GD
Geodynamics

outstanding questions

Enigmas at depth

Enigmas at depth
Dr. Marcel Thielmann

Dr. Marcel Thielmann.

The Geodynamics 101 series serves to showcase the diversity of research topics and/or methods in the geodynamics community in an understandable manner. In this week’s Geodynamics 101 post, Marcel Thielmann, Senior Researcher at the University of Bayreuth, discusses the possible mechanisms behind the ductile deformation at great depths that causes deep earthquakes. 

Earthquakes are one of the expressions of plate tectonics that everybody seems to be familiar with. When I started studying geophysics, people used to ask me what exactly I was studying. As soon as I mentioned earthquakes, I usually got a knowing nod and no further questions were asked (the same goes for volcanoes, but that’s a topic for another day).

Global hypocenter distribution over earthquakes with a magnitude of 5 or larger.

Figure 1: Global hypocentre distribution of earthquakes with a magnitude Mw>5 in the ISC catalogue for the interval 1960-2013. The x-axis has been truncated for better visibility.

 

Most earthquakes occur at the boundaries of tectonic plates, where rock breaks due to the forces originating from the plates’ relative movement. In 1928, Kiyoo Wadati discovered earthquakes that occurred at depths larger than 60 km, which were previously thought to be impossible. Today, we know that these earthquakes are not that extraordinary: about one out of four earthquakes observed on Earth occurs at depths larger than 60 km. At this depth, the pressure inside the Earth reaches values of about 3 GPa and more. Laboratory experiments have shown that at this pressure, rocks do not deform by breaking, but rather by ductile creep, like putty. This kind of deformation should not produce any earthquakes. So, 90 years after their discovery, the question still remains: What causes deep earthquakes?

How do rocks fail at these high pressures?

Proposed ductile failure mechanisms

Figure 2: Schematic view of the three proposed ductile failure mechanisms.

As rocks get transported to larger depths, the minerals making them up can experience phase transformations. Due to these transformations, two things may happen: (1) Previously stored water in the minerals is released. This release may trigger earthquakes due to the released water acting against the pressure of the surrounding rock in a mechanism called dehydration embrittlement (Green and Houston, 1995; Frohlich, 2006). (2) The phase transition renders a fine-grained rock that is easier to deform. If enough of this weak material is produced, rock failure occurs in a process called transformational faulting (Green and Houston,1995; Ferrand et al.,2017). Besides these two mechanisms, a third one called thermal runaway has been thrown into the ring (Hobbs et al., 1986; Ogawa, 1987). This mechanism is a result of shear heating, which describes the generation of heat inside a deforming rock. If heat generation is faster than its transport, temperatures inside the rock will continue to increase and ultimately result in its destabilization, thus causing an earthquake.

The Wind River earthquake

While most of the observed deep earthquakes occur in subduction zones, where one tectonic plate descends beneath another, there are some that occur far from them. One such earthquake hit the Wind River range in Wyoming with a magnitude of MW 4.7 in 2013 (Frohlich et al., 2015; Craig and Heyburn, 2015). This earthquake is not only enigmatic due to its depth of 75 km (making it the second deepest earthquake in such a stable continental region), but also because the Wind River area is considered to be “seismically quiet”. The location of the earthquake is far away from any plate boundary, with the closest tectonic feature being the Yellowstone supervolcano more than 200 km away. Since it occurred, the cause of this earthquake has been a matter of debate, with some scientists preferring a purely brittle origin (Craig and Heyburn, 2015), while others argue for a ductile mechanism (Prieto et al., 2017).

Dehydration embrittlement seems to be an unlikely candidate, since the earthquake is located far away from any subduction zone. How could fluids get down to those depths if not by subduction? Transformational faulting also seems to be unlikely, since this would require a phase transition to take place. The Wind River earthquake occurred in the continental mantle lithosphere, where we would not expect any major phase transitions. Thermal runaway may be a candidate, but studies have shown that very high stresses are required to make this mechanism work, stresses that are very hard to achieve in the Earth.

However, there may be a way out: grain size assisted thermal runaway. Oh no, yet another one you might think. But fear not, this mechanism is essentially the same as the “classical” thermal runaway, just with the effect of small grains included. The consequences of this effect are by no means small however, as it significantly reduces the stresses required for thermal runaway. Indeed, numerical models of this process at the conditions of the Wind River earthquake indicate that it may indeed be a viable mechanism to have generated this earthquake (Thielmann, 2018). However, these models also show that rock deformation has to be sufficiently fast (about 100 times faster than what is commonly assumed) in order to allow for earthquake generation.

Location and mantle structure of the 2013 Wind River earthquake.

Figure 3: Location and mantle structure of the 2013 Wind River earthquake. Inset: Location within the north-western US. Black points represent earthquakes larger than Mw 4.5 from the NEIC catalogue. The red circle indicates the location of the Wind River earthquake. The red box denotes the region of the main figure. Main Figure: Seismic velocity structure and hypocentre location. Tomographic data is taken from Shen et al. (2013). Colours denote seismic velocities, with blue colours indicating faster and red colours slower velocities. Fast seismic velocities are commonly associated with colder and denser material. The red spheres denote the location of the hypo- and epicentre. The grey isosurface at 4.4 km/s delineates the dense body extending to larger depths.

So now we have shifted the question from “How could fluids get down to those depths if not by subduction?” to “How could we deform that fast at those depths?” Here, seismology may come to the rescue: tomographic models of the north-western United States show that the Wind River earthquake lies directly at the transition between two regions with strongly varying seismic wave speeds (Shen et al., 2013; Wang et al., 2016). Fast wave speeds are commonly seen as an indicator for cold material, while slow wave speeds indicate warm material. 3D seismic tomographies such as the one from Shen et al. (2013) show that the 2013 Wind River earthquake occurred in a region where the continental lithosphere may be detaching in the form of a drip (Wang et al., 2016). In such tectonic environments, deformation rates may reach the values needed to initiate grain size assisted thermal runaway (Lorinczi and Houseman, 2009).

Does this now answer all questions we have on the Wind River earthquake and deep earthquakes in general? Certainly not. The example given above was just a single instance of where the combined information from seismology, laboratory experiments and numerical modelling may help us find an answer. We still have to keep in mind G.E.P. Box’s famous expression „Essentially, all models are wrong, but some are useful“. It is certain that deep earthquakes contain a wealth of information that remains to be unlocked. The following quote by Heidi Houston (2015) points the way:

Integration of seismological, laboratory, and modelling effort is needed to bridge the stubborn gap between source properties, which are extracted under strong assumptions and possess substantial intrinsic variability, and physical mechanisms of rupture generation, which are as yet neither well understood nor well constrained. (H. Houston)

 

References

Craig, T. J., and R. Heyburn (2015), An enigmatic earthquake in the continental mantle lithosphere of stable North America, Earth Plan. Sc. Lett., 425, 12–23, doi:10.1016/j.epsl.2015.05.048.

Ferrand, T., N. Hilairet, S. Incel, D. Deldicque, L. Labrousse, J. Gasc, J. Renner, Y. Wang, H. W. Green II, and A. Schubnel (2017), Dehydration-driven stress transfer triggers intermediate-depth earthquakes, Nat. Commun., 8, 15247, doi:10.1038/ncomms15247.

Frohlich, C. (2006), Deep Earthquakes, Cambridge University Press.

Frohlich, C., W. Gan, and R. B. Herrmann (2015), Two Deep Earthquakes in Wyoming, Seismological Research Letters, 86(3), 810–818, doi:10.1785/0220140197.

Green, H. W., and H. Houston (1995), The Mechanics of Deep Earthquakes, Annu. Rev. Earth. Planet. Sci., 23, 169–213.

Hobbs, B. E., A. Ord, and C. Teyssier (1986), Earthquakes in the Ductile Regime, Pure Appl. Geophys., 124(1-2), 309–336.

Houston, H. (2015), 4.13 - Deep Earthquakes, in Treatise on Geophysics (Second Edition), edited by G. Schubert, pp. 329–354, Elsevier, Oxford.

Lorinczi, P., and G. A. Houseman (2009), Lithospheric gravitational instability beneath the Southeast Carpathians, Tectonophysics, 474, 322–336, doi:10.1016/j.tecto.2008.05.024.

Ogawa, M. (1987), Shear instability in a viscoelastic material as the cause of deep focus earthquakes, J. Geophys. Res., 92, 13,801–13,810.

Prieto, G. A., B. Froment, C. Yu, P. Poli, and R. Abercrombie (2017), Earthquake rupture below the brittle-ductile transition in continental lithospheric mantle, Sci. Adv., 3(3), e1602642, doi:10.1126/sciadv.1602642.

Shen, W., M. H. Ritzwoller, and V. Schulte Pelkum (2013), A 3‐D model of the crust and uppermost mantle beneath the Central and Western US by joint inversion of receiver functions and surface wave dispersion, J. Geophys. Res., 118(1), 262–276, doi:10.1029/2012JB009602.

Thielmann, M. (2018), Grain size assisted thermal runaway as a nucleation mechanism for continental mantle earthquakes: Impact of complex rheologies, Tectonophysics, 746, 611–623, doi:10.1016/j.tecto.2017.08.038.

Wang, X., D. Zhao, and J. Li (2016), The 2013 Wyoming upper mantle earthquakes: Tomography and tectonic implications, J. Geophys. Res., 121(9), 6797–6808, doi:10.1002/2016JB013118.

What controlled the evolution of Plate Tectonics on Earth?

Great Unconformity - Immensity River, Grand Canyon
Stephan Sobolev

Prof. Dr. Stephan Sobolev. Head of the Geodynamic Modelling section of GFZ Potsdam.

Plate tectonics is a key geological process on Earth, shaping its surface, and making it unique among the planets in the Solar System. Yet, how plate tectonics emerged and which factors controlled its evolution remain controversial. The recently published paper in Nature by Sobolev and Brown suggests new ideas to solve this problem….

What makes plate tectonics possible on contemporary Earth?

It is widely accepted that plate tectonics is driven by mantle convection, but is the presence of said convection sufficient for plate tectonics? The answer is no, otherwise plate tectonics would be present on Mars and Venus and not only on Earth. The geodynamic community recognized that another necessary condition for plate tectonics is low strength at plate boundaries and particularly along the plate interfaces in subduction zones (e.g. Zhong and Gurnis 1992, Tackley 1998, Moresi and Solomatov 1998, and Bercovici 2003). To quantify the required strength at subduction interfaces, we have used global models of plate tectonics (Fig. 1A) that combine a finite element numerical technique employing visco-elasto-plastic rheology to model deformation in the upper 300 km of the Earth (Popov and Sobolev 2008) with a spectral code to model convection in the deeper mantle (Steinberger and Calderwood 2006). The model reproduces well present-day plate velocities if the effective friction at convergent plate boundaries is about 0.03 (Fig.1B). Low strength corresponds to subduction interfaces that are well lubricated by continental sediments (low friction; Lamb and Davis 2003, Sobolev and Babeyko 2005, or low viscosity; Behr and Becker 2018). In case of sediment shortages in the trenches (corresponding to a friction coefficient of 0.07-0.1), plate velocities would first decrease about two times (Fig. 1C) and then even more because of less negatively buoyant material having subducted into the mantle, leading to less convection driving force.

Effects of sediments on contemporary subduction according to global numerical models.

Figure 1. Global numerical model showing the effect of sediments on contemporary subduction. (A) The global model combines two computational domains coupled through continuity of velocities and tractions at 300 km depth. (B) NUVEL 1A plate velocities in a no-net-rotation reference frame (black arrows) versus computed velocities (blue arrows) for the global model with a friction of 0.03 at convergent plate boundaries. (C) Root mean square of computed plate velocities in the global model versus friction coefficient at convergent plate boundaries.

Hypothesis and its testing

Based on the previous discussion, we infer that continental sediments in subduction channels act as a lubricant for subduction. In addition, the presence of these sediments in trenches is a necessary condition for the stable operation of plate tectonics, particularly earlier in Earth’s evolution when the mantle was warmer and slabs were relatively weak. With this hypothesis we challenge the popular view that secular cooling of the Earth was the only major control on the evolution of plate tectonics on Earth since about 3 Ga. The hypothesis predicts that periods of stable plate tectonics should follow widespread surface erosion events, whereas times of diminished surface erosion should be associated with reduced subduction and possibly intermittent plate tectonics.

We test this prediction using geological proxies believed to identify plate tectonics activity (supercontinental cycles) and geochemical proxies that trace the influence of the continental crust on the composition of seawater (Sr isotopes in ocean sediments; Shields 2007) and continental sediments in the source of subduction-related magmas (oxygen and Hf isotopes in zircons; Cawood et al. 2013, Spencer et al. 2017). All three geochemical markers indeed show that just before or in the beginning of supercontinental cycles the influence of sediments is increasing, while it decreases before periods of diminished plate tectonic activity, like the boring billion period between 1.7 and 0.7 Ga (Cawood and Hawkesworth 2014; Fig. 2). The largest surface erosion and subduction lubrication events were likely associated with the global glaciation evens identified in the beginning (2.5-2.2 Ga) and at the end (0.7-0.6 Ga) of the Proterozoic Era (Hoffman and Schrag 2002). The latter snowball Earth glaciation event terminated the boring billion period and kick-started the modern phase of active plate tectonics.

Another prediction of our hypothesis is that in order to start plate tectonics, continents had to rise above sea level and provide sediments to the oceans. This prediction is again consistent with observations: there are many arguments for the beginning of plate tectonics between 3 and 2.5 Ga (see the review of Condie 2018) and, at the same time, this period is most likely when the continents rose above sea level (Korenaga et al. 2017).

Cartoon summarizing the factors that control the emergence and evolution of plate tectonics on Earth.

Figure 2. Cartoon summarizing the factors that control the emergence and evolution of plate tectonics on Earth. Enhanced surface erosion due to the rise of the continents and major glaciations stabilized subduction and plate tectonics for some periods after 3 Ga and particularly after 0.7 Ga after the cooling of the mantle. Blue boxes mark major glaciations; transparent green rectangles show the time intervals when all three geochemical proxies consistently indicate increasing sediment influence (major lubrication events); and, a thick black dashed curve separates hypothetical domains of stable and unstable plate tectonics. The reddish domain shows the number of passive margins (Bradley 2008), here used as a proxy for plate tectonic intensity.

What was before plate tectonics?

The earlier geodynamic regime could have involved episodic lid overturn and resurfacing due to retreating large-scale subduction triggered by mantle plumes (Gerya et al. 2015) or meteoritic impacts (O’Neill et al. 2017). Retreating slabs would bring water into the upwelling hot asthenospheric mantle, generating a large volume of magma that formed protocontinents. Extension of the protocontinental crust could have produced nascent subduction channels (Rey et al. 2014) along the edges of the protocontinents lubricated by the sediments. In this way, a global plate tectonics regime could have evolved from a retreating subduction regime.

What is next?

Despite of the support from existing data, more geochemical information is required to conclusively test our hypothesis about the role of sediments in the evolution of plate tectonics. Additionally, this hypothesis must be fully quantified, which in turn will require coupled modeling of mantle convection and plate tectonics, surface processes and climate.

References
Behr, W. M. and Becker, T. W. Sediment control on subduction plate speeds. Earth Planet. Sci. Lett. 502, 166-173 (2018).

Bercovici, D. The generation of plate tectonics from mantle convection. Earth Planet. Sci. Lett. 205, 107–121 (2003).

Bradley, D. C. Passive margins through earth history. Earth Sci. Rev. 91, 1-26 (2008).

Cawood, P. A., Hawkesworth, C. J. and Dhuime, B. The continental record and the generation of continental crust. Geol. Soc. Amer. Bull. 125, 14-32 (2013).

Cawood, P. A. and Hawkesworth, C. J. Earth's middle age. Geology 42, 503-506 (2014).

Condie, K. C. A planet in transition: The onset of plate tectonics on Earth between 3 and 2 Ga? Geosci. Front. 9, 51-60 (2018).

Gerya, T.V. et al. Plate tectonics on the Earth triggered by plume-induced subduction initiation, Nature 527, 221-225 (2015).

Hoffman, P. F. and Schrag, D. P. The snowball Earth hypothesis: testing the limits of global change. Terra Nova 14, 129–155 (2002).

Korenaga, J., Planavsky, N. J. and Evans, D. A. D. Global water cycle and the coevolution of the Earth's interior and surface environment. Phil. Trans. R. Soc. Am. 375, 20150393 (2017).

Lamb, S. and Davis, P. Cenozoic climate change as a possible cause for the rise of the Andes. Nature 425, 792-797 (2003).

Moresi, L. and Solomatov, V. Mantle convection with a brittle lithosphere: Thoughts on the global tectonic style of the Earth and Venus. Geophys. J. Int. 133, 669-682 (1998).

O’Neill, C. et al. Impact-driven subduction on the Hadean Earth. Nature Geosci. 10, 793-797 (2017).

Popov, A.A. and Sobolev, S. V. SLIM3D: A tool for three-dimensional thermo mechanical modeling of lithospheric deformation with elasto-visco-plastic rheology, Phys. Earth Planet. Inter. 171, 55-75 (2008).

Rey, P. F., Coltice, N. and Flament, N. Spreading continents kick-started plate tectonics. Nature 513, 405–408 (2014).

Shields, G. A. A normalised seawater strontium isotope curve: possible implications for Neoproterozoic-Cambrian weathering rates and the further oxygenation of the Earth. eEarth 2, 35-42 (2007).

Sobolev, S. V. and Babeyko, A. Y. What drives orogeny in the Andes? Geology 33, 617-620 (2005).

Spencer, C. J., Roberts, N. M. W. and Santosh, M. Growth, destruction, and preservation of Earth's continental crust. Earth. Sci. Rev. 172, 87-106 (2017).

Steinberger, B. and Calderwood, A. Models of large-scale viscous flow in the Earth’s mantle with constraints from mineral physics and surface observations. Geophys. J. Intern., 167 1461–1481 (2006).

Tackley, P. J. Self-consistent generation of tectonic plates in three-dimensional mantle convection. Earth Planet. Sci. Lett. 157, 9-22, (1998).

Zhong, S. and Gurnis, M. Viscous flow model of a subduction zone with a faulted lithosphere: long and short wavelength topography, gravity and geoid. Geophys. Res. Lett. 19, 1891–1894 (1992).

 

Searching for future directions in tectonic modelling

Searching for future directions in tectonic modelling

Geoscientists frequently use forward geodynamic simulations to test hypotheses derived from geophysical and geologic observations. While numerical simulations of lithospheric deformation have lead to key advances in our understanding of tectonic processes, in many cases it remains difficult to ascertain whether numerical models reproduce observations for the correct underlying regions.  This week, John Naliboff and Jolante van Wijk discuss this issue, and talk about a White Paper being prepared by the Computational Infrastructure for Geodynamics (CIG) Long-Term Tectonics Working Group on this topic.

John Naliboff. Assistant Research Scientist in the Department of Earth and Planetary Sciences, UC Davis.

In recent years, advances in numerical methodology, high-performance computing and elucidation of complex geologic observations have enabled 3-D simulations of long-term lithospheric deformation at kilometer-scale resolution and with complex non-linear material behavior. The lithosphere models generally include rheological and compositional layering that delineate a brittle upper crust, ductile (viscous) to brittle mid- to lower crust and a brittle to ductile lithospheric mantle overlying a purely viscous asthenosphere. Inherently, the rheological behavior of distinct layers varies temporally through complex feedbacks between temperature, grain-size, strain-rate, phase transitions, flexural stresses and finite deformation. Across a wide range of tectonic settings, numerical investigations incorporating this type of behavior have qualitatively and in some cases quantitatively reproduced key first- and second-order geologic observations.

Jolante van Wijk. Associate Professor in the Department of Earth and Environmental Science, New Mexico Tech.

Despite these successes, numerous significant challenges remain as the computational tectonics community looks toward investigations that account for physical processes acting across a wide range of spatial and temporal scales (Figure 1). Geodynamic model development currently evolves around modifying existing models to include surface processes, thermodynamically consistent melt and volatile transport, metamorphic reactions, and brittle failure to reproduce characteristic features of the seismic cycle.

While many of these processes or features are active areas of research and have been addressed on an individual basis, it often remains unclear at best as to how one should numerically validate even the simplest models of lithospheric deformation. In other words, one can ask whether numerical models of lithospheric processes are reproducing key observations for the correct underlying reasons. Significantly, this question of validation equally applies to observational studies: given that many geologic processes contain significant feedbacks across vast spatial and temporal scales, to what degree can a set of specific observations be interpreted to meaningfully reflect first-order processes?

Continued close collaboration between observational, experimental, and computational Earth scientists is needed to overcome these challenges. At present, the CIG Long-Term Tectonics Working Group is preparing a white paper draft that outlines a 5-10 year vision for collaboration between computational Earth scientists and experimental and observational communities. Given the vast series of topics and disciplines associated with lithospheric dynamics, the White Paper will be organized around Transitional Domains within the lithosphere. The Transitional Domains include Earth’s surface, the brittle-ductile transition, the Moho, the mid-lithosphere discontinuity, and the lithosphere-asthenosphere boundary. For each of these domains we will address the following questions:

  1. How are these domains characterized in the Earth’s lithosphere?
  2. (How) have these domains been modeled previously?
  3. What steps can we take to improve the characterization of these transitional domains in numerical models?
  4. What new methods need to be developed to implement the domains?

 

Figure 1. Spatial and temporal scales associated with distinct lithospheric processes, which was published in Cooper et al., 2015, GSA Today, v. 25(6), pp. 42-43 (Figure 1a).

 

A new generation of geodynamic models will be developed to include the transitional domains. These models will need to be validated, using datasets from the observational and experimental communities, with newly developed techniques.

The White Paper will also include sections on increasing the value of lithospheric models for other scientific communities, and on a pathway toward increasing societal relevance of our modeling efforts.

Please contact John Naliboff (jbnaliboff@ucdavis.edu) and Jolante van Wijk (jolante.vanwijk@nmt.edu) for suggestions or questions. A draft White Paper will be presented at CIG’s annual meeting at AGU 2019 in San Francisco.

To serve Geoscientists

To serve Geoscientists

The Geodynamics 101 series serves to showcase the diversity of research topics and methods in the geodynamics community in an understandable manner. We welcome all researchers – PhD students to professors – to introduce their area of expertise in a lighthearted, entertaining manner and touch upon some of the outstanding questions and problems related to their fields. For our latest ‘Geodynamics 101’ post, Fabio Crameri, postdoctoral researcher at the Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Norway, joins us again. Continuing from his earlier post on the harmful use of the rainbow colour map, Fabio shares his thoughts on some of the expressions and phrases used in the community that propagate confusion, and how the new “Ocean-Plate Tectonics” concept offers relief for at least some on them. 

Blog author Fabio Crameri – in a shirt that translates from Tamasheq as “deserts” or “empty spaces.” You can expect no empty spaces in your lunchtime conversations after reading this post.

Do you, after reading the title, still wonder what this blog post is all about?
I’ll give you a hint, it’s about the Earth. No, wait, it’s about Earth, or perhaps earth, or isn’t it? And maybe it is a little bit about Moon, I mean the Moon. But also, it is about the Venus, I mean Venus.

It is not confusing, it is just well mixing.

You’ve got it; it is about confusion in the Geosciences. Confusion caused by symbols, letters, words and phrases through misuse, ambiguity or over-interpretation. So, after all, this is a blog post about geo-semantics rather than about culinary excursions.

The Geodynamics community is a diverse group of people with different backgrounds, native languages and customs. This is an attractive breeding ground for semantic related problems, particularly when you throw in some inherent peculiarities of the English language in which we largely operate.

Some use the symbol “a” for years, for years and years.

In line with a widely used standard definition (Holden et al., 2011) – but against the common convention of the Geosciences – the author of this blog post was using the unit of time “a”, or arguably just its symbol, for “years” (and I mean calendar years, neither financial years nor dog years), for years and years. A distinction between discrete points in time and the duration of time is at the heart of this confusion, and indeed has plagued a sub-selection of discussions, working groups and interpretations of the International System of Units (SI; e.g., Christie-Blick, 2012).

Figure 1. An ambiguously phrased situation near the recent end of the Cretaceous.

The symbol “a” for “annus” [year] (“Ma” being the symbol for 106 years, or “mega-annus”) in the Geosciences is most commonly used for a specific time or date in the past as measured from now. For example, “At 65 Ma (which is 65 Myr ago), the dinosaur looked up the sky.” (see Figure 1). On the other hand, “yr” for “year(s)” is commonly used for a duration of time, as in “The Cretaceous period ran for 79 Myr (from approximately 145-66 Ma).”. Other mutations within the convention of time in the Geosciences include “My”, “Myrs”, “Mya” or “m.y.” for “Millions of years”. Thus, the time unit and symbols for multiples of a “year” are likely amongst the most ambiguous expressions in the Earth Sciences, likely because, in contrast to the “second”, a universally applied scientific definition for the “annus” still remains elusive (Thompson and Taylor, 2008).

Such quibbling over semantics may seem petty.

Amongst other examples to cause geodynamic misunderstandings (e.g., Figure 2) might be the misuse of the phrase “stagnant slabs”? Are slabs ever really stagnant? Or are they just being deflected, slowing down, interrupting their downward motion, not directly entering the lower mantle at the same speed and trajectory as before?

Figure 2. One ambiguously phrased geodynamic explanation.

From the literature, you might be forgiven for having the false impression that slabs either fully stagnate around the upper-mantle transition zone or directly and effortlessly penetrate it; they likely do neither of the two (as explained in e.g., in an earlier Geodynamics101 post here).

When these slabs sink, and not temporally stagnate, they induce flow in the surrounding mantle. “Slab suction” is the downward suction induced by the nearby mantle that is set in motion through its dynamic coupling with the slab [e.g., Conrad and Lithgow-Bertelloni 2002]. Or isn’t it? “Slab suction” is also contrarily used as an upward directed force on the slab itself that is induced by the upper plate and might foster low-dipping shallow-depth slab portions in the uppermost upper mantle (unambiguously speaking of which: see again Figure 2).

The downward directed version of “slab suction” can induce “dynamic topography”. Estimates of the maximum amplitude of “dynamic topography” on Earth range from only a few hundred meters up to a few kilometres (see e.g., Molnar et al., 2015 and references therein). Such unusually large ranges of estimates are, as a general rule, a quite solid indicator for an underlying ambiguous definition, or in this case, rather a mix-up of multiple different definitions for the term “dynamic topography”. 

If you’re not confused, you did not pay attention.

As I keep talking about geodynamics, I hope we are all on the same page about subduction, one of the key players: Let’s assume planet XY has one single active subduction zone. Another subduction zone initiates on the opposite side of the same planet. Did “subduction” start once or twice on that planet?

It started once on that planet. Because “subduction” describes a process and not a physical feature; it is nonetheless easily mistaken for a physical feature.

And what about “plate tectonics”, the 50 yr old overarching concept that fascinates us, and for so many of us has become the foundation of our professional lives. Let’s approach this by considering the big question: When did “plate tectonics” start? Serious opinions in the plate tectonics community range from around 850 Ma (Hamilton 2011) all the way back to 4.3 Ga (Hopkins et al., 2008). – Remember what unusually large estimate ranges often indicate? – It is not surprising that the only commonly accepted specific answer everyone seems to agree on currently is that it depends on the very definition of plate tectonics.

So, what is the definition of “plate tectonics”? According to its original formulation, “plate tectonics” is the horizontal relative movement of several discrete and mostly-rigid surface-plate segments (Hess, 1962; see the corresponding visual representation in Figure 3). A generous interpretation of the original formulation might additionally define the plate-interface nature, but that is all.

Figure 3. As long as it is not overinterpreted, there is nothing wrong with the original definition of plate tectonics that solely describes the horizontal motion of several discrete surface plates: It does not discriminate the oceanic from the continental plate, does not consider the important framework of mantle convection, and does not specify the underlying key driver of the surface motion.

Considering the knowledge we have gained about the moving surface plates and their underlying causes and consequences during the past 50 yr, this is an extremely broad definition: As of today, we know that (A) the surface plates with their relative motion are an integral part of whole mantle convection (Turcotte and Oxburgh, 1972), that (B) Earth’s surface has a characteristic bimodal nature due to the partitioning into long-lived continental plates and short-lived oceanic plates (e.g., Wilson, 1966), and that (C) the latter are mainly driven by their very own subducted portions (i.e., all or parts of their slabs; Forsyth and Uyeda, 1975; Conrad and Lithgow-Bertelloni, 2002).

A clear, unambiguous and up-to-date definition for such a crucially important, wide-reaching concept is imperative. It is therefore not surprising that less ambiguous re-definitions have been suggested recently. To avoid propagating confusion, the introduction of alternative phases of plate tectonics that describe the various different possible modes of mantle convection during Earth’s evolution have been cast into the arena (e.g., Sobolev 2016). These include “plate-tectonics phase 1”, in short “PT1”, describing regional, plume-induced plate tectonics (e.g., until 3.0 Ga), “PT2” describing episodic, global plate tectonics (e.g., between 2.5-1.0 Ga), and finally “PT3” describing stable, global plate tectonics (e.g., 1.0-0.0 Ga). Other efforts result in different naming conventions, such as “modern plate tectonics”. However, apart from the fact that “modern” is a time dependent term, “modern plate tectonics” might be a somewhat unfortunate expression, as other planets like Venus might have undergone different, modern styles of plate tectonics than present-day Earth.

Stern and Gerya (2017) then actually suggests an entire update to the definition of “plate tectonics”:

“A theory of global tectonics powered by subduction in which the lithosphere is divided into a mosaic of strong lithospheric plates, which move on and sink into weaker ductile asthenosphere. Three types of localised plate boundaries form the interconnected global network: new oceanic plate material is created by seafloor spreading at mid-ocean ridges, old oceanic lithosphere sinks at subduction zones, and two plates slide past each other along transform faults. The negative buoyancy of old dense oceanic lithosphere, which sinks in subduction zones, mostly powers plate movements.”

Unfortunately, such a re-definition of the same old phrase makes it impossible to know which version of the definition (i.e., the original or the updated one) an author of a subsequent study should be applying and referring to.

In an effort to prevent all of the above problems, we recently introduced an entirely new concept; one that can coexist in harmony with the original definition; one that fully captures the dynamics of the oceanic plate according to our current knowledge. The concept is called “Ocean-Plate Tectonics” or, if you really like the term, “OPT”.

“Ocean-Plate Tectonics is a mode of mantle convection characterised by the autonomous relative movement of multiple discrete, mostly rigid, portions of oceanic plates at the surface, driven and maintained principally by subducted parts of these same plates that are sinking gravitationally back into Earth’s interior and deforming the mantle interior in the process.” – Crameri et al. (2018).

“Ocean-Plate Tectonics” captures not only the relative horizontal surface motion of plates, but crucially also accounts for (A) the importance of the whole mantle framework, (B) the bimodal nature of Earth’s surface plates, and (C) the underlying engine of the surface-plate motion (see Figure 4).

Figure 4. “Ocean-Plate Tectonics”, the unambiguous up-to-date definition describing the dynamics of the oceanic plate that crucially incorporates the bimodal nature of Earth’s surface, the convecting-mantle framework, and the key driver of surface-plate motion (after Crameri et al., 2018).

“Ocean-Plate Tectonics” is here to serve Geoscientists.

The concept of “Ocean-Plate Tectonics” is intended to bring together the extremely diverse research communities, but also the general public, to meet on common, fruitful ground in order to discuss and further develop our understanding of the fascinating dynamics involved in Earth’s plate-mantle system; the unambiguous “Ocean-Plate Tectonics” is here to serve us.

 

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