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EENS1110 PhysicalGeology

TulaneUniversity Prof.StephenA.Nelson

DeformationofRock

Thispagelastupdatedon11Feb2012

MountEverestisthehighestpeakonEarthat29,028feetabovesealevel.Therockatthetopofthepeakisamarinelimestone,depositedontheseafloorabout450millionyearsago!Thisisanamazingfactthatbegsthequestionhowdidthatrockgetthere?Inthisdiscussionwewilltrytoanswerthatquestion.Thetopicswewillcoverinclude:

ReviewofStressandStrainBrittleDeformationFaultsandJointsDuctiledeformationFoldsMountainBuildingProcesses

StressandStrain

Westartourdiscussionwithabriefreviewoftheconceptsofstressandstrain.Recallthatstressisaforceactingonamaterialthatproducesastrain.StressisaforceappliedoveranareaandthereforehasunitsofForce/area(likelb/in2).Pressureisastresswheretheforcesactequallyfromalldirections.Ifstressisnotequalfromalldirectionsthenwesaythatthestressisadifferentialstress.Threekindsofdifferentialstressoccur.

1. Tensionalstress(orextensionalstress),whichstretchesrock

2. Compressionalstress,whichsqueezesrockand

3. Shearstress,whichresultinslippageandtranslation.

Whenrocksdeformtheyaresaidtostrain.Astrainisachangeinsize,shape,orvolumeofamaterial.Weheremodifythatdefinitionsomewhattosaythatastrainalsoincludesanykindofmovementofthematerial,includingtranslationandtilting.

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StagesofDeformation

Whenarockissubjectedtoincreasingstressitpassesthrough3successivestagesofdeformation.

ElasticDeformationwhereinthestrainisreversible.

DuctileDeformationwhereinthestrain

isirreversible.

Fractureirreversiblestrainwhereinthe

materialbreaks.

Wecandividematerialsintotwoclassesthatdependontheirrelativebehaviorunderstress.

Brittlematerialshaveasmallorlargeregionofelasticbehaviorbutonlyasmallregionofductilebehaviorbeforetheyfracture.

Ductilematerialshaveasmallregionofelasticbehaviorandalargeregionofductilebehaviorbeforetheyfracture.

Howamaterialbehaveswilldependonseveralfactors.Amongthemare:

TemperatureAthightemperaturemoleculesandtheirbondscanstretchandmove,thusmaterialswillbehaveinmoreductilemanner.AtlowTemperature,materialsarebrittle.

ConfiningPressureAthighconfiningpressurematerialsarelesslikelytofracturebecausethepressureofthesurroundingstendstohindertheformationoffractures.Atlowconfiningstress,materialwillbebrittleandtendtofracturesooner.

StrainrateAthighstrainratesmaterialtendstofracture.Atlowstrainratesmoretimeisavailableforindividualatomstomoveandthereforeductilebehaviorisfavored.

CompositionSomeminerals,likequartz,olivine,andfeldsparsareverybrittle.Others,

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likeclayminerals,micas,andcalcitearemoreductileThisisduetothechemicalbondtypesthatholdthemtogether.Thus,themineralogicalcompositionoftherockwillbeafactorindeterminingthedeformationalbehavioroftherock.Anotheraspectispresenceorabsenceofwater.Waterappearstoweakenthechemicalbondsandformsfilmsaroundmineralgrainsalongwhichslippagecantakeplace.Thuswetrocktendstobehaveinductilemanner,whiledryrockstendtobehaveinbrittlemanner.

BrittleDuctilePropertiesoftheLithosphere

WeallknowthatrocksnearthesurfaceoftheEarthbehaveinabrittlemanner.Crustalrocksarecomposedofmineralslikequartzandfeldsparwhichhavehighstrength,particularlyatlowpressureandtemperature.AswegodeeperintheEarththestrengthoftheserocksinitiallyincreases.

Atadepthofabout15kmwereachapointcalledthebrittleductiletransitionzone.Belowthispointrockstrengthdecreasesbecausefracturesbecomeclosedandthetemperatureishigher,makingtherocksbehaveinaductilemanner.Atthebaseofthecrusttherocktypechangestoperidotitewhichisrichinolivine.Olivineisstrongerthanthemineralsthatmakeupmostcrustalrocks,sotheupperpartofthemantleisagainstrong.But,justasinthecrust,increasingtemperatureeventuallypredominatesandatadepthofabout40kmthebrittleductiletransitionzoneinthemantleoccurs.Belowthispointrocksbehaveinanincreasinglyductilemanner.

DeformationinProgress

Onlyinafewcasesdoesdeformationofrocksoccurataratethatisobservableonhumantimescales.Abruptdeformationalongfaults,usuallyassociatedwithearthquakesoccursonatimescaleofminutesorseconds.Gradualdeformationalongfaultsorinareasofupliftorsubsidencecanbemeasuredoverperiodsofmonthstoyearswithsensitivemeasuringinstruments.

EvidenceofPastDeformation

Evidenceofdeformationthathasoccurredinthepastisveryevidentincrustalrocks.Forexample,sedimentarystrataandlavaflowsgenerallyfollowthelawoforiginalhorizontality.Thus,whenweseesuchstratainclinedinsteadofhorizontal,evidenceofanepisodeofdeformation.

Sincemanygeologicfeaturesareplanarinnature,weawaytouniquelydefinetheorientationofaplanarfeaturewefirstneedtodefinetwotermsstrikeanddip.

Foraninclinedplanethestrikeisthecompassdirectionofanyhorizontallineontheplane.

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Thedipistheanglebetweenahorizontalplaneandtheinclinedplane,measuredperpendiculartothedirectionofstrike.

Inrecordingstrikeanddipmeasurementsonageologicmap,asymbolisusedthathasalonglineorientedparalleltothecompassdirectionofthestrike.Ashorttickmarkisplacedinthecenterofthelineonthesidetowhichtheinclinedplanedips,andtheangleofdipisrecordednexttothestrikeanddipsymbolasshownabove.Forbedswitha900dip(vertical)theshortlinecrossesthestrikeline,andforbedswithnodip(horizontal)acirclewithacrossinsideisusedasshownbelow.

Forlinearstructures,asimilarmethodisused,thestrikeorbearingisthecompassdirectionandanglethelinemakeswithahorizontalsurfaceiscalledtheplungeangle.

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FractureofBrittleRocks

Aswehavediscussedpreviously,brittlerockstendtofracturewhenplacedunderahighenoughstress.Suchfracturing,whileitdoesproduceirregularcracksintherock,sometimesproducesplanarfeaturesthatprovideevidenceofthestressesactingatthetimeofformationofthecracks.Twomajortypesofmoreorlessplanarfracturescanoccur:jointsandfaults.

Joints

Aswelearnedinourdiscussionofphysicalweathering,jointsarefracturesinrockthatshownoslippageoroffsetalongthefracture.Jointsareusuallyplanarfeatures,sotheirorientationcanbedescribedasastrikeanddip.Theyformfromasaresultofextensionalstressactingonbrittlerock.Suchstressescanbeinducedbycoolingofrock(volumedecreasesastemperaturedecreases)orbyreliefofpressureasrockiserodedabovethusremovingweight.

Jointsprovidepathwaysforwaterandthuspathwaysforchemicalweatheringattackonrocks.Ifnewmineralsareprecipitatedfromwaterflowinginthejoints,thiswillformavein.Manyveinsobservedinrockaremostlyeitherquartzorcalcite,butcancontainraremineralslikegoldandsilver.Theseaspectswillbediscussedinmoredetailwhenwetalkaboutvaluablemineralsfromtheearthinacoupleofweeks.

Becausejointsprovideaccessofwatertorock,ratesofweatheringand/orerosionareusuallyhigheralongjointsandthiscanleadtodifferentialerosion.

Fromanengineeringpointofview,jointsareimportantstructurestounderstand.Sincetheyarezonesofweakness,theirpresenceiscriticalwhenbuildinganythingfromdamstohighways.Fordams,thewatercouldleakoutthroughthejointsleadingtodamfailure.Forhighwaysthejointsmayseparateandcauserockfallsandlandslides.

Faults

Faultsoccurwhenbrittlerocksfractureandthereisanoffsetalongthefracture.Whentheoffsetissmall,thedisplacementcanbeeasilymeasured,butsometimesthedisplacementisso

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largethatitisdifficulttomeasure.

TypesofFaults

Aswefoundoutinourdiscussionofearthquakes,faultscanbedividedintoseveraldifferenttypesdependingonthedirectionofrelativedisplacement.Sincefaultsareplanarfeatures,theconceptofstrikeanddipalsoapplies,andthusthestrikeanddipofafaultplanecanbemeasured.Onedivisionoffaultsisbetweendipslipfaults,wherethedisplacementismeasuredalongthedipdirectionofthefault,andstrikeslipfaultswherethedisplacementishorizontal,paralleltothestrikeofthefault.Recallthefollowingtypesoffaults:

DipSlipFaultsDipslipfaultsarefaultsthathaveaninclinedfaultplaneandalongwhichtherelativedisplacementoroffsethasoccurredalongthedipdirection.Notethatinlookingatthedisplacementonanyfaultwedon'tknowwhichsideactuallymovedorifbothsidesmoved,allwecandetermineistherelativesenseofmotion.

NormalFaultsarefaultsthatresultfromhorizontaltensionalstressesinbrittlerocksandwherethehangingwallblockhasmoveddownrelativetothefootwallblock.

Horsts&GrabensDuetothetensionalstressresponsiblefornormalfaults,theyoftenoccurinaseries,withadjacentfaultsdippinginoppositedirections.Insuchacasethedowndroppedblocksformgrabensandtheupliftedblocksformhorsts.Inareaswheretensionalstresshasrecentlyaffectedthecrust,thegrabensmayformriftvalleysandtheupliftedhorstblocksmayformlinearmountainranges.TheEastAfricanRiftValleyisanexampleofanareawherecontinentalextensionhascreatedsucharift.ThebasinandrangeprovinceofthewesternU.S.(Nevada,Utah,andIdaho)isalsoanareathathasrecentlyundergonecrustalextension.Inthebasinandrange,thebasinsareelongatedgrabensthatnowformvalleys,andtherangesareupliftedhorstblocks.

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HalfGrabensAnormalfaultthathasacurvedfaultplanewiththedipdecreasingwithdepthcancausethedowndroppedblocktorotate.Insuchacaseahalfgrabenisproduced,calledsuchbecauseitisboundedbyonlyonefaultinsteadofthetwothatformanormalgraben.

ReverseFaultsarefaultsthatresultfromhorizontalcompressionalstressesinbrittlerocks,wherethehangingwallblockhasmoveduprelativethefootwallblock.

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AThrustFaultisaspecialcaseofareversefaultwherethedipofthefaultislessthan45o.Thrustfaultscanhaveconsiderabledisplacement,measuringhundredsofkilometers,andcanresultinolderstrataoverlyingyoungerstrata.

StrikeSlipFaultsarefaultswheretherelativemotiononthefaulthastakenplacealongahorizontaldirection.Suchfaultsresultfromshearstressesactinginthecrust.Strikeslipfaultscanbeoftwovarieties,dependingonthesenseofdisplacement.Toanobserverstandingononesideofthefaultandlookingacrossthefault,iftheblockontheothersidehasmovedtotheleft,wesaythatthefaultisaleftlateralstrikeslipfault.Iftheblockontheothersidehasmovedtotheright,wesaythatthefaultisarightlateralstrikeslipfault.ThefamousSanAndreasFaultinCaliforniaisanexampleofarightlateralstrikeslipfault.DisplacementsontheSanAndreasfaultareestimatedatover600km.

EvidenceofMovementonFaults

Sincemovementonafaultinvolvesrocksslidingpasteachothertheremaybeleftevidenceofmovementintheareaofthefaultplane.

FaultBrecciasarecrumbleduprocksconsistingofangularfragmentsthatwereformedasaresultofgrindingandcrushingmovementalongafault.Whentherockisbrokenintoclayorsiltsizeparticlesasaresultofslippageonthefault,itisreferredtoasfaultgouge.

Slickensidesarescratchmarksthatareleftonthefaultplaneasoneblockmovesrelativetotheother.Slickensidescanbeusedtodeterminethedirectionandsenseofmotiononafault.

MyloniteAlongsomefaultsrocksareshearedordrawnoutbyductiledeformation

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alongthefault.Thisresultsinatypeoflocalizedmetamorphismcalleddynamicmetamorphism(alsocalledcataclasticmetamorphism.Theresultingrockisafinegrainedmetamorphicrockshowevidenceofshear,calledamylonite.Faultsthatshowsuchductilesheararereferredtoasshearzones.

DeformationofDuctileRocks

Whenrocksdeforminaductilemanner,insteadoffracturingtoformfaultsorjoints,theymaybendorfold,andtheresultingstructuresarecalledfolds.Foldsresultfromcompressionalstressesorshearstressesactingoverconsiderabletime.Becausethestrainrateislowand/orthetemperatureishigh,rocksthatwenormallyconsiderbrittlecanbehaveinaductilemannerresultinginsuchfolds.

GeometryofFoldsFoldsaredescribedbytheirformandorientation.Thesidesofafoldarecalledlimbs.Thelimbsintersectatthetightestpartofthefold,calledthehinge.Alineconnectingallpointsonthehingeiscalledthefoldaxis.Animaginaryplanethatincludesthefoldaxisanddividesthefoldassymmetricallyaspossibleiscalledtheaxialplaneofthefold.

Werecognizeseveraldifferentkindsoffolds.

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Monoclinesarethesimplesttypesoffolds.Monoclinesoccurwhenhorizontalstrataarebentupwardsothatthetwolimbsofthefoldarestillhorizontal.

Anticlinesarefoldswheretheoriginallyhorizontalstratahasbeenfoldedupward,andthetwolimbsofthefolddipawayfromthehingeofthefold.

Synclinesarefoldswheretheoriginallyhorizontalstratahavebeenfoldeddownward,andthetwolimbsofthefolddipinwardtowardthehingeofthefold.Synclinesandanticlinesusuallyoccurtogethersuchthatthelimbofasynclineisalsothelimbofananticline.

Inthediagramsabove,thefoldaxesarehorizontal,butifthefoldaxisisnothorizontalthefoldiscalledaplungingfoldandtheanglethatthefoldaxismakeswithahorizontallineiscalledtheplungeofthefold.

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Notethatifaplungingfoldintersectsahorizontalsurface,wewillseethepatternofthefoldonthesurface(seealsofigures11.16einyourtext.

DomesandBasinsareformedasaresultofverticalcrustalmotion.Domeslooklikeanoverturnedbowlandresultfromcrustalupwarping.Basinslooklikeabowlandresultfromsubsidence(seefigure11.15inyourtext).Foldsaredescribedbytheseverityoffolding.anopenfoldhasalargeanglebetweenlimbs,atightfoldhasasmallanglebetweenlimbs.

Furtherclassificationoffoldsinclude:

Ifthetwolimbsofthefolddipawayfromtheaxiswiththesameangle,thefoldissaidtobeasymmetricalfold.

Ifthelimbsdipatdifferentangles,thefoldsaresaidtobeasymmetricalfolds.

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Ifthecompressionalstressesthatcausethefoldingareintense,thefoldcancloseupandhavelimbsthatareparalleltoeachother.Suchafoldiscalledanisoclinalfold(isomeanssame,andclinemeansangle,soisoclinalmeansthelimbshavethesameangle).Notetheisoclinalfolddepictedinthediagrambelowisalsoasymmetricalfold.

Ifthefoldingissointensethatthestrataononelimbofthefoldbecomesnearlyupsidedown,thefoldiscalledanoverturnedfold.

Anoverturnedfoldwithanaxialplanethatisnearlyhorizontaliscalledarecumbantfold.

Afoldthathasnocurvatureinitshingeandstraightsidedlimbsthatformazigzagpatterniscalledachevronfold.

FoldsandTopography

Sincedifferentrockshavedifferentresistancetoerosionandweathering,erosionoffoldedareascanleadtoatopographythatreflectsthefolding.Resistantstratawouldformridgesthathavethesameformasthefolds,whilelessresistantstratawillformvalleys(seefigure11.15inyoutext).

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HowFoldsForm

Foldsdevelopintwoways:

Flexuralfoldsformwhenlayersslipasstratifiedrocksarebent.Thisresultsinthelayersmaintainingtheirthicknessastheybendandslideoveroneanother.Thesearegenerallyformedduetocompressionalstressesactingfromeitherside.

Flowfoldsformwhenrocksareveryductileandflowlikeafluid.Differentpartsofthefoldaredrawnoutbythisflowtodifferentextentsresultinginlayersbecomingthinnerinsomeplacesandthickerinouterplaces.Theflowresultsinshearstressesthatsmearoutthelayers.

Foldscanalsoforminrelationshiptofaultingofotherpartsoftherockbody.Inthiscasethemoreductilerocksbendtoconformtothemovementonthefault.

Alsosinceevenductilerockscaneventuallyfractureunderhighstress,rocksmayfolduptoacertainpointthenfracturetoformafault.

FoldsandMetamorphicFoliation

Aswesawinourdiscussionofmetamorphicrocks,foliationisaplanarfabricthatdevelopsinrockssubjecttocompressionalstressduringmetamorphism.Itmaybepresentasflattenedorelongatedgrains,withtheflatteningoccurringperpendiculartothedirectionofcompressionalstress.Italsoresultsfromthereorientation,recrystallization,orgrowthofsheetsilicatemineralssothattheirsheetsbecomeorientedperpendiculartothecompressionalstressdirection.Thus,wecommonlyseeafoliationthatisparalleltotheaxialplaneofthefold.

Shearingofrockduringmetamorphismcanalsodrawoutgrainsinthedirectionofshear.

MountainsandMountainBuildingProcesses

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OneofthemostspectacularresultsofdeformationactingwithinthecrustoftheEarthistheformationofmountainranges.Mountainsfrequentlyoccurinelongate,linearbelts.Theyareconstructedbytectonicplateinteractionsinaprocesscalledorogenesis.Mountainbuilding(orogenesis)involves

Structuraldeformation.Faulting.Folding.IgneousProcesses.Metamorphism.Glaciation.Erosion.Sedimentation

Constructiveprocesses,likedeformation,folding,faulting,igneousprocessesandsedimentationbuildmountainsupdestructiveprocesseslikeerosionandglaciation,tearthembackdownagain.

Mountainsarebornandhaveafinitelifespan.Youngmountainsarehigh,steep,andgrowingupward.Middleagedmountainsarecutbyerosion.Oldmountainsaredeeplyerodedandoftenburied.Ancientorogenicbeltsarefoundincontinentalinteriors,nowfarawayfromplateboundaries,butprovideinformationonancienttectonicprocesses.Sinceorogeniccontinentalcrustgenerallyhasalowdensityandthusistoobuoyanttosubduct,ifitescapeserosionitisusuallypreserved.UpliftandIsostasy

ThefactthatmarinelimestonesoccuratthetopofMt.Everest,indicatesthatdeformationcancauseconsiderableverticalmovementofthecrust.Suchverticalmovementofthecrustiscalleduplift.Upliftiscausedbydeformationwhichalsoinvolvesthickeningofthelowdensitycrustand,becausethecrust"floats"onthehigherdensitymantle,involvesanotherprocessthatcontrolstheheightofmountains.

Thediscoveryofthisprocessanditsconsequencesinvolvedmeasurementsofgravity.Gravityismeasuredwithadeviceknownasagravimeter.Agravimetercanmeasuredifferencesinthepullofgravitytoaslittleas1partin100million.MeasurementsofgravitycandetectareaswherethereisadeficiencyorexcessofmassbeneaththesurfaceoftheEarth.Thesedeficienciesorexcessesofmassarecalledgravityanomalies.

Apositivegravityanomalyindicatesthatanexcessofmassexitsbeneaththearea.Anegativegravityanomalyindicatesthatthereislessmassbeneathanarea.

Negativeanomaliesexistbeneathmountainranges,andmirrorthetopographyandcrustalthicknessasdeterminedbyseismicstudies.Thus,thelowdensitycontinentsappeartobefloatingonhigherdensitymantle.

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Theprotrusionsofthecrustintothemantlearereferredtoascrustalroots.Normalcrustalthickness,measuredfromthesurfacetotheMohois35to40km.Butundermountainbeltscrustalthicknessesof50to70kmarecommon.Ingeneral,thehigherthemountains,thethickerthecrust.

Whatcausesthisistheprincipalofisostasy.Theprincipalcanbedemonstratedbyfloatingvarioussizesoflowdensitywoodblocksinyourbathtuborsink.Thelargerblockswillbothfloathigherandextendtodeeperlevelsinthewaterandmimicthehowthecontinentsfloatonthemantle(seefigure11.26inyourtext).

Itmustbekeptinmind,howeverthatit'snotjustthecrustthatfloats,it'stheentirelithosphere.So,thelithosphericmantlebeneathcontinentsalsoextendstodeeperlevelsandisthickerundermountainrangesthannormal.Becausethelithosphereisfloatingintheasthenospherewhichismoreductilethanthebrittlelithosphere,thesoftasthenospherecanflowtocompensateforanychangeinthicknessofthecrustcausedbyerosionordeformation.

ThePrincipleofisostasystatesthatthereisaflotationalbalancebetweenlowdensityrocksandhighdensityrocks.i.e.lowdensitycrustalrocksfloatonhigherdensitymantlerocks.Theheightatwhichthelowdensityrocksfloatisdependentonthethicknessofthelowdensityrocks.Continentsstandhighbecausetheyarecomposedoflowdensityrocks(graniticcomposition).Oceanbasinsstandlow,becausetheyarecomposedofhigherdensitybasalticandgabbroicrocks.

Isostasyisbestillustratedbyeffectsofglaciation.Duringaniceagecrustalrocksthatarecoveredwithicearedepressedbytheweightoftheoverlyingice.Whentheicemelts,theareaspreviouslycoveredwithiceundergouplift.

Mountainsonlygrowsolongasthereareforcescausingtheuplift.Asmountainsrise,theyareeroded.Initiallytheerosionwillcausethemountainstorisehigherasaresultofisostaticcompensation.But,eventually,theweightofthemountainstartstodepressthelowercrustandsubcontinentallithospheretolevelswheretheystarttoheatupandbecomemoreductile.Thishotterlithospherewillthenbegintoflowoutwardawayfromtheexcessweightandtheabove

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willstarttocollapse.

Thehotterrockscouldeventuallypartiallymelt,resultinginigneousintrusionsasthemagmasmovetohigherlevels,ortheentirehotterlowercrustcouldbegintoriseasaresultoftheirlowerdensity.Theseprocessescombinedwitherosiononthesurfaceresultinexhumation,whichcausesrocksfromthedeepcrusttoeventuallybecomeexposedatthesurface.

CausesofMountainBuilding

Therearethreeprimarycausesofmountainbuilding.1. Convergenceatconvergentplateboundaries.2. ContinentalCollisions.3. Rifting

ConvergentPlateMargins

Whenoceaniclithospheresubductsbeneathcontinentallithospheremagmasgeneratedabovethesubductionzonerise,intrude,anderupttoformvolcanicmountains.Thecompressionalstressesgeneratedbetweenthetrenchandthevolcanicarccreatefoldthrustmountainbelts,andsimilarcompressionbehindthearccreateafoldthrustbeltresultinginmountains.MountainsalongthemarginsofwesternNorthandSouthAmerica,liketheAndesandtheCascaderangeformedinthisfashion.

Islandarcsoffthecoastofcontinentscangetpushedagainstthecontinent.Becauseoftheirlowdensity,theydon'tsubduct,butinsteadgetaccretedtotheedgeofthecontinent.MountainrangesalongthewestcoastofNorthAmericaformedinthisfashion(seefigure11.22inyourtext).

ContinentalCollisions

Platetectonicscancausecontinentalcrustalblockstocollide.Whenthisoccurstherocksbetweenthetwocontinentalblocksbecomefoldedandfaultedundercompressionalstressesandarepushedupwardtoformfoldthrustmountains.TheHimalayanMountains(currentlythehighestonEarth)aremountainsofthistypeandwereformedasaresultoftheIndianPlatecollidingwiththeEurasianplate.SimilarlytheAppalachianMountainsofNorthAmericaandtheAlpsofEuropewereformedbysuchprocesses.

Rifting

ContinentalRiftingoccurswherecontinentalcrustisundergoingextensionaldeformation.Thisresultsinthinningofthelithosphereandupwellingoftheasthenospherewhichresultsinuplift.Thebrittlelithosphererespondsbyproducingnormalfaultswhereblocksofcontinentallithosphereareupliftedtoformgrabensorhalfgrabens.Theupliftedblocksarereferredtoafaultblockmountains.

TheBasinandRangeprovinceinthewesternUnitedstatesformedinthismanner,includingtheSierraNevadaonitswesternedgeandtheGrandTetonsinWyoming..

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CratonsandOrogens

Thecontinentscanbedividedintotwokindsofstructuralunits

Cratonsformthecoresofthecontinents.Theseareportionsofcontinentalcrustthathaveattainedisostaticandtectonicstabilityandhavecooledsubstantiallysincetheirformation.Theywereformedandweredeformedmorethanabillionyearsagoandaretheoldestpartsofthecontinents.Therepresentthedeeprootsofformermountainsandconsistofmetamorphicandplutonicigneousrocks,allshowingextensiveevidenceofdeformation.

Orogensarebroadelongatedbeltsofdeformedrocksthataredrapedaroundthecratons.Theyappeartobetheerodedrootsofformermountainbeltsthatformedbycontinentcontinentcollisions.Onlytheyoungestoftheseorogensstillformmountainranges(seefigure13.10)inyourtext).

Theobservationthattheorogensaregenerallyyoungertowardstheoutsideofanycontinentsuggeststhatthecontinentswerebuiltbycollisionsofplatesthataddedyoungermaterialtotheoutsideedgesofthecontinents,andisfurtherevidencethatplatetectonicshasoperatedforatleastthelast2billionyears.CaseStudyoftheAppalachianMountainsTheAppalachianMountainRangeextendingfromnorthernAlabamatoNovaScotiahaveahistorythatdatesbackabout1billionyears.Thishistorywillbediscussedinclassandiscoveredinsection11.32ofyourtextbook.

Questionsonthismaterialthatcouldbeaskedonanexam.

1. Definethefollowing:(a)faultbreccia,(b)slickensides,(c)mylonite,(d)foldaxis,(e)axialplane,(f)plungingfold,(g)orogenesis,(h)isostasy.

2. Explaininwordsordrawingstheessentialaspectsofthefollowingfeatures:(a)normalfault,(b)reversefault,(c)thrustfault,(d)horstsandgrabens,(e)halfgrabens.

3. Drawcrosssectionsofananticlineandasynclineandlabelthefoldhinge,foldlimbs,foldaxis,andtheaxialplaneofeach.

4. Whatisthedifferencebetweenaflexuralfoldandaflowfold?

5. Isitpossibleforfaultstocausefoldsorfoldstocausefaults?Explain.

6. Inwhichsettingsaremountainsformedandwhatkindofmountainsareformedineach?

7. Whatprocessesareinvolvedinexhumationandwhatisthefinalresultofexhumation?

8. ExplainhowcrustalthicknessvariesontheEarthandhowweknowthis?

9. Whyismetamorphicfoliationoftenparalleltotheaxialplanesoffolds?

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10. Foreachofthefollowingtypesofmountainrangesgivesomeexamples:(a)foldthrust,(b)blockfault,(c)volcanic

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