Research progresses and key scientific issues of pre-plate tectonics and origin of continents

In the solar system, Earth is a unique planet with felsic continents, but controversy has long surrounded the issue of how felsic continents formed. At the center of the controversy is whether or not plate tectonics can be applied back to the Archean eon (>2.5 billion years) since most felsic continents were formed during Archean time, and if not, what was the main geodynamic mechanism that governed the formation and growth of felsic continents? In this review, we have summarized the current research progresses, the challenges, the key scientific questions, and the key research areas in the next 5-10 years. At present, there are three important achievements that have been obtained in this research field: (1) Three new models have been proposed for pre-plate tectonics, including mantle plume, sagduction, and heat pipe; (2) key identification criteria have been summarized to distinguish non-plate tectonics from plate tectonics; and (3) some geological records of early earth's sphere evolution have been discovered. Main evidence supporting Archean plate tectonics are that (1) TTG (Tonalite-Trondhjemite-Granodiorite) rocks dominant in Archean terrains can also form in modern subduction zones; (2) Archean TTG rocks have arc-affinities in geochemistry; and (3) partial melting of mafic rocks to form Archean TTG requires H2O, which favours subduction zones. However, plate tectonics fails to interpret the following features of Archean cratons: (1) Dominant bimodal volcanic assemblages in the Archean greenstones; (2) Archean komatiites whose formation needed a melting process that occurred at 1600-1900 degrees C; (3) nearly coeval emplacement of TTG plutons over a whole craton within a narrow time gap; (4) mass balance problems if Archean TTG rocks were derived from the 10%-30% partial melting of either eclogites or garnet amphibolites; and (5) dominant domiform structures with vertical lineations. Consequently, a number of pre-plate tectonic models have been proposed to interpret the origin of Archean felsic continents, including mantle plume, sagduction, and heat pipe models. However, none of these existing pre-plate tectonics models can successfully explain the origin of Archean continents. For example, both the mantle plume and heat pipe models cannot reasonably explain the issue of how mafic rocks were hydrated before they were partially melt to for TTG magmas. The prerequisite for the sagduction model is the existence of a felsic (TTG) continental crust, and thus it cannot be used to explain the origin of Archean felsic continents. This forms the justifications for holding a NSFC Shuangqing Forum entitled Pre-Plate Tectonics and Origin of Continents on November 11-13, 2022. At the forum, invited experts fully discussed the necessity, research advantages, and risks of conducting research on pre-plate tectonics and the origin of continents in China. The key scientific questions in the research field include: (1) Whether continents or plate tectonics occurred on the Earth first; (2) whether the formation of oceans and continents was synchronous; and (3) how did the Hadean continental crust and TTGs generate? Based on the current development of geosciences, the importance of dynamic thermal modeling and comparative planetology is suggested to be emphasized in the future studies on pre-plate tectonics and the origin of continents. On the basis of these summaries, we suggest that the key research fields in the next 5-10 years should focus on: (1) The origin of continents and their influence on the early Earth's environment and life; (2) the structural patterns of pre-plate tectonics and their formation mechanisms; (3) thermal regime and thermal evolution of Archean tectonics; and (4) pre-plate tectonics and the evolution of Earth-like planets.