Project 8804

The Virgin River Shear Zone in the Careen Lake Area: Field Relationships and Kinematic Indicators 1

J. Carolan2 and K.D. Collerson2

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

....... ... ...

' ', \ Q I

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0

CAREEN

.. :~.

Moraine

. ..; .. •• 'D.

LAKE

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I I I

l? ~, Muskeg "\

~~ ~ '

! J Gneisses in the Western Granulite Domain

Felsic gneisses in the Virgin River Domain

Low deformation zone protolith inlier

B Virgin River Schist Gp.

- Metabasalt

~ Virgin River Shear Zone

0 2 3 km

----

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.

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

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

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

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

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 (S1). 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).

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-

Amphibolif Focies

Figure 3 - Schematic cross-section showing the minor reverse component of the shear zone

Saskatchewan Geological Survey

Z-Type Rolling Structures

Off set D/kes

-:::: I_: _,-::=--- -- I -:;::::-

Asymmetric Pinch and Swells

Transposed Folds

Figure 4 - Kinematic indicators used to establish fault move­ment in the Virgin River Shear Zone

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 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 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.

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.

9. References 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.

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., p46-55.

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