The boring billion? - Lid tectonics, continental growth and environmental change associated with the Columbia supercontinent

The evolution of Earth＇s biosphere,atmosphere and hydrosphere is tied to the formation of continental crust and its subsequent movements on tectonic plates.The supercontinent cycle posits that the continental crust is periodically amalgamated into a single landmass,subsequently breaking up and dispersing into various continental fragments.Columbia is possibly the first true supercontinent,it amalgamated during the 2.0-1.7 Ga period,and collisional orogenesis resulting from its formation peaked at 1.95-1.85 Ga.Geological and palaeomagnetic evidence indicate that Columbia remained as a quasi-integral continental lid until at least 1.3 Ga.Numerous break-up attempts are evidenced by dyke swarms with a large temporal and spatial range; however,palaeomagnetic and geologic evidence suggest these attempts remained unsuccessful.Rather than dispersing into continental fragments,the Columbia supercontinent underwent only minor modifications to form the next supercontinent （Rodinia） at 1.1 -0.9 Ga; these included the transformation of external accretionary belts into the internal Grenville and equivalent collisional belts.Although Columbia provides evidence for a form of ‘lid tectonics’,modern style plate tectonics occurred on its periphery in the form of accretionary orogens.The detrital zircon and preserved geological record are compatible with an increase in the volume of continental crust during Columbia＇s lifespan; this is a consequence of the continuous accretionary processes along its margins.The quiescence in plate tectonic movements during Columbia＇s lifespan is correlative with a long period of stability in Earth＇s atmospheric and oceanic chemistry.Increased variability starting at 1.3 Ga in the environmental record coincides with the transformation of Columbia to Rodinia; thus,the link between plate tectonics and environmental change is strengthened with this interpretation of supercontinent history.

Initiation of plate tectonics in the Hadean: Eclogitization triggered by the ABEL Bombardment

When plate tectonics began on the Earth has been long debated and here we argue this topic based on the records of Earth-Moon geology and asteroid belt to conclude that the onset of plate tectonics was during the middle Hadean（4.37-4.20 Ga）. The trigger of the initiation of plate tectonics is the ABEL Bombardment, which delivered oceanic and atmospheric components on a completely dry reductive Earth, originally comprised of enstatite chondrite-like materials. Through the accretion of volatiles, shock metamorphism processed with vaporization of both CI chondrite and supracrustal rocks at the bombarded location, and significant recrystallization went through under wet conditions, caused considerable eclogitization in the primordial continents composed of felsic upper crust of 21 km thick anorthosite, and 50 km or even thicker KREEP lower crust. Eclogitization must have yielded a powerful slab-pull force to initiate plate tectonics in the middle Hadean. Another important factor is the size of the bombardment. By creating Pacific Ocean class crater by 1000 km across impactor, rigid plate operating stagnant lid tectonics since the early Hadean was severely destroyed, and oceanic lithosphere was generated to have bi-modal lithosphere on the Earth to enable the operation of plate tectonics.Considering the importance of the ABEL Bombardment event which initiated plate tectonics including the appearance of ocean and atmosphere, we propose that the Hadean Eon can be subdivided into three periods：（1） early Hadean（4.57-4.37 Ga）,（2） middle Hadean（4.37-4.20 Ga）, and（3） late Hadean（4.20-4.00 Ga）.

A model of crust-mantle differentiation for the early Earth

The Archean continents,primarily composed of the felsic tonalite-trondhjemite-granodiorite(TTG)suite,were formed or conserved since~3.8 Ga,with significant growth of the continental crust since~2.7 Ga.The difficulty in direct differentiation of the felsic crustal components from Earth’s mantle peridotite leads to a requirement for the presence of a large amount of hydrated mafic precursor of TTG in Earth’s proto-crust,the origin of which,however,remains elusive.The mafic proto-crust may have formed as early as~4.4 Ga ago as reflected by the Hf and Nd isotopic signals from Earth’s oldest geological records.Such a significant time lag between the formation of the mafic proto-crust and the occurrence of felsic continental crust is not reconciled with a single-stage scenario of Earth’s early differentiation.Here,inspired by the volcanism-dominated heat-pipe tectonics witnessed on Jupiter’s moon Io and the resemblances of the intensive internal heating and active magmatism between the early Earth and the present-day Io,we present a conceptual model of Earth’s early crust-mantle differentiation,which involves an Io-like scenario of efficient extraction of a mafic proto-crust from the early mantle,followed by an intrusion-dominating regime that could account for the subsequent formation of the felsic continents as Earth cools.The model thus allows an early formation of the preTTG proto-crust and the generation of TTG in the continent by providing the favorable conditions for its subsequent melting.This model is consistent with the observed early fractionation of the Earth and the late but rapid formation and/or accumulation of the felsic components in the Archean continents,thus sheds new light on the early Earth’s differentiation and tectonic evolution.

The Orosirian(1800–2050 Ma)plate tectonic episode:Key for reconstructing the Proterozoic tectonic record

Eight lines of evidence indicate that the Orosirian Period in mid-Paleoproterozoic time was characterized by plate tectonics:ophiolites,low T/P metamorphism including eclogites,passive margin formation,tall mountains,paleomagnetic constraints,ore deposits,abundant S-type granites,and seismic images of paleo-subduction zones.This plate tectonic episode occurred about 1 billion years earlier than the present plate tectonic episode began in Neoproterozoic time.The two plate tectonic episodes bracket the‘Boring Billion’,which may have been a protracted single lid tectonic episode that began when the supercontinent Nuna or Columbia formed.Recognition of multiple lines of evidence for Orosirian plate tectonics demonstrates that Earth’s tectonic style can be reconstructed with some confidence back to at least Early Paleoproterozoic time,and thus the absence of compelling evidence for Mesoproterozoic plate tectonics is not obvious due to poor preservation.A tectono-magmatic lull2.3 Ga suggests an earlier episode of single lid tectonics.Evidence for two episodes of plate tectonics and two episodes of single lid tectonics indicates that Earth switched between single lid and plate tectonics multiple times during the last 2.4 Ga.