Kinematic Investigation of Part of the Needle Falls Shear Zone 1

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1 Kinematic Investigation of Part of the Needle Falls Shear Zone 1 M.R. Stautte? and J.F. Lewry 3 Stauffer, M.A. and Lewry, J.F. (1988): Kinematic investigation of part of the Needle Falls Shear Zone; in Summary of Investigations 1988, Saskatchewan Geological Survey; Saskatchewan Energy and Mines, Miscellaneous Report The Needle Falls - Parker Lake Shear Zone, located in northern Saskatchewan, is a major Early Proterozoic crustal discontinuity within the western part of the Trans Hudson Orogen. Throughout its exposed strike length of almost 400 km, it defines the western-northwestern margin of the ca Ga Wathaman Batholith. It is also the junction between reworked Archean continental crust and infolded Early Proterozoic miogeoclinal metasediments (Wollaston Group) of the Cree Lake Zone and Peter Lake Domain lying to the northwest and the predominantly juvenile Early Proterozoic arc terrains of the Reindeer Zone to the southeast. Neither sense nor amount of displacement along this regionally significant ductile shear zone have hitherto been adequately documented. We here report preliminary results of detailed kinematic investigation of a well-exposed southern part of the shear zone in the vicinity of Needle Falls on the Churchill River, an area previously mapped by Money (1965) and Munday (1978). In the study area (Figure 1a), protomylonitic to ultramylonitic gneisses (cf. Wise et al., 1984) of the Needle Falls Shear Zone form a subvertical north-northeast-striking belt over 1 km wide. Within this belt, mylonites and ultramylonites displaying dominant 'C' plane (shear foliation) fabrics occur over a width of about 500 m. Mylonite protoliths include megacrystic to nonmegacrystic monzogranite-granodiorite, as well as xenolithic amphibolite-hornblende gneiss screens and later aplite-pegmatite sheets of the Wathaman Batholith, and also augened (megacrystic) granitic gneisses representing Archean basement of the Cree Lake Zone. 1. Relations East of the Shear Zone Potassium feldspar megacrystic monzogranite and granodiorite of the 'main phase' of the Wathaman Batholith (Fumerton et al., 1984) occur in a 1 to 2 km wide belt east of the shear zone. These are variably intruded by leucogranite, aplite and pegmatite sheets ranging from a few centimetres to tens of metres in thickness. Amphibolites, hornblende gneisses and less common psammitic gneisses occur irregularly as subequant to tabular, angular to rounded xenoliths ranging from several centimetres across to major screens tens of metres long and several metres wide. Leucograniteaplite-pegmatite sheets and xenolithic screens are commonly subpara11el, defining a crude layering (So) which may be related to primary magmatic flow within the batholith. Farther east, the Wathaman Batholith grades irregularly into older agmatitic, schlieric and nebulitic granodiorites, tonalites and trondhjemites of the Rottenstone Tonalite-Migmatite Belt across a complex marginal metasomatic and injection aureole. Previous work (Lewry and Slimmon, 1985) shows that the older gneisses are complexly intruded by generally massive to weakly foliated megacrystic and nonmegacrystic granite bodies coeval with the Wathaman Batholith throughout this part of the Rottenstone Domain: Wathaman-related granite sheets postdate at least two deformational phases affecting older rocks. Outside the shear zone, rocks of the Wathaman Batholith have a weak to moderately well developed tectonic foliation (S1) variably subparallel or oblique to So banding. Detailed fabric features suggest that this foliation is predominantly a flattening fabric. Locally, S1 is axial planar to coeval, open to tight F1 folds defined by So banding. A prominent So/S1 intersection lineation (L1), coaxial with fold hinge lines, is widely developed. In the eastern part of the study area, So and S1 generally strike between 040 and 055 and dip moderately to steeply to the southeast. Towards the shear zone, however, both swing progressively towards 025 to 030 and become subvertical (Figure 1 b). F1 told hinge lines and L1 lineation generally plunge at 20 to 30 to the southwest but become more gently south-plunging towards the shear zone (Figure 1 b). Field observations and structural geometry suggest that the structures described above predate, and are kinematically unrelated to, the shear zone itself. 2. Relations West of the Shear Zone One lakeshore outcrop, situated southwest of Needle Falls and about 100 m west of ultramylonites of the shear zone, comprises coarse-grained unfoliated metagabbro/metapyroxenite net-veined by leucogranite which is apparently weakly deformed. However, most of the limited outcrops immediately west of the shear zone are coarsely megacrystic to strongly augened granitic gneisses locally injected by thin aplite or pegmatite veins and contain few xenoliths. Although where least deformed these are superficially similar to the main phase Wathaman monzogranite, both previous work and observed relationships to metasediments of the Wollaston Group indicate that they represent Archean basement. Contact between augen gneisses and Wollaston Group metaconglomerates was noted in one outcrop 200 m west of Needle Falls. The metaconglomerates contain (1) Projec! fun<led by NSERC operating grants to the authors (2) Department of Geologlcal Sciences. University of Saskatchewan (3) Department of Geology. University of Regina 156 Summary of Investigations 1988

2 ... N N ~ 1TS0 x f - L 1Km a. b. N t d. c cm c. Figure 1 - a) Generalized map of the Needle Falls Shear Zone (NFSZ) in the study area. Wathaman Batho/ith lies to the east and Wollaston Domain of the Cree Lake Zone to the west; star shows location of Needle Falls. Solid black, ultramylonite; large dots, mylonite; large squares, protomylonite. b) Stereographic plot showing rotation of F1/L1 /ineation into shear zone stretching direction Lm (a/1 lineations designated Lin diagram) and So into Cm (designatrxf C in diagram) in the shear zone. For 'L': dottrxf field (south-plunging) shows range of data (9) for Wathaman Batholith east of protomylonite, outside NFSZ; 'n' indicates data close to NFSZ (within 1 km); 'f' indicates data further away. Open field shows range of data (4) for protomy/onite and mylonite. Solid field shows range of data (9) for uttramylonite. For -rrso- -rrcm: dotted field shows range of data (15) for Wathaman Batholith east of protomylonite; 'n' indicates data within 1 km of NFSZ; 'f' indicates data farther away. Solid field shows range of data (15) within the NFSZ. c) Diagrammatic illustration of major porphyroclast/tail and C/S relationships in the protomylonite and mylonite which indicate dextral movement during ductile shear strain; top of diagram is to the west. d) Sheath fold in mylonitic layering. e) Lafe. stage brittle-ductile compressional shear band (dotted) and folded Cm. Saskatchewan Geological Survey 157

rounded pebble- to cobble-sized quartz and K-feldspar clasts in a gritty feldspathic quartzite matrix, consistent with local derivation from rocks similar to the adjacent augen granite gneiss.

3 rounded pebble- to cobble-sized quartz and K-feldspar clasts in a gritty feldspathic quartzite matrix, consistent with local derivation from rocks similar to the adjacent augen granite gneiss. The contact is tectonically modified and an unconformity cannot be unequivocally documented. Protomylonitic augen gneisses derived from Archean basement protoliths are well exposed at the west end of the portage south of Needle Falls. To the east, along the portage, there is a high strain gradient to mylonite and ultramylonite, clearly derived from the same protolith, over a distance of 40 m. Granitic basement gneisses, metaconglomerates and orthoquartzites of the Wollaston Group have a strong to intense and steeply west-dipping tectonic foliation striking 020 to 035, subparallel to that of the shear zone. A prominent stretching lineation, plunging to the northnortheast at about 30, is defined by elongation of dynamically recrystallized feldspar augen in the basement gneisses, pebble elongation in the metaconglomerates, and by rodding and pressure shadow tails around small pebbles in orthoquartzites. Strain gradation into mylonitic derivatives of the gneisses is marked by matrix grain-size reduction, increased flattening and elongation of dynamically recrystallized potassium feldspar megacrysts and general intensification of both planar and linear fabric elements with little significant change in attitude. In contrast to mylonitic derivatives of the Wathaman Batholith, described below, rotated tailed porphyroclasts are not developed and there is little other clear indication of rotational strain. 3. Kinematic Features of the Shear Zone Westward strain gradation to protomylonitic gneisses of the shear zone from rocks of the Wathaman Batholith is accompanied by regional rotation and steepening of So and S 1 planar fabrics towards parallelism with the shear zone margins. In the outer margins of the shear zone, 'premylonitic' to protomylonitic gneisses generally show little mesoscopic indication of consistent sense of rotational strain, and the foliation is provisionally interpreted as a predominantly 'S' (flattening) fabric. Possible c (shear) plane development related to the shear zone is evident only locally. Passage into inner parts of the protomylonite subzone is, however, marked by a prominent decrease in matrix grain size, increasing incidence of rotational fabric features such as rotated potassium feldspar porphyroclasts with dynamic recrystallization tails, and increasingly clear development of kinematically related 'C' (shear) and oblique 'S' {flattening) foliations. Strain gradation to fine-grained mylonite and ultramylonite subzones is marked by the relatively abrupt development of an intense subvertical to steeply west-dipping kinematic foliation and a mylonitic lamination parallel to, and in part defined by, transposed So banding which strikes consistently at about 025 to 030. A variety of attendant features, including development of asymmetric, rotated and tailed porphyroclasts as well as klnematically related oblique 'S' planes {flattening fabric), desig- nated Sm, indicate that this dominant mylonitic foliation is a 'C' plane (shear plane) fabric, designated Cm. Welldeveloped oblique Cm-parallel So and coeval Sm planar fabrics were observed in several outcrops in the protomylonite-mylon ite transition zone; in the uttramylonite subzone, there is transition to a general Cm-parallel Smparallel So planar fabric. Throughout the mylonite-ultramylonite subzones, a pronounced penetrative lineation (4n) plunges to the north-northeast at about 25. This fabric element is defined by the preferred orientation of finite elongation axes (X-direction) of triaxially deformed porphyroclasts and general mineral streaking, together with rodding and apparent So/Cm intersection Jineation. The 4n lineation is also commonly parallel to hinge lines of tight to isoclinal intrafolial folds, defined by So layering and axial planar to Cm, and in a few outcrops is paralleled by hinge lines of closed sheath folds (Figure 1d). This fabric element is interpreted as a stretching lineation coeval with the main phase of ductile shear strain and thus indicates slightly oblique transcurrent displacement across the shear zone. Kinematic sense-of-movement indicators related to the main phase of ductile shear strain (Figure 1c) tend to be best developed in transitional protomylonitic-mylonitic parts of the shear zone, and include: 1) Angular relation of oblique C-S planar fabrics (Simpson and Schmid, 1983). 2) Dynamically recrystallized and rotated K-feldspar porphyroclasts (Passchier and Simpson, 1986). These include types with symmetrical dynamic recrystallization tails elongated in either Sm or Cm, highly asymmetric <J'-type porphyroclasts with tails elongate in Cm and highly rotated 6-type porphyroclasts with curving 'winged' tails. The last-mentioned provide the most convincing evidence of sense of movement. Given documented orientation of the stretching lineation (lm), such features consistently and unequivocally indicate oblique dextral shear sense, with a subordinate east-side-up vertical displacement component. Varied small-scale features, developed mainly within the mylonitic and ultramylonitic subzones, attest to continued brittle-ductile to late brittle movement along the shear zone subsequent to development of the ductile strain fabrics described above. These include: 1) Steeply plunging small-scale asymmetric folds, deforming So/Cm mylonitic banding, generating from brittle-plastic deco/laments parallel to the latter. Fold vergence indicates predominant dextral movement sense, but conjugate structures and 'back-fold pile-ups' occur locally in some fold bands. 2) Brittle-ductile to brittle compressional shear bands and listric thrusts (Figure 1 e), generally rooting in decoffements and thin zones of cataclasis parallel to prior So/Cm mylonitic banding. These are commonly, though not exclusively, associated with overturned limbs of small-scale asymmetric folds and 158 Summary of Investigations 1988

4 most show dextral movement sense, although subordinate conjugate thrusts with sinistral displacement occur locally. 3) Extensional shear bands displacing So/Cm, again indicating overall dextral movement sense. 4) Asymmetric boudinage of more competent felsic mylonite layers, showing clockwise rotation of boudins between oblique sinistral extensional shear bands. Geometry and sense of movement in such zones is consistent with overall dextral shear (cf. Simpson and Schmid, 1983). 5) A variety of straight, sigmoidal or highly folded and boudinaged quartz-filled tension gashes, apparently developed at different stages in the kinematic history of the shear zone. Some are developed in late brittle breccia zones. Others occur in overturned limbs of north-verging late asymmetric folds; relations seen in a few outcrops suggest that most of the latter were initially developed normal to the principal extension direction during dextral shear and were rotated into their present attitude during progressive brittle-ductile shear strain. 6) Very late small-scale brittle fractures, including both conjugate thrust sets oblique to and offsetting mylonitic layering, and other sets almost normal to mylonitic layering. 7) Thin pseudotachytrte veins within late cataclastic breccia zones and locally cross-cutting mylonitic banding in the walls of such zones. The orientation and character of most of these structures thus indicates continuation of dominantly transcurrent dextral movement throughout brittle-ductile to brittle stages of shear zone development. 4. Preliminary Interpretation of Geometric Relationships Given unequivocal evidence of dextral displacement during all documented stages of development of the Myl..... WAT HAMAN BATHOLITH Figure 2 - Diagrammatic representation of geometric relations, rotation and development of planar and linear structures in the Needle Falls Shear Zone. Horizontal sections through finite strain ellipsoids shown schematically as so/id tjlack ellipses. So is primary layering in Wathaman Batholith; S, is prrj-shear zone tectonic foliation in Wathaman Batholith and Wol/aston Domain (protjably not of the same age). Sm is flattening plane related to shear zone strain. Cm is shear plane fabric related to shear zone strain. L 1 is pffj-shear zone lineation and fold hinge lines in the Wathaman Batholith. L m is stretching direction related to shear zone strain. Saskatchewan Geological Survey 159

5 Needle Falls Shear Zone, the geometric and age relationships of planar and linear fabric components external to and within the zone are provisionally interpreted as follows (Figure 2): 1) So and 81 in rocks of the Wathaman Batholith to the east of the Needle Falls Shear Zone predate main ductile movement along the shear zone. 2) T awards the shear zone, So and S1 were both initially rotated towards the incipient Cm plane by dominant pure shear flattening rather than by simple shear rotation - that is, they were rotated towards the flattening plane of the progressive strain ellipsoid, which in the outer margins of the shear zone would be oblique to shear zone margins. Before reaching the flattening plane, however, they rotated into the 'C' (slip) direction, at which point a strong Cm fabric typifying the mylonite-ultramylonite subzones developed subparallel to transposed So/S1. It seems likely that both So and S1 may have acted as slip surfaces during this process. This interpretation explains the apparently anomalous sense of rotation of early planar structures, which might otherwise be taken as evidence for sinistral, rather than dextral, displacement. 3) Initially southwest-plunging F1 fold hinge lines and L1 intersection lineation were also progressively rotated and eventually transposed into the northeastplunging Lm stretching direction. Local flow surges within the ultramylonites, oblique to F1 fold hinge lines, produced sheath folds. 5. Magnitude of Displacement No realistic estimate of total displacement across the shear zone is possible on the basis of available data. However, various indirect indicators, including known length of the Needle Falls-Parker Lake zone, widths of protomylonite, mylonite and ultramylonite belts within the shear zone, and preliminary shape fabric analysis of deformed porphyroclasts in various parts of the shear zone, suggest a possible minimum horizontal displacement on the order of 40 to 50 km. True displacement is likely to be considerably greater than this. 6. References Fumerton, S.L., Stauffer, M. R. and Lewry, J.F. (1984): The Wathaman Batholith: largest known Precambrian pluton; Gan. J. Earth Sci., v21, p Lewry, J.F. and Slimmon, W.L. (1985): Compilation bedrock geology, Lac la Ronge, NTS area 73P/731; Sask. Energy Mines, Rep. 225 (1:250,000 scale map with marginal notes). Passchier, C.W. and Simpson, C. (1986): Porphyroclast systems as kinematic indicators; J. Struct. Geel., v8, p Simpson, C. and Schmid, S.M. (1983): AA evaluation of criteria to deduce the sense of movement in sheared rocks; Geel. Soc. Am. Bull., v94, p Money, P.L. (1965): The geology of the area around Needle Falls, Churchill River, Saskatchewan; Sask. Dep. Miner. Resour., Rep. 88, 70p. Munday, R.J.C. (1978): The shield geology of the Oe-a-la Crosse (east) area, Saskatchewan; Sask. Dep. Miner. Resour., Rep. 189, 27p. Wise, D.U., Dunn, D.E. Engelder, J.T., Geiser, P.A., Hatcher, R.D., Kish, S.A., Odom, A.L. and Schamel, S. (1984): Faultrelated rocks: suggestions for terminology; Geology, v12, p Summary of Investigations 1988

Report of Activities 2003 Published by: Manitoba Industry, Economic Development and Mines Manitoba Geological Survey, 2003.

Report of Activities 2003 Published by: Manitoba Industry, Economic Development and Mines Manitoba Geological Survey, 2003. ERRATA: The publisher/department name in the bibliographic reference cited immediately