大三啦-大地构造05plate tectonicsthe divergent boundary

大地构造学

（Tectonics）5.DivergentboundariesPLATEBOUNDARIES板块边界是指两个板块之间的接触带。地球表面最重要构造活动带。火山、地震强烈，占总数85%。从板块间相对运动方式来说，可以将板块边界分为三种类型：离散型板块边界(divergentboundaries)聚敛型板块边界(convergentboundaries)转换断层边界(transformfault)Theearth’sfracturedsurfaceUsuallystartwithincontinents—growstoeoceanbasin5.1DIVERGENTBOUNDARIES

—fromcontinentalriftstomid-oceanridges1.Themid-oceanicriftsystemEastPacificrise:东太平洋洋隆Galapagos:加拉帕戈斯CarlsbergRidge:卡尔斯伯格海岭Bouvetridge:布维岛海岭ScotiaArc:斯科舍岛弧Azores:亚述尔Gibraltar:直布罗陀REYKJANESRIDGE:雷克雅内斯海岭SevernayaZemlya:北地群岛ThemainriftsystemThemid-oceanicriftsystem东太平洋洋隆太平洋-南极洲洋脊澳大利亚-南极洲洋脊东南印度洋洋脊西南印度洋洋脊大西洋洋脊EastPacificrise:东太平洋洋隆Galapagos:加拉帕戈斯CarlsbergRidge:卡尔斯伯格海岭Bouvetridge:布维岛海岭ScotiaArc:斯科舍岛弧Azores:亚述尔Gibraltar:直布罗陀REYKJANESRIDGE:雷克雅内斯海岭SevernayaZemlya:北地群岛（2）SubsidiaryriftsystemGalapagosridgeSouthChileriseCarlsbergridgeRedSeariftAzoresrift劳·哈佛(LauHavre)海槽CaymanRidge:开曼海岭SouthScotiaridgeCaymanRidge:开曼海岭(3)SmallridgesegmentsEastPacificrise:东太平洋洋隆Galapagos:加拉帕戈斯CarlsbergRidge:卡尔斯伯格海岭Bouvetridge:布维岛海岭ScotiaArc:斯科舍岛弧Azores:亚述尔Gibraltar:直布罗陀REYKJANESRIDGE:雷克雅内斯海岭SevernayaZemlya:北地群岛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变质核杂岩（metamorphiccorecomplexesorcorecomplexes,orCordilleranmetamorphiccorecomplexes70年代研究美国西部Cordillera造山带时发现的一种特殊构造单元，并提出mcc的概念，80年代以来研究甚广，并掀起大陆伸展构造研究的高潮。附：关于第三纪变质核杂岩变质核杂岩定义（Concept)：1)Coney(1980):Agroupofgenerallydomalorarchlike,isolatedupliftsofanomalouslydeformed,metamorphicandplutonicrocksoverlainbyatectonicallydetachedanddistendedunmetamorphosedcover.变质核杂岩研究表明，在北美西部Cordillera造山带中（从北部加拿大不列颠哥伦比亚经美国西部向南至墨西哥西北部长约3000km、宽300km的带内，发育有至少27个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).变质核杂岩形成与演化

拆离断层的构造岩组合系列是拆离断层正断抬升作用的结果。拆离断层作用过程中，在不同的构造层次上形成不同的构造岩。随着拆离断层作用的抬升，形成于深部的糜棱状岩石逐渐向上运移，依次经历退化变质剪切作用（角砾岩－绿泥石化角砾岩）、脆性碎裂作用（微角砾岩）。这些岩石依次叠加，最终形成特定的断层构造岩序列。拆离断层带作为一个脆-韧性转变条件下形成的断层带，发育了从碎裂岩—微角砾岩—角砾糜棱岩—绿泥石化角砾糜棱岩—糜棱岩—片麻岩(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(钾盐）layer:Puresylviteandrocksalt(KCl35-40%).Theotherpotashsaltsincreasedownward.③Middleinterlayer:Therearealwayscarnallite（光卤石）fromthetoptothebottom.Potashsaltoccuratthetopandkainiteatthebottom.②

kainite（钾盐镁矾）layer:75%kainiteand25%rocksalt.Layerrocksaltincreasesdownward.Thelayermaydisappearduetofaulting.①Lowerrocksaltlayer:layeredrocksaltwithrareoccurrenceoflayeredanhydrite.⑦Thesalt-bearingstratigraphicsequence①②③④⑤⑥⑦Rocksaltlayer(3)AyoungOceanBasin:TheRedSea古近系火山岩和沉积物，大体分为三个单元：最新的阿法尔组为玄武岩熔岩流;泰勒阿费尔玄武岩组成的玄武岩熔岩流；古近纪高原玄武岩，主要为橄榄石玄武岩与中间的熔岩夹层。(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平缓地形volcanicedifice火山机构(5)OceaniccrustandoceanspreadingcentersSubduedtopography平缓地形volcanicedifice火山机构(5)Oceaniccrustandoceanspreadingcenters2)Magneticanomalypatterns(略过）层1，沉积层，厚度变化大，大洋中脊上往往缺失或作零星分布。层2，火山岩层，沿中脊顶部广泛出露，也广泛分布于洋盆中，纵波速度变化大。深海钻探表明，主要由拉斑玄武岩，部分为固结沉积岩组成。层3为辉长岩或橄榄岩，纵波速度和厚度都十分稳定，厚度在5公里左右，是大洋地壳的主体部分。层3的底面为莫霍面，该面之下为上地幔。莫霍面实际上是海水渗透和热液蚀变的最低界面。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（浮游动物）;

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

(异剥橄榄岩)intrudedintolayeredgabbrosBelowcumulatedunite

withharzburgitexenolithsBelowthisisatectonite

harzburgiteanddunite(unmeltedresiduumoftheoriginalmantle)Serpentinites(weatheredperidotites)青藏高原班公湖的放射虫硅质岩；

日喀则枕状熔岩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