Pre-Neoproterozoic Pre-Neoproterozoic plate tectonics: plate tectonics:

paleomagnetic evidencepaleomagnetic evidence

S.A. PisarevskyS.A. Pisarevsky

Tectonics Special Tectonics Special Research CentreResearch Centre

University of Western AustraliaUniversity of Western Australia

What we need to know to What we need to know to use pre-Neoproterozoic use pre-Neoproterozoic

paleomagnetism?paleomagnetism?•How old is the geomagnetic How old is the geomagnetic field?field?

•Can the Geocentric Axial Dipole Can the Geocentric Axial Dipole model be applied to the ancient model be applied to the ancient geomagnetic field?geomagnetic field?

•Do the ancient rocks carry the Do the ancient rocks carry the primary remanent magnetisation?primary remanent magnetisation?

How old is the geomagnetic How old is the geomagnetic field?field?The oldest remanence:The oldest remanence:• ~3.5 Ga Komati Fm, S. Africa (Hale & ~3.5 Ga Komati Fm, S. Africa (Hale & Dunlop, 1984; Yoshihara & Hamano, 2004). Dunlop, 1984; Yoshihara & Hamano, 2004). However, the intensity probably was only ~ However, the intensity probably was only ~ 0.25-0.30 of the present-day value0.25-0.30 of the present-day value

• ~3.5 Ga Duffer Fm, NW Australia (McElhinny ~3.5 Ga Duffer Fm, NW Australia (McElhinny & Senanayake, 1980) – supported by a & Senanayake, 1980) – supported by a positive fold test (~2 Ga folding)positive fold test (~2 Ga folding)

• ~2.8 Ga Kamiskotia Complex (Irving & ~2.8 Ga Kamiskotia Complex (Irving & Naldrett, 1977) supported by inverse Naldrett, 1977) supported by inverse contact testcontact test

When did the geomagnetic When did the geomagnetic field become dipolar?field become dipolar?

• there are examples of the consistent there are examples of the consistent paleomagnetic results within a rigid paleomagnetic results within a rigid continental blockcontinental block

• paleointensity studies of ~2.5 Ga paleointensity studies of ~2.5 Ga Burakovka intrusion, Russia (Smirnov Burakovka intrusion, Russia (Smirnov et al., 2003) suggest that it was et al., 2003) suggest that it was similar to the present-day intensitysimilar to the present-day intensity

• estimations of paleosecular variations estimations of paleosecular variations of ~2.5 Ga dykes from Superior and of ~2.5 Ga dykes from Superior and Karelia (Smirnov & Tarduno, 2004) are Karelia (Smirnov & Tarduno, 2004) are consistent with the dipolar modelconsistent with the dipolar model

• magnetic reversals were registered as magnetic reversals were registered as early as at ~2.7 Ga (Strik et al., early as at ~2.7 Ga (Strik et al., 2003)2003)

SO:SO:

•it is likely that the geomagnetic it is likely that the geomagnetic field was similar to the present-field was similar to the present-day field since Late Archean – day field since Late Archean – Early ProterozoicEarly Proterozoic

•it means that the model of it means that the model of Geocentric Axial Dipole can be Geocentric Axial Dipole can be applied and paleomagnetic data can applied and paleomagnetic data can be used for testing plate tectonics be used for testing plate tectonics since thensince then

How?How?

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

APWPs for Europeand North America

unfortunately, so far it is just a unfortunately, so far it is just a

dream for > 1 Gadream for > 1 Ga

What can we do with just What can we do with just one pole?one pole?

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

Pre-1000 Ma paleomagnetic Pre-1000 Ma paleomagnetic datadata

1111 results

After “soft” filteringAfter “soft” filtering664 results

After “soft” filteringAfter “soft” filtering

7 pre-3000 Ma results

After “soft” filteringAfter “soft” filtering

150 pre-2000, post-3000 Ma results

After “soft” filteringAfter “soft” filtering

507 pre-1000, post-2000 Ma results

If we have two continents If we have two continents with two coeval pairs of with two coeval pairs of

polespoles

•if these continents moved if these continents moved relative to each other, we can relative to each other, we can prove itprove it

•if they did not, we cannot if they did not, we cannot prove it, but we can prove the prove it, but we can prove the possibility of that 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

ConclusionsConclusions

• paleomagnetism is a valuable tool for paleomagnetism is a valuable tool for checking and building plate tectonic checking and building plate tectonic reconstructions for Paleoproterozoic reconstructions for Paleoproterozoic and (at least) Late Archeanand (at least) Late Archean

• paleomagnetic data show that at least paleomagnetic data show that at least some continents moved relatively to some continents moved relatively to each other during these timeseach other during these times

• paleomagnetism votes for plate paleomagnetism votes for plate tectonics since Late Archean, maybe tectonics since Late Archean, maybe even earliereven earlier