Download The Virgin River Shear Zone in the Careen Lake Area: Field

Project 8804
The Virgin River Shear Zone in the Careen Lake Area:
Field Relationships and Kinematic Indicators 1
2
J. Carolan and K.D. Collerson
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Carolan, J. and Collerson, K.D. (1988): The Virgin River Shear Zone in the Careen Lake area: Field relationships and kinematic
indicators; in Summary of Investigations 1988, Saskatchewan Geological Survey; Saskatchewan Energy and Mines, Miscellaneous Report 88-4.
This paper presents results of fieldwork in the Careen
Lake area of northern Saskatchewan which commenced
this summer (Figure 1). The area was selected for study
because it exhibits a well-exposed section across the Virgin River Shear Zone. This approximately 4.5 km wide
zone of mytonitic rocks separates dominantly granulite
facies quartzofeldspathic gneisses of the Western
Granulite Domain in the west from lower grade supracrustal rocks and falsie gneisses of the Virgin River
Domain in the east. Previous reconnaissance mapping
of the shear zone was carried out by Sibbald and Lewry
(1973 and 1974), Wallis (1970) and Johnson (1968). The
present study forms part of a second-phase mapping
project at a scale of 1:20,000. This study area forms a
segment of a regional transect crossing the Virgin River
Shear Zone from the Cree Lake Zone (Orrell and Bickford, this volume) in the east to the Western Granulite
Domain (Crocker and Collerson, this volume). The study
is being undertaken to evaluate kinematic indicators in
the Virgin River Shear Zone. Such information may be
applied in determining whether this zone represents a
crustal collisional suture, or an intracontinental transcurrent fault related to the Trans-Hudson Orogeny.
1. Regional Setting
The Virgin River Shear Zone (Lewry and Sibbald, 1977),
the Black Lake Shear Zone (Gilboy, 1980) and the
Tulemalu Fault Zone (Tella and Eade, 1986) are components of what has been collectively termed the Snowbird Tectonic Zone (Hoffman, 1988). This major zone,
extending 3000 km from the Rocky Mountains in
Southern Alberta to the Hudson Strait in the Northwest
Territories, separates the Rae and Hearne Provinces
(Hoffman, 1988). The zone has been interpreted as
either a crustal suture or an intracontinental reactivation
structure resulting from Hudsonian thermotectonism
(Lewry and Sibbald, 1977).
In the Careen Lake area (Figure 1), the Virgin River
Shear Zone separates high-grade Archean rocks of the
Western Granulite Domain from lower amphibolite facies
rocks of the Virgin River Domain. Gneissic relationships
in the Western Granulite Domain reflect a complex
lithostructural history (Crocker and Collerson, this
volume). The gneisses appear to have been derived
from a variety of protoliths, including granite, granodiorite, tonalite, gabbro and anorthosite. Although the
gneisses experienced granulite facies metamorphism,
they also show the effect of a lower amphibolite facies
overprint, possibly related to Hudsonian thermotectonism (Lewry and Sibbald, 1980).
The Cree Lake Zone east of the Virgin River Shear
Zone, comprising the Virgin River and Mudjatik
Domains (Figure 1), is dominated by granitic falsie gneisses and interlayered supracrustal rocks. Metamorphic
grades in the Mudjatik Domain range from upper amphibolite to granulite facies, whereas rocks in the Virgin
River Domain are lower amphibolite facies.
Virgin River Shear Zone mylonites at Careen Lake appear to have been derived from rocks of both the
Western Granulite Domain and the Virgin River Domain.
South of Careen Lake, the Virgin River Shear Zone is
cut by the Junction Granite, which gives a U-Pb zircon
crystallization age of 1820 ± 10 Ma (Bickford, 1986).
Lewry and Sibbald (1977) have shown that the Junction
Granite experienced three periods of ductile/brittle deformation associated with movement along the fault zone.
2. Lithology
a) Virgin River Schist Group (VRSG)
The Virgin River Schist Group (Johnson, 1968; Wallis,
1970) consists of psammopelitic metasediments and
possible metatuffs bounding a thin continuous unit of
metabasalt. These fine- to medium-grained metasediments contain biotite, quartz, feldspar, garnet, muscovite, staurolite and andalusite. Locally, a welldeveloped phyllitic texture is developed. The fine- to
medium-grained metabasalt contains actinolite,
hornblende, Fe-Ti oxides and feldspars. In outcropscale areas of low finite strain, relict deformed pillow
structures are recognized. The VRSG is cut by tourmaline-bearing pegmatites, but lacks the mafic dykes
common in the bordering rock units (see below). The
contact between the VRSG and the main mylonite zone
is gradational.
(1) Reid support provided under the Saskatchewan component of the Cenade-Saskatchewan Subsidiary Agreement on Mineral Development
1984-89, In Joint venture w~h the University of California. Santa Cruz
(2) Earth Sciences Board, University of California, Santa Cruz, Gallfornia 95064
92
Summa,y of Investigations 1988
0
CAREEN
LAKE
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Q
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Moraine
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•• 'D.
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~~ ~
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2
3 km
-- --
J Gneisses in the Western
Granulite
Felsic
Domain
gneisses in the
Virgin River Domain
Low deformation zone
protolith inlier
B
Virgin River Schist Gp.
-
Metabasalt
~ Virgin River Shear Zone
Figure 1 - Map showing relationships between major rock types in the Virgin River Shear Zone, Careen Lake allta.
Saskatchewan Geological Survey
93
b) Felsic Gneisses of the Mudjatik Domain
The Mudjatik Domain comprises quartzofeldspathic
gneisses derived from granitic, granodioritic and
tonalitic protoliths (Orrell and Bickford, this volume).
Gneisses derived from these protoliths are also cut by
amphibolite dykes. The gneisses are characterized by
assemblages of quartz, plagioclase, potassium feldspar,
hornblende, biotite and Fe-Ti oxides. The contact between this terrain and the supracrustal rocks of the
VRSG is not exposed in the area mapped.
c) Western Granulite Domain
Rocks of the Western Granulite Domain consist of
quartzofeldspathic gneiss derived from granitic,
granodioritic, tonalitic, gabbroic and anorthositic
protoliths (Crocker and Collerson, this volume). Mineral
assemblages indicative of granulite facies conditions are
widely preserved in these gneisses. The contact between the Western Granulite Domain and the Virgin
River Shear Zone is gradational over a distance of
0.5 km and is reflected in the progressive change from
protomylonitic to mylonitic rocktypes.
zone are sharp and planar. These contact relationships
are probably the result of transposition of units during
ductile deformation.
4. Structural Geology
a) Folding
At least four episodes of folding are recognized in the
Virgin River Shear Zone and the Virgin River Domain
(Figure 1). Rare F1 folds occur as small rootless
isoclinal folds. These intrafolial folds are refolded by a
northeast-plunging folding event (F2). These F2 folds are
tight to isoclinal, with plunges ranging from steep to
shallow. F2 folds occur on both mesoscopic and macroscopic scales. The F2 folds are refolded by northwesttrending F3 folds. Recognition of this D3 event could
only be made on a mesoscopic scale. Postmylonitic
deformation (D4) includes localized reverse kink banding and box folding of the mylonitic fabric. Due to poor
exposure in the Western Granulite Domain, documentation of folding relationships was not possible.
b) Faulting
d) Pegmatites
Crosscutting relationships indicate that four periods of
pegmatite intrusion affected the area. Bodies of pegmatite occur either obliquely to planar fabrics or along
axial planar cleavage traces.
3. Field Characteristics of the Virgin River
Shear Zone
Faulting in the area reflects the effect of ductile and brittle deformation. The main mylonite zone and smallscale oblique shear bands show dextral displacement
due largely to the effect of ductile deformation. Later brittle faulting is both oblique and parallel to the main
mylonitic fabric. Along the mylonitic fabric plane, pegmatite and granitic dykes show dextral offset. Two conjugate brittle fault sets are also present. One set,
oriented at approximately 040° to 080° shows normal
dextral offset and is commonly associated with the
The Virgin River Shear Zone is defined by an approximately 4.5 km wide northeast-trending belt of
mylonitic rock derived from plutonic, volcanic and
sedimentary protoliths. Plutonic components include
granodiorites, diorites, tonalites, granites and diabase
dykes. The volcanic unit consists of a relatively thin sequence of pillow basalts. Metasediments range from
pelitic to psammitic but are generally psammopelitic.
The mylonite zone contains two zones of low finite
strain characterized by a body of tonalitic-granodioritic
gneiss which is cut by granitic augen gneiss and amphibolitic metadiabase dykes. These important zones
are approximately 2.5 by 0.5 km and 0.5 by 0.25 km in
size. The contact between the gneissic enclaves and the
surrounding mylonite is complex and gradational. Along
the eastern boundary of the larger low deformation
zone, the contact is defined by a series of mylonitized
and flattened augen gneisses that grade into protomylonite. The boundary of the smaller low deformation
zone is characterized by a textural path grading from
protomylonite to ultramylonite.
Metasedimetary mylonites occur intermittently throughout the Virgin River Shear Zone. These units are compositionally similar to rocks in the VRSG to the east, suggesting that part of the VRSG has been incorporated
into the mylonite zone. Contacts between these units
and the plutonic and volcanic components of the shear
N
•
•
+
Figure 2 - Plung9s of mineral lineation defined by the alignment of hornblende, biotim and feldspar. Mean plunge is approximately 22°.
Summary of Investigations 1988
development of fault breccia and drag folds. The other
fault set, oriented at approximately 340° to 360°, exhibits
normal sinistral offset. These conjugate faults are interpreted to have resulted from brittle failure of more competent layers during extension along the main shear
zone.
The penetrative foliation of the mylonite zone (S2) is
believed to have formed by reorientation of an earlier
foliation (S 1). The earlier foliation is commonly
preserved as an internal fabric within asymmetric pinchand-swell structures. The dominant foliation (S2) dips to
the northwest at between 35° and 80°, with most dips
being between 55° and 70°. Mineral lineations defined
by aligned hornblende, biotite and feldspar grains
plunge to the northeast at 11° to 40°, averaging approximately 22° (Figure 2). The plunge of the lineation
suggests that the major component of motion is strike
slip with a minor dip slip component. The orientation of
the fault plane, together with the juxtaposition of a highgrade terrain and a low-grade terrain, suggests that a
minor component of reverse faulting is also present (see
Figure 3).
Asymmetric
Z-Type Rolling
Structures
Pinch and Swells
Offset D/kes
Transposed Folds
-::::
I_:
_,-::=-
-- I -:;::::-
Figure 4 - Kinematic indicators used to establish fault movement in the Virgin River Shear Zone
c) Kinematic Indicators
Kinematic indicators of fault movement observed in the
field include rolling structures, asymmetric pinch-andswell structures and offset dykes and veins (Figure 4). Although most rolling structures observed are ambiguous,
those with clear asymmetry are dominately of the Z-type
of Van Den Dreissche and Brun (1987). Rolling structures observed in the area are typically feldspar porphyroblasts with asymmetric tails derived from the porphyroblast. These appear to have formed as a result of
rotation of a rigid object in a ductile matrix during shearing (Van Den Dreissche and Brun, 1987). Rolling structures which display Z-type geometry, similar to those observed in the area, indicate a dextral sense of shearing.
Asymmetric pinch-and-swell structures recognized in the
area are of the Type 2 class of Hamner (1986), and are
developed in layers of amphibolite surrounded by a less
competent quartzofeldspathic matrix. Shear along the
plane of extension commonly produces an antithetic
rotation of the swell section of the structure. Observations of such rotated structures in this area indicate that
the sense of shear was dextral.
Offset granitic and pegmatitic dykes and veins were also
observed in the area. These late dykes cut·the penetra-
tive mylonitic foliation at a high angle and show the effect of dextral offset in the plane of the mylonitic fabric.
This observation indicates that offset was dextral and
there were multiple periods of fault movement.
5. Metamorphism
Mineral assemblages of rock units east of the mylonite
zone indicate lower amphibolite facies metamorphism.
Assemblages include hornblende, biotrte, quartz, plagioclase, K-feldspar, andalusite, garnet and staurolite.
Secondary minerals include epidote and chlorite.
Staurolite inclusions in garnets and synkinematic and
postkinematic andalusite suggest an extensive period of
crystal growth.
Mineral assemblages of rock units west of the mylonrte
zone indicate granulite facies metamorphism with subsequent lower amphibolite facies retrogression. Relic orthopyroxene and blue quartz define the older mineral assemblage. The lower amphibolite mineral assemblage includes hornblende, biotite, quartz, feldspar, magnetite
and ilmenrte. Secondary minerals include goethite, amorphous sphene, chlorite and epidote.
6. Samples Collected for Further Study
Amphibolif
Focies
Figure 3 - Schematic cross-section showing the minor reverse
component of the shear zone
Saskatchewan Geological Survey
Samples of the metasediments from the VRSG, orthogneisses from the major low shear domain, and felsic
gneisses from the Mudjatik Domain have been collected
for U-Pb geochronology. Such information will be used
to determine the relationship of the metasediments to
the surrounding quartzofeldspathic gneisses. Mylonitized pillow basalt and metadiabase dykes have been
sampled for Sm-Nd analysis. Offset granitic and pegmatitic dykes were sampled for Rb-Sr analysis. This information will be necessary in determining timing of brrttle failure along the fault zone.
95
7. Summary
9. References
1) The area exhibits a well-exposed section across the
Virgin River Shear Zone. This approximately 4.5 km
wide zone of mylonitic rocks separates the highgrade Archean rocks of the Western Granulite
Domain from lower amphibolite facies rocks of the
Virgin River Domain.
Bickford, M.E., Van Schmus, W.R., Macdonald, R. , Lewry, J.F.
and Pearson, J.G. (1986): U-Pb zircon geochronology
project for the Trans-Hudson Orogen: current sampling
and recent results; in Summary of Investigations 1986,
Sask. Geol. Surv.• Misc. Rep. 86-4, p101-107.
2) The Virgin River Shear Zone incorporates low-deformation zones in which primary intrusive features of
Cree Lake Zone gneisses are preserved.
3) Relict pillow structures preserved in outcrop-scale
low-deformation areas suggest the possibility that a
early rift sequence may exist in the area.
4) At least four episodes of folding affected the Virgin
River Shear Zone and the Virgin River Domain.
5) Stretching lineations observed in the field plunge at
11° to 40°, averaging approximately 22°. This plunge
suggests that the major component of motion was
strike-slip.
6) Kinematic indicators, including rolling structures,
asymmetric pinch-and-swell structures and offset
veins and dykes, indicate that strike-slip motion was
dextral.
8. Acknowledgements
We are grateful to Saskatchewan Energy and Mines for
financial support of field work. R. Macdonald and
C.T. Harper coordinated, expedited and provided logistics for field work. J.F. Lewry and M.E. Bickford
provided stimulating discussions on interpretations of
field relationships. Able field assistance was provided by
Greg Grimsich. This research was also supported by a
National Science Foundation Grant (EAR 87-20442) to
Collerson and Bickford.
Hamner, S. (1986): Asymmetric pull-aparts and foliation fish
as kinematic indicators; J. Struct. Gaol., v8, no2, p111-122.
Hoffman, P.F. (1988): United plates of America, the birth of a
craton: early Proterozoic assembly and growth of L.aurentia; Annu. Rev. Earth Planet. Sci., v16, p543-603.
Johnson, R.L. (1968): The geology of the Nyberg Lakes area
(west half), Saskatchewan; Sask. Dep. Miner., Rep. 118,
15p.
Lewry, J.F. (1974): Structural relationships in the La Loche
(north) map sheet and adjacent areas; in Summary
Report of Field Investigations by the Saskatchewan
Geological Survey, 1974; Sask. Dep. Miner. Resour., p4655.
Lewry, J.F. and Sibbald, T.1.1. (1977): Variation in lithology and
tectonomorphic relationships in the Precambrian basement of northern Saskatchewan; Can. J. Earth. Sci., v14,
p1453-1467.
Sibbald, T.1.1. (1973): 74-8-NW: Mudjatik (nw); in Summary
Report of Geological Investigations Conducted in the
Precambrian Area of Saskatchewan, 1973; Sask. Dep.
Miner. Resour., p35-42.
Tella, S. and Eade, K.E. (1986): Occurrence and possible tectonic significance of granulite fragments in the Tulemalu
fault zone, Distict of Keewatin, N.W.T., Canada; Can. J.
Earth. Sci., v23, p 1950-1962.
Van Den Dreissche, J. and Brun, J.-P. (1987): Rolling structures at large shear strain; J. Struct. Geol., v9, no5/6,
p691- 704.
Wallis, R.H. (1970): The geology of the Dufferin Lake area
(west half). Saskatchewan; Sask. Dep. Miner. Resour.,
Rep. 132, 59p.
Summary of Investigations 1988