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Hadlari and Rainbird                                                                                                      ...
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Hadlari and Rainbird                                                                                                      ...
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Hadlari and Rainbird                                                                                                      ...
Hadlari and rainbird 2011 baker lake basin tectonic synthesis
Hadlari and rainbird 2011 baker lake basin tectonic synthesis
Hadlari and rainbird 2011 baker lake basin tectonic synthesis
Hadlari and rainbird 2011 baker lake basin tectonic synthesis
Hadlari and rainbird 2011 baker lake basin tectonic synthesis
Hadlari and rainbird 2011 baker lake basin tectonic synthesis
Hadlari and rainbird 2011 baker lake basin tectonic synthesis
Hadlari and rainbird 2011 baker lake basin tectonic synthesis
Hadlari and rainbird 2011 baker lake basin tectonic synthesis
Hadlari and rainbird 2011 baker lake basin tectonic synthesis
Hadlari and rainbird 2011 baker lake basin tectonic synthesis
Hadlari and rainbird 2011 baker lake basin tectonic synthesis
Hadlari and rainbird 2011 baker lake basin tectonic synthesis
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Hadlari and rainbird 2011 baker lake basin tectonic synthesis

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Basin tectonic synthesis: Paleoproterozoic rift basin formed by retro-arc extension.

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Hadlari and rainbird 2011 baker lake basin tectonic synthesis

  1. 1. 1232 Retro-arc extension and continental rifting: a model for the Paleoproterozoic Baker Lake Basin, Nunavut1 T. Hadlari and R.H. RainbirdCan. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Natural Resources Canada on 08/08/11 Abstract: Within Baker Lake sub-basin, the ca. 1.84–1.78 Ga Baker Sequence formed in two stages. At the start of the first stage, during rift initiation, half-graben were host to siliciclastic alluvial, eolian, and lacustrine deposits and to localized fel- sic minette volcanics. Back-stepping of facies indicate high accommodation rates and areal expansion, which, combined with extrusion of voluminous minette volcanic rocks, are interpreted to record increased extension and rift climax. Low ac- commodation post-rift deposits from the second stage of basin development are relatively thin and coeval felsite domes spa- tially restricted. Volcanic rocks and some siliciclastic units correlate between sub-basins, and hence the interpreted history of Baker Lake sub-basin is extended across greater Baker Lake Basin. This implies that the basin formed in response to re- gional extension and crustal thinning. The Baker Lake Basin marks the northern extent of a series of basins that trend north- eastward along the Snowbird Tectonic Zone, including an inlier of the correlative Martin Group in northern Saskatchewan. The high accommodation first stage of basin development is proposed to have been the result of intra-continental retro-arc extension during ca. 1.85–1.84 Ga formation of the Kisseynew back-arc basin of the Trans-Hudson Orogen. Upon closure of the Kisseynew back-arc basin and collision of the Superior Province with the western Churchill Province, Baker Lake Ba- sin was subject to strike-slip faulting. The second, low accommodation stage of basin development and strike-slip faulting is proposed to record lateral tectonic escape between the Saskatchewan–Manitoba and Baffin Island – Committee Bay foci of the western Churchill – Superior Province collision. For personal use only. Résumé : À l’intérieur du sous-bassin du lac Baker, la séquence Baker, ∼1,84–1,78 Ga, s’est formée en deux étapes. Au début de la première étape, durant l’initiation de la distension, des demi-grabens ont reçu des sédiments silicoclastiques allu- viaux, éoliens et lacustres en plus de minettes volcaniques felsiques localisées. Une rétrogradation des faciès indique de forts taux d’accommodation et d’expansion en surface et, combinée à l’extrusion de grands volumes de roches volcaniques minet- tes, cela est interprété comme étant un enregistrement de grande extension et de sommet de la distension. Les dépôts de faible accommodation après la distension de la seconde étape de développement du bassin sont relativement minces et les dômes de roches felsiques contemporaines sont restreints dans l’espace. Des roches volcaniques et quelques unités silicoclas- tiques montrent un rapport entre les sous-bassins et l’historique interprété du sous-bassin du lac Baker est donc appliqué à toute la grande région du bassin du lac Baker. Cela signifie que le bassin s’est formé en réaction à l’extension régionale et à l’amincissement de la croûte. Le bassin du lac Baker marque l’étendue nord d’une série de bassins à tendance nord-ouest le long de la zone tectonique Snowbird et il comprend une enclave du Groupe de Martin (nord de la Saskatchewan) qui lui est corrélée. La première étape de développement de bassin, à niveau élevé d’accommodation, se serait développée en raison de l’extension rétro-arc entre les continents au cours de la formation du bassin d’arrière-arc Kisseynew durant l’orogène trans- hudsonien il y a ∼1,85–1,84 Ga. À la fermeture du bassin d’arrière-arc Kisseynew et lors de la collusion de la province du Supérieur avec l’ouest de la province de Churchill, le bassin du lac Baker a subi des mouvements de coulissage. La seconde étape de mouvements de coulissage et de développement de bassin à faible niveau d’accommodation aurait enregistré l’é- chappement tectonique latéral entre les foyers Saskatchewan–Manitoba et île de Baffin – baie Committee de la collision en- tre l’ouest de la province de Churchill et la province du Supérieur. [Traduit par la Rédaction] Introduction Received 15 June 2010. Accepted 10 January 2011. Published at Deciphering the tectonic history of the western Churchill www.nrcresearchpress.com/cjes on 04 August 2011. Province has been complicated by Paleoproterozoic, broadly Hudsonian-aged, “reworking” of crustal-scale elements that T. Hadlari.* Carleton University, Ottawa, ON, Canada. appear to have originated in the Archean (Davis et al. 2004, R.H. Rainbird. Geological Survey of Canada, 615 Booth Street, 2006; Hanmer et al. 2006). Strata of the ca. 1.84–1.75 Ga Ottawa, ON, Canada. Baker Lake Basin (Rainbird et al. 2003) unconformably over- Corresponding author: Thomas Hadlari (e-mail: lie the Snowbird Tectonic Zone, which is interpreted as the thomas.hadlari@nrcan-rncan.gc.ca). boundary between two Archean cratons (Hoffman 1988). In 1Geological Survey of Canada Contribution 20100436. many ways, the record of Baker Lake Basin is intimately *Current affiliation: Geological Survey of Canada, 3303 - 33rd linked the Paleoproterozoic reworking of the western Church- Street NW, Calgary, AB T2L 2A7, Canada.. ill Province; for example, it is host to the most voluminous Can. J. Earth Sci. 48: 1232–1258 (2011) doi:10.1139/E11-002 Published by NRC Research Press
  2. 2. Hadlari and Rainbird 1233 ultrapotassic volcanic province in the world (LeCheminant et (ca. 2.62–2.6 Ga; e.g., Aspler et al. 2002); (3) high-grade al. 1987; Peterson et al. 1994) and overlaps temporally with metamorphism at ca. 2.56–2.5 Ga (Berman et al. 2002a); the regional ca. 1.85–1.81 Ga Hudson granitoid suite em- and (4) subsequent high-grade metamorphism at ca. 1.9 Ga placed at lower crustal levels (Peterson et al. 2002). Faults (e.g., Sanborn-Barrie et al. 2001). The western Churchill with significant offset during Baker Sequence time have Province is bounded to the west by the 2.02–1.91 Ga The- been identified regionally (e.g., the Tyrrell shear zone, lon–Taltson Orogen and to the southeast by the 1.9–1.8 Ga MacLachlan et al. 2005b). Magnetotelluric data have illumi- Trans-Hudson Orogen (Hoffman 1988; Wheeler et al. 1997). nated the conductive structure of the Rae–Hearne crust, pro- The Rae domain is differentiated from the Hearne domains viding estimates on the depth of the Moho in the vicinity of by the presence of komatiite–quartzite supracrustal succes- Baker Lake Basin (Jones et al. 2002). A Moho depth of 36– sions that extend from Baffin Island to north of Baker Lake. 40 km allows for zero net crustal thickening between meta- In the Rae domain northwest of Baker Lake, older than 2.8Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Natural Resources Canada on 08/08/11 morphism of Kramanituar Complex at pressures of 12–15 Ga basement is unconformably overlain by the Woodburn kbar (1 kbar = 100 MPa; ∼36–45 km depth) at ∼1.9 Ga Lake Group, comprising a 2.735–2.710 Ga komatiite–quartzite (Sanborn-Barrie et al. 2001) and deposition of Thelon For- succession and ca. 2.630 Ga felsic volcanic and terrigenous mation at ca. 1.67 Ga (Davis et al. 2011), yet Barrovian sedimentary rocks (Davis and Zaleski 1998; Zaleski et al. metamorphism has been documented in the “high pressure 1999, 2000; Zaleski and Davis 2002; Fig. 1). corridor” of the northwestern Hearne subdomain and the Ryan et al. (2000) proposed that the Big Lake Shear Zone southern Rae domain between ca. 1.89 and 1.85 Ga (Berman south of Chesterfield Inlet (Fig. 1) represents the northern et al. 2002b, 2005). Those relations imply that significant limit of the Rae–Hearne boundary. This structure formed at crustal thinning occurred between ca. 1.85 and 1.67 Ga. Was ca. 2.5 Ga and was re-activated at 1.9 Ga (Ryan et al. 2000; extension related to the Baker Lake Basin part of this crustal Hanmer et al. 2006). The northeastern extension of the STZ thinning process? is, however, generally indicated to coincide with Chesterfield Herein, we present new fault data that test hypotheses pro- Inlet, usually north of the metamorphic core complexes. Ex- posed by Rainbird et al. (2003) for the structure of Baker tending eastward from the eastern shore of Baker Lake, gran- Lake Basin, and then explore relations between stratigraphic, ulite-facies Kramanituar Complex comprises ca. 1.9 Ga magmatic (e.g., Cousens et al. 2001; Peterson et al. 2002), gabbro, anorthosite, and granitoids (Sanborn-Barrie et al. For personal use only. and structural evolution of Baker Lake Basin. The most com- 2001). Eastward along Chesterfield Inlet, the Uvauk Com- prehensive review of Baker Lake Basin was presented by plex (UCX; Fig. 1) is a similar ca. 1.9 Ga granulite-facies Rainbird et al. (2003). We review more recently published metamorphic complex, but also contains ca. 2.6 Ga anortho- sedimentology (Hadlari et al. 2006), stratigraphy (Hadlari site (Mills 2001; Mills et al. 2007). and Rainbird 2006), and geochronology (Rainbird et al. Southwest of Baker Lake – Chesterfield Inlet, at Angikuni 2006; Rainbird and Davis 2007), and by incorporating results Lake, the Rae–Hearne boundary is represented by the Tule- of previous studies of Baker Lake Basin (e.g., LeCheminant malu fault zone (Tella and Eade 1986; Fig. 1). Angikuni et al. 1981; Peterson et al. 1989; Peterson and Rainbird 1990; Lake area was the site of ca. 2.62–2.61 Ga syntectonic granite Rainbird and Peterson 1990), we propose a new integrated emplacement (Aspler et al. 2002) and 2.56–2.5 Ga high- model of basin evolution. grade metamorphism (Berman et al. 2002a). Monazite in- A new understanding of Baker Lake Basin allows for com- clusions in garnet indicate that 2.56–2.5 Ga and 1.9 Ga parison with time-equivalent events at the scale of the west- high-grade metamorphism and associated deformation ex- ern Churchill Province, that have been documented as tend from Chesterfield Inlet along the trend of the STZ to largely as a result of the western Churchill NATMAP (Na- Angikuni Lake, defining part of a high-pressure corridor tional Geoscience Mapping Program) Project (e.g., Aspler et (Berman et al. 2002a). al. 2001, 2002, 2004; Cousens et al. 2001, 2004a, 2004b; The southwest segment of the STZ, which extends into Berman et al. 2002a, 2002b, 2007; Peterson et al. 2002; Da- Saskatchewan, is represented by crustal-scale shear zones vis et al. 2004, 2005, 2006; Hanmer et al. 2004, 2006; San- that have multistage histories. The oldest deformation is de- deman et al. 2004, 2006; and MacLachlan et al. 2005a). In fined by ca. 2.63–2.6 Ga syntectonic granites (Hanmer et al. focusing on ca. 1.85–1.78 Ga events, the new Baker Lake 1994; Hanmer et al. 1995) and ca. 2.5 Ga granulite-facies Basin model within the context of the western Churchill metamorphism (Mahan et al. 2008), followed by 1.9 Ga Province is combined with tectonic models of the Trans-Hudson mafic intrusions and granulite-facies metamorphism (Baldwin Orogen (e.g., Bickford et al. 1990; Lewry et al. 1994; Ans- et al. 2003; Mahan et al. 2003; Martel et al. 2008; Mahan et dell et al. 1995; Corrigan et al. 2005; Ansdell 2005) to al. 2008). Overlying Rae Province crust west of the Striding present a new tectonic synthesis, particularly for the initia- Athabasca Mylonite Zone, the ca. 1.83–1.82 Ga Martin tion of Baker Lake Basin. Group is considered correlative to the Baker Lake Group (Ashton et al. 2009). The STZ, therefore, appears to have a similar history along its length, from the Kramanituar Com- Regional geology: western Churchill Province plex to the Striding Athabasca Mylonite Zone. The Archean western Churchill Province comprises the From findings of the western Churchill NATMAP project, Rae and Hearne domains (Davis et al. 2000) (Fig. 1), referred the Hearne domain has been subdivided into northwestern, to by Hoffman (1988) as the Rae and Hearne provinces, central, and southern Hearne subdomains (Davis et al. 2000; which are separated by the Snowbird Tectonic Zone. The Hanmer and Relf 2000). Although rocks of the northwestern Rae–Hearne boundary zone is characterized by (1) contrast- Hearne subdomain are isotopically juvenile, rocks from An- ing Archean supracrustal rocks; (2) syntectonic granitoids gikuni Lake, MacQuoid Lake, and Rankin Inlet record inter- Published by NRC Research Press
  3. 3. 1234 Fig. 1. Geology of the western Canadian Shield, including Archean Slave, western Churchill (Rae–Hearne), and Superior provinces (Wheeler et al. 1997; Paul et al. 2002; Tella et al. 2007; Ashton et al. 2009).Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Natural Resources Canada on 08/08/11 For personal use only. Can. J. Earth Sci., Vol. 48, 2011 Published by NRC Research Press
  4. 4. Hadlari and Rainbird 1235 action with older crust (Sandeman, et al. 2000, 2006). This volcanic rocks (e.g., Peterson et al. 2002) that were deposited includes Nd isotopic evidence for older sources (>2.85 Ga; between ca. 1.84–1.78 Ga (Rainbird et al. 2006, Rainbird and Sandeman 2001; Sandeman et al. 2006), and old detrital zir- Davis 2007) contemporaneous with intrusion of Hudson cons from the MacQuoid Lake area (up to 3.4 Ga; Davis et granitoids and Martel syenites (Peterson et al. 2002). Rhyo- al. 2000). Thus, a back-arc setting has been proposed for the lites of the Wharton Group, or Whart Sequence (Rainbird et Archean supracrustal rocks of the northwestern Hearne sub- al. 2003), were coeval with the ca. 1.76–1.75 Ga Nueltin domain (Sandeman et al. 2006). The northwestern Hearne suite of rapakivi granites (Peterson et al. 2002). Rocks of the and southeastern Rae margins share ca. 2.6 Ga granites (e.g., greater Baker Lake Basin are crosscut by normal faults and a LeCheminant and Roddick 1991) and high-pressure meta- conjugate set of strike-slip faults (Peterson et al. 1989; Ha- morphism at ca. 2.5 Ga and 1.9 Ga (Berman et al. 2002a, dlari and Rainbird 2001; Rainbird et al. 2003; Peterson 2005). Tectonic assembly of the Rae and Hearne provinces 2006). Principal offset on the brittle faults occurred beforeCan. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Natural Resources Canada on 08/08/11 is considered to have resulted in 2.5 Ga metamorphism and deposition of the Thelon Formation, originally reported as deformation (Davis et al. 2000), and subsequent reworking ca. 1.720 Ga (Miller et al. 1989) and subsequently redefined after ca. 1.9 Ga has further modified this boundary (Davis et as ca. 1.667 Ga (Davis et al. 2011). al. 2000; Ryan et al. 2000; Sanborn-Barrie et al. 2001; Ma- cLachlan et al. 2005a). Review of greater Baker Lake Basin The boundary between the northwestern and central Hearne subdomains is the Tyrrell shear zone (MacLachlan et General stratigraphy of greater Baker Lake Basin al. 2005a, 2005b; Fig. 1). Earliest deformation along the Tyr- The Dubawnt Supergroup (Wright 1967; Donaldson 1965, rell shear zone is synchronous with emplacement of ca. 2.66– 1966, 1967; Gall et al. 1992) is sub-divided into three uncon- 2.62 Ga granites (MacLachlan et al. 2005b), and youngest re- formity-bounded groups (Fig. 3). The ca. 1840–1785 Ma activation took place at ca. 1.83–1.81 Ga (MacLachlan et al. Baker Lake Group is characterized by arkosic sandstone, pol- 2005a). In the footwall of the Tyrrell shear zone, ca. 1.82 Ga ymictic conglomerate, and alkaline flows and volcaniclastic garnet breakdown in the Nowyak metamorphic complex has rocks (Rainbird and Davis 2007). The Wharton Group is been attributed to Hudson granitoid plutonism and exhuma- characterized by sub-arkose and volcaniclastic sandstone, vol- tion (Ter Meer 2001). caniclastic conglomerate, and ca. 1758–1753 Ma rhyolite For personal use only. The central Hearne subdomain comprises the Tavani and flows (Rainbird and Davis 2007). The Barrensland Group is Kaminak greenstone belts interpreted as intra-oceanic arc represented in the Baker Lake Basin by quartz arenite of the rocks (Cousens et al. 2004a; Davis et al. 2004; Hanmer et Thelon Formation, the overlying Kuungmi and Lookout Point al. 2004; Sandeman et al. 2004) and is typified by isotopi- formations are found within Thelon Basin. Rainbird et al. cally juvenile 2.71–2.68 Ga supracrustal rocks and 2.7– (2003) provided a sequence stratigraphic framework for the 2.65 Ga calc-alkaline granitoid rocks (Davis et al. 2000, Dubawnt Supergroup in which the three groups are equiva- 2004; Hanmer et al. 2004; and Sandeman et al. 2004). lent to the Baker, Whart, and Barrens second-order sequen- Little is known of the southern Hearne subdomain, which ces(Fig. 3). They correspond to the tectonic stages of rift has yielded older U/Pb ages than the central Hearne subdo- associated with ultrapotassic volcanism, a distinct second rift main, particularly where there is basement as old as ca. phase associated with rhyolite volcanism, and thermal relaxa- 3.0 Ga to the Wollaston Group (Bickford et al. 1994), which tion of an intracontinental basin, respectively (Rainbird et al. is a rift to foreland basin succession related to the Trans- 2003). In consideration of the genetic nature of our discus- Hudson Orogen (e.g., Tran et al. 2003). The ca. 3.0 Ga base- sion, we use the sequence stratigraphic framework henceforth. ment of southern Hearne subdomain hints at involvement of Previous detailed studies of the Baker Sequence have fo- an older crustal block, on the south side of the more juvenile cused on volcanology and igneous petrology (e.g., Blake central Hearne subdomain, during assembly of the western 1980; LeCheminant and Heaman 1989; Peterson and LeChe- Churchill Province. minant 1993). Alkaline volcanic and volcaniclastic rocks of At ca. 1.85–1.81 Ga, the Hudson granitoid suite, ranging the Baker Sequence compose the Christopher Island Forma- in composition from monzonite to granite, were intruded tion (Donaldson 1965, 1967). Flows of the Christopher throughout the western Churchill Province (Peterson et al. Island Formation are characterized as porphyritic clinopyrox- 2002). Minimum melt compositions and Nd isotopes indicate ene–phlogopite trachyandesites and K-feldspar-phyric tra- Hudson grantoids were likely derived by melting of late Ar- chytes (LeCheminant and Heaman 1989), and as minette and chean crust (van Breemen et al. 2005). Hudson granitoids felsic minette flows, respectively (Peterson and Rainbird overlap in time with alkaline volcanic rocks of Baker Lake 1990). Minettes are potassic, calc-alkaline, phlogopite + cli- Basin and have locally co-mingled with lamprophyre and re- nopyroxene-phyric lamprophyres (Rock 1984, 1987, 1991). lated spessartite intrusions (Sandeman et al. 2000). Christopher Island Formation minette flows are ultrapotassic, The 1.84–1.67 Ga greater Baker Lake Basin overlies the strongly enriched in light rare-earth elements, and contain northeastern extent of the STZ, unconformably overlying Tu- high abundances of incompatible elements (LeCheminant lemalu fault zone, Kramanituar Complex, and the Chester- and Heaman 1989). They are considered to have been vola- field Fault Zone (Fig. 1). The greater Baker Lake Basin is tile-rich mafic alkaline melts of metasomatized, subcontinen- informally sub-divided into the Kamilukuak, Dubawnt, Whar- tal lithospheric mantle (LeCheminant et al. 1987; Peterson ton, Angikuni, and Baker Lake sub-basins (Fig. 2; Rainbird and LeCheminant 1993; Cousens et al. 2001; Peterson et al. et al. 2003). The Baker Lake Group (Donaldson 1965; Gall 2002). Flows that contain K-feldspar phenocrysts are not et al. 1992), or Baker Sequence (Rainbird et al. 2003), is a minettes sensu stricto, but they are commonly associated continental siliciclastic succession containing ultrapotassic with minettes elsewhere and in the literature have been infor- Published by NRC Research Press
  5. 5. 1236Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Natural Resources Canada on 08/08/11 Fig. 2. Geology of the greater Baker Lake Basin indicating the distribution of sub-basins (modified from Rainbird et al. 2003). Dubawnt sub-basin Christopher Island Formation (CIF) dyke trends from Peterson (2006) and Angikuni sub-basin dyke trends from Aspler et al. (2004). For personal use only. Can. J. Earth Sci., Vol. 48, 2011 Published by NRC Research Press
  6. 6. Hadlari and Rainbird 1237 Fig. 3. Litho- and sequence stratigraphy of Baker Lake Basin (Donaldson 1965; Gall et al. 1992; Rainbird and Hadlari 2000; Rainbird et al. 2003). Geochronology sources: Thelon Formation (Fm.), 1667 ± 5 Ma (Davis et al. 2011); Pitz Fm., Rainbird and Davis (2007); and Baker Lake Group, Rainbird et al. (2006).Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Natural Resources Canada on 08/08/11 For personal use only. mally designated “felsic minettes” (Rock 1987, 1991; Peter- from a lamprophyre dyke southeast of Baker Lake Basin that son and Rainbird 1990; Davis et al. 1996; Feldstein and is considered to be co-magmatic with Baker Sequence vol- Lange 1999). canic rocks (Pb–Pb apatite; MacRae et al. 1996). A lower A generalized volcanic succession for the Christopher Is- age limit for the Baker second-order sequence is delimited land Formation is basal felsic minette (restricted distribution, by 1785 ± 3 Ma, derived from laminated carbonate cements up to 200 m thick), minette (extensive distribution, locally interpreted as travertine, from alluvial deposits of Kamilu- over 500 m thick), and upper felsite flows (localized domes) kuak sub-basin (Pb–Pb isochron from calcite; Rainbird et al. (Peterson and Rainbird 1990; Hadlari and Rainbird 2001; 2006). Based upon the data and discussion in Rainbird et al. Hadlari 2005). Peterson et al. (2002) consider the lower felsic (2006), we refer to the approximate age of the Baker Se- minette flows of Christopher Island Formation to be crustally quence as ca. 1840–1785 Ma. contaminated minette-equivalents. Minette flows represent primary lithospheric mantle melts, and the upper felsite flows Baker Sequence: sedimentary strata of Baker Lake sub-basin are interpreted as differentiates of minette magmas (Peterson In Baker Lake sub-basin (Fig. 4), the Baker second-order et al. 2002; Peterson 2006). Felsite flows locally display flow sequence comprises five third-order sequences and a correla- banding, autoclastic breccia, and sub-centimetre K-feldspar tive tripartite volcanic succession (Fig. 5; Rainbird et al. phenocrysts. This tripartite volcanic subdivision provides the 2003; Hadlari 2005; Hadlari and Rainbird 2006). Third-order basis for correlation across greater Baker Lake Basin and is sequences are interpreted as pulses of accommodation in re- supported by geochronology (Rainbird et al. 2006). Analyses sponse to basin-margin normal faulting and subsequent infill of phlogopite phenocrysts from a minette flow at the base of by the sedimentary system (Hadlari and Rainbird 2006). the Baker Sequence, and a syenite intrusion that intrudes the Across the axis of Baker Lake sub-basin, from Aniguq River lower Baker Sequence, yield 40Ar/39Ar ages of 1845 ± to Thirty Mile Lake, the thickness of the Baker Sequence in- 12 Ma and 1810 ± 11 Ma, respectively (see discussion Rain- creases from ∼500 m to over 2500 m (Fig. 5). Accordingly, bird et al. 2006). A more precise U–Pb zircon age of 1833 ± thicknesses of 3rd-order sequences increase from ∼100– 3 Ma has been obtained from a felsic minette flow from the 150 m to up to 500 m. It is important that 3rd -order sequen- Kamilukuak sub-basin (Rainbird et al. 2006). This age is ces are scaled relative to the total thickness of the Baker within error of an 40Ar/39Ar age of 1837 ± 8 Ma obtained Sequence, because these thicknesses, therefore, reflect differ- from a minette tuff at Aniguq River (Rainbird et al. 2006) ential accommodation and basin asymmetry rather than sub- and is consistent with an age of 1832 ± 28 Ma obtained sequent erosion (Hadlari and Rainbird 2006). Published by NRC Research Press
  7. 7. 1238 Fig. 4. (a) Geology map of Baker Lake sub-basin, showing paleocurrent data derived from cross-set and primary current lineation measurements (modified from Rainbird et al. 2003; Hadlari et al. 2006). (b) North–south schematic cross-section (a–a′) of Baker Lake sub-basin modified after Rainbird et al. (2003).Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Natural Resources Canada on 08/08/11 For personal use only. Can. J. Earth Sci., Vol. 48, 2011 Published by NRC Research Press
  8. 8. Hadlari and Rainbird 1239 Fig. 5. North–south stratigraphic cross-section of Baker Sequence from Baker Lake sub-basin, from Aniguq River to Thirty Mile Lake (Ha- dlari and Rainbird 2006). Sedimentary succession is labeled after third-order depositional sequences (see Fig. 3, after Hadlari and Rainbird 2006). Sedimentary and complementary volcanic-dominated sections are correlated based upon clast lithologies and interfingering relation- ships. Note that the Kunwak River outcrop contains felsite clasts, interpreted to be derived from the youngest volcanic rocks at Thirty Mile Lake (Hadlari and Rainbird 2001; Hadlari 2005).Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Natural Resources Canada on 08/08/11 For personal use only. Basin asymmetry is consistent with the distribution of fa- the basin scale, accommodation (A) > sediment flux (SF). cies — thick alluvial fan deposits near the southeast margin, The progradation of facies at the top of the Baker Sequence braided fluvial deposits toward the centre, and fluvial, flood- indicates that A < SF. The entire succession represents early plain, eolian, and lacustrine deposits near a depocentre at high subsidence followed by low subsidence and sedimentary Christopher Island (Hadlari et al. 2006; Fig. 6). Paleocurrent and volcanic infilling of the basin (Hadlari and Rainbird data from basin margins shown in Fig. 4 were measured from 2006). the full Baker Sequence, indicating that drainage was consis- tently directed first into the basin and then along an axis to- Baker Sequence: sedimentary and volcanic correlation, ward an inferred depocentre. Collectively, the facies, Baker Lake sub-basin paleocurrents, and stratigraphy are thus indicative of a north- Relationships between the sequence stratigraphy and the east–southwest-trending half-graben with a hinged margin to tripartite volcanic stratigraphy are made on the basis of clast the northwest and a master fault approximately parallel to the lithologies and interfingering of sedimentary and volcanic southeastern basin margin (Hadlari et al. 2006). Although units (Fig. 5). During rift initiation, felsic minette flows and this fault is not exposed within the basin, Ryan et al. (2000) equivalent volcaniclastic rocks have a limited geographic ex- identified a northeast–southwest-trending, post-1.9 Ga brittle tent, locally overlying the unconformity at the base of the fault southeast of Baker Lake sub-basin that is crosscut by Baker Sequence (Blake 1980; Hadlari and Rainbird 2001). the post-Baker Sequence South Channel Fault (Blake 1980) Sequences B-1 and B-2 are correlated with felsic minette and postulated that it could be the main normal fault that flows based on interbedding of flows and conglomerates at bounded the Baker Lake sub-basin. Thirty Mile Lake. In addition to numerous felsic minette In the Thirty Mile Lake area, an almost complete compo- clasts, the occurrence of a few minette clasts in conglomer- site section of the Baker Sequence yields a thickness of ates at the base of sequence B-3 suggest that minette volcan- ∼2.5–3.0 km (Fig. 5). The lower succession of 3rd-order se- ism initiated at localized volcanic centres at a time close to quences shows a retrogradation of facies, indicating that at the B-2 – B-3 sequence boundary. At Thirty Mile Lake, the Published by NRC Research Press
  9. 9. 1240 Can. J. Earth Sci., Vol. 48, 2011 Fig. 6. Block diagram of Baker Lake sub-basin half-graben and facies tracts during deposition of the Baker Sequence (from Hadlari et al. 2006). Thick alluvial fan deposits are found along the southeastern basin margin. Transverse braided streams feed a central drainage oriented parallel to the basin axis; the downstream culmination is a lacustrine depocentre.Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Natural Resources Canada on 08/08/11 For personal use only. B-2 – B-3 sequence boundary marks a change in facies from ited geochronology suggests that this is a temporal correla- alluvial fan to gravel-bed braided stream (Hadlari and Rain- tion (Rainbird et al. 2006). bird 2006; Fig. 5). This is consistent with back-stepping of Strike-slip basins grow laterally, leading to diachronous the master fault system, resulting in widening of the basin sedimentation along the basin axis and anomalously thick, and a retreat of alluvial fan facies, a common attribute of nor- laterally stacked stratigraphic successions (e.g., Aspler and mal fault systems (e.g., Morley 2002). The retreat of alluvial Donaldson 1985). The correlation of sedimentary sequences facies, or retrogradation, occurred at the same time as the between the Baker Lake and Angikuni sub-basins, together transition from felsic minette to minette volcanism, and near with contemporaneity of volcanic facies across the greater the beginning of deposition at Aniguq River (Fig. 5). Minette Baker Lake Basin, supports the interpretation that Baker flows overlie part of sequence B-4 at western Thirty Mile Lake Basin formed as an overall extensional or trans-ten- Lake and Nutarawit Lake (Fig. 7), and so by sequence, B-4 sional basin rather than a strike-slip basin. minette volcanism had increased in volume to blanket most of Baker Lake Basin. Representing a low accommodation Structural geology of greater Baker Lake Basin post-rift stage, the occurrence of felsite clasts within con- Strata of greater Baker Lake Basin are crosscut by sets of glomeratic deposits of sequence B-5 is correlated to the multiply reactivated brittle faults, where principal displace- youngest volcanic sub-division of felsite, or fractionated ment probably postdated the Baker and Whart sequences and minette, flows. predated the Barrens Sequence (Rainbird et al. 2003). Ad- dressing the earliest stage of faulting, Aspler et al. (2004) Baker Sequence: greater Baker Lake Basin suggest that northeast-trending lamprophyre dykes parallel to Sequences B-1 to B-4 are correlated from the Angikuni to the Angikuni sub-basin margin indicate that Baker Lake Ba- Baker Lake sub-basins (Fig. 7), consistent with the proposi- sin formed because of overall extension. Two dyke trends, tion by Aspler et al. (2004) that Baker Sequence rocks from primarily east–west and also northeast–southwest, have been these basins are tectonostratigraphic equivalents. The tripar- cited to suggest that the Dubawnt sub-basin formed because tite volcanic succession extends across greater Baker Lake of north–south extension (Peterson 2006). In addition, Kami- Basin from Kamilukuak to Baker Lake sub-basins, and lim- lukuak sub-basin contains a set of dilational faults with ex- Published by NRC Research Press
  10. 10. Hadlari and Rainbird Fig. 7. Sections of sequences B-1 to B-4 from the Thirty Mile Lake area correlated to a section from Nutarawit Lake (see Fig. 2) linking the Baker Lake and Angikuni sub-basins (Rain-Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Natural Resources Canada on 08/08/11 bird et al. 2003). F, mud; S, sand; G, gravel. For personal use only. Published by NRC Research Press 1241
  11. 11. 1242 Can. J. Earth Sci., Vol. 48, 2011 tensive quartz stockwork, interpreted as normal faults, that Quartz stockwork are spatially associated with Baker Sequence basin margins, Linear quartz vein breccia zones, up to 1 m wide, trending but not demonstrated to be syndepositional (Peterson 2006). northwest (∼340°) transect Christopher Island Formation vol- Regional map-scale faults within greater Baker Lake Basin canic rocks (Fig. 8a), parallel to a regional set of northwest- are subdivided into those interpreted as normal and strike-slip trending faults. Minette lithons within the stockwork provide based upon offset of mapped stratigraphy (e.g., Rainbird et a maximum age for dilation and breccia formation. The mini- al. 2003; Peterson 2006). For example, within Kamilukuak mum age of the breccia is determined by the presence of sub-basin strata are north-facing with bedding attitudes dip- quartz stockwork clasts containing minette lithons within ping between 35° and 75° (Peterson 2006). Including the Baker Sequence (B-5) conglomerate at Kunwak River basal unconformity, stratigraphy is repeated across east– (Fig. 8b). The breccia zones crosscut minette flows, are in- west-trending faults that are interpreted as south-side-down cluded as clasts within conglomerate of B-5, and are, there-Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Natural Resources Canada on 08/08/11 normal faults (Peterson 2006). Kamilukuak and Dubawnt fore, considered to be syndepositional, basin-transverse sub-basins also contain a conjugate set of northwest-trending dilational faults. Peterson (2006) speculated that dilational faults and fractures with sinistral offset and north- and north- faults with quartz stockwork in Kamilukuak sub-basin were east-trending map-scale faults with dextral offset (Peterson syndepositional, and our data show that equivalent faults in 2006). Baker Lake sub-basin were in fact syndepositional. Baker Lake sub-basin contains strata of Baker and Whart sequences that dip from over 75° at basin margins to subhor- Normal faults (Figs. 9–11) izontal in central areas (Rainbird et al. 2003; Hadlari et al. The best example of an exposed normal fault is located 2004). Based upon step-wise changes in dip, it has been pro- west of Pitz Lake. There, an east–west-trending, north-dipping posed that blocks bounded and rotated by approximately east- fault is developed within Pitz Formation rhyolite (Fig. 10a). northeast-trending normal faults are responsible for the distri- Fault planes exhibit slickenside lineations that indicate a bution of bedding attitudes (Hadlari and Rainbird 2001; dip-slip, north-side-down, normal sense of displacement Rainbird et al. 2003; Fig. 4b). In addition to east–west trends, (Fig. 10a). This observation supports the proposition by normal faults have also been observed with north–south Rainbird et al. (2003), based upon south-dipping bedding trends, for example on the eastern shore of Baker Lake attitudes and repetition of strata, that the overall structure For personal use only. (Rainbird et al. 1999), indicating an overall extensional fault of northern Baker Lake sub-basin is determined by an regime. Within fault blocks Baker and Whart sequence strata east–west-trending set of normal faults and rotated fault have identical bedding attitudes, indicating that block rotation blocks. postdated the Whart Sequence. Fault blocks are crosscut by a North of Thirty Mile Lake, a fault that trends east–west, conjugate set of northwest-trending faults with dextral offset and dips southward, is associated with subparallel fractures and northeast trending faults with sinistral offset (Rainbird et (Fig. 9). Fault breccia zones had developed; however, no al. 2003). In contrast to those crosscutting relations, changes slickenside lineations were measured, and so the interpreta- in thickness of Whart Sequence strata across northwest-trending tion of normal displacement is based upon the repetition of faults suggest that a component of offset was syndeposi- stratigraphy, change in bedding, and south-dipping attitude of tional (Rainbird et al. 2003). Thelon Formation (Barrens the fault and associated fractures. The orientation of this Sequence) bedding is ubiquitously subhorizontal. Although fault is consistent with the interpretation for southern Baker it is locally crosscut by the main fault sets, offset is minor Lake sub-basin of fault blocks rotated by south-dipping nor- (Rainbird et al. 2003). mal faults (Hadlari and Rainbird 2001; Rainbird et al. 2003; In summary, the Baker Lake Basin is crosscut by three e.g., Fig. 4b). main fault trends with complex crosscutting and stratigraphic At eastern Baker Lake, Baker Sequence conglomerate is relationships. There are indications that at least some of the juxtaposed against anorthosite of the Kramanituar Complex faults were active syndepositionally with both Baker and by an east–west-trending normal fault (Rainbird et al. 1999). Whart sequences, that major displacements occurred after This fault dips south with a normal sense of displacement Whart Sequence, but prior to Barrens Sequence deposition, (Fig. 10b). and that relatively minor reactivation postdated the Barrens Sequence. Strike-slip faults Map-scale strike-slip faults occur at Aniguq River and Results: Baker Lake Basin faults and Christopher Island – South Channel (Fig. 4). The course of Aniguq River follows a northwest-trending fault mapped fractures with ∼1.3 km of dextral offset (Rainbird et al. 2003; Fig. 4). From Dubawnt Lake to Baker Lake, strata of greater Baker Along this fault zone are multiple northwest-trending fault Lake Basin are crosscut by two sets of map-scale faults (Pe- breccia zones generally <20 cm, but up to 80 cm thick. As- terson and Rainbird 1990; Hadlari and Rainbird 2001; Rain- sociated with the fault zone, two slickenside planes trending bird et al. 2003): (1) a set of normal faults that trend northwest–southeast and north–south both have north-plunging approximately east–west; and (2) a conjugate set of strike- (24°–25°) slickenside lineations (Fig. 4). The mapped offset slip faults that trend northwest (∼340°) and northeast of strata is dextral (Fig. 4) and based on slickenside meas- (∼040°). In almost all outcrops fault sets occur together, for urements the sense of displacement is oblique, which, given example, if one set forms the dominant outcrop feature, then the south-facing, ∼40° dip of Baker and Whart sequence the other sets are represented by decimetre to metre-scale off- strata, would magnify the amount of horizontal offset by sets or closely spaced fractures. ∼30%. Published by NRC Research Press
  12. 12. Hadlari and Rainbird 1243 Fig. 8. Strike-slip faults: (a) quartz stockwork in syn-Christopher Island Formation dilational fault; (b) quartz stockwork clast removed from conglomerate of sequence B-5; and (c) strike-slip fault of the 040° set, 80 cm pole on the fault line for scale.Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Natural Resources Canada on 08/08/11 For personal use only. The most prominent northwest-trending fault is South within Baker Lake sub-basin than northwest-trending faults. Channel Fault (Figs. 4, 11), across which the unconformity The most common exposures are of small outcrop-scale at the base of the Baker Sequence is offset by 10 km (Schau faults with sinistral offsets of tens of centimetres up to a few and Hulbert 1977). Fault breccia and slickensides were iden- metres (e.g., Christopher Island). Exceptions are the south- tified on the southern shore of South Channel, along the west part of Baker Lake sub-basin, where the basin margin trend of the South Channel Fault. A vertical slickenside plane is mapped as a northeast-trending fault, and at western Thirty has a lineation plunging 17° trending 330° (Fig. 4), similar to Mile Lake, where offset of stratigraphic units are mapped the fault at Aniguq River. The large offset across the South along northeast-trending faults. Unfortunately, no slickensides Channel Fault of the basement – Baker Sequence contact were measured and so the possibility of oblique-slip remains was probably magnified by the influence of vertical motion untested. during oblique-slip displacement of shallowly dipping strata. In summary, as indicated by slickenside lineations plung- Northeast-trending faults are generally less prominent ing 17°–25° north to northwest, at least some of the north- Published by NRC Research Press
  13. 13. 1244 Can. J. Earth Sci., Vol. 48, 2011 Fig. 9. Geology of the Thirty Mile Lake study area (modified from Hadlari and Rainbird 2001). Stratigraphic sections from Fig. 7 are indi- cated. Note the northward decrease in dip of north-facing strata from over 70° to 30°.Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Natural Resources Canada on 08/08/11 For personal use only. west-trending faults were oblique-slip faults with predomi- ing the incipient formation of fractures that trend ∼340°. nantly strike-slip motion in the northern and eastern parts of Where well developed, the fracture sets have mutually cross- Baker Lake sub-basin. Northeast-trending faults are less com- cutting relationships (Fig. 12d). mon and generally have lesser magnitude sinistral offsets. The observations just mentioned indicate that the succes- sion of events was (1) formation of the 100° fractures; (2) Fractures kinking; (3) formation of 340° fractures initially linking inter- Fracture sets are well developed in massive, volcaniclastic nal kink band fabric, which was generally coincident with (4) mudstones of the Christopher Island Formation, adjacent to formation of 040° fractures and sinistral tension gashes; and the South Channel Fault (Fig. 11). Fractures oriented at (5) small-scale displacement on all fractures, resulting in mu- 340°, 040°, and 100° together with tension gashes and a tually crosscutting relationships. monoclinal set of kink bands are parallel to the fault sets that Kink bands form in rocks that have a strong planar aniso- occur throughout the Baker Lake sub-basin (Figs. 12a–12d). tropy (Ramsay 1967). Experiments indicate that they form Fractures trending 040° show progressive sinistral rotation when maximum stress is at a low angle to the anisotropy that has locally produced tension gashes (Fig. 12a). (Gay and Weiss 1974). However, these results may be best Subvertical fractures trending ∼100° have been rotated to a constrained to conjugate kink bands, not to the monoclinal trend of ∼340° to form a monoclinal set of kink bands set observed here. Sense of shear relative to the initial planar (Fig. 12b). This dextral rotation produced a kink band trend fabric will result in a sympathetic sense of rotation for a of ∼040°. Figure 12c shows two kink bands formed by dex- monoclinal kink set (Cruikshank et al. 1991). So, relative to tral rotation of fractures trending ∼100°; these are linked by a the 100° fractures, the rock was subject to dextral shear. This set of fractures parallel to the rotated segment of the kink may have been related to development of the dextral 340° band and parallel to other fractures trending ∼340°. Figures fractures, to transtension related to presumably extensional 12b, 12c are considered to represent progressive states show- forces that formed the 100° fractures, or to both. Subsequent Published by NRC Research Press
  14. 14. Hadlari and Rainbird 1245 Fig. 10. Normal faults: (a) north-dipping fault located west of Pitz Lake, inset (i) shows dip-slip slicken sides, and inset (ii) shows intense fractures parallel to the fault plane; (b) normal fault that juxtaposes Baker Sequence conglomerate against anorthosite of the Kramanituar Complex.Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Natural Resources Canada on 08/08/11 For personal use only. linkage of the internal fabric between kink bands by fractures tually crosscutting relations suggest that the east–west set was as part of incipient formation of the 340° fracture set favours reactivated after the conjugate fracture array formed, the former explanation. although the sets probably were broadly contemporaneous In summary, tension gashes indicate that the 040° fractures (e.g., Zhao and Johnson 1991). were characterized by sinistral kinematics, and kink bands suggest that the 340° fractures were dextral. Thus, the frac- Relations between fractures and faults tures sets trending 040° and 340° are antithetic, comprising Trends of the fractures are parallel to the regional post- a conjugate array. Generally, the conjugate fracture array Whart Sequence faults and their kinematics are identical to crosscuts or deforms the east–west-trending set (∼100°). Mu- the offsets indicated by the regional faults. Fractures are best Published by NRC Research Press

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