GD
Geodynamics

Luca Dal Zilio

Luca Dal Zilio is a PhD student and part of the SNF project “SINERGIA-AlpArray” at the Swiss Federal Institute of Technology (ETH–Zürich). He obtained his master’s degree at University of Padua (Italy), with the specialisation in Tectonics and Numerical Geodynamic Modelling. His PhD project focuses on earthquake cycles in collisional orogens. By studying the
characteristics of mountain belts, he aims to better constrain the seismic behaviour of mountain belts. Based on numerical modelling, he is describing this behaviour, which can then be used to improve the assessment of earthquake hazard of large urban areas close to mountain belts. You can reach Luca via email.

Going with the toroidal mantle flow

Going with the toroidal mantle flow

Subduction zones host one of the most complex and fascinating tectonic systems on the planet. Numerical models by Király and colleagues recently published in Earth and Planetary Science Letters reveal that the strength of the toroidal flow depends on the mantle viscosity and the magnitude of the slab pull force while the characteristic size of the toroidal cells mainly depends on the size of the convecting mantle.

 

The motion of Earth’s tectonic plates—the lithosphere—is driven by the subduction of relatively cold and dense oceanic plates into the mantle. Subduction zones are some of the most striking features on Earth. They represent one of the two types of convergent plate boundaries, in which one tectonic plate sinks underneath another one into the Earth’s mantle. The resulting forces induce mantle flow around the subducting plate, but the manner in which this happens is still a matter of debate.

Below the tectonic plates, the mantle moves in a slow and unseen manner—as cold slabs sink, hot upwellings rise, and convection slowly rids the Earth’s interior of its primordial heat. Much of our knowledge of mantle flow patterns is indirect, inferred from geodynamical models or seismic tomography images.

Seismological and geochemical investigations suggest 3D subduction-induced mantle flow around lateral slab edges from the sub-slab zone towards the mantle wedge[1]. Such flow has also been observed in laboratory experiments[2] and numerical models of subduction[3]. In particular, geodynamic numerical models of subduction demonstrate that back-arc extension at narrow subduction zones is driven by rollback-induced mantle flow[4].

Observations of seismic anisotropy—the directional dependence of seismic wave speed—provide us with tantalizingly direct information about mantle flow direction. For example, crystals of olivine, which is the most common upper-mantle mineral, tend to become aligned by the mantle flow, and seismic anisotropy is an indicator of this alignment.

To apply the laboratory-derived viscosity laws to nature, they must be extrapolated over 10 orders of magnitude, which introduces uncertainty. Moreover, it is unclear to what extent experiments on centimeter-scale samples are representative of the crust and lithosphere.

The viscosity distribution of the lithosphere and the surrounding mantle is therefore one of the least certain parameters in geodynamics. Alternative ways to determine viscosity on geological time scales are thus needed.

Schematic diagram of a subduction zone, showing the dominance of 3D flow beneath the slab and the competing influence of 2D and 3D flow fields in the mantle wedge. Credit: Long and Silver, 2008, Science.

 

Writing in Earth and Planetary Science Letters, Király and colleagues[5] present a cutting-edge numerical model that shows how the strength and length scale of the toroidal flow vary with the mantle viscosity and the magnitude of the slab pull force. Király and co-workers highlight some remarkable implications of these effects: around subducting plates, the characteristic length, axis, and shape of the toroidal cell are almost independent of the slab’s properties and mainly depend on the thickness of the convecting mantle. The independence of the shape of the toroidal cell on the slab width can appear controversial with respect to previous studies[6–7] that showed that the overall mantle flow is dependent on the slab width. However, as Király et al. point out, these differences can be explained with the model setup adopted and the range of slab widths investigated.

In order to characterize the flow—in terms of its geometry and strength—Király and co-workers analysed the vertical component of the mantle vorticity as well as the ratio between the vertical and the trench parallel component of the vorticity vector. With a series of numerical experiments, they find that subduction-induced mantle flow is highly three-dimensional, and that the toroidal component is sub-horizontal with some vertical flow components. According to the authors, this vertical flow around the slab edges has significant implications, as it can be responsible for the presence of the off-arc volcanism around several laterally confined subduction zones.

The numerical experiments by Király et al. represent another step forward in our understanding of how mantle circulation plays a relevant role in the shaping of tectonic features around subduction zones, and they provide evidence that the slab properties only impact the vigour of the flow around subducting slabs. The extension of these numerical experiments to a range of different parameters will allow for a fuller characterization of the olivine fabric, which in turn will allow seismologists to relate their measurements of seismic anisotropy to flow directions and, ultimately, to mantle processes.

 

References:

1. Long, M. D., and P. G. Silver (2008), The subduction zone flow field from seismic anisotropy: A global view, Science, 319, 315-318.
2. Funiciello, F., C. Faccenna, and D. Giardini (2004), Role of lateral mantle flow in the evolution of subduction systems: insights from laboratory experiments, Geophy. J. Int., 157, 1393-1406.
3. Jadamec, M. A., and M. I. Billen (2010), Reconciling surface plate motions with rapid three-dimensional mantle flow around a slab edge, Nature, 465, 338-342.
4. Sternai, P., L. Jolivet, A. Menant, and T. Gerya (2014), Driving the upper plate surface deformation by slab rollback and mantle flow, Earth Planet. Sci. Lett., 405, 110-118.
5. Király, A. Capitanio, F.A., Funiciello, F., Faccenna, C. (2017). Subduction induced mantle flow: Length-scales and orientation of the toroidal cell. Earth Planet. Sci. Lett., 479, 284–297.
6. Funiciello, F., Moroni, M., Piromallo, C., Faccenna, C., Cenedese, A., Bui, H.A., 2006. Mapping mantle flow during retreating subduction: laboratory models analyzed by feature tracking. J. Geophys. Res., Solid Earth 111, 1–16.
7. Stegman, D.R., Freeman, J., Schellart, W.P., Moresi, L., May, D., 2006. Influence of trench width on subduction hinge retreat rates in 3-D models of slab rollback. Geochem. Geophys. Geosyst. 7.

NetherMod Day 3 – Science & Windsurfing

NetherMod Day 3 – Science & Windsurfing

Let’s keep going with the programme: today’s talks dealt with the following topics:

(1) Lower mantle rheology–what do we really know?

(2) Modelling plasticity and fractures in porous media: everything goes wrong!

(3) Complexities of subduction dynamics.

The first keynote talk was on the rheology of the lower mantle, an important part of the Earth. John Hernlund spent his 45 min describing what the lower mantle is and what it is made of. He also went over where you can find surface expressions and its importance in shaping our planet.

How can we possibly know what’s in the lower mantle if it is so far below the surface? 

Geodynamic modellers have to get creative to figure out what the lower mantle is made of.

Although uncertainties of present data preclude precise numerical predictions, several interesting achievements in this area have recently been published. So, stay tuned!

The second talk–by René de Borst–was about the mechanical behaviour of geomaterials. He gave an overview of how complex these materials are, from almost the onset of loading to shear deformation and the volumetric response. Modellers have been warned about the possibility that non-uniqueness occurs as a consequence of strain softening and plasticity. Since different options exist, we should keep discussing the physical consequences carefully!

Thorsten Becker had the last talk of the day. What did we learn from him? Very briefly, we can summarise it in three key-points:

(1) Rock rheology is a usual suspect for uncertainty;

(2) Oceanic slabs reign supreme in controlling plate motions;

(3) Two slabs are better than one!

Free afternoon – Introduction to windsurfing 

This afternoon the NetherMod crew had the chance to stop talking about slabs, rifting, plasticity, and partial differential equations. One group went to the lake for an awesome windsurfing lesson.

Why is windsurfing awesome?

After this unforgettable experience, geodynamic modellers have heaps of reasons why they would say that windsurfing is the best sport ever. All you have to do is ask any modeller “Why windsurfing?” and they will give you explanations of what makes windsurfing great.

A young geodynamic modeller ready for windsurfing!

 

Windsurfing is easy

There are tons of sports that are hard to learn: bike riding, snowboarding, rock climbing and the list goes on. Windsurfing has a great advantage in that you can make the learning environment ideal, even if you are a nerdy scientist! You only need flat water and light constant winds.

Windsurfing is a great exercise

Obviously all sports end up being good for fitness. However, the great thing of windsurfing is that the intensity varies along with your level, in the same way as you work on numerical codes!

So, do you believe it? Two more geodynamicists try to convince you:

Simon Preuss (PhD student, ETH Zürich, Switzerland):

“Wow, what a great day. I mean, usually I don’t do sports and every sport I start gets boring really quickly.
But you know… I got the feeling windsurfing is different: it’s a sport that you do alone on your own board with your own sail–sounds depressing, I know. But actually we went with a group of 20 people and everybody managed to catch the wind right away. And well, I guess after ~1 hour we were ~20 meters away from the coastline–it was great!!
If I had the chance to do it again I would do it right away. It’s just surprising how fast you can learn sports. Definitely easier then understanding geodynamic models.”

Anna Gülcher (MSc student, ETH Zürich, The Netherlands and windsurfing teacher):

“I must say the geophysicists pleasantly surprised me in how fast they picked up the windsurfing. Within half an hour everyone stood up straight and was actually windsurfing. It’s a pity the wind was not strong enough to fully challenge everyone!”

Very happy geodynamicists after their windsurfing experience

 

Pre-plate-tectonics on early Earth: How to make primordial continental crust

Pre-plate-tectonics on early Earth: How to make primordial continental crust

The sequence of events before continental crust formation has long been contested. Numerical simulations performed by Rozel and colleagues imply that the key to the puzzle could lie in the intrusive magmatism.

Despite several decades of research on the topic, the trigger of proto-continental crust formation on early Earth remains an enigma. However, magmatic processes may hold the key to unravelling what controlled the pre-plate tectonic dynamics of Earth.

The hidden shreds of evidence narrating Earth’s geological history are located deep within the interiors of old and thick cratons or even deeper in the Earth’s mantle – there lies the answer to the non-trivial question: how does the Earth machine work? Synthesizing these clues requires gathering a vast array of data, and geodynamic simulations are required in an attempt to reproduce the processes that gave rise to continents on early Earth.

Writing in Nature, Antoine Rozel (ETH Zürich) and colleagues provide a first global picture of Archean geodynamics by exploring continental crust formation on the early Earth with numerical modelling. Attempting global simulations of the early Earth is challenging as the models must satisfy petrological constraints and must be able to explain field data in Archean (~4 to 2.5 billion years ago) provinces.

Rozel and colleagues use cutting-edge software (the global convection code StagYY) to investigate different magmatism scenarios. Eruptive or intrusive magmatism differs in the way in which magma is deposited, namely cold at the surface or warm at the bottom of the crust. This is important as the primordial continental crust can only be produced in small windows of pressure-temperature conditions.

Rozel and colleagues simulate the thermomechanical evolution of early Earth (0–1 billion years), and find that the tectonics model dominated by volcanism is not able to produce Earth-like primordial continental crust, because of a too low crustal temperature. In contrast, a tectonics regime dominated by intrusive magmatism results in a higher crustal temperature, resulting in a stable continental crust. Commenting on the research, Antoine Rozel said:

Our models explain how continental crust might have been formed.

This work illustrates that the sophistication of numerical models has now reached a level where crusts of various types as well as depleted mantle can form self-consistently. Such models have the potential to distil geological and geophysical knowledge into complex process models that allow geoscientists to investigate a very exotic, pre-plate tectonics Earth. However, the specific results of Rozel and colleagues depend on the details of the pre-defined hypothesis assumed in their model. Specifically, they assume two critical parameters: the eruption efficiency (as opposed to intrusion efficiency) and the strength of the lithosphere.

Via a systematic parameter study, they found that a volumetric melt eruption efficiency of <40% leads to the production of the primordial crustal, both in terms of quantity and composition. Despite inevitable uncertainties, co-author Charitra Jain (ETH Zürich) said:

The production of the expected amount of the main primordial crustal compositions is in agreement with data constrained from fieldwork.

The impact of the emplacement mechanism (eruption vs. intrusion) on the geotherm (modified from Rozel et al., 2017, Nature – doi: 10.1038/nature22042). (a) 100% eruption efficiency; (b) mix of eruption and intrusion and (c) intrusive emplacement only. The grey envelopes in the right-hand-side plots show the temporal evolution of the geotherm.

Whether or not the formation of primordial crust gave birth to plate tectonics will no doubt remain contested. Scientists have previously suggested numerous ways to understand when and how plate tectonics began. For instance, the apparently quiescent phase between the Paleo- and Meso-Archean (~3.2 billion years) seems to have been followed by a geological era of intense deformation. This has been interpreted as the onset of plate tectonics, although this question is beyond the scope of the work presented here.

Rozel and colleagues have thus provided a fresh perspective on the long-standing problem of understanding the sequence of events that led to the formation of continental crust on early Earth. However, whether these events have driven the initiation of plate tectonics is likely to remain controversial for some time. An important missing ingredient in this study is the formation of strong continental roots (cratons), which is now under investigation in the Geophysical Fluid Dynamics team at ETH Zürich.

 

REFERENCE:

Rozel, A. B., Golabek, G. J., Jain, C., Tackley, P. J., & Gerya, T. (2017). Continental crust formation on early Earth controlled by intrusive magmatism. Nature, doi: 10.1038/nature22042