THESE TERMS GOVERN YOUR USE OF THIS DOCUMENT
Your use of this Ontario Geological Survey document (the “Content”) is governed by the
terms set out on this page (“Terms of Use”). By downloading this Content, you (the
“User”) have accepted, and have agreed to be bound by, the Terms of Use.
Content: This Content is offered by the Province of Ontario’s Ministry of Northern Development and
Mines (MNDM) as a public service, on an “as-is” basis. Recommendations and statements of opinion
expressed in the Content are those of the author or authors and are not to be construed as statement of
government policy. You are solely responsible for your use of the Content. You should not rely on the
Content for legal advice nor as authoritative in your particular circumstances. Users should verify the
accuracy and applicability of any Content before acting on it. MNDM does not guarantee, or make any
warranty express or implied, that the Content is current, accurate, complete or reliable. MNDM is not
responsible for any damage however caused, which results, directly or indirectly, from your use of the
Content. MNDM assumes no legal liability or responsibility for the Content whatsoever.
Links to Other Web Sites: This Content may contain links, to Web sites that are not operated by MNDM.
Linked Web sites may not be available in French. MNDM neither endorses nor assumes any
responsibility for the safety, accuracy or availability of linked Web sites or the information contained on
them. The linked Web sites, their operation and content are the responsibility of the person or entity for
which they were created or maintained (the “Owner”). Both your use of a linked Web site, and your right
to use or reproduce information or materials from a linked Web site, are subject to the terms of use
governing that particular Web site. Any comments or inquiries regarding a linked Web site must be
directed to its Owner.
Copyright: Canadian and international intellectual property laws protect the Content. Unless otherwise
indicated, copyright is held by the Queen’s Printer for Ontario.
It is recommended that reference to the Content be made in the following form: <Author’s last name>,
<Initials> <year of publication>. <Content title>; Ontario Geological Survey, <Content publication series
and number>, <total number of pages>p.
Use and Reproduction of Content: The Content may be used and reproduced only in accordance with
applicable intellectual property laws. Non-commercial use of unsubstantial excerpts of the Content is
permitted provided that appropriate credit is given and Crown copyright is acknowledged. Any substantial
reproduction of the Content or any commercial use of all or part of the Content is prohibited without the
prior written permission of MNDM. Substantial reproduction includes the reproduction of any illustration or
figure, such as, but not limited to graphs, charts and maps. Commercial use includes commercial
distribution of the Content, the reproduction of multiple copies of the Content for any purpose whether or
not commercial, use of the Content in commercial publications, and the creation of value-added products
using the Content.
Contact:
FOR FURTHER
INFORMATION ON
PLEASE CONTACT:
The Reproduction of
Content
MNDM Publication
Services
The Purchase of
MNDM Publications
MNDM Publication
Sales
Crown Copyright
Queen’s Printer
BY TELEPHONE:
Local: (705) 670-5691
Toll Free: 1-888-415-9845, ext.
5691 (inside Canada,
United States)
Local: (705) 670-5691
Toll Free: 1-888-415-9845, ext.
5691 (inside Canada,
United States)
Local: (416) 326-2678
Toll Free: 1-800-668-9938
(inside Canada,
United States)
BY E-MAIL:
[email protected]
[email protected]
[email protected]
LES CONDITIONS CI-DESSOUS RÉGISSENT L'UTILISATION DU PRÉSENT DOCUMENT.
Votre utilisation de ce document de la Commission géologique de l'Ontario (le « contenu »)
est régie par les conditions décrites sur cette page (« conditions d'utilisation »). En
téléchargeant ce contenu, vous (l'« utilisateur ») signifiez que vous avez accepté d'être lié
par les présentes conditions d'utilisation.
Contenu : Ce contenu est offert en l'état comme service public par le ministère du Développement du Nord
et des Mines (MDNM) de la province de l'Ontario. Les recommandations et les opinions exprimées dans le
contenu sont celles de l'auteur ou des auteurs et ne doivent pas être interprétées comme des énoncés
officiels de politique gouvernementale. Vous êtes entièrement responsable de l'utilisation que vous en faites.
Le contenu ne constitue pas une source fiable de conseils juridiques et ne peut en aucun cas faire autorité
dans votre situation particulière. Les utilisateurs sont tenus de vérifier l'exactitude et l'applicabilité de tout
contenu avant de l'utiliser. Le MDNM n'offre aucune garantie expresse ou implicite relativement à la mise à
jour, à l'exactitude, à l'intégralité ou à la fiabilité du contenu. Le MDNM ne peut être tenu responsable de tout
dommage, quelle qu'en soit la cause, résultant directement ou indirectement de l'utilisation du contenu. Le
MDNM n'assume aucune responsabilité légale de quelque nature que ce soit en ce qui a trait au contenu.
Liens vers d'autres sites Web : Ce contenu peut comporter des liens vers des sites Web qui ne sont pas
exploités par le MDNM. Certains de ces sites pourraient ne pas être offerts en français. Le MDNM se
dégage de toute responsabilité quant à la sûreté, à l'exactitude ou à la disponibilité des sites Web ainsi reliés
ou à l'information qu'ils contiennent. La responsabilité des sites Web ainsi reliés, de leur exploitation et de
leur contenu incombe à la personne ou à l'entité pour lesquelles ils ont été créés ou sont entretenus (le
« propriétaire »). Votre utilisation de ces sites Web ainsi que votre droit d'utiliser ou de reproduire leur
contenu sont assujettis aux conditions d'utilisation propres à chacun de ces sites. Tout commentaire ou toute
question concernant l'un de ces sites doivent être adressés au propriétaire du site.
Droits d'auteur : Le contenu est protégé par les lois canadiennes et internationales sur la propriété
intellectuelle. Sauf indication contraire, les droits d'auteurs appartiennent à l'Imprimeur de la Reine pour
l'Ontario.
Nous recommandons de faire paraître ainsi toute référence au contenu : nom de famille de l'auteur, initiales,
année de publication, titre du document, Commission géologique de l'Ontario, série et numéro de
publication, nombre de pages.
Utilisation et reproduction du contenu : Le contenu ne peut être utilisé et reproduit qu'en conformité avec
les lois sur la propriété intellectuelle applicables. L'utilisation de courts extraits du contenu à des fins non
commerciales est autorisé, à condition de faire une mention de source appropriée reconnaissant les droits
d'auteurs de la Couronne. Toute reproduction importante du contenu ou toute utilisation, en tout ou en partie,
du contenu à des fins commerciales est interdite sans l'autorisation écrite préalable du MDNM. Une
reproduction jugée importante comprend la reproduction de toute illustration ou figure comme les
graphiques, les diagrammes, les cartes, etc. L'utilisation commerciale comprend la distribution du contenu à
des fins commerciales, la reproduction de copies multiples du contenu à des fins commerciales ou non,
l'utilisation du contenu dans des publications commerciales et la création de produits à valeur ajoutée à l'aide
du contenu.
Renseignements :
POUR PLUS DE
RENSEIGNEMENTS SUR
VEUILLEZ VOUS
ADRESSER À :
la reproduction du
contenu
Services de
publication du MDNM
l'achat des
publications du MDNM
Vente de publications
du MDNM
les droits d'auteurs de
la Couronne
Imprimeur de la
Reine
PAR TÉLÉPHONE :
Local : (705) 670-5691
Numéro sans frais : 1 888 415-9845,
poste 5691 (au Canada et aux
États-Unis)
Local : (705) 670-5691
Numéro sans frais : 1 888 415-9845,
poste 5691 (au Canada et aux
États-Unis)
Local : 416 326-2678
Numéro sans frais : 1 800 668-9938
(au Canada et aux
États-Unis)
PAR COURRIEL :
[email protected]
[email protected]
[email protected]
A STRUCTURAL ANALYSIS
OF THE
CENTRAL PART OF THE SHEBANDOWAN METAVOLCANIC - METASEDIMENTARY BELT
Final Report to the Ontario Geoscience Research Grant Program
G.M. Stott
W.M. Schwerdtner
Department of Geology
University of Toronto
TABLE
O F
CONTENTS
Abstract
Introduction
Methods
Results
1
2
3
A.
D^ Phase
3
3
5
D2 Phase
8
Strain Domains
9
D- Phase
10
Plutons
Contact Strain Aureoles
10
11
Field Analysis
General Overview
Tectonic. Sequence
B.
Magnetic Susceptibility Anisotropy Analysis
11
C.
Strain Domain Boundaries
Discussion
Coherent and Incoherent
Boundaries
13
13
D.
13
The Greenwater Pluton and Other Late Felsic Intrusions 16
Introduction
16
Late to Posttectonic Plutons
IQ
The Greenwater Pluton
19
Discussion
21
Summary of Conclusions
23
References
27
ABSTRACT
The central part of the Shebandowan Greenstone belt is found to comprise
mappable domains of tectonic strain.
These domains are a consequence of two
successive phases of regional deformation with the second phase, D2 , superimposed
upon the first only within megascopically - discrete zones.
D, domains possess
strain fabrics characterized by westerly-plunging stretching directions and
a somewhat less oblateness of strain than the easterly-plunging D- domains.
The boundaries between these contrasting domains of strain appear to be generally
coherent (not dislocated) but some faulting along these boundaries may have
occurred.
Two models are proposed that apply to different settings.
The concomitant
development of strain domains appears to be typified by the Larder Lake "break"
(Downes 1980).
The sequential development of strain domains is represented
in the Shebandowan belt.
The variations in the strain pattern across the belt
have been confirmed using magnetic susceptibility anisotropy.
The results from
this method also suggest that an increase in the oblateness of strain in D, strained rocks occurs close to the strain domaxn boundaries; this probably
serves to lower the contrast in strain across the boundaries and avoid sharp
dislocations in most areas.
Dislocations along domain boundaries may have concentrated precious metals
where such boundaries juxtapose lithostratigraphic sources of these metals.
is suggested that discontinuities between tectonic strain domains should be
considered when investigating exploration targets.
A detailed study of the crescent-shaped Greenwater Pluton is outlined.
The intrusion, by comparison with idealized strain models, appears to possess
a primary strain fabric consistent with an antiformal sheet emplacement.
i l
It
jntreduction
So little is known of the pattern of cumulative tectonic strain within
greenstone belts or whether such strain patterns can be correlated with those
in adjacent subprovinces or gneissic terrains.
This study documents the tectonic sequence of events and the pattern of
strain within the central part of the Shebandowan greenstone belt.
The pattern
of strain domains is confirmed by magnetic susceptibility anisotropy.
A
detailed study of a crescent-shaped pluton is also outlined to demonstrate the
close correspondence between an idealized emplacement model and the strain pattern
within an actual pluton.
The presence of strain domain boundaries and their bearing upon gold
exploration is discussed.
-lethods
The magnetic susceptibility anisotropy (MSA) method has been used to
1) verify the general orientation of mineral lineations observed in the
field and 2) estimate the range of variation of the tectonic fabric between
the regional strain domains shown in Figure l and Maps A and B.
The method
is also being used at present to test the penetrativeness of mineral
lineations in handspecimen.
For example, oppositely plunging lineations
locally prevail on separate foliation planes within handspecimens in areas
particularly
close to strain domain boundaries.
The non-penetrativeness
of lineations would imply a locally non-homogeneous superimposition of a
later deformation upon a pre-existing tectonic fabric.
Such observations
have an important bearing on the nature of strain domain boundaries.
The
mafic to intermediate volcanic rocks of the Shebandowan belt were found to
possess a very low magnetic susceptibility.
As a consequence, the increased
time factor required on a spinner magnetometer for acceptable precision was
prohibitive for suitable time-sharing of the equipment.
Instead, a more
readily available low-field torque meter (King and Rees 1962) was used.
This
method is much slower but may be somewhat more precise than the spinner
magnetometer (Ellwood 1978).
Analysis in this study has been limited to the
prolateness factor and orientation of the principal directions of the MSA
ellipsoid (Stone 1963).
Successful application elsewhere of MSA as an
estimate of strain (e.g. Rathore, 1979) encourages us to use the approach
to estimate strain variations in the belt.
This is especially valuable in
volcanic terrains where suitable primary strain gauges are sparsely preserved.
The field examination has concentrated on measuring the orientations of
mineral lineations and estimating the relative strength of mineral liireations
and foliations following a procedure outlined in Schwerdtner et al (1977) .
Results
A.
Field Analysis
General Overview
The regional fabric in the central part of the Shebandowan belt is
characterized by a steeply dipping mineral foliation consistently parallel
to bedding.
This is also observed in other relatively narrow linear greenstone belts
and in the continuation of the Shebandowan belt into Minnesota (D. Davidson,
pers.comm.).
Mesoscopic (outcrop-scale) fold hinges are not widely observed
except in the area just south of Lower Shebandowan Lake and in a thick zone
of mafic tuff west of Squeers Lake, approximately 8kn southwest of Greenwater
Lake.
A major westerly-plunging fold hinge lies in the vicinity of Horseshoe
Lake just east of Greenwater Lake.
Folds are generally tight to isoclinal with some exceptions immediately
south of Lower Shebandowan Lake where the intensity of deformation is so weak
as to preserve angular hyaloclastite breccia fragments for example and open
folds in hematized volcanic breccia and lahar units.
All observed fold axes
closely parallel the orientation of the local mineral lineation.
Interpreted megascopic fold axes are shown in Figure 1.
The structural
evidence does not support the contention of Beakhouse (1974) that the
greenstone belt is a large homocline facing consistently to the north.
His suggestion is probably induced in part by the observation that the
stratigraphic north-south section east of Greenwater Lake shows a general
asymmetry with tholeiites and komatiites in the southern half of the belt and
a more differentiated suite of andesites, dacites and rhyolites to the north.
However, it appears that this asymmetry is a primary feature of this and other
belts (Trowell etal 1980). The pillow lavas along the north edge of the
Shebandowan belt north of Burchell Lake stratigraphically underlie the
turbidite sediments of the Quetico subprovince (Giblin 1964 and Bau 1975).
Structural interlayering of mafic volcanics with the consistently N.W.-facing
Quetico sediments close to the belt margin appears to reflect contemporaneity of
the units.
The evidence is less clear in the area of Kashabowie Lake and
eastwards where the Postans Fault defines the northern belt margin.
North
of Middle Shebandowan Lake the younging directions in the mafic tuffs south
of the fault and Quetico sediments north of the fault suggest the Postans
Fault locally represents the "broken" hinge line of a syncline along the
belt margin.
~~
South of Lower Shebandowan Lake Morton (1979) has conducted detailed
mapping and logging of core on the Shebandowan Mine property.
The petrologic
results plus chromite analysis in ultramafic sills and flows support the
structural interpretations in the present study: the Shebandowan mine
section is on the north limb of an anticline that lies about 300 metres
south of the mine shaft.
The syncline to the north was partly truncated by
the intrusion of the Shebandowan pluton.
The megascopic fold axes east of Greenwater Lake (Figure 1) are based on
few pillow top directions and their locations are tentative.
It should be noted
that the fold locations are consistent with a stratigraphic feature observed by
Morton (ibid) and in the present study.
some coarse
Ultramafic flows and sills (?) and
grained mafic flows typically have magnetite - chert iron
formation deposited on or close to their upper surfaces as determined by
independent younging criteria elsewhere.
This feature may be fortuitous.
Mineral lineations are observed in this study throughout the region.
Indeed
with sufficient core they may be observed in virtually every outcrop in
the region including the very weakly deformed rocks south of Lower Shebandowan
Lake which Morin (1973) has described as "massive, with no megascopic
penetrative (tectonic) fabric" p!6.
The ubiquitous presence
of visible
mineral lineations encourages us to suggest that such structural elements
can and should be more widely measured and recorded by field geologists
and used effectively for structural interpretation.
The sparse presence of bedding/cleavage relations, mesoscopic folds, polyphase
folding, younging direction indicators, boudins, and suitable primary strain
gauges in mafic volcanic rocks has forced us to concentrate on lineation
orientations and variations in the nature of the tectonic strain fabric.
The following is based mainly on the results of these measurements.
Tectonic Sequence
Three generations of regional structures are distinguished within the
Shebandowan belt.
These structures are attributed to successive phases
of regional deformation.
In addition, there is a generation of structures
related to contact strain aureoles around individual late to posttectonic
plutons.
The first two deformation phases constitute the dominant tectonic
stages in the Shebandowan belt.
The sequence of structures is shown in
Table l and the interpreted sequence of tectonic events is summarized in
Table 2.
Stereograms of D-, and D~ - domain structural elements are shown in
Figures 2 and 3.
D. Phase
The first recognizable deformation phase, D., is characterized by consistent
west - to southwest-plunging
Table l
MESOSCOPIC STRUCTURAL ELEMENTS AND THEIR CHARACTERISTICS
Deformation
Phase
Associated
Structural
Elements
D
l
D
z
Dj
Fl
F2
F3
s,
So
S,
Fold Profile
tight to isoclinal
Fold
Orientation
plunging upright
folds with near
verticle axial
surfaces and
moderate to steep
rake of fold axes;
axes plunge W. to
S.W. -
Fold
Distribution
Surface
Folded
tight to isoclinal to
open in sediments and
"Timiskaming" volcanics
plunging upright
folds with near
vertical axial
surfaces and
gentle to steep
rake of fold axes;
axes plunge E. to
N.E.
plunging upright
folds with near
vertical axial
surfaces and
generally steep
rake of fold axes;
axes plunge variably
from W., N. to E.
Characterizes D,
Characterizes D2
widely observed in
strain domains but
sparsely observed;
best seen in mafic
tuffs S.W. of Geenwater Pluton.
Probably occur in D
domains, but not
observed in outcrop
domains but
sparsely observed
except in sediments,
tuffs and "Timiskaming"
volcanics.
north half of belt
and in Quetico
metasediments;
most prominent in
thinly bedded units.
SQ bedding
S
S bedding and
S, S foliations
S.. almost
Axial
Surface Foliation always parallel
bedding
S2 parallel to
S. weak and
S .j and bedding;
confined to narrow
hinge of F3
bedding/cleavage
to bedding; bedding/
cleavage intersections intersection locally
rarely observed.
observed in sediments,
tuffs.
Axial
Lineations
kink folds;
dominantly Z
folds
L- mineral lineations
"L
mineral lineations
parallel to fold
axes and plunging
W. to S.W.
parallel to fold
axes and plunging
E. to N.E.
folds; commonly
a fracture cleavage.
L^ only locally
observed, confined
to F^ fold hinges as
Granulations and
mineral alignment.
Table 2
Tectonic Sequence of Events
STRUCTURAL
INTRUSIVE
D,, kink folding
CRITERIA
D^ superimposed on
/^concentrated in N.
half of belt and in
Quetico sediments;
related to late E.-W.
compression parallel
to foliation
the late trachytic
dykes
Late felsic plutons and
dykes
D~ heterogeneously concentrated
D2 predominent
in zones (domains) across
the belt; predominent in
less competent sediments and
areas of lithologically complex
stratigraphy, typical of N.
half of belt. D2 is superimposed
in less competent
units that are
expected to be
affected by last
major deformation.
upon D. and strain elements are
composite of D., plus D2 deformations.
D, westerly-plunging
folding of supracrust across
Shebandowan belt and
southern Quetico sediments.
Shebandowan pluton
possesses steep W.-plunging
primary lineation in N.
half of pluton but
D2 - deformed in south
half; no contact strain
aureole.
D. folds and lineations
locally preserved in
Quetico sediments
(E. Sawyer, personal
communication);
Inliers of D, in D2
domain of belt.
mineral lineations end mesoscopic fold axes.
The lineations are typically
moderately - well developed, at least within the major D, domain in the
southern half of the belt and are more pronounced than lineations observed
in Dj domains.
the bedding.
The tectonic foliation, S-,, is almost invariably parallel to
Megascopic F., folds are defined from lithologic patterns
and facing directions notably east and north of Greenwater Lake.
D, synclines
and anticlines possess plunging fold axes with upright to steeply northward dipping axial planes.
Just east of Greenwater Lake they are deflected around
a crescent-shaped pluton and tonalitic gneiss dome.
L^ mineral lineations are typically prominent and plunge generally 50
to 65 degrees westward.
This degree of plunge is consistent throughout the D. strain
domains (Figure l and Map A) except close to the late to posttectonic plutons
where they steepen within the contact strain aureoles, and except close to the
boundary of D~ strain domains (Figure 1) where the lineations in places
become either shallower along some boundaries or steeper.
This is discussed later.
D0 Phase
The second deformation phase, D2 is characterized by weakly developed
east-plunging mineral lineations.
The tectonic foliation is generally more
prominently developed and mineral lineation more weakly developed than in
areas affected exclusively by D, .
The plunge of mineral lineations within
the northern half of the belt changes from relatively shallow, 10 to 30 degrees,
in the Burchell Lake - Kashabowie Lake area to 55 to 65 degrees further east
in the area north of Middle Shebandowan Lake.
The plunge of lineations within
the Quetico metasediments is consistently shallow.
Within the core of the belt,
1,2 lineations range from 35 to 70 degrees in plunge with the steeper lineations
typically found close to strain domain boundaries.
Strain Domains
The greenstone hele has been subdivided into domains of tectonic
strain.
These are separately characterized by either predominance of D, ~
induced structures (D., domains) with westerly-plunging lineations and folds
or by the predominance of easterly-plunging D2 -induced structures (D ? domains)
These domains are separated by rather sharp boundaries or discontinuities
(Maps A, B) across which the mineral lineation changes orientation.
This
change may occur sharply or at least within 60 metres (200 feet) as seen along
the ramp section of the Shebandowan Mine (Figure 8).
Steep mineral lineations
are widely observed in the vicinity of such boundaries.
Or the lineation
orientation may change progressively through shallow plunges as seen along
the south shore of Upper Shebandowan Lake, northwest of Greenwater Lake
(Figure l, Map A).
The discontinuities tend to follow the stratigraphy
closely but have been observed to obliquely cross-cut mappable units.
For
example, Figure 4 shows a volcanic breccia unit, south of lower Shebandowan
Lake, transected by the strain boundary.
Faulting has not been observed to follow these strain discontinuities.
Instead, a relative increase in oblateness of strain is observed within the
larger D, domain in the south close to the adjacent discontinuities.
Narrow
domains occur within the northern part of the belt especially around Upper
Shebandowan Lake ( see Maps A and B).
Both D- and D2 domains are generally
characterized there by oblateness of strain with relatively weak mineral
lineations.
(See also-results of MSA analyses).
These naxrower D^ domains
appear to be tectonic "inliers" within the major D2 strain domain.
Although the bulk of D2 deformation is confined to clearly mappable D ?
strain domains, narrow, probably discontinuous D. zones typically less than
\o
300m wide, occur within the najor D., domain close to the mutual boundary.
These are most prominent between Greenwater and Upper Shebandowan Lakes.
Within such narrow zones, both L, and L9 lineations are observed within
handspecimens.
The regional strain pattern is not restricted to the Shebandowan belt
but extends across the Quetico subprovince(an easterly-plunging D2 domain)
to the north and there is evidence that at least the margin of the tonaliticgranodioritic gneiss terrain to the south is also affected.
However, posttectonic
plutons have intruded the belt's southern margin and distorted the original
strain field so that correlation of the D2 strain donain into the gneiss terrain
is limited.
D- Phase
A third phase of deformation, D~, appears confined to the northern half
of the Shebandowan belt and at least the southern
subprovince.
part of the Quetico
It is identified by steeply, almost vertically-plunging kink
folds within well foliated supracrustal units, especially pryroclastic deposits
and sediments.
This phase is locally imposed upon, and thus postdates, the
otherwise posttectonic syenitic-trachytic dykes that transect the stratigraphy.
The kink folding appears to have been generated by a late east-west compression
and post-dates all other deformation and intrusive events.
Plutons
Discussion of the plutons
is presented later.
These bodies are late to
posttectonic and each possesses primary stretching lineations which appear
related to the pluton's internal strain during its emplacement.
Pluton is given as an example in detail.
The Greenwater
\\
Contact Strain Aureoles
Subsequent Co the D^ deformation phase, the rise c;f late to posttectonic
plutons deflected the stratigraphy and imposed contact strain aureoles upon
the supracrustal rocks typically less than 600m in width.
The most prominent
contact strain envelopes enclose the Greenwater and Kekekuab Plutons.
The regional pattern of stretching lineations in the belt is deviated in
proximity to these late intrusions.
The width of contact strain around a
pluton is taken to begin where the regional lineation trend begins to deviate
and plunges steeply toward the pluton.
Such changes in lineation orientation
are revealed by changes in lineation pitch relative to the strike of the
foliation.
No separate lineation is superimposed upon the regional fabric
indicating that, as expected, sufficiently high pressure-temperature conditions
prevailed to cause a simple re-orientation of the pre-existing stretching
direction.
Outside the contact strain
aureoles, the plutons (e.g. Greenwater and
Peewatai) also mechanically rotated the stratigraphy and its regional stretching
lineations.
The width of such effects appears to show a direct correspondence
with the horizontal width of the pluton involved.
B. Magnetic Susceptibility Anisotropy Analysis
Oriented cores one inch in diameter and 0.85 inch long were cut from one
hundred and thirty-one samples across the belt.
Fifteen axial measurements
were obtained for each of generally three cores per specimen
with the low field
torque meter and from these were computed the principal axial directions of
susceptibility and a prolateness factor - P.
P is used as an estimate of prolateness,one aspect of the shape of the
susceptibility ellipsoid where P - (^ - k2 ) l (k ? - * 3 ) and k^ k^ > k^
are the principal semi-axes of the ellipsoid.
The P value as a prolateness
factor is strictly valid only for quasi- isotropic rocks, <:>'^ anisotropy,
or highly lineated rocks (Schwerdtner 1976), but it nevertheless serves
as a rapid measure to quantify regional tectonic fabric changes.
The results of the MSA analysis are displayed on Map B.
The maximum
directions of susceptibility are consistent with the mineral extension directions
measured in the field within the corresponding strain domains.
The exceptions
are to be found generally close to the boundaries between the strain domains
particularly within the D^ domains where it is believed relict D., fabrics
may be locally preserved but easily missed in the field.
The orientation of the
MSA for each strain domain verifies the existence of these domains and the field
observations recorded on Map A.
The P value for 87 stations is shown in Figure 6 corresponding to azimuth
direction of plunge of the major axis of the magnetic susceptibility ellipsoid.
There is a general increase in P from susceptibility ellipsoids plunging
eastward in the D2 domains, to those plunging westward, characteristic of D
domains.
This is more clearly evident if we disregard the D,-oriented samples close
to the strain domain boundaries which possess low P values and typically steep
plunges in their extension directions.
These are interpreted to have acquired
increased flattening strain as a consequence of the D2 deformation.
Figure 5
illustrates the orientations of magnetic lineations for all supracustal samples
measured.
Orientations within D, and D2 domains are comparable with those of
mineral lineations shown in Figure 2.
We m.iy consider the D., domain to be characterized by a wider variation
in the prol:iteness of strain th-an is evident: in D, dor-ains.
The higher P values
correspond to the field observation of relatively more pronounced mineral
lineations in the major D, domain.
Note that unusually high P values in D2 domains are from samples of chilled
pillow margins wherein a primary component of the magnetic lineation has
probably been preserved.
C.
Strain Domain Boundaries
Discussion
To our knowledge, this study constitutes the first discussion of strain
domains and their boundaries on a regional scale in a greenstone belt.
Discontinuous interfaces between contrasting strain domains have been observed
locally in other areas:
in Saskatchewan (Schwerdtner 1970) and in the Larder
Lake area in Ontario (Downes 1980).
Both W.M. Schwerdtner and M.J. Downes have
suggestad for their respective areas that rotational faulting may account for
the discordance in the plunge values of the fold axes and mineral lineations
between the adjacent domains.
The difficulty with this is that the amount of
rotation required to bring the two orientations in line can be quite considerable;
the lack of major lithologic discontinuities across these boundaries in many
areas suggests that two other models to explain these features should be
considered.
Before we discuss these models, some basic implications of these
strain boundaries are considered.
Coherent and Incoherent Boundaries
No faulting or shearing has been observed along the strain domain boundaries
in the Shebandowan belt.
boundaries are said to be
Ideally, if slip has not occurred, then domain
coherent
(Means, 1976) since the strain in all
directions within the plane of the boundary is the same on both immediate sides
14
of the boundary.
The failure of two strain domains to produce a mutually
composite strain within the ideal boundary results in slipping along the
boundary to relieve the contrast in strain.
as being incoherent.
Such a boundary is referred to
Thus, when two contrasting strain domains develop adjacent
to each other, they have a choice to resolve their differences: either come to
a mutual "agreement" at the interface or create a space-problem that can only
be resolved by slipping.
These conditions are illustrated in Figure 7.
It appears that in the Shebandowan belt, nature has attempted to retain
coherency at the boundaries.
The evidence from figures 6 and 8 suggest that
the effects of superimposition of D2 upon D, go beyond the boundaries as shown
on Maps A and B.
The boundaries are defined where the visible lineation reverses
plunge direction; but, the "effective boundary" appears to be a narrow zone
of undetermined width wherein the oblateness of cumulative strain in D., domains
is increased in proximity to the "new" D2 domain.
is not severe and slippage is avoided.
Thus the strain adjustment
We could conceptualize the domain
boundaries as "strain fronts" at the leading edge of a conversion of D, strained
rocks to a new cumulative strain induced by D^.
In essence, the D2 domains
possess a cumulative strain (D.. 4- 02).
We cannot be certain that all such boundary surfaces have avoided slippage
entirely in this greenstone belt since it is conceivable that a spectrum of
adjustments may have developed.
Peewatai Pluton follows faults).
(Note also that a boundary west and north of
The implication here is that boundaries on the
surface may be cryptic traces of local fracture surfaces or shear zones at depth.
Thus, dislocations may have developed without any evidence of such at the surface,
The distribution of fracture zones has an important bearing on the
hydrothermal concentration of precious metals and the tendency for such
15
favourable zones to be developed ac planar discontinuities of strain should be
considered for future investigation.
An example of a boundary that has been dislocated and within which gold
has been concentrated is outlined by Dovmes (1980).
Along the Larder Lake
"break" Downes has located a major discontinuity between strain domains that
juxtaposes lithostratigraphic hosts for gold.
Nevertheless, there are some basic differences between the Larder Lake
area and the Shebandowan belt.
Downes notes that his "structural discordance
B" is characterized by simple megascopic folds of the sar:e apparent generation
having contrasting plunge directions on opposite sides of the discordance.
In the Shebandowan belt, different generations of folds are involved with
a complex of strain domains, some merely 100 metres wide, that reflect a
localized superimposition of one generation upon the older.
These settings suggest two models beyond a simple rotational fault
model.
1)
Theconcomitant development of two strain domains with a single major
generation of folds as in the Larder Lake setting.
There is no local interaction of one strain domain upon the other;
the boundary between the two is sharp.
The difference in the stretching
direction on each side of the fault may be related to "mega-release" directions
on a large scale, possibly governed by the position of batholiths.
The movement
on the fault itself would be the resultant of the strains on each side of the
fault.
In the Larder Lake case, this would presumably be dip-slip although
this may not be evident in the field if subsequent strike-slip has occurred.
2)
The sequential development of strain domains with a sequence of folding
exemplified by the Shebandowan belt.
Fault discontinuities mayor may not be present.
However, it is likely
that during the progressive localized development of the second strain domain
(D^) the entire deforming system is more able to maintain coherency of strain
domain boundaries.
The absence of two "actively gener--icing" strain domains
avoids the dramatic interfacing of domains that produces slip.
The major distinguishing features of this model situation are: the
presence of narrow zones where D2 is non-penetratively superimposed on D ,
close to major boundaries of strain; the presence of "relict-inliers" of
smaller D, domains within a major D2 field; and the tendency for D., rock
fabrics to resemble the more oblate anisotropy of strain typical of D^ as
one approaches the domain boundary.
The smaller D., domains in the Shebandowan
belt have undergone a more uniform increase in oblateness although they still
retain their signature of a westerly-plunging lineation (only preserved nonuniformly in some areas).
The mechanisms for producing the deformations observed here still lie
in the realm
of speculation and will not be commented upon.
D. The Greenwater Pluton and other Late Felsic Intrusions
Introduction
A set of late to posttectonic felsic intrusions is located within or
marginal to the Shebandowan greenstone belt: this set extends from the Saganaga
intrusion on the Minnesota border to several small intrusions northwest of
Thunder Bay.
Many, if not all, plutons shown in Figure l may be comagmatic.
With the exception of the pluton north of Burchell Lake, all possess
hornblende laths as the predominant mafic mineral, and vary compositionally
from tonalite to adamellite .(granodiorite predominant) with some monzonitic
to syenitic outer margins.
Some of the intrusions tend to be quartz
porphyritic such as the Saganaga (Davidson, Jr. 1980), Qreenwater and
Shebandowan bodies with plagioclase and/or K-feldspar phenocrysts dominant
towards the outer margins.
Other intrusions such as the Peewatei granite
and Hood syenite plutons and some smaller syenite bodies south of Burchell
Lake are exclusively K-feldspar porphyritic and markedly homogeneous.
Dark greenish-red to red trachyandesitic-trachytic dykes are spatially
associated with the Greenwater and Shebandowan bodies and sharply transect
the local stratigraphy.
These dykes are most prominent north of Greenwater
Lake (Hodgkinson 1968).
Dykes associated with the Greenwater pluton contain
steeply plunging mineral lineations that are clearly unrelated to the
regional D., and D2 strain domains of Figure l, attesting to their generally
late to posttectonic emplacement.
The internal strain fabric of the Greenwater Pluton, described in detail
below, is typical in one sense of all but the Shebandowan Pluton: it appears
that these late intrusions possess primary strain fabrics related to the
emplacement of the bodies and do not possess any regional tectonic overprint.
It is critical to note that this holds true for the marginal phases of such
intrusions and associated dykes or apophyses extending into the surrounding
host rocks where any regional deformation would certainly have been recorded.
The one exception is the compositionally variable Shebandowan Pluton and
its associated small gold-bearing porphyry intrusions (see Morin 1973).
Not
only is it an elongate body, typical of syntectonic intrusions, but it appears
.o straddle two regional strain domains (Figure 1).
The northern half of the body displays steep westerly-plunging
primary mineral lineations accentuated by quartz that appears to parallel
the prevailing regional D, strain fabric.
The southern half of the body
shows a marked flattening strain with shearing heterogeneously concentrated in
zones.
Correspondingly hornblende, prominent in the north, is progressively
retrograded to chlorite in the south.
A contact strain aureole around each of the late to posttectonic
intrusions is superimposed upon the'volcanic envelope and this contrasts
with an apparent lack of such an aureole around the Shebandowan body.
The
separation in time of emplacement between the Shebandowan and Greenwater
Plutons is further evident from the contrasting internal strain fabric of
their respectively associated trachyandesite dykes.
The evidence suggests that if the various intrusions are comagmatic, there
is a protracted timing to the final emplacement of related felsic intrusions
from at least the D, deformation phase to a post - D^, pre-D~ period.
The
relatively older emplacement age of the Shebandowan pluton suggests that the
pluton is structurally an acceptable candidate as the sub-volcanic equivalent
of the late "Timiskaming" calc-alkaline volcanic suite (Shegelski 1980) to the
south and east.
Petrologic confirmation of this is outside the scope of this
study but, is recommended in the light of gold showings within porphyry
phases of the Shebandowan Pluton (Morin 1973) and the proximity of the
calc-alkaline volcanic suite that appears to be tectonically and lithologically
comparable with the gold-enriched (Tihor and Crocket 1977) Timiskaming Group
at Kirkland Lake.
Late to Posttectonic Plutons
Most other plutons postdate the bulk of the regional supracrustal deformation
and preserve their primary unmetaiaorphosed fabric.
Several intrusions
(the Greenwacer, Pinecone and Kekekuab Flutens) are crescent-shaped and are
semi-concordantly emplaced between the granitoid gneiss terrain to the south
and the greenstone belt.
Such intrusions have become more widely recognized
in recent years with detailed mapping of the granitoid complexes (Schwerdtner
et al. 1979).
Because of their distinctive shape, these plutons possess internal strain
patterns that are distinctive of either diapirism or a sheet intrusion
mechanism.
In the following section, we shall demonstrate that the internal
strain pattern of the Greenwater Pluton more closely resembles a concordant
intrusive sheet.
The Greenwater Pluton
The Greenwater Pluton (Figure 9) is a single intrusion grading outwards
from a hornblende-biotite granodiorite with quartz megacrysts tov/ards the
margins of feldspar porphyritic syenodiorite to diorite.
Typically, the mafic
minerals are concentrated in elongated clots or xenoliths of amphibolite
or meta-hornblendite.
The pluton is separated from a tonalitic gneiss dome to the south by
a narrow screen of amphibolite.
The intrusion and its apophyses display
clear evidence of late to posttectonic emplacement.
The body appears to have
advantageously invaded along the curved interface between a gneiss dome and the
volcanic belt.
Figure 9 illustrates that the southern, concave boundary of the
pluton dips away from the gneiss dome.
The convex boundary to the north and east
dips near vertically.
Natural strain gauges
in the form of quartz aggregates (l-8mm across)
have been used for strain analysis using a method developed by Robin (1977).
The results (Table 3, Figure 10) show a symmetric strain pattern with a zone
GREENWATER PLUTON STRAIN DATA
TABLE 3
1
A/ B
B/C
A/C
k
X,
1.99
1.19
2.03
1.42
1.35
1.91
2.98
2.54
1.84
1.28
1.33
3.39
1.56
1.71
1.55
1.12
1.14
1.20
1.23
3.23
2.14
3.60
2.00
2.79
2.29
3.55
2.81
2.73
1.93
2.01
3.75
2.33
2.96
2.47
17
18
19
1.62
1.80
1.77
1.40
2.06
1.20
1.19
1.10
1.48
1.51
1.51
1.11
1.49
1.72
1.60
1.61
1.17
1.75
1.59
0.6
4.2
0.7
1.0
3.0
0.2
0.1
0.07
0.6
1.8
1.5
0.05
0.9
1.0
1.1
4.9
1.2
3.7
2.6
0.96
0.65
1.05
0.56
0.86
0.71
1.12
0.93
0.82
0.54
0.57
1.20
0.69
0.88
0.74
0.51
0.23
0.63
0.56
20
21
22
23
24
25
26
27
28
29
30
31
1.67
1.61
1.07
1.26
1.36
1.50
1.59
1.28
1.73
1.17
1.10
1.21
1.10
1.12
3.35
1.65
1.22
1.41
3.62
1.56
1.18
1.21
1.95
1.85
1.84
1.79
3.61
2.14
1.47
2.11
6.15
1.99
2.05
1.43
2.14
2.25
6.7
5.2
0.03
0.4
1.6
1.2
0.2
0.5
4.0
0.8
0.1
0.2
0.53
0.50
1.17
0.63
0.35
0.61
1.51
0.57
0.61
0.29
0.68
1.40
32
1.11
1.45
1.62
0.2
0.41
SAMPLE
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1.80
1.33
2.10
1.95
|
i
of constriction (extension as opposed to flattening deformation) extending
along the central length of the pluton, progressing to flattening deformation
near the pluton margins.
The k value of Figure 10, is a measure of the
prolateness of the strain ellipsoid (Flinn 1965) where:
k s
C(A - B)
B(B - C)
and A > B > C are the principal semi-axes of the strain
ellipsoid.
The symmetric strain pattern is inconsistent with an arcuate diapiric
ridge model (Ramberg 1967, Fig. 71; Schwerdtner and Troeng 1978).
This model
shows, among other features, a distinct asymmetry of the total strain pattern
in a radial cross-section with k < l near the concave boundary of the arcuate
ridge and k > l near the convex boundary.
In addition, secondary domes
typically emerge within the end regions of the ridge.
These tend to cause
bilateral flow of the magma towards domes, producing a central region of
subhorizontal prolate strain ellipsoids.
Such features are not present in the
Greenwater Pluton.
Rather, tha structural trends (Figure 9) display steeply-plunging prolate
ellipsoids that reflect longitudinal extension in the direction of material
transport.
This plus the symmetry of the strain pattern are consistent with
the pattern of an ideal antiformal intrusive sheet.
Furthermore, the trajectory
of lineation trends (Figure 11) shows a steep fanning of plunge directions
which approximately parallels the direction of material transport within an
antiformal sheet (Figure 13).
Discussion
This is to our knowledge the first documented example of an arcuate antiformal
sheet intrusion.
Other known arcuate intrusions possess characteristics of
synformal sheets or arcuate diapirs (Schwerdtner 1976, Sutcliffe 1977,
Schwerdtner et al in preparation).
These studies of granitoid bodies demonstrate that posttectonic plutons
often retain strained mineral fabrics as a consequence of emplacement.
Those
bodies which appear "massive" may possess a fabric that can be confirmed only
by testing the magnetic fabric (e.g. King 1966).
The presence of a strain fabric implies that granitoid magmas continue
to flow during consolidation.
The distribution of flow during the very last
stage of consolidation may be evident from a map of strain intensity across
the pluton.
Figure 12 shows the approximately symmetric variation in the
octahedral logarithmic shear strain, a measure of strain intensity where
V - 2^ /"(In A ) 2 * (In IJ ) 2 * (In C^ 2 i ^ .
0
3 'B
C
A -
It appears that the central core of the
pluton preserved a much lower strain intensity as though rising as a single
arcuate "plug", transferring much of the shear strain towards the margins
during at least the final stage of consolidation.
Finally, the study of the Greenwater Pluton demonstrates that proposed
mechanisms for generating observed strain patterns can at least be qualitatively
rppraised.
We anticipate similar success in greenstone belts as strain
models become available,
(e.g. Dixon and Summers 1980).
Summary of Conclusions
We have examined the structural record of the Shebandowan belt by
emphasizing the orientation and relative strength of development of mineral
lineations.
A review of Archean literature and government survey reports
demonstrates that lineations are not consistently recorded (even by structural
geologists) and are otherwise generally neglected.
This may in part be due
to the extra time required to obtain accurate preasurements in weakly-fissile
rocks.
Most lineations measured within the Shebandowan belt are visually
subtle and easily dismissed during rapid examination.
However
subtle, these
features constitute an intrinsic element of the total strain fabric and have
provided us, in conjunction with other types of data, with the following
general results and conclusions at this time:
1.
A major phase of folding, D , appears to have developed across the full
width of the belt and into the Quetico subprovince.
This phase is characterized
by a total strain fabric with its extension direction plunging to the west.
2.
This was superimposed by a second regional deformation, the strain effects of
which are confined
to megascopically discrete
domains and dominating the
structurally more ductile lithologies including the Quetico sediments,
3.
A third deformation, D-, is represented by mesoscopic kink folds within
the northern half of the belt.
This is related to late east-west compression
and one should expect to find more bedding-parallel shearing in the northern
half of the belt than in the south.
4.
Most felsic intrusions have invaded the belt and its southern margin following
the D^ phase of deformation.
Their timing is characterized by their
primary internal strain fabrics and contact strain aureoles imposed upon
the volcanic envelopes.
The one exception is the Shebandowan Pluton, which
aqprobably intruded during D .
5.
A regional pattern of strain domains is defined across the belt and the
adjacent Quetico subprovince.
This pattern features domains of D. total strain and other domains
of cumulative strain arising from the superimposition of D~ upon D-.
6.
The existence of these strain domains is confirmed by magnetic susceptibility
anisotropy (MSA).
Further, there is a general contrast in the relative
prolateness of the susceptibility ellipsoid, and by inference, the rock strain
ellipsoid; the major D. domain is characterized by less oblate total strain
than the D2 domains.
7.
The boundaries or discontinuities between domains appear, from field and
MSA results, to have retained their coherency and avoided major dislocations.
This seems to have been accomplished by increasing the oblateness of
strain in the D. rocks close to the boundaries so as to conform more closely
with the typical oblateness of strain in D2 domains.
then is a zone.
The effective boundary
It is not as sharp as the rapid change in mineral lineation
orientation would suggest.
8.
Two models are suggested to accommodate different settings in which strain
domains are observed:
i)
The concomitant development of strain domains
producing a dislocation along the boundary; this is illustrated by the
"
M
Larder Lake break along which gold is locally concentrated,
ii)
The
sequential development of strain domains in which a deformation is locally
superimposed upon an earlier regional deformation and produces strain domain
patterns with coherent (non-dislocated) boundaries.
9.
The Greenwater Pluton possesses a primary strain fabric that strongly
suggests the body was emplaced as an antiformal crescentic sheet intrusion
and not as an arcuate ridge diapir.
This demonstrates a successful confirmation
of an idealized strain model with a field example.
As idealized models for
greenstone belt strain patterns become more complex, it is hoped that similar
comparisons may be applied to documented total strain patterns of actual
greenstone belts.
10.
The bearing of this study upon mineral exploration is as follows:
a)
The presence of fractures suitable for the hydrothermal
concentration of precious metals may be explored in the vicinity
of strain domain boundaries where they juxtapose lithostratigraphic
hosts of metals.
Their surface expression may not be exposed
or their presence at depth may be inferred only from a coherent
strain domain boundary at the surface.
Such boundaries would
probably not appear as lineaments and would require detailed
mapping to define them.
Many suitable targets may not be as
pronounced in their surface expression as the Larder Lake "break".
b)
The stratigraphic and structural setting south of Lower Shebandowan
Lake displays features that bear some limited resemblance to the
Kirkland Lake area.
Major strain domain boundaries lie in the
proximity of the Shebandowan pluton and a Keewatin/"Timiskaming"
unconformity.
What is required for a viable exploration "mix"
is the presence of suitable lithostratigraphic sources of gold
For example, southwest of the Shebandowan Mine are found ultramafic
units and associated carbonate facies iron formation (Morton 1979)
that lie on or very close to a major strain discontinuity that has
been locally offset by faulting.
Carbonatization of stratigraphy
has not been do c lament: id in this study, but the example
above, nevertheless, illustrates the potential significance
of juxtaposed discontinuities of regional strain.
c)
Ore deposits are likely to be rotated and extended in directions
closely paralleling local extension directions of cumulative strain,
Two examples in the Shebandowan belt appear to confirm this.
The now abandoned North Coldstream Mine (Cu, Au, Ag) on
Burchell Lake contained lenticular (baudinaged?) orebodies
plunging eastward at about 50
(Giblin 1964) within an
easterly-plunging D,, domain.
The Shebandowan Ni-Cu Mine
contains a long ribbon-shaped tabular orebody that plunges
eastward parallel to the local extension directions of strain.
Undiscovered orebodies in this belt can be predicted to have
responded similarly to the local cumulative strain-
This has
obvious implications for geophysical exploration and drilling
programs.
11.
Finally, it is emphasized that this study is to our knowledge the first:
to define regional pattern of strain domains for a greenstone belt; to
demonstrate the value of magnetic susceptibility anisotropy for examining
variations of strain on a regional scale; and (of more academic interest) to
record the presence of an antiformal crescentic sheet intrusion.
We are at present, continuing to examine additional aspects through
support from an NSERCC Grant.
REFERENCES
Bau, A.F.S., 1975: Structures in the Kashabowie Upsala region, northwestern
Ontario; Proceedings of the 1975 Geotraverse Workshop,Precambrian
Research Group, University of Toronto, paper 14, p. 78-80.
Beakhouse, G.P., 1974: Volcanic stratigraphy in the Greenwater-Upper Shebandowan
Lakes Area, Northwestern Ontario; 1974 Annual Report, part 2, Centre for
Precambrian Studies, University of Manitoba, p. 89-99.
Ellwood, B.B., 1978: Measurement of anisotopy of magnetic susceptibility:
A comparison of the precision of torque and spinner magnetometer systems
for basaltic specimens; J. Phys. E. Sci. Instrum., vol. 11, p. 71-75.
Davidson, D.M., Jr., 1980: Emplacement and deformation of the Archean Saganaga
batholith, Vermilion District, northeastern Minnesota; Tectonophysics,
v. 66, p. 179-195.
Dixon, J.M., and Summers, J.M., 1980: A centrifuged model study of the tectonic
development of Archean greenstone belts; Grant 68, p. 58-71. in Geoscience
Research Grant Program, Summary of Research, 1979-1980, edited by
E.G. Pye, Ontario Geological Survey, MP93, p. 262.
Downes, M.J., 1980: Structural and stratigraphic aspects of gold mineralization
in the Larder Lake area, Ontario; p. 92-103 in Roberts, R.G. (editor):
Genesis cf Archean, Volcanic-Hosted Gold Deposits, Ontario Geologies.!
Survey Open File Report 5293, p. 387.
Flinn, D., 1965: On the symmetry principle and the deformation ellipsoid.
Geological Magazine, v. 102, p. 36-45.
Giblin, P.E., 1964:
Rep. 19, 39 p.
Burchell Lake Area; Ontario Department of Mines
Geol.
Hodgkinson, J.M., 1968: Geology of the Kashabowie Area; Ontario Department
of Mines Geol. Rep. 35 p.
King, R.T., 1966:
p. 43-66i
Magnetic fabric of some Irish granites;
Geol. J., v.5,
King, R.T., and Rees, A.I., 1962: The measurement of the anisotropy of
Magnetic susceptibility of rocks by the Torque method; Jour. Geophys.
Res., v. 67, p. 1565-1572.
Means, W.D., 1976: Stress and Strain - Basic Concepts of Continuum Mechanics
for Geologists; Springer-Verlag, New York, p. 339.
- 23-
Morin, J.A. 1973: Geology of the Lower Shebandowan Lake Area, District
of Thunder Bay; Ontario Division of Mines Geol. Rep. 110,
45p.
Morton, P., 1979: Volcanic stratigraphy in the Shebandowan Ni-Cu Mine
Area, Ontario; p. 39-43 in Current Research, Part B, Geological
Survey of Canada, Paper 79-1B.
Ramberg, H., 1967: Gravity, deformation and the earth's crust (as studied
by centrifuged models). Academic Press. London, England,
.214p.
Rathore, J.S., 1979:
Magnetic susceptibility anisotropy in the Cambrian
Slate Belt of North Wales and correlation with strain; Tectonophysics,
v. 53, p. 83-97.
Robin, P.-Y.F., 1977: Determination of geological strain using randomly
oriented strain markers of any shape; Tectonophysics, v. 42, p. T7-T16.
Schwerdtner, W.M., 1970:
Hornblende lineations in Trout Lake area,
Lac La Ronge map sheet, Saskatchewan; Can. J. Earth Sciences, vol. 7,
p. 884-899.
Schwerdtner, W.M., 1976: Remanent magnetization and magnetic susceptibility
anisotropy measurement in Lac des Mille Lacs area of northwestern
Ontario: Discussion; Can. J. Earth Sciences, v. 13, p. 493-494.
Schwerdtner, W.M., Bennett, P.J., and Janes, T.W., 1977: Application of
' L-S fabric scheme to structural mapping and paleostrain analysis;
Can. J. Earth Sciences, v. 14, p. 1021-1032.
Schwerdtner, W.M., and Troeng, B., 1978: Strain distribution within
arcuate diapiric ridges of silicone putty, Tectonophysics, v. 50, p. 13-28.
Schwerdtner, W.M., Stone, D., Osadetz, K., Morgan, J., and Stott, G.M., 1979:
Granitoid complexes and the Archean tectonic record in the southern
part of northwestern Ontario; Can. J. Earth Sciences, v. 16, p. 1965-1977.
Schwerdtner, W.M., Sutcliffe, R.H., and Stott, G.M., in preparation.
Crescentic granitoid plutons in metavolcanic-metasedimentary terrains
of the southern Canadian Shield.
Shegelski, R.J., 1980: Archean cratonization, emergence and red bed
development, Lake Shebandowan area, Canada; Precambrian Research,
v. 12, p. 331-347.
Stone, D.B., 1963:
Anisotropic magnetic susceptibility measurements
on a phonolite and on a folded metamorphic rock.
Geophys. J., v. 7, p. 375-390
Sutcliffe, R.H., 1977: Geology and emplacement of the Jackfish Lake pluton,
a major intrusion in the Rainy Lake dome. Proceedings of the 1977
Geotraverse Workshop, Precambrian Research Group, University of Toronto,
p. 146-154.
Tihor, L.A., and Crocket, J.H., 1977: Gold distribution in the Kirkland
Lake - Larder Lake area, with emphasis on Kerr-Addison-type ore
deposits - a progress report; p. 363-369 in Report of Activities,
Part A, Geological Survey of Canada, Paper 77-1A.
Trowell, N.F., Blackburn, C.E., and Edwards, G.R., 1980: Preliminary
Geological Synthesis of the Savant Lake - Crow Lake Metavolcanic Metasedimentary Belt, Northwestern Ontario, and Its Bearing upon
Mineral Exploration; Ontario Geological Survey, Miscellaneous
Paper 89, 30p.
30
FIGURE CAPTIONS
Figure l :
Generalized summary of major strain domains, megascopic folds,
lineation and foliation orientations.
Figure 2 :
Equal area stereograms showing a) D- donain lineations and
b) D. domain lineations in the eastern half of the belt.
Figure 3 :
Equal area stereograms showing a) D 0 domain foliations and
b) D domain foliations in the eastern half of the belt.
Figure 4 :
Sketch showing the discontinuity between strain domains
obliquely transecting the stratigraphy southeast of Shebandowan
Mine, (stratigraphy from Morton, 1979).
Figure 5 :
Equal area stereogram showing the westerly (D-,) and easterly (D.-)
concentrations of MSA lineations for supracrustal samples.
Steep north and south - plunging lineations tend to occur close
to strain domain boundaries.
Figure 6 :
Variation of prolateness of the magnetic fabric with lineation
plunge direction for mafic volcanic, gabbro and ultramafite
samples. Note higher tendency to higher P value for rocks
within the major D2 domain and decrease in P value close to
strain domain boundaries.
Figure 7 :
Sketch illustrating the difference between coherent and incoherent
strain domain boundaries. See text for description, (from
Means, 1976).
Figure 8 :
Cross section of the Shebandowan Mine ore haulage incline
showing change from westerly-plunging to easterly-plunging
magnetic lineations as one crosses the boundary between strain
domains from south to north. Note the greater variation in
orientations within the D domain suggesting relict D., fabrics
are locally preserved. P value increases for westerly-plunging
lineations. The major discontinuity between strain domains
probably lies close to the steep easterly-plunging lineations
as shown.
Figure 9 :
Map showing the general geology and structural elements within
and around the crescent-shaped Greenwater Pluton.
-3! -
Figure 10:
Map of k values determined at stations within the Greenwater
Pluton. Note the two zones of constriction. The smaller
zone to the north is apparently due to the separate rise of
magma behind a thin screen of partly assimilated volcanics
along the north edge of Shelter Island., The strain pattern
within the pluton is otherwise notably symmetric and contrasts
with the asymmetric pattern of arcuate ridge diapirs (see
text).
Figure 11:
Map showing the fanning pattern of steeply plunging mineral
lineations within the Greenwater Pluton. This is typical
of an idealized antiformal sheet intrusion. Note shallow
lineations plunging away from the core of the gneiss dome
which steepen towards the outer margins as shown in
Figure 9.
Figure 12:
Map showing the distribution of strain intensity within the
rock fabric of the Greenwater Pluton. Asymmetric increase
in intensity towards the margins suggests that during the
final stage of emplacement, the bulk of the shear strain
was transferred towards the margins.
Figure 13:
A sketch illustrating the contrast in flow lines and resulting
extension directions of the strain fabric between synformal
and antiformal sheet instrusions.
o
2a
141 Stations
2b
169 Stations
3a
178 Stations
3b
192 Stations
D1 domain
D2 domain
116 Stations
coe
ic to 'S
O*
a
E
SS
0
g 0)
5 i: o .2
4) Q t.
'
"O'O g t 2 cf
? ? ? S o 2 i
l A O w w i o
O CM h.
CO
t- (/) CO
S*\
O
O
o
O
p
UJ
l
i
O
o
UJ
i* 5ac
Points
UJ
o
(0
O
^
00
(O
UJ
ac
D
O
iZ
aoiovd d
Ul X
(C CC
Ul ^
ft Q
lu *
09
lu 2
2 S
O O
O 00
ffi
ft:
D
O
.E!
ffi
c
.Eg
cog E
i
o
O) "O
j*
,* O as
o
2:
E
2
o ~
'S
c
— (Q
3
-S
**
—
o *-O)
M-O
o
Of
LU
O
O
l
^ o
"J
z o
ffi 3
9?z^-
gz^
2*
^
O
u
p
>-
i
iu
u-g o y gSgs:|
g ia yy^ i| s| 5
il 8 e l si lil 1111
of O S DEG CSZ3 DiD CSD of
"H s0|^ si
A^^^o./
^^ J
*n is.
<
o *u s: — Q. — Q.
UU
C UJ
o
h- Q. CO
O ir JD 1Z
-
-
Q)
"
U *'g
g
W
•J-f ^ 5^
O 1 o o -s
H- O. CO
y
SYNFORMAL SHEET INTRUSION
ANTIFORMAL SHEET INTRUSIO
FIQUBG. 13