A GRAVITY STUDY OF THE NORTHWESTERN BOUNDARY FAULT OF THE SOUTHERN KAPUSKASINO STRUCTURAL ZONE

A thesis rubmitted in confonnity Mai the requirements for the degm of Master of Science Graduate Department of Geology

University of Toronto

@ Copyright by -dan Nitscu 2000

3 ubbns and Acquisitions et 0' bgraphic Senficas wtvicer bibliographiques

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A GRAVITY STUDY OF THE NORTHWESTERN BOUNDARY FAULT OF THE SOUTHERN KAPUSKASINO STRUCTURAL ZONE

M.Sc. Thesi8 by Bogdan Nitescu Department of Geology, University of Toronto, 2ûûû

ABSTRACT

The northwstem boundary of the Kapuskasing Structural Zone is fotmed by the

Saganash Lake Fault, which was previously interpreted as a noraiwest dipping normal fault. A

gravity study was initiated in the Liile Missinaibi Lake - Racine Lake orna with the purpose of

detemining the nature of this fouît at its southern end. Gnvity observations mm, made with a

Lacoste-Rombeg M&l G gravity meter along a 40 km pmfile that is normal to t h trace of

the fault. The Bougwr gmvity values show an kicrease of over 35 rngal aaoss the faut

refîecting the presenœ of uplifted deep cnist southeast of the Saganash Lake Fault. 2.5D

gmvity modeling ruggests that at its souaiern end the Saganash Lake Fault is a reverse fsult

with a dip of 60°-100 SE and a depth extent of 10-15 km. This msult supports the mdel of a

popup structure for the souttwm Kapuskasing Structural Zone.

l thank and espedally acknowkdge rny supenrisor Prof. Henry Halls for offering me the

opponwiity to wok on this project and for the enniuriastic and excellent guidanœ h provided.

Hi8 uqui8ite scieMc knowkdge. vast experienœ and intuition wre much appreciated and of

great bendit.

I thank Geodetic Survey Division of Geomatics Canada for the Ioan of a Lacoste-

Romkg gmvity metet. I am gratefül to Mr. Bryne Hearty for coordinathg the loan proœdunt

und t h technical training l mived, and for dl hi8 support and advice. l also aiank Mr. Carey

Gagnon for instructions on the operation of the grovity meter and MI. Diane Jobin for offaring

dviw on th use of the data reâuction software.

I thnk Dr. Chrd Guôaîa hom the bpartmrnt of Geography, University of Toronto pl

Miwissauga, who contributad significantly to the succes8 of this pmject, by pmviding the GPS

equipment and giving 80 fiwly hi8 time to hdp with b opration in the field and with the

pmœming of the w GPS daîa.

I would al= Iike to thrnk the mernbers of my sdvisory wmmit&ee Pmf. Richard Baiky

and Prof. PLmYvea Robin for hdpful swebkns at dHennt stages of the project.

l uko wish to thank and acknowkdge Mt. Sheryl Bunting for her enthmiastic

a8irstanœ in the @Id; MI. Warner Miles fmm Geological Surwy of Canaâa for providing

digital omvily and mromagnetic data sets fmm the Kapuskaring Structural Zone ana; Dr.

Monika ûaiky for hdp(ul discussions about the m i i c h project; Ms. JennWr W s z n i ~ k i for

hrlp with wcmtarial rnattn; Mt. Adam Sorin for making thin 8ections; and MI. Ken T u m for

the ban of runny equipment.

Tha fesearch m# rupporteâ by an NSERC gmnt awardrd to Prof. Henry HaWr.

TABLE OF CONTENTS

1. INtROOUCTlON

2. GEOLOGICAL BACKGROUND

2.1 Geological wtting and structural fiamework of the

Kapuskaring Sttllctunl Zone

2.2 Rock units of centmtleastem Wawa Gneiss Oomain

2.3 Upiift history of the Chapleau blodc

3. GRAVITY DATA ACQUISITION AND PROCESSING

3.1 The gravity suwey

3.2 The GPS rurvey

3.3 The teduction of the field observations

3.4 Analysis of mon

3.5 Qualitative evaluation of the Bauguer anomaly

4. INTERPRETATION OF THE GRAVITY DATA

4.1 ûenrity âetminationo

4.2 Thu source gwmetry and the position of the faul

at the surface

4.3 2.50 f m r d modeling

4.4 The validity of the p m p d grovity m a l

5. GEOLOOICAL IMPLICATIONS OF THE GRAVITY MOOEL

5.1 An d m a t e of the components of displacement

on wutham Saganash Lake Fault

5.2 The cmrcwdonal geomeûy and the ex!ent

of the KapuskQICing Sbudural Zone

6. SUMMARY AND CONCLUSIONS

APPENDlX A A TEST OF THE MODE LING SOFTWARE

APPENDlX 8 OYKE MODELS

APPENDlX C THE DETERMINATION OF THE STRlKE SEPARATION

OF THE MATACHWAN DYKES CAUSED BY DIP SLIP

ON THE SAGANASH LAKE FAULT

APPENDIX D DATA TABLES

REFERENCES

LIST OF FIGURES

Fig. 1-1. Regional geology of the Suprior Provinœ.

Fig. 1-2. Geobgical map of the central Superior Provinœ showing

the fault-bounded blocks of the Kapuskasing Structural Zone.

Fig. 1-3. Liaioprobe ngional seismic reflection data from southem KSZ

Fig. 2-1. Geological maps showing the location of the Pineal Lake blodc

Fig. 2-2. Geological map of the study ama.

Fig. 3-1. Shaâed relief ammagnetic map of the southern KSZ showhg

the location of the gmvity profile.

Fig. 3-2. Cornparison between the GPS eîevations and the resub of a

conventional survey along a segment of the gravity profile.

Fig. 3-3. The elevation and the Bougwr gravity values along

the grwity profile.

Fig. 34. Cornparison betmnm the Bouguer gravity cuwes obtained for

hno different values of Bouguer density.

Fig. 3-5. The extended Bouguer grwity profile, the smooth gravity

cuwe and the local anomalies removed through smoothing.

Fig. 38. Shadeâ relief aeromagnetic map shuwing the positions of most

of the gravity stations.

Fi. Cl. Dens~ty distriMion rnap anâ density chart showing the location

of a# rock mmpkr cdlectd in the area of the gmvity profile

and their dennt~ty.

Fig. 4-2 Denrity hirtognms for the cwnpîes from Wawa G ~ i u Oomain

and muthem KSZ.

Fig. 4-3. Coloutdnped shded relief aeromagnetic map rhowing

the podaon of the gravily stations with respect to the

Saganarh Lake Fault

Fi. 4-4. Interpretation of the Bouguer gmvity data.

Fig. 4-5. Compariwn of the o h w e d Bouguer gravity with the gravity

M W 8 of t h 2.50 step rnodds having difbmnt depth extents.

Fig. 4-6. The ôest-M 2.5D $tep mode!.

Fig. 4-7. The gravity afed in the case of a vertical fouît.

Fig. 44. The horizontal gradient of gmvity dong the gravity profile.

Fig. 5-1. Colour-drapecl shaâed r d i i ammagnetic map showhg

a group of six dykes which am &et along the Saganadi

Lake Fault in the ama of the gmvity survey.

Fi. 5-2. Sketch rhowing the components of t h strike o f k t of the

Fi. 53. Scfmmatic representation of the cross-sectional geornetry of

the C ha pleau block.

Fig. 5 4 . A possible mode1 for the cross-wdional geometry of

the Gmundhog River and Val Rita blocks.

Fig. 5-5. Major mstal fwb in the Midcontinent rift - Kapuskaring Structural Zone mgion.

vii

1. INTRODUCTION

The Kapudrasing Structural Zone (KSZ) is a northeast-trending, fault-bounded,

di~ntinuous kit of Archean high grade (granulite to upper arnphibdite) rnettamorphic rocks,

which cuts diagonally acmss the gerierally east-uwst subprovinœ stnictum of t h south-

centrai Superior Province, and edends 500 km southwstward fiom James Bay (Fig. 1-1). The

high grade rocks m i n this discordant structure am characterized by high denslies, high

manetic susceptibilities and high seismic velocities. The strong wntrast in physical properties

between the rodu of the KSZ and thom in adjacent amas produces prominent gmvity,

magnetic and mismic anomalies. Thom geophysical anomalies attenuate in a region situated

about 20 km w s t of Chapleau, which corresponds to the disappearanœ of the granulite to

upper amphibol% rocks.

Various interpietatiocw have been proposal for the KSZ. The wily mode18 included

thinning of the gnniüc upper c m (Gorland, 1950), mafic intrusions along a rift system (Innes

et ai., 1967), a horst (bnnett et al., 1967), an intercontinental suture (Wilson, 1 W), a failed

a m d a triple juncticn a880~1*ated with the Keweenawan rift (Burke and Dewey, 1973), and a

Jnistral ûanrcumnt fauk zone (Watson, 1980). Based on evidence obtained from geobgical

mrpping, geobarornetric measumments, and grwity mockling, Percival and Card (1983,

1Q85), and Perdval and McGrath (1986) have interpreted the KSZ as an east-verging

Pmterc~oic thnirt hl aptem. In this model, a slab of deep cru& was uplifted akng the

nofthwe8tdipping Ivanhoe L a b Fauît, which repments the southeastem boundary faul of

the KSZ (Fi. 1-2). The thiui2 madel was confirmeci by the m u b of the wimic surveys

conductd in KSZ unâet the auspices of Liioprok (Bdmd and Ellis, 198Q; Pefcival et al.,

1980; Geis et al., IWO; Clowes et al., 1982). On tha b i s d the configuration of ais

Mstichsmn dyke swarm, West and Ernst (1 991) pro- a modiication of the simpk thrust

modd by inf@mng uiktsnüal Protefozoic ôex&al displacement

Fig. 1-2. Geobgieil mg dthe œntnl Sur#ibr Pmgnœ showing the liuRboundrd bkdrr of the Kapuslca8ing Structural Zone (rlbi Bunnil et al.. 1994).

The noiaiwstem boundary fauit of the œnW and southem KSZ is npmwnted by the

Sagaiush Lake ~ault'. In central KSZ the Saganath Lake Fault separates the Kapuskasing

grwrlites Rom the amphiboîite facies rocks of the Wawa Subprovinœ to the wst, wherear in

routhem KSZ thore i8 lithological and metamorphic continuity amos8 the Ssgan~h Lake

Fault, the tmn8ition fmm amphiboiite facies to gmnulite facies rocks Wng gradational (Fig. 1-

2)-

The geophysical data have led to contradictory interpretations regarding the nature and

chsracteristics of the Saganash Lake Fauk On aie basis of gmvity and magnetic

inve8tigationi in central and southem KSZ, Perchral and Card (1983). Percival and Mdiriath

(1086) and Atekwana et al. (1994) have suggesteâ that Saganash Lake Fault is a nor(hw8st

dipping normal fwk fomed as a coHapse structure in msponm to crusta1 thickening which

occurnd during upliR akng the Ivanhoe Lake Fault. B a W on Lithopmôe wismic reflection

data hwn t h southem KSZ (Fig* 1-3), Manson and Hals (1997) suggested that Saganash

Lake Fault dipr to the southeart, $0 that the KSZ in the vicinity of Chapkau ha8 the gemmetry

of a pop.up slrudum bouideâ by inwarbdipping reverse faultt.

The diierent interpretations of the Saganash Lake Fault indicate the nec8ssity of new

grophysiicil inveutigationu amsr this fauk The gravity study pmmnteâ in this thesis was

iniüutd wiai the purpose of providing a better unckmtsnding of the nature of Saganaah Lab

Fauk a1 it8 southem end. The gmity measuremenb w m conduded in the surnmer of 1998

abng a piofik aorring thu fault in th6 mgion of graâaüonal transition bdween the amphibolite

grade rodu of Wawa Subprovinœ and the gmnulite grade rocks of southm KSZ The M y

attmpts to ciam two main probbms conceming the southem Saganarh Lake Fault: (11 the

frult attbde nd (2) îhe &@h extent of the fauk

' T M nme of the hult k mneous since the hult does not go thmugh or nearby Sagansrh Laâe, kit it mr uwâ previou8îy in the litmature (e.g. Pwval and McGrath, 1986; BunnaII et al.. 1oe4; PmivaI anci Wmt, lm).

Fi. 1-3. Lithoprobe mgional wimic Medion data h m touthem KSZ suggesting that the

Saganarh Lake Faul dipr to the SE. Seismic lime locations a n indicated on the map.

2. GEOLOGICAL BACKGROUND

The Superior Province of the Canadian Shield f o m the corn of the Nocai American

continent. It i8 the krgest expowd Archean crston, and consists of igneous and metamorphic

rocks with U-Pb rinon dates of 3.5-2.6 Ga (Percival and McGrath, lm). The remnt

inteqmtatiom of the formation of the Supeflor Province suggest that it is the resuît of r e p d

plate tectonic interactions (Card, IWO; Williams, lm: Williams et al., 1992). However,

opinion8 non-suppo~tive of a plate tectonic origin for the Archean aust wn abo expreswd

m n U y (Hamilton, I998). T h Supwior Provinœ was stabilized ca. 2.5 Ga ago, but its

gwlogical evolution continueâ in Proterozoic mVi a protractecl period of faulting and dyk8

emmplscement (Williams et al., 1992).

Tha southem Supior Provinœ is corn- of wbparalkl east-west trending granite-

greenstom and metasedimentary subprovinces (Fig. 1-1). The granite-gmnstone

rubprovinœs a n dominated by granitoid and gneissic rocks and contain wpracnistal

aswrnbîagm of minly volcanic origin, which am ~ m o r p h o s e â to gmn8chirtsmphibolite

facies. The metasedimentary bels consist pmdominantly of maagreywackes and defived

migmatite and granite. The subpiovinœ boundaries an generally zoner of compkx tectonic

intemûion8 and roine of them are markeâ by steep itrike-slip faultcr (Clowes et al., 1992,

Perdval and West, 1994).

In routh-centtel Supiar Province the east-wmt tnnding Quetiw and Opatica

mtmeâimentary-plutonic subprovincsr in the norai, and Wawa and Abiibi gnnitsgreensbm

rubprwim in the south am sepanted by the northeaat rtiiking Kapuskaring Struduml Zorn

(Fi. 1-2), c h a ~ e ~ by hbh grade metamorphic rocks and positive gravity and

aemrnagnetic anamaIies- &maci on similat 8tmtignphic seqwnces, agm of rock unb and

the chmcter of ryn- and porhrolanic plutonic rocks, the Wawa and Abitibi m~bprovinces can

8

k comlatd scms the KSZ, this correlation indicating the intra-ctatonic character of the KSZ

structure (Percival and Card, 1985. Percival and West, 1994). In Wowa Subprovince a

wntinuous transition is obserwd from the low gmde metavolwnic rocks of the Michipicoten

kt, through amphiboll facies tonalitic gneiss and feiiic plutons of the eastem Wawa

Subprovince (Wawa Gneiss Domain). to gmnulb fwies rocks of the KSZ, which am

juxtaposed b the east against the low grade gmnstone units and gnnitic intrusions of the

Abitibi Subprovince. This transition is interpreted (Penival and Catd, 1983, 1985) as

repmsenting an exposed oblique cross-section through the Archean mst, the difïerent

metamo~hic grades conesponding to â i imn t crustal leveb: Michipicoten k l t - uppr crust,

W w a Gneiss Domain - middie crust, KSZ - lower m8t.

On the basis of combineci geological (distribution of high grade rocks) and geophysical

(gmvity and aeromagnetic anomalies) attributes, Percival and Mffirath ( l m ) identified three

distinct tactonic Mocks within the KSZ Aithough the KSZ is characterized by both gravity and

aemmsgnetic anomalies, the two typs of anomalies am not prfectly coincident. Positive

rromagnetic and gravity anomalies are coextenrive in southem and northern KSZ. whenas

in the centrai KSZ the anomalies diverge by up to 45 km. The spatial distribution of the

aemmagmtic anomaly coincides wiai the surfaœ ocaimnœ of granulite grade rocks. The

aeromagnetic anomaly is rnoâerate (250.500 nT) in wuthern KSZ and is strong (750-1000 ni')

in central and northern KSZ. The gravity anomaly is sûong (40-50 mgal) in boai nortbrn and

w u h m KSZ, but i8 not conünuad in central KSZ. It deviates to the wsst of the central KSZ in

a region of thu Wawa Subprovinœ (Val Rita blo&) chmderized by amphibolRe facies rocks.

The northern block of the K S t is the FnurdribMlirrrrnw #O&, which is bounded

by the Fowvilk fauk to the umst and by the Bad River hult to the e88t (Fig. 1-2). It consists

mainly of gnnuîite facies pusgneiss and mr(lt gneiss with 8 to 10 kbar p a k o ~ u r e s ,

indicathg about 13 km d upliR with mpct to the lower-pmsure rodu (4-5 kbar) d t h

Qwüco bdt ûawd on gmRy modding, the bkck w# inte-ed a8 a popup anrduro

(Plnhral anâ McGmth. 1PB8).

9

The central Mocû of the KSZ i8 the Gioundhog Rivet biock, which is separated fmm

th northem FmserdaIe-Mwionee blde by a 65-km gap without granulbs. The block is

bounâeâ by b r W faub to the eaat (Ivanhw Lake Fauit) and wmt (Saganash Laûe Fault),

and is characterked by an intense positive aoromagnutic anornaly and negligibie gravity

exprmsion. The Groundhog River block conaists of granulite fa-s mafii gneiss, tonalite

gneim and puoeneiu. The estirnates of the mettamorphic paleopres8ure are in a range of 7 to

g &bar (Percival and McGroth, 1986) implying at lesot 15 km of upiift almg the ivanhoe Lake

Fauît, which juxtapows the Kapuskasing granuliti against the low grade rocks of the Abitibi

Subprovinœ. B s d on potential field data, the Groundhog River block was interpnted as a

thin (24 km thidr) thnirt wuâge of granulb, boundeâ to the east by a wsstdippirig thrust fauit

and to the w s t by a wmtdipping normal fault (Perdval and McGroth, 1886; Atdmna et al.,

1894).

The Chaphu blook in southern KSZ is the Iargest part of the Kapurkasing rtnictum.

it is mparated from the Gmundhog River block by the Waîcusimi Rivet Fauit (Fig. 1-2) and is

bounded by the lvanhw Lake Fault to the souaieast and the Saganash Lake Fauit to the

northwmt. The notthem part of the Chapleau biocû wnsists of no&east-riking kb of upper

amphibolb to granule facies parsgneisr, mafic gneiss and tonalb gneiss similar to those of

the Gmundhog River blocû, and a layerad anorthoiite-gabbm intrusion, the Shawmere

cornpbx. T b Modt bro8âens to th8 routhwust and merges mai the central Wawa Gneiss

Domain akng an imgular, north-trending, lû-ûm-wide zone east of Chaplrau, which ir

definrd by îithological, ttnidursl and metamoiphic change8 refiecting th8 transition to the

mphiboiite i b c h rodo of Waum Subprovince (Fi. 1-2). Aaorr this tranriüon the estimateâ

pmmum of metamorphism decreme fi#n 9 1 1 kbar in the Chapieau bkd< granuiites to 56

ûbar in the W m Gneiss Domain (Penivrl and WestB 1994). The wum of the large gnvity

anomaly r#ocidrd with the Chiplwu bkck was modrkd a8 o hutt-baindeâ dab of deme

roda u p î i i along th8 norWmtdipping lvanhoe Laice Fauit (Perdval and Card, 1983,1985;

PvcM anâ McGmth, 1086; Atdcwana et al., 1994). This interpretatbn ir rupportod by the

10

Liihoprobe wismic reftaction and mfiection data (Boland and Ellis, 1989; Penival et al., 1989;

Geis et al., lm). A fourth uplifteâ block, the Pineal Lake block (Fig. 2-l), was reœntly discoverad,

bawâ on changes ohewed in certain physical and petrolagical properties (paleomagnetic

polarity, feldspar douding intensity, feldspar hydrous aiteration level) of the Proterozoic

Matachewan dykes that indicate variations in their emplacement Ievel (Halls and Zhang,

1995a, 1998; ïhang, 1999). The Pineal Lake block is 20 km sinistnlly offset from the

Chapleau block and extends 60 km to the southwest. It teminates to the m t along a north-

south fault (McEwan Lake Fault), which on the basis of gravity data was interpreted as a west-

verging thnist having a depth extent of 5-7 km (Halls and Mound, 1998).

2.2 Rock u n b of contnl+rsbrn W w a Gnaiss Oomrln

The central-eastern Wawa Gneiss Domain consists of vanably foliated tonalitic to

grenodioritic orthogneisses and pkitons (mainly granitic and granodioritic) surrounding

kilometre-sale b i ts of amphibolite to granulite grade mafic gneiss and paragneiss. The

tonaliüc gneiss is the preâominant rock type in the central Wawa Gneiss Dornain, making up

approximately 65% of the exposeâ orna (Moser, 1994). Near Chapleau, the tonalitic units

extend from the Wawa Gneiss Oomain into the KSZ. The oldest rocks of the Wawa Gneiss

Domain am tonalitic gneiss components, da td by U-Pb on zircon at 2920 Ma (Mowr et al.,

1991; Percival and West, 1994). The distribution of these rocks is not known bcause they are

indistinguishaôk petrognphically from younger tonalitic gnebses in the area. The U-Pb age

deteminations show a widb range of ages for the tonalitic rocks of the Wawa Gneiss Domain.

from 2020 Ma to 2860 Ma (Mowr, 1984). In the units from southwestern Chapkau Mock, the

metamorphiwn of tonalitic gneisa ha8 producd kuco~omal segrqations with U-Pb zircon

agas of 2880 and 2645 Ma (Mmr, 1894). The bodies of pîutonic rocks in Waws Gneiss

Domain have ages from 2690 to 2633 Ma (PeccivaI and West, 1894).

Fia. 2-1. Geolqical mapr showhg the location of Uia Pined Lake blodc (PB), which is ofFset

sinistmliy fmm the Chopleau blodc (CB) along the Nagasin Lake Fauk (NLF). The blodr is

boundeâ to the weit by the McEwan Lake Fault (MLF). B U , MRF and ACF Mer to the Budd

Lake Fault, Montnal River Fauît and Agawa Canyon Fauk.

a. The dots npmsent sites whem pakomagnetic poiarity data have been obtriined. The r d

amas cormponâ to âeep cwstal levelr Were the Matachewatt dykes have normal

magnetic polar@. The blue and purpk amas npmwnt rhallowr crustal levels whem the

dykes have moitiy m v e d magnetic pdarity.

b. The dots mpmwnt sites when fadrpar clouding data have bwn obtaineâ. The blue and

green amas cornapond to deep crustal levels whem the Matachewan dykes have cloudy

bldspan. The yellow ana8 mpmmnt shallc~wer cnistal levelr when the dykes have dear

bldspam.

The figures are modified after Halls and Zhang (lm) thmugh the addition of new data and

corrections for mi8placd points (H.C. Halls. unpubliihed map).

14

The ama d the grwity study is located 30 km nom of Chapleau, in t h region of

cornplex transition between the amphibolb grade rocks of the centralaistem Wawa Gneiss

Domain and the gnnulites of the Chapbau #O& (Fig. 2-2). It extends from Little Missinaibi

Lake in the northwst to Racine Lake in the routheast. In this ama the tonalitic gneiss

genemlly con8i8ttr of hombknde-biotite-plagiodasequartz * K-feldrpar asm#sgcw. Mafic

gneiss ocwn as endaves within the tonalitic gneiss Pt scales nnging from 1 m to several tens

of meters, the abundance of the enclaves inaeasing eatwîrds (Moser, 1994). The mafic

gneiss contains a dominant assemblage of finegrainexi amphibole, plagiodase, and minor

biotite and quartz Southeast of the Saganash Lake Fault, there are two kilomebscale bodies

of mMc tonalite (15%-20% amphibole) clos-aiffi*ng the tonalitic gneiss. M o , an east-west

tmnding, kilometre-scak bet of migmatitic paragneiss ocwm routheast of the fault (Fig. 2-2).

Northwest of the Saganash Lake Fault the gravity profile crosses the Windemere Batholiai,

which is a 15 km x 20 km cornplex of massive, kucoaoüc gnnitoid rocks including tonalb,

gmnodiorite, and granite. In the whole ama of study granbid pegmatites am ubiquitous as

centimetre- to metre-cpk veinr and dykm.

The cuntral-eastern Wawa Gneiss domain i8 transecteâ by north tmnding dykes

ôeionging to the 2.4501 Matachewan Swan, which am ckarly visibk on t h aeromagnetic

map. The dyûm am iron rich thoieiites &en charaderizeâ by calcic plagiocIam phenocrysts

(Zhang, 1999). Northawt Wnding dykm bdonging to the 2.û4-G~ Kapusbsing wann occur

routheast of the Saganash Lake Fault, and are also visibk on the aeromagnetic map. Both

type8 of dyke8 have prodominant dinopyroxem and lakadorite, with minot caidc amphibok,

ilmenite, magnetite a d quark (PedvPI et al., lm).

23 upim NI- dm chplwu bkok

Constrainta on the upiift hiaory of the $outhm KSZ are piovidaâ by a variety of

Wopk aga data, the M-n and Kapuskasing dyîces, and hno sets of aikalit rock -

16

The U-Pb geochmnology suggests that the Kapuskasing rocks in southem Chapleau

bloclc remained at high temperature and signifiant depth until about 2500 Ma. The U-Pb

zircon ages demase with incmasing paleodepth, from 2863 Ma near the western limit of the

gmnulite facies to 2582 Ma in the east. and the U-Pb tiianb agas decrease eastward fiom

2550 Ma at the amphibolite-gnnulite transition, to 2514-2493 Ma at the deepest stnictuni

kvels within a b u kikmetres of the Ivanhoo Lake Fault Zone (Krogh, 1993; Penival and

W&, 1994).

a ~ ~ P 8 ~ r homblende and biotite ages as old as 2500-2450 Ma from the deepest

8trucRinl Ievds of the KSZ indicate that earfy uplift has occunad in this time interval. More

the emplacement of the Matachewan dyke Swan at 2454 Ma (Penival and West, 1994). On

the &sis of the difbrems betwwn the cryatallization pressures of the Matachwuan dykes

and the metimorphic pressures of their country rocks in the Chapkau block, Percival et al.

(1W) ertimated that the pre-Matachman uplift has a magnitude of 0-8 km.

Th- are several lines of evidence for uplR after the emplacement of the Matachewan -

dyke swam: deformation of the Matachewan dyke swarm (Bates and Halls, 1991; West and

Ernst, 1991), tilting and offset of the Kapdasing dykes in the Ivanhoo Lake Fault Zone

(Perdval and West, 1 W4), paleomagnetic polarity reversais and mineralogical (feldspar

douding and hornbknde Al content) variations in the Matachewan dykes indicating signifiant

diibmnces in the âepth of emplacement across KSZ (Halls et al., 1994).

Geobemmetiic data on clowly spacd Matachman and Kapuska8ing dykes suggest

that mm8 uplift (4-7 km) may have occumd in the intenral 2.45-2.04 Ga ôetween the

empkœrnent of the Matachewan swarm and the emplacement of the Kapuskasing swam

(Perdval et al., 1994). An episoôe of fouît madvation in this interval is also wpported by ages

in the range 2.2-2.3 Ga for micas dom to the fault m m ( H a m et al., 1994).

The main upiift postdates the emplacement d the Kapuskasing dyke swam and,

bawâ on the aystallitiaon pnewms of the Kspu8kasing dykes e%posed in KSZ, hm a

mlgnituôe of 10-17 km (Pemba1 et al., lm). The A M r bise ages in KSZ (PefcivaI and

17

Peteman, 1994) dacreaw eastward fmm 2500 Ma in Wawa Gneiss Domain to 1930 Ma near

hnnhoe Lake Faut Zone, implying that temçumtuiw in the demp cnist remaiMd above 280°C

until 1930 Ma. This pattern ruggests a post-IWWa uplift (Pecivol and Wmt. 1994). The

Iomr age bracket for the uplM io uncetain, but the 1 .&Ga Cargil alkalic rock - carbonalite

cornpiex emplaced on the Lepage faut in northem Wawa Gneiss ûomain is only rlightly *t,

suggmting that t h major KSZ deformation had occurreâ by this tirne (Percival and West,

1894). The defornation st ca. 1.M.85 Ga is probably the d k c t of an Early Proterozoic

orogenic collision at the magins of the Superior Province, possibly the Trans3ludson orogen

(Manson and Halls, 1997) or the Penokean orogen (Percival and Peteman, 1994; Riller et al.,

1 999).

Late magmatic acüvity a8sociated wiai the Kapurkasing structure is represented by the

intrusion d alkalic rock - carbonlite complexes and lamprophym dykes in the intenral 1.14-

1 .O1 Ga, and minor fauîting a1 1.1 Ga, which may be relateci to alkaline igneous adivity, or to

distant stniaunl &CM of the Gmnville orogen (Percival and West, 1994).

3. GRAVITV DATA ACQUISITION AND PROCESSINO

3.1 The g m i ( y 8umy

The gmvity survey was conduded along a logging road which crosses the Saganash

Lake Fault about 30 km north of Chapleau (line AB on Fig. 3-1). In total, fw-seven gravity

stations wre establirhed, seven of which (stations 51-57) are situateâ on the shores of

Radne Lake (Fig. 2-2) and have no road access. n i e gravity stations are spaced at distances

of 0.3-1 km when projected on a straight line orthogonal to the stnke of the fault The gravity

oôservaüonr wn made during a fiveday period, with repeat mearrurements at the beginning

and end of each day at Control Station 900244 of the Canadian Gravity Standardization Net

located at Chapleau Airport, and having a pravity value of 980751 .û6 mgal.

The instrument ured to measure the gravity diffemnœs betwwn the gravity stations

and the barn station rituated Pt Chapleau Airport was a Lacoste-Romôeg Model G gravity

meter loaned from the Geoûetic Division of Geornatics Canada. The accuracy of the mode1 G

meter is 0.01 mgal. The gmvity meter has a thermostat for temperature control and achieves

drift rates of 0.5 mgaUmonth. Given this smaY drift rate, the largest sourœ of errom in the field

cornes from the irregular drift produced by the vibrations and the various bump encountered

during the transport of the instrument- In order to r d u œ these unwanted effadrr, the meter

was tnnaported in a canying case equippsd with a shodcmountd cradk.

The drift contml for the gravity survey was achieveâ by occupying the base station at

the bginning and end of each suwey day. For the road traverws excellent msuL were

obtained, the Iargest dowm m r Wng only 0.03 mgal. In the case of the lake traverse, the

gmvity mrtrrwas tranrported by boat maiout the shock-mountetd cradle and, as a resuît d the

wave action, the irmgulst drift of the meter was larger, a closura mt d 0.1 m ~ a l k i n g

obtaid.

Fig. 3-1. Shaâeâ relief mromagnetic map (total field intenrity) of the souaiem KSZ rhowing

the location of the gmvity profle (lin8 AB). T h aeromagndic enprer8ion of the Saganash

Lake Fauk (SLF) is ckady visibk. Illumination uimuth: 295'. lllurnination indination: 29'.

3.2 fh. G?$8UWay

The duction of the grwity obsewationr requires the determination of the rlevation

and porition (latitude) of each of the gravity stations. For this purpose a semi-kirnatic

difbrenüal GPS rurwy was conducteâ akng the gmvity profik. The GPS rurvey involved two

P u M e c h GPS mœivam, one mmaining pnnanently fixed at a base station and the other

tracking the movement of the vehick along the road. 60th reœivem acquired signals

simuttaneously fmm the same satellites and measured the (pseudo-)range to each satellite by

rnultiplying the rignal propagation time by the speed of light. A 2-3-minutes ocacpancy time at

each d the maâ gravity stations with the mobik reœiver enrureâ an inaeawd occuracy as a

msutt of the accumulation and aversging d several measunment epochs. The roîe of the

stationary mœiver was to pmvide emr corrections (dock, orbit, propagation), improving thus

considonbly the accuracy of the rnrarumments.

The elevations of the gmvity stations locatd on the shores of Racine Lake in amas not

accessible by land wem measured in the field with respect to the watw level uring a simple

mkr. For the chtermination of the absolute ekvation of the lako level a lalteshore point

rituateâ pl the water kvel and accessible by mad was ocaipied with the GPS maiver. The

poritionr (Iatituôas) of the lake stations were determined from the NTS 150000 map.

The accuracy of the GPS piocesseâ data (both elevations and horizontal positions) is

~ * r n a h â to be at the decimette level. In the case of the stations located on the shores of the

I a k the uncortainty in their horizontal position is considerd to be about 25 m.

In order to test the accuracy of the GPSdetemined elevations a conventional suwey

using a WYd NA2 Univesol Automatic kvd was undeiken along a segment of t h gravity

prdik. The dillbrencm behen the values of relative elevation obtaineâ h m the

convonthal wnny and those pn,viCkd by GPS m m about 0.2 m (Fi. 3-2), conlnming the

e8ümated accuracy d th8 GPS mlts.

The iWmüon4 of al1 of the gravity rtaüonr am pfesented in Fi. 3.3.

A mries of mâudions are made to the rneasumâ gravity values in orckr to obtain the

Bougmr gravity values, which teflect only lateral density variations in the suburfaœ.

A pmliminary q w n œ of oprations is usually appliexi to the gravity meter readings,

in odet to d e h the abrdute gravity values in each of the gravity stations of a survey. Fint,

the metw reading are convaited from instrument unb to gravity wits (usually milligals) by

multiplyhig anmi with a sca!e constant supplied by aie manufacturer of aie gravity meter. The

values obtaind are corrected for the tidal effeds of the sun and moon, whieh are both tirne-

and latitude- dependent and con have amplitudes of the orâer of 0.3 mgal. Also, bared on the

dosum emm (diierencer obtaineâ between reaâings taken at the base station, comcted for

nie Mal efbcts), a correction for the irregular drift is applied to th8 observations in each of the

stations by linear interpolation. The values conected for earai-tides and drift allow the

âetennination of the gravity differences betwwn each of t h gravity stations and the base

station, and the atmlute valw of gravity in each gravity station can h obtained if the absolute

value is known for the base station.

The usual mdudion procedum of the values of obsewved grsvity attempts to accwnt

for the total mass of the Earth, the ellipsoidal shape of the €am, the rotation of the Edh, the

ekvation of the observation point above the sea fevel and the attraction of t h 'nonnaln mass

situateâ above ma level (NeMeton, 1976; Blakdy, 19g5; Sirnpn and Jachens,lQ89). The

Bougwt anomsly (BA) values are wrnputd according to the folIwbng equation:

25

FA is the air tem. It adjusts the aiemetical value of gravity at sea level to account for

the elevotion d the g r a m station. The rate for the vertical d m a s e d gravity mth

inmasing devation is 0.3088 rngallrn, which repmnts the theoretical vertical gradient of

gmity.

ga= 0.04193*a.h is the Bouguer correction which accounts for the masa between the

oôservation point and sea kvel by appmimating P with an infinitey extendeci horizontal

dab of density o and thicknem h equal to the height of the obwwation point above sea

kvel. In moit standardireci gravity retduction oprations, the density value uwd for the

Bouguer correction ii a = 2.87 g/cm3, which is considend to k an average crustal density.

The purpose of the Bouguer correction is to odjust the theoretical value of gmvity at the

point of obswation to include the e c t of an idealueci mars laye.

g~ i8 the terrain conecfion, Which accounts for the departuns of the Earth's surface from

an idealkd flat rurf8œ (as it was considereâ in the Bouguer correction).

The Bouguer anomaly as definrd by Equation [l] repewnts the gravity effeds of the

subQurf8w geology at the point of measurement resulng Rom the diffennco ôetwwn the

gravity oô8erved at that point and the gmvity (ako at the rame point of observation) c o u d by

a definad wrth modsl. The Bouguer anomaly may al80 include M8Cts cauwd by deficiencies

in the earth mode1 (the infinb horizontal layer conriderd for the Bouguer correction or the

hct mat the -air and Bo-r aâjustrnents use the ma level as the Merence efevation

mthr than the surface of th8 ellipoid).

The gnvity obwwationr conrickmd in the study presented in this thesis wre

comcted br the WaI variations, i q u l a r dm, lotrude and eavaion MUS. using a software

a p p i i i n (PCGRAV) provideâ by the Godetic Suwey Division of Geomatics Canada. For

the oilcukaon al the thoretid gmiity, the PCGRAV appliïon u w the Gmvity Fonnula

1967, which ybîds theoretical gravity values on the su- d th &nnœ ellipooid definecl

by the GmWc Relhnce S-rn lQû7 (Geomatîcs Canada, unpr#hhed manmaipt):

[2] % ($1 = 978031.85 [1 .O + 0.005278895 sin2(+) + 0.000023462 sin4(+)] (mgai),

whem 4 is the station latitude in dqrees.

The Bougwr correction was applied for the standard ctustal density of 2.67 9/cm3. Given the

small topognphic imgularities in the ama of study, terrain cottections wte omitted, as they

were nogligibb.

The Bouguer grwity profile (Fig.3-3) was produceâ by projecting the positions of the

gnvity station8 on a straight line ppndicular to the strike of the faut the orientation of the

huît having been infemd from s mromagnetic expression (Fig. 3-1).

3.4 Ani lyrk of amors

The final unœrtainty in the Bouguer gmvity values was estimated by considering the

variour sources of error occumng in the ptoc8ss of gmity, elevation and position data

acquisition, and also the enon causeci by the assumptions made for the redudion of the

gravity observations.

The irmgular drift of the gravity meter causd an unœrtainty in the Bouguer grwity

v a l m of maximum 0.03 mgal for the land stations (stations 1-50) and 0.1 mgal for the lake

rtations (stations 51 -57).

For the standard mstal density value of 2.87 glcm3, the elevation factor (the combined

frw-air and Bougwc corrodions) hm a value of 0.20 mgaum. Therefom the unœr!ainty of

about 0.25 m componding to the GPSdetermined ekvdkns translates into approamately a

0.0Smgai uncertaint'y in the Bougwr gmity.

lhu horizontal positions of the land st8üons won d8teimitwd mth the GPS and have

~~ accumcy. The horizontal positions of the k k -*on8 wem ôutmined fmm tha

1 :50000 to~ognphic map, and have an uncertainty of 25 m. Sinœ at 48" Iaütude (üm ruwey

Iaütude) a variation of 100 m in the naiüçwuth diredion produces a thange of about 0.08

mgal in the theoretical value of gravity (Nettkton,

theoretical gravity terni is less than 0.001 mgal

8btions.

27

IQ76), the largest error in the value of the

for land stations and 0.02 mgal for lake

The omission of temin corrections in the case of a mgional relief of less than 80 m

over 20 km is estimated to have causeci an enor in the Bouguer gravity which is not greater

than 0.1 mgal (Jones, 1973).

In summary, oie maximum expected error for the Bouguer gravity is less than 0.2 mgal

for the land stations and less than 0.3 mgal for the lake stations.

A topic which ha8 to k addressd in a discussion of the emrs in the final gravity

values is that of the Bouguer density. As mentioneû in a previous section, the Bouguer

cormdon was opplid for t h standard density of 2.67 $/cm3. The density deteminations

bawd on the wllected rocû samples (Chapter 4) suggest a higher average surfaœ density

(2.73 @an3), but this result doem not consider the less dense seâiments of glacial origin

oocurring in the ana of the gravity survey. Howsver, due to the small variations in topography,

the shap of the Bouguer anomaly curve computed for a Bouguer density of 2.73 g/cm3 is

similar to the curve obtained for the standard density of 2.67 g/cm3 (Fig. 34). If one of the

cunnr is translateâ over the other so that the points conesponding to the fint station overiap,

then the owrîapping ermrs for the nst of the stations are less than 0.1 5 mgal. B a d on the

prmiour considerations and on the observation that the Bouguer anomaly computeâ for the

rtandarâ Bouguer density show littk cornlotion with topography (Fi. 3-3), it can be

conriâed that the density value of 2.67g/cm3 was appropriate for the Bouguer consdion, the

enor ôeing negligibk if compored to the amplitude (> 35 mgal) of the anomaly investigatd.

T h Bougwr gmvity values show an inmase of owr 35 mgal fmm the nocthwsrt to

ttm of the profile, which Mects the presence of upîifted &nw m s t wutheast of

Sagana& Lake Fauk Due to the lack of acu38s1 th8 profile could not be continued beyond

29

Lake Radne (Fi. 2-2) inride the KSZ and as a msutt, the top of the anomaly is not well

M n d . In orckr to nmow this unœrtainty, six pmviously established gravity stations situated

southeast of Lake Racine on the d i m o n of the profile wem used to extend the gravity profile.

The Bouguer gravity values comsponding to these stations were obtained from the federal

gmvity database. The extended gravity profik ha8 a total length of 57 km and defines very well

the gmity anomaly auocrated with the nodhwe~tern boundary fault of the Chapleau block

(FIg. 35). It can k observed thot the Bouguef gnvity rises from the regional background

gravity of approximately -65.5 mgal in Wawa Gneiss Domain to a maximum of about -28 rngal

wutheart of the Saganarh Lake Fault, inside wuthern Chapleau blodr. The large amplitude

of the anomaly indicotes the existence of a significant mam contrast in the wkurfaœ.

Another charaderistic of the Bouguer anomaly cwve is represented by the positive

local anomalies of short lateral o n t and low amplitude (les8 than 3 mgal) wprimposed on

the large anomaly associated with the Saganash Lake Fauit. In order to remove these shnt

waveiength anomals, the Bouguer anomaly wwe was smootheâ gnphically (Fig. 3-5). The

moothing proceâuie WBS unequivocal sin- most of the local anomalies are ckarly

disting uhhable h m the background gravity.

The aeromagnetic data indicate that in most of the cases the= local gravity anomalies

are oô8enmd in the stations situated in the vicinity of the Matachewan and Kapuskasing dykes

Mich are trossed by the gmvw praik (Fi. 34). Based on this observation it is inferred that

the local anomalks am producd by dykes. In -me cases the anomalies might indude the

dfe& of more than om dyke. For example, the miâual anomaly o b w e d in station 45 ir

very likely th8 sum of the gnvity M&ts of both a Matachewan and a Kapuskasing dyke which

intemec2 mat mis staüon. It can be notiœâ that the local anomalies obsenred n o r t b t of the

Saganaah Lake Fault have Iwger amplitudes thui th- o b n m d wutheart of ais Saganash

Lake Fuît (Fi. 3-5). T M wriaüon in the average amplitude of the local anomalies saorr t h

Saganaah Laka Fauit cou# be explrid by (1) a variaüon of the dansity contraa b twwn the

dykm and the h W iodu awss the huR, and (2) a dihmnce in the dylte thicknem, the dykw

Fig. 36. Shaded n l M aeromagnetic map showing the positions of moat of the gravity stations.

Illumination azimuth: 295'. liiumination Nidination: 22'. The yellow dots correspond to the

rtitions whem the local grawty anornalias have maximum amplitudes.

33

inskie KSZ k i n g thinner than those obseweû in Wawa Gneies Domain (Ernst and Halls,

1983; Halls and Nitesa, field ohwations). In order to confimi the comlation obesrvd

between th local anomalies and the dykes crosseci by the gravity profile, the gravity M a s of

several dyîce mockk having parameten similar to those of the dykea Rom the study area m m

calculatd using a 2.50 fornard moâeling program (Appendir 6). The maximum amplitudes

obtainad for the cornputeci dyke anomalies (0.5-1.8 mgal) a n similar to the amplitudes of most

of the local anomalies. However, the anomalies observed in stations 20 and 29 are too large

(2.5-3.5 mgsl) to be pproducci only by the dykes visibîe on the aeromagndc image. These

anomalba nprewnt ekhet the cumulateci efbct of a largw numbei of dykes, not al1 of them

having a mpatate aeromqnetic egpression, or indude the Mects of some unknown shallow

doniity contm8b.

4. INTERPRETATION OF THE GRNlTY DATA

The objective of the gnvity data intepreûation is the detemination of the mass

distribution msponsiblu for the o h ~ e d gravity anomaly. A source of unmtainty in the

intefpretation of a granty momaly n s u b fmm the inherent non-uniqwness (ambiguity) of the

invetw pmbkm for potentiol I l ds . An unlimitd nurnôer of possibk mass distributions can

8 o i f y the^ observacl gravity anomaly. In order to constrain the gravity mode1 and to reduœ the

ambiguity it is essential to obtain independent information (density contrasts, geometry of the

8ouros, positions of contacts) fmrn geological okenrationr or other typs of geophysical data.

In the srsa of the gravity wnrey the outcrops are scarce, most of the surfa- b ing

covered by dense vegetation and surficial deposits of glacial origin. In order to inveatigate if

thon is any surface deniity contmst aaoss Saganash Lake Fault, the densiües of 48 mcû

sampks collocted from the outcrops ocanlng in the study ana were determined. In t m s of

lithology, m a t of the mmples ftom Wswa Gneiss Domain are tonalite-gnnodionta gneisses

and a (bw a n massive gmnitoids fmm the Windermem batholith. Inside the Chapîeau blodc

the rodc unib sampîed w m of mafic tonaïte and also tonaliteqnnodiorite gneiss. The

rnacroscop'c analysir of the rock sampbs and a study of 27 thin sections revealed that the

rocks contain quark, fskbpar piagioclatm, bioüte andlor amphibole. Common acwuary

mimnlr are rphum ancl rnagnajfe. Smal amounts of potassium feldrpar a n present in m e

r m p k r on both tiikr of the fault No pyioxem or gamet war found in the sampks collected.

T h gnnitoid and tonalite gneim s a m m h m Wawa Gneiss Oomain contain 5%-10%

amphibok and biotite. Wrea8 tha mafic tonalite and tonalite gneim sampies from the

Chphau bbck contain a Iarger pmporüon of msfic mineralr (15%-20% amphibok and

Wite), and am Chamderizeâ by a covrrr grain size. In the ana of study them is no change

35

in the rnetamo~hic grade of the rock units across the fault, al1 the rocks k ing in the middk

amphibdite facies. Hydrous alteration products like sencite, epidote and chlorite are more

evidrnt in the rocks from Wawa Gneiss Domain, but they are also pmsent in some samples

h m Km. The 1-r dogme of hydrous abration obrswed in the $amples from southern KSI

indicetes a deeper crusta1 level for the rocks situated southeast of the Saganash Lake Fault.

The density deteminations comprised mam and volume meawrements for each of the

samplrs wnsidemd. The mass measunrnents were made with an Ohaus balance readable to

0.5 g. For the volume determinations a bucket having a downward dipping overflow pipe was

u d . Each sampk was immemed in the bucket C l k d with water up to the pipe level, and the

diaplad amount of water was coilectd in a calibrated cylinder. In order to validate a volume

detwmination, the same resuit had to be obtained in three measurernents. The enor in volume

cauW by the fluide trapped in dosed poms is negligible due to the very small porosity (less

than O.?%) of the rock sarnples. The volume values are reliable to 2 cm3, Wch is the reading

accuracy of the calibrateû cylinder useâ to meaaum the volume of displaœd M e r . The

unœflainty in the âensity values is conaidercd to ôe 0.02 @cm3.

The msub of the density deteninetions have brought direct evidence for tha

existence of a surface density contrast associateci with the southem segment of Saganash

Lake Fault (Fi. Cf). The main causes of the contrast are considerd to be the presenœ of an

inmawâ proportion of rnsfic minefalr (5%-15% more biotite and amphibole) and a lower

degiw of hydrous aîteaüon for t h rocks situateci southeast of the fault. The density

hiatogramr for t h sampîes h m Wawa Gneiss Domain and for those ftom the Chapkau bl&

am pmented in Fi. 4-2. It a n k obsewed that most of the $amples collected northwest of

the huit have denaies in a ronge betwwn 2.62 and 2.73 glan3, mlh a peak at 2.-2.60

g/m3. This wido range is cauwd by both the hetemgeneity of the tonalite gneiss (difbmnces

in and wnphibok content) and variations in tttu degree of hydrow altefation. In the case

of the r n i p h from muthem KSZ. densitks between 2.74 and 2.88 @m3 am o b w w û for

75% of the umpkr. Thme rirnples show l i i or no hydrous aIteraüon. The nmainïrtg 25%

WAWA ONUS8 00MAlM

Fig. 4-2. bnsity histogmms k r the samples from W a Gneiss Domsin and rauthem KSZ.

38

of th8 mmpb am chamderiad by a higher degiw of hydrous alteration and have densities

in a range similar to that of the samples fmm Wawa Gneiss Domain.

The denrity ~tenninrüons peomd for the wllectd samples cannot Iead to a

pf'ecim estimate d the magnitude of the âensity contmst ocrosa the faun. This is due to a Iack

d pmdae kn-e of both the proportions of different rock types in the ama and the

proportions of rocks rtrongly affectecl by hydrous alrotion. Howcrwr, sinœ the sampks wen

collecteâ from as many different locations along the gravity profile as possible, a simple

dansity average cou# give an appmximate estirnate of the average surfaœ density. The

average density of the rock samples from Wawa Gneiss Domain is 2.69 glcm3 and for t h m

from KSZ is 2.75 gkm3, suggesting a s u h œ density contmst of 0.08 glcm3. This value of th8

surface âenrity contraut could in fact underestimate the mal surface density contrast, sina it

dom not take into account the southeastward inmase in abundance of the mafic gneiss

enclaves withh the tonalite gneiss. If a contact similar to the one obsewed al surface across

n o m Saganash Lake Fault between granulites of the northem Chapleau block and

amphibdite f a c h gneiss88 of the Wawa Gneiss Dornain is buried in southem KSZ, then the

density contntt at âepth is likely higher than that obsecved at surface across southem

Sagana8h Lake Faut whem the rocks separated by the faul have the crame metamorphic

g d e . Thorefore, the value of the âensity contra& obtained Rom the density determinations

ha8 to k regardeâ as a lower lima for the average Lnsity contrsrt bettwe8n Wawa Gneiss

Domain and southern KSZ

4.2 Tho roum Ilronirby n d the porWon oltha kult

An important conmint a n be hi- on a gravity modd if independent arwmptions

o n k made about thb gmmeûy of the roufce. This requim an understanding of the

gdogicil featuril pnr«rt in the sukurface. Uswlly, the coniûucüon of a modal ir bawd on

hnro aiteria: (1) ths mode1 h n to be malistic and (2) the mockl shouîâ be kept as simpk as

pouibk. A complkated modd having many detaik is not jusMW ewn if it giwr a pmf6ct

39

comspondenœ Wween calculated and observed gravity values, unless other geological or

geophysical constroints can ôe brought to support an incrsosed level of detail.

The mode1 of a fouît-bouncîd slab of uplif'ted deep cnwt for the KSZ. b a W on the

Lithoprobe seismic sumyr and the pmvious grovity interpretations (Percival and Card, 1985;

Pedval and McGrath, lm; AteWana et al., 1994; Percival and West, l m ) , suggests a step

structure pomeûy for the sourw of the gravity anornaly associateâ with the northwstem

boundary of this structure in southem Chapleau block. Although the assumption of a tnincated

horizontal slab cou@ ôe a simplification of the geological reality, such a model is suitable for

the detenination of the Sagjanash Lake Fault parameters, which constitute the purpose of this

study. The possible average inclination of 8' NW (Percival and Card, 1985) for the dab of

denwi uplifted roda ia too rmal to proâuœ a gravity M6ct significantly different from that of a

horizontal slab, and would not requins a change of the fault parameten in order to ft the

obeweâ anomaly.

The unequivocal determination of the fault attitude requires a pnciw specification of

t h poslion of the fault at the surface with respect to the gravity profile. By udng the fomrrd

modeling capsbility il was detennined that an unconstrained position of the fault can lead to

equally acceptable steprtructws models incorporeting a normal, vertical or reverse fauk.

Sinœ the statude of the fault constitutes one of the main objectives oî this study, the position

of the tmœ of the huk reprewnts the mort critical constriaint which ha$ to be imposed on the

modsl. Although the fault ha8 no topognphic expression, its position with respect to the gravity

profile can ôe wcœ8sfully deteminad h m the analysis of the d i g i t i i fderal-provincial

mmmagnaic data. 10 enhance the short-wavelength. near-surface magnetic featum a

coloutdrapad shrckd nlkf image d the tcW magnetic fidd intensity in the w m y area was

producad, the illumination direction and indination king mtected to best illustrate the position

of th8 Saganash Lake Fauît (Fig. 4-3). The ammagnetic expression of the huit ir reveakd as

a northeaa-wuamrSa Iinear region along which the Motrchewan dykm a n LnincaW and

alha. By piOtthg on the aeromrgr#ot image th8 positions of the gravity stations, it a n be

Foi. 4-3. Colourdmped rhodsd mW mrnagneüc map showing t h position of the gravity

stations with respect to the Saganash Lake Fouît. Points A and 6 indicate the location of two

Matacbuvan dyke sampkr for which diffemnt kveh of fddspar douding w m obseweû. The

colOum indicate the intemity of the total field snomaly (rd: low; pu-: high). Illumination

azimuth: 295'. Illumination inclination: 24'.

42

observed that the point of intersection bahmen the giavity profile and the trace of the fault is

situated in the vicinity of station 38, which corresponds to the point of maximum horizontal

gradient of the magnetic anomaly asociatad with the W. The uncertsinty in the position of

tha fault is of I 0-3 km about station 36.

A m n t l y developecl methad which can k used in hie# amas to constrain the

position of the fsuîts acrrwr which diffemntial uplift occurred consists in the study of the

fddspar douding intsmity in Protemzoic mafic dyke swanns (Hdls and Zhang, 1995b; Zhang,

1998). The method ir based on the oôsenration that the ckudiness, which is caused in part by

rubmicro8copic magnetite fomd through exsolution of Fe from t h feldspar structure during

slow coding, increams with the depth of dyke emplacement. Thesfore. sudden variations in

the lave1 of douding obsewd within a dyke swam should wincide with th8 location of major

faub across which difbnntial uplift occurrsd- In the region of the gravity suwey, two

Matachewan dyke sniplds from the fauk orna (points A and B in Fig. 43) nveal a significant

diffemnœ in the ievd d btdspar douding (H.C. Halls, personal communication). The feldspar

fmm the dyke rarnpkd at point A is not clouâed, whereas the dyke sampleâ a1 point 6 is

chamctefized by ciouôeâ hfdspar, which indicotes a dwper level of ernplaœmcmt Based on

this observation, it con be infewed that points A and B are separatecl by the Saganash Lake

Fault, this mult Wng confirmed by Me aeromognetic image. Due to the locations of the hno

sampling points, this method gives a poorer conrrtraint for the position of the fault (unœitainty

of 5.5 km) men compared to the mult obtainad h m the aeromagnetic data. Hmver, the

o h n i d variation in feldspar douding across the southem Saganaah Lake Fault

demonstrates thd the rock wparated by it correspond to different crustol Ievels, and confimis

4.3 2.W fwward moôoîing

The formrd modding approech for the quantitative intefpmbathn of th8 gmvity data

invdvw the direct calcultition of the gnvity o f k t d a n initial modd assumed for the wum

43

body, and th8 ~0mpsri80n of the calculatecl and obsewed values. Then, the model parameten

a n adju8ted and the whoi proces8 i8 npeatd until a ratisfactory level of agreement between

the calculateci and the obsewed anomaly is obtained.

The gravity anomaly obsewed across southern Sagnarh Lake Fault w s interpreted

wing a 2.5D non-lrative modeling somniore produced by Karl Bouchard (Ecde

Polytechnque, Montmal). The program can k uwd b calculate the gravity effects of

horbonâl bodies having an arbitnty polygonal cross-section and finite stike. In order to test

the a#urcy of the moûeling prognm, the theoretical gravity effed of a two-dimrnsional

vertical shoot was compared with the gravity e d calculateâ by the modeling software

(Appendix A). The dimpancy at sny point htwemn the theoretical value of the gravity M e d

and th8t obtoineâ wiai the modeling prognm was lm8 than 0.2% of t h theoretical efïect at

that point.

The geometry considered for the source of the gravity anomaly is that of a horizontal

tnincated plate (section 4.2). The parometen of t h mode1 are the depths to the top and

bottom of the plate, t h density contrast, the fault dip, the wrfaœ position of the fault with

respect to the gravity profile and the strike and lateral (pependicular to the strike) ectant of the

wum body. The values for some of these parameten mm detemineâ from independent

geological and geophysical data. Thus, the w ~ a œ position of the fault was ml1 constrained

b a d on t h aemmagnetic data. The strike kngth of the mockl wao estimateci ftom the

Bouguer gmvity map by conaideflng the kWh d the gravity anomaly amodated with the

Chapkw ffod<, and the lateral extent of the source body wae obtaineâ h m the aemmagnetic

NP by considering the width of the KSZ in t h sunmy ana. It was aho asrumed, ôawâ on

the exi8tenœ d an obwrved surface denaity contisrt, that the top of the anomalou8 body

maches the rufice. The only parameters which haâ to be detmnineâ (iom the modding of

the anomaly m m the dip of th fauît, the depth extent of the anomalous body and t b den8ity

44

The mdeling procedure consistai in fucing the surfaœ position of the fautt to coincide

mth the position of its aemmagnetic expression and determining the values of the fault dip and

cknsity contrsrt which produco the best agreements for various depths of the model. The

pfefemd mockl was that in which a b e t fh was obtained for both the amplitude of the

anomaly and the segment of steep gradient. Thus, the density contrast was varied until the

calculateâ wwe had an amplitude similar to the observed anomaly and the dip of the fault was

changeâ untif at least the point of steepst gradient on the oalculated cunre wincided with the

point of steepest gradient on the observed curve. In a fint stage, this procedure was

prfonneâ for models having depth extents of 5, 10, 15, and 20 km (Fig. 44). For the models

having depths of 5 km and 20 km the ft between the cdculated and obwwed cuwes is poor

(RMS emr > 1 mgal). The best agreements (RMS errorcr of 0.5 mgal) were obtaind for the

maddt with depths of 10 and 15 km. These two models indicated density contrasts of 0.07

and 0.1 g/cm3 and a dip of 60'-70' to the SE. The range of dip values corresponds to the

range of possibk positions of the fault trace in the uncertainty intewal (* 0.3 km about station

36)-

The cornparison of the observed anomaly with the gravity effects obtained for t h four

modelr suggests that the best fit to the observeâ anomaly should correspond to a mode1

having a âepth betvueen 10 and 15 km (Fig. 4-5). The analysis of various models having depth

extents between 10 and 15 km showed that the b s t ft (RMS error of 0.35 mgal) is obtaind

for a mde l chamcterizeâ by a depth of 12 km. a density contrast of 0.085 g/cm3, and a dip of

60'-70' to the SE (Fi. 4-8).

All the m&h indicate that the Saganash Lake Fauk is reverse. For any depth modd,

if the anomaly is modekd with a vertical or normal fauk whose surfaœ position is wmtraineâ

by the aeromagnetic data, a signifiant o f b t Uween the calculated and the obwrved cuve

ir pmducd (Fi. 4-7). In order to mode1 the obsefvd anomaly with a vertical or nomal fauk

the trace of the huit must be positionad soutfmast of staüon 39, whidi on a di-on

peiprndicular to the trace is situatd about 2 km away fmm t h position indicated by the

Fig .44 . Interpmtation of the Bouguer gmvity data. The solid lin8 shows the gravity

wmsponding to a 2.50 &ep model.

a. ûepth extemt of the step mockl: 5 km. Fault dip: 62' SE.

b. ûepth extent of the $tep model: 10 km. Fauît dip: g60 SE.

c. Dopai extant of the step m a l : 15 km. Fault dip: 68' SE.

d. Dipth extent of the step madsl: 20 km. Fault dip: 68' SE.

53

aemmagnetic data. This obecrmation demonstrates that the gravity anornaly associated with

southern Saganash Lake fauit cannot be produω by a normal or veacd fauit model.

4.4 Tho valMHy of thm pro- gmvity m a l

In orâer to k accepted as malistic, the gmvity mode1 ha8 to produœ not only a good fit

betwwn the calculated and observed cuwes, but it also has to be characterized by

parameten which aie compatible with other geological and geophysical obseivations.

The resub of the foiward modeling show that the gravity anomaly can k successfully

modelecl by a step model having a depth extent in the range 10-15 km, a density contrast of

0.074.1 g/cm3 and a fault dipping approximately 65' I 5' to the SE. The depth extent and the

density contra& ranges obtained for this model are consistent with the panmeten d other

geophysical rnodels of t h Chapleau block. The depth of the gravity mode1 corresponds to the

depth of the geophysical modd pmsented by Mand and Ellis (1991) basai on refndion and

gmvity data, as WH as to the depth of the gnvity rnoâel obtained by Atekwana et al. (1994).

The Ionni limit of the density conûast range is dightly higher than the value of the surface

denrity contrsst. The higher values of the average density contma obtained fiom gravity

modeling confimi that the value of the wrfaœ density contraa b a d on denMy

deteminations undemstimatm the average density contrort across southm Saganarh Lake

Fault. The upper limit of the density range (0.1 glcm3) is clow to the density contrast of the

gmvity model prswnteâ by Percival and Card (1985) for northem Chaplaw block (0.12

gr^').

T h major d i m g m e n t behmn the gravity mode1 prewnted in this study and the

previour guophysical rnodels conœmr the attitude of the Saganaah Lake fault. Independent

information on the dipping direction of the fault bounding a truncatetâ plate con b oMained

fiom the horizontal gnvity gradient cunm if the position of the edge of the 8tnidun is knom

(Gnuch and Cordrll, 1Q87). Thur, for a horhonbl rlob tnnmtecl by a vertical margin the

maximum gmûiint would ôe position& abow the dge, whmas in the case of a dipping

54

mafgin the point of maximum horizontel gmdiknt would k displacecl sligMly towards the

dipping direction. f h SE dip of the Saganaah Lake fauk suggested by gravity mdeling (Fi.

4-6) i8 confkmed by the slight displacement towards SE of the maximum horizontal gradient of

gruvity wmi mapect to the surface position d the fauit (Fi. 4-8).

The pmmnt study, which ir based on a detaileâ gravity sunmy, ha8 indicated that an

unœrtainty of more thon 2 km in the fauk position wwld have led to equally acceptable

m u s of a nomal. vertical or nverse faut TheMore. an inaccurate positioning of the fault

and a Iarger spacing of gmvity station8 migM explain the different dipping direction of the

Saginwh Lake Faut obtained in prwious gravity studies.

6. GEOLOGICAL IMPLICATIONS OF THE GRAVITY MODEL

6.1 An 08timata of th. cmponrnb of dlrpliœmnt on $outhem Siginirh Wta Fault

On the buis d the distortions and the offsets obwrved in the aeromagnetic pattern of

the Matachewan dykos, West and Ernst (1991) have inferrd that port-Matachewan dextrd

tmnswmnt defornation ha8 occumâ in the KSZ and surrounding ama. The horizontal sûain

ruffemd by the KSZ (8 tnainly a northeast-southwest tremding band of dextral deformation,

which in the northeat is conœntrated along the boundary fautts of northem and central KSZ

(total horizontal offwt 80-80 km) and in the aouthwst widenr into an - 8 M m wide zone of

dirtiibuted $train. The western rukwam of the Matachmuan dyke swann shows an open Z-

shaped k n d as it cro8ms the Chapleau block. this di8tortion of the dyke swan k ing

attributrd to a rmooth, continuour pattern of shear sûain. AJthough no major fauit offset is

mwakd in southern KSZ, a small discontinuous trsnscurrent deformation occuned along

muthem Sogmash Lake Fault, pouibly accompanied by differential uplR. However, the

lithdogical similarity oôse~eâ acmss the fauit suggests that the total vertical displacement is

minimal in this ama. In Wr section an attempt is maâe to emtimate both the dextral strike slip

and the vertical component of slip on clouthem Saganash Lake Fauit.

The mrmqnetic data show that in the area of the gravity wrvey the Matachewan

dykm are truncated and ofha along Saganash Lake Fault. The amount of the strike ofhet con

k d8Wmimd only if at kast one dyke can be comlated acrou, the fouît. The aerornagrmtic

pattern of the Matadmuan dykes in the study area indicates that a group of six dyk- on th

northwmtem ride of the huit may correspond to a sirnilar gmup of dykes on the routheastem

riôe d th fault (m. 5-1). Th8 poeribie cornlotion betwemn th8 two group of dykes is

suggeatd by the fehtbly 8imilat intetdyûe distancm oôwfvaâ for conmponding pain of

dyfces liom each group. On the pîausibk auumption that the two groupl of dykes mprewnt

th8 same gmup, w h i i was truncatd and dbet along Ssgsnash Lake Fwtt, the obmvd

Fig. 5-1. Cokurdmped shaded relief aemmagndc map shwing a group of six dykes whW

am off&& along the Saganaah Lake Fautt in the a m of the gisvity wwey. The coloun

indicab th8 intenrity of total fdd anomaly (md: low; purple: high).

Illumination azimuth: 295'. Illumination inclination: 260.

59

dextml strike septaration of the dykes along the Saganash Lake Fauk is approxirnately 5 km.

This estimate agms ~ M l y with the strie ofbet indicated for this segment of the feult on

the map of the strain pattern producd by West and Emst (1 QQl ).

The s W b setpsration of the Matachewan dykes is not necesrarily only the mutt of

dextml sûikadp deformation. It is important to notice that if the Saganash Lake Fault is a SE-

dipping revenu, fauk as suggested by the gtavity interpmtation, any post-Matachmn dipslip

movement would cause a dextral strike eeparation of the Matachewan dykes (Fig. 5-2). The

obwrveû strike sepration is likely the sum of hno cornponents: the strike &et produced by

the po8t-Matachewan dip slip and the strike slip component of displacement. Therefore, the

stdke slip component cannot be assessed b a d on the o b w w d strike olha of the

Matachewan dykes, unlem the amount of pst-Matachewan differential uplM aaoss t h fautt

is k n m .

The pot-Matachewan vertical component of slip on southem Saganash Lake Fauk

con be mtimated on the basis of geobarometric and seimic data. The country rock

paleopm8sures indiate a contrast of maximum 0.15 GPa across southem Saganash Lake

Fault (Percival et al., 1994; Percival and W-4, 1994)) which suggeots a total differential up l i

of 4-5 km. On the seisrnic section of the L i i r o b e refiection Iine 4 in Chapleau block (Fig. 1-

l), the pairs of M e d o n A-A' and EB', which presurnably mpresent o W segments of the

same initial rofiectom, indicate a total vertical wmponent of diaplamment of 4 km, in perfact

agreement with the gaoôarometric data. The diffemnœa be-n the country rock

pakopnuure a d the empbœment p m ~ u m of the MatacWuan dykm on both sides of the

hult (Penhral et al., 1994) sugget that the d i i n œ in the pmMatachewan uplM ruffereâ by

western Wawa Gndrr Domain and the Chapkau blodc is botwuen O and 1.5 km. This mal l

differ8nœ wuld be expîaimâ by a 2' pre-Matachewan rotation sufkmcî by the crusta1 slab

uplifted along tha Ivanhoe Wu Faut, its kaâing dge undorgoing a la- amount of upWt lt

is likdy that the Saganash Lake Fault is a poat-Matacbuan feature, oll thu displacements

obwnmâ along this huit king of post4abchann og.. If thir as8umpüon is comd, then the

&pom@: the drike &perdkn pioduœd by the dip Jip and ~ l l l Y A ~ i p oompomt

61

poot-Matachewan vertical slip wmponent on southem Saganash Lake Fault is in fad the total

vertical slip infened from geobarometric and seismic data. A differential upHR of 4-5 km would

have pmduced a dextral stnke separation of the Matachewan dykes of 1.5-3 km (Appendiw C).

Sinœ th total o h w e d rtrike offset of the Matacheuvan dykes along southem Saganarh Lake

Fait is of 5 km, the strike cornponent of slip is approximately 2-35 km.

5.2 The crort=sactional geometry and thr extent of the KSt

m e gravity mockl of the souaiern Saganash Lake Fault shows that at least in

Chapieau block the noraiwrtern boundary fault could not have formed as a collap~e fault in

msponw to upliR along lvanhoe Lake Fault, as it was suggestd in pteviour interpretations. It

is likely that Saganash Lake Fauît was initiated at a late stage during the compressive regime

mponribk for the main Kapuskasing uplii, which occurred at 1.91.85 Ga. The cnwtal

shortening in Chapleau Mo& was accommodateâ in the upper brittle cwst thtough thnist

duplication mainly along lvanhoe Lake Fauit crust, and to a much lesoer extent, akng

Saganash Lake Fault. The development of the Saganash Lake Fauft as a reverse fauit paralkl

to the main thrust contributed to the accommodation of the upper crustal uplift caused by the

thicûening of the lowr cru& as a resuît of shortening, and of the isostatic uplift produc8d aftet

etorrion.

The diiemntial uplnt of about 4-5 km acmcu, southem Saganash Lake Fault ha8

kought in contact rocks h m different crustal levels, cmatiq thus a density contrast which,

hwâ on the gmvity intemation, pmpagated to a depth of 10-15 km. If the fault extends

beyond 10-15 km, then the Iack of a signifiant density contrast below this depth could k

explainrd either by a much smalkr component of vertical displacement at this W h , or by a

maIl density gradient in the middk to loww wst. The depth extent of the den8ity conûast

obtaineâ fiom gmvity mockling gives a minimum estimate for t h extent of the Saganash Lake

Fauît. T b wi8mic rstkcaon data f i the Chapîeau M d suggest that the main Kapuskasing

Ihrust huit ha8 a nmp and fîat gmetry, king situated at a depth of 10-12 km in the wsitem

62

part of the Chapkuu blodc (Geis et 8k8 1990). B a d on this interpmtation il is concluded that

the Saganaah Lake Fadt rnay intemct the main detachment surface and may the

dipping wstal slab of dense rocks uplif'td along the Ivanhue Lake Fauh (Fig. 5-3).

The popup geometry inferrad for both southem and northem KSZ rairas the problem

of the structure of central KSZ The reverse character of southem Saganash Lake Fault and

the smooth continuity of its aemmagnetic expmssion along southem and central KSZ, as wll

as mœnt wmagnetic data modeling (Famkhi, 1999) suggest that the fault may also have a

reverse nature in Groundhog River block. If this i8 the case, then the convergence of the

Saganash Lake Fault and Ivanhoe Lake Fauk in the Groundhog River block a m has ptobably

Id to the faulting of the tip of the uplifted dab of den- rocks and to its wparation fmm the

nrt of t h slab through upli along a reverse Saganash Lake Fault (Fig. 54). T b small

thickness of the perched tip of dense rocks explains the Iack of a gravi& anomaly in

Groundhog Riwr block. A~so, the compssive n!gime which led to the development of the

Saganarh Lake Fault, wuld have producd the uplR of the northwstern Val Rita block akng

a mwrw Lepage Fault (Fi. 5 4 ) . This would explain the large gravity anomaly obmwd in

Val Rita Mock and the gmnulb ocainanws at o W localities along the Lepage Fault. In

onkt to confin this possible mode1 for the central KSZ, a reinterpretation of the gravity data,

b a d on detaikd sampling and accurate positioning of the fault traces, is necessary.

The m8uB of the present gmvity study confinn the cross-sectional similarity between

the KSZ and the compressional bdt wiai central uplift f o m d in Lake Supior through the

inversion of the 1.141 Midcontinent Mt. 8 a d on the oôsecvation that the inwarddipping

boundaty f i ub of muthem KSZ are almo8t continuou8 akng strike with th8 two main

boundiry fauits aIrt ddim the inversion d the Midcantinent rift in Lake Superior, Mamon and

Halls (1007) hava wggested that the Lake Suparior faulb, whom Iast major phase of Pctivity

was during the Grenvilie O w e n (1.1 Ga), may mprewnt macüvotion of much older h u b that

beîonged to an etsnâed Kapuskasing ttnictum (Fi. 56). It is dMiaiit at this stage to aswss

the vaIidity of air hypdhesit, sinœ a di- phpical connaetion ôdwaen the KSZ and the

66

Midcontinent Rift har not been establisheâ betwwn their northwestem boundaries. Alaiough

nlatively deep gneimes with an Archean history dmilar to that of the rocks m i n the

ChapkHl btock occur along the eastern rhon of Lake Superior (Card, 1979; Corfu, 1987),

euidence for a fault-boundeâ Protemzoic uplift L lacking in the mgion btwssn th8 Pineal Lake

bkdt and Lake Suprior. The a h n œ of gravity and magnetic anomalies in this a m and aie

minimal diffemntial upl i inferrd for the southem Chapleau bkck and for the Pineal Lake

Mo& rugget thot the KSZ structure could end in fact at the north-south trsnding McEwan

Lake Fault, which mp~~wnts the western boundary of the Pineal Lake Mo&. At pnsent it is

not knomi if the McEwan Lab Fauit tmncates the KSZ or represents the southerly

continuation of the northem boundary faun of Pineal Lake block, which in tum is preiumd to

b the olha continuation of the Saganash Lake Fault.

6. SUMMARY AND CONCLUSIONS

The purpose of the thidy premnted in this thesis was that of understanding the nature

of the S~ganash Lake Fault in southem KSZ, bawd on the modeling of gravity data. A total of

57 gmvity station8 wre estaMished along a profile which cmsms approximately

perpndicularly the trace of southem Saganaah Lake Fouît. For the determination of the

ehmtions and positions of the gravity stations, a GPS surwy was conducted along the gravity

profik. The GPS measurements have a decimetre level accuracy, enwrin~ an unœrtainty of

lesa than 0.05 mgal in the Bougwr anomaly values. This excellent resuft confims that the

GPS tschnology i8 a mliabk tod for the posiüoning of the gravity stations, which dfa also

the advantoge of k ing much faster than the conventional sunreying. The final unœrtainty in

the Bouguer gmvity values due to the various soums of errors occurring in the proœsrr of

data acquisition and data reduction b les8 than 0.2 mgal.

An incmase of over 35 mgal in the Bouguef anomaly values is observecl from the

northuuast to the wuthw8t of the graviiy profik, Meeting the presenœ of uplRed dense crust

wuthead of Saganash Lake Fauit. In order to mode1 this gmvity anomaly, two constnintr

won impomd on the gravity mockl. The geornetry of the source was assumed to be that of a

fault bou- horizontal slab, b a W on the prwious gravity and seismic i n t e~ re t~on t of the

Chapkw ôiock. and the surfece position of the Saganash Lake Fouît was spacAed precimly,

ô a d on the mmagnetic expression of the fauît. A 2.50 gmvity madeling pmgnm wrw useâ

for th interpmWon of the grwity anomaly o b w w ocrom touthem Saganash Lake Fauk

Thu gmvity madrling msub have indcated th* th S m a s h Lake Fautt is reverse, having a

dip of 600-70" SE and a depth extent of 10-15 km.

Br& on th fsul paratneters obtained fiom gravity modding, the geobamain and

mismit data (Som the ama of $My, and the obmmd &et of the Matadwuvan dykm, it ir

68

mtimated that on southem Ssganash Lake Fauk the vertical component of slip is of 4-5 km,

and the rtrike component of slip is of 2-3.5 km.

The rimikrity between the depth extent of the southem Saganash Lake Fauit obtained

h#n gravtty rnoûeling and the depth of the Kapuskaring thrust in western Chapkau blocû

inhmd from wismic nfkcaon data, indicates that the Sagenarh Lake Fauit fauk migM

intemect and o ib t the main âetachrncmt rurfaœ along which the KSZ uplift occurrsd.

The reverse chander of southem Saganash Lake Fauk supports the mode1 of a pop

up structure for the Chapleau block and indicates that in this region the Saganash Lake Fault

cannot k interpreted as an extensional fwlt.

APPENDIX A

A TEST OF THE MODELING SOFTWARE

70

The theomtical gmvity etW of a two-âimensional vertical ih-t is compared with that

calculatd by the moâeling pmgram. The formula uwd for the calculation of the aieoretical

gnvity o f k t of a two dimen8ionol vertical s h a ici (Nettleton, 1976):

g (x) = 30.7 d t (log n - 0.5 - log [(l + 2@)/(1 + r'ln?)]}

whem

d - density of the vettical shwt in glcm3

0 t - the Viickness of the vertical sheet in km; it has to be substantially leur than the other

dimensions

z - the depth to the top of the vertical shwt

a n = hh, *ers h is the âepth to the bottom of the vertical sheet

a K - the horizontal distance h m the center of the vertical shed

The charadaristics of the vertical sheet are:

d=0.1glan3;t=0.5km;z=2km;n=4

The values of the theoretical effed and t h value8 obtoined with the modeling software are

compred in the following taôle:

lt ir o h w d mat at any point. the diflbmnœ ôdwen th two valm is less than 0.2% of the

APPENDIX 6

DYKE MODELS

72

In orâer to estimate the gnvity efk ts of the Proterozoic dykes crossed by the gravity profile,

thme dyke mode18 wem considemci. Rie density values considecsd for the host rocks an,

bamd on the denrities of the rock sample8 collectai in the ama of the gravity sunmy. The

value u s d for the dyke density comsponds to the density of a dyke sarnple from Racine Lake

ama. The valuus of thickness considered rire based on field observations. The mode1

parametom and the maximum gravity M e oôtaineâ for each dyke mode1 am pwnted

below .

DEL 1 Dvke inside KSZ

Thfckmrs: 20 m

ûepth extent: 10 km

Dyke denaity: 3.1 g/m3

Htmt rad< ôemity: 2.85 glcm'

Maximum grwity e W : 0.5 mgal

EL 2 Ovke in Wawa Gneiss Domain

Thicknees: 30 m

ûepth extmt: 10 km

Dyke dmaity: 3.1 9/cm3

Hoet rad< âendty: 2.7 glarS

Maximum gmity effect: 1.2 mgal

Two pvrlkl dykes having parsmeter8 aimilsr to thme of tha dyke mode1 2,

Dbtam b&mn dyltes: 1 km

W m m gmity Wéct 1.6 mgal

APPENDIX C

THE DETERMINATION OF THE STRIKE SEPARATION OF

THE MATACHEWAN DYKES CAUSED BY DIP SLIP

ON THE SAGANASH LAKE FAULT

74

If Saganash Lake Fault is reverse and d i p to the SE. any dip slip movement on the fault plane

would produce a duRn1 offset of a north-south ûending Matochewan dyke, assumed to be

vsrlicaî (Fi. 52).

The relation behnmn the horizontal comporurnt of t h dip slip (heeve) and the vertical dip

component is:

H =VItand

wtiete

H - horizontal component of the dip slip

V - vertical component of slip

d - dip of ais fault

ïhe dation htwmn the strike wparation of a Matachewan dyke produwd by the dip slip and

the horizontal component of the dip slip ir:

S=H/tani

whem

S - strike separation of the dyke

H - horizontal wmponent of the dip slip

i - angle between the dyke and the fauit measured in a horizontal plane

For

APPENDIX D

DATA TABLES

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