticallyduetoextension;normalfaultsdevelopinuppercrust;ductiledeformationinthelowercrust(mylonites)andmantleHighheatflow,buoyantuplift,volcanism,andplutonism(volcanics,intrusions,floodbasalts,rhyolite)(2)AfricaandtheEastAfricanRiftContinentallithosphereisstretchedhorizontallyandthinnedverticallyduetoextension;normalfaultsdevelopinuppercrust;ductiledeformationinthelowercrust(mylonites)andmantleHighheatflow,buoyantuplift,volcanism,andplutonism(volcanics,intrusions,floodbasalts,rhyolite)EastAfricariftsystem(extendsN-S~3000km)TheriftseparatestheAfricanplatetothewestandtheSomalianplatetotheeast(2)AfricaandtheEastAfricanRiftContinentallithosphereisstretchedhorizontallyandthinnedverticallyduetoextension;normalfaultsdevelopinuppercrust;ductiledeformationinthelowercrust(mylonites)andmantleHighheatflow,buoyantuplift,volcanism,andplutonism(volcanics,intrusions,floodbasalts,rhyolite)EastAfricariftsystem(extendsN-S~3000km)(2)AfricaandtheEastAfricanRiftContinentallithosphereisstretchedhorizontallyandthinnedverticallyduetoextension;normalfaultsdevelopinuppercrust;ductiledeformationinthelowercrust(mylonites)andmantleHighheatflow,buoyantuplift,volcanism,andplutonism(volcanics,intrusions,floodbasalts,rhyolite)EastAfricariftsystem(extendsN-S~3000km)(2)AfricaandtheEastAfricanRiftContinentallithosphereisstretchedhorizontallyandthinnedverticallyduetoextension;normalfaultsdevelopinuppercrust;ductiledeformationinthelowercrust(mylonites)andmantleHighheatflow,buoyantuplift,volcanism,andplutonism(volcanics,intrusions,floodbasalts,rhyolite)EastAfricariftsystem(extendsN-S~3000km)SomaliSubplateAfricanPlateDivergentBoundariesContinentalPlateSeparationEastAfricanRiftValleySomaliSubplateAfricanPlateEastAfricanRiftValleySomaliSubplateAfricanPlateParallelvalleys;volcanoesandearthquakes.RiftSegmentationEastAfricanriftshowingstep-likesegmentsobliquetotheriftaxis—proto-ridgesforfutureseafloorspreading?NatureGeoscience:4/08變質(zhì)核雜巖（metamorphiccorecomplexesorcorecomplexes,orCordilleranmetamorphiccorecomplexes70年代研究美國西部Cordillera造山帶時發(fā)現(xiàn)的一種特殊構(gòu)造單元，并提出mcc的概念，80年代以來研究甚廣，并掀起大陸伸展構(gòu)造研究的高潮。附：關于第三紀變質(zhì)核雜巖變質(zhì)核雜巖定義（Concept)：1)Coney(1980):Agroupofgenerallydomalorarchlike,isolatedupliftsofanomalouslydeformed,metamorphicandplutonicrocksoverlainbyatectonicallydetachedanddistendedunmetamorphosedcover.變質(zhì)核雜巖研究表明，在北美西部Cordillera造山帶中（從北部加拿大不列顛哥倫比亞經(jīng)美國西部向南至墨西哥西北部長約3000km、寬300km的帶內(nèi)，發(fā)育有至少27個mcc。MetamorphiccorecomplexDistributionofCordilleran-typemetamorphiccorecomplexesinwesternNorthAmerica39(from:Fossen2010)A)ListerandDavis(1989)model:detachmentfaults“fire”fromoldersubhorizontalmyloniticshearzone;shearzoneiscapturedbymasterdetachmentfaultandisostaticallybowedupwards;B)SpencerandReynolds(1986,1989)model:myloniticshearzonerepresentsmid-crustalcontinuationofinitiallylow-angledetachmentfault;detachmentfaultandshearzoneareisostaticallybowedtosubhorizontal;C)“Rollinghinge”modelofBuck(1988)andWernickeandAxen(1988):myloniticshearzonerepresentsmid-crustalcontinuationofhigh-anglenormalfault;normalfaultandshearzoneisostaticallyrotatetosubhorizontal(figureadaptedfromBartleyet

al.,1990);D)Domino-faultingmodel:similartorolling-hingemodelexceptmechanismofrotationisdomino-stylenormalfaulting(e.g.Proffett,1977;Davis,1983;GansandMiller,1983;WongandGans,2008).變質(zhì)核雜巖形成與演化

拆離斷層的構(gòu)造巖組合系列是拆離斷層正斷抬升作用的結(jié)果。拆離斷層作用過程中，在不同的構(gòu)造層次上形成不同的構(gòu)造巖。隨著拆離斷層作用的抬升，形成于深部的糜棱狀巖石逐漸向上運移，依次經(jīng)歷退化變質(zhì)剪切作用（角礫巖－綠泥石化角礫巖）、脆性碎裂作用（微角礫巖）。這些巖石依次疊加，最終形成特定的斷層構(gòu)造巖序列。拆離斷層帶作為一個脆-韌性轉(zhuǎn)變條件下形成的斷層帶，發(fā)育了從碎裂巖—微角礫巖—角礫糜棱巖—綠泥石化角礫糜棱巖—糜棱巖—片麻巖(3)AyoungOceanBasin:TheRedSeaTheUpperProterozoicArabian-NubianShield(ANS)(3)AyoungOceanBasin:TheRedSeaGeographyFromBosworthetal.,2005(3)AyoungOceanBasin:TheRedSeaGeography⑦Sediments:Gypsum,gravelsandfinesandsinalluvialfans.Beneaththislayeristherocksaltlayer.⑥Upperrocksaltlayer:Layeredrockssaltwithrareoccurrenceofthin-layeredgypsum,anhydriteandclays.NamedastheHoustonlayer.⑤

Markerlayer:Rocksaltwithinterlayersofanhydrite.④

Sylvite(鉀鹽）layer:Puresylviteandrocksalt(KCl35-40%).Theotherpotashsaltsincreasedownward.③Middleinterlayer:Therearealwayscarnallite（光鹵石）fromthetoptothebottom.Potashsaltoccuratthetopandkainiteatthebottom.②

kainite（鉀鹽鎂礬）layer:75%kainiteand25%rocksalt.Layerrocksaltincreasesdownward.Thelayermaydisappearduetofaulting.①Lowerrocksaltlayer:layeredrocksaltwithrareoccurrenceoflayeredanhydrite.⑦Thesalt-bearingstratigraphicsequence①②③④⑤⑥⑦Rocksaltlayer(3)AyoungOceanBasin:TheRedSea古近系火山巖和沉積物，大體分為三個單元：最新的阿法爾組為玄武巖熔巖流;泰勒阿費爾玄武巖組成的玄武巖熔巖流；古近紀高原玄武巖，主要為橄欖石玄武巖與中間的熔巖夾層。(3)AyoungOceanBasin:TheRedSea(3)AyoungOceanBasin:TheRedSeaSeafloorspreadingAnomaliesinthesouthernRedSeaPresenttoamaximumageof5MaMaximumagevariesalongaxisDifferentnucleationcentersthathavelinkeduptoformacontinuousaxisNucleatedat~17°NandpropagatedbothnorthandsouthfromCochran(1983)RiftingHistoryRiftingprecededbymassivevolcanisminEthiopiaandsouthernYemenAr-Ardatingshowsthatmosteruptedfrom31-29.5MaSomecontinuedactivityto25MaCoevalflowsasfarnorthassouthernSudanPrimarilybasalticvolcanism,butsomerhyolitesfromKeneaetal(2001)fromBosworthetal(2005)AreawasnearsealevelatbeginningofvolcanismMassivevolcanismwasnotpaniedbysignificantextensionCurrentlyactiveRedSeawidthvs.openingrateCurrentlyactiveFormedwellawayfromactiveplateboundariesCutsthroughPreCambrian(~700Ma)Pan-AfricanlithosphereNearlyorthogonalextensionPresentanexampleofoceanfloorspreadingTheRedSeariftOceansarebornwhencontinentsbreakapart(e.g.,deconstructionofPangeaandformationofAtlanticOcean)Continentsbreakapartatacontinentalrift—aregionofcrustalextension(4)Matureriftedcontinentalmargins:passiveorAtlanticstylemarginsOceansarebornwhencontinentsbreakapart(e.g.,deconstructionofPangeaandformationofAtlanticOcean)Continentsbreakapartatacontinentalrift(4)Matureriftedcontinentalmargins:passiveorAtlanticstylemargins(1)(2)(3)(4)(5)(6)(5)Oceaniccrustandoceanspreadingcenters(5)Oceaniccrustandoceanspreadingcenters1)Topography(5)OceaniccrustandoceanspreadingcentersSubduedtopography平緩地形volcanicedifice火山機構(gòu)(5)OceaniccrustandoceanspreadingcentersSubduedtopography平緩地形volcanicedifice火山機構(gòu)(5)Oceaniccrustandoceanspreadingcenters2)Magneticanomalypatterns(略過）層1，沉積層，厚度變化大，大洋中脊上往往缺失或作零星分布。層2，火山巖層，沿中脊頂部廣泛出露，也廣泛分布于洋盆中，縱波速度變化大。深海鉆探表明，主要由拉斑玄武巖，部分為固結(jié)沉積巖組成。層3為輝長巖或橄欖巖，縱波速度和厚度都十分穩(wěn)定，厚度在5公里左右，是大洋地殼的主體部分。層3的底面為莫霍面，該面之下為上地幔。莫霍面實際上是海水滲透和熱液蝕變的最低界面。3)Seismology3)Seismology3)Seismology4)Gravityandheatflow4)Gravityandheatflow5)Compositionofoceaniccrustandtheophiolitemodel4layersdistinguishedviaseismicvelocitiesDeepSeaDrillingProgramDredgingoffracturezonescarpsOphiolitesOceanicCrustandUpperMantleStructureTypicalOphioliteLithologyandthicknessofatypicalophiolitesequence,basedontheSamialOphioliteinOman.AfterBoudierandNicolas(1985)EPSL,76,84-92.Layer1

Athinlayerofpelagicsediment

OceanicCrustandUpperMantleStructure

ModifiedafterBrownandMussett(1993)TheInaccessibleEarth:AnIntegratedViewofItsStructureandComposition.Chapman&Hall.London.

Pelagicsediment

or

pelagite

isafine-grainedsedimentthathasaccumulatedbythesettlingofparticlesthroughthewatercolumntotheoceanfloorbeneaththeopenoceanfarfromland.Theseparticlesconsistprimarilyofeitherthemicroscopic,calcareousorsiliceousshellsofphytoplankton

（浮游植物）or

zooplankton（浮游動物）;

clay-size

siliciclastic

sediment;orsomemixtureofthese.Traceamountsofmeteoricdustandvariableamountsofvolcanicashoccurwithinpelagicsediments.Baseduponthecompositionoftheoozearethreemaintypesofpelagicsediments.Theyare

siliceousoozes,

calcareousoozes,andredclays.Thecompositionofpelagicsedimentsiscontrolledbythreemainfactors.Thefirstfactoristhedistancefrommajorlandmasses,whichaffectstheirdilutionbyterrigenous,orland-derived,sediment.Thesecondfactoriswaterdepth,whichaffectsthepreservationofbothsiliceousandcalcareousbiogenicparticlesastheysettletotheoceanbottom.Thefinalfactorisoceanfertility,whichcontrolstheamountofbiogenicparticlesproducedinsurfacewaters.Layer2isbasaltic

Subdividedintotwosub-layers

Layer2A&B=pillowbasaltsLayer2C=verticalsheeteddikesOceanicCrustandUpperMantleStructure

PillowlavasintheSemailOphioliteSheetedDykesinSemailOphioliteBasalticpillowsSubmarineeruptionsandpillowsSheetedDyke/LavaTransitionTheverticalslabsofrockaredikesintrudingintolavasthateruptedontheseafloor.ThissectionrepresentsthetransitionfromlavastosheeteddikesandisthoughttocorrespondtoseismicLayer2BLayer3

morecomplexandcontroversial

Believedtobemostlygabbros,crystallizedfromashallowaxialmagmachamber

(feedsthedikesandbasalts)Layer3A=upperisotropicandlower,somewhatfoliated(“transitional”)gabbrosLayer3B

ismorelayered,&mayexhibitcumulatetexturesLayeredGabbrosandMohoSemailDiscontinuousdioriteandtonalite(“plagiogranite”)

bodies=latedifferentiatedliquidsOceanicCrustandUpperMantleStructure

PlagiogranitesLayer4=ultramaficrocksOphiolites:baseof3Bgradesintolayeredcumulatewehrlite&gabbroWehrlite

(異剝橄欖巖)intrudedintolayeredgabbrosBelowcumulatedunite

withharzburgitexenolithsBelowthisisatectonite

harzburgiteanddunite(unmeltedresiduumoftheoriginalmantle)Serpentinites(weatheredperidotites)青藏高原班公湖的放射蟲硅質(zhì)巖；

日喀則枕狀熔巖TheAlpsDistributionofophiolitesintheworldObductionObduction

istheoverthrustingof

continentalcrust

by

oceaniccrust

or

mantle

rocksata

convergentplateboundary,suchasclosingofanoceanora

mountainbuilding

episode.Thisprocessismonasthedenseroceaniclithosphereusuallysubductsunderneaththelessdensecontinentalplate.Obductionoccurswhereafragmentofcontinental

crust

iscaughtina

subductionzone

withresultingoverthrustingofoceanicmafic

andultramaficrocksfromthemantleontothecontinentalcrust.Obductionoftenoccurswhereasmall

tectonicplate

iscaughtbetweentwolargerplates,withthecrust(both

islandarc

andoceanic)weldingontoanadjacent

continent

asanew

terrane.Whentwo

continentalplatescollide,obductionoftheoceaniccrustbetweenthemisoftenapartoftheresultingorogeny.Mostobductionsappeartohaveinitiatedatback-arcbasinsabovethesubductionzonesduringtheclosingofanoceanoranorogeny.Oman(Semail)OphioliteGreenschistfaciesshearzonesLayered…massivegabbrosDykesPillows(6)IcelandTheMid-AtlanticRidge,whichsplitsnearlytheentireAtlanticOceannorthtosouth,isprobablythebest-knownandmost-studiedexampleofadivergent-plateboundary.

AerialviewoftheareaaroundThingvellir,Iceland,showingafissurezone(inshadow)thatistheon-landexposureoftheMid-AtlanticRidge.Rightofthefissure,theNorthAmericanPlateispullingwestwardawayfromtheEurasianPlate(leftofthefissure).

(7)TheWilsoncycle-EarlystagesThesimpleidealmodeloftectonicevolutionWilsonCycle–OpeningPhaseRiftingeventsandtheformationofdivergentplateboundariesaredrivenbymantleconvectioncells.Whereamantleplumeorconvectioncellrisestowardthelithosphere(redzonesinthediagramabove)heatistransferredtothelithospherecausing