ON THE EMERGENCE OF PLATE TECTONICS

The critical condition for plate tectonics (PT) is the creation and maintenance of a global network of narrow plate boundaries separating multiple plates. Localized subduction not part of a network of plate boundaries, such as exist on Venus, is not evidence of PT. To argue that PT operated before the Jurassic requires more than a record of subduction, it requires evidence of a global network of plate boundaries and multiple plates. A case can be made for PT back to the Paleoproterozoic, when a cycle of breakup and collision led to formation of the supercontinent Columbia, but before then PT is contentious. Further back the rock record is limited, particularly before the Neoarchean, and may only provide a lower limit on the emergence of PT. Earth’s mantle was warmer in the past, although by how much is unclear, and could have precluded stable subduction. Sporadic evidence of intermittent subduction is insufficient to identify PT and an alternative mode of heat loss would have been required before PT. The widespread appearance of two thermobaric types of metamorphism during the Neoarchean likely records the dual thermal regimes that are the hallmark of subduction and collision at convergent plate margins. Prior to this, the Archean lithosphere likely comprised a more-or-less continuous ‘squishy’ lid, and any subduction was intermittent. This lithosphere would have been strongly affected by plumes, which potentially could have triggered subduction in which retreating slabs surrounded expanding cells of plate-like behavior. Influx of asthenosphere beneath the ‘plates’ could have generated plateau-like crust, partial melting of which would have generated TTGs, forming protocontinents. Collision between cells and protocontinents due to slab retreat could have amalgamated the protocontinents to form the first continents. Initiation of subduction was likely linked to the rise of these continents above sea level, increased erosion, accumulation of sediments at the continental edges, and GPE-driven spreading of continental over oceanic crust, depressing it into eclogite facies. This process could have initiated and then stabilized subduction due to the ongoing availability of lubricating sediments, ultimately enabling the emergence of a globally-linked network of plate boundaries during a Neoarchean transition to PT.