Upload
venkatalakshmikorrapati
View
228
Download
4
Embed Size (px)
DESCRIPTION
rocks
Citation preview
4/13/2015 DeformationofRock
http://www.tulane.edu/~sanelson/eens1110/deform.htm 1/18
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.
4/13/2015 DeformationofRock
http://www.tulane.edu/~sanelson/eens1110/deform.htm 2/18
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,
4/13/2015 DeformationofRock
http://www.tulane.edu/~sanelson/eens1110/deform.htm 3/18
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.
4/13/2015 DeformationofRock
http://www.tulane.edu/~sanelson/eens1110/deform.htm 4/18
Thedipistheanglebetweenahorizontalplaneandtheinclinedplane,measuredperpendiculartothedirectionofstrike.
Inrecordingstrikeanddipmeasurementsonageologicmap,asymbolisusedthathasalonglineorientedparalleltothecompassdirectionofthestrike.Ashorttickmarkisplacedinthecenterofthelineonthesidetowhichtheinclinedplanedips,andtheangleofdipisrecordednexttothestrikeanddipsymbolasshownabove.Forbedswitha900dip(vertical)theshortlinecrossesthestrikeline,andforbedswithnodip(horizontal)acirclewithacrossinsideisusedasshownbelow.
Forlinearstructures,asimilarmethodisused,thestrikeorbearingisthecompassdirectionandanglethelinemakeswithahorizontalsurfaceiscalledtheplungeangle.
4/13/2015 DeformationofRock
http://www.tulane.edu/~sanelson/eens1110/deform.htm 5/18
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
4/13/2015 DeformationofRock
http://www.tulane.edu/~sanelson/eens1110/deform.htm 6/18
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.
4/13/2015 DeformationofRock
http://www.tulane.edu/~sanelson/eens1110/deform.htm 7/18
HalfGrabensAnormalfaultthathasacurvedfaultplanewiththedipdecreasingwithdepthcancausethedowndroppedblocktorotate.Insuchacaseahalfgrabenisproduced,calledsuchbecauseitisboundedbyonlyonefaultinsteadofthetwothatformanormalgraben.
ReverseFaultsarefaultsthatresultfromhorizontalcompressionalstressesinbrittlerocks,wherethehangingwallblockhasmoveduprelativethefootwallblock.
4/13/2015 DeformationofRock
http://www.tulane.edu/~sanelson/eens1110/deform.htm 8/18
AThrustFaultisaspecialcaseofareversefaultwherethedipofthefaultislessthan45o.Thrustfaultscanhaveconsiderabledisplacement,measuringhundredsofkilometers,andcanresultinolderstrataoverlyingyoungerstrata.
StrikeSlipFaultsarefaultswheretherelativemotiononthefaulthastakenplacealongahorizontaldirection.Suchfaultsresultfromshearstressesactinginthecrust.Strikeslipfaultscanbeoftwovarieties,dependingonthesenseofdisplacement.Toanobserverstandingononesideofthefaultandlookingacrossthefault,iftheblockontheothersidehasmovedtotheleft,wesaythatthefaultisaleftlateralstrikeslipfault.Iftheblockontheothersidehasmovedtotheright,wesaythatthefaultisarightlateralstrikeslipfault.ThefamousSanAndreasFaultinCaliforniaisanexampleofarightlateralstrikeslipfault.DisplacementsontheSanAndreasfaultareestimatedatover600km.
EvidenceofMovementonFaults
Sincemovementonafaultinvolvesrocksslidingpasteachothertheremaybeleftevidenceofmovementintheareaofthefaultplane.
FaultBrecciasarecrumbleduprocksconsistingofangularfragmentsthatwereformedasaresultofgrindingandcrushingmovementalongafault.Whentherockisbrokenintoclayorsiltsizeparticlesasaresultofslippageonthefault,itisreferredtoasfaultgouge.
Slickensidesarescratchmarksthatareleftonthefaultplaneasoneblockmovesrelativetotheother.Slickensidescanbeusedtodeterminethedirectionandsenseofmotiononafault.
MyloniteAlongsomefaultsrocksareshearedordrawnoutbyductiledeformation
4/13/2015 DeformationofRock
http://www.tulane.edu/~sanelson/eens1110/deform.htm 9/18
alongthefault.Thisresultsinatypeoflocalizedmetamorphismcalleddynamicmetamorphism(alsocalledcataclasticmetamorphism.Theresultingrockisafinegrainedmetamorphicrockshowevidenceofshear,calledamylonite.Faultsthatshowsuchductilesheararereferredtoasshearzones.
DeformationofDuctileRocks
Whenrocksdeforminaductilemanner,insteadoffracturingtoformfaultsorjoints,theymaybendorfold,andtheresultingstructuresarecalledfolds.Foldsresultfromcompressionalstressesorshearstressesactingoverconsiderabletime.Becausethestrainrateislowand/orthetemperatureishigh,rocksthatwenormallyconsiderbrittlecanbehaveinaductilemannerresultinginsuchfolds.
GeometryofFoldsFoldsaredescribedbytheirformandorientation.Thesidesofafoldarecalledlimbs.Thelimbsintersectatthetightestpartofthefold,calledthehinge.Alineconnectingallpointsonthehingeiscalledthefoldaxis.Animaginaryplanethatincludesthefoldaxisanddividesthefoldassymmetricallyaspossibleiscalledtheaxialplaneofthefold.
Werecognizeseveraldifferentkindsoffolds.
4/13/2015 DeformationofRock
http://www.tulane.edu/~sanelson/eens1110/deform.htm 10/18
Monoclinesarethesimplesttypesoffolds.Monoclinesoccurwhenhorizontalstrataarebentupwardsothatthetwolimbsofthefoldarestillhorizontal.
Anticlinesarefoldswheretheoriginallyhorizontalstratahasbeenfoldedupward,andthetwolimbsofthefolddipawayfromthehingeofthefold.
Synclinesarefoldswheretheoriginallyhorizontalstratahavebeenfoldeddownward,andthetwolimbsofthefolddipinwardtowardthehingeofthefold.Synclinesandanticlinesusuallyoccurtogethersuchthatthelimbofasynclineisalsothelimbofananticline.
Inthediagramsabove,thefoldaxesarehorizontal,butifthefoldaxisisnothorizontalthefoldiscalledaplungingfoldandtheanglethatthefoldaxismakeswithahorizontallineiscalledtheplungeofthefold.
4/13/2015 DeformationofRock
http://www.tulane.edu/~sanelson/eens1110/deform.htm 11/18
Notethatifaplungingfoldintersectsahorizontalsurface,wewillseethepatternofthefoldonthesurface(seealsofigures11.16einyourtext.
DomesandBasinsareformedasaresultofverticalcrustalmotion.Domeslooklikeanoverturnedbowlandresultfromcrustalupwarping.Basinslooklikeabowlandresultfromsubsidence(seefigure11.15inyourtext).Foldsaredescribedbytheseverityoffolding.anopenfoldhasalargeanglebetweenlimbs,atightfoldhasasmallanglebetweenlimbs.
Furtherclassificationoffoldsinclude:
Ifthetwolimbsofthefolddipawayfromtheaxiswiththesameangle,thefoldissaidtobeasymmetricalfold.
Ifthelimbsdipatdifferentangles,thefoldsaresaidtobeasymmetricalfolds.
4/13/2015 DeformationofRock
http://www.tulane.edu/~sanelson/eens1110/deform.htm 12/18
Ifthecompressionalstressesthatcausethefoldingareintense,thefoldcancloseupandhavelimbsthatareparalleltoeachother.Suchafoldiscalledanisoclinalfold(isomeanssame,andclinemeansangle,soisoclinalmeansthelimbshavethesameangle).Notetheisoclinalfolddepictedinthediagrambelowisalsoasymmetricalfold.
Ifthefoldingissointensethatthestrataononelimbofthefoldbecomesnearlyupsidedown,thefoldiscalledanoverturnedfold.
Anoverturnedfoldwithanaxialplanethatisnearlyhorizontaliscalledarecumbantfold.
Afoldthathasnocurvatureinitshingeandstraightsidedlimbsthatformazigzagpatterniscalledachevronfold.
FoldsandTopography
Sincedifferentrockshavedifferentresistancetoerosionandweathering,erosionoffoldedareascanleadtoatopographythatreflectsthefolding.Resistantstratawouldformridgesthathavethesameformasthefolds,whilelessresistantstratawillformvalleys(seefigure11.15inyoutext).
4/13/2015 DeformationofRock
http://www.tulane.edu/~sanelson/eens1110/deform.htm 13/18
HowFoldsForm
Foldsdevelopintwoways:
Flexuralfoldsformwhenlayersslipasstratifiedrocksarebent.Thisresultsinthelayersmaintainingtheirthicknessastheybendandslideoveroneanother.Thesearegenerallyformedduetocompressionalstressesactingfromeitherside.
Flowfoldsformwhenrocksareveryductileandflowlikeafluid.Differentpartsofthefoldaredrawnoutbythisflowtodifferentextentsresultinginlayersbecomingthinnerinsomeplacesandthickerinouterplaces.Theflowresultsinshearstressesthatsmearoutthelayers.
Foldscanalsoforminrelationshiptofaultingofotherpartsoftherockbody.Inthiscasethemoreductilerocksbendtoconformtothemovementonthefault.
Alsosinceevenductilerockscaneventuallyfractureunderhighstress,rocksmayfolduptoacertainpointthenfracturetoformafault.
FoldsandMetamorphicFoliation
Aswesawinourdiscussionofmetamorphicrocks,foliationisaplanarfabricthatdevelopsinrockssubjecttocompressionalstressduringmetamorphism.Itmaybepresentasflattenedorelongatedgrains,withtheflatteningoccurringperpendiculartothedirectionofcompressionalstress.Italsoresultsfromthereorientation,recrystallization,orgrowthofsheetsilicatemineralssothattheirsheetsbecomeorientedperpendiculartothecompressionalstressdirection.Thus,wecommonlyseeafoliationthatisparalleltotheaxialplaneofthefold.
Shearingofrockduringmetamorphismcanalsodrawoutgrainsinthedirectionofshear.
MountainsandMountainBuildingProcesses
4/13/2015 DeformationofRock
http://www.tulane.edu/~sanelson/eens1110/deform.htm 14/18
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.
4/13/2015 DeformationofRock
http://www.tulane.edu/~sanelson/eens1110/deform.htm 15/18
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
4/13/2015 DeformationofRock
http://www.tulane.edu/~sanelson/eens1110/deform.htm 16/18
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..
4/13/2015 DeformationofRock
http://www.tulane.edu/~sanelson/eens1110/deform.htm 17/18
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?
4/13/2015 DeformationofRock
http://www.tulane.edu/~sanelson/eens1110/deform.htm 18/18
10. Foreachofthefollowingtypesofmountainrangesgivesomeexamples:(a)foldthrust,(b)blockfault,(c)volcanic
ReturntoEENS1110Page