GEODYNAMICS THROUGH TIME

Voluminous evidence is incompatible with popular geodynamic speculations. For example, the passive-margined Atlantic slowly widens at about the same areal rate as the rapidly spreading Pacific shrinks between inward-migrating subduction hinges. This disproves concepts of whole-mantle convection with semifixed components, including ideal-liquid-convection cartoons. Archean cratons have sharp, flat Mohos between TTG crust (wet-melted from now-missing mafic sources after 4.0 Ga) and ancient refractory dunite (Fo93) mantle. This disproves geochemists’ still-mostly-unfractionated mantle, and geologists’ steady-state Earth. And vastly more.

 Earth’s U, Th, and 40K are enough even now to melt the mantle if distributed uniformly, and their 5-times-more young-Earth heat melted it during accretion. They partitioned into rising melts, and left mantle fractionated at high T, e.g. the dunite, overlain by thick mafic radioactive protocrust, the TTG source. Hadean zircon compositions accord with dry impact melts from such crust. Hydration enabling oceans, atmosphere, and Archean TTG melting may have come in a barrage of icy outer asteroids with disrupted orbits circa 4.0 Ga. Archean mafic-lava regional sheets (not belts) erupted on TTG, which itself yielded secondary melts. Garnet-rich residual protocrust sank through dunite, beginning re-enrichment of the lower part of upper mantle. Proterozoic orogens developed from basin fills on Archean felsic crust where protocrust delamination was incomplete. Basin strata lap on to cratons on both sides. Proterozoic igneous rocks represent radiogenic heating of basin fills, older felsic basement, and remaining protocrust.

 Indicators of subduction, arcs, and oceanic crust postdate about 800 Ma. Top-down upper-mantle re-enrichment ultimately enabled asthenosphere and plate tectonics. Circulation is confined to upper mantle and driven by the density inversion of top-cooled oceanic lithosphere. Subvertically sinking slabs are plated down on the profound 660-km seismic discontinuity. They push all sub-lithospheric upper mantle back under incoming oceanic plates, producing rapid spreading (Pacific), while “sucking” overriding plates toward rolling-back hinges. Overridden downplated slabs enable slow spreading (Atlantic) behind overriding plates.