CanadianShieldandBasementRocksMountainBelts

TheInteriorPlainsandPlateausofNorthAmerica

EarthHistory:notesfromtheLabManual,chapters15,16,and17,integratedwithoriginalcontributions

AlessandroGrippo,Ph.D.PrecambrianmetamorphicrocksfromtheCanadianShield(withglacialstriations)

IngrahamTrail,Yellowknife,NorthwesternTerritories,Canada

©AlessandroGrippo

Cratonsvs.Orogens

•  CratonicandorogenicsettingsweredefinedinLabManualchapter5as:– Cratonicsetting

•  Stableareaswithsmallverticalmovements,withslowupliftassociatedwithslowsubsidence–  Thesearethestablepartofacontinent

– Orogenicsetting•  Unstableareaswithbigverticalmovements,withrapidupliftassociatedwithslowsubsidence–  Thesearethosecontinentaledgeswhereplatetectonics,namelymountainbuilding,isgoingon

TheNorthAmericanCraton

•  InNorthAmerica,thestablepart,orthecraton,consistsofrocksthathavenotundergonesignificantorogenicdeformationforhundredsofmillionsofyears

•  Thismeansthat–  orogenesis,ormountainbuilding,hasnothappenedinthisareaforalongtime

–  rockscanpotentiallybevery,veryold–  onlysubtlechangeshaveoccurredintheseareas

Acraton:shield+platform

•  Acratoncanthuseithershowveryoldrocksatthesurface,orbecoveredbyyoungerrocks

•  Precambriancrystallinerocks(igneousormetamorphicrocks)exposedatthesurfacemakeuptheshield

•  Phanerozoicsedimentaryrockscoveringtheshieldmakeuptheplatform

•  Ashieldissocalledbecauseofitsarchedshapeinprofile

•  Therocksmakinguptheshieldarepartofthecrystallinebasement

•  ThecrystallinebasementisbydefinitionPrecambrianinageandmadebyeitherigneousormetamorphicrocks–  TherearealsosmallamountsofPrecambriansedimentaryrocks

CrystallineBasement

•  TheshieldoftheNorthAmericancontinentiscalledtheCanadianShield– MostoftheNorthAmericanshieldshowsupinCanada,fromtheNorthwesternTerritoriesallthewaytoQuebecandNewfoundland

–  ItalsocropsoutinlimitedareasinMinnesota,Michigan,WisconsinandupperstateNewYork

CanadianShield

TheredcolorationsaroundHudsonBaydenotetheextensionoftheCanadianShield

Platform

•  TheCanadianShieldissurroundedbygenerallyflat-lyingsequencesofsedimentaryrocksformedbecauseoftheerosionoftheshielditself

•  So,theshieldisshowingbecauseithasbeeneroded,andthesedimentthatformsoutofthaterosionconstitutestheplatform

•  TheplatformisPhanerozoicinage

•  Sothecrystallinebasementsalwaysexistsunderneaththeplatform

•  Iftheplatformismissingbecauseoferosion,therocksofthecrystallinebasementmakeuptheshield

•  Thismeansthatifwedrillthroughtheplatform,weareabletoreachandfindthecrystallinebasement

Wheredoweseethecrystallinebasement?

•  Asamatteroffact,youcanseeorencounterthecrystallinebasementinthesefourgeologicalcontext:– Directlyexposedatthesurface,asitisintheCanadianShield

– Atvariabledepths,bydrilling(from800to3000mfromthesurface)

– AtthefrontofthrustfaultsintheCanadianRockiesandintheRockiesofMontanaandWyoming

–  Becauseofupliftinganddeeperosion,atthebottomoftheGrandCanyoninArizona

•  MostcrystallinerocksintheCanadianShieldaregranitegneisses(meaning,agneisscontainingthesamemineralsofagranite)

•  TheoldestrockintheworldistheAcastaGneissfromtheNorthwesternTerritories,Canada,datedat3.96billionyears

TheAcastaGneiss

NorthwesternTerritoriesProvincialMuseum,Yellowknife,NT,Canada©AlessandroGrippo

TheCanadianShieldismadeofsmallercontinentsthatcollidedinthepast

•  TheCanadianShieldisnotuniforminage,butitshowsregionswherecertainagesareclustered

•  ThesearedefinedasseparatedPrecambriangeologicprovinces

•  Everyoneoftheseprovincesisinferredtorepresentanorogenicbelt

•  Sothesewereeachtheirownmicrocontinentwhich,uponcollisionovergeologictime,cametoformtheNorthAmericancontinentoftoday

Accretionarytectonics

•  Whensmallercontinents(microcontinents)collide,theybuildintimebiggercontinents

•  Whentheyarejustaddedatthemarginofacontinent,wetalkaboutaccretion