Retro-arc extension and continental rifting: amodel for the Paleoproterozoic Baker Lake Basin,Nunavut1

T. Hadlari and R.H. Rainbird

Abstract: Within Baker Lake sub-basin, the ca. 1.84–1.78 Ga Baker Sequence formed in two stages. At the start of the firststage, 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, combinedwith 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 historyof 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-arcextension during ca. 1.85–1.84 Ga formation of the Kisseynew back-arc basin of the Trans-Hudson Orogen. Upon closureof 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 isproposed to record lateral tectonic escape between the Saskatchewan–Manitoba and Baffin Island – Committee Bay foci ofthe western Churchill – Superior Province collision.

Résumé : À l’intérieur du sous-bassin du lac Baker, la séquence Baker, ∼1,84–1,78 Ga, s’est formée en deux étapes. Audébut 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 fortstaux 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 defaible accommodation après la distension de la seconde étape de développement du bassin sont relativement minces et lesdômes 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 àtoute la grande région du bassin du lac Baker. Cela signifie que le bassin s’est formé en réaction à l’extension régionale et àl’amincissement de la croûte. Le bassin du lac Baker marque l’étendue nord d’une série de bassins à tendance nord-ouest lelong de la zone tectonique Snowbird et il comprend une enclave du Groupe de Martin (nord de la Saskatchewan) qui lui estcorrélée. La première étape de développement de bassin, à niveau élevé d’accommodation, se serait développée en raison del’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 duSupérieur avec l’ouest de la province de Churchill, le bassin du lac Baker a subi des mouvements de coulissage. La secondeétape 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]

IntroductionDeciphering the tectonic history of the western Churchill

Province has been complicated by Paleoproterozoic, broadlyHudsonian-aged, “reworking” of crustal-scale elements thatappear to have originated in the Archean (Davis et al. 2004,2006; Hanmer et al. 2006). Strata of the ca. 1.84–1.75 GaBaker Lake Basin (Rainbird et al. 2003) unconformably over-lie the Snowbird Tectonic Zone, which is interpreted as theboundary between two Archean cratons (Hoffman 1988). Inmany ways, the record of Baker Lake Basin is intimatelylinked the Paleoproterozoic reworking of the western Church-ill Province; for example, it is host to the most voluminous

Received 15 June 2010. Accepted 10 January 2011. Published atwww.nrcresearchpress.com/cjes on 04 August 2011.

T. Hadlari.* Carleton University, Ottawa, ON, Canada.R.H. Rainbird. Geological Survey of Canada, 615 Booth Street,Ottawa, ON, Canada.

Corresponding author: Thomas Hadlari (e-mail:thomas.hadlari@nrcan-rncan.gc.ca).1Geological Survey of Canada Contribution 20100436.*Current affiliation: Geological Survey of Canada, 3303 - 33rdStreet NW, Calgary, AB T2L 2A7, Canada..

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ultrapotassic volcanic province in the world (LeCheminant etal. 1987; Peterson et al. 1994) and overlaps temporally withthe regional ca. 1.85–1.81 Ga Hudson granitoid suite em-placed at lower crustal levels (Peterson et al. 2002). Faultswith significant offset during Baker Sequence time havebeen identified regionally (e.g., the Tyrrell shear zone,MacLachlan et al. 2005b). Magnetotelluric data have illumi-nated the conductive structure of the Rae–Hearne crust, pro-viding estimates on the depth of the Moho in the vicinity ofBaker Lake Basin (Jones et al. 2002). A Moho depth of 36–40 km allows for zero net crustal thickening between meta-morphism of Kramanituar Complex at pressures of 12–15kbar (1 kbar = 100 MPa; ∼36–45 km depth) at ∼1.9 Ga(Sanborn-Barrie et al. 2001) and deposition of Thelon For-mation at ca. 1.67 Ga (Davis et al. 2011), yet Barrovianmetamorphism has been documented in the “high pressurecorridor” of the northwestern Hearne subdomain and thesouthern Rae domain between ca. 1.89 and 1.85 Ga (Bermanet al. 2002b, 2005). Those relations imply that significantcrustal thinning occurred between ca. 1.85 and 1.67 Ga. Wasextension related to the Baker Lake Basin part of this crustalthinning process?Herein, we present new fault data that test hypotheses pro-

posed by Rainbird et al. (2003) for the structure of BakerLake Basin, and then explore relations between stratigraphic,magmatic (e.g., Cousens et al. 2001; Peterson et al. 2002),and structural evolution of Baker Lake Basin. The most com-prehensive review of Baker Lake Basin was presented byRainbird et al. (2003). We review more recently publishedsedimentology (Hadlari et al. 2006), stratigraphy (Hadlariand Rainbird 2006), and geochronology (Rainbird et al.2006; Rainbird and Davis 2007), and by incorporating resultsof previous studies of Baker Lake Basin (e.g., LeCheminantet al. 1981; Peterson et al. 1989; Peterson and Rainbird 1990;Rainbird and Peterson 1990), we propose a new integratedmodel of basin evolution.A new understanding of Baker Lake Basin allows for com-

parison with time-equivalent events at the scale of the west-ern Churchill Province, that have been documented aslargely as a result of the western Churchill NATMAP (Na-tional Geoscience Mapping Program) Project (e.g., Aspler etal. 2001, 2002, 2004; Cousens et al. 2001, 2004a, 2004b;Berman et al. 2002a, 2002b, 2007; Peterson et al. 2002; Da-vis et al. 2004, 2005, 2006; Hanmer et al. 2004, 2006; San-deman et al. 2004, 2006; and MacLachlan et al. 2005a). Infocusing on ca. 1.85–1.78 Ga events, the new Baker LakeBasin model within the context of the western ChurchillProvince is combined with tectonic models of the Trans-HudsonOrogen (e.g., Bickford et al. 1990; Lewry et al. 1994; Ans-dell et al. 1995; Corrigan et al. 2005; Ansdell 2005) topresent a new tectonic synthesis, particularly for the initia-tion of Baker Lake Basin.

Regional geology: western Churchill ProvinceThe Archean western Churchill Province comprises the

Rae and Hearne domains (Davis et al. 2000) (Fig. 1), referredto by Hoffman (1988) as the Rae and Hearne provinces,which are separated by the Snowbird Tectonic Zone. TheRae–Hearne boundary zone is characterized by (1) contrast-ing Archean supracrustal rocks; (2) syntectonic granitoids

(ca. 2.62–2.6 Ga; e.g., Aspler et al. 2002); (3) high-grademetamorphism at ca. 2.56–2.5 Ga (Berman et al. 2002a);and (4) subsequent high-grade metamorphism at ca. 1.9 Ga(e.g., Sanborn-Barrie et al. 2001). The western ChurchillProvince is bounded to the west by the 2.02–1.91 Ga The-lon–Taltson Orogen and to the southeast by the 1.9–1.8 GaTrans-Hudson Orogen (Hoffman 1988; Wheeler et al. 1997).The Rae domain is differentiated from the Hearne domains

by the presence of komatiite–quartzite supracrustal succes-sions that extend from Baffin Island to north of Baker Lake.In the Rae domain northwest of Baker Lake, older than 2.8Ga basement is unconformably overlain by the WoodburnLake Group, comprising a 2.735–2.710 Ga komatiite–quartzitesuccession and ca. 2.630 Ga felsic volcanic and terrigenoussedimentary rocks (Davis and Zaleski 1998; Zaleski et al.1999, 2000; Zaleski and Davis 2002; Fig. 1).Ryan et al. (2000) proposed that the Big Lake Shear Zone

south of Chesterfield Inlet (Fig. 1) represents the northernlimit of the Rae–Hearne boundary. This structure formed atca. 2.5 Ga and was re-activated at 1.9 Ga (Ryan et al. 2000;Hanmer et al. 2006). The northeastern extension of the STZis, however, generally indicated to coincide with ChesterfieldInlet, usually north of the metamorphic core complexes. Ex-tending eastward from the eastern shore of Baker Lake, gran-ulite-facies Kramanituar Complex comprises ca. 1.9 Gagabbro, anorthosite, and granitoids (Sanborn-Barrie et al.2001). Eastward along Chesterfield Inlet, the Uvauk Com-plex (UCX; Fig. 1) is a similar ca. 1.9 Ga granulite-faciesmetamorphic complex, but also contains ca. 2.6 Ga anortho-site (Mills 2001; Mills et al. 2007).Southwest of Baker Lake – Chesterfield Inlet, at Angikuni

Lake, the Rae–Hearne boundary is represented by the Tule-malu fault zone (Tella and Eade 1986; Fig. 1). AngikuniLake area was the site of ca. 2.62–2.61 Ga syntectonic graniteemplacement (Aspler et al. 2002) and 2.56–2.5 Ga high-grade metamorphism (Berman et al. 2002a). Monazite in-clusions in garnet indicate that 2.56–2.5 Ga and 1.9 Gahigh-grade metamorphism and associated deformation ex-tend from Chesterfield Inlet along the trend of the STZ toAngikuni Lake, defining part of a high-pressure corridor(Berman et al. 2002a).The southwest segment of the STZ, which extends into

Saskatchewan, is represented by crustal-scale shear zonesthat have multistage histories. The oldest deformation is de-fined by ca. 2.63–2.6 Ga syntectonic granites (Hanmer et al.1994; Hanmer et al. 1995) and ca. 2.5 Ga granulite-faciesmetamorphism (Mahan et al. 2008), followed by 1.9 Gamafic intrusions and granulite-facies metamorphism (Baldwinet al. 2003; Mahan et al. 2003; Martel et al. 2008; Mahan etal. 2008). Overlying Rae Province crust west of the StridingAthabasca Mylonite Zone, the ca. 1.83–1.82 Ga MartinGroup is considered correlative to the Baker Lake Group(Ashton et al. 2009). The STZ, therefore, appears to have asimilar history along its length, from the Kramanituar Com-plex to the Striding Athabasca Mylonite Zone.From findings of the western Churchill NATMAP project,

the Hearne domain has been subdivided into northwestern,central, and southern Hearne subdomains (Davis et al. 2000;Hanmer and Relf 2000). Although rocks of the northwesternHearne subdomain are isotopically juvenile, rocks from An-gikuni Lake, MacQuoid Lake, and Rankin Inlet record inter-

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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).

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action with older crust (Sandeman, et al. 2000, 2006). Thisincludes Nd isotopic evidence for older sources (>2.85 Ga;Sandeman 2001; Sandeman et al. 2006), and old detrital zir-cons from the MacQuoid Lake area (up to 3.4 Ga; Davis etal. 2000). Thus, a back-arc setting has been proposed for theArchean supracrustal rocks of the northwestern Hearne sub-domain (Sandeman et al. 2006). The northwestern Hearneand southeastern Rae margins share ca. 2.6 Ga granites (e.g.,LeCheminant and Roddick 1991) and high-pressure meta-morphism at ca. 2.5 Ga and 1.9 Ga (Berman et al. 2002a,2005). Tectonic assembly of the Rae and Hearne provincesis considered to have resulted in 2.5 Ga metamorphism anddeformation (Davis et al. 2000), and subsequent reworkingafter ca. 1.9 Ga has further modified this boundary (Davis etal. 2000; Ryan et al. 2000; Sanborn-Barrie et al. 2001; Ma-cLachlan et al. 2005a).The boundary between the northwestern and central

Hearne subdomains is the Tyrrell shear zone (MacLachlan etal. 2005a, 2005b; Fig. 1). Earliest deformation along the Tyr-rell shear zone is synchronous with emplacement of ca. 2.66–2.62 Ga granites (MacLachlan et al. 2005b), and youngest re-activation took place at ca. 1.83–1.81 Ga (MacLachlan et al.2005a). In the footwall of the Tyrrell shear zone, ca. 1.82 Gagarnet breakdown in the Nowyak metamorphic complex hasbeen attributed to Hudson granitoid plutonism and exhuma-tion (Ter Meer 2001).The central Hearne subdomain comprises the Tavani and

Kaminak greenstone belts interpreted as intra-oceanic arcrocks (Cousens et al. 2004a; Davis et al. 2004; Hanmer etal. 2004; Sandeman et al. 2004) and is typified by isotopi-cally juvenile 2.71–2.68 Ga supracrustal rocks and 2.7–2.65 Ga calc-alkaline granitoid rocks (Davis et al. 2000,2004; Hanmer et al. 2004; and Sandeman et al. 2004).Little is known of the southern Hearne subdomain, which

has yielded older U/Pb ages than the central Hearne subdo-main, particularly where there is basement as old as ca.3.0 Ga to the Wollaston Group (Bickford et al. 1994), whichis a rift to foreland basin succession related to the Trans-Hudson Orogen (e.g., Tran et al. 2003). The ca. 3.0 Ga base-ment of southern Hearne subdomain hints at involvement ofan older crustal block, on the south side of the more juvenilecentral Hearne subdomain, during assembly of the westernChurchill Province.At ca. 1.85–1.81 Ga, the Hudson granitoid suite, ranging

in composition from monzonite to granite, were intrudedthroughout the western Churchill Province (Peterson et al.2002). Minimum melt compositions and Nd isotopes indicateHudson grantoids were likely derived by melting of late Ar-chean crust (van Breemen et al. 2005). Hudson granitoidsoverlap in time with alkaline volcanic rocks of Baker LakeBasin and have locally co-mingled with lamprophyre and re-lated spessartite intrusions (Sandeman et al. 2000).The 1.84–1.67 Ga greater Baker Lake Basin overlies the

northeastern extent of the STZ, unconformably overlying Tu-lemalu fault zone, Kramanituar Complex, and the Chester-field Fault Zone (Fig. 1). The greater Baker Lake Basin isinformally sub-divided into the Kamilukuak, Dubawnt, Whar-ton, Angikuni, and Baker Lake sub-basins (Fig. 2; Rainbirdet al. 2003). The Baker Lake Group (Donaldson 1965; Gallet al. 1992), or Baker Sequence (Rainbird et al. 2003), is acontinental siliciclastic succession containing ultrapotassic

volcanic rocks (e.g., Peterson et al. 2002) that were depositedbetween ca. 1.84–1.78 Ga (Rainbird et al. 2006, Rainbird andDavis 2007) contemporaneous with intrusion of Hudsongranitoids and Martel syenites (Peterson et al. 2002). Rhyo-lites of the Wharton Group, or Whart Sequence (Rainbird etal. 2003), were coeval with the ca. 1.76–1.75 Ga Nueltinsuite of rapakivi granites (Peterson et al. 2002). Rocks of thegreater Baker Lake Basin are crosscut by normal faults and aconjugate set of strike-slip faults (Peterson et al. 1989; Ha-dlari and Rainbird 2001; Rainbird et al. 2003; Peterson2006). Principal offset on the brittle faults occurred beforedeposition of the Thelon Formation, originally reported asca. 1.720 Ga (Miller et al. 1989) and subsequently redefinedas ca. 1.667 Ga (Davis et al. 2011).

Review of greater Baker Lake Basin

General stratigraphy of greater Baker Lake BasinThe Dubawnt Supergroup (Wright 1967; Donaldson 1965,

1966, 1967; Gall et al. 1992) is sub-divided into three uncon-formity-bounded groups (Fig. 3). The ca. 1840–1785 MaBaker Lake Group is characterized by arkosic sandstone, pol-ymictic conglomerate, and alkaline flows and volcaniclasticrocks (Rainbird and Davis 2007). The Wharton Group ischaracterized by sub-arkose and volcaniclastic sandstone, vol-caniclastic conglomerate, and ca. 1758–1753 Ma rhyoliteflows (Rainbird and Davis 2007). The Barrensland Group isrepresented in the Baker Lake Basin by quartz arenite of theThelon Formation, the overlying Kuungmi and Lookout Pointformations are found within Thelon Basin. Rainbird et al.(2003) provided a sequence stratigraphic framework for theDubawnt Supergroup in which the three groups are equiva-lent to the Baker, Whart, and Barrens second-order sequen-ces(Fig. 3). They correspond to the tectonic stages of riftassociated with ultrapotassic volcanism, a distinct second riftphase associated with rhyolite volcanism, and thermal relaxa-tion of an intracontinental basin, respectively (Rainbird et al.2003). In consideration of the genetic nature of our discus-sion, we use the sequence stratigraphic framework henceforth.Previous detailed studies of the Baker Sequence have fo-

cused on volcanology and igneous petrology (e.g., Blake1980; LeCheminant and Heaman 1989; Peterson and LeChe-minant 1993). Alkaline volcanic and volcaniclastic rocks ofthe Baker Sequence compose the Christopher Island Forma-tion (Donaldson 1965, 1967). Flows of the ChristopherIsland Formation are characterized as porphyritic clinopyrox-ene–phlogopite trachyandesites and K-feldspar-phyric tra-chytes (LeCheminant and Heaman 1989), and as minette andfelsic minette flows, respectively (Peterson and Rainbird1990). Minettes are potassic, calc-alkaline, phlogopite + cli-nopyroxene-phyric lamprophyres (Rock 1984, 1987, 1991).Christopher Island Formation minette flows are ultrapotassic,strongly enriched in light rare-earth elements, and containhigh abundances of incompatible elements (LeCheminantand Heaman 1989). They are considered to have been vola-tile-rich mafic alkaline melts of metasomatized, subcontinen-tal lithospheric mantle (LeCheminant et al. 1987; Petersonand LeCheminant 1993; Cousens et al. 2001; Peterson et al.2002). Flows that contain K-feldspar phenocrysts are notminettes sensu stricto, but they are commonly associatedwith minettes elsewhere and in the literature have been infor-

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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).

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mally designated “felsic minettes” (Rock 1987, 1991; Peter-son and Rainbird 1990; Davis et al. 1996; Feldstein andLange 1999).A generalized volcanic succession for the Christopher Is-

land Formation is basal felsic minette (restricted distribution,up to 200 m thick), minette (extensive distribution, locallyover 500 m thick), and upper felsite flows (localized domes)(Peterson and Rainbird 1990; Hadlari and Rainbird 2001;Hadlari 2005). Peterson et al. (2002) consider the lower felsicminette flows of Christopher Island Formation to be crustallycontaminated minette-equivalents. Minette flows representprimary lithospheric mantle melts, and the upper felsite flowsare interpreted as differentiates of minette magmas (Petersonet al. 2002; Peterson 2006). Felsite flows locally display flowbanding, autoclastic breccia, and sub-centimetre K-feldsparphenocrysts. This tripartite volcanic subdivision provides thebasis for correlation across greater Baker Lake Basin and issupported by geochronology (Rainbird et al. 2006). Analysesof phlogopite phenocrysts from a minette flow at the base ofthe Baker Sequence, and a syenite intrusion that intrudes thelower Baker Sequence, yield 40Ar/39Ar ages of 1845 ±12 Ma and 1810 ± 11 Ma, respectively (see discussion Rain-bird et al. 2006). A more precise U–Pb zircon age of 1833 ±3 Ma has been obtained from a felsic minette flow from theKamilukuak sub-basin (Rainbird et al. 2006). This age iswithin error of an 40Ar/39Ar age of 1837 ± 8 Ma obtainedfrom a minette tuff at Aniguq River (Rainbird et al. 2006)and is consistent with an age of 1832 ± 28 Ma obtained

from a lamprophyre dyke southeast of Baker Lake Basin thatis considered to be co-magmatic with Baker Sequence vol-canic rocks (Pb–Pb apatite; MacRae et al. 1996). A lowerage limit for the Baker second-order sequence is delimitedby 1785 ± 3 Ma, derived from laminated carbonate cementsinterpreted as travertine, from alluvial deposits of Kamilu-kuak sub-basin (Pb–Pb isochron from calcite; Rainbird et al.2006). Based upon the data and discussion in Rainbird et al.(2006), we refer to the approximate age of the Baker Se-quence as ca. 1840–1785 Ma.

Baker Sequence: sedimentary strata of Baker Lake sub-basinIn Baker Lake sub-basin (Fig. 4), the Baker second-order

sequence comprises five third-order sequences and a correla-tive tripartite volcanic succession (Fig. 5; Rainbird et al.2003; Hadlari 2005; Hadlari and Rainbird 2006). Third-ordersequences are interpreted as pulses of accommodation in re-sponse to basin-margin normal faulting and subsequent infillby the sedimentary system (Hadlari and Rainbird 2006).Across the axis of Baker Lake sub-basin, from Aniguq Riverto Thirty Mile Lake, the thickness of the Baker Sequence in-creases from ∼500 m to over 2500 m (Fig. 5). Accordingly,thicknesses of 3rd-order sequences increase from ∼100–150 m to up to 500 m. It is important that 3rd -order sequen-ces are scaled relative to the total thickness of the BakerSequence, because these thicknesses, therefore, reflect differ-ential accommodation and basin asymmetry rather than sub-sequent erosion (Hadlari and Rainbird 2006).

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 BakerLake Group, Rainbird et al. (2006).

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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).

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Basin asymmetry is consistent with the distribution of fa-cies — thick alluvial fan deposits near the southeast margin,braided fluvial deposits toward the centre, and fluvial, flood-plain, eolian, and lacustrine deposits near a depocentre atChristopher Island (Hadlari et al. 2006; Fig. 6). Paleocurrentdata from basin margins shown in Fig. 4 were measured fromthe full Baker Sequence, indicating that drainage was consis-tently directed first into the basin and then along an axis to-ward an inferred depocentre. Collectively, the facies,paleocurrents, and stratigraphy are thus indicative of a north-east–southwest-trending half-graben with a hinged margin tothe northwest and a master fault approximately parallel to thesoutheastern basin margin (Hadlari et al. 2006). Althoughthis fault is not exposed within the basin, Ryan et al. (2000)identified a northeast–southwest-trending, post-1.9 Ga brittlefault southeast of Baker Lake sub-basin that is crosscut bythe post-Baker Sequence South Channel Fault (Blake 1980)and postulated that it could be the main normal fault thatbounded the Baker Lake sub-basin.In the Thirty Mile Lake area, an almost complete compo-

site section of the Baker Sequence yields a thickness of∼2.5–3.0 km (Fig. 5). The lower succession of 3rd-order se-quences shows a retrogradation of facies, indicating that at

the basin scale, accommodation (A) > sediment flux (SF).The progradation of facies at the top of the Baker Sequenceindicates that A < SF. The entire succession represents earlyhigh subsidence followed by low subsidence and sedimentaryand volcanic infilling of the basin (Hadlari and Rainbird2006).

Baker Sequence: sedimentary and volcanic correlation,Baker Lake sub-basinRelationships between the sequence stratigraphy and the

tripartite volcanic stratigraphy are made on the basis of clastlithologies and interfingering of sedimentary and volcanicunits (Fig. 5). During rift initiation, felsic minette flows andequivalent volcaniclastic rocks have a limited geographic ex-tent, locally overlying the unconformity at the base of theBaker Sequence (Blake 1980; Hadlari and Rainbird 2001).Sequences B-1 and B-2 are correlated with felsic minetteflows based on interbedding of flows and conglomerates atThirty Mile Lake. In addition to numerous felsic minetteclasts, the occurrence of a few minette clasts in conglomer-ates at the base of sequence B-3 suggest that minette volcan-ism initiated at localized volcanic centres at a time close tothe B-2 – B-3 sequence boundary. At Thirty Mile Lake, the

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 Rainbird2006). 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 MileLake (Hadlari and Rainbird 2001; Hadlari 2005).

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B-2 – B-3 sequence boundary marks a change in facies fromalluvial fan to gravel-bed braided stream (Hadlari and Rain-bird 2006; Fig. 5). This is consistent with back-stepping ofthe master fault system, resulting in widening of the basinand a retreat of alluvial fan facies, a common attribute of nor-mal fault systems (e.g., Morley 2002). The retreat of alluvialfacies, or retrogradation, occurred at the same time as thetransition from felsic minette to minette volcanism, and nearthe beginning of deposition at Aniguq River (Fig. 5). Minetteflows overlie part of sequence B-4 at western Thirty MileLake and Nutarawit Lake (Fig. 7), and so by sequence, B-4minette volcanism had increased in volume to blanket mostof Baker Lake Basin. Representing a low accommodationpost-rift stage, the occurrence of felsite clasts within con-glomeratic deposits of sequence B-5 is correlated to theyoungest volcanic sub-division of felsite, or fractionatedminette, flows.

Baker Sequence: greater Baker Lake BasinSequences B-1 to B-4 are correlated from the Angikuni to

Baker Lake sub-basins (Fig. 7), consistent with the proposi-tion by Aspler et al. (2004) that Baker Sequence rocks fromthese basins are tectonostratigraphic equivalents. The tripar-tite volcanic succession extends across greater Baker LakeBasin from Kamilukuak to Baker Lake sub-basins, and lim-

ited geochronology suggests that this is a temporal correla-tion (Rainbird et al. 2006).Strike-slip basins grow laterally, leading to diachronous

sedimentation along the basin axis and anomalously thick,laterally stacked stratigraphic successions (e.g., Aspler andDonaldson 1985). The correlation of sedimentary sequencesbetween the Baker Lake and Angikuni sub-basins, togetherwith contemporaneity of volcanic facies across the greaterBaker Lake Basin, supports the interpretation that BakerLake Basin formed as an overall extensional or trans-ten-sional basin rather than a strike-slip basin.

Structural geology of greater Baker Lake BasinStrata of greater Baker Lake Basin are crosscut by sets of

multiply reactivated brittle faults, where principal displace-ment probably postdated the Baker and Whart sequences andpredated the Barrens Sequence (Rainbird et al. 2003). Ad-dressing the earliest stage of faulting, Aspler et al. (2004)suggest that northeast-trending lamprophyre dykes parallel tothe Angikuni sub-basin margin indicate that Baker Lake Ba-sin formed because of overall extension. Two dyke trends,primarily east–west and also northeast–southwest, have beencited to suggest that the Dubawnt sub-basin formed becauseof north–south extension (Peterson 2006). In addition, Kami-lukuak sub-basin contains a set of dilational faults with ex-

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 orientedparallel to the basin axis; the downstream culmination is a lacustrine depocentre.

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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-bird et al. 2003). F, mud; S, sand; G, gravel.

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tensive quartz stockwork, interpreted as normal faults, thatare spatially associated with Baker Sequence basin margins,but not demonstrated to be syndepositional (Peterson 2006).Regional map-scale faults within greater Baker Lake Basin

are subdivided into those interpreted as normal and strike-slipbased upon offset of mapped stratigraphy (e.g., Rainbird etal. 2003; Peterson 2006). For example, within Kamilukuaksub-basin strata are north-facing with bedding attitudes dip-ping between 35° and 75° (Peterson 2006). Including thebasal unconformity, stratigraphy is repeated across east–west-trending faults that are interpreted as south-side-downnormal faults (Peterson 2006). Kamilukuak and Dubawntsub-basins also contain a conjugate set of northwest-trendingfaults and fractures with sinistral offset and north- and north-east-trending map-scale faults with dextral offset (Peterson2006).Baker Lake sub-basin contains strata of Baker and Whart

sequences that dip from over 75° at basin margins to subhor-izontal in central areas (Rainbird et al. 2003; Hadlari et al.2004). Based upon step-wise changes in dip, it has been pro-posed that blocks bounded and rotated by approximately east-northeast-trending normal faults are responsible for the distri-bution of bedding attitudes (Hadlari and Rainbird 2001;Rainbird et al. 2003; Fig. 4b). In addition to east–west trends,normal faults have also been observed with north–southtrends, for example on the eastern shore of Baker Lake(Rainbird et al. 1999), indicating an overall extensional faultregime. Within fault blocks Baker and Whart sequence stratahave identical bedding attitudes, indicating that block rotationpostdated the Whart Sequence. Fault blocks are crosscut by aconjugate set of northwest-trending faults with dextral offsetand northeast trending faults with sinistral offset (Rainbird etal. 2003). In contrast to those crosscutting relations, changesin thickness of Whart Sequence strata across northwest-trendingfaults suggest that a component of offset was syndeposi-tional (Rainbird et al. 2003). Thelon Formation (BarrensSequence) bedding is ubiquitously subhorizontal. Althoughit is locally crosscut by the main fault sets, offset is minor(Rainbird et al. 2003).In summary, the Baker Lake Basin is crosscut by three

main fault trends with complex crosscutting and stratigraphicrelationships. There are indications that at least some of thefaults were active syndepositionally with both Baker andWhart sequences, that major displacements occurred afterWhart Sequence, but prior to Barrens Sequence deposition,and that relatively minor reactivation postdated the BarrensSequence.

Results: Baker Lake Basin faults andfracturesFrom Dubawnt Lake to Baker Lake, strata of greater Baker

Lake Basin are crosscut by two sets of map-scale faults (Pe-terson and Rainbird 1990; Hadlari and Rainbird 2001; Rain-bird et al. 2003): (1) a set of normal faults that trendapproximately east–west; and (2) a conjugate set of strike-slip faults that trend northwest (∼340°) and northeast(∼040°). In almost all outcrops fault sets occur together, forexample, if one set forms the dominant outcrop feature, thenthe other sets are represented by decimetre to metre-scale off-sets or closely spaced fractures.

Quartz stockworkLinear quartz vein breccia zones, up to 1 m wide, trending

northwest (∼340°) transect Christopher Island Formation vol-canic rocks (Fig. 8a), parallel to a regional set of northwest-trending faults. Minette lithons within the stockwork providea maximum age for dilation and breccia formation. The mini-mum age of the breccia is determined by the presence ofquartz stockwork clasts containing minette lithons withinBaker Sequence (B-5) conglomerate at Kunwak River(Fig. 8b). The breccia zones crosscut minette flows, are in-cluded as clasts within conglomerate of B-5, and are, there-fore, considered to be syndepositional, basin-transversedilational faults. Peterson (2006) speculated that dilationalfaults with quartz stockwork in Kamilukuak sub-basin weresyndepositional, and our data show that equivalent faults inBaker Lake sub-basin were in fact syndepositional.

Normal faults (Figs. 9–11)The best example of an exposed normal fault is located

west of Pitz Lake. There, an east–west-trending, north-dippingfault is developed within Pitz Formation rhyolite (Fig. 10a).Fault planes exhibit slickenside lineations that indicate adip-slip, north-side-down, normal sense of displacement(Fig. 10a). This observation supports the proposition byRainbird et al. (2003), based upon south-dipping beddingattitudes and repetition of strata, that the overall structureof northern Baker Lake sub-basin is determined by aneast–west-trending set of normal faults and rotated faultblocks.North of Thirty Mile Lake, a fault that trends east–west,

and dips southward, is associated with subparallel fractures(Fig. 9). Fault breccia zones had developed; however, noslickenside lineations were measured, and so the interpreta-tion of normal displacement is based upon the repetition ofstratigraphy, change in bedding, and south-dipping attitude ofthe fault and associated fractures. The orientation of thisfault is consistent with the interpretation for southern BakerLake sub-basin of fault blocks rotated by south-dipping nor-mal faults (Hadlari and Rainbird 2001; Rainbird et al. 2003;e.g., Fig. 4b).At eastern Baker Lake, Baker Sequence conglomerate is

juxtaposed against anorthosite of the Kramanituar Complexby an east–west-trending normal fault (Rainbird et al. 1999).This fault dips south with a normal sense of displacement(Fig. 10b).

Strike-slip faultsMap-scale strike-slip faults occur at Aniguq River and

Christopher Island – South Channel (Fig. 4). The course ofAniguq River follows a northwest-trending fault mappedwith ∼1.3 km of dextral offset (Rainbird et al. 2003; Fig. 4).Along this fault zone are multiple northwest-trending faultbreccia zones generally <20 cm, but up to 80 cm thick. As-sociated with the fault zone, two slickenside planes trendingnorthwest–southeast and north–south both have north-plunging(24°–25°) slickenside lineations (Fig. 4). The mapped offsetof strata is dextral (Fig. 4) and based on slickenside meas-urements the sense of displacement is oblique, which, giventhe south-facing, ∼40° dip of Baker and Whart sequencestrata, would magnify the amount of horizontal offset by∼30%.

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The most prominent northwest-trending fault is SouthChannel Fault (Figs. 4, 11), across which the unconformityat the base of the Baker Sequence is offset by 10 km (Schauand Hulbert 1977). Fault breccia and slickensides were iden-tified on the southern shore of South Channel, along thetrend of the South Channel Fault. A vertical slickenside planehas a lineation plunging 17° trending 330° (Fig. 4), similar tothe fault at Aniguq River. The large offset across the SouthChannel Fault of the basement – Baker Sequence contactwas probably magnified by the influence of vertical motionduring oblique-slip displacement of shallowly dipping strata.Northeast-trending faults are generally less prominent

within Baker Lake sub-basin than northwest-trending faults.The most common exposures are of small outcrop-scalefaults with sinistral offsets of tens of centimetres up to a fewmetres (e.g., Christopher Island). Exceptions are the south-west part of Baker Lake sub-basin, where the basin marginis mapped as a northeast-trending fault, and at western ThirtyMile Lake, where offset of stratigraphic units are mappedalong northeast-trending faults. Unfortunately, no slickensideswere measured and so the possibility of oblique-slip remainsuntested.In summary, as indicated by slickenside lineations plung-

ing 17°–25° north to northwest, at least some of the north-

Fig. 8. Strike-slip faults: (a) quartz stockwork in syn-Christopher Island Formation dilational fault; (b) quartz stockwork clast removed fromconglomerate of sequence B-5; and (c) strike-slip fault of the 040° set, 80 cm pole on the fault line for scale.

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west-trending faults were oblique-slip faults with predomi-nantly strike-slip motion in the northern and eastern parts ofBaker Lake sub-basin. Northeast-trending faults are less com-mon and generally have lesser magnitude sinistral offsets.

FracturesFracture sets are well developed in massive, volcaniclastic

mudstones of the Christopher Island Formation, adjacent tothe South Channel Fault (Fig. 11). Fractures oriented at340°, 040°, and 100° together with tension gashes and amonoclinal set of kink bands are parallel to the fault sets thatoccur throughout the Baker Lake sub-basin (Figs. 12a–12d).Fractures trending 040° show progressive sinistral rotationthat has locally produced tension gashes (Fig. 12a).Subvertical fractures trending ∼100° have been rotated to a

trend of ∼340° to form a monoclinal set of kink bands(Fig. 12b). This dextral rotation produced a kink band trendof ∼040°. Figure 12c shows two kink bands formed by dex-tral rotation of fractures trending ∼100°; these are linked by aset of fractures parallel to the rotated segment of the kinkband and parallel to other fractures trending ∼340°. Figures12b, 12c are considered to represent progressive states show-

ing the incipient formation of fractures that trend ∼340°.Where well developed, the fracture sets have mutually cross-cutting relationships (Fig. 12d).The observations just mentioned indicate that the succes-

sion of events was (1) formation of the 100° fractures; (2)kinking; (3) formation of 340° fractures initially linking inter-nal kink band fabric, which was generally coincident with (4)formation of 040° fractures and sinistral tension gashes; and(5) small-scale displacement on all fractures, resulting in mu-tually crosscutting relationships.Kink bands form in rocks that have a strong planar aniso-

tropy (Ramsay 1967). Experiments indicate that they formwhen maximum stress is at a low angle to the anisotropy(Gay and Weiss 1974). However, these results may be bestconstrained to conjugate kink bands, not to the monoclinalset observed here. Sense of shear relative to the initial planarfabric will result in a sympathetic sense of rotation for amonoclinal kink set (Cruikshank et al. 1991). So, relative tothe 100° fractures, the rock was subject to dextral shear. Thismay have been related to development of the dextral 340°fractures, to transtension related to presumably extensionalforces that formed the 100° fractures, or to both. Subsequent

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°.

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linkage of the internal fabric between kink bands by fracturesas part of incipient formation of the 340° fracture set favoursthe former explanation.In summary, tension gashes indicate that the 040° fractures

were characterized by sinistral kinematics, and kink bandssuggest that the 340° fractures were dextral. Thus, the frac-tures sets trending 040° and 340° are antithetic, comprisinga conjugate array. Generally, the conjugate fracture arraycrosscuts or deforms the east–west-trending set (∼100°). Mu-

tually crosscutting relations suggest that the east–west set wasreactivated after the conjugate fracture array formed,although the sets probably were broadly contemporaneous(e.g., Zhao and Johnson 1991).

Relations between fractures and faultsTrends of the fractures are parallel to the regional post-

Whart Sequence faults and their kinematics are identical tothe offsets indicated by the regional faults. Fractures are best

Fig. 10. Normal faults: (a) north-dipping fault located west of Pitz Lake, inset (i) shows dip-slip slicken sides, and inset (ii) shows intensefractures parallel to the fault plane; (b) normal fault that juxtaposes Baker Sequence conglomerate against anorthosite of the KramanituarComplex.

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developed in proximity to major faults, such as the SouthChannel Fault, and are, therefore, interpreted as mesofault-type fractures formed as part of the regional macrofault sets(cf. Peacock 2001).The implication of the fracture data that the east–west-

trending normal faults and the conjugate set of strike-slipfaults may be coeval is problematic because they indicate op-posite orientations of extension. The east–west-trending nor-mal faults accommodated north–south extension, whereas theconjugate strike-slip faults accommodated north–south short-ening and east–west extension. It is unlikely that the conju-gate fault set was driven by north–south contraction, becausethat would have re-activated the east–west-trending normalfaults as reverse faults, leading to basin inversion, a modelfor which there is presently no evidence. As discussed previ-ously, the strike-slip faults are parallel to pre-existing struc-tures. So, if the primary component of extension wasaccommodated by the normal faults, then the conjugatestrike-slip set may have accommodated a secondary compo-nent of extension in the east–west direction by reactivating apre-existing fault system. It is also possible that the orienta-tion of extension changed over time and that this deviationwas accommodated by the conjugate strike-slip system.Thus, a finite strain ellipse accommodating both fault sys-tems would be oriented with the long axis inclined to thenormal of the 100° fault set.In summary, this episode of faulting and related fracturing

represents extension that was oriented primarily north–south.

A component of east–west extension is inferred to have beenmostly accommodated by conjugate strike-slip displacementon a pre-existing fault system, but also by certain north–south-trending normal faults. This occurred as multiple dis-crete events, during transtension, or both, probably over aprotracted phase of overall extension. Although there is evi-dence for syn-Whart activity, principal displacement on thenormal and conjugate strike-slip fault systems postdated dep-osition of the Whart Sequence (ca. 1760–1750 Ma) and pre-dated the Barrens Sequence (1667 ± 5 Ma; Davis et al. 2011).

Discussion

Baker Lake Basin

Intra-Baker Sequence faultsThe only faults with conclusive evidence for syn-Baker Se-

quence displacement are the ∼340°-trending dilational faultsand quartz stockwork. Our understanding of syn-Baker Se-quence faulting relies mainly upon inference.Based on stratigraphic relationships, sedimentary facies,

and paleocurrents, Baker Lake sub-basin is interpreted tohave formed as a half-graben with a bounding fault along itssouthern margin (Rainbird et al. 2003; Hadlari et al. 2006).Minette dykes trend northeast parallel to the Angikuni sub-basin margin, therefore, a component of northwest–southeastextension transverse to the basin axis during Christopher Is-land Formation volcanism (Aspler et al. 2004).

Fig. 11. Geologic map of the Christopher Island study area showing paleocurrent data. St, fluvial; Sw, wave ripple crests; Ste, eolian cross-sets. Modified from Hadlari et al. (2006).

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Dubawnt sub-basin trends northeast–southwest (Fig. 2) andis bounded by faults that postdate deposition of the Baker Se-quence (Rainbird et al. 2003). It has been proposed to haveformed as a small strike-slip basin during Baker Sequencetime (Rainbird and Peterson 1990; Rainbird et al. 2003);however, bimodal lamprophyre dyke trends oriented approxi-mately northeast–southwest and east–west (Fig. 2) supportnorth–south extension or dextral transtension (Peterson2006).Southwest of Baker Lake Basin (Fig. 1), the Tyrrell shear

zone was active during deposition of the Baker Sequence.Tyrrell shear zone is northeast trending, northwest dipping,and juxtaposes low metamorphic-grade rocks of the Yathkyedgreenstone belt that preserve Archean K–Ar ages (K–Arhornblende, 2430–2485 Ma; Tella and Eade 1986) in thehanging wall, against footwall gneisses and granitoids(MacLachlan et al. 2005a). Although early dip-slip motionwas Archean, footwall rocks that were at amphibolite meta-morphic grade at ca. 1835 Ma were juxtaposed againstgreenschist-facies hanging wall rocks by ca. 1816–1810 Ma.There is, however, no evidence preserved of deformation be-tween ca. 1835 and ca. 1816 Ma, probably because of trans-position by dextral-normal ductile shearing between ca. 1816and ca. 1810 Ma (MacLachlan et al. 2005a). This time inter-val overlaps with the ca. 1840–1810 Ma age of volcanic andintrusive rocks from Baker Lake Basin. In addition to con-

temporaneous normal sense of displacement, the northwestdip of the fault relative to Angikuni sub-basin is consistentwith the Tyrrell shear zone being a mid- to upper crustal ex-posure of the fault network that bounded Baker Lake Basin.In that sense, and considering the ca. 1800–1700 Ma K–Arand Ar–Ar cooling ages in the Yathkyed area (Sandeman2001), we do not expect ca. 1840–1810 Ma surface depositsto have been preserved close to Tyrrell shear zone, and we,therefore, cannot distinguish between the Tyrrell shear zoneas the basin bounding fault or as part of the fault networkoutside of the basin.At Baker Sequence time, LeCheminant et al. (1981) pro-

posed that Wharton sub-basin formed as a northwest-trendinghalf-graben, oriented at high angle to the east-northeast trendof the greater Baker Lake Basin, and that Wharton sub-basinwas an allocogen, or triple point junction within the rift sys-tem. Since adjacent strike-slip basins would be expected tohave parallel trends, northeast and northwest trending half-graben further reinforce the interpretation of an overall exten-sional rather than strike-slip setting. Furthermore, extensionalfaulting on the margins of both basin trends (trending north-east and west-northwest) requires oblique-slip on both faultsystems resolving into north–south or east–west overall ex-tension. Considering the dextral-normal kinematics of thenortheast-trending Tyrrell shear zone (MacLachlan et al.2005b) and lamprophyre dyke trends from Baker Lake Basin

Fig. 12. Structures developed within relatively homogeneous volcaniclastic mudstone: (a) tension gashes indicating sinistral rotation of frac-tures trending ∼040°; (b) kink band (lens cap for scale); (c) fractures trending ∼340° linking rotated, internal kink band fractures; and (d)three fracture sets with mutually crosscutting relations. (a, c, and d) Pen for scale.

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(Aspler et al. 2004; Peterson 2006) overall extension wasprobably directed north–south.In summary, available data, in large part indirect, suggest

that Baker Lake Basin formed in response to regional north–south-directed extension. Just as the Tyrrell shear zone is are-activated older structure, so then are the structures thatbounded the Baker Lake Basin, resulting in a series of north-east-trending half-graben. In this predominantly extensionalenvironment, footwall rocks to the Tyrrell shear zone wereexhumed from amphibolite to greenschist metamorphic gradebetween ca. 1835 and ca. 1816 Ma, which we correlate to thehigh accommodation basin phase accompanied by volumi-nous eruption of primary mantle melts. The shallowly plung-ing lineation and dextral-normal shear sense of the Tyrrellshear zone between ca. 1816 and 1810 Ma (MacLachlan etal. 2005b) is correlated to the low accommodation phase ofthe Baker Sequence accompanied by fractionated minettevolcanism, and we, therefore, propose that this transition isbecause of a change in regional faulting from extensional topredominantly strike-slip.

Post-Whart-, pre-Barrens Sequence deformationWhereas the geographic disposition of Baker Lake Basin is

because of intra-Baker Sequence faults, the present structureof the Baker Lake Basin was largely determined by post-Whart-, pre-Barrens Sequence displacement on brittle faults.Fault blocks were rotated up to 70°, analogous to normalfaults systems within the Basin and Range (e.g., Brady et al.2000). We propose that the orientation of those faults was in-herited from pre-existing structures and that the system ac-commodated a primary component of regional north–southextension and secondary component of east–west extension.

Post-Barrens Sequence reactivationThe Thelon Formation of the Barrens Sequence is crosscut

by the three main fault sets described earlier in the text,where post-Barrens reactivation resulted in minor offset (gen-erally metre-scale), and minimal tilting as indicated by sub-horizontal bedding attitudes. Some of the uraniummineralization in the Thelon Basin has been related to thesefaults (Fuchs and Hilger 1989; Kyser et al. 2000). For example,the Boomerang Lake Prospect lies along a northwest-trend-ing fault zone and, at outcrop-scale, the zones of alterationand mineralization are associated with small-scale faults(Davidson and Gandhi 1989). We interpret the outcrop-scalefaults within the Barrens Sequence as reactivations of themain, pre-existing faults sets that dissect Baker Lake Basin.

Integrated summary of Baker Lake BasinThe greater Baker Lake Basin formed during overall

north–south extension (Fig. 13). Half-graben that werebounded by oblique-slip faults trend primarily northeast–southwest, parallel to the STZ, except for the Wharton sub-basin,which trended northwest–southeast. Lithospheric extensionled to decompression melting of the lithospheric mantle,likely beginning at ∼1850 Ma (e.g., Sandeman et al. 2000;Cousens et al. 2001). Eruption of felsic minettes at local-ized volcanic centres within half-graben initiated at∼1840 Ma (Rainbird et al. 2006). Experimental data showthat minette melts that have equilibrated at crustal pressureshave a stable phenocryst assemblage similar to that of felsic

minettes (Esperança and Holloway 1987). Proper minetteshave a phenocryst assemblage indicating phenocryst-meltstability at mantle pressures (Esperança and Holloway1987) and, therefore, indicate rapid transport of melt fromthe lithospheric mantle. It follows that the felsic minetteflows of the Christopher Island Formation indicate extrusionof alkaline melts that were initially derived from a lithosphericmantle source and had probably resided within the crust.Eruption of mantle melts that probably had resided within,

and were possibly contaminated by, continental crust (Peter-son 1994; Peterson et al. 1994, 2002) is recorded by strati-graphic sequences B-1 and B-2, the rift initiation phase ofthe Baker Lake Basin (Fig. 13a). Within the Baker Lakesub-basin, alluvial fans at the basin margins fed transverseand axial braided streams leading to lacustrine and eolian de-pocentres. By the time sequence B-3 was deposited, the basinhad widened and accommodation increased, which is specu-latively attributed to back-stepping of the normal fault sys-tem. Increased accommodation is correlated with a renewedpulse of primary mantle melts recorded by the transitionfrom felsic minette to minette volcanism near the B-2 – B-3sequence boundary. By the late stages of sequence B-4,minette volcanics had virtually blanketed the entire basin,marking the rift climax (Fig. 13b). This extension-dominatedoblique-slip phase is considered to overlap with exhumationrecorded in the footwall of the Tyrrell shear zone after meta-morphism at ca. 1835 Ma (MacLachlan et al. 2005b). Se-quence B-5 represents clastic infilling of remainingaccommodation space after minette volcanism in a low ac-commodation setting, during a post-rift stage (Fig. 13c). Dur-ing this post-rift stage, oblique-slip along faults bounding theBaker Lake Basin likely became dominated by strike-slip aslithospheric thinning decreased or ceased. Magmatism wasrepresented by felsite domes of fractionated minettes, ca.1810 Ma syenite plugs, and the youngest intrusions of theHudson granitoid suite. This time interval probably over-lapped with dextral-normal oblique shear on the Tyrrell shearzone between ca. 1816 and ca. 1810 Ma (MacLachlan et al.2005b). The youngest deposits of the Baker Sequence areolder than ca. 1785 Ma (Rainbird et al. 2006).After deposition of the ca. 1760–1750 Ma Whart Se-

quence, the Baker Lake Basin was subject to another phaseof north–south extension. Strata were rotated north and southby east–west-trending listric normal faults (e.g., Fig. 4b). Ex-tension oblique to the main set of normal faults was accom-modated by a pre-existing conjugate strike-slip system,involving faults that were active during the Baker and Whartsequences, and local north–south-trending normal faults. Fol-lowing extensional faulting, uplift, extensive erosional plana-tion, and regolith development, the ca. 1667 Ma base of theBarrens Sequence (Davis et al. 2011) was deposited over theregion and has almost everywhere retained subhorizontalbedding attitudes.

Tectonic synthesis

Pre-Baker Lake Basin history of the western ChurchillProvinceThere are two end-member tectonic assembly models for

the western Churchill Province (e.g., Davis et al. 2006). Inan Archean assembly model (Davis et al. 2000; Hanmer and

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Relf 2000), supracrustal rocks of the Woodburn Group in thesouthern Rae domain were deposited in a continental settingat ca. 2735–2710 Ma (Zaleski and Davis 2002). Juvenile,2700–2650 Ma supracrustal belts of the northwestern Hearnesub-domain, though influenced by older crust of uncertain af-finity and considered to represent a back-arc setting (Sande-man et al. 2006), were amalgamated with the Rae domainprior to widespread intrusion of ca. 2640–2580 Ma gran-itoids. Supracrustal rocks of the central Hearne sub-domainwere deposited at 2710–2670 Ma in an intra-oceanic arcsetting (Cousens et al. 2004a; Hanmer et al. 2004; Sandemanet al. 2004). Ca. 2550–2500 Ma metamorphism in the

northwestern Hearne, also recorded on the Tyrrell shear zone(MacLachlan et al. 2005b), a boundary between the north-western and central Hearne sub-domains, is postulated topostdate final assembly of a combined Rae–Hearne Archeancraton (Hanmer et al. 2006).Within the Archean assembly model, Baker Lake Basin

formed on a major crustal break, the Snowbird TectonicZone (STZ), which formed by ca. 2600 Ma and was reacti-vated in the Paleoproterozoic at 2500 and 1900 Ma (Hanmeret al. 1994; Sanborn-Barrie et al. 2001; and Mills et al.2007). Deformation and metamorphism related to northwest-vergent shortening at ca. 1890–1850 Ma (Berman et al. 2002a,

Fig. 13. Paleogeographic evolution of the Baker Lake Basin illustrating three principal phases of of basin development during deposition ofthe ca. 1.84–1.78 Ga Baker Sequence.

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2002b, 2005) within the northwestern Hearne sub-domain isanalogous to Laramide-style foreland deformation (Aspler etal. 2002; e.g., Brewer et al. 1982), which occurred hundredsof kilometres from the related convergent margin.Alternatively, in a two phase Archean–Proterozoic assem-

bly model for the western Churchill Province (e.g., Davis etal. 2006; Berman et al. 2007), the Rae domain and north-western Hearne sub-domain were amalgamated by ca.2600 Ma, with metamorphism at ca. 2550–2500 Ma. Begin-ning at ca. 1900 Ma, the central Hearne sub-domain was ac-creted, and subsequent northwest-vergent 1890–1850 Madeformation and metamorphism in the northwestern Hearnesub-domain and southern Rae domain, thus, record shorten-ing related to microcontinent–continent collision prior to ca.1865 Ma (Berman et al. 2007). This interpretation was ap-plied to the segment of the STZ near the southeastern bor-der of the Northwest Territories at Snowbird Lake (Martelet al. 2008), where the Hearne Province was postulated tohave accreted to the Rae domain beginning at 1920 Ma(Martel et al. 2008). In contrast, detailed pressure–tempera-ture–time–deformation (P–T–t–D) work from the east Atha-basca mylonite triangle of the STZ have also identified twophases of high-pressure metamorphism at 2550 Ga and1900 Ma, but were interpreted to indicate that the rocks“experienced long-term lower crustal residence from 2550to 1900 Ma” (Mahan et al. 2008, p. 669).Problems with the Archean–Proterozoic assembly model

include lack of evidence for a magmatic arc to consume crustthat would have separated the two crustal blocks, the absenceof contemporaneous south-vergent fold and thrust deforma-tion of the central Hearne sub-domain, and most signifi-cantly, the Archean history of structures bounding thenorthwestern and central Hearne sub-domains (e.g., Tyrrellshear zone, MacLachlan et al. 2005a, 2005b) and other partsof the STZ (e.g., Hanmer et al. 1994; Mahan et al. 2008). AnArchean–Proterozoic assembly model would provide a muchmore dynamic tectonic setting at ca. 1850–1840 Ma than areactivated continental interior, but in consideration of evi-dence to the contrary, we proceed with the Archean assemblymodel in mind.

Origin of Baker Lake BasinSeveral observations suggest that Baker Lake Basin formed

in an extensional tectonic environment. These include the ap-parent synchroneity of deposition along the longitudinal axisof the Baker Lake sub-basin, coeval juxtaposition of amphib-olite- and greenschist-facies rocks against a potential basin-bounding fault (the Tyrrell shear zone), and the orientationof lamprophyre dykes parallel to interpreted trend of the ba-sin axis.The Kramanituar Complex underwent ca. 1900 Ma exhu-

mation from the lower to middle crust, although final unroof-ing likely postdated ca. 1890–1850 Ma deformation andmetamorphism but preceded the Baker Sequence, as indi-cated by the unconformable contact. Within the NowyakComplex, located in the footwall to Tyrrell shear zone, TerMeer (2001) considered that post-tectonic garnet breakdownand ca. 1820 Ma matrix monazite were related to ca.1820 Ma granite plutonism and exhumation. This relationalso is hinted at within the Uvauk Complex, where Mills etal. (2007) postulate that garnet breakdown and retrograde

metamorphism were related to Hudson granite intrusion.Thus, unroofing of the Kramanituar Complex preceded BakerLake Basin formation (Sanborn-Barrie et al. 2001), but it ap-pears that other metamorphic core complexes southeast of thebasin (Uvauk Complex; Mills et al. 2007), including Tyrrellshear zone footwall rocks (Nowyak Complex; Ter Meer2001), underwent exhumation coeval with intrusion of gran-ites and minette lamprophyres (Peterson et al. 2002) and dep-osition of the Baker Sequence (Rainbird et al. 2006).Lamprophyre flows and dykes equivalent to the Christo-

pher Island Formation occur along the STZ from Baker Laketo within 20 km of the Martin Group in Saskatchewan(Fig. 1; Donaldson 1968; Ashton et al. 2009). The MartinGroup comprises coarse siliciclastic deposits that are consid-ered correlative to those in Baker Lake Basin and were de-posited in an areally restricted, high accommodation basin(Donaldson 1968; Fraser et al. 1970; Macey 1973; Ashton etal. 2009). Basaltic volcanic rocks dated at 1820 ± 4 Ma havegeochemical characters indicating derivation from an en-riched mantle source (Morelli et al. 2009). Correlation alongthe STZ implies that greater Baker Lake Basin is the mostnortherly of a series of fault-bounded basins with volcanicrocks sourced from the lithospheric mantle that parallel theSTZ and formed at ca. 1840–1820 Ma. As the CommitteeBay supracrustal belt (Fig. 1) was undergoing ca. 1850–1820 Ma tectonometamorphism related to deformation of theBaffin Island segment of the Trans-Hudson Orogen at thistime (Carson et al. 2004), this zone of extension didn’t ex-tend much farther northeast than the Baker Lake sub-basin.Models for passive rifting involve stresses on lithospheric

plates exerted from plate margins, as opposed to upwellingof mantle below active rifts (e.g., Sengör and Burke 1978;Ruppel 1995; Sengör 1995); thus, the cause of extension thatproduced the Baker Lake Basin was probably related to tec-tonic events at the continental margin. At ca. 1850 Ma, thewestern cratonic margin was on the west side of Slave Prov-ince, represented by the ca. 1950–1840 Ma Wopmay Orogen(Hoffman 1988). Closer to the Baker Lake Basin in the Sas-katchewan–Manitoba segment of the Trans-Hudson Orogen,the 1865–1850 Ma Wathaman Batholith records continental-arc magmatism after accretion of the La Ronge – Lynn Lakearc (Meyer et al. 1992; Corrigan et al. 2001, 2005; Fig. 14a).Convergent margins can induce hinterland extension by hingeretreat owing to slab rollback, which is accompanied byback-arc extension (Forsyth and Uyeda 1975; Molnar and At-water 1978). Ansdell et al. (1995) has proposed that cessa-tion of Wathaman Batholith magmatism was because of ca.1850 Ma accretion of the Flin Flon – Glennie Complex tothe southern Hearne craton (Fig. 14b). Subduction continuedon the southeastern side of the Flin Flon – Glennie Complex,and the Kisseynew back-arc basin formed between 1850 and1840 Ma because of subduction rollback. Felsic and maficretro-arc magmatism continued from 1840 to 1820 Ma (Hol-lings and Ansdell 2002). Basin inversion occurred during theterminal collision phase of the Trans-Hudson Orogen begin-ning at ∼1830 Ma, and rocks of the Kisseynew basin under-went amphibolite-facies metamorphism at ca. 1815–1800 Ma, during collision of the Superior craton (Ansdell etal. 1995; Hollings and Ansdell 2002).With respect to Trans-Hudson Orogen, extension in the

Baker Lake Basin and, thus, along the STZ, coincided with

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the initiation of the Kisseynew back-arc basin and couldsimilarly be a response to subduction rollback (Fig. 14c).Late Tertiary extensional basins of eastern China that formedas part of the rifting event responsible for opening of the Ja-pan Sea (Tian et al. 1992; Ren et al. 2002) provide appealinganalogues. These basins contain alkaline basalts that are in-terpreted as melts of the upper mantle (Fan and Hooper1991), similar to the alkaline basalts of the Martin Group.After rifting of a paleo-Honshu continental arc, back-arc ba-sin spreading began at ca. 22 Ma, and most of the Japan Sea

ocean floor had formed by ca. 15 Ma (Taira 2001). This wasaccompanied by the migration of arc magmatism toward thetrench, indicating slab rollback and subduction retreat. Clo-sure of the Japan Sea is postulated to be presently underway(Tamaki and Honza 1985), suggesting an opening–closingcycle of ∼30 Ma duration.The distance from the Baker Lake Basin to the Trans-Hudson

Orogen seems large (∼500 km); however, the classic retro-arc extension model was proposed based on the Rio Granderift on the east side of the Colorado Plateau (Lawton and

Fig. 14. Cartoon cross-section of the western Churchill Province illustrating a proposed tectonic model for the Baker Lake Basin evolution.(a) After accretion of the LaRonge – Lynn Lake arc to the southern Hearne margin, the Wathaman Batholith records Andean-style continen-tal-arc magmatism from ca. 1865 to 1850 Ma (Meyer et al. 1992). (b) By ca. 1850 Ma, the Glennie – Flin Flon domain collided with theallochthonous southern Hearne margin, resulting in cessation of Wathaman Batholith magmatism (Ansdell et al. 1995; Ansdell 2005). (c)Post-1850 Ma subduction retreat led to back-arc extension and opening of the Kisseynew back-arc basin (Ansdell et al. 1995; Ansdell 2005;Hollings and Ansdell 2002). Retro-arc extension resulted in intracontinental rifting along the Snowbird Tectonic Zone, including and recordedwithin Baker Lake Basin. Minette flows derived from the lithospheric mantle were extruded throughout the basin. (d) Closure of the Kissey-new basin began at ca. 1830 Ma, peak metamorphic conditions were achieved at ca. 1815–1800 Ma (Hollings and Ansdell 2002), the MartinGroup was deformed shortly after ca. 1820 Ma (Ashton et al. 2009), and Tyrrell shear zone was shallowly oblique at ca. 1816–1810 Ma(MacLachlan et al. 2005b). Subsidence ceased in Baker Lake Basin as faults transformed from normal to strike-slip. Since extension hadceased, melting of the upper mantle also ceased and fractionated minette magmas were extruded as felsite domes within the basin.

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McMillan 1999), ∼1000 km from the west coast of NorthAmerica (considering 50%–100% extension of the Basinand Range, this was probably 600–700 km). The RioGrande rift formed during subduction along the west coastof Laurentia, but prior to subduction of the East PacificRise and large-scale Basin and Range extension (Wernickeand Snow 1998; Lawton and McMillan 1999). Based inpart on the Rio Grande rift, Lawton and McMillan (1999)suggested a three phase model for formation of passive con-tinental rifts behind arc systems. During phase 1, continen-tal-arc magmatism weakens the crust. Phase 2 involves slabrollback causing continental extension, lithospheric melting,and deposition in rift basins. During phase 3, slab founder-ing, mantle upwelling, and local decompression melting ofthe asthenosphere leads to eruption of ocean-island-like ba-salts within the rift basins. Phase 2 fits the Baker Lake Ba-sin (BLB) well, but phases 1 and 3 do not.First, although the earliest Hudson granites predate the

Baker Lake Basin, they are generally coeval and so thermalweakening of the crust probably does not account for thespecific location of the BLB. A crustal-scale discontinuity,such as the STZ, provides a favourable mechanical explana-tion. Second, the absence of asthenospheric melts of phase 3can be accounted for if the lithosphere remained sufficientlythick to prevent decompression melting of upwelling astheno-sphere. Thick lithosphere under this region of westernChurchill Province is supported by magnetotelluric data(Jones et al. 2002).In considering the minette – calc-alkaline granite associa-

tion (MCG) of the Christopher Island Formation lampro-phyres, Peterson et al. (2002) have speculated on retro-arcprocesses including slab breakoff, asthenospheric upwelling,and post-orogenic extension. The favoured model for genera-tion of the Hudson granite suite was anatexis related to ca.1900 Ma shortening. Subsequent reports indicating ca.1900–1850 Ma Barrovian metamorphism from the southernRae domain and northwestern Hearne sub-domain are sup-portive (Zaleski et al. 2001; Berman et al. 2002a, 2002b), asthere was sufficient time for geotherms to rise and promotemelting of the lower crust beginning at ca. 1850 Ma. Crustalthickening, however, was discounted as the sole cause oflithospheric melting because of the high liquidus tempera-tures of minettes, and it was noted that the lack of evidencefor a juvenile mantle source is an outstanding problem withrespect to a heat source (Peterson et al. 2002), such as up-welling asthenosphere owing to slab foundering (e.g., Hol-lings and Ansdell 2002). For generation of minette melts,Cousens et al. (2001) favour a model of decompression melt-ing of the lithospheric mantle during lateral escape tectonicsalong the STZ as a response to terminal collision of theTHO. Additionally, Sandeman et al. (2000) propose thatmelting of the lower crust to produce the Hudson granitesuite was a response to advective heat flux by minette mag-mas, and so an anatectic model for the Hudson granites is nota necessity.Combining arguments offered by previous authors, a modi-

fied model would be retro-arc extension owing to slab roll-back during formation of the Kisseynew back-arc basin(Hollings and Ansdell 2002), decompression melting of thelithospheric mantle (Cousens et al. 2001), which resulted inadvective heat flux by alkaline magmas to melt the lower

crust, thus producing the Hudson granite suite (Sandeman etal. 2000). Moreover, thick lithosphere indicated by the mag-netotelluric survey (Jones et al. 2002) would have preventedmelting of potential upwelling asthenosphere. Extension post-dates a regional shortening event (Berman et al. 2002a,2002b), and so it is possible that thickened crust and elevatedgeotherms contributed to generation of the Hudson granitoids(Peterson et al. 2002).The retro-arc model has advantages over other models.

With respect to lateral escape tectonics along the STZ (Cou-sens et al. 2001; Rainbird et al. 2003), retro-arc extensionprovides a better, more widespread mechanism for litho-spheric extension and therefore generation of melts via adia-batic decompression and also for subsequent thermalsubsidence to form the intracratonic Thelon Basin. Comparedto a heat source provided by upwelling asthenosphere (Peter-son et al. 2002) owing to slab foundering (Hollings and Ans-dell 2002), which would certainly have aided in meltgeneration, a retro-arc extension model provides strong ra-tionale for the occurrence of a not just magmatism but also asedimentary basin preserved along a crustal-scale discontinu-ity, namely the STZ.

Evolution of Baker Lake Basin: Baker SequenceContinent–continent collision of the Superior and Rae–

Hearne cratons led to tectonic shortening in Saskatchewan–Manitoba (ca. 1830–1800 Ma) and Baffin Island segmentsof the Trans-Hudson Orogen (e.g., Lewry et al. 1994; St.Onge et al. 2006), including the Committee Bay supracrustalbelt located west of Baffin Island (ca. 1850–1820 Ma; Car-son et al. 2004; Fig. 1). The Kisseynew back-arc basin under-went inversion and metamorphism by ca. 1820 Ma (Ansdellet al. 1995; Hollings and Ansdell 2002), shortly before fold-ing and thrusting of the Martin Group (Ashton et al. 2009).However, it is proposed that because the Baker Lake Basinwas situated between the deformational foci of the Superior– western Churchill Province collision, it was not subject tosignificant shortening-related deformation.Orogenic shortening associated with Trans-Hudson Orogen

terminal collision, however, may have driven lateral escapetectonics in the Baker Lake Basin area, characterized bystrike-slip faults and basins, as described from southeastAsia adjacent to the Himalayan Orogen (Tapponier and Mol-nar 1979). Notably, the latest motion on the normal fault sys-tems that bounded the Baker Lake Basin would have beenprimarily strike-slip, contemporaneous with the dextral-normalshear sense associated with a lineation plunging shallowlynortheast on the Tyrrell shear zone between 1816 and1810 Ma (MacLachlan et al. 2005b). Because strike-slipfault systems shorten and extend without a change in thick-ness, widespread decompression melting of lithosphericmantle would have ceased, consistent with eruption of frac-tionated felsite flows. In Baker Lake sub-basin, there areonly a few thin deposits younger than the minette flows,suggesting that subsidence was minor. In the late stages of ba-sin evolution, sediment and volcanic flux, therefore, outpacedaccommodation, which resulted in the post-rift signature ofsequence B-5 (Fig. 14d). This is consistent with the two phasehistory of the Baker Lake Basin outlined previously: initiallywithin a high accommodation, extension-dominated setting

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(Fig. 14c), followed by a low accommodation, strike-slip-dominated system (Fig. 14d).

Evolution of Baker Lake Basin: Whart SequenceRhyolite flows, dated at 1757.6 ± 3.4 and 1753.0 ±

1.7 Ma (Rainbird and Davis 2007), and sedimentary depositsof the Whart Sequence unconformably overlie the Baker Se-quence. Rainbird et al. (2003) considered this to be a secondrift phase, subsequent to initial rifting that formed the BakerLake Basin. Detrital zircon populations from the Whart Se-quence include ca. 1830 Ma ages likely derived from theHudson granitoids (Rainbird and Davis 2007). Exhumationand erosion of the middle crustal levels into which the Hud-son granitoids were intruded (Peterson et al. 2002), is alsoreflected in ca. 1795–1713 Ma Ar–Ar ages in northwesternHearne between Yathkyed and Kaminak lakes (Sandeman2001) and ∼1.75 Ga K–Ar ages in the Rae Province (Lover-idge et al. 1988).Deposition of the Whart Sequence was followed by exten-

sional faulting, in part reactivating syn-Whart Sequencefaults. These sets of faults occur throughout the greater BakerLake Basin and, therefore, represent regional post-Whart, pre-Barrens sequence extension. This was probably a continua-tion of syn-Whart Sequence faulting, where final attenuationwas accompanied by uplift and erosional planation prior todeposition of the Barrens Sequence. Such a stage wouldhave been analogous to extension in the uplifted Basin andRange (Wernicke and Snow 1998; Stewart and Diamond1990), but on a smaller scale.

Evolution of Baker Lake Basin: Barrens SequenceFollowing normal faulting, broad uplift, and chemical

weathering, the Thelon Formation of the Barrens Sequencewas deposited over a well-developed regolith across BakerLake and Thelon basins (Ross and Chiarenzelli 1985; Gall1994; Rainbird et al. 2003). Thelon Basin is interpreted asan intracontinental basin with slow, long-lived subsidencethat probably was contemporaneous with formation of theAthabasca Basin in northern Saskatchewan (Gall et al. 1992;Rainbird et al. 2003). Thelon Basin postdates rifting of theBaker and Whart sequences and so it fits models involvingpost-rift thermal subsidence (e.g., DeFrito et al. 1983), asproposed by Rainbird et al. (2003).

ConclusionRift initiation within the Baker Lake sub-basin resulted in

deposition of coarse siliciclastic alluvium and localized extru-sion of crustally contaminated, mantle-derived magmas (Pe-terson et al. 2002) within northeast-trending half-graben.During rifting, increased areal extent of the basin because ofback-stepping of the normal fault system, as inferred fromretrogradation of the sedimentary facies, was accompaniedby voluminous and widespread extrusion of near primaryupper mantle melts (Peterson and LeCheminant 1993; Cou-sens et al. 2001; Peterson et al. 2002). Basin-bounding faultsare considered to have been oblique-slip, with overall exten-sion oriented approximately north–south. This is consideredto have been contemporaneous with normal offset along apotential basin-bounding fault, the Tyrrell shear zone, be-tween ca. 1835 and ca. 1816 Ma (MacLachlan et al. 2005b).The greater Baker Lake Basin is considered to have been

part of a series of extensional, volcanic-influenced basinsspanning the length of the STZ, including the Martin groupin northern Saskatchewan. Intracontinental extension is pro-posed as a far-field response to extension associated with ca.1850–1840 Ma opening of the Kisseynew back-arc basin onthe southeastern margin of the Rae–Hearne craton (Fig. 14c).Suggested analogs for this tectonic setting include the RioGrande Rift (Lawton and McMillan 1999) and Tertiary ex-tensional basins of eastern China related to late Tertiaryopening of the Japan Sea (Tian et al. 1992; Ren et al. 2002).Upon ca. 1820 Ma closure and inversion of the Kisseynew

back-arc basin during terminal collision of the Superior andRae–Hearne cratons, extension along the Snowbird TectonicZone transformed into post-ca. 1820 Ma shortening in theMartin Basin and strike-slip lateral escape tectonics in theBaker Lake Basin (Fig. 14d). Minimal subsidence in theBLB and extrusion of fractionated products of minette mag-mas (Peterson 1994; Peterson et al. 2002) accompanied thisphase. This final, post-rift, phase is postulated to be contem-poraneous with dextral-normal kinematics and shallow north-plunging lineations on the Tyrrell shear zone between ca.1816 and 1810 Ma and marking the lower ca. 1810 Ma agerange of the Hudson granites and Martell syenites.Following a second rifting event at ca. 1760–1750 Ma, re-

corded by rhyolites of the Whart Sequence, variable north–south extension was accommodated by east–west-trendingnormal faults, north-trending normal faults, and a conjugateapproximately strike-slip fault set. Extensional faulting andregional exhumation recorded by Ar–Ar cooling ages be-tween 1800–1700 Ma, was followed by deposition of theBarrens Sequence beginning at ca. 1667 Ma in a broad intra-continental basin.

AcknowledgementsThis work composes part of a Ph.D. thesis by T. Hadlari at

Carleton University. Support was provided by the GeologicalSurvey of Canada as part of the western Churchill NATMAPproject and the thesis was funded by an Natural Sciences andEngineering Research Council Discovery Grant to R.H. Rain-bird. Derek Smith provided field assistance and completed arelated B.Sc. thesis. We also thank Sandra Ookout for fieldassistance. The ideas presented here have evolved from dis-cussions with Hamish Sandeman, Brian Cousens, Larry Asp-ler, Kate MacLachlan, Simon Hanmer, Jim Ryan, SallyPehrsson, Kevin Ansdell, and Andrea Mills. An early versionof the manuscript benefited from comments by Al Donald-son, Charlie Jefferson, Brian Cousens, and Tony Peterson.The manuscript was vastly improved with comments by K.M. Ansdell, M.E. Bickford, and two anonymous reviewers.

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