GEOLOGICTIME

Lab14principlesof

STRUCTURALGEOLOGYandTECTONICS

AlessandroGrippo,Ph.D

StructuralGeologyandTectonics

•  Branchesofgeologythatdealwiththereconstruc;onofmovementsthathaveoccurredover;meinEarth’sLithosphere

•  fromLa;n(struere)andGreek(tektos)=tobuild

•  movementsinclude–  simplemo;on–  bending–  breaking

StructuralGeologyandTectonics

•  Ifweknow–  underwhatcondi;onsmo;onoccurs–  howdeforma;onsareoriginated

•  Thenwe–  getinforma;onforreconstruc;ngEarth’sHistory

•  bigscale:platemo;on(Tectonics)•  medium-sizescale:mountainbuilding(Tectonics)•  smallscale:localdeforma;on,ascausedforinstancebyearthquakes(StructuralGeology)

•  micro-andsubmicroscopicscale:deforma;oninrocksandminerals(StructuralGeology)

CompressionandTension

Shear

HowdowestudyLithosphere’sdeformaHon?

•  Qualita;veandquan;ta;veanalysis–  con;nuummechanics–  laboratorydeforma;ontes;ng– mathema;calmodels–  studyofgeophysicaldata

•  seismic,gravity,magne;c

–  satelliteimages,airborneandspacebornedata–  petrologyandgeochemistry–  sedimentology,stra;graphy,paleontology

LithosphericdeformaHonsoriginateStructures

•  Aforce(stress)causesadeforma;on(strain)– elas;cdeforma;on

– briOledeforma;on•  joints•  faults

– duc;ledeforma;on•  folds

ElasHc,BriLle,andDucHleDeformaHon

ANtudes:strikeanddip

•  Inordertounderstandhowtectonicdeforma;onoccursweneedtounderstandtheorienta;onofstructuresinspace

•  Thissystemisbasedonthestrikeandthedipofasurface(itsaQtude)

•  Mostlyusedforgeologicmapping,strikeanddipareusefulinunderstandingstructures

TopographicMapvs.GeologicMap

•  TheSTRIKEisalinethatcomesfromtheintersec;onofaninclinedplane(a;ltedlayer,forinstance)withanimaginaryhorizontalsurface

•  Itsdirec;oncanbemeasuredinthefieldwithacompass

•  TheDIPisissimplytheangleofmaximuminclina;onofoursurface(layer)

•  Thedipisalwaysat90°fromthestrike

•  Thedippointsinthedirec;onofthe;lt

StrikeandDipsymbols

•  AQtudeisrepresentedbyaT-shapedsymbol

–  thelongarmoftheTrepresentsthestrike

–  theshortarmoftheTrepresentsthedip

–  anumberisassociatedwiththesymbol,indica;ngtheangleofdip

Ver;callayers:dipis90°

Duluth,Minnesota

©AlessandroGrippo

•  A:the“normal”symbolforstrikeanddip:alayer;ltedby10°towardsENE

•  B:ver;callayers

•  C:horizontallayers

FOLDS

DomesandBasins

FRACTURES,orJOINTS

•  Fracturesaresurfacesalongwhichrocksormineralshavebroken

•  Fracturesgeneratetwofreesurfaceswherenoneexistedbefore

•  Systema;cJoints–  planar,parallelto

eachother,regularlyspaced

JointSets

•  Otherkindsofjoints

Exfolia;onJoints

HalfDome,YosemiteNa;onalPark,California

ColumnarJoints

Devil’sTowerNa;onalPark,Wyoming

FAULTS

•  Faults:fracturesalongwhichmo;onoccurs

•  Centuries-oldminingterminologyused–  Footwall

•  Blockbelowthefaultplane•  Minerwouldstandhere

– Hangingwall•  Blockabovethefaultplane•  Hangalanternhere

Faulttypes

•  Faulttypes– Dis;nguishedbydirec;onofrockdisplacement

•  Normaldip-slip•  Reversedip-slip

•  Strike-slip

•  Dip-slipfaults”– Ver;calmo;on

– NormalFaults•  thehangingwallmovesdownwardwithrespecttothefootwall(causedbyextension)

– ReverseFault•  thehangingwallmovesupwardwithrespecttothefootwall(causedbycompression)

•  Normal(dip-slip)Fault– Hangingwallmovesdownrela;vetofootwall

•  Reverse(dip-slip)Fault– Hangingwallmovesuprela;vetofootwall

ThrustFaults

•  Transform(Strike-slip)Fault– Crustmovesinhorizontaldirec;on

•  Someac;vefaultsdonotextendtothesurface– BlindFaults

StructuralGeologyandTectonics

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