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Harold Williams Series GEOSCIENCE CANADA Volume 41 2014 313 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- Geoscience Canada, v. 41, http://dx.doi.org/10.12789/geocanj.2014.41.049 © 2014 GAC/AGC® 314 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 exhu- mation 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 oro- gen. 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.
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