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

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

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 ~ of the present-day value ~3.5 Ga Komati Fm, S. Africa (Hale & Dunlop, 1984; Yoshihara & Hamano, 2004). However, the intensity probably was only ~ of the present-day value ~3.5 Ga Duffer Fm, NW Australia (McElhinny & Senanayake, 1980) – supported by a positive fold test (~2 Ga folding) ~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 ~2.8 Ga Kamiskotia Complex (Irving & Naldrett, 1977) supported by inverse contact test

When did the geomagnetic field become dipolar? there are examples of the consistent paleomagnetic results within a rigid continental block 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 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 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) magnetic reversals were registered as early as at ~2.7 Ga (Strik et al., 2003)

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

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

APWPs for Europe and North America

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

What can we do with just one pole?

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

Pre-1000 Ma paleomagnetic data 1111 results

After “soft” filtering 664 results

After “soft” filtering 7 pre-3000 Ma results

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

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

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

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

or by analysing the movements of their palaeomagnetic poles

Another example

For younger times fragments of APWPs can be reconstructed Laurentia Baltica

Conclusions paleomagnetism is a valuable tool for checking and building plate tectonic reconstructions for Paleoproterozoic and (at least) Late Archean 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 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 paleomagnetism votes for plate tectonics since Late Archean, maybe even earlier