Early Proterozoic Rocks of the Great Lakes Region: Sudbury, Ontario, Canada to Duluth, Minnesota, U.S.A, July 1-7, 1989

71 Pages·5.357 MB·English

by  G. B. Morey, Ken Card, John S. Klasner, Klaus Schulz, Richard W. Ojakangas(auth.)

Early Proterozoic Rocks of
the Great Lakes Region
Sudbury, Ontario, Canada to
Duluth, Minnesota, U.S.A.
July 1-7, 1989
Field Trip Guidebook T14S
Leader: G. B. Morey
Associate Leaders:
Ken Card Klaus Schulz
John S. KJasner Richard W Ojakangas
Ameri<;an Geophysical Union, Washington, D.C.
Copyright 1989 American Geophysical Union
2000 Florida Ave., N.W., Washington, D.C. 20009
ISBN: 0-87590-625-7
Printed in the United States of America
OncolitesofthegroupOsagiaTwenhofelfromtheapproximatelytwo-billion-year-oldBiwabikIron
Fonnation(AnimikieGroup) ofnorthernMinnesota. Theseunattachedstromatolitesoccurat
severalstratigraphiclevels intheiron-fonnationasmillimeter-tocentimeter-sizespheroidsinjasper
richbedsthatalso containprominentcolumnarandbranchingstromatolites. Theoncolitesfigured
here areenlargedabout10timesfrom aglass-platenegativemadebyJohnW. Grunerin1919-121
andnow inthefiles oftheMinnesotaGeologicalSUNey. Thisphotographconstitutesoneof'first
piecesofevidenceforbiologicactivityduringtheprecipitationofthis iron-fonnation.
CONTENTS
Introduction 1
GeneralGeologicSetting 1
Segment 1 - Geology of the North Shore of Lake Huron:
Penokean Fold Belt and Sudbury Structure, by K.D. Card .3
Basement Rocks 3
PenokeanFoldBelt 5
SudburyStructure 9
StructureandMetamorphism 12
RegionalTectonicHistory 13
Summary 14
StopDescriptions- Day1 ' 16
StopDescriptions- Day2 17
Segment 2 - Geology of the Marquette Range Supergroup
and the Penokean Fold Belt in Northern Michigan,
by John S. Klasner, William F. Cannon, and Richard W. Ojakangas 19
MetaIIlorphism 22
Structure 22
AgeofThrusting 24
StopDescriptions- Day3 24
StopDescriptions- Day4 29
. Segment 3 - Wisconsin Magmatic Terrane and Marquette Range
Supergroup in Wisconsin and the Western Part of Northern Michigan,
byK.J. SchulzandRichardW. OJakangas 31
Wisconsin Magmatic Terrane 31
Tectonic Evolution 33
StopDescriptions- Day5 33
StopDescriptions- Day6 38
Segment 4 - The Penokean Orogen in East-central and
Northern Minnesota, by G.B. Morey, D.L. Southwick, and
RichardW. Ojakangas 43
RegionalStructuralFramework 43
Stratigraphic Implications 50
Conclusions 50
StopDescriptions- Day7 51
References 57
v
Leader:
G. B. Morey
Minnesota Geological Survey
2642 University Ave.
St. Paul, MN 55114
Associate Leaders:
Ken Card
Geological Survey of Canada
588 Booth St.
Ottawa, Ontario
Canada K1A OE4
Klaus Schulz
U.S. Geological Survey
National Center, Mail Stop 954
Reston, VA 22092
John S. Klasner
Department of Geology
Western Illinois University
Macomb, IL 61455
Richard W. Ojakangas
Department of Geology
University of Minnesota, Duluth
Duluth, MN 55112
IGC FIELD TRIP T145:
EARLY PROTEROZOIC ROCKS OF THE GREAT LAKES REGION
INTRODUCTION Thepurposeofthisfieldtripistoexamineseveralaspectsof
EarlyProterozoiccrustalevolutionintheLakeHuronand
ThisguidebookwaspreparedforfieldtripT145, "Early LakeSuperiorregionsandtogivetheparticipantsanideaof
ProterozoicRocksoftheGreatLakesRegion," aspartofthe ourcurrentthoughts abouttherocks aswestandonthe
28thInternationalGeologicalCongressinWashington, D.C., outcrop. Manymorelocalitieshavebeenidentifiedforthis
July9-19,1989. Separatesegmentsofthetripwereprepared fieldtripthancanreasonablybeexaminedinthetimeallotted;
bythefieldtripleadersandassembledbyG.B.Moreyofthe thespecificlocalitiestobevisitedwillbeestablishedincon
MinnesotaGeologicalSurvey. TheSurvey,PriscillaC. sultationwiththeparticipants.
Grew,Director,providededitorialandcartographicassistance
andundertookpreparationofthecamera-readycopy.
IndividualsfromtheGeologicalSurveyofCanada,theU.S. GENERAL GEOLOGIC SETTING
GeologicalSurvey,theUniversityofMinnesota,Duluth,and
SouthernIllinoisUniversityalsocontributedtheirtimetoward EarlyProterozoicsupracrustalsequencesintheLake
thepreparationofthisguidebook. HuronandLakeSuperiorregionsconstituteadiscontinuous
Becauseoftheirmajormineralresources,includingiron, linearfoldbeltsome 1300kmlong,whichextendsfrom
uranium,andcopper-nickel,theEarlyProterozoicrocksofthe easternOntariointoMinnesotaalongthesouthernmarginof
GreatLakesregionwereformanyyearsthedomainofmineral theSuperiorProvinceoftheCanadianShield(Fig. 1). These
explorationists,andmuchofthegeologythatwasaccom sequencescomposethemajorpartoftheSouthernProvince
plishedwasdoneinsupportofthoseactivities. Sincethe andarereferredtohereasthePenokeanFoldBeltorthe
1960s,however,theserockshavebeenstudiedbyawide Penokeanorogen. TheEarlyProterozoicsequences are
varietyofgeologicdisciplines, andnewideashaveemerged. transectedatbothendsofLakeSuperiorbytheMiddlePro-
o
I
o
MINNESOTA
IOWA
FIGURE 1GeneralizedGeologicMapShowingtheDistributionofEarlyProterozoi~StrataintheWesternGreatLakes
Region. Stipple,mainlysedimentaryrocks; black,mainlyvolcanicrocks. Modifiedfrom Young [1983].
T145: 1
terozoicMidcontinentriftsystemand,innortheasternOntario, plutoniccomplexes,allofEarlyProterozoicage(1880-1830
bytheLateProterozoicGrenvilleFront TectonicZone,which Ma). TheWisconsinmagmaticterraneapparentlyaccumulated
largelyobliteratestheprimaryfeaturesoftheEarlyProterozoic inacomplexisland-arcenvironmentandisseparatedfromthe
rocksoftheGrenvilleProvince. TheEarlyProterozoicrocks epicratonicrocksoftheMarquetteRangeSupergroupbythe
areoverlaintothesouthbyflat-lyingPhanerozoicstrata. Niagarafault, abroadlyarcuate,complexsystemof
Fromwesttoeast,thesupracrustalsequencesconsistof southward-dippingductileshearsasmuchas 10kmwide,
theAnimikie,MilleLacs,andNorthRangegroupsin whichisinterpretedasapaleosuture.
Minnesota[Southwicketal., 1988];theMarquetteRange ApossiblecorrelationofthestratifiedrocksintheU.S.
SupergroupinnorthernMichiganandWisconsin; andthe withpartoftheHuronSupergroupinOntariohasbeen
HuronSupergroupinnortheasternOntario. Thesestratawere debatedfornearly 100years [James, 1958]. Thebasalpartof
depositedbetween2500and 1900m.y. andweresub theChocolayGroupatplacesinMichigan(Fig. 2)contains
sequentlydeformed,metamorphosed, andintrudedby coarseclasticdeposits,namedtheReanyCreek,Enchantment
plutonicrocks. ExceptforthoseEarlyProterozoicrocksin Lake, andFernCreekFormations,whichcouldbepartly
theGrenvilleProvince,themajortectoniceventaffectingthese glacialinorigin[Ojakangas, 1984;seeLarue, 1981,for
successionswasthePenokeanOrogeny(1900-1800m.y.). contraryviews]. Young[1983], amongothers,hassuggested
TheEarlyProterozoicsupracrustalsequencesare thattheReanyCreek-EnchantmentLake-FernCreekstrata
composed-mainlyofclasticsedimentaryandvolcanicrocks correlatewiththeglaciogenicGowgandaFormationinthe
withsubordinatechemicalsediments,chieflyiron-formation. HuronSupergroup. Young[1983]has alsosuggestedthat
Theythickensouthwardfromathinerosionaledgetopossibly otherunitsoftheCobaltGroupcanbecorrelatedonaone-for
asmuchas 15lan,althoughthesectionmayhavebeenover onebasiswithunitsintheChocolayGroup(Fig. 3).
thickenedinplacesbythrustfaulting. Ingeneraltheybear Isotopicagesdonotprecludethesuggestedcorrelation
somesimilaritiestoPhanerozoicgeosynclinalsequences,but betweenthelowerpartoftheChocolayGroupandthe
theylackrockscharacteristicofsynorogenicflysch, andpost uppermostHuronSupergroup. TheHuronSupergroupwas
orogenicmolassesuitesofPhanerozoicorthogeosynclines. depositedduringtheinterval2480-2200Ma;volcanicrocks
Additionally,theEarlyProterozoicsuccessionsbeginwith nearthebasehaveU-Pbzirconagesofapproximately2480
shallow-waterandendwithdeep-watersedimentaryand Ma,andthesequenceiscutbyNipissingDiabasedikesthat
volcanicmaterial,theoppositeofmanyPhanerowicgeo havebeendatedat2219±4Ma. IntheLakeSuperiorregion,
synclinalsequences. Thesedimentswerederivedmainlyfrom isotopicagesfromthestratifiedrocksarepoorlyconstrained.
theadjacentArcheancratontothenorthandconsequently BanksandVanSchmus [1971, 1972]havereportedaU-Pb
seemtolackthedistinctivebipolaritycharacteristicofmany zirconageof1910±10Maforasampleofrhyoliteinthe
youngergeosynclines. HemlockFormation(seeFig. 3),butnootherdirectagesare
IntheLakeSuperiorregion,thedominantlyclastic available. Possiblylowerlimitsontheageofdepositionare
sequencesareborderedonthesouthbytheWisconsin givenbySm-Ndmodelagesof2120±67Ma[Beckand
magmaticterrane,azoneofdominantlyvolcanicrocksand Murthy, 1982] onmaficdikesinthebasementofnorthern
NORTHWEST SEGMENT SOUTHEAST SEGMENT
Gunflintrange Mesabirange Cuyuna range GWageestbeicrnrapnagret GEoagsetbeircnrapnagrte WesternranMgearquette EasternranMgearquette Amasa Oval CrysItraolnFRalilvserrange Menominee range
Goodwin,1956 White,1954 Morey,1978 Aldrich,1929 PTrreinnzt,,11997766 KClaosnnneorn,1a9n7d6 Gairan1d96T8haden, James19a6nd8others, James,1968 Bayley1a9n6d6others,
FortuneLakeSlote
StomboughFormation
~
HiawathaGraywacke 0:
Riverton
IronFormation ~
DunnCreekSlate
slatembr. BadwaterGreenstone BadwaterGreenstone BadwaterGreenstone
iron-fm.mbr
MichigammeSlate MichigammeSlate MichigammeSlate
~
FoRrmovaetion Amasa Formation FFecnrcmeaRtiiovner
lD
GoodrichQuartzite
Gunflint
IronFormation
~~~~br_
~ PQoukaergtzaimtea MFoarhmnoamtioenn QPuaarlmtzsite
AjibikQuartzite
'I-------"--..r--~...........-.--'"'f'-~~~~--------~..-./"'--~~--------~..-./"'--~~~~~~~~""'___I
WeweSiate
TFroorumtaLtiaokne BadRiJerDolomite BadRiverDolomite KonoDolomite SaundersFormation RandvilleDolomite
~I----:-L""""-ittl"---;ef;=--:-all;--s--+------+----------1
Formation SundayQuartzite SundayQuartzite Mesnard Quartzite SturgeonQuartzite ~
:;: GI~gr~~~i~~hip ".........,..~~~...--r--~~~--------I EnchontmentLake
~ DenhamFormation Formation
~?~?~?'-""'--
RFeaonrmyaCtiroenek FernCreekFormation
Archean Rocks, undivided
FIGURE2CorrelationChartofEarlyProterozoicRocksintheLakeSuperiorRegion. AdaptedfromMorey[1983a].
T145: 2
Nipissing diabase
~g~ Bar River Fm.
~ Dolomite Kona Dolomite ~g- ~~ Gordon Lake Fm. g-
~ §-,~~~~~:on ~fin~r_d ~~ 8~_L_or_ra_in_F_m' -t+_~
__
~ ~ ~
Gowganda Fm. ::;-
~~~~ ~
::!: Fern Creek\ \ \\-..-- \ Quirke Lake, cc:
Fm. ERneacnhyanCtmreeenkt/LakeFms.\ '\\ Hough Lake, oc:
\ and Elliot Lake ::~I:
\ Groups, undivided
Archean
\
FIGURE3MapShowingLocation·ofEarlyProterozoicSupracrustalRocksintheGreatLakesRegionandSchematic
RepresentationofProposedCorrelationbetweentheHuronianandMarquetteRangeSupergroups. Adaptedfrom
Young [1983].
MiImesotathatdirectlyunderliestrataoftheAnimikieGroup youngerthantheHuronSupergroupinOntario. Similarly,
[SouthwickandDay, 1983],andbyanageof1970Ma[Van rocksintheWisconsinmagmaticterrane,whichaccumulated
Schmusetal., 1978]onremetamorphosedbasementrocks duringtheintelVal1840-1880Ma,aresomewhatyoungerthan
thatunderlietheupperpartoftheChocolayGroupintheFelch theepiclasticrocksoftheMarquetteRangeSupergroup.
trougharea,upperMichigan. Thesedataimplythattheentire Granitoidrocks,rangingfromgabbroanddioritethrough
AnimikieGroupinMinnesotaandmostoftheMarquette quartzmonzoniteandgranite,intrudedthevolcanicsuccession
RangeSupergroupinWisconsinandMichiganaredistinctly from about 1870to 1760Ma.
SEGMENT 1 - GEOLOGY OF THE NORTH SHORE OF LAKE HURON:
PENOKEAN FOLD BELT AND SUDBURY STRUCTURE
K.D. Card
GeologicalSUNeyofCanada,Ottawa,Ontario
EarlyProterozoicsupracrustalandintrusiverocksinthe massive,post-kinematicgranitebatholiths;tIleAlgon1a
northeasternpartofthePenokeanFoldBeltalongthenorth granites;earlierpre-tolate-kinematicintrusionsrangingfrom
shoreofLakeHuronarecomprisedmainlyofsedimentaryand quartzdioritetogranodiorite;metavolcanicandmeta
volcanicrocksoftheHuronSupergroup. Theserocksonlap sedimentaryrocksoftheAbitibiandothergreenstonebelts;
Archeanbasementtothenorth,areboundedonthesoutheast metasedimentarygneisses; andhigh-gradegneissesofthe
bytheGrenvilleFrontTectonicZone-thezoneofcontact KapuskasingStructuralZoneandLevackGneissComplex.
betweentheGrenvilleandSouthern/SuperiorProvinces,and MajorArcheandefonnationandmetamorphismoccurredin
areoverlaintothesouthbyPhanerozoicstrataborderingthe this areabetween2700and2680m.y. ago,the agesofthe
Michiganbasin(Fig. 1-1). TheHuronSupergroupwas youngestdefonnedvolcanicrocksandoflate-kinematic
depositedduringEarlyProterozoictimesome2.4Gaago as a intrusions~respectively [FrareyandKrogh, 1986],andwere
southward-thickeningwedgeofcraton-derivedclastic followedbyupliftandcratonization,whichwasessentially
sedimentswithlocalbasalvolcanicaccumulations. Itwas completedby2630Ma,theapproximateRb-Srageofthe
subsequentlyaffectedbyasuccessionofdefonnational, Matachewandiabasedikeswann [GatesandHurley, 1973].
metamorphic,andintrusiveevents,includingthefonnationof TheageandnatureofthePrecambrianrockstothesouthof
theenigmaticSudburyStructure,duringtheProterozoic. theHuronianoutcroparea,beneathPaleozoicstrataof
ManitoulinIslandandthenorthernmarginoftheMichigan
basin,canonlybeinferredfromgeophysicaldataandlimited
BASEMENT ROCKS drilling. VanSchmusetal. [1975a] concludedthatrocksof
theHuronSupergroupextendsouthwardonlytothemiddleof
ArcheanrocksoftheSuperiorProvincearebasementtothe ManitoulinIsland, wheretheyarejuxtaposedagainstgneisses
HuronSupergroupandconstitutethesourceterraneforits ofunknownprimaryagethatyieldeddiscordantProterozoic
sediments. IntheregionnorthoftlJePenokeanFoldBeltand Rb-Srdates. ThecontactbetweentheHuronianrocks and
theCobaltEmbayment,theArcheanbasementconsistsof gneisses-theManitoulinIslanddiscontinuity-trendseast-
T145: 3
Nipissing diabase
~g~ Bar River Fm.
~ Dolomite Kona Dolomite ~g- ~~ Gordon Lake Fm. g-
~ §-,~~~~~:on ~fin~r_d ~~ 8~_L_or_ra_in_F_m' -t+_~
__
~ ~ ~
Gowganda Fm. ::;-
~~~~ ~
::!: Fern Creek\ \ \\-..-- \ Quirke Lake, cc:
Fm. ERneacnhyanCtmreeenkt/LakeFms.\ '\\ Hough Lake, oc:
\ and Elliot Lake ::~I:
\ Groups, undivided
Archean
\
FIGURE3MapShowingLocation·ofEarlyProterozoicSupracrustalRocksintheGreatLakesRegionandSchematic
RepresentationofProposedCorrelationbetweentheHuronianandMarquetteRangeSupergroups. Adaptedfrom
Young [1983].
MiImesotathatdirectlyunderliestrataoftheAnimikieGroup youngerthantheHuronSupergroupinOntario. Similarly,
[SouthwickandDay, 1983],andbyanageof1970Ma[Van rocksintheWisconsinmagmaticterrane,whichaccumulated
Schmusetal., 1978]onremetamorphosedbasementrocks duringtheintelVal1840-1880Ma,aresomewhatyoungerthan
thatunderlietheupperpartoftheChocolayGroupintheFelch theepiclasticrocksoftheMarquetteRangeSupergroup.
trougharea,upperMichigan. Thesedataimplythattheentire Granitoidrocks,rangingfromgabbroanddioritethrough
AnimikieGroupinMinnesotaandmostoftheMarquette quartzmonzoniteandgranite,intrudedthevolcanicsuccession
RangeSupergroupinWisconsinandMichiganaredistinctly from about 1870to 1760Ma.
SEGMENT 1 - GEOLOGY OF THE NORTH SHORE OF LAKE HURON:
PENOKEAN FOLD BELT AND SUDBURY STRUCTURE
K.D. Card
GeologicalSUNeyofCanada,Ottawa,Ontario
EarlyProterozoicsupracrustalandintrusiverocksinthe massive,post-kinematicgranitebatholiths;tIleAlgon1a
northeasternpartofthePenokeanFoldBeltalongthenorth granites;earlierpre-tolate-kinematicintrusionsrangingfrom
shoreofLakeHuronarecomprisedmainlyofsedimentaryand quartzdioritetogranodiorite;metavolcanicandmeta
volcanicrocksoftheHuronSupergroup. Theserocksonlap sedimentaryrocksoftheAbitibiandothergreenstonebelts;
Archeanbasementtothenorth,areboundedonthesoutheast metasedimentarygneisses; andhigh-gradegneissesofthe
bytheGrenvilleFrontTectonicZone-thezoneofcontact KapuskasingStructuralZoneandLevackGneissComplex.
betweentheGrenvilleandSouthern/SuperiorProvinces,and MajorArcheandefonnationandmetamorphismoccurredin
areoverlaintothesouthbyPhanerozoicstrataborderingthe this areabetween2700and2680m.y. ago,the agesofthe
Michiganbasin(Fig. 1-1). TheHuronSupergroupwas youngestdefonnedvolcanicrocksandoflate-kinematic
depositedduringEarlyProterozoictimesome2.4Gaago as a intrusions~respectively [FrareyandKrogh, 1986],andwere
southward-thickeningwedgeofcraton-derivedclastic followedbyupliftandcratonization,whichwasessentially
sedimentswithlocalbasalvolcanicaccumulations. Itwas completedby2630Ma,theapproximateRb-Srageofthe
subsequentlyaffectedbyasuccessionofdefonnational, Matachewandiabasedikeswann [GatesandHurley, 1973].
metamorphic,andintrusiveevents,includingthefonnationof TheageandnatureofthePrecambrianrockstothesouthof
theenigmaticSudburyStructure,duringtheProterozoic. theHuronianoutcroparea,beneathPaleozoicstrataof
ManitoulinIslandandthenorthernmarginoftheMichigan
basin,canonlybeinferredfromgeophysicaldataandlimited
BASEMENT ROCKS drilling. VanSchmusetal. [1975a] concludedthatrocksof
theHuronSupergroupextendsouthwardonlytothemiddleof
ArcheanrocksoftheSuperiorProvincearebasementtothe ManitoulinIsland, wheretheyarejuxtaposedagainstgneisses
HuronSupergroupandconstitutethesourceterraneforits ofunknownprimaryagethatyieldeddiscordantProterozoic
sediments. IntheregionnorthoftlJePenokeanFoldBeltand Rb-Srdates. ThecontactbetweentheHuronianrocks and
theCobaltEmbayment,theArcheanbasementconsistsof gneisses-theManitoulinIslanddiscontinuity-trendseast-
T145: 3
It,·
lEuEND
R --- .
~>'8 Approllimatelocationofcros~-.sr.etion
,.44 Lim·ltofPhaneroxo·1tandM·,ddleProteroloiccover,
N TG•Timiskaming&raben;LC.S-Lake5up.,.ior
!»truC+ure.
/
/ FaultiMFl.-Murra!jFaultlone.iFLF-FIQtk
LakeFault)TF-TimiskamingFauIhj
OF-OnapingFaults
""" ThrustjILF-Ivanhoe.LakeFoult)GtFTI
Grenville FrontTec.tonic.ton..
&0 Man;toulinIslond Disc.ontinuit~
/(ilom~+,..es PaleozoIc 5rr4.+a..
-MIDD-LE.P-RO-TER-OI.-oIC
~ ~c.0_Oc;o~:~~:;_Mon~l::~inn$;:~o~:7P'tlUi
k-l'Iillar'n..y
E-AR-l'l -PRO-TER-OI.O-IC
A LFaklaeckFault QSuyirnkcelinLeakr. B [fi] ~:~!~:e:~~~~~~~~-_~uut:;:~tL.thieflokI.)
Mttr~:o:rt""":llllF~~~~~l""tlt-'~~ti~~~~~ttt---:t1 II Sudbur~IgneousComplell;Nlp'Hln,DI4.(,.Q.se
3000 t t
't500 t ~ Whitewo+erG.roup
Huron Supergroup
CobaltGroup
QuirkeLoke,HoughLoke)£lIiotLgke.Groups
Lower'volc.anic.s)gabbro-anorthositeintrus·,ons
C. FMauultrZra.yone LaSytnlco.tlihnee IDnutnrulospions-E6L-Eost8ulllakej50-Shakespeore
Voleanic\-5-SUdbury)51.-Salma!:lLake;
1500 1-The"alon
3't050000 t ARCHEAN
Fe-Isic.pluton·lc.andmetavolcanic.roc.ks
Lote-toP0:lt-kinematic.~el.sic.plutons)
E Murray M~s~~t~~lin Grenville AG-Algomagranite
kFmault Zoneo~ Manitoulin Discontinuity Island FrontF Pre-tosynkinemoticfelsicplutonic.ondgneiuic
roc.ks
_
~:::~:~:~:~~~ High-grade gnei",ik5I.-Kopu.skosingstructural
"LoneiLroC.-Levoc.kgn.isscomplex
15 [E Metovolcanics
FIGURE 1-1 GeologyoftheNorthShoreoftheLakeHuron-SudburyRegion.
\vestandmaybeamajorfault. Recentreflectionseismicdata wasthrustovertheSuperiorcratonduringEarlyProterozoic
from surveys [Greenetal., 1988] conductedontheGreat orogeniceventsabove. TheHuronianrocksandgneissesare
LakessuggeststhattInsis acrustal-scalestructure. A intrudedbyseveralplutons,prevalentthroughthemidcon
prominentzoneofgentlyeast-dippingseismicreflectionsata tinentregion, ofa1500-Ma-old"anorogenicsuite" (Fig. 1-1).
depthof18Ianseparatesalowerlayerwithnumerous ThecharacterofthebasementbeneaththeHuron
reflectorsfrom anupper,lessreflectivecrustallayer. Allare SupergroupintheareabetweentheManitoulinIsland
truncatedontheeastbyaseriesofstrong,moderately discontinuityandthefirstexposuresofSuperiorProvince
southeast-dippingreflectionsthatmarktheGrenvilleFront ArcheanbasementalongthenorthshoreofLakeHuronis
TectonicZone. Theeast-dippingreflectionsmayrepresenta unknown. Muchofthisareaismarkedbyanintense
thrustzoneordecollementbetweenattenuatedSuperior magneticlow,theNorthChannelanomaly[Cardetal., 1984].
Provincecrustbelowandacomplexallocthonousterranethat Attemptstomodelthisanomalyasresultingfrom athickcover
T145: 4