Rae Craton As the Backstop for Proto-Laurentian Amalgamation by Slab Suction Paul F

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Geoscience Canada Journal of the Geological Association of Canada Journal de l’Association Géologique du Canada

The Origin of Laurentia Rae Craton as the Backstop for Proto-Laurentian Amalgamation by Slab Suction Paul F. Hoffman

Volume 41, Number 3, 2014 Article abstract Proto-Laurentia (i.e. pre-Grenvillian Laurentia) is an aggregate of six or more URI: https://id.erudit.org/iderudit/1027247ar formerly independent Archean cratons that amalgamated convulsively in geons 19 and 18 (Orosirian Period), along with non-uniformly distributed areas See table of contents of juvenile Paleoproterozoic crust. Subduction polarities and collision ages have been provisionally inferred between the major cratons (and some minor ones), most recently between the Rae and Hearne cratons. The oldest Orosirian Publisher(s) collisions bound the Rae craton: 1.97 Ga (Taltson- Thelon orogen) in the west, and 1.92 Ga (Snowbird orogen) in the southeast. All other Orosirian collision The Geological Association of Canada ages in proto-Laurentia are < 1.88 Ga. The Rae craton was the upper plate during (asynchronous) plate convergence at its western and, tentatively, ISSN southeastern margins. Subsequent plate convergence in the Wopmay and Trans-Hudson orogens was complex, with the Rae craton embedded in the 0315-0941 (print) lower plate prior to the first accretion events (Calderian, Reindeer and Foxe 1911-4850 (digital) orogenies), but in the upper plate during major subsequent convergence and terminal collisions, giving rise to the Great Bear and Cumberland magmatic Explore this journal arcs, respectively. The ‘orthoversion’ theory of supercontinental succession postulates that supercontinents amalgamate over geoidal lows within a meridional girdle of mantle downwellings, orthogonal to the lingering Cite this article superswell at the site of the former supercontinent. If the downwelling nodes develop through positive feedback from the descent of cold oceanic slabs, then Hoffman, P. F. (2014). The Origin of Laurentia: Rae Craton as the Backstop for viscous traction should contribute to drawing the cratons together over the Proto-Laurentian Amalgamation by Slab Suction. Geoscience Canada, 41(3), downwelling node. Viewed in this way, the Rae craton was the first to settle 313–320. over the downwelling node and became the backstop for the other cratons that were drawn towards it by subduction. It was, literally, the origin of Laurentia. Whether the Rae craton was also the origin of Nuna, the hypothetical cogenetic supercontinent, depends on ages and subduction polarities of Orosirian sutures beyond proto-Laurentia.

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HAROLD WILLIAMS SERIES

east. All other Orosirian collision ages moins six cratons archéens indépen- in proto-Laurentia are < 1.88 Ga. dants qui se sont amalgamés convul- The Rae craton was the upper sivement durant les géons 19 et 18 plate during (asynchronous) plate con- (Orosirien), le long de zones de croûtes vergence at its western and, tentatively, juvéniles paléoprotérozoïques réparties southeastern margins. Subsequent plate de manière hétérogène. Les polarités convergence in the Wopmay and de subduction et les âges de collision Trans-Hudson orogens was complex, entre les grands cratons (et d’autres with the Rae craton embedded in the moins grands) ont été provisoirement lower plate prior to the first accretion déduits, le plus récemment entre le cra- events (Calderian, Reindeer and Foxe ton de Rae et le craton de Hearne. Les orogenies), but in the upper plate dur- plus anciennes collisions orosiriennes The Origin of Laurentia: ing major subsequent convergence and ont soudé le craton de Rae : 1,97 Ga Rae Craton as the terminal collisions, giving rise to the (orogène de Taltson-Thelon) dans Great Bear and Cumberland magmatic l’ouest, et 1,92 Ga (orogène de Snow- Backstop for Proto- arcs, respectively. bird) dans le sud-est. Tous les autres Laurentian Amalgamation The ‘orthoversion’ theory of âges de collision en proto-Laurentie by Slab Suction supercontinental succession postulates sont inférieurs à 1,88 Ga. that supercontinents amalgamate over Le craton de Rae constituait la geoidal lows within a meridional girdle plaque supérieure durant la conver- Paul F. Hoffman of mantle downwellings, orthogonal to gence de plaque (asynchrone) à sa the lingering superswell at the site of marge ouest, et peut-être aussi à ses 1216 Montrose Ave the former supercontinent. If the marges sud-est. La convergence de Victoria, British Columbia downwelling nodes develop through plaque subséquente dans les orogènes V8T 2K4, Canada positive feedback from the descent of de Wopmay et Trans-Hudson a été E-mail: [email protected] cold oceanic slabs, then viscous trac- complexe, le craton de Rae étant encas- tion should contribute to drawing the tré dans la plaque inférieure avant les SUMMARY cratons together over the downwelling premiers événements d’accrétion Proto-Laurentia (i.e. pre-Grenvillian node. Viewed in this way, the Rae cra- (orogènes caldérienne, de Reindeer et Laurentia) is an aggregate of six or ton was the first to settle over the de Fox), puis dans la plaque supérieure more formerly independent Archean downwelling node and became the durant la grande convergence sub- cratons that amalgamated convulsively backstop for the other cratons that séquente et les collisions terminales, ce in geons 19 and 18 (Orosirian Period), were drawn towards it by subduction. qui a créé les arcs magmatiques de along with non-uniformly distributed It was, literally, the origin of Laurentia. Great Bear et de Cumberland respec- areas of juvenile Paleoproterozoic Whether the Rae craton was tivement. crust. Subduction polarities and colli- also the origin of Nuna, the hypotheti- La théorie de « l’orthoversion sion ages have been provisionally cal cogenetic supercontinent, depends » de la succession des supercontinents inferred between the major cratons on ages and subduction polarities of présuppose que les supercontinents s’a- (and some minor ones), most recently Orosirian sutures beyond proto-Lau- malgament au-dessus de creux géoï- between the Rae and Hearne cratons. rentia. daux en deça d’une gaine méridienne The oldest Orosirian collisions bound de convections mantéliques descen- the Rae craton: 1.97 Ga (Taltson- SOMMAIRE dantes, à angle droit d’un super-renfle- Thelon orogen) in the west, and 1.92 La proto-Laurentie (c.-à-d. la Laurentie ment persistant au site d’un ancien Ga (Snowbird orogen) in the south- pré-grenvillienne) est un agrégat d’au supercontinent. Si le nœud de convec-

Table 1. Estimated collision ages and subduction polarities for Orosirian sutures in proto-Laurentia Orogen Collision Upper plate Lower plate References Thelon - Taltson 1.97 Ga Rae Slave van Breemen et al. (1987); Bostock et al. (1987); James et al. (1988); van Breemen and Henderson (1988); Grotzinger and Royden (1990); Tirrul and Grotzinger (1990); Bowring and Grotzinger (1992); Thériault (1992); McDonough et al. (2000) Snowbird 1.92 Ga Rae Hearne Berman et al. (2007); Martel et al. (2008) Wopmay 1.88 Ga Hottah Slave Hildebrand et al. (2010); Hoffman et al. (2011) Foxe - Rinkian 1.88 Ga Rae Meta Incognita- Connelly et al. (2006); St-Onge et al. (2007) Disco Great Falls 1.87 Ga Medicine Hat Wyoming Mueller et al. (2002) Torngat - Nagssugtoqidian 1.86 Ga North Atlantic SE Churchill- Scott (1998); van Gool et al. (2002); Kolb (2014) Disco Penokean 1.85 Ga Marshfield Superior Schulz and Cannon (2007) Vulcan 1.84 Ga Medicine Hat Hearne Eaton et al. (1999) Wopmay 1.84(?) Ga Hottah Nahanni Cook (2011) Trans-Hudson (southern) 1.83 Ga Hearne Superior Corrigan et al. (2009); Maxeiner and Rayner (2011) New Quebec 1.82 Ga SE Churchill Superior Wardle et al. (2002) Trans-Hudson (northern) 1.82 Ga Rae Superior Machado et al. (1993); St-Onge et al. (2006, 2007); Berman et al. (2013) tion descendante s’établit par rétroac- called the North and South Keewatin Martel et al. 2008). tion positive de la descente de plaques cratons (Williams et al. 1991). With the Outside of largely ice-covered océaniques froides, la traction disappearance of the District of Kee- Greenland, the Snowbird tectonic zone visqueuse devrait contribuer à entraîner watin as a political entity, Hank’s cra- was the last of the major Orosirian les cratons ensembles au-dessus du tons reverted to their synonyms, the (2.05 – 1.80 Ga, Gradstein et al. 2012) nœud de convection descendante. Vu Rae and Hearne cratons respectively geosutures in proto-Laurentia to be de cette façon, le craton de Rae a été le (Hoffman 1988; Eglington et al. 2013). dated. The age sequence and inferred premier à s’établir au-dessus du nœud The collision zone between the two subduction polarity of sutures can now de convection descendante, ce qui en a cratons, originally inferred from recon- be tabulated (Table 1 and Fig. 1). Sub- fait la butée des autres cratons naissance geology, gravity gradient duction polarity refers to the time of entraînés par la subduction. Littérale- maps, aeromagnetic discontinuities, and large ocean closure leading to craton- ment, telle a été l’origine de la Lauren- seismic soundings (Taylor 1963; Sharp- craton collision, not to earlier arc- and tie. ton et al. 1987; Hoffman 1990; Ross et continental ribbon-accretion events Quant à savoir si c’est le cra- al. 1995), has been confirmed in the that typically involve subduction flips ton de Rae qui a été à l’origine de last decade by inter-cratonic compara- (e.g. Suppe 1984), nor to post-collision- Nuna, cet hypothétique surperconti- tive geology, igneous petrology, meta- al convergence when structural ver- nent cogénétique, cela dépend des âges morphic thermobarometry, and U–Pb gence may flip as a result of retrochar- et des polarités de subduction des geochronology (Mahan et al. 2006; riage (Roeder 1973). The suture sutures orosiriennes au-delà de la Berman et al. 2007; Martel et al. 2008). sequence and polarity patterns shed proto-Laurentie. Although no well-defined magmatic light on the dynamics of cratonic con- arc is recognized, a subduction zone vergence and amalgamation (Faccenna INTRODUCTION dipping northwest under the Rae cra- et al. 2013; see also Hager and O’Con- In his postcards showing the tectonic ton is tentatively inferred from mag- nell 1978; Hager et al. 1983). If plate elements of the North American con- matic and metamorphic asymmetry convergence was driven primarily by tinent, Hank Williams divided the across the suture zone (Berman et al. ‘slab rollback’ (Fig. 2A), we should Churchill Province roughly in half 2007). The best estimate of the age of expect that suture ages would decrease along a hypothetical suture, the Snow- collision between the Rae and Hearne with distance from the Superior craton, bird tectonic zone, separating what he cratons is 1.92 Ga (Berman et al. 2007; which was the lower plate with respect GEOSCIENCE CANADA Volume 41 2014 315

is marked by the change from steady- state arc magmatism to short-lived slab-breakoff magmatism (Davies and von Blanckenburg 1995), followed by post-collisional crustal anatexis, often accompanied by rapid tectonic exhumation through gravitational collapse and lateral extrusion of tectonically thickened crust.

Taltson-Thelon Orogen The oldest Orosirian sutures bound the Rae craton (Table 1 and Fig. 1). To the west, it was impacted by the Slave craton along the Thelon-Taltson orogen. To the southeast, it collided with the Hearne craton along the Snowbird tectonic zone. The age of the Rae/ Slave collision is constrained by a U–Pb (ID–TIMS) zircon date of 1968.7 ± 1.1 Ma from a tuff in the basal part of the Bear Creek foredeep on the Slave margin (Bowring and Grotzinger 1992; Grotzinger and Roy- den 1990). This is a tight minimum age constraint on collision as the onset of foredeep subsidence marked the arrival Figure 1. Tectonic sketch map of Laurentia showing Orosirian orogens (blue) and of the passive-margin shelf (Kimerot Archean cratons, differentiated according to the subduction polarities of their colli- platform) at the trench (Tirrul and sional boundaries. ‘Middle plates’ were the upper plate on one side and the lower Grotzinger 1990). The marine-to-non- plate on the other. ‘Upper’ and ‘lower’ plates had the same polarities on all sides. marine transition in the Bear Creek Estimated collision ages in 109 years before present (Ga). Names mentioned in the foredeep occurred around 1.93 Ga text or Table: BH, Buffalo Head terrane; CP, Cumberland-Prøven arc; Ht, Hottah (Bowring and Grotzinger 1992). On terrane; MI, Meta Incognita terrane; MCR, Mid-Continent Rift (1.1 Ga); Mf, the upper plate, subduction-related Marshfield terrane; NAtl, North Atlantic craton; Ng, Nagssugtoqidian orogen; Nh, (including slab-breakoff?) magmatism Nahanni terrane; NQ, New Quebec orogen; Rk, Rinkian orogen; SEC, Southeast (2.03 – 1.96 Ga) and associated high- Churchill (Core Zone) hinterland; S, Snowbird orogen; Sg, Sugluk terrane; Sk, Sask temperature granulite-grade metamor- terrane; T, Thelon orogen; Ta, Taltson arc; Tg, Torngat orogen; W, Wathaman arc. phism switched around 1.96 Ga to anatectic magmatism (1.96 – 1.91 Ga) to the adjacent cratons. Alternatively, if plate convergence rates of 2–10 cm and exhumation (Bostock et al. 1987; the cratons were drawn together by yr-1, foredeep subsidence at any point van Breemen et al. 1987; James et al. ‘slab suction’ (Fig. 2B), suture ages begins 1–10 Myr before its passage 1988; van Breemen and Henderson should decrease with distance from the beneath the leading edge of the upper 1988; Thériault 1992; McDonough et Rae craton, which was the upper plate plate (the trench axis). The onset of al. 2000). The Rae/Slave collision age with respect to its neighbours. All foredeep subsidence is sedimentologi- estimate of 1.97 Ga, determined over other cratons are known to have mixed cally most apparent when the outer 20 years ago, remains an excellent polarities with respect to adjacent cra- shelf of a passive margin enters the example of apparent consilience tons (Fig. 1). trench, by which time the continental between lower and upper plate rise will have already passed beneath chronologies. OROSIRIAN COLLISION AGES AND the trench axis in most cases. Passive- SUBDUCTION POLARITIES margin shelf-to-foredeep transitions of Snowbird Tectonic Zone Collision ages can be constrained by Orosirian age have been recognized As described in the Introduction, the the chronology of the lower plate, the throughout proto-Laurentia (Hoffman Rae craton is bounded on the south- upper plate, or preferably the con- 1987; Partin et al. 2014) and some are east by the Snowbird suture zone, with silience of both. On the lower plate, tightly constrained chronometrically an estimated collision age with the the onset of foredeep subsidence from U–Pb dating of tuffs (Bowring Hearne craton of 1.92 Ga based on marks the arrival of a passive margin at and Grotzinger 1992). Metamorphic dates of high-pressure metamorphism a subduction zone (Bradley 2008). For dates from passive-margin protoliths followed by high-temperature exhuma- typical foredeep widths (trench axis to provide minimum constraints on colli- tion (orogenic collapse) around 1.90 forebulge crest) of 100–200 km and sion age. On the upper plate, collision Ga (Mahan et al. 2006; Berman et al. 316

1882.50 ± 0.95 Ma from a tuff in the basal part of the Recluse foredeep on the Slave margin (Bowring and Grotzinger 1992; Hoffman et al. 2011). Hottah accretion was immediately followed by eastward subduction, produc- ing the epi-orogenic Great Bear magmatic arc, active between 1.88 and 1.84 Ga (Hildebrand et al. 2010; Cook 2011, 2013; Hayward and Oneschuk 2011). Terminal collision in Wopmay orogen involved accretion of the buried Nahanni – Fort Simpson terrane ~1.84 Ga, also as a consequence of east-dipping subduction as inferred from seismic profiling (Cook 2011, 2013). In summary, Orosirian collisions become younger with distance from the center of the Rae craton (Fig. Figure 2. Four time steps (t1 to t4) in multicratonic assembly by slab rollback (A), 3B). No such age relation exists with in which slabs sink gravitationally into a passive medium, and slab suction (B), in respect to the Superior craton (Fig. which convergent mantle flow (downwelling) has traction with the plates. In each 3A). Except for the first accretion case, the founding craton of the assembly is tracked by the open diamonds. In A, events in the Trans-Hudson and Wop- the founding craton remains on the lower plate and subduction zones dip away may orogens, the Rae craton was pre- from it. In B, the founding craton remains on the upper plate and subduction dominantly on the upper plate during zones dip toward it. The amalgamation of proto-Laurentia is better described by convergence of the Slave, Hearne, (B). North Atlantic, Nahanni and Superior cratons. These patterns suggest that 2007; Martel et al. 2008). South of Hudson Bay, the slab suction (Faccenna et al. 2013), Trans-Hudson orogen borders the rather than slab rollback, drove the Trans-Hudson Orogen Hearne craton (St-Onge et al. 2006; amalgamation of proto-Laurentia (Fig. Beyond the northeastern limit of the Corrigan et al. 2009; Maxeiner and 2). Hearne craton, the Rae craton is Rayner 2011; Corrigan 2013). The first bounded by the Trans-Hudson orogen accretion event, the Reindeer orogeny DISCUSSION of Southampton and southern Baffin ~1.88 Ga, was an arc-continent colli- The Rae craton was the founding cra- islands, and central West Greenland. sion involving the juvenile La Ronge – ton, the ‘origin’ around which the This segment of the orogen includes Lynn Lake island arc. The Rae craton, other cratons amalgamated (Fig. 3). three episodes of collision (Wardle et conjoined with the Hearne craton, was There are, of course, older pre-Orosiri- al. 2002; van Gool et al. 2002; St-Onge in the lower plate with respect to this an (mostly Neoarchean) sutures within et al. 2006, 2007; Berman et al. 2013; arc-continent collision. Following arc- the constituent cratons, inherited from Corrigan et al. 2009; Corrigan 2013; accretion, subduction flipped to north- older episodes of amalgamation (e.g. Eglington et al. 2013; Pehrsson et al. westward-dipping beneath the Hearne- Pehrsson et al. 2013b), but they are not 2013a; Partin et al. 2014). The first, Rae cratons, accommodating conver- relevant to the Orosirian assembly of ~1.88–1.86 Ga, involved the accretion gence and collision of a second juve- proto-Laurentia. of Archean microcontinents (Sugluk, nile terrane, the Flin Flon – Glennie Contrasting modes of super- Meta Incognita, Southeast Churchill complex, ~1.85 Ga. The arrival of the continental reassembly have been Core Zone, Disco and North Atlantic) western Superior craton ~1.83 Ga was described. Introversion and extraver- to the Rae craton. Second, ~1.845 Ga, preceded by the accretion of an sion are kinematic end-members (Mur- was the addition of a juvenile arc-fore- Archean microcontinent, Sask craton, phy and Nance 2004). Intraversion pre- arc terrane (Narsajuaq arc). Last was to the southeast-facing forearc of the dicts that the present Atlantic and Indi- the arrival of the Superior craton, Flin Flon – Glennie complex ~1.84 an oceans will close and the ghost of ~1.82 Ga. Accretion progressed out- Ga. Pangea will reappear, with modifica- wards from the Rae craton and, tion. Extraversion predicts a future although subduction dipped southeast- Wopmay Orogen Atlantic-Indian Panthalassa at the ward prior to the first arrivals (e.g. In the Wopmay orogen, west dipping expense of the remaining half of the Meta Incognita terrane), northwest- subduction led to collisional accretion Pacific. Like plate tectonics, these are ward-directed subduction, manifested of the Hottah terrane on the present kinematic theories only. They do not by the Cumberland-Prøven arc western margin of the Slave craton at address dynamics, nor are they explicit- batholith, accommodated subsequent 1.88 Ga (Hildebrand et al. 2010), based ly tied to the geoidal coordinates of convergence and accretion. on a U–Pb (ID–TIMS) zircon date of first-order mantle convection. GEOSCIENCE CANADA Volume 41 2014 317

tion (Fig. 2A). In proto-Laurentia, suture chronology (Fig. 3) and subduction polarities (Fig. 1) support slab suction as the dominant assembly mechanism. In one sense, it is an unfair comparison: negative buoyancy of old oceanic slabs operates in both regimes (Fig. 2A and B). Slab suction is simply slab rollback with the addition of mantle flow and significant plate-mantle traction. Viewed this way, proto-Lau- rentian amalgamation history implies long-lived convergent (downwelling) Figure 3. Orosirian collision ages (black dots) in proto-Laurentia as a function of mantle flow and significant plate trac- distance (where the age was determined) from (A) the center of the Superior cra- tion. A lingering manifestation of this ton (James Bay, 52°00’N, 80°00’E) and (B) the center of the Rae craton (Aberdeen mantle supersucker was dynamic subsi- Lake, 64°30’N, 100°00’E). Open circle in B is the Penokean orogen, which docked dence, accommodating widespread with the southern margin of the Superior craton at an unknown distance from Rae post-orogenic sheet sandstones in craton. It was therefore excluded from the calculated correlation line. Collision ages proto-Laurentia (e.g. Thelon, Athabas- do not diminish with distance from the center of the Superior craton, as predicted ca, Hornby Bay and Baraboo sand- by slab rollback (Fig. 2A), but they do become systematically younger with distance stones) and beyond. from the Rae craton, consistent with slab suction (Fig. 2B). Proto-Laurentia is a megacon- tinent, arguably cogenetic with the Orthoversion (Mitchell et al. approximately 90° from the center of supercontinent Nuna (Zhang et al. 2012; Evans 2003; Li and Zhong 2009) the former supercontinent and its lin- 2012). Whether the Rae craton was by contrast is a dynamical theory of gering superswell. If the reassembly also the founding craton of Nuna supercontinental reassembly coupled occurs away from the paleoequator, depends on the Orosirian subduction with large-scale mantle convection. As then superswell development will incite polarities and collision ages throughout a dynamical theory, it can and has been true polar wander to restore inertial Nuna as a whole (Zhang et al. 2012). tested with numerical simulations (e.g. stability. If the reassembly is equatorial, Zhong et al. 2007), as well as by paleo- true polar wander is unnecessary. CONCLUSIONS magnetic data (Mitchell et al. 2012). In orthoversion, superswells The collision age and subduction Paleomagnetic testing requires addi- are long-lived (100s Myrs) but imper- polarity of each of the major Orosiri- tional constraints on paleolongitude, manent. This contrasts with the neofix- an geosutures in proto-Laurentia have which can be obtained from Ceno- ist view that the antipodal superswells been estimated. The oldest sutures zoic–Mesozoic plume trails and Paleo- of today have persisted since the bound the Rae craton and sutures zoic–Proterozoic true-polar-wander Moon-forming impact (Burke et al. become progressively younger with rotation axes (Steinberger and Torsvik 2012). Whether order-1 or order-2, distance from its center. Subduction in 2008; Mitchell et al. 2012). Orthover- prone to true polar wander or not, both the bounding orogens dipped sion postulates that long-lived super- orthoversion predicts that superconti- (asynchronously) beneath the Rae cra- continents bring about their own nents assembled over first-order man- ton, which remained on the upper destruction through the mantle super- tle downwellings. Such downwellings plate essentially for the duration of swells engendered by continental insu- must develop in areas of long-lived proto-Laurentia’s Orosirian amalgama- lation and absence of slab injections. subduction through positive feedback tion. From the patterns of suturing This was basically Arthur Holmes’ between cold slabs and thermal con- and subduction polarity I infer that the (Holmes 1928, 1931, 1944) dynamical vection. Rae craton was captured above a long- explication of the breakup of Wegen- Orthoversion assumes that lived, subduction-induced, downwelling er’s Pangea, ruled inadmissable by the mantle flow plays an active role in plate in the mantle, after which it served as many who claimed no mechanism motions. It predicts the style of multi- the backstop for other cratons swept in existed for continental drift. According cratonic amalgamation described as by continued subduction beneath it. to orthoversion, in order-1 convection ‘slab suction’ (Faccenna et al. 2013), An active role for mantle convection in the supercontinental fragments go with where the cratonic backstop is an cratonic drift is implied: slab suction the flow and reassemble above the upper plate and subduction plunged prevailed over slab rollback. Whether mantle downwelling located approxi- inward beneath the founding craton the Rae craton and slab suction played mately 180° distant. In order-2 convec- (Fig. 2B). Conversely, dominance of the same roles in the assembly of tion, like the present, the fragments slab rollback, where slabs collapse supercontintent Nuna remains unan- reassemble somewhere along the gravitationally into a passive medium, swered. An active role for mantle con- meridional girdle of mantle down- predicts a lower plate as the cratonic vection and plate traction is also con- welling, today the circum-Pacific ring, backstop and outward dipping subduc- sistent with orthoversion dynamics of 318 supercontinental reassembly. However, and Torsvik, T.H., 2012, Why is the based digital summary of the evolu- it remains uncertain if Nuna assembled areoid like the residual geoid?: Geo- tion of the Palaeoproterozoic of under order-1 or order-2 convection, physical Research Letters, v. 39, North America and Greenland and or if it was ever preceded by a conti- L17203, http://dx.doi.org/ associated unconformity-related urani- nental assembly large enough to grow a 10.1029/2012GL052701. um mineralization: Precambrian superswell. It appears possible that Connelly, J.N., Thrane, K., Krawiec, A.W., Research, v. 232, p. 4–26, and Garde, A.A., 2006, Linking the http://dx.doi.org/10.1016/j.precam- these questions may be answerable. Palaeoproterozoic Nagssugtoqidian res.2013.01.021. and Rinkian orogens through the Evans, D.A.D., 2003, True polar wander ACKNOWLEDGEMENTS Disko Bugt region of West Green- and supercontinents: Tectonophysics, This paper originated with an invita- land: Journal of the Geological Socie- v. 362, p. 303–320, http://dx.doi.org/ tion from Christian Böhm as Technical ty, v. 163, p. 319–335, 10.1016/S0040-1951(02)000642-X. Program co-chair to attend the 2013 http://dx.doi.org/10.1144/0016- Faccenna, C., Becker, T.W., Conrad, C.P., GAC-MAC Joint Annual Meeting in 764904-115. and Husson, L., 2013, Mountain Winnipeg. Cees van Staal encouraged Cook, F.A., 2011, Multiple arc develop- building and mantle dynamics: Tecton- me to write it up as a tribute to Hank ment in the Paleoproterozoic Wopmay ics, v. 32, p. 80–93, http://dx.doi.org/ Williams, who I first knew as a noon- orogen, northwest Canada, in Brown, 10.1029/2012TC003176. hour table-tennis hustler with GSC in D., and Ryan, P.D., eds., Arc-Continent Gradstein, F.M., Ogg, J.G., Schmitz, M.D., Collision: Springer-Verlag, Berlin, p. and Ogg, G.M., 2012, The Geologic Ottawa, and later as a fiddler whose 403–427. Time Scale 2012 (2 volumes): Elsevier, playing grew more assured with each Cook, F.A., 2013, Paleoproterozoic assem- Amsterdam, 1144 p. Trans-Newfoundland field trip. 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