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2010 Lithospheric architecture and tectonic evolution of the Hudson Bay region

Eaton, David W.; Darbyshire, Fiona

Elsevier

Eaton, D. W. & Darbyshire, F. "Lithospheric architecture and tectonic evolution of the Hudson Bay region". Tectonophysics 480 (2010) 1-22. http://hdl.handle.net/1880/47836 journal article

Downloaded from PRISM: https://prism.ucalgary.ca Lithospheric architecture and tectonic evolution of the Hudson Bay region

David W. Eaton 1 ,Fiona Darbyshire 2

University of Calgary, Department of Geoscience, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4

Université du Québec à Montréal, Centre GEOTOP, CP 8888, succ. Centre-Ville, Montréal, Québec, Canada H3C 3P8

E-mail: [email protected] E-mail: [email protected]

Tectonophysics 480 (2010) 1-22

Keywords : Abstract

Trans-Hudson orogen; Hudson Bay; Hudson Bay conceals several fundamental tectonic elements of the intracratonic basin; lithospheric root North American continent, including most of the ca. 1.9-1.8 Ga Trans- Hudson orogen (THO) and the Paleozoic Hudson Bay basin. Formed

due to a collision between two cratons, the THO is similar in scale and tectonic style to the modern Himalayan-Karakorum orogen. During collision, the lobate shape of the indentor (Superior craton) formed an orogenic template that, along with the smaller Sask craton, exerted a persistent influence on the tectonic evolution of the region resulting in anomalous preservation of juvenile Proterozoic crust. Extensive products of 2.72-2.68 Ga and 1.9-1.8 Ga episodes of subduction are preserved, but the spatial scale of corresponding domains increases by roughly an order-of-magnitude (to 1000’s km, comparable to modern subduction environments) from the Archean to the Proterozoic. Based on analysis of gravity and magnetic data and published field evidence, we propose a new tectonic model in which Proterozoic crust in the southeastern third of Hudson Bay formed within an oceanic or marginal-basin setting proximal to the Superior craton, whereas the northwestern third is underlain by Archean crust. An intervening central belt truncates the southeastern domains and is interpreted to be a continental magmatic arc. Thick, cold and refractory lithosphere that underlies the Bay is well

imaged by surface-wave studies and comprises a large component of the cratonic mantle keel beneath North America. The existence of an

unusually thick mantle root indicates that subduction and plate collision during the Trans-Hudson orogeny were ‘root-preserving’ (if not ‘root-forming’) processes. Although the Hudson Bay basin is the largest by surface area of four major intracratonic basins in North America, it is also the shallowest. Available evidence suggests that basin subsidence may have been triggered by eclogitization of lower crustal material. Compared to other basins of similar age in North America, the relatively stiff lithospheric root may have inhibited subsidence of the Hudson Bay basin. 1 Introduction that the Nastopoka arc may have formed as part of an extensive Precambrian continental collisional Hudson Bay is a vast inland sea that orogen, linked to the closure of an ancient ocean penetrates deeply into north-central Canada, basin. spanning an area of 1350 × 1050 km and forming a conspicuous element of the North American Almost a decade and a half later, the term coastline. The Bay conceals several fundamental Trans-Hudson orogen (THO) was coined by Hoffman tectonic elements of North America, including most (1981), in reference to the orogenic belt about which of the Paleoproterozoic Trans-Hudson orogen and the Wilson had speculated. The THO is now recognized as Paleozoic Hudson Bay basin. Thick, cold and a ca. 1.9-1.8 Ga collisional orogen of continental refractory lithosphere that underlies the Bay proportions; it extends through the centre of North comprises a large component of a cratonic mantle America, from its subsurface truncation beneath the keel that forms the nucleus of the continent. With its Great Plains by the slightly younger North American central location within Laurentia, knowledge of the Central Plains orogen, northward through lithospheric structure and tectonic development of Saskatchewan and Manitoba, thence bending Hudson Bay is of fundamental importance for dramatically eastward across Hudson Bay and understanding the assembly and evolution of the northward into the Ungava peninsula of North American continent. northernmost Quebec (Fig. 1). Although there is disagreement about whether the label THO is strictly The first comprehensive, multidisciplinary applicable to orogenic belts of similar age in Baffin set of studies of Hudson Bay was published in a series Island, Labrador and parts of Greenland, the THO of volumes associated with an Earth Science nevertheless represents the dominant component of Symposium sponsored by the National Research a series of Paleoproterozoic orogens that welded Council of Canada in 1968 (Beals, 1968a; Hood, 1969). together a group of Archean cratons, including the Among contributions ranging from Inuit culture to Superior and Western Churchill cratons 1, to create ecosystems, these volumes contain hallmark Laurentia, the North American protocontinent investigations of regional geology, geophysics and (Hoffman, 1988). Approximately coeval with the theories of the origin of Hudson Bay. Lee (1968) Svecofennian orogen of the Fennoscandian Shield reviewed the Quaternary geology of the region, (e.g., Nironen, 1997) and intensively studied in areas focusing on the ca. 18-10 ka retreat of the Laurentide of exposure on both sides of Hudson Bay, the THO is Ice Sheet, inundation of the region by the Atlantic one of the best-preserved Proterozoic orogenic belts Ocean via Hudson Strait to form the Tyrell Sea, and in the world. In view of the vast extent of (and glacial isostatic uplift since ca. 7 ka. Seismic refraction voluminous literature about) this orogenic system, surveys acquired between 1963 and 1965 (Hobson, the present study does not consider areas north and 1968; Ruffman et al., 1968; Hajnal, 1969) revealed up east of the Ungava peninsula. to 2 km of Paleozoic sedimentary rocks beneath the centre of Hudson Bay and a two-layer crust with a It has long been known from global and regional thickness of approximately 40 km. Ship-borne seismic tomographic studies that the present-day magnetic surveys covering the western half of the upper mantle beneath Precambrian cratons is Bay (Morley et al., 1968) and seafloor gravity generally characterized by high seismic velocities measurements (Innes et al., 1968) provided a basis (e.g., Jordan, 1975; 1978). The Hudson Bay region is for extrapolating major structural elements. Innes et no exception; indeed, a characteristic fundamental- al. (1968) noted a systematic reduction in free-air mode Raleigh-wave dispersion curve for a number of gravity values toward the centre of the Bay, and Canadian Shield paths including traverses across postulated that about 190 m of glacial isostatic Hudson Bay (Brune and Dorman, 1963) has served for rebound remains to occur in the future. Beals decades as a reference model for shield areas (1968b), noting the paucity of impact structures on worldwide. The origin and vertical extent of high- Earth in relation to the Moon and Mars, was struck by velocity roots beneath Precambrian cratons, the nearly circular shape of the southeastern shoreline (the Nastapoka arc) and speculated that 1 The name Western Churchill craton refers collectively to this feature may have been caused by a giant the (dominantly) Archean Rae and Hearne domains, together meteorite impact. Wilson (1968) commented on this with juvenile Proterozoic terranes accreted to the Rae- interpretation, and offered the alternative suggestion Hearne landmass prior to its collision with the Superior craton. however, remains a matter of significant debate (Eaton et al., 2009 and references therein), with evidence pointing toward offsetting thermal and compositional contributions to explain the observed seismic-velocity structure (Pedersen et al., 2006)

Figure 1 . Simplified tectonic map showing currently understood crustal subdivisions of Hudson Bay and adjacent regions of the Canadian Shield (after Bleeker and Hall, 2007). Abbreviations are as follows: THO, Trans-Hudson orogen; MP, Manitoba Promontory; QP, Quebec Promontory (Ungava Peninsula), HBB, Hudson Bay basin; HSG, Hudson Strait graben; FB, Foxe Basin; MRB, Moose River Basin; NACP, North American Central Plains orogen, STZ, Snowbird Tectonic Zone. Upper left inset compares the shape and extent of the THO with the modern Himalayan orogen (St. Onge et al., 2006).

This paper compiles and synthesizes published geophysical investigations of the Bay. Our numerous diverse, multidisciplinary investigations, review concludes with a discussion of several spanning nearly half a century, of the lithospheric hypotheses that are potentially testable using data structure and evolution of Hudson Bay. By combining currently being acquired in several ongoing studies undertaken at various scales and in different investigations of the Hudson Bay region. Earth Science disciplines, our objective is to develop a holistic perspective that emphasizes features linking the tectonic evolution of the upper crust, lower crust, lithospheric and sublithospheric mantle. To provide a regional geological framework for this study, we begin by reviewing the lithotectonic setting of the areas that surround Hudson Bay. We then focus on 2 Regional Geological Framework are thought to be continental remnants, dominated by plutonic material and containing exposures of > 2.1 Superior craton 3.0 Ga rocks. Between these superterranes, the Oxford-Stull domain comprises lithologies consistent With an area of approximately 1.6 million with an ancient continental margin, imbricated with square kilometers, the Superior craton is the largest fragments of oceanic crust. It is thought that this Archean craton on Earth (e.g. Thurston et al., 1991). domain represents the North Caribou margin, The craton is composed of a complex assemblage of trapped between the two superterranes during their terranes including granite-greenstone belts, accretion. From this ancient nucleus, growth of the metasedimentary sequences, and volcanic and Superior craton progressed from north to south along plutonic complexes. Assembly of the Superior craton a series of north-dipping subduction zones, took place from ~3.0 to ~2.6 Ga, through a culminating in the accretion of the Minnesota River progression of orogenic events that are interpreted Valley domain ca. 2.68 Ga (Percival et al., 2006). to be analogous to present-day plate-tectonic mechanisms. Evidence for subduction-like signatures In common with the western Superior within the Superior crust and upper mantle can be craton, the central-southern region is characterized seen in both controlled- and passive-source seismic by largely east-west trending subprovinces. The images, largely collected through the LITHOPROBE largest of these is the Abitibi terrane, composed of programme (e.g. Clowes et al., 1992; 1999). The multiple generations of volcanic and plutonic western and central regions of the Superior craton material, including the largest greenstone belt are characterized by roughly east-west trending exposure on Earth. The structure and composition of domains (e.g. Percival 1996; Percival et al., 2006), the Abitibi igneous suites are consistent with the whereas in the northeastern region of northern accretion of oceanic arcs through subduction Quebec, domains have a predominantly north-south processes (e.g. Chown et al., 2002; Daigneault et al., strike (e.g. Percival et al., 1994). 2002). However, geochemical evidence, in particular the presence of komatiite suites, also suggests the The western Superior craton is largely involvement of mantle-plume processes in the composed of east-west-trending igneous (plutonic formation of the Abitibi terrane (e.g. Sproule et al., and volcano-plutonic), metasedimentary, and high- 2002). grade gneissic subprovinces (Fig. 2). These terranes are thought to have been assembled by a process of The most northerly of the east-west trending subduction and accretion of continental and oceanic terranes of the eastern Superior are the Opinaca (or fragments over a time period of 2.72-2.68 Ga. Nemiscau-Opinaca) subprovince, a predominantly Continental remnants within the Western Superior metasedimentary assemblage (e.g. Telmat et al., have ages of up to 3.7 Ga, whereas the domains of 2000), and the volcano-plutonic La Grande oceanic affinity are typically younger. This region has subprovince. The La Grande region is a complex been studied in considerable detail over a number of assemblage of tonalitic Mesoarchean basement decades, amassing a huge body of literature on scales rocks, metasediments, volcanic strata and granitoid from local geological studies to province-wide intrusives. Most of the basement is dated at ~2.88- tectonic reconstructions. Over time, the classification 2.79 Ga, though older zircons suggest that the area of the terranes and domains making up the western has also been influenced by the tectonic reworking of Superior has evolved, resulting in a number of older crustal material (e.g. Telmat et al., 2000; Benn changes of nomenclature. An exhaustive review of and Moyen, 2008). studies of the western Superior can be found in Percival et al. (2006). In the far east of the Superior craton, the Ashuanipi Complex contains predominantly high- The initial formation of a composite Superior grade metasedimentary rocks and a number of craton comprised the collision of two superterranes plutonic assemblages. Both easterly and north-north- in the northwest of the province, the Northern westerly structural trends are observed. There is Superior and North Caribou superterranes. Within the evidence for correlation between the Ashuanipi rocks former, granulite-grade gneisses in the northwestern and the lower-grade metasediments of the La margin of the Superior craton are referred to as the Grande, Opinaca and Opatica belts, and with terranes Pikwitonei gneiss terrane (Weber, 1990). Both blocks

Figure 2. Simplified tectonic map of the Superior Province and environs (modified from Percival, 2007). The entire region from the northern Bienville Subprovince northwards is referred to in the literature as the Minto Block. Within this block, the Lake Minto, Goudalie, Utsalik and Douglas Harbour domains together make up the Rivière Arnaud Terrane; remaining domains within the Minto Block are grouped together into the Hudson Bay Terrane.

further west such as the Quetico terrane, leading to The Minto Block is the largest-scale tectonic the interpretation that parts of these terranes component of the Northeast Superior craton. It is represent a large-scale Neoarchean accretionary recognized as one of the largest high-grade prism (e.g. Guernina and Sawyer, 2003). Analysis of metamorphic regions of the Canadian Shield (e.g. the igneous suites contained within the Ashuanipi Percival et al., 1994). The region is largely composed Complex suggests a history of subduction of young of Neoarchean plutonic suites, but its overall oceanic crust, ridge-trench collision, and exposure of evolution is thought to span a period of at least 3.8- the lower crust to asthenospheric temperatures 1.9 Ga. On a broad scale, the Minto Block has been through the formation of a slab window in the partitioned into two major lithotectonic domains, subduction system (Percival et al., 2003). based on both lithological and potential field data (e.g. Boily et al., 2009 and references therein). In the To the northwest, the Bienville subprovince east, the Rivière Arnaud terrane consists of (Fig. 2) is primarily composed of granodiorites and isotopically juvenile tonalite-trondhjemite- granites with crystallization ages of ~2.73-2.68 Ga and granodiorite (TTG) crust of age < 3.0 Ga. To the west sources of up to 3.3 Ga in age (e.g. Percival, 2007). and south, the Hudson Bay terrane is made up of Structural trends, magnetic anomalies and isotopically enriched TTG crust and greenstone belts, geochemical signatures suggest a common magmatic largely spanning an age range of ~3.9-2.9 Ga. The two history with the domains of the Minto Block in the terranes are separated by a metasedimentary northeastern Superior (Percival et al., 2001). domain. The two major terranes of the Minto Block leading to the exhumation of lower-crustal rocks from can be further subdivided into constituent lithologic ~30 km depth to the surface. Shortening in the lower domains (Fig. 2). In contrast to those of the rest of crust was accommodated by the formation of a the Superior craton, the Northeast Superior tectonic crustal root, resulting in crustal thicknesses of~48-52 domains are characterized by a north-south trend. km, in contrast to the ~40 km average in surrounding Along the margin of Hudson Bay, the Inukjuak and regions (e.g. Boland and Ellis, 1989). Tikkerutuk domains are composed of suites of TTG and monzodiorite-granite. (e.g. Stevenson et al., Magnetic anomalies in the northwestern 2006). Geochemical data suggest that the parental Superior craton terminate abruptly against the Winisk melts of these suites interacted with older (pre-3.0 River fault (WRF), a geophysically-defined lineament Ga) crustal reservoirs. Recently the Nuvvuagittuq that extends for 300-600 km in the Hudson Bay greenstone belt on the Hudson Bay coast, which lies lowlands of northern Ontario. This feature has been within the Inukjuak domain, has come under intense interpreted as a major dextral strike-slip fault with > scrutiny. This region consists of a suite of volcanic and 100 km of displacement (Gibb, 1976), based on a metasedimentary rocks including amphibolites, reconstruction of horizontally-displaced gabbroic and ultramafic dykes and banded iron aeromagnetic and gravity anomalies on either side of formations. The suite was dated at 3.8 Ga, with the the fault zone. Strike-slip motion along the WRF has surrounding tonalites dated at ~3.6 Ga (e.g. O’Neil et been interpreted to link Proterozoic collisional al., 2007). More recently, detailed analysis of the so- deformation at the edge of the Superior craton to called “faux-amphibolites” within the greenstone uplift of the Kapuskasing structure (Gibb, 1983). belt, using 142-Nd, has suggested an age of 4.28 Ga, making these units the oldest intact rocks so far found on Earth (O’Neil et al., 2008). 2.3 Superior Boundary Zone

Baragar and Scoates (1981) coined the term 2.2 Kapuskasing Uplift and WInisk River Fault Circum-Superior belt in reference to Paleoproterozoic supracrustal sequences of broadly similar age, The Archean Wawa, Abitibi, Quetico and stratigraphy and lithology, distributed unevenly in Opatica belts are cut by the northeast-southwest- discrete segments around the periphery of the trending Kapuskasing Uplift (Kapuskasing Structural Superior craton. These segments include the Zone), a zone of high-grade metamorphic rocks Thompson Belt, Fox River Belt (Halden, 1991), Sutton stretching for over 500 km across the central Superior Inlier, Belcher Fold Belt, and Cape Smith Belt (Fig. 3) craton. This complex structure and its tectonic history as well as others on the southern and eastern is described in detail in a geotectonic synthesis by margins of the Superior craton. Although significant Percival and West (1994) based on LITHOPROBE studies; regional differences exist, each of these areas share here we summarize the main features of the region. certain features: namely, the base of the supracrustal The zone is characterized by outcrops of granulite- sequence rests unconformably upon Archean facies rocks of lower-crustal origin (e.g. Percival and basement gneisses of the Superior craton; ultramafic McGrath, 1986), positive aeromagnetic and Bouguer rocks and iron formation are common, and gravity anomalies, and anomalously thick crust. While sedimentary units exhibit a transition from shallow to the overall evolution of the Kapuskasing Zone can be deep water facies, both up-section and away from traced over a period from ~2.8 to ~1.1 Ga, it is the craton. On the basis of these similarities, Baragar thought that the main phase of deformation and and Scoates (1981) postulated that the Circum- uplift occurred in the time period 1.95-1.85 Ga. The Superior belt represents vestiges of a continental deformation likely occurred in response to terrace prism deposited along a once-continuous transmission of stresses into the interior of the passive margin that fringed the entire Superior Superior craton, caused by orogenic events at the microcontinent, in a manner analogous to the Superior margins (Trans-Hudson and Penokean margins of modern Greenland. Rocks of similar age orogens). Dextral transpression and thrusting caused and tectonic setting also occur in the Fennoscandian approximately 27 km of shortening and 50 km of Shield, such as the Jormua ophiolite in central Finland transcurrent motion across the Kapuskasing Zone, (Peltonen and Kontinen, 2004).

Figure 3 . Compilation map showing the currently understood geological framework of the Trans-Hudson orogen and Hearne domain (after Roksandic, 1987; Hoffman, 1990; Böhm et al., 2000; St.Onge et al., 2001; Berman et al., 2005). Abbreviations are as follows: ORSZ, Owl River shear zone; SH, southern Hearne domain; CHSB, central Hearne supracrustal belt; NWH, northwest Hearne domain, CD, Chesterfield domain; STZ, Snowbird tectonic zone.

More commonly known as the Superior northeast shore of Hudson Bay, known as the Ottawa boundary zone (SBZ) (or Churchill-Superior boundary Islands (Baragar and Scoates, 1981). zone), this belt forms a tectonic foreland to the Trans- Hudson orogen in several locations. Within the Farther south, Paleoproterozoic supracrustal present study area, the SBZ extends more than 2800 rocks on the east side of Hudson Bay are divisible into km from the subsurface in Manitoba to the Quebec three parts (Baragar and Scoates, 1981; Chandler, promontory. In the Cape Smith belt (Fig. 3), 1984): (1) the arkosic, alluvial Richmond Gulf group, parautochthonous rocks of the SBZ are imbricated preserved only within a younger failed rift arm, the and thrust onto the Superior craton within a foreland- Richmond Gulf aulacogen (Fig. 3); (2) carbonate-rich, verging fold-thrust belt. The lowermost part of the shallow-marine sediments (Nastapoka Group) SBZ is composed of the Povungnituk Group, deposited onto a stable platform after rifting ceased containing basal clastic sediments and volcanogenic in the Richmond Gulf aulacogen; (3) widespread iron rocks ranging in age from 2038 to 1959 Ma (St. Onge formation, flysch and arkosic molasse (also part of et al., 2000). These units are overlain by a younger the Nastapoka Group). The Richmond Gulf group was succession of tholeitic basalts and komatiites derived from the west, suggesting that the edge of (Chukotat Group) that range from 1887 to 1870 Ma the Superior basement extends considerably farther (St. Onge et al., 2000). The older age range has been west than the current shoreline of Hudson Bay interpreted to represent the time of initial rifting, (Chandler, 1984). The Nastapoka Group thickens to whereas the younger age is interpreted as a the west and, in the Belcher Islands, subgreenschist subsequent rifting event within a marginal (back arc?) metasediments intercalated with tholeiitic basalts basin setting (St. Onge et al., 2000). Between Cape have a net stratigraphic thickness of > 6 km Smith and the Belcher fold belt, komatiitic rocks (Chandler, 1984). Folding within this belt is correlative with the Chukotat Group are well exposed characterized by broad synclines and narrow along a string of small offshore islands near the anticlines with upright axial planes (Baragar and Scoates, 1981). In the Sutton Inlier (Fig. 3), correlative In the Thompson belt, like the Cape Smith rocks are exposed within a relatively thin (~300m), belt, the Superior craton formed the lower plate flat-lying and unmetamorphosed sequence above which a fold-thrust belt developed. Within the comprising a lower carbonate and an upper Thompson belt, tectonized supracrustal rocks of the greywacke / iron formation, both intruded by mafic Ospwagan group exhibit strong lithologic similarities sills (Bostock, 1971). to the Fox River belt, although the metamorphic grade and degree of deformation in the Thompson The Fox River belt is a north-facing belt is much higher. These rocks comprise homocline, 15-20 km wide, containing a thick (~ 7 km) metasedimentary, metavolcanic and ultramafic sequence of greenschist-grade clastic and dolomitic sequences that occur as infolded and/or fault- rocks, divided into Lower, Middle and Upper bounded slivers within overprinted Archean formations, intercalated with basalts with an migmatites of the Superior craton (Baragar and aggregate thickness of 5 km (Baragar and Scoates, Scoates, 1981; Weber, 1990). In this area, the nature 1981). The middle unit hosts the 1883 ± 2 Ma Fox of the SBZ evolved over a ~ 200 Ma convergent River sill (Heaman et al., 1986), a differentiated margin history from a lower-plate collisional thrust intrusion containing rocks ranging in composition belt setting at > 1.88-1.81 Ga, through inferred from dunite to mafic granophyre. This tabular body lithospheric delamination at ~ 1.82-1.81 Ga, to a has an average preserved thickness of 2 km and steep transpressive plate boundary at 1.80-1.72 Ga extends for 152 km, with a 12 km gap in the middle; (White et al., 1999). The geometry of the Manitoba aeromagnetic anomalies suggest that the sill promontory appears to have played a primary role in continues to the east, beneath Paleozoic sediments, the tectonic evolution of the region, allowing for at least another 100 km (Baragar and Scoates, preservation of a foreland thrust belt within the SBZ 1981). Although it has rift-type chemical affinities, as a result of reduced collisional convergence and the Fox River sill (as well as the associated Molson subsequent exhumation (White et al., 2002). dyke swarm) was not formed during initial rifting of the Superior craton, as suggested by Baragar and Scoates (1981), because it is coeval with, or younger than, arc volcanic rocks in the THO (Weber, 1990). 2.4 Trans-Hudson Internides Like the Chukotat Group of the Cape Smith belt, the The internides of the THO, well exposed synorogenic Fox River sill most likely formed within a northwest of the Quebec and Manitoba marginal or back-arc basin setting (Heaman et al., Promontories, represent an assemblage of juvenile 1986; Halden, 1991). Weber (1990) suggested that intraoceanic and continental arc terranes thought to the rifted margin of the Superior craton lies an have formed within or adjacent to a Pacific-scale unknown distance north and west of the Fox River ocean basin (Symons and Harris, 2005), termed the sill. Manikewan ocean (Stauffer, 1984). During closure of Weber’s (1990) suggestion is supported by the ocean basin, these terranes were accreted to the more recent field mapping and geochronology in the Hearne domain to form the Churchill plate, which area north and west of the previously accepted subsequently collided with the Superior craton. suture between the Superior craton and the THO. In northern Quebec, closure of the Based on U-Pb zircon and Nd model ages, Böhm et al. Manikewan ocean is marked by the Bergeron suture (2000) reported evidence for very old (> 3.5 Ga) (Fig. 3), which separates allocthonous terranes from exotic crust within metasedimentary units that had Archean basement and overlying parautochthonous previously been correlated to the Proterozoic rocks of the Cape Smith belt (St. Onge et al., 2001). Kisseynew domain. These results imply that long- The hanging wall of the suture contains the Watts standing models for the location of the suture in this Group, an obducted 1998 ± 2 Ma ophiolite complex area (Lenton and Corkery, 1981) require revision. In with characteristics of both mid-ocean ridge and the preferred model of Böhm et al. (2000), the suture ocean-island environments (Scott et al., 1992, 1999). correlates with Owl River shear zone (Gibb, 1976), a To the north, the Narsajuaq Island Arc terrane forms dextral strike-slip fault that extends northeast from the leading edge of the Churchill plate; it contains the Manitoba promontory (Fig. 3). The broader fore-arc clastic deposits, two suites of volcano- implications of this interpretation are discussed plutonic rocks and a rifted sliver of Archean below. continental crust (Thériault et al., 2001). The older the Wathamun-Chipewyan batholith postdates ca. volcano-plutonic suite (1863-1845 Ma; Dunphy and 1.88 Ga accretion of the La Ronge arc to the Hearne Ludden, 1998) is interpreted as an island arc craton. Maxeiner et al. (2005), however, argued assemblage built on Proterozoic oceanic crust (Watts against this model based on evidence for younger Group ophiolite). A younger (1842-1820 Ma) cross- tectonic interaction of the La Ronge arc with cutting suite of plutons is interpreted to have formed turbidites in an oceanic realm. in an Andean-type continental arc setting (Dunphy and Ludden, 1998; Thériault et al., 2001). Due to lack of exposure, little is known about the crustal architecture of the THO beneath Hudson In Saskatchewan and Manitoba, the Bay. Based on interpretation of gravity and magnetic Reindeer zone (Stauffer, 1984; Fig. 3) is a collage of data, coupled with a 200-km industry seismic profile juvenile supracrustal belts and associated that imaged the upper 8 km of the crust in eastern sedimentary basins that formed and accreted to the Hudson Bay, Roksandic (1987) proposed a model for Hearne domain during the interval 1.92-1.84 Ga tectonic subdivisions and crustal structure beneath (Hoffman, 1988; Lewry and Collerson 1990; Lewry et the Bay. This model includes a belt extending ~ 300 al., 1994, Ansdell 1995). Two distinct composite km offshore from the Ontario and Quebec shorelines, island-arc terranes are known to have amalgamated composed of two major domains: the Belcher fold and accreted at different times: the La Ronge arc and belt (considered ensialic in nature) and the Winisk the Flin Flon / Glennie complex (Corrigan et al., 2005). structural low, considered to be of oceanic affinity, A backarc ocean floor assemblage (Stern et al., 1995), possibly a back-arc basin. Roksandic (1987) a supra-subduction zone ophiolite (Wyman, 1999) speculated that an Archean microcontinent (the and a deformed ophiolite formed within a backarc Hudson Bay protocontinent) exists beneath the environment (Maxeiner et al. 2005) have thus far centre of Hudson Bay, mantled to the south and east been recognized within the Reindeer Zone. Terminal by the Winisk structural low. Berman et al. (2005) collision occurred between 1.83 and 1.79 Ga recently invoked collision with the Hudson Bay throughout the Reindeer Zone (Lewry, 1981; Bickford protocontinent to explain 1.85 Ga metamorphism et al., 1990; Lewry and Stauffer, 1990), but complete observed in the Rae domain. Hoffman (1990) convergence of the Western Churchill and Superior conjectured that the Severn arc, defined by magnetic continental blocks was precluded by the presence of anomalies in Hudson Bay, correlates with continental the Sask craton (Fig. 1), resulting in the preservation arc magmatism (the Wathamun-Chipewyan batholith) of anomalous amounts of juvenile material (Hajnal et along the northwestern external domain of the THO. al., 2005). Although exposed only in a few isolated tectonic windows, the Archean-Proterozoic Sask craton is known to extend beneath large areas where only Proterozoic rocks occur at the surface (Bickford 2.5 Cree Lake Zone et al., 2005; Hajnal et al., 2005). The Cree Lake zone represents the reworked margin of the Archean Hearne domain, including The 1865–1850 Ma Wathaman Batholith is remnants of a ca. 2.10–1.90 Ga continental margin an Andean-type continental magmatic arc that sedimentary prism (Lewry and Sibbald, 1980). It is formed along the Hearne margin. The largest known comprised of two main subdivisions: an interior zone Precambrian pluton (Fumerton et al., 1984), it (Mudjatik Domain), dominated by ca. 2.7 Ga extends from north-central Saskatchewan to granitoids and layered granitic gneiss, with minor northeastern Manitoba, where it is named the metasedimentary screens, and a marginal zone Chipewyan Batholith. The batholith is relatively (Wollaston Domain), which contains abundant homogeneous in composition and generally coarse Proterozoic supracrustal rocks infolded with Archean grained, dominated by K-feldspar porphyritic granite basement and isolated ca. 2.2-2.0 Ga felsic intrusions with lesser amounts of granodiorite and quartz (Corrigan et al., 2005). The supracrustal rocks are monzonite (Fumerton et al., 1984; Meyers et al., interpreted to overlie Archean basement 1992). The Baldock batholith is a magnetite-bearing unconformably (Lewry et al., 1985). The boundary granitoid unit that is similar in appearance to the between the Mudjatik and Wollaston domains is Wathamun-Chipewyan magmatic complex (Halden et diffuse, corresponding mainly to a variation in the al., 1991). Bickford et al. (1990), Meyers et al. (1992) relative abundance of granitoid over and Corrigan et al. (2005) suggested that formation of metasedimentary rocks (Corrigan et al., 2005). Both Under the auspices of the Western Churchill domains were penetratively deformed and NATMAP project (Hanmer and Relf, 2000), renewed metamorphosed at moderate to high grade during field investigations of the Hearne domain are yielding the THO (Lewry and Sibbald 1980). fundamental new data to support tectonic subdivisions and models for the craton. Two major Taken together, the Reindeer Zone, subdivisions of the Hearne domain are now Wathaman-Chipewyan Batholith and Cree Lake Zone recognized: the central Hearne supracrustal belt form an orogen-parallel, shallow to moderately (CHSB) and the southern Hearne domain. The CHSB, northwest-dipping homoclinal stack. Syn- and post- exposed within a 700 km-long swath from northern collisional (1.8-1.7 Ga) intracontinental deformation Saskatchewan to Hudson Bay, is interpreted to have during the Trans-Hudson orogeny produced a series formed within an extensional, suprasubduction zone of high-angle faults and folds parallel to the axis of setting (Hanmer et al., 2004; Davis et al., 2004). The the orogen. The faults are thought to be a CSHB is comparable in size to the better-known consequence of the boundary conditions imposed by Abitibi subprovince, making it one of the largest the northeast trend of both the Superior and Hearne Neoarchean greenstone terranes in the world. cratons, and resulted in orogen-parallel extrusion of Archean rocks are overlain by abundant erosional the internides relative to the bounding Archean remnants of the 2.45-1.91 Ga Hurwitz Group, which is continental blocks (Hajnal et al., 1996). comprised of two major sequences and formed within a shallow-water intracratonic basin setting (Sandeman et al., 2003). The southern Hearne domain contains 2.72-2.70 Ga tonalitic orthogneiss 2.6 Western Churchill craton with a juvenile isotopic signature (van Breemen et al., The Western Churchill craton is one of the 2007), as well as crustal components as old as 3.5 Ga largest but least understood tracts of Archean crust (van Breemen et al., 2005). An abrupt change in 2.68 on Earth. Originally distinguished from the Ga magmatism in the southern Hearne domain has neighboring Archean Superior and Slave cratons been interpreted to represent collision with the CHSB based on abundant Paleoproterozoic K-Ar ages and a change from oceanic to continental arc (Stockwell, 1982), it is dominated by Neoarchean, magmatism (van Breemen et al., 2007). ampibolite- to granulite-grade gneisses and To the north and west, the Rae domain is supracrustal (greenstone) belts, overlain by 2.45-1.75 characterized by areas of Mesoarchean basement Ga volcano-sedimentary sequences and intruded by with a supracrustal cover succession that includes 1.83 and 1.75 Ga granite suites (Hanmer et al., 2004 komatiite-quartzite assemblages, intruded by and references therein). Extensive Paleoproterozoic extensive 2.6 Ga plutons (Zaleski et al., 2001). The reworking of Archean lithosphere reflects the Chesterfield block is similar in character and inferred disposition of the Western Churchill craton in composition to the Rae domain, but underwent 2.56- the upper plate of both the 2.0-1.9 Ga Thelon-Taltson 2.50 Ga tectonomorphism that is thought to orogen to the west and the 1.9-1.8 Ga Trans-Hudson represent collisional reworking of the composite Rae- orogen to the south and east (Berman et al., 2005). Chesterfield margin (Davis et al., 2006). Both domains The geophysically defined Snowbird tectonic zone contain several strongly deformed Archean (STZ) (Fig. 3; Gibb and Walcott, 1971; Hoffman, 1988) supracrustal belts such as the MacQuoid belt, divides the Western Churchill craton into the Hearne interpreted to reflect a geodynamic setting similar to and Rae domains (previously termed cratons). The modern back-arc marginal basins such as the Sea of age and nature of the STZ are controversial. Hanmer Japan (Sandeman et al., 2006). et al. (1995) argued that it is an Archean intracontinental shear zone with limited Unlike most of the Superior craton, the Western Paleoproterozoic reworking, but Berman et al. (2007) Churchill craton is overlain by weakly deformed present new geochronologic and microdiamond data Paleoproterozoic sedimentary sequences within the suggesting that this feature represents a 1.9 Ga 1.85-1.79 Baker Lake Basin and ca. 1.72 Ga collisional suture between two distinct Archean intracratonic basins (Athabasca and Thelon Basins; microcontinents. Fig. 3). The Baker Lake group may have formed due to (Aspler et al., 2004) during the Trans-Hudson lateral escape (Rainbird et al., 2003) or extension orogeny, whereas the two intracratonic basins are interpreted to represent the far-field response of the a system of broad, low-relief arches (Sanford, 1990). continental interior to accretion and collision Lower Paleozoic units within all of these basins are processes (Hoffman, 1988) dominated by carbonate and evaporitic successions, which record marine transgressions that can be linked with tectonic activity along the edges of North America (Sanford, 1990). Seismic-reflection profiles 2.7 Paleozoic Basins acquired in Hudson Bay by the oil industry in the

1970’s and 1980’s (Dimian et al., 1983) reveal In Hudson Bay and surrounding areas to the biohermal reefs, block faulting and salt dissolution north and south (Fig. 1), Archean and Proterozoic features that resemble hydrocarbon-bearing crust is unconformably overlain by relatively structures in other North American intracratonic undeformed lower Paleozoic and minor Cretaceous basins. A series of normal faults with a NNW-strike sedimentary rocks with a composite thickness of ~2.5 direction bound a horst-like uplift that divides the km (Sanford, 1990 and references therein). The basin into two nearly symmetrical sub-basins (Fig. Hudson Bay basin is one of four roughly 4a). This structure extends for more than 300 km and contemporaneous intracratonic basins in North has a maximum amplitude of more than 500 m. America (Quinlan, 1990), and is the largest by surface Testing of some of these structures by exploration area. Three peripheral sub-basins, the Foxe and drilling has not yet revealed commercial quantities of Moose River basins and the Hudson Strait graben oil and gas (Sanford, 1990). (Fig. 1), are separated from the Hudson Bay basin by

Figure 4 . A) Mapped faults and total sediment isopach contours in km (from Sanford, 1990) superimposed on bathymetry and elevation based on model etopo2 (National Geophysical Data Center, 2006). B) Subsidence history of Hudson Bay basin compared to the Michigan basin (from Hamdani et al., 1991). C) Cross section A-A’ across the centre of the Hudson Bay basin (Sanford, 1990). PC, O, S, SD, D,C and Q denote Precambrian, Ordovician, Silurian, Siluro-Devonian, Devonian, Carboniferous and Quaternary, respectively. Hanne et al. (2004) reviewed the GGM02 (Tapley et al., 2005) are revolutionizing our Phanerozoic vertical motions of Hudson Bay and understanding of long-wavelength (> 600 km) modelled its subsidence trends. The focus of their anomalies and their temporal variation, maps derived study was to test a hypothesis, based on global free- from conventional surface measurements are still air gravity observations (Simons and Hager, 1997), necessary to resolve smaller-scale features of interest that the Hudson Bay basin was produced by an for tectonic interpretations. Canadian Federal- unusual mantle downwelling; they found no evidence Provincial map series have long been used as a guide to support this model. In fact, relative to other for geological mapping (e.g., Gibb, 1983; Hoffman, intracratonic basins of similar size in North America, 1988), since they offer a practical means for the Hudson Bay basin shows reduced subsidence (Fig. extrapolating tectonic domains and structures into 4b; Hamdani et al., 1991). This difference may be due areas that cannot be mapped directly. Accordingly, to the greater effective thickness of the elastic potential-field maps are employed here as a guide for lithosphere in the Hudson Bay region (Hanne et al., extrapolating major geological units beneath Hudson 2004) – i.e., the stiffer lithosphere beneath Hudson Bay. Bay inhibited subsidence and deep basin formation. Figure 5 shows a Bouguer gravity map created using 145,631 individual Bouguer gravity observations, representing an average density of 2.8 3 Geophysical Data stations per 100 km 2. Within Hudson Bay, this database mainly comprises seafloor measurements 3.1 Gravity and magnetic data acquired in 1965 during a set of cruises that There is generally an excellent correlation transected most of the Bay (Innes et al., 1968), between regional magnetic and gravity anomaly together with subsequent winter acquisition patterns and major geological structures on the programs conducted on the ice-covered surface of Canadian Shield and contiguous platform regions Hudson Bay and the southern littoral zone (Green et al., 1985; Villeneuve et al., 1993; Leclair et (Mukhopadhyay and Gibb, 1981; Earth Physics al., 1997; White et al., 2005). Although recent Branch, 1982). A regular grid was created from the satellite-derived gravity datasets such as model irregular point distribution using a triangle-based cubic-spline interpolation algorithm.

Figure 5 . Bouguer gravity anomaly map of the Hudson Bay region. Boxes show locations for Figures 6 and 7. Anomalies G1- G4 are discussed in the text. Being situated mainly within the Hudson Bay density, a bulk property of crustal rocks, magnetic gravity low (Simons and Hager, 1997), Bouguer maps within shield regions primarily reflect variations gravity values throughout our study area are almost in magnetic susceptibility (Beck, 1991), which is uniformly negative (Fig. 5). The lowest gravity values controlled mainly by accessory mineral phases are peripheral to our study region and occur within (especially magnetite). In addition, because magnetic the Western Churchill craton west of Hudson Bay, the anomalies are fundamentally dipolar in nature, they Labrador trough, and the Grenville Front gravity low exhibit generally greater sensitivity to shallow (Eaton et al., 1995). Superimposed on the overall features due to the 1/ r3 falloff of dipole fields negative Bouguer gravity values are significant compared to the 1/ r2 falloff of monopole fields. regional gravity anomalies that fall below the short- wavelength cut-off of satellite gravity observations, The Manitoba promontory is characterized but are nevertheless significant for tectonic by a conspicuous east-west trending magnetic- investigations. Gravity anomaly G1 closely tracks the anomaly fabric (Kenoran magnetic fabric, Fig. 9). inferred position of the SBZ. In our study region, this Within this domain, regionally extensive magnetic gravity high is nearly continuous from southern highs correspond to greenstone belts that contain Manitoba to at least as far as the Ottawa Islands magnetite-rich mafic rocks (Green et al., 1985). The (offshore from the Ungava peninsula) in eastern Kenoran magnetic fabric terminates abruptly against Hudson Bay (Fig. 5). There are several locations along a north-south linear band of negative magnetic values the trend, particularly in the southeast corner of that delineates the western margin of the Superior Hudson Bay, where piecewise linear segments of craton, including the Thompson belt and correlative anomaly G1 define local culminations, as well as units in the subsurface (Green et al., 1985). sharp bends that are suggestive of triple-junction features (Fig. 6). In the Quebec promontory a similar map pattern exists. Magnetic anomalies in the Superior Another positive gravity anomaly (G2) is craton consist of distinctive, high-amplitude, 50-150 situated in the northeastern part of Hudson Bay. This km linear features trending N-S to NW-SE, referred to feature is elongate in a northeast-southwest direction here as the Ungava fabric. This fabric terminates and extends beneath the Narsajuaq Island Arc terrane abruptly against a regional magnetic low within the at the tip of the Ungava peninsula. Within Hudson Paleoproterozoic Cape Smith belt. The Cape Smith Bay, anomaly G2 appears to truncate (or overprint) magnetic low extends offshore for ~ 150 km into anomaly G1. Near the centre of the Hudson Bay Hudson Bay, where it abuts a linear magnetic high basin, an approximately circular gravity high (G3; Fig. (M1). Where offshore aeromagnetic coverage is 7) is situated near the region of the central horst available, the western limit of the Ungava fabric complex; a simple gravity model, however, appears to follow closely the modern shoreline of demonstrates that this gravity high cannot be Hudson Bay. explained by uplifted basement rocks, but may be caused by a high-density block near the Moho (Fig. 8). On the basis of potential-field anomaly Finally, a linear gravity anomaly (G4) extends patterns, the interior of Hudson Bay is herein eastward from the northeastern shoreline of Hudson partitioned into distinct domains (Fig. 9). A U-shaped Bay. A possible extension of this feature farther east, region in southeastern third of the Bay, here named towards Hudson Strait, is obscured since it lies near the Belcher domain after Roksandic (1987), straddles the edge of gravity-station coverage. gravity anomaly G1 and is characterized (where data are available) by generally negative magnetic- The aeromagnetic anomaly map in Fig. 9 was anomaly values with isolated positive anomalies, created using 400m gridded data. These data were including anomaly M1. Enclosed within the arc of the digitized by the Geological Survey of Canada from Belcher domain, a wedge-shaped region named the original analogue records and leveled to a common Severn domain (after Hoffman, 1990) contains datum. The flight-line spacing over Hudson Bay is intense positive magnetic anomalies. Although more more than 1600 m, so the inherent spatial resolution clearly discernible in the magnetic map, subtle gravity of the aeromagnetic grid is comparable to the gravity trends also mimic the wedge shape of this domain. map. Aeromagnetic anomaly data are The Severn and Belcher domains are separated from complementary to gravity maps for several reasons. each other by several prominent magnetic highs (M2, Whereas gravity anomalies reflect the distribution of M3) with intervening discontinuous magnetic lows. A domain that runs along a NE-SW trending corridor and/or Baldock batholiths (Fig. 9). Finally, the through the centre of Hudson Bay, here named the northwestern Hudson Bay (NWHB) domain is a zone central Hudson Bay domain (CHBD), contains a characterized by curved, bifurcating magnetic lows, negative anomaly labeled M4. This anomaly appears resembling the anomaly pattern within the Hearne to truncate M2 and M3. To the west, anomaly M4 domain west of Hudson Bay. The boundary between appears to merge with the Wathamun-Chipewyan the CHBD and the NWHB domain is unclear.

Figure 6 . Enlargement of the Bouguer gravity anomaly map for southeastern Hudson Bay. Gravity high (G1) is interpreted to represent (or lie close to) the ancient passive margin of the Superior craton. Piecewise linear segments of this anomaly exhibit geometrical similarities to arms of rift triple junctions.

3.2 Controlled-source seismic data about 26 km west of the Ottawa Islands, as well as close to anomaly G4 near Chesterfield Inlet (Hunter Controlled-source seismic data and Mereu, 1967; Ruffman and Keen, 1967). Abrupt (multichannel reflection and refraction/wide-angle changes in crustal thickness of more than 10 km may reflection) have been acquired in Hudson Bay and, be related to sutures within the Bay, but Ruffman and more recently, across the THO in Saskatchewan and Keen (1967) note large uncertainties due to Manitoba as part of Canada’s LITHOPROBE program inadequacies in the marine navigation equipment (Clowes et al., 1992; 1999). In Hudson Bay, seismic- available at that time. Using observations of chemical refraction data were acquired as part of a major explosions in Hudson Bay and Lake Superior, Barr experiment in 1965 (Hobson, 1967). Analysis of first- (1967) reported evidence for seismic discontinuities arrival data indicates significant variations in crustal at 126 and 366 km depth, noting that the thickness, from about 40 km in the southeastern observations do not require a low-velocity zone in the Hearne domain and northern Superior craton to upper mantle. Figure 7 . Enlargement of the Bouguer gravity anomaly map over the Hudson Bay basin. Sediment isopach contours and faults are from Sanford (1990). Profile A-B is shown in Fig. 8.

Figure 8 . Observed Bouguer gravity profile A-B (heavy line, top panel) across Hudson Bay, with linear trend removed. Shaded region around the observed profile shows +/- one standard deviation obtained using the ensemble method of Hope and Eaton (2002). Solid and dashed lines show calculated profiles (shifted by a constant value so that end points taper to zero) for the models indicated below. Model 1 depicts a basement uplift, derived from section A-A’ in Figure 4. Density contrast is set to an extreme value (0.5 Mg/m 3) to provide an upper bound on the amplitude of the gravity anomaly predicted for this structure. Model 2 depicts an idealized high-density (eclogitized) crustal root with a thickness of 15 km. The density contrast is based on Hamdani et al. (1991) and the shape of the root is similar to that observed beneath the Sask craton (Fig. 10).

Seismic exploration by the oil and gas the entire width of the orogen (Fig. 10) show a industry between 1973 and 1982 yielded ~ 14,400 km broadly symmetric structure, with reflections linked of seismic-reflection data, mainly concentrated near to island-arc domains dipping beneath bounding the central horst of the Hudson Bay basin (Roksandic, Archean cratons. The observed reflection geometries 1987). These seismic profiles are strongly imply doubling of crustal thickness during the contaminated by multiple reverberations, seismic collision, along crust-penetrating faults (Lucas et al., energy trapped within the water layer. Nevertheless, 1993). A crustal-scale culmination in the western part careful processing and interpretation have revealed of the Reindeer zone is cored by Archean basement key features of these data, including evidence for a (the Sask craton). West-dipping structures below the thin-skinned fold-thrust belt in the Belcher domain Wathaman Batholith and reworked Hearne craton (Roksandic, 1987). may reflect subduction polarity in this part of the orogen, whereas east-dipping structures below the Here, we focus on the newer seismic data Superior craton are largely related to late post- acquired by LITHOPROBE in Saskatchewan and collisional deformation, rather than prior oceanic Manitoba, which are of much higher quality than the subduction polarity (Lewry et al., 1994; White et al., data in Hudson Bay. Seismic reflection images across 1994).

Figure 9 . Magnetic anomaly map of the Hudson Bay region, showing inferred crustal domains. Abbreviations are as follows: RZ, Reindeer Zone, TB, Thompson Belt, ORSZ, Owl River Shear Zone, WB, Wathamun-Chipewyan Batholith, WRF, Winisk River Fault, KU, Kapuskasking Uplift, SD, Severn Domain, BD, Belcher Domain, CHBD, Central Hudson Bay domain, NA, Narsajuaq Arc, CSB, Cape Smith Belt, NWHB, northwest Hudson Bay domain, K, Kenora fabric, U, Ungava fabric. Anomalies M1-M4 are discussed in the text.

Figure 10 . Seismic profile across the Trans-Hudson orogen (THO) in Saskatchewan and Manitoba. Top panel shows crustal- domain boundaries or prominent structures (vertical exaggeration = 2:1) interpreted from a depth-migrated composite seismic- profile (modified from White et al., 2005). WB denotes Wathamun batholith, SBZ denotes Superior Boundary Zone. The lower panel shows migrated seismic data from LITHOPROBE line 9 (modified from Hajnal et al., 2005), illustrating the high quality of the data.

In addition to seismic-reflection profiles, apparent velocities of 8.4-8.7 km/s were also noted LITHOPROBE acquired approximately 2000 km of on the record sections. seismic refraction / wide-angle reflection data in 1993. The data, modelling and interpretation are Based on the seismic velocity models, summarized by Németh et al. (2005). In this survey, Németh et al. (2005) divided the region into two the subsurface of the THO region is investigated using distinct sections. In the south and east, crustal one east-west profile, running across the entire THO seismic velocities are higher than average, with little and covering parts of the Hearne domain and lateral variation and a poorly reflective Moho. The Superior craton at its extremities, and two north- north and west is characterized by lower and more south profiles in the east and west of the THO. variable crustal velocities, a more distinct upper- Seismic velocity models show that the crust is broadly lower crust boundary and a highly-reflective Moho. divided into two layers, with upper crustal thickness The transition zone between these two regions is of 12-14 km and Moho depth generally 40-45 km. The marked by the zone of anomalous uppermost mantle, models also show a high-velocity wedge-shaped block thought to be an anisotropic region and interpreted in the northeast of the orogen, which is interpreted as a mantle suture zone. as a portion of oceanic lower crust trapped during the Superior-Hearne collision. Beneath this block, the Moho deepens to 55 km. Sub-Moho seismic velocities 3.3 Surface-wave observations are variable across the profiles, ranging from 8.0 to 8.6 km/s, with the anomalously high velocities Surface waves can be analyzed to obtain confined to distinct zones in the centre of the two information along their travel paths, and so are well north-south profiles. Secondary mantle arrivals with suited to the study of Hudson Bay, within which no seismograph stations are located. The first major study of the Canadian Shield region as a whole was Seismic anisotropy, the variation in seismic carried out by Brune and Dorman (1963) using arrival- wavespeed as a function of wave polarization or time picks of surface wave peaks and troughs in long- propagation direction, is also key to understanding period analogue seismograph records. Among the the structure and evolution of the continental data used to construct the “CANSD” composite lithosphere (e.g., Eaton et al., 2009 and references dispersion curve were three 2-station paths and two therein). Radial anisotropy, related to mantle earthquake-station paths crossing Hudson Bay. The shearing without a preferred orientation, is observed resulting dispersion curve has been used as a as a difference in shear wavespeed between SV and reference data set for comparison with continental SH polarizations, and is thought to be prevalent shield regions worldwide. The “CANSD” phase beneath continental roots (e.g. Gung et al., 2003). velocities lie significantly higher than the global Azimuthal anisotropy indicates a fabric with a average to periods of ~130 seconds, suggesting the preferred orientation, and is observed as a variation presence of a high-velocity structure in the in seismic wavespeed with propagation direction. uppermost mantle which may be interpreted as the Significant radial anisotropy is restricted to depths < seismic signature of the subcratonic lithosphere. 200 km (Shapiro and Ritzwoller, 2002; Nettles and Dziewonski, 2008). Seismic evidence for the nature of The shear-wave velocity model produced by the lithospheric root can also be found in models of Brune and Dorman (1963) using the “CANSD” surface wave azimuthal anisotropy, though these dispersion curve (Fig. 11) shows a pronounced high- data are currently restricted to global-scale models. velocity lid, with a base at 110 km depth. This lid In the upper 200 km, the Rayleigh wave fast- thickness is small in comparison with modern models propagation direction lies east-west (Debayle et al., of the subcratonic lithosphere, but this is most likely 2005) to northwest-southeast (Marone and to be a product of the model calculation technique of Romanowicz, 2007). A depth slice from a global the time. Inversion of the same dispersion curve model at 300 km depth - considered to be below the using modern modelling techniques produces a lid seismological lithosphere for Hudson Bay - exhibits a thickness of approximately 150-200 km (Fig. 11). change in seismic fast direction to NNE-SSW (Marone and Romanowicz, 2007). Changes in fast-propagation Global and continental-scale mantle models, directions beneath continents have been cited as whether based on body waves or surface waves, SH evidence for a change from “frozen” lithospheric or SV velocities, all show a pronounced high-velocity anisotropy to structure related to present-day anomaly in the mantle beneath the Canadian Shield, asthenospheric flow (e.g. Debayle and Kennett, generally centred beneath Hudson Bay (Fig. 12). The 2000), but more detailed models are required to depth to the base of this anomaly varies significantly constrain the deeper fast-propagation directions depending on the data set and type of seismic beneath Hudson Bay. wavespeed modelled. Typically, V SV models indicate anomalously high seismic velocities to depths of 200- 300 km, whereas V SH and V P models suggest high velocities to at least 400 km depth. Some of the most 3.4 Other geophysical studies detailed global/continental-scale mantle models presently available rely primarily on data from Regional free-air gravity data provide surface waves, providing information on SV and SH another way to investigate the properties of the wavespeeds, radial anisotropy and azimuthal mantle beneath Hudson Bay. It has long been anisotropy (e.g. Shapiro and Ritzwoller, 2002; van der recognized that a major free-air gravity anomaly with Lee and Frederiksen, 2005; Lebedev and van der Hilst, an amplitude of about -50 mGal is located in the 2008; Nettles and Dziewonski, 2008). The average region of greatest thickness of the Laurentide Ice depth extent of the high-velocity anomaly beneath Sheet (Fig. 2b). Decomposition of this anomaly using Hudson Bay is 250 km, and the centre of the anomaly wavelet methods has demonstrated that it has a is typically beneath central or western Hudson Bay globally unique expression in the wavenumber-space (Fig. 12a). Most models suggest that the Hudson Bay domain (Simons and Hager, 1997). The spatial region is characterized by the thickest lithospheric correlation of the free-air gravity anomaly with the root in North America. former ice sheet led to a hypothesis (Innes et al., 1968; Walcott, 1970) that the anomaly is caused by incomplete glacial isostatic adjustment (GIA). Figure 11: CANSD dispersion curve (upper right) for fundamental-mode Rayleigh waves that traverse Hudson Bay (Brune and Dorman, 1963), compared with a modern global reference model (ak135; Kennett et al., 1995). Graph on the left shows inversion of the CANSD using a Monte-Carlo method (Shapiro and Ritzwoller, 2002). In contrast to the model proposed by Brune and Dorman (1963), the new modelling method suggests a lithospheric thickness between 140km (2% velocity contour above ak135) and 200km (crossover point with ak135).

This interpretation conflicts, however, with simple Bay. A negative correlation is observed between heat models for GIA based on the uplift record and flow and elastic thickness in the central and western estimates of mantle viscosity (Cathles, 1975; James, Canadian Shield (Mareschal et al., 2005) suggesting 1992). Layered viscosity models that give rise to that, even in stable continents, the elastic thickness is multiple relaxation times (Peltier and Wu, 1982) and largely controlled by the lithospheric temperatures. a gravity anomaly associated with dynamic topography arising from a mantle downwelling The advent of satellite-derived gravity data (James, 1992; Simons and Hager, 1997) have been has opened up new possibilities for measuring and proposed to account for this discrepancy, but these removing time-varying signals from regional gravity interpretations are controversial (Mitrovica, 1997). maps. Using the time rate of change of free-air gravity obtained from the GRACE satellite mission, Large lateral variations in lithospheric Tamisiea et al. (2007) found that only 25-45% of the thickness are significant for interpreting GIA signals, gravity low in the Hudson Bay region is caused by GIA. including relative sea level and long-wavelength The remaining gravity anomaly (Fig. 12b) is therefore gravity data (Wang and Wu, 2006). Effective elastic due to mantle density variations, mainly in the thickness (Te) is a proxy for lithospheric thickness lithosphere. This residual gravity anomaly also that is based on the equivalent flexural behavior of a correlates well with the seismic high-velocity anomaly homogeneous elastic layer. Audet and Mareschal at 200 km depth (Fig. 12a), in addition to a region of (2004) used spectral methods to estimate Te and high Te. found that the thickest region in the Canadian Shield occurs within the THO, beneath and west of Hudson 4 Discussion magnetic fabric in the CHBD. This relationship suggests that the Bergeron suture, which is exposed 4.1 Interpretation of potential-field anomalies in the Cape Smith belt, follows the path of magneti c- anomaly truncations across Hudson Bay (Fig. 9). The The acquisition of nearly complete concordance of magnetic anomaly M4 with the aeromagnetic (Coles and Haines, 1982) and gravity Wathamun-Chipewyan batholiths suggests that it coverage in the Hudson Bay region prompted the represents a continuation of the magmatic arc, which development of an elegant double-indentor model developed on the Churchill plate. The slightly younger for the tectonic evolution of the THO (Gibb, 1983). age of Andean-type magmatism in the Cape Smith The symmetrical distribution of gravity anomalies belt (1842-1820 Ma; Dunphy and Ludden, 1998; parallel to the southern and eastern shorelines of Thériault et al., 2001) compared to the Wathamun- Hudson Bay outlines a deep embayment of the Chipewyan batholith (1865-1850 Ma; Halden et al., Superior boundary zone (Fig. 5). Gibb (1983) 1991), suggests that magmatism might have been interpreted this embayment, together with the diachronous across the Bay. adjacent Manitoba and Quebec promontories, to have formed by ca 2.1-2.0 Ga rifting of the Superior Our proposed correlation of the Bergeron craton. According to this model, the overall shape of Suture with truncations along the southern margin of the Superior margin largely controlled the the CHBD implies that the Severn and Belcher subsequent development of the THO during domains were accreted to the Superior craton prior collisional orogenesis. This model predicts that to collision. The Belcher domain is most likely a collision initiated at the Manitoba promontory, with continental terrace wedge that was deformed in a partial accommodation of progressive NW (present- foreland fold-thrust belt that developed during the day co-ordinates) convergence of the Superior craton Trans-Hudson orogeny (Roksandic, 1987). Based on relative to the Western Churchill craton along a series the presence of arcuate magnetic highs, which are of transform faults linked to the Kapuskasing typically associated with calc-alkaline magmatic arcs structure (Fig. 3). As convergence proceeded, the (Villeneuve et al., 1993), we interpret the Severn onset of collision at the Quebec promontory and domain as an accreted island-arc terrane. Gravity development of associated transform structures high G3 is located beneath Hudson Bay, but cannot along its western margin led to two-sided crustal be explained simply by uplift of basement rocks extrusion into the Hudson Bay embayment. Gibb within the central horst (Fig. 8). One possible (1983) speculated that plugging of the embayment explanation is that this region is underlain by rocks with extruded material ultimately contributed to a similar to the Narsajuaq Island Arc terrane, which is change in plate motion. also marked by a pronounced gravity high (G1). The spatial coincidence of this anomaly with the Hudson Based on its close spatial association with Bay basin invites speculation that the anomaly is the SBZ, we interpret anomaly G1 to correlate with related to formation of the basin. Although there is the rifted margin of the Superior craton. Similar evidence from 1965 refraction data for significant elongate positive gravity anomalies are commonly Moho topography (Hunter and Mereu, 1967; Ruffman observed along modern passive margins (e.g., and Keen, 1967), the crust does not appear to thin Stephenson et al., 1994). In addition to the spatial beneath this region. Modelling of this feature as a correlation of this anomaly with rift-related rocks in dense body beneath the basin is described in section the Cape Smith region (Thomas and Gibb, 1977), an 4.4, below. offshoot anomaly from G1 near the Belcher Islands (Fig. 6) links this feature with the Richmond Gulf Finally, we note that gravity anomaly G4 is aulacogen, which is interpreted as part of the rift located near the currently interpreted location of the (Baragar and Scoates, 1981). The geometry defined Snowbird Tectonic Zone (Berman et al., 2007), which by the gravity anomaly near the Richmond Gulf is marks a proposed suture between the Hearne and strongly suggestive of a failed rift arm at a rift triple Rae domains of the Western Churchill craton. This junction. feature was first interpreted as a suture by Gibb and Walcott (1971) based on a large gravity gradient The pattern of magnetic anomalies across signature. There is evidence from the 1965 Hudson the central part of Hudson Bay indicates that the Bay refraction experiment (Hunter and Mereu, 1967) Belcher and Severn domains are truncated by a for an abrupt NW step in crustal thickness (from ~ 26 to 40 km) near the offshore location of G4, which other evidence (aside from the circular shoreline) to supports this interpretation. Ongoing studies of support such a hypothesis. By fitting Bouguer gravity Hudson Bay may yield data that will allow testing of data averaged within a cylindrical co-ordinate system these crustal models. (Fig. 13), Hynes (1991) demonstrated that the flexural model provides a viable explanation. It is interesting to note, however, that the effective elastic thickness (15 km) required to fit this flexural model is far less 4.2 Flexural origin of the Nastapoka Arc than estimates of the present-day Te (> 80 km), which Hynes (1991) argued that the nearly circular are based on coherency of topographic and free-air shape of the shoreline in southeastern Hudson Bay gravity data (Audet and Mareschal, 2004). It is likely (the Nastapoka arc), as well as a weakly developed that this difference in Te simply reflects higher crustal temperatures during the Paleoproterozoic within the peripheral forebulge in the Superior craton, is the deforming orogen, since if conditions were result of lithospheric flexure due to tectonic loading sufficiently hot to produce partial melt and channel during the Trans-Hudson orogeny. Previous authors have suggested that the Nastopoka arc may be a flow within the lower crust, then the effective elastic large astrobleme, but Hynes (1991) noted the lack of thickness would correspond only to the upper crust (C. Beaumont, pers. comm., 2009).

Figure 12. A) Isotropic (Voigt average) Vs at 200 km depth from global surface-wave inversion (Nettles and Dziewonski, 2008). High-velocity anomaly beneath Hudson Bay approximates the distribution of cold, refractory lithospheric mantle. B) Residual free air gravity anomaly field derived from GRACE satellite observations, with effects of glacial isostatic adjustment (GIA) removed (Tamisiea et al., 2007). Co-location of the lithospheric keel inferred from seismic models and a large negative gravity anomaly in the Hudson Bay area suggests that compositional and thermal buoyancy effects in the lithosphere do not completely cancel.

One element of this model that is not high degree of circular symmetry. In this context, the explained by Hynes (1991) is a plausible mechanism geometrical consequences of Gibb’s (1983) extrusion for localizing the tectonic load, which was situated at model are considered in the next section. We suggest the centre of curvature of the Nastapoka arc. Hynes that the geometry of the embayment produced an (1991) argued that, on the surface of a sphere, depth accretionary tectonic environment, similar in size and contours of a flexural foredeep in front of an kinematics to the modern Gulf of Alaska, which (by advancing orogen tend to occur along a small-circle analogy) could explain the localization of the tectonic path; most foredeeps, however, do not exhibit such a load. Figure 13 . Bouguer gravity map (left) showing best-fitting small circle to the Nastapoka arc (Hynes, 1991). Topographic map of the Gulf of Alaska (right) is plotted at the same scale, and shows development of localized massifs (Mt. St. Elias and Denali) within a modern terrane accretion setting near the junction between the Queen Charlotte fault (QCf) and the Aleutian trench. We propose that this setting is a modern analogue for the Hudson Bay region prior to collision of the Superior craton with the Churchill plate. In addition to the QCf, dextral strike-slip displacement between the Pacific and North American plates is accommodated inboard of the plate boundary along the Denali fault (Df) and other strike-slip structures; these may be analogous to the Winisk River fault (WRF). Upper inset shows a loading solution (Hynes, 1991) that best fits the geometry of the Nastapoka arc and the mean gravity profile (dashed curve) determined in cylindrical co-ordinates within the 155 o arc shown. Height of the topographic load is plotted against distance from center of arc. The load was built up from disk-shaped sheets 1 km thick (drawn every 2 km). The apparently massive relief seaward of the inflection point is not physically meaningful (Hynes, 1991), but implies a large localized load (massif) near the centre of curvature.

4.3 Proposed tectonic evolution of the THO arc terrane developed along the margin of the Hudson Bay protocontinent during the Trans-Hudson Figure 14 summarizes three scenarios for orogeny. This model is not convincing, however, for tectonic subdivisions of the crust beneath Hudson several reasons. First, it implies a fortuitous keystone- Bay. Part (a) combines models proposed by Roksandic like fit between the proposed Hudson Bay (1987) and Hoffman (1991). Roksandic (1987) protocontinent and the embayment in the Superior suggested that the most of Hudson Bay is underlain craton (arguably possible, but not likely, if the by an Archean crustal block, referred to as the protocontinent were originally rifted from the Hudson Bay protocontinent. Hoffman (1991) Superior and then collided again with negligible net conjectured that an extension of the Wathamun- rotation during a full Wilson cycle). There is also little Chipewyan batholith occupies the wedge-shaped or no evidence from potential-field data in Hudson region of the Severn domain (this study). Taken Bay to support the interpreted outlines of the Hudson together, these models imply that a large magmatic Bay protocontinent.

Figure 14 . Scenarios for tectonic subdivisions of the Trans-Hudson orogen beneath Hudson Bay, with selected gravity and magnetic anomalies shown for reference. (A) In this model, most of Hudson Bay is underlain by an Archean protocontinent, as postulated by Roksandic (1987). Hoffman (1991) conjectured that continental arc magmatism and the Churchill-Superior suture follows the geophysically-defined Severn Arc. (B) In this model, the suture extends from the Owl River shear zone (southwest side of Hudson Bay) and follows the central Hudson Bay domain (this study). Gravity anomaly G3 is conjectured to be related to a high-density anomaly (eclogite?) beneath the Hearne domain. (C) As in (B), but G3 underlies an Archean continental fragment (Hudson Bay craton) that is smaller than that envisioned by Roksandic (1987). This is our preferred interpretation.

Figure 14b depicts another scenario, in consistent with all of the available data from the which the Severn domain represents an island-arc region. terrane that accreted to the Superior craton prior to terminal collision. Products of Andean-style A simplified model for the tectonic continental arc magmatism, exposed on either side evolution of the THO, based on our preferred of Hudson Bay in the Wathamun-Chipewyan scenario, is presented in Fig. 15. This model batholith and the Narsajuaq Island Arc terrane, are incorporates many features of the double-indentor interpreted to occur within the CHBD. This model is model of Gibb (1983), but it also contains new more consistent with potential-field anomaly elements that attempt to reconcile the model with patterns in the Bay than the first model, and is also current understanding of tectonic relationships consistent with the back-arc chemistry of the Fox within the orogen. In the first panel (Fig. 15a), which River sill (Heaman et al, 1986; Halden, 1991). In this represents an age range of 1.9-1.88 Ga, the Superior scenario, the NWHB domain represents an extension and Western Churchill cratons and their fringing of the Hearne domain. continental terrace wedges are separated by the Manikewan Ocean, a Pacific-scale ocean basin Figure 14c depicts a third alternative, in (Symons and Harris, 2005). The Reindeer zone, which the NWHB domain contains a distinct crustal composed of the La Ronge arc and the Flin Flon / block that is smaller than, but otherwise similar to Glennie complex (Corrigan et al., 2005), has not yet the Hudson Bay protocontinent of Roksandic (1987). assembled. Other tectonic elements on the Hearne Evidence to support this interpretation includes the side of the Manikewan Ocean include the Sask presence of a geophysically defined region bounded craton and the Narsajuaq Island Arc terrane. The by curved magnetic anomalies (Fig. 9) as well as the Severn domain is represented as an island arc that presence of gravity anomaly G3 beneath the centre formed on the Superior side of the Manikewan of Hudson Bay. As discussed below, gravity Ocean. modelling suggests that this feature could be an eclogitized crustal root, similar in size to the root In the second panel (Fig. 15b), which that underlies the Sask craton (Fig. 10). The third represents an age range of 1865-1850 Ma, the span scenario is our preferred model, since it is most of the Manikewan Ocean has diminished substantially as a result of convergence of the Superior and Rae-Hearne cratons. The Reindeer deformation pattern may reflect large-scale Zone and Sask microcontinents have assembled; in rheologic segmentation of the craton, in which the addition, continental arc batholiths (the Wathamun- Kapuskasing uplift was localized near the edge of a Chipewyan batholith of Saskatchewan and rigid cratonic block. Manitoba, and the Cumberland batholith of Baffin Island) are being formed (Ansdell, 2005). The By analogy to the Alaskan Bight in the amalgamation of terranes on this side of the modern northeast Pacific, we propose that the Manikewan Ocean produced the Churchill Plate, accretionary setting combined with the sharp which has both Archean and Proterozoic oroclinal bend in the Hudson Bay embayment may components. On the other side of the ocean basin, have produced a local culmination (topographic double-indentor collision involving the Manitoba and load) near the restraining bend. A similar tectonic Quebec promontories of the Superior craton with setting in the modern Gulf of Alaska region (Redfield the Severn arc causes extrusion of juvenile arc et al., 2007) has produced large local topographic material into the Hudson Bay embayment (Gibb, loads in the foreland, including the Denali massif in 1983) and obduction of the Fox River belt. According Alaska and the St. Elias massif in the Yukon. The to this model, the extrusion process is largely flexural response to a similar, relatively localized responsible for the present-day wedge-shaped tectonic load could have contributed to the unusual geometry of the Severn domain and was partly circular shape of the Nastapoka Arc (Hynes, 1991). accommodated by strike-slip motion along the This model also resembles accretion/collision Winisk River Fault, structurally linked to thrusting of models proposed for the Svecofennian orogeny the Kapuskasing uplift (Gibb, 1983). Given recent (Nironen, 1997; Korja and Heikkinen, 2005), evidence for faster (and presumably stronger) although in Hudson Bay there is no evidence for lithosphere in the western Superior (Frederiksen et preservation of anomalously thick crust (up to 65 al., 2007; Darbyshire et al., 2007), this far-field km) as seen in Finland (Lahtinen and Huhma, 1996).

Figure 15. Tectonic reconstruction of the Trans-Hudson orogen, based on our preferred model. (A) Manikewan Ocean is similar in scale to the modern Pacific Ocean. Rae, Chesterfield and Hearne domains have assembled (Berman et al., 2007) to form the Western Churchill craton; disparate crustal terranes (Sask, Reindeer and Hudson) are located on the Hearne side of the ocean basin. The Severn domain represents an island arc terrane on the Superior side of the ocean. The Fox River sill forms in a back- arc basin associated with this arc setting. (B) Hudson Bay craton has collided with the western Churchill craton, producing a far- field metamorphic signature in the Rae domain (Berman et al., 2005). The Narsajuaq arc begins to form within an oceanic setting. On the Superior side, extrusion of the Severn domain into the large embayment within the Superior craton is partly accommodated along intracratonic fault zones (Gibb, 1983). By analogy with the Gulf of Alaska, the terrane accretionary setting results in localized loading due to the restraining bend in the craton, causing the flexural signature observed in the Nastapoka arc (Hynes, 1991). (C) Terminal collision of the Superior and Churchill plates, accompanied by strike-slip motion within the THO of Manitoba and Saskatchewan. Generalized age constraints are from Heamen et al. (1986), Dunphy and Ludden (1998) and Ansdell (2005). The terminal collision phase of the orogen basins due to the greater elastic thickness (and (Fig. 15c) preserved unusually large tracts of juvenile therefore stiffness) of the underlying lithospheric Paleoproterozoic crust. This preservation may be mantle in this region (Hanne et al., 2004). attributed to several factors, including the presence of the rigid Sask craton between the Superior and Churchill plates, as well as the pre-collisional shape of the Superior craton with its large central 4.5 Lithospheric mantle root and the influence of subduction embayment. The last increments of tectonic activity included strike-slip motion along the Owl River shear An unusually thick lithospheric root is zone, adjacent to the Manitoba Promontory, as well inferred to underlie Hudson Bay and the adjacent as other strike-slip faults within the Reindeer Zone Hearne domain (Fig. 12). This feature has been (Hajnal et al., 2005). Böhm et al. (2000) proposed imaged via inversion of high-resolution global that the Owl River shear zone represents the surface-wave data (e.g. Nettles and Dziewonski, primary suture between the Superior and Churchill 2008) as a pronounced high-velocity anomaly that plates, an interpretation that is potentially testable extends to > 200 km depth. Global compilations within exposed domains north of the Fox River belt. show that such seismic images correlate well with surface tectonics and tectono-thermal age

(Artemieva, 2006) and can be used to obtain 4.4 Formation of the Hudson Bay basin lithospheric thickness estimates that are consistent with global thermal modelling (Artemieva and As shown in Fig. 4, initial subsidence within Mooney, 2001). In a recent study in which non- the Hudson Bay basin commenced at roughly the thermal signals are removed from global seismic same time as the intracratonic Michigan basin. Both tomographic data (Artemieva, 2009), the Hearne basins experienced a second phase of subsidence, domain also appears as a conspicuous high-velocity but these were not synchronous. Hanne et al. (2004) anomaly that extends to more than 200 km. Using simulated the subsidence history of the Hudson Bay satellite-derived gravity observations, Tamisea et al. basin based on a simple thermal model. They (2007) separated the long-wavelength effects of considered, but rejected, the possibility that the glacial isostatic adjustment from the compositional Hudson Bay basin formed as the result of dynamic signature of the mantle, providing an independent surface response to a convective mantle geophysical image of the mantle root beneath downwelling, marked by the Hudson Bay gravity low Hudson Bay. (Fig. 12b). Hamdani et al. (1991) suggested that a pronounced second phase of subsidence in the Whole rock geochemistry and mantle Michigan basin (Fig. 4b) was caused by a flexural xenoliths provide further constraints on the load introduced by density increase due to formation and evolution of the lithospheric root. transformation of lower crustal material to eclogite Based on analysis of xenoliths hosted by facies. We have tested this model for Hudson Bay ultrapotassic rocks in the western Churchill craton, (Fig. 8c) by assuming the presence of a hypothetical Cousens et al. (2001) suggested that an Archean subcrustal load, produced by transformation to mantle root in this region was substantially eclogite of a 15-km lower-crustal root (similar to that metasomatized during Proterozoic subduction. beneath the Sask craton, Fig. 10) using the mantle Sandeman et al. (2003) show that mafic rocks of the density contrast employed for modelling by Hamdani Hurwitz Group have isotopic signatures of enriched et al. (1991). These simple assumptions yield a (metasomatized) Archean mantle. Citing evidence reasonable fit to gravity anomaly G3, suggesting that from ~ 200 Ma kimberlite dikes in the Rankin Inlet such a model may be viable for Hudson Bay. area, Sandeman et al. (2003) concluded that the lithospheric root was substantially thick from at least We suggest that the reduced subsidence the Paleoproterozoic to the present. within the Hudson Bay basin relative to the Michigan basin and other intracratonic basins in North Numerous studies have noted the unusual America may reflect the rheological properties of the Fe-depleted composition of Archean subcontinental underlying mantle lithosphere; i.e., Hudson Bay may lithospheric mantle (e.g., Griffin et al., 1999 and be broader and shallower than other intractronic references therein). This composition is difficult to produce from conventional melting models for the mantle (Arndt et al., 2009); it leads to both a for a secular change in the spatial scales of tectonic reduction in density and an increase in effective processes, from the Neoarchean (ca. 2.7-2.6 Ga) to viscosity, properties that are essential for long-term the Paleoproterozoic (ca. 1.9-1.8 Ga). Although preservation of continental lithosphere (Jordan, some differences in scale may simply be due to less 1975; 1978). However, mantle metasomatism, as complete preservation of Archean terranes than documented for the western Churchill craton Proterozoic terranes, we interpret most of this (Cousens et al., 2001), could lead to density and distinction to be primary, implying a profound shift viscosity values that are closer to typical (post- in plate dimensions and mantle geodynamic Archean) subcontinental lithosphere (Arndt et al., processes between 2.6 and 1.9 Ga. 2009). A number of models have been proposed to explain the formation of Archean subcontinental mantle lithosphere (Lee, 2006), including: (1) residue from extremely hot mantle plumes (Boyd, 1989), (2) 5 Conclusions accretion and stacking of oceanic lithosphere This paper provides a review and synthesis (Helmstaedt and Schulze, 1989), and (3) processing of the lithospheric architecture of the Hudson Bay of material in the mantle wedge above a subduction region. Three major tectonic elements are reviewed; zone (Jordan, 1988). Each of these models is from top to bottom, these are the Paleozoic Hudson supported by some observations but is incompatible Bay basin, the Paleoproterozoic Trans-Hudson with others; critical reviews are provided by Lee orogen, and the thick lithospheric mantle root that (2006) and Arndt et al. (2009). These models underlies much of this region. Our review of the generally appeal to processes that are expected to regional geologic framework around Hudson Bay have operated differently during the Archean than in provides a context for new or revised tectonic subsequent eras, associated with a significantly models, which are principally based on potential- hotter thermal regime (Michaut et al., 2009). field interpretations (gravity and magnetic) and velocity models derived from regional and global In light of the foregoing considerations, surface-wave studies. Hypotheses proposed in this several features of the Hudson Bay region deserve study are potentially testable using geophysical data special mention. First, based on geophysical from ongoing and future lithospheric investigations observations, the thickest part of the lithospheric in Hudson Bay. root (among the thickest on Earth) occurs in a region of the mantle that was strongly affected by The THO has been extensively studied in Proterozoic subduction of a Pacific-scale ocean areas adjacent to Hudson Bay, in Saskatchewan and basin. This spatial association not only indicates that Manitoba or the Ungava peninsula in northern ultra-thick high-velocity mantle roots beneath Quebec. Virtually all of these previous studies have Precambrian shields are not necessarily confined to considered these regions in isolation, leaving Archean domains, it also lends support to root important questions about the correlation of belts formation models that involve subduction (i.e., and sutures across Hudson Bay and the nature and models 2 and 3, above). Alternatively, if model 1 is evolution of the orogen at a continental scale largely correct, this association indicates a remarkable unanswered. A sinuous gravity high, which can be degree of ‘subduction tolerance’ for highly correlated over a distance of > 2800 km, is refractory Archean roots. interpreted to mark the edge of the Superior craton along its sparsely exposed northwestern margin. This Neoarchean subduction signatures are now margin contains two large-scale promontories widely recognized within Archean cratons (Superior, (Manitoba and Quebec) and a large-scale re-entrant Western Churchill) and elsewhere in the world (Nastapoka); this double-indentor geometry exerted (Cawood et al., 2006), and are preserved in a profound influence on the evolution of the THO supracrustal belts at spatial scales of ~ 100 km in (Gibb, 1983). width and < 1000 km in length. The spatial scale of the Trans-Hudson orogen, on the other hand, is The Bergeron suture in the eclogite-bearing approximately an order-of-magnitude larger and is Cape-Smith belt juxtaposes the Churchill and comparable with modern continent-continent Superior plates. Offshore gravity and magnetic collision systems (St. Onge et al., 2006). The regional anomalies suggest that this suture extends across geology around Hudson Bay thus preserves evidence the centre of the Bay, where it merges with prominent magnetic anomalies that, in turn, shallowest. Subsidence may have been inhibited by correlate with the Wathamun-Chipewyan stiff lithosphere beneath Hudson Bay. A positive continental magmatic arc on the southwest side. The gravity anomaly near the centre of the basin cannot herein-defined Severn and Belcher domains in be explained by uplifted basement within the central Hudson Bay, south and east of the inferred suture, horst (Fig. 8); we speculate that this anomaly reflects are interpreted to have formed, respectively, within the presence of dense eclogitic material at upper- an island arc and/or backarc setting, and a mantle depths, the formation of which could have continental margin setting. Both domains are accelerated or triggered basin subsidence (Hamdami interpreted to have formed proximal to the Superior et al., 1991). craton. This interpretation is consistent with interpretations of the Fox River belt and adjacent The existence of unusually thick lithospheric exotic domains (Weber, 1990; Böhm et al., 2000) mantle beneath extensive juvenile Proterozoic crust and is based on magnetic anomalies (Fig. 9), vintage of the Trans-Hudson orogen has implications for the industry seismic profiles (Roksandic, 1987), and origin of the continental lithospheric roots. Recent stratigraphic and provenance data in the Belcher global and continental-scale surface-wave studies Islands and Richmond Gulf (Chandler, 1984). that cover the Canadian Shield generally place the thickest part of the root beneath or just west of A tectonic reconstruction is herein central Hudson Bay. Taken together with evidence proposed in which a large lithospheric block (Severn for rapid subduction of the Pacific-scale Manikewan domain) accreted to the Superior craton prior to the Ocean (Stauffer, 1984) during Churchill-Superior Superior-Churchill collision. This relationship convergence, this association suggests that contrasts with the Reindeer Zone (Saskatchewan and subduction processes are ‘root-friendly’, if not ‘root Manitoba) and Narsajuaq Island Arc terrane forming’. (Quebec-Baffin), which accreted first to the Western Churchill craton. The geometry of the Superior craton, coupled with the presence of the rigid Sask craton, enabled preservation of large tracts of juvenile Proterozoic crust. Lateral transport and extrusion of the accreted Severn domain was partly accommodated by dextral strike-slip motion along the Winisk River fault, during which the western margin of the Quebec promontory (Superior craton) formed a rigid buttress that ultimately led to plugging of the Nastapoka embayment. Our model contains several elements of Gibb’s (1983) double- indentor interpretation, as well as elements of a new extrusion tectonic (orogenic flow) model for the present-day North Pacific Rim of North America (Redfield et al., 2007). This proposed modern analog renders equivalent the Nastapoka arc and the Denali region near the Gulf of Alaska, and provides a model for the enigmatic “point load” invoked by Hynes (1991) to explain the nearly circular Nastapoka arc.

The Ordovician-Devonian Hudson Bay basin is located above the THO and fits neatly between the Manitoba and Quebec promontories of the Superior craton. The basin contains up to 2 km of sediments and is characterized by an unusual central horst, which effectively splits older basin sediments into two sub-basins. Although the largest by surface area of four roughly coeval Paleozoic intracratonic basins in North America, the Hudson Bay basin is also the 6 Acknowledgements Audet, P. and Mareschal, J.-C., (2004). Variations in elastic thickness in the Canadian Shield. Earth Planet. This work was funded by NSERC Discovery Sci. Lett., 226, 17-31. grants to DWE and FD. We thank other participants in the Hudson Bay Lithospheric Experiment (HuBLE) Barager, W.R.A. and Scoates, R.F.J., (1981). The for stimulating discussions that gave rise to many of circum-Superior belt: a Proterozoic plate margin? In: the ideas expressed in this paper. Several figures A. Kröner (Ed.), Precambrian Plate Tectonics. were prepared using GMT software (Wessel and Elsevier, Amsterdam, pp. 297-330. Smith, 1991). Gravity and magnetic data were obtained from the GSC’s Geophysical Data Cantre Barr, K.G., (1967). Upper mantle structure in Canada (http://gdcinfo.agg.nrcan.gc.ca/ ). This paper from seismic observations using chemical explosions, benefitted from constructive reviews by Ron Clowes Can. J. Earth Sci., 4, 961-975. and an anonymous reviewer, as well as Associate Editor Hans Thybo. Beals, C.S., (1968a). Science, History and Hudson Bay, volumes 1 & 2, Department of Energy Mines and Resources, Ottawa, 1057 pp.

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