Paper No. 1
Presentation Time: 8:05 AM


HAMILTON, Warren B., Department of Geophysics, Colorado School of Mines, Golden, CO 80401,

Abundant evidence shows modern-style plate tectonics since ~600 Ma. No viable evidence has been presented for lithospheric oceans, arcs, or subduction sutures, in Laurentia or anywhere else, before a transition to plates began ~850 Ma.

Thermodynamics requires fractionation of Earth as it accreted, and precludes plumes at any time. Ancient high upper mantle is buoyant refractory Fo~93 dunite. A 4.5 Ga thick mafic protocrust since mostly vanished, sourced more-fractionated crust. Now-associated fertile upper-mantle ultramafics formed by incremental top-down re-enrichment by delaminated residual protocrust and, much later, subducted slabs. Upper mantle has been progressively enriched, not depleted; primitive lower mantle never existed.

The shallower and weaker the radioactivity, the cooler the interior. Venus, Mars, and Moon froze at thick-protocrust stage, and preserve Hadean (4.5-3.9 Ga) bombardment products. Earth had Venus-type impact basins and impact-melt constructs to 2500 km in diameter, complicating its subsequent unique mobility.

Archean crust (now mostly TTG in sharp contact with high-Mg dunite) formed from protocrust that intermittently yielded “juvenile” TTG and volcanics while becoming denser, and delaminating. Most Archean crust is little eroded and has a near-flat Moho because it flowed; plates were impossible. Lower crust was kept hot by radioactivity + insulating upper crust, and flowed beneath, dragged, and rose into upper crust as dense supracrustals sank synformally. Cratonization resulted where and when (variously 2.9-2.2 Ga) all residual protocrust had sunk through dunite.

Only older continental basement is proved beneath Paleoproterozoic and Mesoproterozoic orogens, which formed from collapsed thick-filled ensialic basins, not subducting oceans. Internal TTG formed where subjacent protocrust was still present. Abundant high-K granites came from radioactively heated buried basement and sediments. All margins with Archean cratons were passive onlaps, not subducting. Stratal wedges and thickened interior complexes were thrust cratonward from some orogens.

Re-enrichment of upper mantle in fusibles ultimately enabled plate tectonics despite decreasing radioactivity. Oceans perhaps formed where protocrust foundered without having produced much TTG.