Upload
others
View
4
Download
0
Embed Size (px)
Citation preview
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
The author has gnmted a non- L'auteur a accordé une licence non exclusive licence allowing the exclusive permettant à la National Liôrary of Canada to Bibliothhque nationale du Canada de reproduce, loan, distniute or sell reproduire, prêter, distriiuer ou copies of this thesis in microform, vendre des copies de cette thèse sous paper or electronic formats. la forme de microfiche/fiIm, de
reproduction sur papier ou sur format électronique.
The author retains ownership of the L'auteur conserve la propriété du copyright in tbis thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts fimn it Ni la thèse ni des extraits substantiels may be printed or o t h d s e de ceiieci ne doivent être imprimés reproduced without the authof's ou autrement reproduits sans son permission. autorisation.
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
REFERENCES
Atdmsna, E-A*, Salisbury, M.H., Vehoef, J., and Culshaw, N. 1994. Ramp-Rat geometry
wiaiin the central Kapuskasing upli? Evidence from potentiel field modeling msuits.
Canadien Journal of Earth Sciences, 31 : 1027-1041.
Bates, M.P., and Halls, H.C. 1901. Broad-scale Proterozoic defornation of l)ie central
Supedor Province teveaîeâ by paleomagnetism of the 2.45 Ga Matachewan dyice
swarm. Canaâian Jownal of Earth Sciences, 28: 1780-1796.
ûennett, O., B m , D.D., George, P.T., and Leahy, E. J. 1967. Operation Kapushsing.
Ontario üeptment of Mines, MisœWaneour Paper 10.
Blakely, R. l9QS. Potential theov in gmvity and magnetic applications. Cambridge University
Pmss, 441 p.
Bduid, A.V., and Ellis, R.M. 1WQ. Velodty structure of the Kapuskasing upR, northem
Ontario, from wismic nhd ion studies. Journal of Geophysical Research,
84: 71 8S7204.
Bdnd, AN., and Ellis, R.M. 1901. A geophysical madel for the Kapuskaring uplift from
mismic and gmvity studies. Canadian Journal of Earth Sciences, 28: 342-354.
Burice, K., and Wmy, J.F. 1973. Plume-generated triple junctions: Key indicaton in
applying plate tedonia to old rocks. Journal of Geology, 81 : 408433.
Bunnall, J.T., Leciair, AD., M w r , DE., and Percival, J.A. 1994. Structural correlation
within the Kapukasing uplift. Canadian Journal of Earth Sciences, 31 : 1081-1 095.
Cwd, KD. 1979. Regional geobgical synthesis, central Suprior Province. ln: Cumnt
meorch, part A. Geological Survey of Canaâa. Paper 791A, pp. 87-90.
Cwd, KD. 1990. A mview of the Superior Province of the Canadian shield, a product of
Archean accretion. Precambrian Remarch, 48: 99-156.
Ches , R.M., Cookt FA, Green, AG., Keen, C.E., Luâden, J.N., Pedvsl, J.A.. Quinlan,
G.M., and West, G.F. 1992. Lithoprobe: new perspectives on crustal evolution.
Canadian Journal of Earth Sciences, 29: 1 81 3-1 864.
Cotfu, F. 1987. Inverse age stratification in the Archean cmst of the Supen'or Provinœ:
eviâenœ for infto and sub«wtal am t i on from high molution U-Pb zircon and
monBUjte agas. Pmcambrian Remarch, 36: 259-275.
Ernst, RE., and Halls, H.C. 1983. Structural variation of Proterozoic dykes in the Central
Superior Province - a possible refi8CÜdn of pst-Archean shield defornation.
In: Workrhop on o cross-section of the archean crust. Edited by L.D. khwal and
K.O. Card. Lunar and Planetory Institute, Houston, pp. 4246.
Famkhi, A. 1998. Potential &Id interpmtation in the Kapuakaring Structural Zone.
B.A.Sc. thesis, University of Toronto.
Garland, GD. 1950. Interpretation of gravimetric and mwnetic anomalies on traverses in the
Canadian shield of northem Ontario. Publications of the Dominion Observatory
(O#owa), No 16.
a i s , W.T., Co&, FA., Gmn, AG., Percival, J.A., West, GE, and Milkenit, B. 1990. Thin
thmst rhwt formation of the Kapuskasing structural zone revealed by Lithoprobe
seismic &ledion data. Gwlogy, 18; 51 3-516.
Grauch, V.J.S., and Cordell, L. 1987. Limitations of detemining density or magnetic
boundariea lrom the horizontal gradient of gravity or pseudogravity data.
Geophysics, 52: 1 184 21.
Halb, H.C., and Mound, J. 1998. The McEwon Lake fauît: grave evidence for a iwm
rttuduml riement d tho Kapuskasing zone. Canadian Journal of Earai Sciences,
35: 696-701.
Halb, H.C., and Zhang, B. 1QQSa. Magnetic polarity domains in ~e eaily Protmroic
W-n dyûe mrm, Canada: A novd mahod for mapping major faub.
In: Phpics and Chernirtry of ûykw. €&id by O. Baw and A. Heimann.
AA. Baîkma, Roîterdam, W. 166-1 70.
Halls, H.C. and Zhang, B. 1995b. Tectonic implications of clouded feldcp9r in Protemzoic
mafic dyke 8wam8. Mernoir Gemlogicsl Society af India, 33: 6580.
Halls, H.C. and Zhang, B. 1QW. UplR structure of the southem Kapurkasing Zone from
2.45 Ga dyke s w m displaœment. G~oIo~Y, 26: 67-73.
Huk, H.C., Palmr, HeC., Botes, M-P., and Phinney, W.C. 1984. Constraints on the nature
of the Kapuakasing ritnimiral zone frorn the study of Proterozoic dyke swans.
Canadian Joumal of Earth Sciences, 31 : 1 182-1 196.
Hamikon, W.B. 1908. Archean magmatism and deformation wen, not products of plate
tactonics. Precambrian Research, O1 : 143-1 79.
Ham, J.A., hhibald, D.Ael Qwen, M., and Farrar, E. 1994. Constraints from 'O~r/%r
gwchronology on the tectonothemal evolution of the Kapwkaaing uplin in the
Canadian Suprrior P mince. Csnadian Joumal of Earth Scienœs, 31 : 1 148-1 171.
I n m , M.J.S., Goodacm, A.K., Weôer, J.R., and McConnell, R.K. 1987. StnichiraI
implications of the gmrity field in Hudson bay and vicinity. Canadian Journal of Esrai
Sciences, 4: 977-993.
Jones, B.A. 1973. A gravity survey and interpratation in nocthw88tern Ontario. M.Sc. thmis,
University of Toronto.
Krogh, T.E. 1993. High pntision U-Pb ages for granulite metamofphhm and drfonaüon in
th8 Archean Kapuskaring structural zone, Ontario: implications for structure and
âewbpment of the lowet mst. Eorth and Planetary Suence Letters, 1 1 Q: 14 8.
LaFehr, T.R. 1991. Standardkation in gravity mdudion. Gwphysics, 56: 1 170-1 178.
Manton, ML., and Halls, H.C. 1997. Protemzoic madivation of the routhm Suprior
Province and ib rok in the evolution of the Midconüwnt rift. Canadian Joumal d Earth
Sciences, 34: 582-575.
Mom, D.E. 1994. Thu gdogy anû anictum dm mideusta1 Wowr gneiss domain: a key
to undemtanding Wonic variation with depth and timo in t h late Arcban Abitibi - Wmrs orogen. Canadian Journal of E8M Sciisnœs, 31 : 1-1080.
81
Mowr, DE., Kiogh, TE., Heaman, L.M.. Hanes, J.A., and Helmstaedt, H. 19Q1. Evidenoo for
2920 Ma gneimes and the timing of mid-cnistal extension in the Wawa gneiss domain,
Supeiior Province, Ontario. Geological Association of Canada, Prognm with Abstracts,
16: AM.
Nettieton, L.L. 1 976. Gtavity and magnetics in oil prospcüng. McGraw-Hill, New-York, 484 p.
PeciMI, J.A. 1990. Archean tectonic setting of granulite tertanes of the Superior Provinœ,
Canada: a view h m the bottom. In: GtanulLs and Cmstal Evolution. Edited by
D. Velzeuf and P. Vdal. K l w r Acodemic Publishets, pp. 171-1 93.
Perdval, J.A., and Carci, K.D. 1983. Archean cn#d as mvealed in the Kapuskasing uplift,
Superior Provinœ, Canada. Geology , 1 1 : 323-326.
Perdval, J.A., and Card, K.D. 1985. Structure and evolution of Archean crust in central
Suprior Provinœ, Canada. In: Evolution of Archean wpracrurtal seqwnws. Edited
by L.D. Ayns, P.C. Thumton, KD. Card, and W. Weber. Geological Association of
Canada, SWal Paper 28, pp. 179-1 92.
Percival, J.A., and Marsth, P.H. 1988. Deep crusta1 sûucturo and tectonic history of the
northem Kapuskasing uplift of Ontario: An integrated petroIogical-geophy8icaI study.
Tectonics, 5: 553472.
Parcival, J.A, and Peterman, Z.E. 1994. Rb-Sr biotite and whole-rock data from the
Kapuskasing uplift and their bearing on the cooling and exhumation history. Canadian
Journal of Earth Scisnœs, 31 : 1 1724 181 .
Parcival, J.A., and W-, G.F. 1994. The Kaputkaring uplM: a geobgical and geophyrical
rynthmis. Canaâian Journal of Earth Scknœs, 31: 125&1286.
fercival, J.A., Green, AG., Milkel , B., Cook, FA.. Geit, W., and West, G.F. 1989. Seismic
rdbdon proflb aaow ôeep continental crut exposed in th8 Kapuskasing uplilt
stnicture. Nature, 342: 416-420.
Pefcival, JA., Palmer, H.C.. and Bamett, R.L. 1994. Quantitathe mürnatm of emplacsment
M l of postm~ocphic mdic dykw and rubwqwnt m i o n magnitude in the
routhem Kapuskasing uplift. Canadian Journal of Earth Sciences, 31: 1218.1 226.
Rilkr, U., Schmrdtner, W.M., Halls. H.C.. and Card, ICD. 1999. Transpressive tectonisrn in
the eastem Penokean omgen, Canada. Consequenwo for Proterozoic wstal
kinematics and continental fngmentation. Pmcamkian Rewarch, 93: 51-70.
Simpson, W.R., and Jacfiens, R.C. 19ûQ. Gravity methods in regional studies. In: Geophysicol
fmmrwoik of the continental United States. Editd by L.C. Pakiwr and W.D. Mooney.
Gwlogical Sodety of America Memoir 172. Boulder, Colorado, pp. 35-44.
Watson, J. IWO. The origin and history of the Kapuskasing structural zone, Ontario, Canada.
Canadian Journal of Earth Scienœs, 17; 8-75.
West, G.F., and Ernst, RE. 1W1. Eviâenca from aeromagnetics on the configuration of
Mutachewan dykrr and the tdonic evolution of the Kapuskasing Strucutunl Zone,
Ontario, Canada. Canadian Journal of Earth Sciences, 28: 1797-181 1.
Williams, H.R. 1990. Subprovince acuetion tectonics in the southcentral Suprior Province.
Canadian Journal of Earth Sciences, 27: 570-581.
William, H.R-, Stott, O.M.. Thunton, P.C., Sutdilfa, R.H.. knnett, G., Easton, R.M., and
Armstrong, O.K. 1992. Tectonic evolution of Ontario: summary and qnthesis.
In: Geology of Ontario. Edited by P.C. Thumton, H.R. Williams, R.H. Sutclif%, and
G.M. Stott. Ontario Gwlagical Suwey, S ~ * a l Volume 4, Part 2, pp. l2SW 332.
Wlkon, J.T. 1Qû8. Cornparison of the Hudson Bay arc with some 0 t h féatwes. In: Science,
history and Hudson Boy. Edited by C.S. mals and DA. Shenrtone. ûepartmnt of
Emrgy, Mines and Resources, ûttawa, pp. 1015-1033.
Zhang, B. 1BW. A study of austal uplM along the Kapuskasing zone using 2.45 Ga
M m n âyke8. Ph-D. thesis, University of Toronto.