Seminar: A mantle overturn model for the Archaean Earth and a new hypothesis to explain the Meso- to Neo-Proterozoic transition to Plate Tectonics
Presenters: Jean H Bédard (Geol. Surv. Canada) & Lyal Harris (INRS-ETE)
Seminar Series: Virtual Seminar for Precambrian Geology
Date and Time: 8am PST, 11am EDT – 21st April
Geological data imply plate tectonics was not active on the Archaean Earth (pre-2.5 Ga). Key arguments include: 1) the absence from the Archaean rock record of ophiolites and high-pressure metamorphic rocks; 2) the rarity of Archaean andesites and lahars; 3) the absence of arc-like, source-metasomatic, trace element signatures in Archaean calc-alkaline magmatic suites; and 4) fundamental differences in overall structural style. Conversely, unstable stagnant-lid models better account for many features of Archaean geology. Thermomechanical models imply many unstable stagnant-lid planets would have experienced transient mobile-lid phases linked to periodic mantle overturns of 30–100 My duration, separated by stable-lid phases lasting 100–300 My. Mantle overturn upwelling zones would be characterized by high magma fluxes that may have generated continental nuclei and would have reworked, disaggregated and resurfaced tracts of pre-existing oceanic and continental lithosphere. Overturns would also have generated large-scale lateral mantle flow patterns, with traction forces acting on the SCLM (sub-continental lithospheric mantle) keels that underlie most cratons to induce continental drift and orogenesis, despite the absence of plate-boundary forces such as slab pull and ridge push. The isotopically juvenile Abitibi-Wawa Greenstone Belt may represent typical, crust-dominated, buoyant, Archaean-Style Oceanic Lithosphere (ASOL), that would probably have covered 80% of the Archaean Earth’s surface. Volcanic resurfacing during overturns would have had negative impacts on biota, perhaps retarding the apparition of metazoans until the initiation of the more efficient plate tectonic cooling system.
Larger and more linear Proterozoic orogens are frequently interpreted as the result of plate tectonics, yet ophiolites and high P/T metamorphic rocks of this age are rare to absent, and many Proterozoic orogens contain large volumes of AMCG-type rocks (anorthosites-mangerites-charnockites-granites/gabbros), which are absent from Archaean and Phanerozoic tectonic environments. We speculate that: 1) the Rhyacian-Siderian quiet period (from ~2.4 to ~2.2 Ga) represents a stagnant-lid episode that followed the Neoarchaean paroxysmal overturn; 2) a mantle overturn at ~2.2-1.8 Ga ruptured the crustal lid and triggered formation of the 1st arcs when drifting continents overrode denser ASOL tracts; 3) where continental masses diverged, the first Atlantic-style spreading ridges formed, yielding subductable, modern-type oceanic lithosphere. Inverse trace element models show that the melts from which the Proterozoic massif-type anorthosites formed resembled adakites, not basalts, requiring a crustal source and garnet-bearing residues. Plausible source rocks for such voluminous melts resemble ASOL similar to the bulk Abitibi stratigraphy. We speculate that subcretion and extensive melting of buoyant tracts of ASOL beneath the leading edges of drifting Proterozoic continents generated the melts from which Proterozoic massif-type anorthosite complexes formed. Although subduction zones and ridges occurred as early as the Paleoproterozoic, it is only after most of the relict ASOL had been destroyed (~1-0.8 Ga) that modern-style plate tectonics became the dominant, world-girdling system.