Plate Tectonics in the ArcheanOn Archean granites, greenstones,

cratons and tectonics: does the evidence demand a verdict? M. J. de Wit (1998)

Nick Cowan

UW Astronomy

January 2006

Game Plan

• Review everything we know about Archean (3.8-2.5 Ga) terrains everywhere on Earth.

• Wave our hands about what it all means for the origin of plate tectonics and continents.

• Poke holes in some widely accepted geophysical ideas.

Superior Province(part of the Canadian Shield)

Late Archean Tectonism

• Superior Province is the biggest, baddest Archean craton.

• It lasted so long because of its deep mantle keel of relatively buoyant material.

• Did all Archean terrains have keels or are they simply the only survivors?

• In any case, modern terrains don’t have keels so tectonics must have changed between then and now.

Slave Craton

Early Archean: Granites• The oldest rocks (~4 Ga) in the Slave Craton are

continental and formed from hydrated, chemically depleted mafic precursors and earlier granitoids.

• Geochemical evidence implies that they formed by partial melting 20-100 km underground, at temperatures of 750-1000 K, by some sort of subduction process.

• Many have assumed that Archean subduction was shallow and at a low-angle.

• Some now believe that subduction may have been steeper, so that some mantle would end up sandwiched between the subducting crust and the overlying lithosphere.

• The data is ambiguous.

Kaapvaal Craton (South Africa) & Pilbara Craton (Australia)

Early Archean: Greenstones

• Greenstones show a range of tectonic boundary environments and evidence of shallow mantle melting.

• The oceanic plates must have been rigid and plate tectonics played an important role.

• Preliminary evidence implies that oceanic crust was at least as thin as its modern-day counterpart but theoretical arguments imply that it was thicker...

Archean Tectonics• Hadean ocean crust got subducted unaltered and

dry. Mid-ocean ridges were above sea level.• Mantle degassing (4.5-4.0 Ga) increased the sea

level, submerging and hydrating spreading ridges.• As this warm, hydrated ocean crust was subducted at

a shallow angle, it would have formed the first TTG (old granite).

• The upper layer of serpentinized lithosphere would have delaminated during subduction. The stacking of such layers would eventually form continental keels (as opposed to the conventional idea of building them from below).

Transition to Modern Tectonics• Continental keels lead to steep

convection and mantle-wedge hydration.

• Trench retreat, thin continent formation

• Slab pull begins driving plate tectonics

• Water flux switches from degassing of mantle to hydration of mantle.

• By the Late Archean, these continental kernels were colliding and forming bona fide continents.

Theoretical Models: Issues• There was more radiogenic heating in the Archean,

but different tectonics may have led to more effective heat loss, too.

• Komatiites formed deep in the mantle at temperature hundreds of degrees warmer than today. Or maybe they were just soggy, in which case the temperature could have been similar to today’s.

• Water was not completely degassed early in the Hadean (there are still pockets of volatiles in the mantle today).

• In fact, just a modest amount of water decreases the viscosity of the mantle by 2 orders of magnitude, making all of this hard to model...

• A wet mantle mimics a hot mantle.

Theoretical Models: More Issues

• More generally, the fact that there is more young continental crust around today doesn’t mean that the amount of continental crust has been increasing in time. It may be that old crust gets destroyed/recycled. (“growth vs recycling problem”)

• Given the rate at which the Superior Province was created, there should be hundreds of similar sized cratons on the planet... recycling is manifestly important.

The Golden Mean

• Some argue that tectonics in the Archean was only quantitively different from in the Phanerazoic.

• Others argue that the lack of certain rocks in the Archean indicate a completely different tectonic activity.

• de Wit takes the Middle Path: Archean tectonics were basically the same, except that sea floor recycling wasn’t quite as efficient back then.