Pre-Neoproterozoic plate tectonics: paleomagnetic evidence

1. Pre-Neoproterozoic plate tectonics: paleomagnetic evidence S.A. Pisarevsky Tectonics Special Research Centre University of Western Australia

2. What we need to know to use pre-Neoproterozoic paleomagnetism? • How old is the geomagnetic field? • Can the Geocentric Axial Dipole model be applied to the ancient geomagnetic field? • Do the ancient rocks carry the primary remanent magnetisation?

3. How old is the geomagnetic field?The oldest remanence: • ~3.5 Ga Komati Fm, S. Africa (Hale & Dunlop, 1984; Yoshihara & Hamano, 2004). However, the intensity probably was only ~ 0.25-0.30 of the present-day value • ~3.5 Ga Duffer Fm, NW Australia (McElhinny & Senanayake, 1980) – supported by a positive fold test (~2 Ga folding) • ~2.8 Ga Kamiskotia Complex (Irving & Naldrett, 1977) supported by inverse contact test

5. When did the geomagnetic field become dipolar? • there are examples of the consistent paleomagnetic results within a rigid continental block • paleointensity studies of ~2.5 Ga Burakovka intrusion, Russia (Smirnov et al., 2003) suggest that it was similar to the present-day intensity • estimations of paleosecular variations of ~2.5 Ga dykes from Superior and Karelia (Smirnov & Tarduno, 2004) are consistent with the dipolar model • magnetic reversals were registered as early as at ~2.7 Ga (Strik et al., 2003)

6. SO: • it is likely that the geomagnetic field was similar to the present-day field since Late Archean – Early Proterozoic • it means that the model of Geocentric Axial Dipole can be applied and paleomagnetic data can be used for testing plate tectonics since then

7. How? • if it is possible to construct Apparent Polar Wander Paths for several continents, the plate tectonic reconstructions can be built with good precision

8. APWPs for Europe and North America

9. unfortunately, so far it is just a dream for > 1 Ga

10. What can we do with just one pole?

11. At pre-Neoproterozoic times we are somewhere between these two extremes

12. Pre-1000 Ma paleomagnetic data 1111results

13. After “soft” filtering 664results

14. After “soft” filtering 7 pre-3000 Ma results

16. After “soft” filtering 150 pre-2000, post-3000 Ma results

18. After “soft” filtering 507 pre-1000, post-2000 Ma results

19. If we have two continents with two coeval pairs of poles • if these continents moved relative to each other, we can prove it • if they did not, we cannot prove it, but we can prove the possibility of that

20. This can be done by showing the variety of their possible mutual configurations

21. or by analysing the movements of their palaeomagnetic poles

24. Another example

28. For younger times fragments of APWPs can be reconstructed Laurentia Baltica

29. Conclusions • paleomagnetism is a valuable tool for checking and building plate tectonic reconstructions for Paleoproterozoic and (at least) Late Archean • paleomagnetic data show that at least some continents moved relatively to each other during these times • paleomagnetism votes for plate tectonics since Late Archean, maybe even earlier