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Geodynamics
Diogo Lourenço & Antoine Rozel

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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.

Iron volcanism on metallic asteroids?

Metallic asteroid

This week, Francis Nimmo, professor in the Department of Earth and Planetary Sciences (University of California Santa Cruz), tells us about volcanism on metallic asteroids! Around and after the formation of the solar system (4.5 billion years ago onwards), volcanoes on some of the gigantic bodies of the asteroid belt might have erupted … iron. Explanations.

Francis Nimmo

Francis Nimmo

One way in which we can learn about the insides of planetary bodies is by looking for signs of volcanism. Volcanism transports molten material from the interior to the surface of a body, where it solidifies. Something similar may happen on icy bodies, where cryovolcanism is thought to occur. Together with Coby Abrahams, one of my grad students, I recently wrote a paper proposing “ferrovolcanism”, which is the eruption of metallic iron. We suggested that this may have happened in the past on metallic asteroids.

 

What are metallic asteroids? Some asteroids, such as 16 Psyche are thought to be made mainly of iron and nickel, on the basis of their spectra and radar signatures. They probably originated from a catastrophic collision. The picture is that a proto-planet, which had already formed a dense metallic core, was broken apart so violently that its core was fragmented, and the fragments then cooled to form free-floating metallic bodies. Such an event would also explain the large number of iron meteorites in our collections. So far we have never seen a metallic asteroid up close, but NASA is planning to send an orbiter which will reach 16 Psyche in 2026.

Iron meteorite

Artistic view of an iron meteorite with ferrovolcanism (Elena Hartley)

How would “ferrovolcanism” work? Immediately before the catastrophic collision, the core would have been liquid (because the silicate mantle is a good insulator). After the collision, the surface of a free-floating iron blob would have initially cooled very rapidly. Analysis of iron meteorites tells us that, in some cases, the blob developed a strong, cold iron crust. The melt (liquid iron) beneath the crust would have been less dense than the crust – just like molten magma on Earth. And so, just like magma on Earth, the liquid iron would tend to rise to the surface – hence ferrovolcanism.

There are a few factors that make ferrovolcanism less likely than silicate volcanism. One is that, as the iron crust cools, it contracts, putting the whole body into compression and making it more difficult for melt-filled fractures to open up. Another is that iron is both ductile and strong, so that fracture propagation is harder. And iron is also much more thermally conductive than rock, so that melt-filled fractures will cool and solidify rapidly. Nonetheless, we don’t think any of these factors is fatal. For instance, Mercury also experiences global compression but still shows signs of volcanism. Iron eruptions might be aided by volatiles coming out of solution, just as with terrestrial eruptions, although the catastrophic impact may have removed some volatiles.

How do we test this hypothesis? One way is to wait for images of 16 Psyche! If we see things that look like iron volcanoes, that would be interesting. But identifying signs of ancient volcanism is notoriously hard – this paper documents several proposed volcanic or cryovolcanic features which turned out to be something else on further investigation. And our calculations show that the volume of erupted material will be only about 0.1 percent of the total volume of Psyche, so these features may be hard to spot – especially after 4 billion years of erosion by impacts.

Another way is to look at the iron meteorites in our collections, because ferrovolcanism should produce anomalous-looking meteorites. For instance, one might find vesicle-rich samples, or ones happening late in the crystallization sequence but showing extremely rapid cooling (because they were erupted to the surface).

How did we write the paper? It was more or less accidental. Coby was working on how iron asteroids might lose their volatiles early when one day he turned to me and said “I think they’ll erupt!”. I thought this was a neat idea, and so we started to look at how the eruptive process might work.

The strangest part of writing our paper was that at the LPSC conference someone came up to me and said “I just invented iron volcanism!”. It turned out that another group had been working on ferrovolcanism completely independently. They suggested that pallasite meteorites might be evidence of ferrovolcanism (something that I had completely missed). This simultaneous but independent development of ideas seems to be quite common in the Earth Sciences – but that is a topic for another day.

 

How the EGU works: Experiences as GD Division President

How the EGU works: Experiences as GD Division President

In a new regular feature, Paul Tackley,  president of the EGU geodynamics division, writes about his role as a president, and gives us an insider’s view on how EGU works and is preparing for the future. 

Paul Tackley. Professor at ETH Zürich and EGU geodynamics division president. Pictured here giving an important scientific talk, or maybe at karaoke. Your pick.

Stepping into the role of GD Division President has given me a big learning experience about how the European Geosciences Union is run and about how members are represented and can participate. Here I convey some impressions, give a quick overview of how EGU functions and the role of division presidents, and mention a few other activities you may not be aware of.

Firstly, I was impressed just how much a bottom-up organisation the EGU is – how it is run by members for the benefit of members. EGU employs only 7 full-time staff – very few compared to the 140+ employed by the American Geophysical Union! Thus, most of the organisation is run my volunteers, including the big jobs of President, Vice-President, Treasurer and General Secretary, and also the presidents of the 22 scientific divisions and members of eight committees. Of course, the fact that so few staff are needed is helped by the fact that Copernicus (the company) deals with publishing all the journals and organising the General Assembly (GA), and Copernicus has 54 employees.

Secondly, I now appreciate that EGU does a lot more beyond organising the General Assembly and publishing 18 open access journals. In particular, EGU is active in the areas of Education and Outreach, and supports various Topical Events, with each area coordinated by a committee. Additionally, a Diversity and Equality working group was recently set up. I encourage you to read more about these various activities on EGU’s web site.

What must a division president do?  The main tasks are to organise the division’s scientific programme at the General Assembly, and to attend three EGU Council + Programme Committee meetings per year: short ones at the General Assembly, and longer (2-3 days) ones in October near Munich, and in January somewhere warm (such as Nice or Cascais). Practically, this involves sitting in a darkened room for 2-3 days with a lot of other people (there are many other members in addition to the division presidents, including early career scientists) listening to information of variable interest level and discussing and making decisions (voting) when necessary. The EGU Council discusses the full range of EGU activities, so meetings consist of a series of reports: from the president, the treasurer, the various committees, the ECS representative, etc., often with much time spent discussing and voting on new points and developments that arise. Programme Committee meetings are focussed on the General Assembly, both discussing general issues and accomplishing the specific tasks of finalising the list of sessions (October meeting) and the session schedule (January meeting). Throughout all these meetings, I have found the council members to be very collegial and constructive in trying to do what is best for improving EGU activities and making optimal arrangements for the General Assembly (although of course, opinions about what is best can vary). Additionally, Copernicus is continually improving their online tools to make scheduling easier.

The President Alberto Montanari, Programme Committee Chair Susanne Buiter and Copernicus Managing Director Martin Rasmussen, celebrating the EGU General Assembly.

I am happy that there are several other people actively taking care of various tasks in the GD Division. Division officers stimulate sessions in their respective areas of the GA programme and judge the Outstanding Early Career Scientist Award nominations, while judging of the OSPP (Outstanding Student Poster and Pico) awards is organised by an Early Career Scientist (now Maelis Arnould). Our Early Career Scientists are incredibly active, maintaining this blog and the Facebook page, and organising social events at the GA. Finally, the Medal committee decides the winner of the Augustus Love Medal.

Changes are ongoing at EGU! In a multi-year process the finances are being moved from France to Germany, a complicated process as described by our Treasurer at the GA Plenary session. Moving the EGU office (where the 7 people work) from a confined space on the campus of Ludwig Maximilian University of Munich to a much larger modern office premises is happening around now and will allow some expansion of the staff and a suitable space to greet visitors. In the longer term, it may be necessary to move the location of the General Assembly from Vienna due to the ever-increasing number of attendees!

To conclude, EGU is our organisation and we can contribute to the running of it and the decision-making process, so I encourage you to get involved and to make your views about possible future improvements or other issues known to your representative (i.e. me, or our Early Career Scientist representative Nicholas Schliffke). And if anyone wants to take over as the next GD Division President, (self-)nominations can be submitted starting in September with the vote coming in November!

Programme Committee of EGU, which includes its chair, all the division presidents, the executive board, key people from Copernicus and Programme Committee Officers including the ECS representative and OSPP coordinator.

On the resolution of seismic tomography models and the connection to geodynamic modelling (Is blue/red the new cold/hot?) (How many pixels in an Earth??)

What do the blobs mean?

Seismologists work hard to provide the best snapshots of the Earth’s mantle. Yet tomographic models based on different approaches or using different data sets sometimes obtain quite different details. It is hard to know for a non specialist if small scale anomalies can be trusted and why. This week Maria Koroni and Daniel Bowden, both postdocs in the Seismology and Wave Physics group in ETH Zürich, tell us how these beautiful images of the Earth are obtained in practice.

Daniel Bowden and Maria Koroni enjoying coffee in Zürich

Seismology is a science that aims at providing tomographic images of the Earth’s interior, similar to X-ray images of the human body. These images can be used as snapshots of the current state of flow patterns inside the mantle. The main way we communicate, from tomographer to geodynamicist, is through publication of some tomographic image. We seismologists, however, make countless choices, approximations and assumptions, which are limited by poor data coverage, and ultimately never fit our data perfectly. These things are often overlooked, or taken for granted and poorly communicated. Inevitably, this undermines the rigour and usefulness of subsequent interpretations in terms of heat or material properties. This post will give an overview of what can worry a seismologist/tomographer. Our goal is not to teach seismic tomography, but to plant a seed that will make geodynamicists push seismologists for better accuracy, robustness, and communicated uncertainty!

A typical day in a seismologist’s life starts with downloading some data for a specific application. Then we cry while looking at waveforms that make no sense (compared to the clean and physically meaningful synthetics calculated the day before). After a sip, or two, or two thousand sips of freshly brewed coffee, and some pre-processing steps to clean up the mess that is real data, the seismologist sets up a measurement of the misfit between synthetics and observed waveforms. Do we try to simulate the entire seismogram, just its travel time, its amplitude? The choice we make in defining this misfit can non-linearly affect our outcome, and there’s no clear way to quantify that uncertainty.

After obtaining the misfit measurements, the seismologist starts thinking about best inversion practices in order to derive some model parameters. There are two more factors to consider now: how to mathematically find a solution that fits our data, and the choice of how to choose a subjectively unique solution from the many solutions of the problem… The number of (quasi-)arbitrary choices can increase dramatically in the course of the poor seismologist’s day!

The goal is to image seismic anomalies; to present a velocity model that is somehow different from the assumed background. After that, the seismologist can go home, relax and write a paper about what the model shows in geological terms. Or… More questions arise and doubts come flooding in. Are the choices I made sensible? Should I make a calculation of the errors associated with my model? Thermodynamics gives us the basic equations to translate seismic to thermal anomalies in the Earth but how can we improve the estimated velocity model for a more realistic interpretation?

What do the blobs mean?

Figure 1: A tomographic velocity model, offshore southern California. What do the blobs mean? This figure is modified from the full paper at https://doi.org/10.1002/2016JB012919

Figure 1 is one such example of a velocity model, constructed through seismic tomography (specifically from ambient-noise surface waves). The paper reviews the tectonic history of the crust and upper mantle in this offshore region. We are proud of this model, and sincerely hope it can be of use to those studying tectonics or dynamics. We are also painfully aware of the assumptions that we had to make, however. This picture could look drastically different if we had used a different amount of regularization (smoothing), had made different prior assumptions about where layers may be, had been more or less restrictive in cleaning our raw data observations, or made any number of other changes. We were careful in all these regards, and ran test after test over the course of several months to ensure the process was up to high standards, but for the most part… you just have to take our word for it.

There’s a number of features we interpret here: thinning of the crust, upwelling asthenosphere, the formation of volcanic seamounts, etc. But it wouldn’t shock me if some other study came out in the coming years that told an entirely different story; indeed that’s part of our process as scientists to continue to challenge and test hypotheses. But what if this model is used as an input to something else as-of-yet unconstrained? In this model, could the Lithosphere-Asthenosphere Boundary (LAB) shown here be 10 km higher or deeper, and why does it disappear at 200km along the profile? Couldn’t that impact geodynamicists’ work dramatically? Our field is a collaborative effort, but if we as seismologists can’t properly quantify the uncertainties in our pretty, colourful models, what kind of effect might we be having on the field of geodynamics?

Another example comes from global scale models. Taking a look at figures 6 and 7 in Meier et al. 2009, ”Global variations of temperature and water content in the mantle transition zone from higher mode surface waves” (DOI:10.1016/j.epsl.2009.03.004), you can observe global discontinuity models and you are invited to notice their differences. Some major features keep appearing in all of them, which is encouraging since it shows that we may be indeed looking at some real properties of the mantle. However, even similar methodologies have not often converged to same tomographic images. The sources of discrepancies are the usual plagues in seismic tomography, some of them mentioned on top.

410 km discontinuity

Figure 2: Global models of the 410 km discontinuity derived after 5 iterations using traveltime data. We verified that the method retrieves target models almost perfectly. Data can be well modelled in terms of discontinuity structure; but how easily can they be interpreted in terms of thermal and/or compositional variations?

In an effort to improve imaging of mantle discontinuities, especially those at 410 and 660 km depths which are highly relevant to geodynamics (I’ve been told…), we have put some effort into building up a different approach. Usually, traveltime tomography and one-step interpretation of body wave traveltimes have been the default for producing images of mantle transition zone. We proposed an iterative optimisation of a pre-existing model, that includes flat discontinuities, using traveltimes in a full-waveform inversion scheme (see figure 2). The goal was to see whether we can get the topography of the discontinuities out using the new approach. This method seems to perform very well and it gives the potential for higher resolution imaging. Are my models capable of resolving mineralogical transitions and thermal variations along the depths of 410 and 660 km?

The most desired outcome would be not only a model that represents Earth parameters realistically but also one that provides error bars, which essentially quantify uncertainties. Providing error bars, however, requires extra computational work, and as every pixel-obsessed seismologist, we would be curious to know the extent to which uncertainties are useful to a numerical modeller! Our main question, then, remains: how can we build an interdisciplinary approach that can justify large amounts of burnt computational power?

As (computational) seismologists we pose questions for our regional or global models: Are velocity anomalies good enough, intuitively coloured as blue and red blobs and representative of heat and mass transfer in the Earth, or is it essential that we determine their shapes and sizes with greater detail? Determining a range of values for the derived seismic parameters (instead of a single estimation) could allow geodynamicists to take into account different scenarios of complex thermal and compositional patterns. We hope that this short article gave some insight into the questions a seismologist faces each time they derive a tomographic model. The resolution of seismic models is always a point of vigorous discussions but it could also be a great platform for interaction between seismologists and geodynamicists, so let’s do it!

For an overview of tomographic methodologies the reader is referred to Q. Liu & Y. J. Gu, Seismic imaging: From classical to adjoint tomography, 2012, Tectonophysics. https://doi.org/10.1016/j.tecto.2012.07.006