Chapter 5 - Wiley.ppt

Chapter9 HowRockBends Buckles andBreaks HowIsRockDeformed Tectonicsforcescontinuouslysqueeze stretch bend andbreakrockinthelithosphere ThesourceofenergyistheEarth sheat whichistransformedintotomechanicalenergy Stress Uniformstressisaconditioninwhichthestressisequalinalldirections Inrocksitisalsoconfiningstressbecauseanybodyofrockinthelithosphereisconfinedbytherockaroundit Differentialstressisstressthatisnotequalinalldirections DifferentialStress Thethreekindsofdifferentialstressare Tensionalstress whichstretchesrocks Compressionalstress whichsqueezesthem Shearstress whichcausesslippageandtranslation Figure9 1 StagesofDeformation Straindescribesthedeformationofarock Whenarockissubjectedtoincreasingstress itpassesthroughthreestagesofdeformationinsuccession Elasticdeformationisareversiblechangeinthevolumeorshapeofastressedrock Ductiledeformationisanirreversiblechangeinshapeand orvolumeofarockthathasbeenstressedbeyondtheelasticlimit Fractureoccursinasolidwhenthelimitsofbothelasticandductiledeformationareexceeded Figure9 2 Figure9 3 DuctileDeformationVersusFracture Abrittlesubstancetendstodeformbyfracture Aductilesubstancedeformsbyachangeofshape Thehigherthetemperature themoreductileandlessbrittleasolidbecomes RocksarebrittleattheEarth ssurface butatdepth wheretemperaturesarehighbecauseofthegeothermalgradient rocksbecomeductile Figure9 4 ConfiningStress Confiningstressisauniformsqueezingofrockowingtotheweightofalloftheoverlyingstrata Highconfiningstresshinderstheformationoffracturesandsoreducesbrittleproperties Reductionofbrittlenessbyhighconfiningstressisasecondreasonwhysolidrockcanbebentandfoldedbyductiledeformation Fracture Alltheconstituentatomsofasolidtransmitstressappliedtoasolid Ifthestressexceedsthestrengthofthebondbetweenatoms Eithertheatomsmovetoanotherplaceinthecrystallatticeinordertorelievethestress or Thebondsmustbreak andfractureoccurs StrainRate Thetermusedfortime dependentdeformationofarockisstrainrate Strainrateistherateatwhicharockisforcedtochangeitsshapeorvolume StrainratesintheEarthareabout10 14to10 15 s Thelowerthestrainrate thegreaterthetendencyforductiledeformationtooccur Figure9 6 EnhancingDuctility Hightemperatures highconfiningstress andlowstrainrates characteristicofthedeepercrustandmantle Reducebrittleproperties Enhancetheductilepropertiesofrock CompositionAffectsDuctility 1 Thecompositionofarockhaspronouncedeffectsonitsproperties Quartz garnet andolivineareverybrittle Mica clay calcite andgypsumareductile Thepresenceofwaterinarockreducesbrittlenessandenhancesductileproperties Wateraffectspropertiesbyweakeningthechemicalbondsinmineralsandbyformingfilmsaroundmineralsgrains CompositionAffectsDuctility 2 Rocksthatreadilydeformbyductiledeformationarelimestone marble shale phyllite andschist Rocksthattendtobebrittleratherthanductilearesandstoneandquartzite granite granodiorite andgneiss RockStrength 1 RockstrengthintheEarthdoesnotchangeuniformlywithdepth Therearetwopeaksintheplotofrockstrengthwithdepth Strengthisdeterminedbycomposition temperature andpressure Rocksinthecrustarequartz rich sothestrengthpropertiesofquartzplayanimportantroleinthestrengthpropertiesofthecrust Rockstrengthincreasesdowntoadepthabout15km Above15kmrocksarestrong theyfractureandfailbybrittledeformation RockStrength 2 Below15km fracturesbecomelesscommonbecausequartzweakensandrocksbecomeincreasinglyductile Rocksinthemantleareolivine rich Olivineisstrongerthanquartz andthebrittle ductiletransitionofolivine richrockisreachedonlyatadepthabout40km Figure9 7 RockStrength 3 Byabout1300oC rockstrengthisverylow Brittledeformationisnolongerpossible Thedisappearanceofallbrittledeformationpropertiesmarksthelithosphere asthenosphereboundary Inthecrustlargemovementshappenssoslowly lowstrainrates thattheycanbemeasuredonlyoverahundredormoreyears AbruptMovement Abruptmovementresultsfromthefractureofbrittlerocksandmovementalongthefractures Stressbuildsupslowlyuntilfrictionbetweenthetwosidesofthefaultisovercome whenabruptslippageoccurs Thelargestabruptverticaldisplacementeverobservedoccurredin1899atYakutatBay Alaska duringanearthquake AstretchoftheAlaskanshoreliftedasmuchas15mabovethesealevel Abruptmovementsinthelithospherearecommonlyaccompaniedbyearthquakes GradualMovement Gradualmovementistheslowrising sinking orhorizontaldisplacementoflandmasses Tectonicmovementisgradual Movementalongfaultsisusually butnotalways abrupt Figure9 9 EvidenceOfFormerDeformation Structuralgeologyisthestudyofrockdeformation Thelawoforiginalhorizontalitytellsusthatsedimentarystrataandlavaflowswereinitiallyhorizontal Ifsuchrocksaretilted wecanconcludethatdeformationhasoccurred DipandStrike Thedipistheangleindegreesbetweenahorizontalplaneandtheinclinedplane measureddownfromhorizontal Thestrikeisthecompassdirectionofthehorizontallineformedbytheintersectionofahorizontalplaneandaninclinedplane Figure9 10 Figure9 11 DeformationByFracture Rockinthecrusttendstobebrittleandtobecutbyinnumerablefracturescalledeitherjointsorfaults Mostfaultsareinclined Todescribetheinclination geologistshaveadoptedtwooldminingterms Thehanging wallblockistheblockofrockaboveaninclinedfault Theblockofrockbelowaninclinedfaultisthefootwallblock Theseterms ofcourse donotapplytoverticalfaults Figure9 12 ClassificationofFaults 1 Faultsareclassifiedaccordingto Thedipofthefault Thedirectionofrelativemovement Normalfaultsarecausedbytensionalstressesthattendtopullthecrustapart aswellasbystressescreatedbyapushfrombelowthattendtostretchthecrust Thehanging wallblockmovesdownrelativetothefootwallblock Figure9 13 Figure9 13B ClassificationofFaults 2 Adown droppedblockisagraben orarift ifitisboundedbytwonormalfaults Itisahalf grabenifsubsidenceoccursalongasinglefault Anupthrustblockisahorst Theworld smostfamoussystemofgrabensandhalf grabensistheAfricanRiftValleyofEastAfrica Thenorth southvalleyoftheRioGrandeinNewMexicoisagraben ThevalleyinwhichtheRhineRiverflowsthroughwesternEuropefollowsaseriesofgrabens Figure9 14 ClassificationofFaults 3 Reversefaultsarisefromcompressionalstresses Movementonareversefaultissuchthatahanging wallblockmovesuprelativetoafootwallblock Reversefaultmovementshortensandthickensthecrust ClassificationofFaults 4 Thrustfaultsarelow anglereversefaultswithdiplessthan15o Suchfaultsarecommoningreatmountainchains Strike slipfaultsarethoseinwhichtheprincipalmovementishorizontalandthereforeparalleltothestrikeofthefault Strike slipfaultsarisefromshearstresses TheSanAndreasisaright lateralstrike slipfault Apparently movement morethan600km hasbeenoccurringalongitforatleast65millionyears Figure9 17 Figure9 18 ClassificationofFaults 5 Whereoneplatemarginterminatesanothercommences theirjunctionpointiscalledatransform J T Wilsonproposedthatthespecialclassofstrike slipfaultsthatformsplateboundariesbecalledtransform faults Figure9 19 EvidenceofMovementAlongFaults Movementofonemassofrockpastanothercancausethefault ssurfacestobesmoothed striated andgrooved Striatedorhighlypolishedsurfacesonhardrocks abradedbymovementalongafault arecalledslickensides Inmanyinstances faultmovementcrushesrockadjacenttothefaultintoamassofirregularpieces formingfaultbreccia DeformationbyBending Thebendingofrockisreferredtoasfolding Monocline thesimplestfold Thelayersofrockaretiltedinonedirection Anticline anupfoldintheformofanarch Syncline adownfoldwithatrough likeform Anticlinesandsynclinesareusuallypaired Figure9 21 Box9 1 TheStructureofFolds 1 Thesidesofafoldarethelimbs Themedianlinebetweenthelimbsistheaxisofthefold Afoldwithaninclinedaxisissaidtobeaplungingfold Theanglebetweenafoldaxisandthehorizontalistheplungeofafold Animaginaryplanethatdividesafoldassymmetricallyaspossibleistheaxialplane Figure9 22C D E Figure9 22A B TheStructureofFolds 2 Anopenfoldisoneinwhichthetwolimbsdipgentlyandequallyawayfromtheaxis Whenstressisveryintense thefoldclosesupandthelimbsbecomeparalleltoeachother Suchafoldissaidtobeisoclinal TheStructureofFolds 3 Eventually anoverturnedfoldmaybecomerecumbent meaningthetwolimbsarehorizontal Commoninmountainousregions suchastheAlpsandtheHimalaya thatwereproducedbycontinentalcollisions Anticlinesdonotnecessarilymakeridges norsynclinesvalleys Figure9 23 Figure9 24 Figure9 25 Figure9 26 ExamplesofFaults 1 IntheValleyandRidgeprovinceofPennsylvania as