PRECAMBRIAN PLATE-TECTONIC EVOLUTION: BOTTOM-UP AND TOP-DOWN MANTLE CIRCULATION

Intense devolatilization + differentiation attended planetesimal accretion on the primordial Earth, but declined after solidification of a magma ocean. By 4.3 or 4.2 Ga, water oceans were present, so surface temperatures had fallen far below low-P solidi of dry peridotite, basalt, and granite, ~1300, ~1120, and ~950 °C. At less than half their solidi, rocky materials existed as thin lithospheric slabs in the Hadean Earth. Bottom-up mantle convection, including voluminous plume ascent, efficiently rid the Earth of massive amounts of deep-seated heat; it declined over time as cooling and top-down lithospheric sinking increased. Thickening, lateral extension and contraction typified the post-Hadean lithosphere. Stages of geologic evolution included: (a) 4.5-4.4 Ga, magma ocean overturn involving ephemeral, surficial rocky platelets; (b) 4.4-2.7 Ga, growth of oceanic and small continental plates, obliterated by return mantle flow prior to ~4.0 Ga; sial gradually began to accumulate in largely sub-sea, continental crust-capped collages; (c) 2.7-1.0 Ga, progressive suturing of old shields + young orogenic belts into cratonal plates typified by emerging continental freeboard, increasing sedimentary differentiation, and episodic glaciation during transpolar drift, and periods of stagnant-lid convection beneath growing supercontinents; (d) 1.0 Ga-present, laminar-flowing asthenospheric cells capped by giant, stately moving plates. Near-restriction of komatiites to the Archean, and appearance of multicycle sediments, ophiolite complexes, alkaline igneous rocks, and HP/UHP metamorphic belts in progressively younger Proterozoic and Phanerozoic orogens reflecting increasing negative buoyancy of cool oceanic lithosphere, decreasing subductability of enlarging, more buoyant continental plates. Attending supercontinental assembly, density instabilities of thickening oceanic plates began to control overturn of suboceanic mantle as cold, top-down convection. Over time, the scales and dynamics of hot asthenospheric upwelling versus lithospheric foundering + mantle return flow (bottom-up plume-driven ascent versus top-down plate subduction) evolved gradually, reflecting planetary cooling. These plate-tectonic processes have accompanied the Earth’s thermal history since ~4.4 Ga.