Wouter Bleeker, Richard Ernst & Ken Buchan Geological Survey of Canada, Ottawa

1. Evidence for plate behaviour at 2.1-1.8 Ga: break-up, dispersal & suturing of Archean cratons Wouter Bleeker, Richard Ernst & Ken Buchan Geological Survey of Canada, Ottawa Did Plate Tectonics begin in Paleoproterozoic time?…well before, but scale & style become more modernduring the Paleoproterozoic Penrose, June 2006

2. “The 2.7 Event” No data Bleeker & Enst, in prep. Significant secular change?...Yes, of course! • Higher heat production • Weaker lower crust • Always more basalt in the system …more significant density inversions • Smaller plate scales • Faster recycling Penrose, June 2006

3. Precambrian geology of North America A Paleoproterzoic collage of micro-plates and inter-vening arcs terranes Modified after Hoffman, 1989 ; based on a century of geological research Penrose, June 2006

4. Penrose, June 2006

5. The ~1.8 Ga collage: plate tectonics? 7. Large strike-slip faults? …Yes. LITHOPROBE’S SNORCLE Transect Great Slave Lake Shear Zone • Relevant questions: • Was there significant lateral movement? ...Yes. • Are now adjacent blocks unrelated (exotic)? ...Yes, commonly. • Plate behaviour: rifting, break-up, convergence? ...Yes. • Did blocks behave (quasi) rigid? ...Yes, some. • Strong asymmetry across suturing orogens? ...Yes. • Are time spans and rates similar? …Yes, comparable. Penrose, June 2006

6. Bleeker, 2002 Cook et al., 1999 Penrose, June 2006

7. Accretionary structure along westernmargin of Slave craton, 1.9-1.7 Ga: Cook et al., 1999 Penrose, June 2006

8. Successivesutures: Penrose, June 2006

9. Suture geometries: e.g., White et al., 2002 Penrose, June 2006

10. Laurentia within~1.8 Ga “Nuna”: ??? ??? Baltica e.g., Buchan et al., 2000 ??? Siberia? Australia- Antarctica? Long-lived active margin Penrose, June 2006

11. Nuna to Rodinia: Penrose, June 2006

12. ~1 Ga Rodinia (conceptual only): Rifted Nuna fragments Intact core of ~1.8 Ga Nuna Stray fragments Penrose, June 2006

13. Further back in time: before Nuna Bleeker, 2005 Penrose, June 2006

14. Others Others Others “Break-out” of the Superior craton,out of ancestral landmass Superia: Ancestral landmass “Superia” Kola Wyoming Karelia Hearne Penrose, June 2006

15. Superia: Superia Wyoming Bleeker & Ernst, 2006 Penrose, June 2006

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

17. From late Archean supercratons to Nuna:break-up & independent drift ofcratonic fragments Not to scale! Penrose, June 2006

18. ~5 cm/yr Did things move? Penrose, June 2006

19. Ophiolites? Sparse but present!! Kontinen, Peltonen et al. Penrose, June 2006

20. Diagnostic rock associations: Arc Foredeep sequence Passive margin sequence Rift sequence Slave basement -Plume-assisted extension & break-up? …Yes, definitely. -Rift & passive margin sequences? …Yes. Penrose, June 2006

21. 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? Penrose, June 2006

22. Conclusions:The Paleoproterozoic preserves a clear record of (small) plate tectonics, resulting in Earth’s first “modern” supercontinent Nuna Not to scale! Penrose, June 2006

23. Penrose, June 2006

24. Hearne – southern Superior link: 2446 Ma dykes 2440-2450 Ma (2.45-2.5 Ga) Kaminak dykes 2446 Ma (2.45-2.5 Ga) Matachewan dykes Dates by Heaman Penrose, June 2006

25. Solution allowed by current paleomagnetic data: 2110 Ma Penrose, June 2006 Bleeker, 2002, 2004

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

27. “Fragmentation tree”of Archean fragments Karelia Labr. Hearne Nain Kaapvaal Slave Lew. Superior Sclavia Superia Vaalbara The Archean family tree Penrose, June 2006

28. “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. Penrose, June 2006