1. Penrose, June 20061 Did Plate Tectonics begin in Paleoproterozoic time? …well before, but scale & style become more modern during the Paleoproterozoic Wouter Bleeker, Richard Ernst & Ken Buchan Geological Survey of Canada, Ottawa Evidence for plate behaviour at 2.1-1.8 Ga: break-up, dispersal & suturing of Archean cratons Evidence for plate behaviour at 2.1-1.8 Ga: break-up, dispersal & suturing of Archean cratons

2. Penrose, June 20062 No data Bleeker & Enst, in prep. Significant secular change?...Yes, of course! Higher heat productionHigher heat production Weaker lower crustWeaker lower crust Always more basalt in the system …more significant density inversionsAlways more basalt in the system …more significant density inversions Smaller plate scalesSmaller plate scales Faster recyclingFaster recycling “The 2.7 Event”

3. Penrose, June 20063 Precambrian geology of North America Modified after Hoffman, 1989 ; based on a century of geological research A Paleoproterzoic collage of micro- plates and inter- vening arcs terranes

4. Penrose, June 20064

5. 5 Great Slave Lake Shear Zone 7. Large strike-slip faults?…Yes. The ~1.8 Ga collage: plate tectonics? L ITHOPROBE’S SNORCLE Transect Relevant questions: 1.Was there significant lateral movement?...Yes. 2.Are now adjacent blocks unrelated (exotic)?...Yes, commonly. 3.Plate behaviour: rifting, break-up, convergence?...Yes. 4.Did blocks behave (quasi) rigid?...Yes, some. 5.Strong asymmetry across suturing orogens?...Yes. 6.Are time spans and rates similar?…Yes, comparable. Relevant questions: 1.Was there significant lateral movement?...Yes. 2.Are now adjacent blocks unrelated (exotic)?...Yes, commonly. 3.Plate behaviour: rifting, break-up, convergence?...Yes. 4.Did blocks behave (quasi) rigid?...Yes, some. 5.Strong asymmetry across suturing orogens?...Yes. 6.Are time spans and rates similar?…Yes, comparable.

6. Penrose, June 20066 Bleeker, 2002 Cook et al., 1999

7. Penrose, June 20067 Accretionary structure along western margin of Slave craton, 1.9-1.7 Ga: Cook et al., 1999

8. Penrose, June 20068 Successive sutures:

9. Penrose, June 20069 Suture geometries: e.g., White et al., 2002

10. Penrose, June 200610 Baltica Siberia? Australia- Antarctica? ??? Laurentia within ~1.8 Ga “Nuna”: e.g., Buchan et al., 2000 Long-lived active margin

11. Penrose, June 200611 Nuna to Rodinia:

12. Penrose, June 200612 ~1 Ga Rodinia (conceptual only): Rifted Nuna fragments Intact core of ~1.8 Ga Nuna Stray fragments

13. Penrose, June 200613 Bleeker, 2005 Further back in time: before Nuna

14. Penrose, June 200614 Ancestral landmass “Superia” “Break-out” of the Superior craton, out of ancestral landmass Superia: Karelia Hearne Wyoming Kola Others

15. Penrose, June 200615 Superia WyomingSuperia:Superia: Bleeker & Ernst, 2006

16. Penrose, June 200616 Correlating multiple events: Bleeker & Ernst, 2006

17. Penrose, June 200617 From late Archean supercratons to Nuna: break-up & independent drift of cratonic fragments Not to scale!

18. Penrose, June 200618 Did things move? ~5 cm/yr

19. Penrose, June 200619 Ophiolites? Sparse but present!! Kontinen, Peltonen et al.

20. Penrose, June 200620 Diagnostic rock associations: -Plume-assisted extension & break-up?…Yes, definitely. -Rift & passive margin sequences?…Yes. -Plume-assisted extension & break-up?…Yes, definitely. -Rift & passive margin sequences?…Yes. Slave basement Passive margin sequence Rift sequence Foredeep sequence Arc

21. Penrose, June 200621 Diagnostic rock associations: -Plume-assisted extension & break-up?…Yes, definitely. -Rift & passive margin sequences?…Yes. -Arcs? Arc batholiths (at plate scale)?…Yes. -Ophiolites?…Yes. -Elevated P/T metamorphic facies series?…Yes,…but UHP?? -Blueschists…No…or? -Plume-assisted extension & break-up?…Yes, definitely. -Rift & passive margin sequences?…Yes. -Arcs? Arc batholiths (at plate scale)?…Yes. -Ophiolites?…Yes. -Elevated P/T metamorphic facies series?…Yes,…but UHP?? -Blueschists…No…or?

22. Penrose, June 200622 Conclusions: The Paleoproterozoic preserves a clear record of (small) plate tectonics, resulting in Earth’s first “modern” supercontinent Nuna Not to scale!

23. Penrose, June 200623

24. Penrose, June 200624 Matachewan dykes 2446 Ma (2.45-2.5 Ga) Hearne – southern Superior link: 2446 Ma dykes 2440-2450 Ma (2.45-2.5 Ga) Kaminak dykes Dates by Heaman

25. Penrose, June 200625 Bleeker, 2002, 2004 2110 Ma Solution allowed by current paleomagnetic data:

26. Penrose, June 200626 Bleeker & Ernst, 2006 Barcodes:Barcodes:

27. Penrose, June 200627 KaapvaalSlaveSuperiorLew. Nain Labr. Karelia Hearne Superia Sclavia Vaalbara “Fragmentation tree” of Archean fragments The Archean family tree

28. Penrose, June 200628 “Evidence for plate behaviour at 2.1-1.8 Ga: break-up, dispersal, and suturing of Archean cratons” Wouter Bleeker & Richard Ernst Presentation style: oral is preferred. I will trace the origin of Archean cratons within the context of much larger supercratons in the late Archean. These may or may not have been connected in a ca. 2.6 Ga supercontinent. The mininum length scale of supercratonic landmasses was many thousands of kilometres. Whatever the details, fundamental heterogeneity of Archean cratons demands that horizontal movements and terrane juxtaposition must have played a major role in building supercratonic aggregations. Following emplacement of numerous LIPs, and their plumbing systems, the supercratonic landmasses broke up diachronously between ca. 2.2 Ga and 1.9 Ga, spawning most of the ca. 35 known Archean cratons (s.s.). After a dispersal phase, these cratonic fragments and intervening juvenile terranes aggregated and collided between 1.9 and 1.8 Ga to form Earth’s first “modern” supercontinent Nuna (a.k.a. Columbia). I call Nuna the fist “modern” supercontinent because its geodynamics and tectonics show mostly familiar aspects (e.g., incorporation of sediment-rich passive margins, the first bonafide ophiolites, large coherent arcs, and undisputed sutures). Also, it was large enough to start dominating, for the first time, geochemical cycles with continental signatures (e.g., the seawater Sr isotopic record). The only major “tectonic innovation” yet to come were blueschists. From 1.8 Ga to ca. 1.0 Ga, Nuna evolved into Rodinia. Details remain murky but general systematics suggest themselves. Going back in time, component Archean cratons within 1.8 Ga Nuna can be restored into their ancestral supercratonic aggregations. We show that there is enough information in the system to do so for many of the ca. 35 cratons. In fact, a concerted international effort could accomplish this in less than a decade. Only then will we be able to test whether late Archean supercratons were ever connected in a pre-Nuna supercontinent and make general statements about the early part of the supercontinent cycle.