The inception of plate tectonics: a record of failure

Abstract

The development of plate tectonics from a pre-plate tectonics regime requires both the initiation of subduction and the development of nascent subduction zones into long-lived contiguous features. Subduction itself has been shown to be sensitive to system parameters such as thermal state and the specific rheology. While generally it has been shown that cold-interior high-Rayleigh-number convection (such as on the Earth today) favours plates and subduction, due to the ability of the interior stresses to couple with the lid, a given system may or may not have plate tectonics depending on its initial conditions. This has led to the idea that there is a strong history dependence to tectonic evolution-and the details of tectonic transitions, including whether they even occur, may depend on the early history of a planet. However, intrinsic convective stresses are not the only dynamic drivers of early planetary evolution. Early planetary geological evolution is dominated by volcanic processes and impacting. These have rarely been considered in thermal evolution models. Recent models exploring the details of plate tectonic initiation have explored the effect of strong thermal plumes or large impacts on surface tectonism, and found that these 'primary drivers' can initiate subduction, and, in some cases, over-ride the initial state of the planet. The corollary of this, of course, is that, in the absence of such ongoing drivers, existing or incipient subduction systems under early Earth conditions might fail. The only detailed planetary record we have of this development comes from Earth, and is restricted by the limited geological record of its earliest history. Many recent estimates have suggested an origin of plate tectonics at approximately 3.0 Ga, inferring a monotonically increasing transition from pre-plates, through subduction initiation, to continuous subduction and a modern plate tectonic regime around that time. However, both numerical modelling and the geological record itself suggest a strong nonlinearity in the dynamics of the transition, and it has been noted that the early history of Archaean greenstone belts and trondhjemite-tonalite-granodiorite record many instances of failed subduction. Here, we explore the history of subduction failure on the early Earth, and couple these with insights from numerical models of the geodynamic regime at the time.This article is part of a discussion meeting issue 'Earth dynamics and the development of plate tectonics'.

Keywords: Archaean geodynamics; mantle convection.

Figure 1.

(a) Static force balance on an incipient subduction zone. RP, ridge push; CR, collision resistance; DF, mantle drag force; SP, slab pull; SR, slab resistance; SU, slab suction. (b) Development of incipient subduction in a numerical simulation, in response to a mantle upwelling. Colours represent log of mantle viscosity (in Pa s). Here, mantle stresses (grouped DF, SP, SU and SR forces) generate thickening in the adjacent lithosphere and eventually downwelling. (Online version in colour.)

Figure 2.

Development of a transient subduction zone in an early Earth model (after [50]). Here, downwellings promote over-thickening of the lithosphere, which eventually yields (b) and develops into a short-lived subduction zone (c,d), characterized by frequent necking and slab break-off. (Online version in colour.)

Figure 3.

Development of incipient subduction in response to the arrival of a thermal plume at the base of the lithosphere. The thermal buoyancy of the plume drives lateral spreading at the surface (26.25 Myr), and the development of localized subduction at the edges (29.5 Myr). Also shown is a compositional field equating to ‘MORB’ concentration (purple is depleted, harzburgitic mantle; yellow equates to more MORB-like compositions). The eclogitization of dense basaltic drips facilitates subduction in over-thickened regions.

Figure 4.

Development of subduction in response to the thermal effect of a giant (1000 km diameter) impactor (after [50]). (Online version in colour.)

Figure 5.

APW velocities for the Pilbara (a) and Kaapvaal (b) cratons, as well as a combined path (c). APW estimates (blue diamonds) and uncertainties are calculated using a Monte Carlo approach (see text), incorporating both spatial uncertainties in the poles and age uncertainties. We have also used a propagating one-dimensional Kalman filter on the data (magenta line, uncertainties shaded magenta). The Kalman filter implements a motion model (here velocity depends on previous velocities—see text), which both reduces variance when it encounters noisy observations, and facilitates the propagation of uncertainties into regions of sparse data sampling (shown as increasing uncertainty regions between APWV measurements). The plots show estimates of APWV far above background levels at certain periods of time (e.g. 3.5, 3.2, 2.7–2.8 and 2.1 Ga). In between these peaks, it is difficult to ascertain any clear APW motion above uncertainty levels (particularly prior to 3.0 Ga). Compiled poles/data and scripts are available from online repositories ([78]: https://doi.org/10.5281/zenodo.1310984). (Online version in colour.)

Figure 6.

Plot of SiO₂ (wt%) versus age using the database (approx. 70 000 analyses) of Keller & Schoene [94]. Grey dots are individual analyses with a running mean shown by the black line.

Figure 7.

Classification of amphibolites from the oldest proposed boninites, from Nuvvuagittuq (reproduced from [93]). Panels (a)–(e) demonstrate the importance of chemically screening the altered rocks using the methodology detailed in Pearce & Reagan [93]. Panels (f) and (g) use carefully filtered samples from the Lower, Middle and Upper Nuvvuagittuq Units to confirm basaltic affinities for the Lower Unit and boninite affinities for the Middle Unit, and indicate SHMB affinities for the Upper Unit. The trend from basalts to LSB to SHMB is consistent with the concept of Archaean subduction initiation [33]. BA, basaltic andesite; A, andesite; D, dacite; LSB, low-Si boninite; HSB, high-Si boninite; HMA, high-Mg andesite; SHMB, siliceous high-Mg basalts; LOTI, low-Ti basalts. (Online version in colour.)

Figure 8.

Th/Yb versus Nb/Yb diagram (after [9]) for calculated melts in equilibrium with 3.3–4.2 Ga Jack Hills zircons (data from [101]). Black circles used trace-element partition coefficients from Burnham & Berry [102]; grey circles used partition coefficients from Nardi et al. [103]. Thus, the observation is not dependent on whose partition coefficients are used. (Online version in colour.)

Figure 9.

Major element data for natural and experimental TTG compositions: (a) CaO versus Na₂O and (b) MgO versus SiO₂. Open symbols are Archaean TTGs. Closed circles are experimental partial melts of basaltic starting material, from 11 different studies, from 17 different starting materials and over 126 individual experiments (data from our compiled database). The experimental melts form a much wider distribution than the natural TTGs, showing that there are only specific compositions that produce TTG-like melts. Natural data are from Moyen & Martin [112], Nutman et al. [113], Huang et al. [114], Laurent et al. [115] and O'Neil & Carlson [116]. Natural compositional data are filtered after Moyen & Martin [112].

Figure 10.

Timeline of major observational constraints on subduction initiation and plate tectonic proxies discussed in the text. Included (upwards from bottom) are an example of the proposed onset of plate tectonics [30], evidence of meteoritic bombardment (see [50]), earliest subduction stratigraphy [33], Archaean Nb/Th from the Pilbara and Yilgarn ([19]; black indicates no subduction involvement, orange indicates subduction involvement, red indicates possible subduction initiation), palaeomagnetics (see text; red indicates episodic APW velocities, blue indicates possible plate tectonic speeds), the Magmatic gap identified by Condie et al. [84], the ‘Boring billion’ [121], and evidence for UHP metamorphism since the Neoproterozoic [14]. (Online version in colour.)