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Geodynamics of the Early Earth: Quest for the Missing Paradigm RESEARCH FOCUS © 2019 Geological Society of America. For permission to copy, contact [email protected]. https://doi.org/10.1130/focus102019.1 Geodynamics of the early Earth: Quest for the missing paradigm Taras Gerya* Institute of Geophysics, Department of Earth Sciences, ETH Zurich, Sonneggstrasse 5, 8057 Zurich, Switzerland In contrast to modern-day plate tectonics, geodynamics of the early • During Archean-Proterozoic transitional tectonics between 3 Ga and Earth presents a unique challenge, as currently there is no consensus on a 1 Ga, squishy-lid tectonics gradually evolved toward a plate tectonics global paradigm concerning the mantle dynamics and lithosphere tecton- regime (e.g., Fischer and Gerya, 2016b; Chowdhury et al., 2017; Sobo- ics in the Precambrian (Benn et al., 2006; Gerya, 2014). This challenge lev and Brown, 2019). Numerical models suggest that the transitional is mainly due to the severe objective restrictions of obtaining geological tectonics occurred at mantle temperatures ∼100–200 °C higher than and/or geophysical observations constraining Earth’s surface and interior present-day values, and was triggered by stabilization of rheologically dynamics back in geological time (Fig. 1). strong plates (e.g., Sizova et al., 2010, 2014; Fischer and Gerya, 2016b). The subject of geodynamics can be schematically represented by the Plume-induced subduction was likely very common in the beginning, time-depth diagram (see Fig. 1) covering the entire Earth’s history and and triggered the onset of this transitional plate-tectonic–like regime interior. In theory, the entire diagram should be “covered” by data points (Gerya et al., 2015). Elements of squishy-lid (plume-lid) tectonics were characterizing the physical-chemical state of Earth at different depths, for also locally present (Fischer and Gerya, 2016b). Delamination of the different moments in geological time. However, in practice, observations mantle lithosphere in long-lived ultra-hot accretionary orogens con- are only available along two axes: (1) geophysical data for Earth’s internal trolled silicification and rising of the continents due to recycling of structure at all ranges of depths, but only for the very short present-day mafic lower crust (Perchuk et al., 2018; Chowdhury et al., 2017). The time, and (2) the geological record preserved in rocks formed over a broad episodic (short-lived), rapidly retreating subduction was associated with range of geological times, but only at a very shallow depth range. As a massive decompression melting of the mantle resulting in formation of result, the importance of well-constrained geological and geophysical data, oceanic plateau basalts (Perchuk et al., 2019). and thoroughly studied present-day geodynamic regime (modern-style • The establishment of modern-style plate tectonics at ca. 1–0.5 Ga was plate tectonics) is almost unavoidably exaggerated and “stretched” toward attained by a combination of (Bercovici and Ricard, 2014; Gerya, 2014; the Precambrian Earth. This “plate tectonics trap” can only be avoided Gerya et al., 2015; Sobolev and Brown, 2019) (1) further cooling of by further calibrating our geological intuition on the basis of numerical the mantle to temperatures ∼50–100 °C higher than present-day values, geodynamic modeling that integrates available geological, geochemical, (2) emergence of a global mosaic of rigid plates divided by localized, petrological, and geochronological records (Gerya, 2014). long-lived, week boundaries, (3) rise of the continents, and (4) growing An emerging holistic view of the evolution of early Earth geodynam- intensity of surface erosion, providing weak sediments that lubricated ics (e.g., Gerya, 2014; Sobolev and Brown, 2019) can be summarized as subduction trenches. Widespread development of modern-style (cold) follows (Fig. 1): continental collision started during the Neoproterozoic (Sizova et al., 2014) and created favorable conditions for the generation of ultrahigh- • Hadean-Archean squishy-lid tectonics before ca. 3 Ga was characterized pressure metamorphic complexes. The transition to modern-style plate by mantle temperatures ∼200–250 °C higher than present-day values, tectonics followed a long period of reduced tectonomagmatic activity— and was dominated by widespread plume-induced processes under con- the boring billion (years) (Sobolev and Brown, 2019). The unprece- ditions of an internally deformable (squishy) Venus-like global lid (e.g., dented scale of surface erosion following the snowball Earth glaciations Van Kranendonk, 2010; Gerya et al., 2015; Harris and Bédard, 2014; possibly initiated the modern-style plate tectonics, and triggered the Rozel et al., 2017). In this pre–plate tectonics regime, both proto-oceanic Cambrian explosion of life on Earth (e.g., Stern, 2016; Sobolev and and proto-continental lithospheres were formed by tectonomagmatic Brown, 2019). differentiation processes (e.g., Sizova et al., 2015). These lithospheres were rheologically weak due to the high Moho temperature and melt Further progress in deciphering Precambrian geodynamics clearly percolation from hot, partially molten, sublithospheric mantle (Sizova requires cross-disciplinary efforts, with a special emphasis placed upon et al., 2015). The lid evolution was driven by episodic regional-scale numerical models that are thoroughly compared to the available lim- tectonomagmatic activity combining (Sizova et al., 2015; Fischer and ited observational record. The paper by Capitanio et al. (2019, p. 923 in Gerya, 2016a) (1) a longer (80–100 m.y.) and relatively quiet ‘growth this issue of Geology) follows this coupled modeling-observation–based phase’ which is marked by growth of crust and lithosphere, followed by approach by focusing on the thermal regimes of Hadean-Archean geo- (2) a short (∼20 m.y.) and catastrophic ‘removal phase’, where unstable dynamics that are recorded in oldest magmatic and metamorphic rocks. parts of the crust and mantle lithosphere are removed by eclogitic drip- Our understanding of the period of Earth’s earliest history (ca. 4.5–3.0 Ga) ping and delamination. Plume- and impact-induced retreating subduction is strongly biased and relies on very limited observational data from respec- and delamination contributed to the episodic regional-scale lid recycling tive preserved continental terrains (e.g., Kamber, 2015). Geothermobaromet- (Gerya et al., 2015; O’Neill et al., 2017). ric estimates for mineral assemblages formed in these terranes recognized widespread variations in metamorphic gradients (from cold to hot), as far back as ca. 3.7 Ga. These gradients become pronounced and grouped into *E-mail: [email protected] CITATION: Gerya, T., 2019, Geodynamics of the early Earth: Quest for the missing paradigm: Geology, v. 47, p. 1006–1007, https://doi.org/10.1130/focus102019.1 1006 www.gsapubs.org | Volume 47 | Number 10 | GEOLOGY | Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/10/1006/4830257/1006.pdf by guest on 25 September 2021 The relatively simple models of Capitanio et al. replicate fundamental P-T regimes preserved in the Archean rock record through non–plate tec- tonic processes. This result clearly warns against exaggeration of the roles of modern-style subduction and plate tectonics for early Earth geodynamics, and calls for further thorough development of the self-consistent Precam- brian geodynamics paradigm without falling into the “plate tectonics trap.” REFERENCES CITED Benn, K., Mareschal, J.-C., and Condie, K.C., 2006, Archean Geodynamics and Environments: American Geophysical Union Geophysical Monograph Series, v. 64, 320 p., https://doi .org/10.1029/GM164. Bercovici, D., and Ricard, Y., 2014, Plate tectonics, damage and inheritance: Nature, v. 508, p. 513–516, https://doi .org/10.1038/nature13072. Brown, M., and Johnson, T.E., 2018, Secular change in metamorphism and the onset of global plate tectonics: The American Mineralogist, v. 103, p. 181–196, https://doi .org/10.2138/am-2018-6166. Capitanio, F.A., Nebel, O., Cawood, P.A., Weinberg, R.F., and Chowdhury, P., 2019, Reconciling thermal regimes and tectonics of the early Earth: Geology, v. 47, p. 923–927, https://doi .org/10.1130/G46239.1. Cawood, P.A., Hawkesworth, C.J., Pisarevsky, S.A., Dhuime, B., Capitanio, F.A., and Nebel, O., 2018, Geological archive of the onset of plate tecton- ics: Philosophical Transactions of the Royal Society: A, v. 376, p. 20170405, https://doi .org/10.1098/rsta.2017.0405. Chowdhury, P., Gerya, T., and Chakraborty, S., 2017, Emergence of silicic con- tinents as the lower crust peels off on a hot plate-tectonic Earth: Nature Geoscience, v. 10, p. 698–703, https://doi .org/10.1038/ngeo3010. Fischer, R., and Gerya, T., 2016a, Early Earth plume-lid tectonics: A high- resolution 3D numerical modelling approach: Journal of Geodynamics, v. 100, p. 198–214, https://doi .org/10.1016/j .jog.2016.03.004. Figure 1. The evolution of terrestrial tectonic styles (top), and time- Fischer, R., and Gerya, T., 2016b, Regimes of subduction and lithospheric dynamics depth diagram for the availability of data for constraining geodynamic in the Precambrian: 3D thermomechanical modeling: Gondwana Research, relationships for Earth (bottom). Modified from Gerya (2014) using v. 37, p. 53–70, https://doi .org/10.1016/j .gr.2016.06.002. numerical models by Fischer and Gerya (2016a, 2016b). Gerya, T.V., 2014, Precambrian geodynamics: Concepts and models: Gondwana Research, v. 25, p. 442–463, https://doi .org/10.1016/j .gr.2012.11.008. Gerya, T.V., Stern, R.J., Baes, M., Sobolev, S., and Whattam, S.A., 2015, Plate tec- a bimodal distribution from Neoarchean time onward, <2.8 Ga (Brown tonics on the Earth triggered by plume-induced subduction initiation: Nature, and Johnson, 2018). Because this paired association of thermal gradients v. 527, p. 221–225, https://doi .org/10.1038/nature15752. is a distinct characteristic of modern-style plate tectonics, it has been sug- Harris, L.B., and Bédard, J.H., 2014. Crustal evolution and deformation in a non- gested that its appearance marks the establishment of plate-tectonics–like plate-tectonic Archaean Earth: Comparisons with Venus, in Dilek, Y., and Furnes, H., eds.
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