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User:Mikenorton/Tectonics of the Early Earth

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The tectonics of the Early Earth, before the establishment of plate tectonics, are poorly constrained due to the limited outcrop extent of rocks of Archean age. Several different geodynamic mechanisms have been proposed, working either independently or in combination. The main control on the likely tectonics is the temperature of the mantle, which is expected to have been 200–250°C hotter than current values during the Archean.[1]

Forms of evidence

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At least six types of evidence have been used to determine the type of tectonics that operated during the early stages of Earth's history.[2]

Rock record

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The types of rocks preserved in crust of Archaean to Proterozoic age provide the main constraint on the tectonics that operated at the time of their formation and subsequent deformation and metamorphism. The presence of blueschists for instance are held as unequivocal indicators of active subduction similar to plate tectonics as it operates now. Their disappearance from the rock record in crust older than the Neoproterozoic suggests that subduction (and therefore plate tectonics) did not occur earlier than that. However, a hotter mantle and therefore higher geothermal gradients in the early Earth may mean that these high temperature - low pressure assemblages were not stable before the Neoproterozoic.[3][2]

Detrital zircons

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Trace element and isotope geochemistry

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Atmosphere-crust-mantle exchange

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Paleomagnetic data

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Paleomagnetic analysis has the capability of providing strong evidence of relative movement between different crustal blocks over time, although it can only show differences of paleolatitude as changes in longitude cannot be ruled out using this technique. For the oldest periods the method is also constrained by the limited number of samples available from rocks that still retain their original magnetisation.[3]

Numerical modelling

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Hypotheses for the tectonic activity in the early Earth have been tested using thermomechanical numerical modelling. Although the results of such modelling provide some constraints on the possible mechanisms, they necessarily rely on assumptions regarding the starting point that are mostly unconstrained by data.[2]

Archean crust

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Crust where the dominant formation, deformation and metamorphism are all of Archean age represents only 8% of continental crust, suggesting that most of the crust of this age has been recycled.[4] This means that the surviving Archean crust may not be at all representative, being only the parts most resistant to reworking.[2]

Several rock types are restricted to Archean crust or are unusually abundant there, including komatiites, lavas unusually rich in Magnesium that are almost unknown in younger rocks, banded iron formations (BIFs) found in Eoarchean rocks, with the main development in Neoarchean rocks, and tonalite–trondhjemite–granodiorite associations (TTGs), representing most of the plutonic granitic igneous rocks found in Archean crust. These rock types have been the subject of intensive study in attempts to understand the geological processes active during their formation.

Komatiites

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These high MgO lavas are restricted to the Archean and Paleoproterozoic. The water content of komatiites is hotly debated, with initial models for formation suggesting that they resulted from anhydrous melting of anomalously hot mantle, presumed to be associated with mantle plumes.[5] Significant water content has been measured for several examples, although probably less than 1% wt, which still suggests anomalously high melting temperatures and that they are plume related. The origin and size of the proposed plumes are also debated with smaller plumes originating from the mantle transition zone (MTZ) at the base of the upper mantle being proposed,[6] rather than the larger plumes originating at the core-mantle boundary (CMB), as is generally thought to be the case for more recent plumes. An origin at the MTZ would explain the presence of a significant water content. Alternatively, it has been suggested that plumes from the CMB picked up water as they passed through the MTZ. The presence of a continuous relatively water-rich layer at the MTZ would argue against whole-mantle convection at this stage in Earth's history.

BIFs

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TTGs

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Tectonic mechanisms

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Heat-pipe tectonics

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Main article: Heat-pipe tectonics

This mechanism envisages an early earth in which melt rises through an early lithosphere in relatively narrow pipe-like structures, forming thick lava flows on the surface. Each stage of eruption would be accompanied by downward advection (subsidence) of earlier flows.[7]

Lid tectonics

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Main article: Lid tectonics

The early history of the Earth and the current state of many of the rocky planets is described in terms of lid tectonics. The "lid" is a strong outermost layer, the equivalent of the lithosphere. This may be "stagnant" where the strength of the lid is sufficient to prevent it becoming involved in the underlying convecting mantle. Plate tectonics can be regarded as a form of "mobile" lid tectonics. The effect of a single continuous stagnant lid would be to reduce the amount of heat escaping from the Earth and numerical modelling suggests that this would result in episodic mantle-overturn events that would substantially rework the lid.[8]

Plate tectonics

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The timing of the onset of plate tectonics in its current form remains uncertain, with some workers favouring a period in the middle of the Archean, at about 3.2 Ga,[9] while others suggest that it occurred between about 1.0 to 0.5 Ga.[1] This later age is supported by the analysis of apparent polar wander paths that suggests the change to full plate tectonics occurred shortly after 0.6 Ga. The same analysis does, however, also support periods of mobile tectonics affecting just the periphery of a single supercontinent from the late Archean up to the Neoproterozoic.[10]

The possibility that some early form of plate tectonics (mobile lid) may have operated in the Eoarchean or even earlier has been proposed based on field relationships within crust of this age in west Greenland, the Itsaq Gneiss Complex, and that these rocks lack any evidence of the proposed alternatives of stagnant lid or pipe tectonics.[11]

[12]

[13]

References

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  1. ^ a b Gerya, T. (2019). "Geodynamics of the early Earth: Quest for the missing paradigm" (PDF). Geology. 47 (10): 1006–1007. doi:10.1130/focus102019.1.
  2. ^ a b c d Harrison, T.M. (2024). "We don't know when plate tectonics began". Journal of the Geological Society. 181 (4). doi:10.1144/jgs2023-212.
  3. ^ a b Palin, R.M.; Santosh, M. (2021). "Plate tectonics: What, where, why, and when?". Gondwana Research. 100: 3–24. doi:10.1016/j.gr.2020.11.001.
  4. ^ Korenaga, J (2021). "Was There Land on the Early Earth?". Life. 11 (11): 1142. Bibcode:2021Life...11.1142K. doi:10.3390/life11111142. PMC 8623345. PMID 34833018.
  5. ^ Grosch, E.G.; Wilson, A. (2023). "The discovery and petrogenetic significance of komatiites". Journal of African Earth Sciences. 205. doi:10.1016/j.jafrearsci.2023.105002.
  6. ^ Wyman, D. (2020). "Komatiites From Mantle Transition Zone Plumes". Frontiers in Earth Science. 8. doi:10.3389/feart.2020.540744.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  7. ^ Moore, William B.; Webb, A. Alexander G. (2013). "Heat-pipe Earth". Nature. 501 (7468): 501–505. Bibcode:2013Natur.501..501M. doi:10.1038/nature12473. ISSN 0028-0836. PMID 24067709.
  8. ^ O'Neil C.; Roberts N.M.W. (2018). "Lid tectonics – Preface". Geoscience Frontiers. 9 (1): 1–2. doi:10.1016/j.gsf.2017.10.004.
  9. ^ Pease, V.; Percival, J.; Smithies, H.; Stevens, G.; van Kranendonk (2008). "When did plate tectonics begin? Evidence from the orogenic record". In Condie, K.C.; Pease, V. (eds.). When Did Plate Tectonics Begin on Planet Earth?. Special Paper. Vol. 440. Geological Society of America. doi:10.1130/2008.2440(10). ISBN 9780813724409.
  10. ^ Piper, J.D.A. (2018). "Dominant Lid Tectonics behaviour of continental lithosphere in Precambrian times: Palaeomagnetism confirms prolonged quasi-integrity and absence of supercontinent cycles". Geoscience Frontiers. 9: 61–89. doi:10.1016/j.gsf.2017.07.009.
  11. ^ Nutman, A.P.; Bennett, V.C.; Friend, C.R.L.; Polat, A.; Hoffmann, E.; Van Kranendonk, M. (2021). "Fifty years of the Eoarchean and the case for evolving uniformitarianism". Precambrian Reearch. 367. doi:10.1016/j.precamres.2021.106442.
  12. ^ Hawkesworth, C.A.; Cawood, P.A.; Dhuime, B.; Kemp, T. (2024). "Tectonic processes and the evolution of the continental crust". Journal of the Geological Society. 181 (4). doi:10.1144/jgs2024-027.
  13. ^ Bédard, J. (2024). "A gradual Proterozoic transition from an unstable stagnant lid to the modern plate tectonic system". Journal of the Geological Society. 181 (4). doi:10.1144/jgs2024-023.
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