EARTH'S FIRST TWO BILLION YEARS—THE ERA OF INTERNALLY MOBILE CRUST

Archean rocks differ strikingly in individual and collective composition, occurrence, association, and structure from modern rocks. Archean plate-tectonic conjectures are based on chemical analogies that are vague, forced, and inappropriate. Geologic features that characterize continental sundering, separation, convergence, subduction, and arc magmatism are lacking. The only known basement to Archean supracrustal rocks, including ultramafic lavas, consists of tonalite-trondhjemite-granodiorite (TTG) migmatites and gneisses. No Archean oceanic crust or mantle has been found. Before an areally-varying threshold of 3.5-2.9 Ga, the crust flowed and mixed vertically and laterally with extreme mobility. Later, dense lavas were erupted on the surface but caused a density inversion upon cooling, and sank in synforms between which deeper felsic crust ballooned up, while simultaneously the granite-and-greenstone upper crust flowed laterally with high mobility. Archean crust was far too weak and mobile to behave as rigid plates. Granite/greenstone/shearzone tectonics gradually gave way to more rigid continents, and by ~2.1 Ga a form of proto-plate-tectonics began to operate. A thick and global basaltic protocrust may have formed by ~4.45 Ga, and from it TTG suites were extracted by partial melting over much of the next 2 b.y. Delamination and sinking of the increasingly dense restite of the mafic protocrust enabled rise of deeper lighter and hotter mantle that enabled more crustal melting. Earth may have had a dense greenhouse atmosphere, not a hydrosphere, before 3.5 Ga, for the oldest proved supracrustal rocks and mafic and ultramafic dikes are of that age, and older felsic crust may have been too hot to permit rise of dense melts. The mantle beneath preserved cratons generated ultramafic melts that required a temperature ~300 degrees C hotter than modern asthenosphere ca. 3.5 Ga. The products of Paleoproterozoic plate tectonics differ markedly from those of the Phanerozoic Earth, and record a transitional era of cooling and stiffening lithosphere.