THE ONSET OF PLATE TECTONICS IN THE EOARCHEAN

A major controversy of Earth Sciences today concerns the time of onset of modern-style plate tectonics in the Archean. There are two principal times: ca. 3.9-3.6/3.5 Ga and ca. 3.3-3.0 Ga.

1.Field-based geological relations in >14 areas worldwide supported by geochemical-isotopic-age data provide compelling evidence that modern-style plate accretion and subduction was operating at 3.9-3.6/3.5 Ga. Emphasis is placed on: Lower crust: horizontal-thrusted juvenile TTG gneisses; thrusted slices of volcanic supracrustal belts with arc-like calc-alkaline-tholeiitic basalts, boninites-picrites; ophiolitic oceanic crust (Isua); imbricate-stacking/downward accretionary growth, layer-parallel thrusted ocean plate stratigraphy; HP granulites (Itsaq), anorthosite-gabbro-UM complexes (arc roots), local modern geothermal gradients (15-20°C/km, Barberton). These rocks are subducted-accreted dismembered oceanic/continental arcs that underwent collision tectonics. Lower crustal high-T metamorphism, partial melting of supracrustal rocks, and granitoid intrusions reflect processes operating in hotter Archean subduction zones. Upper crust: arc-dominated greenstones, rare komatiites, TTGs, anorthositic complexes, diapiric granitoid plutons, all the result of subduction-accretion tectonics

2.Experimental modellers propose that the onset of plate tectonics was at 3.3-3.0 Ga after a time of vertical tectonics. Interpretation of the models, based mainly on numerical simulations, is in terms of gravity-driven vertical tectonics involving hot pipes, stagnant-lids, drips, plumes, and sagduction-diapirism. However, the field-based geological-geochemical group considers that statements like: “Archean arcs are imaginary constructs with no objective existence”, “absence of thrusting is a key characteristic of Archean crust”, and “geological-geochemical data are inconsistent with modern-style ridges and arcs in the Archean” are factually wrong. A lack of understanding of Eoarchean (and Paleoarchean) geology, and use of inappropriate non-geologically-based boundary conditions to model the very geology that the models are supposed to test, invalidate the experimental approach. The few experimental models are inconsistent with the well-documented Eoarchean geology worldwide.