A gradual Proterozoic transition from an unstable stagnant lid to the modern plate tectonic system

Abstract

When did Plate Tectonics begin on Earth, and what preceded it? Published thermo-mechanical mantle evolution models imply that the early history of planets with a composition and size similar to Earth and Venus should be characterized by periodic mantle overturns of 30-100 million years duration, separated by stable lid phases of 100-300 My. I argue this is best described as an Unstable Stagnant Lid, because this term captures the Jekyll-and-Hyde duality of such worlds, which alternate between a Stagnant Lid ss phase between mantle overturns, and a Mobile Lid phase during overturns. Mantle overturn upwelling zones would rework and resurface large tracts of pre-existing Hadean crust and basalt-dominated Archean-Style Oceanic Lithosphere (ASOL). Basal anatexis of ASOL ∼40-50 km thick (or melting within down-drips) could generate tonalite-trondhjemite melts (TTGs) and create proto-continental nuclei, while garnet pyroxenite restites delaminate into the mantle. With further reworking, low-K tonalitic rocks would remelt to produce granodiorite and granite, completing the transfer of radioactive elements out of the lower crust. Mantle overturns would generate large-scale lateral currents in the upper mantle that would push against Archaean-aged sub-continental lithospheric mantle keels, causing continental drift and orogenesis despite the absence of plate-boundary forces like slab pull. The validity of this is corroborated by the observed displacement of Lakshmi Planum (>1000 Km) on Venus, a planet with no arcs or ridges. Recent models suggest the Abitibi Greenstone Belt formed as an oceanic tract behind a detached ribbon continent during partial breakup of the Southern Superior craton; and represents a possible sample of Archaean oceanic lithosphere. The Abitibi has ∼50 km of apparent stratigraphy composed of 2-10 My mafic-felsic bimodal volcanic cycles that follow assimilation-fractionation trends indicating contamination of mantle-derived basalts with TTG-like anatectites derived from older basalts. ASOL of this type would be difficult to subduct because of its weakness and buoyancy, but would be fertile and could generate large amounts of second-stage melts. There are no sheeted dykes, precluding a seafloor-spreading model, while the absence of basal cumulates or attached mantle means this type of ASOL should not be called an ophiolite. Archaean/Proterozoic unconformities are followed by deposition of Fe-formations, clastic and volcanic rocks that are only rarely affected by sagduction. The increase in siliciclastic input and decreasing sagduction reflect near-global late Archean emergence from the water of stiffening granitic continents due to secular cooling and intra-continental differentiation. Albeit associated with continent-derived siliciclastic debris, many Paleo-Proterozoic volcanic (and plutonic) rocks resemble Archaean ones geochemically. The similarity of magmatic rocks and hot orogenic styles in the Archaean and Paleo-Proterozoic could imply the overall geodynamic regime was similar in both. The Siderian-Rhyacian Quiet Period could therefore represent a Stagnant Lid phase that followed the 2.5 Ga Archaean overturn. When the next mantle overturn ruptured the lid at ∼2.2-2.0 Ga (and again at ∼1.9-1.8 Ga), continents would have been set into motion, forming arcs and ridges. Once initiated, arc and ridge segments would have needed to multiply and propagate to create a world-girdling system. Meso-Proterozoic rocks preserve clear evidence of plate mobility, subduction, and orogenesis; but inexplicably, ophiolites, the geological record of seafloor-spreading, are extremely rare prior to 1 Ga. Earth at 2.0 Ga was probably still largely covered by ASOL, possibly similar to the Abitibi, but how and where it was all destroyed and replaced by modern oceanic lithosphere are mysteries. Given the volume of ASOL involved, recognizable by-products of this global-scale reworking process should exist. Voluminous anorthosite-mangerite-charnockite-granite/gabbro suite rocks (AMCG) are mostly of Proterozoic age, requiring either an ephemeral source, or a unique process. Trace element inversion models applied to massif anorthosites imply they crystallized from high-La/Yb melts that do not resemble tholeiitic basalts, invalidating the notion that they are floatation cumulates from basaltic underplates. Model anorthosite-forming melts can, however, be explained by high-pressure melting of an ASOL-like basalt source with garnet-bearing residues. I posit that massif anorthosites record destruction at Proterozoic convergent margins of an ephemeral source: ASOL. When the last ASOL was crushed between converging continents or consumed by an overprinting arc (∼0.8-1 Ga), AMCG rocks ceased to form, and Earth became a modern Plate Tectonic planet.