Taras Gerya, Institute of Geophysics, ETH-Zurich, Switzerland
Busy person’s summary: An egg does not look like a chicken. Therefore, even if we look at the chicken from 1000 different perspectives it will not help us to understand what the egg looked like. Modern plate tectonic Earth is like the chicken, whereas pre-plate tectonic Earth is like the egg. It will not be easy to visualize the pre-plate tectonic Earth.
No successful global physics-based geodynamic paradigm yet exists for Earth before plate tectonics due to the scarceness and fundamental unavailability of necessary data (e.g. global picture of the unperturbed Archean Earth interior and surface). Finding modern physical extraterrestrial/extrasolar planetary analogues and developing realistic numerical models of pre-plate tectonic Earth are thus critical for further progress. This blog is aimed to provoke thinking and discussion on that subject. Based on recent geological-geochemical literature and numerical experiments it must be that a distinct Venus-like plume-lid tectonics regime operated on Earth before plate tectonics, which was associated with widespread tectono-magmatic heat and mass exchange between crust and mantle. This regime was characterized by weak internally deforming highly heterogeneous lithosphere, massive juvenile crust production from mantle derived melts, mantle-flows-driven crustal deformation, episodic magma-assisted crustal overturns and widespread development of lithospheric delamination and eclogitic drips. Both proto-continental and proto- oceanic domains formed in this regime by a combination of eclogitic drips and ultra-slow proto-oceanic spreading without deep subduction. Subduction was episodic and short-lived and could have led to rapid global resurfacing of proto-oceanic domains associated with disaggregation and reworking of proto-continental regions. It therefore makes sense to use Venus as the “visible” modern analogue for Archean Earth and combining observations from the two planets underpinned by numerical modeling is a fruitful foundation for reconstructing Earth’s missing global pre-plate tectonic paradigm.
Complete Blog: This the second of four blogs written in advance of a meeting at the ETH conference facility in Ascona, Switzerland this July to discuss four related questions about the evolution of the solid Earth: “When did Plate Tectonics begin on Earth?”, “How did Plate Tectonics begin?”, “What was Earth’s tectonic style before Plate Tectonics began?, and “Why is it important to understand when and how Plate Tectonics began on Earth, and What came before?” In this blog, the focus is on the third question: “What was Earth’s tectonic style before Plate Tectonics began?” The March blog post “When did Plate Tectonics begin on Earth?”, can be found here wp.me/pNy1x-uv.
Pre-plate tectonic Earth is a challenge. An egg does not look like a chicken. Therefore, even if we look at the chicken from 1000 different perspectives it will not help us to understand what the egg looked like. Modern plate tectonic Earth is like the chicken, whereas pre-plate tectonic Earth is like the egg. It will not be easy to visualize the pre-plate tectonic Earth. It is not, therefore, surprising that understanding Earth geodynamics before plate tectonics is important and currently represents a fundamental barrier in furthering our understanding of how the Earth evolved through time. The lack of consensus regarding this geodynamics and the continuing controversy primarily can be associated to the scarcity and fundamental unavailability of data (Fig. 1).
Here, we aim at understanding the evolution of the Earth’s interior and surface over time. A time–depth diagram (Fig. 1) covering the entire Earth’s history and interior schematically represents this evolution. For a systematic characterization of geodynamic relationships, the entire diagram should be “covered” by data points characterizing the physical–chemical state of the Earth at all depths (ranging from 0 to 6000 km), for entire geological time (ranging from 0 to around 4.5 billion years ago). However, the unfortunate fact is that observations for such a systematic coverage are only available along two axes: (A) direct observations for the modern Earth surface structure (B) the geophysical snapshot for the present-day Earth’s interior and (C) the geological record preserved in rocks formed close (typically within few tens of kilometers) to the Earth’s surface. The rest of the diagram is fundamentally devoid of observational data. Not surprisingly, therefore, the topic of early Earth geodynamics remains controversial but fruitful.
The special challenge here is that we do not have a successful physics-based paradigm of global geodynamics and lithosphere tectonics for the early Earth (comparable to modern plate tectonics), which we can validate and elaborate using continually expanding set of observational and analytical data (Benn et al, 2006; Gerya, 2016). One obvious consequence is that in the absence of the paradigm for pre-plate tectonic Earth we will always tend to “overstretch” the successful paradigm of modern plate tectonics back in time. To improve this situation and calibrate our intuition better, numerical modeling should naturally play an ever increasingly important role in developing and testing geodynamic hypotheses based on robust observations aimed at explaining the evolution of the early Earth. Here, we should also recognize critical significance of global Earth’s surface observations for the development of modern plate tectonics paradigm. We can speculate, therefore, that finding and investigating modern extraterrestrial (or even extrasolar) planetary analogue(s) for and developing realistic numerical models of pre-plate tectonic Earth are two critical components for future progress.
Venus as the key to pre-plate tectonic Earth. A number of recent studies pointed out Venus as an observable analogue for the Archean Earth (e.g. Van Kranendonk, 2010; Harris and Bédard, 2014, 2015; Gerya et al., 2015). As Venus does not have plate tectonics, it should then obviously portrait the pre-plate tectonic Earth. By looking at the surface of Venus we can learn a number of surprising facts:
- Continental-like domains with thick crust and positive topography and oceanic-like regions with thin crust and negative topography can form without plate tectonics (Fig. 2). This is in strong contrast to modern Earth where oceanic lithosphere only form by oceanic spreading resulting from plate tectonics and compensating oceanic subduction.
- Significant lateral motions of continental-like domains (presumably) driven by mantle currents can be observed, leading to rifting, indentation and strike slip motions at their margins. On Earth continental motions are dominantly driven by subduction.
- Mantle plume-lithosphere interactions critically affected the evolution of Venus’ surface. Examples of plume-lithosphere interactions include 512 corona and 64 nova. These plume-related structures vary in size from few tens to >1000 km (Fig. 3). This again contrasts the modern Earth where amount of plume-related LIPs and hot spots is an order of magnitude less. The style of Venus lithospheric tectonic style can be therefore coined as plume-lid tectonics (Gerya et al., 2015).
- Presence of significant volumes of felsic rocks on Venus (Fig. 5) is indicated by various data (e.g., Fink et al., 1993; Harris and Bédard, 2015 and references therein) implying that felsic continental-like crust can form without subduction and plate tectonics. On modern Earth such crust predominantly forms in subduction related volcanic arcs.
- Last but not least, Venus was subjected to global episodic tectono-magmatic “resurfacing” events that obliterated previous surface record such as meteoritic cratering, tectonic structures etc. (Strom et al., 1994; Armann and Tackley, 2012). The exact mechanisms (massive volcanism caused by subduction? delamination? dripping?) of resurfacing are debatable but the process was presumably driven by the negative buoyancy of aging Venusian lithosphere and affected both continental-like and oceanic-like domains. In contrast, resurfacing of the modern Earth by volcanism accompanying plate tectonics is continuous, driven by subduction and predominantly involving oceanic domains.
What do numerical models tell? Geological-geochemical evidence point towards higher mantle potential temperature and massive juvenile magmatism in the early Earth (>3.2 Ga) compared to the present day global plate tectonics (e.g. review by Gerya, 2014 and references therein). From the perspective of numerical geodynamic modeling present day geodynamics, plate tectonics, are predominantly driven by subduction, whereas plume processes have subordinate significance. Thermomechanical models are therefore sufficient to understand modern geodynamics. In the early pre-plate tectonic Archean Earth, however, the increased mantle temperature led to massive heat and mass advection by mantle-derived magmas and widespread melt-induced weakening of the lithosphere (Lenardic et al., 2005; Gerya et al., 2015, and references therein). Archean geodynamics (plume-lid tectonics) follow an overall tectono-magmatic style and require more sophisticated magmatic-thermomechanical models (Johnson et al., 2014; Sizova et al., 2015; Fisher and Gerya, 2016).
Results of recent 2D and 3D numerical experiments show that plume-lid tectonic regime was characterized by (Sizova et al., 2015; Fisher and Gerya, 2016):
- weak internally deformable highly heterogeneous lithosphere,
- massive juvenile crust production from mantle derived melts,
- mantle-flows-driven crustal deformation,
- episodic magma-assisted crustal overturns,
- widespread development of lithospheric delamination and eclogitic drips,
- continental crust production and segregation
Both proto-continental and proto- oceanic domains were formed in this regime by a combination of eclogitic drips and ultra-slow proto-oceanic spreading without subduction (Fig.6). Subduction was episodic and short-lived and could have led to rapid global resurfacing of proto-oceanic domains associated with disaggregation and reworking of proto-continental regions (Fig. 6).
Two distinct phases in coupled crust-mantle evolution are identified (Fisher and Gerya, 2016), which compare well with some Archean granite-greenstone records (Hickman and Van Kranendonk, 2004): (1) A longer (80-100 Myr) and relatively quiet ‘growth phase’ which is marked by growth of crust and lithosphere, Followed by (2) a short (<20 Myr) and catastrophic ‘removal phase’ (Fig. 7), where unstable parts of the crust and mantle lithosphere are removed by eclogitic dripping and later delamination. Modeling thus suggests that the early Earth plume-lid tectonic regime followed a pattern of episodic growth and removal also called episodic overturns with a periodicity of ca. 100 Myr (Fisher and Gerya, 2016; Sizova et al., 2015).
The results of the numerical experiments are thus surprisingly similar to inferred characteristics of Venus surface dynamics, which makes this planet even more attractive analogue for the pre-plate tectonic Earth. It is worth considering that the huge pulses of crustal growth seen for Earth at 2.7 and 1.9 Ga (Fig. 4 of the previous blog post) are reflections of a Venus-like resurfacing event affecting pre-plate tectonic Earth.
Conclusions. A distinct Venus-like plume-lid tectonics regime must have operated on Earth before plate tectonics. Using Venus as the “visible” modern analogue for Archean Earth and combining observations from two planets based on numerical modeling is a fruitful way of developing the missing global pre-plate tectonic paradigm.
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