Did plate tectonic begin in Early Archean times?

Hugh SmithiesMartin Van KranendonkDave ChampionGeological Survey

of Western Australia

Back then Now> 3 billion years

Paleoarchean Neoproterozoic

Earth time scale

Blue schists

Accretionary wedge deposits

Classic complete ophiolite sections

Horizontal tectonics

Major strike-slip movements

Arc-like geochemistry

Uniformitari

anism

arc, arc….arc, arc, backarc

Archaean Branch

tectonic fence

Barberton GSB – Kaapvaal Craton

M. J. deWit and colleagues

1982: Interpreted large recumbent folds with overturned stratigraphy, and early thrusts and glide planes

1983: Interpreted a zone of thrust-stacked continental basement slices beneath the Komati Fm, and a major thrust separating the Komati Fm from underlying rocks

1987: Interpreted the Komati Fm as an ophiolite (Jamestown ophiolite complex)

1990: Age dating of older over younger rock packages

Isua (Greenland): North Atlantic Craton

1974: Bridgwater et al. Proposed a large-scale horizontal tectonic regime with nappe-like folding accompanied by voluminous granite magmatism

1989: Nutman et al. Recognised that "homogeneous grey gneisses” had

variable geochronologic and isotopic histories, and could be divided into distinct terranes separated by thin mylonite zones

1997: Nutman et al. Recognised that Isua was comprised at least

two chronologically and lithologically different supracrustal belts, juxtaposed in the early Archaean

2002: Nutman et al. Interpreted early thrust imbrication of greenstones

and gneisses at 3600-3650 Ma

Courtesy A. Nutman

Old (3.53-3.2 Ga) east Pilbara nucleus – contains no clear evidence for modern-style plate tectonics. This thickcrustal block most likely began as some form of oceanicplateau type crust.

The West Pilbara SuperterraneA younger (3.3 – 3.05 Ga) amalgamation of terranes – containsextensive set of features that collectively present a compelling case for modern steep-style subduction at ~3.2 Ga.This represents accretion peripheral to the East Pilbara nucleus.

~20 km thick (10 km min) basalt dominated sequence.

Uniformly upwards Younging packages bound by unconformities.

No clear evidence for tectonic boundaries – NOT a collection of accreted terrains

Just a simple pile of autochthonous groups.

•Basalts show no evidence for a subduction enriched source

•No boninites

•Felsic volcanics in lower part are not TTG or calc-alkaline rocks – they are fractionated tholeiites

East Pilbara

• we can not unequivocally rule out a modern-style subduction involvement for the Pilbara Supergroup, but there is no evidence supporting it and there are alternative scenarios that better explain the data. If subduction was involved here, it must have differed significantly from modern steep-style subduction.

Vivid contrast with the East Pilbara•Linear, structurally bound packages•Geochronolgically and isotopically distinct terrains with independent stratigraphic histories – exotic•Sholl Shear Zone accommodates 100’s km of sinistral movement•Achieved this configuration before 3.0 Ga

The 3.12 Ga Whundo Group essentially forms a ~10 km thickgeochronologically and lithologically exotic terrain comprising a mafic to felsic volcanic sequence with very juvenile isotopic

compositions and with little physical, chemical or isotopic evidence for older felsic basement

i.e. it was not deposited on continental crust.

Whundo

Calc-alkaline basalt and andesite

Trace element enrichments cannot be accounted for through contamination by any locally or regionally available crustalcomponent.

Boninites

Tholeiitic basalt

Adakitehigh-Nb basalt

high-Nb basaltAdakite

Calc-alkaline basalt and andesite

Boninites

Incr

easi

ng m

elti

ng

Flux melting – differs from normal melting of the mantle (decompression melting) and is confined to subduction zones because it needs an external volatile input

Increasing slab component

Ba/La

Enriched in fluid

Enriched in melt

Ba/La

Ba (ppm)Relative stratigraphic height (up )

Siliceous high-Mg basalts

Distinctive LREE enrichments that can’t be accounted for via contamination of any locally or regionally available crust.Unusually consistent isotopic and trace element ratios over a very wide region.

Derived from a mantle source metasomatized by a homogeneous mix of ‘old’ Pilbara crust and Whundo crust

West Pilbara Superterrane

THE COMPLETE PACKAGE

We have several lines of evidence that combine to present a

compelling case that modern-style subduction occurred at least by

3.2 Ga.

• an overall linear architecture with distinct terrains and boundaries that include strike-slip shears that account for 100’s km movement – Exotic terrains.

• isotopically juvenile crust

• a 3.12 Ga volcanic sequence free of any exotic continental material

• Whundo calc-alkaline basalts which require an enriched mantle source

• Whundo calc-alkaline basalts which reflect flux-melting

• Whundo boninites

• Whundo adakite/NEB association

• Whundo volcanic/geochemical association/architecture

• Later basalts independently derived from a modified mantle source, flanking the East Pilbara

Did plate tectonic begin in Early Archean times?

That’s not clear – but modern steep-style subduction was certainly active, at least

locally, by ~ 3.2 Ga