Earth's first two billion years—The era of internally mobile crust

Abstract

The magmatic and tectonic processes of the pre-2.5 Ga hot, young Earth differed profoundly from those of the modern planet. The ancient rocks differ strikingly in individual and collective composition, occurrence, association, and structure from modern rocks. Widespread forcing of Archean geology into plate-tectonic frameworks refl ects unwarranted faith in uniformitarianism and in inappropriate chemical discriminants, and disregard for the lack of features that characterize plate interactions. Archean crust records extreme and prolonged internal mobility and was far too weak and mobile to behave as rigid plates, required, by defi nition, for plate tectonics. None of the geologic indicators of subduction, arc magmatism, and continental sundering, separation, and convergence have been documented. No Archean oceanic crust or mantle has been recognized, and the only known basement to supracrustal rocks, including the thick basalts, high-Mg basalts, and ultramafi c lavas that typify greenstone successions, consists of tonalite-trondhjemite-granodiorite (TTG) migmatites and gneisses. A thick global melabasaltic protocrust likely formed by ca. 4.45 Ga, and from it TTG suites were extracted by partial melting over the next 2 b.y. Delamination of the increasingly dense restitic protocrust enabled rise of lighter and hotter depleted mantle and hence more melting. The oldest known crustal materials are zircons, which scatter in age back to 4.4 Ga and are recycled in migmatites whose fi nal crystallization was after 3.8 Ga, and in ancient sediments. Earth may have had a dense greenhouse atmosphere, not a hydrosphere, before 3.6 Ga, for the oldest proved supracrustal rocks are of that age, and older felsic crust may have been too hot to permit rise of dense melts. Rigid plates of lithosphere did not stabilize until a billion years after that and then were mostly small and local. Dense lavas erupted atop mobile felsic crust after 3.6 Ga produced a density inversion that was partly righted by sinking of the volcanic rocks and rising of the subjacent TTG. In some places, the early dense rocks retained cohesion and sank as synclinal keels between rising domiform diapiric batholiths. In others, the early dense rocks sank deep into mobile TTG crust, and only later in Archean time was the felsic substrate strong enough to enable dome-and-keel style. The TTG substrate rose slowly, with variable amounts of partial melting to generate more-fractionated melts

Ultra-Deep Drilling through 3.5-Billion-Year-Old Crust in South Africa

“…These data form the backbone for models that represent the ISZ as a 3.2-3.3 Ga paleo-suture zone, within which evidence is preserved for a low Archean geothermal gradient of 10°C-20°C km -1 that was subsequently overprinted by higher temperatures at lower pressures, indicative of collision and exhumation tec-tonics. This is contrary to conventional theories that all Archean environments had high geotherms (Hamilton, 2007). The drilling through the ISZ is thus a prime target for the study of a range of early Earth processes in an environment similar to those in modern subduction and suture zones.…”

Drilling the solid earth: global geodynamic cycles and earth evolution

“…As a result, continental crust contains the only accessible record of Earth processes and events prior to 200 Ma, and in many areas, the full length of this record can only be studied by scientific drilling. The processes by which the early continental crust formed are still not well understood and debate continues over the timing of the onset of plate tectonics and over the geodynamic processes that operated in the first part of Earth's history (e.g., Shervais 2006;Hamilton 2007;Stern 2008). In a modern context, the initiation and development of subduction zones can only be studied in a handful of places; continental scientific drilling offers the opportunity to investigate both active and fossil subduction zones in greater detail.…”

Plate-tectonic evolution of the Earth: bottom-up and top-down mantle circulation

“…But how and when did plate-tectonic processes commence, very early in Earth history (Kröner 1981;Ernst 1983;Sleep 1992Sleep , 2005Komiya et al 1999;Parman et al 2001;Polat et al 2002;Smithies et al 2003Smithies et al , 2005aCondie 2005;Cawood et al 2006;Dilek and Polat 2008;Nutman and Friend 2009;Sleep et al 2014;Turner et al 2014) or much later (Davies 1992;Hamilton 1998Hamilton , 2003Hamilton , 2007Stern 2005Stern , 2007Stern , 2008Dewey 2007;Brown 2008;Shirey and Richardson 2011;Shirey et al 2013)? In a sense, this is just a terminological problem, for the early, hot mantle must have circulated beneath a mobile outer rind since solidification of the postulated magma ocean.…”

“…The decomposition of methane and escape of H 2 into space may also have contributed to progressive oxidation (Catling et al 2007;Pope et al 2012). Alternatively, some greenstone complexes might have been generated at shallow, slightly more oxidized depths by fractional crystallization of basaltic magma (i.e., Bowen 1928;Sisson et al 2004), through partial fusion of high-T eclogite and (or) garnet amphibolite (Green and Ringwood 1967;Holloway and Burnham 1972;Boettcher 1973;Pertermann and Hirschmann 2003;Dufek and Bergantz 2005;Hamilton 2007;Nair and Chacko 2008), or by incipient melting of hydrous, fertile upper mantle peridotite (Kushiro et al 1968;Mysen and Boettcher 1975;Arculus 1981;Ulmer 2001). The sweeping together, decapitation, and suturing of near-surface portions of oceanic crust ± oceanic plateaus against primitive island arcs could have produced the ubiquitous Archean granite-greenstone belts.…”