In contrast to modern-day plate tectonics, geodynamics of the early Earth presents a unique challenge, as currently there is no consensus on a global paradigm concerning the mantle dynamics and lithosphere tectonics in the Precambrian (Benn et al., 2006; Gerya, 2014). This challenge is mainly due to the severe objective restrictions of obtaining geological and/or geophysical observations constraining Earth’s surface and interior dynamics back in geological time (Fig. 1).
The subject of geodynamics can be schematically represented by the time-depth diagram (see Fig. 1) covering the entire Earth’s history and interior. In theory, the entire diagram should be “covered” by data points characterizing the physical-chemical state of Earth at different depths, for different moments in geological time. However, in practice, observations are only available along two axes: (1) geophysical data for Earth’s internal structure at all ranges of depths, but only for the very short present-day time, and (2) the geological record preserved in rocks formed over a broad range of geological times, but only at a very shallow depth range. As a result, the importance of well-constrained geological and geophysical data, and thoroughly studied present-day geodynamic regime (modern-style plate tectonics) is almost unavoidably exaggerated and “stretched” toward the Precambrian Earth. This “plate tectonics trap” can only be avoided by further calibrating our geological intuition on the basis of numerical geodynamic modeling that integrates available geological, geochemical, petrological, and geochronological records (Gerya, 2014).
Hadean-Archean squishy-lid tectonics before ca. 3 Ga was characterized by mantle temperatures ∼200–250 °C higher than present-day values, and was dominated by widespread plume-induced processes under conditions of an internally deformable (squishy) Venus-like global lid (e.g., Van Kranendonk, 2010; Gerya et al., 2015; Harris and Bédard, 2014; Rozel et al., 2017). In this pre–plate tectonics regime, both proto-oceanic and proto-continental lithospheres were formed by tectonomagmatic differentiation processes (e.g., Sizova et al., 2015). These lithospheres were rheologically weak due to the high Moho temperature and melt percolation from hot, partially molten, sublithospheric mantle (Sizova et al., 2015). The lid evolution was driven by episodic regional-scale tectonomagmatic activity combining (Sizova et al., 2015; Fischer and Gerya, 2016a) (1) a longer (80–100 m.y.) and relatively quiet ‘growth phase’ which is marked by growth of crust and lithosphere, followed by (2) a short (∼20 m.y.) and catastrophic ‘removal phase’, where unstable parts of the crust and mantle lithosphere are removed by eclogitic dripping and delamination. Plume- and impact-induced retreating subduction and delamination contributed to the episodic regional-scale lid recycling (Gerya et al., 2015; O’Neill et al., 2017).
During Archean-Proterozoic transitional tectonics between 3 Ga and 1 Ga, squishy-lid tectonics gradually evolved toward a plate tectonics regime (e.g., Fischer and Gerya, 2016b; Chowdhury et al., 2017; Sobolev and Brown, 2019). Numerical models suggest that the transitional tectonics occurred at mantle temperatures ∼100–200 °C higher than present-day values, and was triggered by stabilization of rheologically strong plates (e.g., Sizova et al., 2010, 2014; Fischer and Gerya, 2016b). Plume-induced subduction was likely very common in the beginning, and triggered the onset of this transitional plate-tectonic–like regime (Gerya et al., 2015). Elements of squishy-lid (plume-lid) tectonics were also locally present (Fischer and Gerya, 2016b). Delamination of the mantle lithosphere in long-lived ultra-hot accretionary orogens controlled silicification and rising of the continents due to recycling of mafic lower crust (Perchuk et al., 2018; Chowdhury et al., 2017). The episodic (short-lived), rapidly retreating subduction was associated with massive decompression melting of the mantle resulting in formation of oceanic plateau basalts (Perchuk et al., 2019).
The establishment of modern-style plate tectonics at ca. 1–0.5 Ga was attained by a combination of (Bercovici and Ricard, 2014; Gerya, 2014; Gerya et al., 2015; Sobolev and Brown, 2019) (1) further cooling of the mantle to temperatures ∼50–100 °C higher than present-day values, (2) emergence of a global mosaic of rigid plates divided by localized, long-lived, week boundaries, (3) rise of the continents, and (4) growing intensity of surface erosion, providing weak sediments that lubricated subduction trenches. Widespread development of modern-style (cold) continental collision started during the Neoproterozoic (Sizova et al., 2014) and created favorable conditions for the generation of ultrahigh-pressure metamorphic complexes. The transition to modern-style plate tectonics followed a long period of reduced tectonomagmatic activity—the boring billion (years) (Sobolev and Brown, 2019). The unprecedented scale of surface erosion following the snowball Earth glaciations possibly initiated the modern-style plate tectonics, and triggered the Cambrian explosion of life on Earth (e.g., Stern, 2016; Sobolev and Brown, 2019).
Further progress in deciphering Precambrian geodynamics clearly requires cross-disciplinary efforts, with a special emphasis placed upon numerical models that are thoroughly compared to the available limited observational record. The paper by Capitanio et al. (2019, p. 923 in this issue of Geology) follows this coupled modeling-observation–based approach by focusing on the thermal regimes of Hadean-Archean geodynamics that are recorded in oldest magmatic and metamorphic rocks.
Our understanding of the period of Earth’s earliest history (ca. 4.5–3.0 Ga) is strongly biased and relies on very limited observational data from respective preserved continental terrains (e.g., Kamber, 2015). Geothermobarometric 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 a bimodal distribution from Neoarchean time onward, <2.8 Ga (Brown and Johnson, 2018). Because this paired association of thermal gradients is a distinct characteristic of modern-style plate tectonics, it has been suggested that its appearance marks the establishment of plate-tectonics–like behavior of the lithosphere since the Neorchean (Brown and Johnson, 2018). Capitanio et al. challenge this interpretation by using numerical models of mantle convection and melting performed under Archean mantle temperature conditions. Based on these relatively simple models, they were able to show that different tectonic modes can coexist and alternate (both spatially and temporally) within a single, global, non–plate tectonics regime of mantle circulation. The authors document the development of two tectonomagmatic domains: (1) the vertical tectonics domain in which lithospheric generation and recycling occurs at sites of localized vertical drips, and (2) the horizontal tectonics domain, in which the coupling of mantle and stiffened lithospheric proto-plates results in large-scale lateral motion, and the proto-plates’ recycling back into the mantle along inclined planes.
The horizontal tectonics domain replicates several key features of plate tectonics, including stable divergent and convergent zones, long-lived environments for calc-alkaline magmatic activity inboard of zones of dipping lithospheric recycling, and paired metamorphic belts with contrasting pressure-temperature (P-T) gradients. On the other hand, this domain differs significantly from modern-style plate tectonics in that it does not form strongly localized plate margins and a globally linked plate-mosaic system (cf., Bercovici and Ricard, 2014; Cawood et al., 2018). The P-T gradients obtained by Capitanio et al. compare well with the P-T estimates made for tonalite-trondhjemite-granodiorite (TTG) series rocks and the paired metamorphic belt record, thereby supporting the feasibility of their formation within a mobilized, yet laterally continuous (i.e., non–plate tectonic), lithospheric lid. Comparisons of the numerical modeling results with the crustal production and reworking record led Capitanio et al. to conclude that the suggested mobilized lid regime had emerged in the Hadean. This regime also bears similarities with the squishy-lid regime predicted by more-complex tectonomagmatic numerical models (Sizova et al., 2015; Fischer and Gerya, 2016a; Rozel et al., 2017).
The relatively simple models of Capitanio et al. replicate fundamental P-T regimes preserved in the Archean rock record through non–plate tectonic 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 Precambrian geodynamics paradigm without falling into the “plate tectonics trap.”