Field Relationships and Kinematic Indicators in the Virgin River Shear Zone

James Carolan 1 and Kenneth D. Collerson 1

Carolan, J. and Collerson, K.0. (1989): Fteld relationships and kinematic indicators in the Virgin River Shear Zone; in Summary of Investigations 1989, Saskatchewan Geological Survey; Saskatchewan Energy and Mines, Miscellaneous Report 89-4.

The Virgin River Shear Zone (VRSZ), an Early Proterozoic transcurrent fault located in northwest Sas­katchewan, is defined by a 4 km-wide belt of mytonitic rocks derived from metamorphosed plutonic, volcanic and sedimentary protoliths. The zone separates dominantly granulite facies quartzofeldspathic gneisses of the Western Granulite Domain from lower grade supracrustal rocks and felsic gneisses of the Virgin River Domain to the east. Previous reconnaissance mapping of the shear zone was carried out by Sibbald (1973) and Lewry (1974), Wallis (1970), and Johnson (1968). The present study, a 1 :20 000 scale mapping project, is being undertaken to evaluate kinematic indicators in the Virgin River Shear Zone and account for its role in the tectonic superposition of the Rae and Hearne Provinces (Hoffman, 1988).

1. Regional Setting The VRSZ (Lewry and Sibbald, 1977), the Black Lake Shear Zone (Gilboy, 1980) and the Tulemalu Fault Zone (Tella and Eade, 1986) are components of what is now collectively called the Snowbird Tectonic Zone {Hoffman, 1988). This major zone, extending 3 000 km from the Rocky Mountains in southern Alberta to the Hudson Strait in the Northwest Territories separates the Rae and Hearne Provinces. The zone has been pre­viously been interpreted as an intracontinental reactiva­tion structure resulting from Hudsonian thermotectonism (Lewry and Sibbald, 1977) or as a crustal suture (Hoffman, 1988).

In the Careen lake area (Figure 1 ), the VRSZ forms a 4 km-wide northeast-trending complex of interbanded protomytonites and ultramylonites derived predominant­ly from the Western Granulite Domain and, to a lesser extent, from the Virgin River Domain.

2. Field Characteristics West of the Shear Zone Western Granulite Domain gneisses west of the shear zone have been derived from a variety of protoliths, in­cluding granite, granodiorite, tonalite, gabbro, anor­thosite and complexly intercalated supracrustals. Mineral assemblages indicate that these rocks ex­perienced a granulite facies event which was later over­printed by lower amphibolite to greenschist facies metamorphism. Recognition that the intensity of this

(1) Ealth Sciences Board, University ot Calllornla, Santa Cruz. California

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~ Viqiin River \di~ Shea r ?.out'

L - - -- - - - · .... ... -· - .

Figure 1 - Geological sketch map of the Virgin Rill9r Shear Zone at ~n Lake.

overprint increases with proximity to the shear zone sug­gests that the retrogressive metamorphic event may have been contemporaneous with movements along the shear zone.

Lakeshore outcrops along the western margin of the area indicate a progressive dextral rotation and flatten­ing of regional gneissic layering with proximity to the shear zone (Carolan, Crocker and Collerson, this volume). The same sense of rotation can be recognized on a plot of aeromagnetic highs and lows in the area. (Sibbald and Lewry, 1977)

Summary of Investigations 1989

3. Field Characteristics East of the Shear Zone

Rocks of the Virgin River Domain consist of amphibolite to granulite grade felsic gneisses flanked on the western margin by the Virgin River Schist Group (Johnson, 1968; Wallis, 1970). The falsie gneisses are composed of granite, tonalite, granodiorite, and minor amphibolite.

The well exposed Virgin River Schist Group (VRSG) con­sists of psammopelitic metasediments and possible metatuffs bounding a thin continuous unit of metabasalt. The VRSG is cut by tourmaline-bearing pegmatites but lacks the mafic dykes common in the shear zone. The contact between the VRSG and the main mylonite zone is gradational. The progressive rotation of fabric recog­nized on the western margin of the shear zone was not recognized on the eastern margin.

4. Field Characteristics of the Shear Zone

The VRSZ is defined by an approximately 4 km-wide northeast-trending belt of mylonitic rock derived from plutonic, volcanic and sedimentary sources. Plutonic components include granodiorites, gabbros, tonalites, and augen granites. The volcanic component is a rela­tively thin sequence of pillow basalts at the eastern edge of the shear zone. Metasedimentary mylonites, which occur intermittently throughout the shear zone, are com­positionally similar to rocks of the VRSG to the east, in­dicating 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, probably due to transposi­tion of units during ductile deformation. Mineral as­semblages within the shear zone indicate that greenschist to lower amphibolite facies metamorphism prevailed.

Most rocks in the shear zone appear to have been derived from Western Granulite Domain gneisses and less from the Virgin River Domain. Of particular interest are a body of nepheline syenite and a cross-cutting mafic dyke swarm:

a) Nephellne Syenite

The nepheline syenite crops out as a 1000 m long len­soid body lying in a low shear enclave of the main mylonitic zone. The body strikes parallel to the shear zone and has a complex contact with the surrounding mylonite. Where significant shear has taken place within the nepheline syenite, fluids from the main mylonite zone have precipitated quartz, producing anamolous quart-rich zones.

b) Mafic Dyke Swarm

The mafic dyke swarm in the shear zone was previously recognized by Wallis (1970) along a section of the shear zone north of the present study area and identified regionally by Macdonald (1980). The dyke swarm, which generally parallels the trend of the shear zone, com­prises mafic mylonites in the main shear zone and rela-

Saskatchewan Geological Survey

tively undeformed gabbros in areas of low strain. In many places in the areas of low strain the gabbros preserve relict igneous textures and cross-cut the gneis­ses.

c) Strain Features

Strain variation within the VRSZ is very complex and characterized by a complicated intercalation of low strain enclaves within the main mylonite zone. The tow strain enclaves, generally composed of granodioritic­tonalitic gneiss with subordinate cross-cutting augen gneiss and gabbros, are kilometre-scale lensoid bodies exhibiting primary intrusive relationships. Contacts be­tween these zones and the surrounding mylonite range from relatively simple textural gradations to a complex series of flattened gneiss-mylonite interbands.

Observations of grain morphologies indicate that flatten­ing in the main mylonite zone was due to a significant component of pure as well as simple shear.

5. Structural Geology

a) Folding

At least tour episodes of folding have been recognized in the VRSZ and the Virgin River Domain. Rare F1 folds are small, rootless and isoclinal. They are refolded by northeast-plunging folds (F2), which are tight to isoclinal with steep to shallow plunges, and are recognized at both mesoscopic and macroscopic scales. The F2 folds are in turn refolded by northwest-trending F3 folds, which have only been recognized at a mesoscopic scale.

Postmylonite deformation (04) includes localized reverse kink banding and box folding of the mylonitic fabric.

b) Faulting

Faulting was produced by both ductile and brittle defor­mation. The dominant fabric within the shear zone preserves a history of dextral displacement due largely to 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 recognized in the area:

1) oriented at 040° to 080° with near vertical dips, they show normal dextral offset and are commonly as­sociated with fault breccia and drag folds; and

2) faults oriented at approximately 340" to 360° exhibit normal sinistral offset. These conjugate faults are in­terpreted as resulting from brittle failure of more com­petent layers during extension along the main shear zone.

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c) Foliation In the VRSZ

The dominant penetrative foliation of the myfonite zone ($3) is believed to have formed by the reorientation of an earlier foliation ($1) which in places is preserved as an internal fabric within asymmetric pinch and swell structures. An oblique fabric (S2) is also commonly preserved as a component in C-S fabrics.

The dominant foliation ($3) dips to the northwest at 35°-80°, with most dips at 55"-70". Mineral lineations are defined by aligned hornblende and feldspar grains and plunge northeast at 5°-45°, averaging approximately 20°. The plunge of the lineation suggests that the major com­ponent of motion is strike-slip with a minor east-side-up dip-slip component.

d) Kinematic Indicators

Observed kinematic indicators include: rolling struc­tures, asymmetric pinch-and-swell, asymmetric folds, C­S structures, and offset veins and dykes. Although most asymmetric grains are somewhat ambiguous, all the roll­ing structures possessing a clear asymmetry consistent­ly show a Z-type geometry, indicative of a dextral shear sense (Van Den Dreissche, 1987). The observed rolling structures are typically feldspar porphyroblasts with asymmetric tails.

Asymmetric pinch-and-swell structures recognized in the area are of the Type 2 class of Hamner (1986), and occur in layers of amphibolite surrounded by a less com­petent quartzofeldspathic matrix. Shear along the plane of extension has commonly produced an antithetic rota­tion of the swell section of the structure. Observations of such rotated structures in the area indicate that the sense of shear was dextral.

C-S structures observed in the area are largely am­biguous. Near the margins of the shear zone a sinistral shear sense is largely indicated, but in the core of the shear zone the shear sense is consistently dextral.

Late movement in the shear zone is indicated by asym­metric folds and offset pegmatitic dykes. The asym­metric folds are fairly common and exhibit a consistent dextral shear sense. Late dykes cut the penetrative mylonitic foliation at a high angle and are dextrally offset within the plane of the mylonitic fabric. This observation indicates that there were multiple periods of fault move­ment along the VRSZ.

6. Interpretation Gross similarities in rock types and deformation style suggest that the VRSZ may have resulted from proces­ses analogous to the Cenozoic Eurasian-Indian tectonic closure. Recognizing the similarities between Himalayan deformation processes and the plasticity problem of plane indentation, Molnar and Tapponier (1984) were able to develop a reasonable model for Himalayan tec­tonic processes. We believe a similar analogy can be drawn for the Early Proterozoic Wopmay and Trans-Hud­son Orogens.

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In consideration of the plasticity problem, slip lines dis­playing strike-slip motion form when a rigid indentor con­verges upon a semi-infinite plastic body. These slip lines, acting as escape structures, respond to the im­posed strain by allowing material in the semi-infinite plas­tic body to move away from the rigid indenter. Molnar and Tapponier (1984) believe that such escape struc­tures account for much of the deformation north of the Himalayas. Applying this to the Wopmay and Trans-Hud­son Orogens, we believe that the Great Lake Shear Zone, the Needle Falls Shear Zone, and the VRSZ may represent similar escape structures. Using the plasticity problem analogy, we believe that the Superior and/or Slave Cratons may have acted as rigid indentors con­verging upon the semi-plastic Trans-Hudson Orogen. The resulting slip geometry and the relative positions of these faults to the orogen should produce dextral strike­slip movement along these faults. Recent fieldwork in the VRSZ (Carolan and Collerson, 1988) , the Needle Falls Shear Zone (Stauffer and Lewry, 1988) and the Great Slave lake Shear Zone (Hamner, 1980) indicates that relative motion along these faults is indeed strike­slip and dextral. Thus, recent evidence suggests that the tectonic juxtaposition of the Rae and Hearne Cratons oc­curred in response to intracratonic transcurrent move­ment rather than to continental collision.

Offset dykes that yield Rb-Sr model ages of 1910 Ma, as well as cross-cutting relationships between the ca. 1820 Ma (Bickford et a/.,1986) Junction Granite and the VRSZ provide loose constraints indicating that both the Slave and Superior Cratons may have played roles in the movement history of the VRSZ. Kinematic data are consistent with the interpretation of the shear zone as an intracratonic transcurrent fault, possibly related to col­lisional indentation of either or both the Slave Craton to the northwest or the Superior Craton to the southeast. The VRSZ may also be an older Archean feature, such as an early rift, that was reactivated during Early Proterozoic continental collision. Further geochronologic work in progress by the authors will, it is hoped, further constrain the timing of fault movement and the relative roles of the Slave and Superior Cratons in the formation of the VRSZ.

7. Sampling Samples of the metasediments and volcanics from the VRSG, orthogneisses from low shear enclaves, and fal­sie gneisses from the Virgin River Domain have been collected for U-Pb geochronology. Such information will be used to determine the relationship of the metasedi­ments to the surrounding quartzofeldspathic gneiss. The metabasalt and gabbros from low shear domains have been sampled for Sm-Nd geochronology. Offset granitic dykes have been sampled for Rb-Sr analysis. This infor­mation will be necessary in constraining the movement history of the VRSZ.

8. Summary 1) The area exhibits a well exposed section across the

VRSZ, an approximately 4 km-wide zone of mylonitic rocks which separates the high grade Archean rocks

Summa,y of Investigations 1989

of the Western Granulite Domain from lower am­phibolite facies rocks of the Virgin River Domain.

2) The VRSZ is characterized by a complex intercala­tion of low shear enclaves within the main mylonitic body.

3) At least four episodes of folding affected the VRSZ and the Virgin River Domain.

4) Observed stretching lineations plunge to the north­east at 5° to 45°, averaging approximately 20°. This plunge indicates that the major component of fault movement was strike-slip with a slight east-side-up component.

5) Kinematic indicators, including rolling structures, asymmetric pinch-and-swell structures, C-S struc­tures, asymmetric folds, and offset granitic dykes in­dicate the strike-slip motion was dextral.

6) Relict pillow structures preserved in outcrop-scale low shear domains suggest the possibility of an early rift sequence in the area.

7) The movement history of the VRSZ during the Early Proterozoic may be modelled using the plasticity problem of plane indentation. That the VRSZ was an Archean discontinuity reactivated by Early Proterozoic thermotectonism has not yet been resolved.

9. Acknowledgements

We are grateful to Saskatchewan Energy and Mines for financial support of fieldwork. We appreciate J.F. Lewry for his invaluable aid in expediting fieldwork and provid­ing guidance in the field. Excellent field assistance was provided by Alden Hough. This research was also sup­ported by a National Science Foundation Grant (EAR 87-20442) to Collerson and Bickford.

10. References Bickford, M.E., Van Schmus, W.R., Macdonald, R., Lewry, J.F.

and Pearson,J.G. (1986): U-Pb 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.

Carolan, J . and Collerson, K.D. (1988): The Virgin River Shear Zone in the Careen Lake Alea: field relationships and kinematic indicators; in Summary of Investigations 1988, Sask. Geol. Surv., Misc. Rep. 88-4, p92-96.

Saskatchewan Geological Survey

Gilboy, C.F. (1980): Reconnaissance bedrock geology: Cham­beuil Lake east area (part of NTS area 74P); in Summary of Investigations 1980, Sask. Geol. Surv., Misc. Rep. 80-4, p14-16.

Hamner, S. (1986): Asymmetric pull aparts and foliation fish as kinematic indicators; J. Struct. Geol., v8, no2, p111-122.

Hoffman, P.F. (1988): United Plates of America, the birth of :: craton: Early Proterozoic assembly and growth of Lauren­tia; 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. Resour., 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 1974, Sask. Geol. Surv., 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.

Macdonald, R. (1980): New edition of the geological map of Saskatchewan, Precambrian Shield area; in Summary of Investigations 1980, Sask. Geol. Surv., Misc. Rep. 80-4, p19-21 .

Sibbald, T.1.1. (1973): 74-8-NW: Mudjatik (NW); in Summary Report of Geological Investigations Conducted in the Precambrian Alea of Saskatchewan, 1973, Sask. Oep. Miner. Resour. , p35-42.

Stauffer, M.P. and Lewry, J.F. (1988): Kinematic investigation of part of the Needle Falls Shear Zone; in Summary of In­vestigations 1988, Sask. Gaol. Surv., Misc. Rep. 88-4, p156-160.

Tella, $ . and Eade, K.E. (1986): Occurrence and possible tec­tonic significance of granulite fragments in the Tulemalu Fault Zone, District of Keewatin, N.W.T., Canada; Can. J. Earth. Sci., v23, p1950-1962.

Van Den Dreissche, J . and Brun, J.-P. (1987): Rolling struc­tures at large shear strain; J. Struct. Geol., v9, no5/ 6, p 691-704.

Wallis, R.H. (1970): The geology of the Dufferin lakes (west half) , Saskatchewan; Sask. Dep. Miner. Resour., Rep. 132, 59p.

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