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Tectonics and Structural Geology

Modelling

Meeting Plate Tectonics – Barbara Romanowicz

Meeting Plate Tectonics – Barbara Romanowicz

These blogposts present interviews with outstanding scientists that bloomed and shape the theory that revolutionised Earth Sciences — Plate Tectonics. Get to know them, learn from their experience, discover the pieces of advice they share and find out where the newest challenges lie!


Meeting Barbara Romanowicz


Barbara Romanowicz studied mathematics and applied physics and did two PhDs, one in astronomy from Pierre and Marie Curie University and one in geophysics from Paris Diderot University. After her postdoctoral studies at the Massachusetts Institute of Technology, she researched at the Centre national de la Recherche Scientifique (CNRS), where she developed a global network of seismic stations known as GEOSCOPE to study earthquakes and the interior structure of the earth. She currently splits her time between a professorship at UC Berkeley, California, where she does research, and a teaching position as the Chair in Physics of the Earth’s interior at Collège de France, in Paris, where she teaches to the public.

I go between theory and observations, back and forth.

What is your main research interest and which approach do you use in your research?

Barbara Romanowicz in class. Credit: Barbara Romanowicz

My main research interest is the Earth’s interior: figuring out the dynamics and the evolution of the Earth by providing constraints from seismic imaging at the global and continental scale, from the lithosphere to the inner core of the Earth. The methodology that we use is primarily tomography. In my team, we develop new techniques in tomography, so we can achieve higher resolution. But also other types of seismic waveform modelling.

What would you say is the favorite aspect of your research?

What I find most exciting is that I go between theory and observations, back and forth. This brings different types of excitements. For example, developing a method that works is exciting, and so is finding something new in the data. Making progress and discovering something new, basically through a lot of attempts at modelling, and commonly after a lot of time, is very rewarding.

If we do not contribute to it, we will not have any more data.

Why is your research relevant? What are the possible real world applications?

The research is relevant because we are trying to understand the driving mechanisms of plate tectonics. And plate tectonics is what causes earthquakes, volcanoes, tsunamis, and all other natural disasters related to the solid Earth. It is not directly relevant, of course, because of the different timescales; the dynamics of the interior of the Earth are in millions of years, and people are interested in timescales of decades, maybe hundreds of years. So this is a bit of a challenge, but if we do not understand the causes of natural disasters, it is not possible to mitigate them.

Depth cross-sections through model SEMUCB_WM1 (French and Romanowicz, Nature – 2015, doi:https://doi.org/10.1038/nature14876) highlighting broad low velocity “plume-like” conduits beneath major hotspot volcanoes in the central Pacific.

What do you consider to be your biggest academic achievement?

I was asked this question recently, and I did not hesitate to say that I was able to make some impact with my research, but also to contribute to the infrastructure of research. I have been involved since very early in my career, in the development of seismic networks at a global and later regional scale, or trying to put stations in the oceans… Developing the infrastructure to collect data for research is a very recurrent issue that people should keep in mind: if we do not contribute to it, we will not have any more data. If the younger generation of researchers keeps on considering that the data is granted, and do not take up this challenge, the good situation that we’re at will not last.

I thought it is kind of cool that we could show that.

What would you say is the main problem that you solved during your most recent project?

In a fairly recent project, we were able to not only to confirm that there is an ultra slow velocity zone at the base of the Iceland plume near the core-mantle boundary, but also to determine that it is circular in shape. This required being able to illuminate it from different sides, and showing that the same model works for whichever way you look at it. I think that the fact that we can show that is kind of cool, as it combined modelling of seismic waveforms, as well as some imagination in 3D geometry.

Seasonal changes in the dominant locations of the sources of the earth’s low frequency “hum” (top) as inferred from seismic data, compared to the distribution of significant ocean wave height (bottom).

We are not doing enough to raise funds [to build a seismic network infrastructure].

What would you change to improve how science in your field is done?

In my field, which is global seismology, we really rely on a large network of stations, and we need a lot of instruments. Ideally, we would like to cover the entire Earth with instruments, which is not only logistically difficult but also very expensive. I think we are not doing enough to raise funds to build this better infrastructure. The astronomical community, for example, develop decadal plans to build the next generation instruments. In a way, it is easier for them because they need perhaps only a small number of telescopes, whereas our systems are completely distributed, so it is harder for us to join forces. Nevertheless, we are not doing enough of that.

3D rendering of a portion of upper mantle shear velocity model SEMum2 (French, Lekic and Romanowicz, 2013 – Science, doi:10.1126/science.1241514) showing interaction of mantle plume conduits with the asthenosphere beneath the south Pacific superswell (A) and the presence of quasi-periodic low velocity “fingers” aligned in the direction of absolute plate motion extending below the oceanic low velocity zone (B).

What do you think are the biggest challenges right now in your field?

There are several computational challenges, in the sense that we are moving increasingly towards modelling the complete seismic wavefield using numerical methods that are computationally very expensive. One has to think about how big the computer is that you can use, and balance that by finding smart ways to speed up computations in a way that doesn’t rely too much on big computers.

Another really big challenge is to reach the ocean floor and to cover the oceans with broadband seismic observatories. We don’t have enough such stations, and two-thirds of the Earth is covered by oceans. We have less resolution in the southern hemisphere and in the middle of the ocean just because we do not have enough seismic stations on the ocean floor. This is a problem for research on ocean basin structure and deeper upper mantle structure beneath the oceans, but also for research on the very deep Earth, including the inner core. Ocean Bottom Seismometers are great, but we really need very broadband recording, with good coupling to the ground and for long enough times (several years), as well as really large aperture arrays to be able to catch seismic waves over a large azimuth and depth range.

I never really worried about my career.

Barbara Romanowicz. Credit: Barbara Romanowicz

When you were in the early stages of our career, what were your expectations? Did you always see yourself staying in academia?

I think times have changed a lot. When I was doing my Ph.D., I really didn’t have any expectations. I never worried about my career. I simply did not think about it. Probably because I was naive, but also because there was less of a concern at that time… maybe it was easier to find jobs. The landscape was quite different.

Primarily thinking about their [ECS] research will get them where they want to be.

What is the best advice you ever received?

I think the best advice I received is to be daring, to think broadly and about the big picture. So, my best advice to Earth Career Scientists (ECSs) is the same. I would recommend ECSs not to worry too much about their immediate results or about their citation index, but to really think about their research. Primarily thinking about their research will lead them where they want to be. Otherwise, their thinking can be polluted by practical worries. Also, you will always get into situations where you cannot do all the work that you need to do for your research because you have other demands on your time. So my other advice to ECSs is to always keep a couple of hours (the best ones) during the day to completely isolate yourself and work on your research. It is very important. Everything else is easier, but the research itself is the hardest, and if you get distracted you will end up frustrated by not being able to accomplish much.

 

Barbara Romanowicz. Credit: Barbara Romanowicz

 

Interview conducted by David Fernández-Blanco

Meeting Plate Tectonics – Jean-Philippe Avouac

Meeting Plate Tectonics – Jean-Philippe Avouac

These blogposts present interviews with outstanding scientists that bloomed and shape the theory that revolutionised Earth Sciences — Plate Tectonics. Get to know them, learn from their experience, discover the pieces of advice they share and find out where the newest challenges lie!


Meeting Jean-Philippe Avouac


Prof. Jean-Philippe Avouac initially studied mathematics and physics during his undergraduate and graduate degrees. Later he got more inclined towards geophysics and then he discovered Earth Sciences. During his Ph.D. at the Institut de Physique du Globe de Paris, advised by Paul Tapponnier, he immersed himself in geology and tectonic geomorphology. Currently, Jean-Philippe Avouac is a Professor of Geology at the California Institute of Technology.

Like living organisms, earthquakes have a life cycle: they nucleate, grow and arrest. There can be some lineage but each earthquake is a different being.

Fieldwork along the Kali Gandaki (Nepal) in 1999. Credit: Barbara Avouac

Where lies your main research interest?

I study crustal dynamics: How the crust is deforming as a result of earthquakes, but also as a result of viscous processes. I study the process of stress accumulation on faults, the release of this stress by earthquakes, as well as how earthquakes and other mechanisms of deformation are contributing to building the topography and geological structures in the long run.

 

How would you describe your approach and methodology?

In my group, we develop techniques to measure crustal deformation using in particular remote sensing and seismology. We were using radar images initially, and we have moved toward using more optical images with time and also GPS data… We try to reproduce the observations (geodetic deformation, kinematic models of seismic ruptures, gravity field…) using dynamic models to determine what are the forces and rheologies needed.

 

What would you say is the favorite aspect of your research?

What I like most about my research is mentoring Ph.D. students and postdocs. I love matching their skills with good problems, problems that will be attractive to them and that will resonate with the currently hot questions in Earth Sciences. I really love doing that.

The other thing I love is to use what I learned as I student (maths and physics) to answer science questions arising from natural observations. I love that part when you look at nature, you observe something and try to measure it quantitatively and then you try to explain the observation with dynamic models. I really enjoy going back and forth between observations and modelling. And the field! I really like being in the field… This is an aspect of the job that really attracted me initially.

We built from what other researchers had done before, but we reached quite different conclusions […] that’s exciting!

Jean-Phillipe Avouac leading a field excursion in the Dzungar basin, 2006. Credit: Aurelia Hubert-Ferrari

 

Why is your research relevant? What are the possible real-world applications?

A significant fraction of my research is relevant to seismic hazards. After my Ph.D., I worked for the Commissariat à l’Energie Atomique (CEA) for 10 years. I was conducting seismic hazard assessment studies for nuclear facilities. So, I have been exposed to the applied side of earthquake science and I like that some of the research we do in my group can help to improve the way we do seismic hazard assessments.

But what I really want to say is that I do not think relevance should drive academic research. In that regard, I should say that I don’t like much the way the funding system works today. I think there is too much emphasis on relevance to society. The idea that you start from stating problems of societal relevance, and only then see what kind of research we can do to solve this problem is not a good approach, in my opinion. I don’t think this is the way important scientific discoveries are made. You make discoveries by being curious, by observing nature with an open mind, by exploring new ideas and coming up with new concepts, or by observing something that is not explained in the current theoretical framework that we have and then you make use of the knowledge that you build after looking at these problems. There is no way you can clearly anticipate where the joyful exploration of an intriguing idea or observation can lead but we know from experience that the society benefits from curious scientific exploration. So, although I think there is relevance in what I am doing, I do not think that, in general, relevance to society should be driving academic research.

 

An outcome of Jean-Phillipe Ph.D Thesis, later published in Kinematic model of active deformation in Central Asia (Avouac and Tapponnier, GRL – 1993; doi: https://doi.org/10.1029/93GL00128).

I do not think relevance to society should drive academic research

What would you say is the main problem that you solved during your most recent project?

People in my group work on many different projects that are all very exciting to me. I’m going to mention just one project though because I can not possibly list them all.

We have done a lot of work in the past to develop techniques to invert geodetic measurements for fault slip at depth. A postdoc and a graduate students in my group have moved on to improve the technique and use it to document slow slip events in Cascadia over the last 15 years. That was a daunting work but their hard work and perseverance have really paid back. The end result is amazing! We see how the slow slip event initiate, propagate, arrest, trigger one another… We built from what other researchers had done before us, but we reached quite different conclusions given that we now have a more complete view of the behaviour of the system –that’s exciting! I anticipate that we are going to learn a lot about the dynamics of slow-slip events, and maybe it will have important implications for regular earthquakes!

What do you consider to be your biggest academic achievement?

The research for which my group is probably best known is that we have done in the Himalaya. In particular, we have built a model of the seismic cycle that explains the observations that we have from seismology, geodesy, geomorphology and geology. We worked a lot on the Himalaya, in part because I love mountains, but also because it is a very unique setting to study orogenic processes which are still active today. There is really no better place where you can get geological constraints on the thermal and structural evolution of the range. There is a lot of erosion and it has been going on for a long time, so the rocks that have been brought to the surface have recorded the thermal and deformation history over tens of million years. Our research has helped understand how the Himalaya has formed as a result of seismic and aseismic deformation, and I think it has yielded important insight on orogenic processes and the seismic cycle in general.

By the way, I don’t mean that earthquakes are periodic. Like living organisms, earthquakes have a life cycle: they nucleate, grow and arrest. There can be some lineage but each earthquake is a different being.

Animation showing the process of stress build up and release associated to earthquakes along the Main Himalayan Thrust fault, along which India is thrust beneath the Himalaya and Tibet. Credit: Jean-Philippe Avouac, Tim Pyle and Kristel Chanard.

We tend to build walls between disciplines […] We would not have been able to discover plate tectonics without a deep cross-disciplinary dialogue

What do you think are the biggest challenges right now in your field?

As I mentioned before, the funding system is an issue. Funding agencies are clearly making a big mistake in prioritizing social relevance as a criterion to evaluate proposals. Aside from that, the challenge that we have in the Earth Sciences is that we tend to build walls between disciplines. Specialization is a natural drift, and you can make a very successful career in a particular field pushing further a particular analytical or modelling technique. Also, it is easier to get funding for what you are known to be good at. As a result, walls between disciplines are building with time. The vocabulary is evolving in each individual discipline and it is increasingly difficult to make major advancements that can bridge different disciplines. In my research, I try to navigate from one discipline to the other… but it is a challenge –while it can be key to make significant discoveries, it takes time and effort. There are fewer and fewer people making a carrier this way. It can be dangerous because of a dilution effect, but at some point, it is needed. Look at plate tectonics for example: it happened because of advances in different disciplines but most importantly because some scientists were aware of these advances and were able to connect them and derive a coherent global framework. We would not have been able to discover plate tectonics without a deep cross-disciplinary dialogue.

Another challenge is that nowadays we have a lot more data than we used to have. This is both an opportunity and a threat. There is a trend to produce more and more publications, that look very solid because they use a lot of data, but that are in fact very incremental. More of the same is not necessarily advancing knowledge at a fundamental level. We have to be imaginative with regard to how to process the increasing flux of data, but it should not come at the cost of being imaginative with regard to what they mean.

I do not like the way the funding system works today

When you were in the early stages of our career, what were your expectations? Did you always see yourself staying in academia?

After my Ph.D. I did not stay in academia. But even when outside academia, I kept doing research, because I had an appetite for it and was working in an environment where scientific curiosity was valued, even if science was not the main objective. Although I was not unhappy at all outside academia, I decided to go back to it since I found it more exciting for myself: I like to solve scientific questions but there is not so much I could solve without the help of students and postdocs. I didn’t consider staying in academia after my PhD because there were sides of the academic life I did not feel comfortable with… I was finding people in academia to be a bit… difficult sometimes, with big egos and not so open minded. Also, we are a very conservative community. There’s a reason for that, for we as scientists have to be sceptical and to push back new ideas and new observations. I guess I have now become now one of those crazy and conservative academic guys (laughs)!

 

Mapping and sampling Holocene terraces abandoned by rapid climate-driven incision in the Tianshan. Credit: Luca Malatesta

If you have a new idea… you will probably have a hard time

What advice would you like to share with Early Career Students?

My first advice is to be aware of the important questions that we should try to solve. Not because they are relevant but because they are interesting and because they are timely, given the tools and data that we have access to. Being aware of the really big questions is important because we tend to forget them sometimes as we become more specialized. And be also aware of the new techniques available, especially those that you could draw from other fields; computer science or medical imagery for example… It is important to be curious and see what is happening in other fields so that you can transfer new ideas and new techniques to your own field and give a try at answering big science questions.

Be curious, be adventurous. Take risks. Try things that might not work. You might be losing your time but it is also an opportunity to make real fundamental advancements. You can make a career by increments, but I think it is not as rewarding as taking risks and really solving a difficult problem.

Follow your own dreams and don’t be intimidated by peer pressure. If you put a new idea on the table, a really new one, first, you will probably have a hard time expressing it clearly… And second, peers will most probably push back, as they should. So do not be intimidated, believe in your ideas, and keep adjusting and pushing them forward. I see too many times students or postdocs who meltdown and get discouraged if they receive a negative comment after a presentation… – I would say, that could, in fact, be a good sign! You may be doing something different and maybe people are not understanding because there is something disturbing and really new!

 

Jean-Phillipe Avouac. Credit: Trish Reda.

 

Interview conducted by David Fernández-Blanco

Meeting Plate Tectonics – Nicolas Coltice

Meeting Plate Tectonics – Nicolas Coltice

These blogposts present interviews with outstanding scientists that bloomed and shape the theory that revolutionised Earth Sciences — Plate Tectonics. Get to know them, learn from their experience, discover the pieces of advice they share and find out where the newest challenges lie!


Meeting Nicolas Coltice


Nicolas Coltice graduated with a PhD from the École Normale Supérieure of Lyon, France. He then became assistant professor at the Université Claude Bernard in Lyon, and ultimately, full professor. As of last year, he also holds a professorship position at ENS Paris, France. He has received an ERC grant for the project AUGURY and he is one of the co-founders of the manifesto ’Did this really happen?’, which addresses sexual harassment and inequality issues within sciences.

 

Nicolas Coltice. Credit: Eric Le Roux / Université Claude Bernard Lyon 1.

I think it is extremely important that models are supported by evidence or data.

Hi Nicolas, could you tell us about your research interests and the methods you use to solve your problems?

Sure! My research interest is focussed on mantle convection and geochemistry. The research I do is strongly directed to combine models and observations to understand, for example, the geochemical cycle. I also combine observations and inverse models to build tectonic reconstructions and 3D spherical models. I work a bit with geologists and so I sometimes go into the field. I think it is extremely important that models are supported by evidence (or data) and so I try to combine this as much as I can in my research.

You have been active on different topics. What achievement in your carer are you most proud of?

The one thing I’m most proud of is setting up an ERC team for the project ‘AUGURY’, which happened to have more women than men, which is quite rare in our field. I feel we made quite some progress on undermining the patriarchy within sciences with this ERC project. I’m very proud to work with my team. One of the good things that came out of ‘AUGURY’ is our manifesto ’Did this really happen?’. It is a website where we tell the stories on sexual harassment and gender inequality that women in sciences using comics. Besides advocating gender equality science I also teach, which I find very fulfilling and my teaching is well-received.

Good research needs time.

Did this really happen?. Courtesy of www.didthisreallyhappen.net.

It’s fantastic that you are making the community aware of these more social issues. In terms of research, how does that benefit society?

The application of my research to society is first of all doing the job by itself. Every day that scientists invest in understanding parts of our planet is beneficial to society, just by the very act of it. Publishing my work might give a breach to society and offer perspectives that were not thought of before. I guess a more concrete way my research benefits society would be in the reserve or resources industry, where we always like to understand better where resources form and why they form under certain condition. This will eventually help to actually find them and exploit them and the better we understand that, the less impact it will eventually have on the environment.

Every day that scientists invest in understanding parts of our planet is beneficial to society, just by the very act of it.

 

How do you see the future in geoscience?

In my opinion, good research needs time. Currently we are given very little time to do good research. If we want to change the publishing-focussed mentality, we need to start at the bottom. We do not necessarily have to create a big revolution, but from the inside we can collaborate and slowly change the system. For example, if you publish, public money is used to pay for your publication. This public money then often goes to stakeholders, which is not good! We can change this by publishing in different journals with different ethics. This way, we can slowly lower the pressure we feel on publishing nowadays. So in terms of future, I think we need to get back to the core, do good research.

Selected 3-D view state of the model. Continental material is highlighted in yellow. Figure from Coltice & Shephard, 2018 “Tectonic predictions with mantle convection models”, Geophysical Journal International, 10.1093/gji/ggx531.

When you feel it gets rough, stick with your plan and keep your relationship with your colleagues positive.

One last question, what advice would you like to give to Early Career Scientists?

When I was hired 15 years ago, times were different. If recruiters had the choice, they would always go for the youngest person, not necessarily the best. Nowadays productivity is the factor that counts most and is imposed on people which makes it very difficult to maintain an interesting profile at an early stage in your career. I would advise to find time and space to feel good and let go of the pressure you might feel in your work. I believe there is room for everyone, just keep the spirit up. When you feel it gets rough, stick with your plan and keep your relationship with your colleagues positive.

Nicolas Coltice. Credit: Nicolas Coltice.

 

Interview conducted by Anouk Beniest

Minds over Methods: What controls the shape of oceanic ridges?

Minds over Methods: What controls the shape of oceanic ridges?

In this edition of Minds over Methods, Aurore Sibrant, postdoc at Bretagne Occidentale University (France) explains how she studies the shape of oceanic ridges, and which parameters are thought to control this shape. By using laboratory experiments combined with observations from nature, she gives new insights into how spreading rates and lithosphere thickness influence the development of oceanic ridges. 

 

Credit: Aurore Sibrant

What controls the shape of oceanic ridges? Constraints from analogue experiments

Aurore Sibrant, Post-doctoral fellow at Laboratoire Géosciences Océans, Bretagne Occidentale University, France

Mid-oceanic ridges with a total length > 70 000 km, are the locus of the most active and voluminous magmatic activity on Earth. This magmatism directly results from the passive upwelling of the mantle and decompression melting as plates separate along the ridge axis. Plate separation is taken up primarily by magmatic accretion (formation of the oceanic crust), but also by tectonic extension of the lithosphere near the mid-ocean ridge, which modifies the structure of the crust and morphology of the seafloor (Buck et al., 2005). Therefore, the morphology of the ridge is not continuous but dissected by a series of large transform faults (> 100 km) as well as smaller transform faults, overlapping spreading centres and non-transform offsets (Fig. 1). Altogether, those discontinuities form the global shape of mid-ocean ridges. While we understand many of the basic principles that govern ridges, we still lack a general framework for the governing parameters that control segmentation across all spreading rates and induce the global shape of ridges.

Geophysical (Schouten et al., 1985; Phipps Morgan and Chen, 1993; Carbotte and Macdonald, 1994) and model observations (Oldenburg and Brune, 1975, Dauteuil et al., 2002, Püthe and Gerya, 2014) suggest that segmentation of oceanic ridges reflects the effect of spreading rate on the mechanical properties and thermal structure of the lithosphere and on the melt supply to the ridge axis. To understand the conditions that control the large-scale shape of mid-ocean ridges, we perform laboratory experiments. By applying analogue results to observations made on Earth, we obtain new insight into the role of spreading velocity and the mechanical structure of the lithosphere on the shape of oceanic ridges.

 

Laboratory experiments

The analogue experiment is a lab-scale, simplified reproduction of mid-oceanic ridges system. Our set-up yields a tank filled from bottom to top by a viscous fluid (analogous to the asthenosphere) overlain by the experimental “lithosphere” that can adopt various rheologies and a thin surface layer of salted water. This analogue lithosphere is obtained using a suspension of silica nanoparticles which in contact with the salted water emplaced on the surface of the fluid causes formation of a skin or “plate” that grows by diffusion. This process is analogous to the formation of the oceanic lithosphere by cooling (Turcotte and Schubert, 1982). With increasing salinity, the rheology of the skin evolves from viscous to elastic and brittle behaviour (Di Giuseppe et al., 2012; Sibrant and Pauchard, 2016).

The plate is attached to two Plexiglas plates moving perpendicularly apart at a constant velocity. The applied extension nucleates fractures, which rapidly propagate and form a spreading axis. Underlying, less dense, fresh fluid responds by rising along the spreading axis, forming a new skin when it comes into contact with the saline solution. By separately changing the surface water salinity and the velocity of the plate separation, we independently examine the role of spreading velocity and axial lithosphere thickness on the evolution of the experimental ridges.

 

Figure 2. Close up observations of analogue mid-oceanic ridges and schematic interpretation for different spreading velocity. The grey region is a laser profile projected on the surface of the lithosphere: the laser remains straight as long as the surface is flat. Here, the large deviation from the left to centre of the image reveals the valley morphology of the axis. Credit: Aurore Sibrant.

 

Analogue mid-oceanic ridges

Over a large range of spreading rates and salinities (Sibrant et al., 2018), the morphology of the axis is different in shape. The ridge begins with a straight axis (initial condition). Then during the experiment, mechanical instabilities such as non-transform offset, overlapping spreading centres and transform faults develop (Fig. 2) and cause the spreading axis to have a non-linear geometry (Fig. 3). A key observation is the variation of the shape of the analogue ridges with the spreading rate and salinities. For similar salinity and relative slow spreading rates, each segment is offset by transform faults shaping a large tortuous ridge (i.e. non-linear geometry). In contrast, at a faster spreading rate, the ridge axis is still offset by mechanical instabilities but remains approximately linear.

Figure 3. Ridge axis morphology observed in the experiments and schematic structural interpretations of the ridge axis, transform faults (orange ellipsoids) and non-transform faults (purple ellipsoids). Measurements of lateral deviation (LD) correspond to the length of the arrows. For comparison, white squares represent the size of closeup shows in Fig 2. Credit: Aurore Sibrant.

We can quantify the ridge shape by measuring the total lateral deviation, which is the total accumulated offset of the axis, when the tortuosity amplitude becomes stable. For cases with similar salinities, the results indicate two trends. First, the lateral deviation is high at slow spreading ridges and decreases within increasing spreading rate until reaching a minimum lateral deviation value for a given critical spreading rate (Fig 4A). Then the lateral deviation remains constant despite the increasing spreading rate. Experiments with different salinities also present a transition between tortuous and linear ridges. These two trends reflect how the lithosphere deforms and fails. In the first regime, the axial lithosphere is thick and is predominantly elastic-brittle. In such cases, the plate failures occur from the surface downwards through the development of faults: it is a fault-dominated regime. In contrast, for faster spreading rate or smaller salinities, the axial lithosphere is thin and is predominantly plastic. Laboratory inspection indicates that fractures in plastic material develop from the base of the lithosphere upwards: it is a fluid-intrusion dominated regime.

 

 

Comparison with natural mid-oceanic ridge

In order to have a complete understanding of the mid-oceanic ridge system, it is essential to compare the laboratory results with natural examples. Hence, we measure the lateral deviation of nature oceanic ridges along the Atlantic, Pacific and Indian ridges. The measurements reveal the same two regimes as found in laboratory data. The remaining step consists of finding the appropriate scaling laws to superpose the natural and experiment data. This exercise requires dynamics similarity between analogue model and real-world phenomena which is demonstrated using dimensionless numbers (Sibrant et al., 2018). Particularly, the “axial failure parameter – πF” describes the predominant mechanical behaviour of the lithosphere relative to its thickness. Low-πF accretion is dominated by fractures in a predominantly elastic-brittle lithosphere: the lateral deviation of the ridges is tortuous, while at higher pF, accretion is dominated by intrusion in a predominantly plastic lithosphere: the shape of the mid oceanic ridges is mostly linear (Fig 4B).

 

Figure 4. (A) Lateral deviation values measured in the experiments in function of the spreading rate velocities and salinities. (B) Evolution of the lateral deviation of the ridge axis, normalized by the critical axial thickness (Zc) relative to the axial failure parameter. Dark grey is the laboratory experiments and the colored circles are the Earth data. Adapted from Sibrant et al., 2018.

 

Our experiments give insight into the role of axial failure mode (fault-dominated or intrusion-dominated) on the shape of mid-oceanic ridges. In the future, we want to use this experimental approach to investigate the origin of mechanical instabilities, such as transform faults or overlapping spreading centres. This experimental development and results are a collaborative work between Laboratoire FAST at Université Paris-Saclay and Department of Geological Sciences at the University of Idaho and involves E. Mittelstaedt, A. Davaille, L. Pauchard, A. Aubertin, L. Auffray and R. Pidoux.

 

 

References
Buck, W.R., Lavier, L.L., Poliakov, A.N.B., 2005. Modes of faulting at mid-ocean ridges. Nature 434, 719-723.
Schouten, H., Klitgord, K.D., Whitehead, J.A., 1985. Segmentation of mid-ocean ridges. Nature 317, 225-229.
Carbotte, S.M., Macdonald, K. C., 1994. Comparison of seafloor tectonic fabric at intermediate, fast, and super fast spreading ridges: Influence of spreading rate, plate motions, and ridge segmentation on fault patterns. J. Geophys. Res. 99, 13609-13631.
Phipps Morgan, J., Chen, J., 1993. Dependence of ridge-axis morphology on magma supply and spreading rate. Nature 364, 706-708.
Oldenburg, D.W., Brune, J.N., 1975. An explanation for the orthogonality of ocean ridges and transform faults. J. Geophys. Res. 80, 2575-2585.
Dauteuil, O., Bourgeois, O., Mauduit, T., 2002. Lithosphere strength controls oceanic transform zone structure: insights from analogue models. Geophys. J. Int. 150, 706-714.
Püthe, C., Gerya, T., 2014. Dependence of mid-ocean ridge morphology on spreading rate in numerical 3-D models. Gondwana Res. 25, 270-283.
Turcotte, D., Schubert, G., Geodynamics (Cambridge Univ. Press, New York, 1982).
Di Giuseppe, E., Davaille, A., Mittelstaedt, E., Francois, M., 2012. Rheological and mechanical properties of silica colloids: from Newtonian liquid to brittle behavior. Rheologica Acta 51, 451-465.
Sibrant, A.L.R., Pauchard, L., 2016. Effect of the particle interactions on the structuration and mechanical strength of particulate materials. European Physics Lett., 116, 4, 10.1209/0295-5075/116/49002.
Sibrant, A.L.R., Mittelstaedt, E., Davaille, A., Pauchard, L., Aubertin, A., Auffray, L., Pidoux, R., 2018. Accretion mode of oceanic ridges governed by axial mechanical strength. Nature Geoscience 11, 274-279.

 

Minds over Methods: Experimental seismotectonics

Minds over Methods: Experimental seismotectonics

For our next Minds over Methods, we go back into the laboratory, this time for modelling seismotectonics! Michael Rudolf, PhD student at GFZ in Potsdam (Germany), tells us about the different types of analogue models they perform, and how these models contribute to a better understanding of earthquakes along plate boundaries.

 

Credit: Michael Rudolf

Experimental seismotectonics – Seismic cycles and tectonic evolution of plate boundary faults

Michael Rudolf, PhD student at Helmholtz Centre Potsdam – German Research Centre for Geosciences GFZ

The recurrence time of large earthquakes that happen along lithospheric-scale fault zones such as the San Andreas Fault or Chile subduction megathrusts, is very long (≫100 yrs.) compared to human timescales. The scarcity of such events over the instrumental record of around 60 years is unfortunate for a statistically sound analysis of the earthquake time series.

So far, only few megathrust events have been monitored in detail with near-field seismic and geodetic networks. To circumvent this lack of observational data, we at Helmholtz Tectonic Laboratory use analogue modelling to understand plate boundary faulting on multiple time-scales and the implications for seismic hazard. We use models of strike-slip zones and subduction zones, to investigate several aspects of the seismic cycle. Additionally, numerical simulations accompany and complement each experimental setup using experimental parameters.

 

Seismotectonic scale models
In my project, we develop experiments that can model multiple seismic cycles in strike-slip conditions. Our study employs two types of experimental setups both are using the same materials. The first is simpler (ring shear setup) and is able to show the on-fault rupture propagation. The second is geometrically more similar to the natural system, but only the surface deformation is observable.

To model rupture propagation, we introduce deformable sliders in a ring shear apparatus. Two cylindrical shells of ballistic gelatine (Ø20 cm), representing the side walls, rotate against each other, with a thin layer (5 mm) of glass beads (Ø355-400µm) in between representing an annular fault zone. A see-through lid connected to force sensors holds the upper shell in place, whereas the machine rotates the lower shell. Through the transparent lid and upper shell, we directly observe the fault slip. We can vary the normal stress on the fault (<20 kPa) and the loading velocity (0.0005 – 0.5 mm/s).

The next stage of analogue model, features depth-dependent normal stress and a rheological layering mimicking the strike-slip setting in the uppermost 25-30 km of the lithosphere (see also Mehmet Köküm’s blog post). A gelatine block (30x30cm) compressed in uniaxial setting represents the elastic upper crust under far-field forcing. Embedded in the block is a thin fault filled with quartz glass beads. The ductile lower crust is modelled using viscoelastic silicone oil. The model floats in a tank of dense sugar solution, to guarantee free-slip, stress-free boundaries.

 

Figure 2 – Setup and monitoring technique during an experiment. Several cameras record the displacement field and the ring shear tester records the experimental results. Credit: Michael Rudolf

 

Analogue earthquakes
Both setups generate regular stick-slip cycles including minor creep. Long phases of quiescence, where no slip or very slow creep occurs, alternate with fast slip events sometimes preceded by slow slip events. The moment magnitude of analogue earthquake events is Mw -7 to -5. The cyclic recurrence of slip events is an analogue for the natural seismic cycle of a single-fault system.

 

Figure 3 – Detailed setup and results from the ring shear tester experiments. The upper right image shows a snapshot of an analogue earthquake rupture along the fault zone. The plot shows the recorded shear forces and slip velocities over one hour of experiment. Credit: Michael Rudolf

 

Optical cameras record the slip on the fault and the deformation of the sidewalls. Using digital image correlation techniques, we are able to visualize accurately deformations on the micrometre scale at high spatial and temporal resolution. Accordingly, we can verify that analogue earthquakes behave kinematically very similar to natural earthquakes. They generally nucleate where shear stress is highest, and then propagate radially until the seismogenic width is saturated. In the end, the rupture continues laterally along the fault strike. Our experiments give insight into the role of viscoelastic relaxation, interseismic creep, and slip transients on the recurrence of earthquakes, as well as the control of loading conditions on seismic coupling and rupture dynamics.

 

Figure 4 – Setup and Results for the strike-slip geometry. The surface displacement field is very similar to natural earthquakes. The plot shows that due to technical limitations of this setup, fewer events are recorded but the slip velocities are higher. Credit: Michael Rudolf

 

Future developments
Together with our partners in the Collaborative Research Centre (CRC1114 – Scaling Cascades in Complex Systems) we employ a new mathematical and numerical description of the fault system, to simulate our experiments and get a physical understanding of the empirical friction laws. In the future, we want to use this multiscale spatial and temporal approach to model complex fault networks over many seismic cycles. The experiments serve as benchmarks and cross-validation for the numerical code, which in the future will be using natural parameters to get a better geological and mathematical understanding of earthquake slip phenomena and occurrence patterns in multiscale fault networks.

Minds over Methods: Block modeling of Anatolia

 

How can we use GPS velocities to learn more about present-day plate motions and regional deformation? In this edition of Minds over Methods, one of our own blogmasters Mehmet Köküm shares his former work with you! For his master thesis at Indiana University, he used block modeling to better understand the plate motion and slip rates of Anatolia and surrounding plates.

 

credit: Mehmet Köküm

Using block modeling to constrain present-day deformation of Anatolia and slip rates along the North Anatolian Fault

Mehmet Köküm, researcher at Firat University, Turkey

Until the late 1980’s, geological features such as offset of geomorphological markers were mainly used to determine historical slip rates along faults. Since the mid 1990’s, however, GPS has been widely used since it gives more accurate estimates of present-day slip rates by calculating strain accumulation at the crust. In this work, I use a GPS derived velocity field of Anatolia including data from 1988 to 2005 by Reilinger et al. (2006).

Turkey (Anatolian Plate) is located in the center of the Alpine fold and thrust belt. Due to the closure of different branches of the Neo-Tethys Ocean, main tectonic features of the Anatolian Plate are complicated by interactions between several tectonic plates.  The Arabian plate collides with the African plate in the south and the Eurasian plate in the north while the African plate subducts beneath the Anatolian plate along the Hellenic-Cyprus trench. As a result of these complex tectonic structures, the Anatolian plate displays various tectonic styles simultaneously.

Modeling and Data
Kinematic block modeling of interseismic surface motions has been used in different formats by several authors (e.g., McClusky et al. 2000; Westaway 2000; Barka and Reilingier 1997, 2006). The block modeling approach used here is described by Johnson and Fukuda (2010). In this study we used an elastic block model, which is a traditional block model that assumes no long-term deformation of the blocks. For simplicity, all faults are vertical, plates are considered as blocks and are assumed to be rigid. Block boundaries are defined from historic earthquakes, mapped faults and seismicity. Many of the major structures in Anatolia are well known except for a few submarine structures.

 

Map showing selected block model including of 14 blocks (or plates). Credit: Mehmet Köküm

 

Locking Depth
Locking depths indicate the depth for which a fault is completely locked above and creeping below. Estimates of these locking depths are output of the modeling studies and should correlate with the depth of major earthquakes along related faults. Meade and Hager (2005) suggest that there is a relation between locking depth and fault slip rates. Shallower locking depths correlate with slower slip rate estimates; therefore, GPS velocities near locked faults have slower velocities (Reilinger et al., 2006).

 

Elastic-half-space model showing fault creep at surface, locked (nonslipping) fault at depth, and freely sliding zone at great depth. (source: SFSU CREEP Project)

 

Results
On the basis of the GPS velocity field, the Anatolia and Aegean blocks show counterclockwise motion with respect to the Eurasian plate and the rate of the motion increases towards the west. The locking depth variations of the work are between 20-25 km, which correlates with the focal depths of significant earthquakes. The major fault slip rates are consistent with some of the geological slip rate estimates.

 

Results of the model. Figure shows Anatolian plate motion and slip rate estimates of major faults. Credit: Mehmet Köküm

Minds over Methods: Reconstruction of salt tectonic features

Minds over Methods: Reconstruction of salt tectonic features

What is the influence of salt tectonics on the evolution of sedimentary basins and how can we reconstruct such salt features? Michael Warsitzka, PhD student at the Friedrich Schiller University of Jena, explains which complementary methods he uses to better understand salt structures and their relation to sedimentary basins. Enjoy!

 

Credit: Michael Warsitzka

Reconstruction of salt tectonic features from analogue models and geological cross-sections

Michael Warsitzka, PhD student, Institute of Geosciences, Friedrich Schiller University Jena

Salt tectonics, as a sub-discipline of structural geology, describe deformation structures developing due to the special deformation behaviour of salt (as synonym for a sequence of evaporitic rocks). Salt behaves like a viscous fluid over geological time scales and, therefore, it may flow due to lateral differences in thickness and density of the supra-salt layers. This influences the structural evolution of sedimentary basins, because salt flow can modify the amount of regional subsidence of the basin. Local sinks (“minibasins”) develop in regions from where salt is squeezed out and salt structure uplifts, e.g. diapirs or pillows evolve in regions of salt influx. Unfortunately, temporal changes of salt flow patterns are often difficult to reconstruct owing to enigmatic ductile deformation structures in salt layers. Understanding the evolution of salt-related structures requires either forward modelling techniques (e.g. physically scaled sandbox experiments) or restoration of sedimentary and tectonic structures of the supra-salt strata.

In my PhD thesis, I tried to integrate both, analogue modelling and restoration, to investigate salt structures and related minibasins developed in the realm of extensional basins. The sandbox model is a lab-scale, simplified representative of natural salt-bearing grabens, e.g. the Glückstadt Graben located in the North German Basin (Fig. 1). A viscous silicone putty and dry, granular sand were used to simulate ductile salt and brittle overburden sediments. Cross sections were cut through the model at the end of each experiment to conduct reconstruction of the final experimental structures. The material movements were monitored with a particle tracking velocimetry (PIV) technique at the sidewalls of the experimental box.

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Fig 1: 2D restoration of the supra-salt (post-Permian) strata in the Glückstadt Graben (Northern Germany). Credit: Michael Warsitzka

Using experimental and geological cross sections, structures in the overburden of the ductile layer can be reconstructed, if present-day layer geometries and lithologies of the overburden strata can be identified. From natural clastic and carbonatic sediments we know that they compact with burial, reducing the layer thickness. Therefore, the reconstruction procedure sequentially removes the uppermost layer and layers beneath are decompacted and shifted upwards to a horizontal surface (Fig. 2). The sequence of decompaction and upward shifting is then repeated until the earliest, post-salt stage is reached (Fig. 1). It intends to restore the initial position, shape and thickness of each reconstructed layer.

In analogue experiments, no decompaction is necessary, because the compressibility of the granular material is insignificant for depths of a few centimetre. Restoration can be directly applied to coloured granular layers revealing detailed layer geometries for each experimental period (Fig. 2a). The PIV technique displays coeval material movement and strain patterns occurring during the subsidence of the experimental minibasins (Fig. 2b). Based on the observation that the experimental structures resemble those reconstructed from the natural example (Glückstadt Graben during the Early Triassic, Fig. 1), it can be inferred that strain patterns observed in the experiments took place in a similar manner during the early stage of extensional basins. This demonstrates the advantage of applying both methods. First, original geometries of basin structures can be determined from the restoration and then reproduced in the model. If the restored geometries are suitably validated by the models, the kinematics observed in the model can be translated back to nature and help to understand the effect of salt flow on the regional subsidence pattern.

Fig 2: Result of an analogue model showing (a) reconstructed sand layers restored from a central cross section, and (b) monitored displacement and strain patterns in the viscous layer above the left basal normal fault. Credit: Michael Warsitzka

Minds over Methods: Sensing Earth’s gravity from space

Minds over Methods: Sensing Earth’s gravity from space

How can we learn more about the Earth’s interior by going into space? This edition of Minds over Methods discusses using satellite data to study the Earth’s lithospere. Anita Thea Saraswati, PhD student at the University of Montpellier, explains how information on the gravity of the Earth is obtained by satellites and how she uses this information to get to know more about the lithosperic structure in subduction zones.

 

Sensing Earth’s gravity from space

Anita Thea Saraswati – PhD student, Géosciences Montpellier

From the basic physics we all know that the value of the gravity is a constant 9.81 meter per second squared. This assumption would be true if the Earth were a smooth nonrotating spherical symmetric body made of uniform element and material. However, because of the Earth’s rotation, internal lateral density variation, and the diversity of the topography (including mountains, valleys, oceans and glaciers), the gravity  varies all over the surface. These tiny changes in gravity due to the mass variations could be a crucial hint for understanding the structure of the Earth, both on the surface and at depth.

The determination of Earth’s gravity field has benefited from various gravity satellite missions that have been launched recently. Among them are the Challenging Minisatellite Payload (CHAMP) (2000-2010), the Gravity Recovery and Climate Experiment (GRACE) (2002-recent), and most recently the Gravity field and steady-state Ocean Circulation Explorer (GOCE) (2009-2013). From these missions, finally a global high quality coverage of Earth’s gravity field became available. (Yay!)

GRACE observation data are very useful for the temporal analysis of changes in gravity. For example to detect the gravity signal before and after a big earthquake, like the Sumatra Mw 9.1 (2004) and Tohoku Mw 9.1 (2011) ones. By analyzing the changes of gravity signal during a certain period of time, it could also be used to detect the drought over a large scale area, which is used in several areas in Africa and Australia.

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Design of GOCE satellite observation. A geoid’s shape is showed on the bottom left. On the top right, the GOCE gravity gradients in six components. (Source : ESA)

 

Meanwhile, GOCE is very suitable for the construction of a static model of Earth’s gravity field. Since this satellite has a very low orbit, ~250 km above mean sea level, it has a better spatial resolution. Its accuracy is also better than the previous missions, up to 1 mGal. GOCE is equipped with a gradiometer, which measures the gravity acceleration in three directions (x, y, and z). Afterwards this information is processed into a gravity-gradient dataset containing six components (XX, XY, XZ, YY, YZ, ZZ).

This gravity gradient is the first derivative of the gravity acceleration, which provides us better information about the geometry of the earth’s structure than the gravity acceleration itself. For my PhD, I use this gravity gradient dataset to analyze the lithospheric structure of subduction zones. Before treating the GOCE observation data, I am developing a computational code to calculate the gravity and gravity gradient due to the effect of topography, also called the topographic reduction. The observed gravity and gravity gradient values will be reduced by this topography effect in order to get the anomaly signal. This means that only the signal due to other geodynamic phenomena over the observed area (e.g. slab, isostasy, mantle plum, etc.) is left. By doing further processing, we can obtain the lateral variations of the lithospheric structure in the study areas and then investigate the correlation with the occurrence of mega-earthquakes in these subduction zones.

Since there is still some ambiguity about the information that is produced by gravity data only, it is better to combine the use of them with others geophysical or geological measurements, e.g. seismic tomography measurements and magnetic field observations.

 

Global coverage of GOCE gravity gradient (in milliEötvös) in radial direction (ZZ) (Panet, I. et al., 2014)

 

Reference:

Panet, I., Pajot-Métivier, G., Greff-Lefftz, M., Métivier, L., Diament, M. and Mandea, M., 2014. Mapping the mass distribution of Earth/’s mantle using satellite-derived gravity gradients. Nature Geoscience7(2), pp.131-135.

Minds over Methods: studying dike propagation in the lab

Minds over Methods: studying dike propagation in the lab

Have you ever thought of using gelatin in the lab to simulate the brittle-elastic properties of the Earth’s crust? Stefano Urbani, PhD student at the university Roma Tre (Italy), uses it for his analogue experiments, in which he studies the controlling factors on dike propagation in the Earth’s crust. Although we share this topic with our sister division ‘Geochemistry, Mineralogy, Petrology & Volcanology (GMPV)’, we invited Stefano to contribute this post to ‘Minds over Methods’, in order to show you one of the many possibilities of analogue modelling. Enjoy!

 

dscn0024Using analogue models and field observations to study the controlling factors for dike propagation

Stefano Urbani, PhD student at Roma Tre University

The most efficient mechanism of magma transport in the cold lithosphere is flow through fractures in the elastic-brittle host rock. These fractures, or dikes, are commonly addressed as “sheet-like” intrusions as their thickness-length aspect ratio is in the range of 10-2 and 10-4 (fig.3).

Understanding their propagation and emplacement mechanisms is crucial to define how magma is transferred and erupted. Recent rifting events in Dabbahu (Afar, 2005-2010) and Bardarbunga (Iceland, 2014, fig.1) involved lateral dike propagation for tens of kilometers. This is not uncommon: eruptive vents can form far away from the magma chamber and can affect densely populated areas. Lateral dike propagation has also been observed in central volcanoes, like during the Etna 2001 eruption. Despite the fact that eruptive activity was mostly fed by a vertical dike to the summit of the volcano, several dikes propagated laterally from the central conduit and fed secondary eruptive fissures on the southern flank of the volcanic edifice (fig.2). Lateral propagation can hence occur at both local (i.e. central volcanoes) and regional (i.e. rift systems) scale, suggesting a common mechanism behind it.

fig-3mario-cipollini

Fig. 2 Lava flow near a provincial road, a few meters from hotels and souvenir shops, during the 2001 lateral eruption at Etna. Credit: Mario Cipollini

Therefore, it is of primary importance to evaluate the conditions that control dike propagation and/or arrest to try to better evaluate, and eventually reduce, the dike-induced volcanic risk. Our knowledge of magmatic systems is usually limited to surface observations, thus models are useful tools to better understand geological processes that cannot be observed directly. In particular, analogue modelling allows simulating natural processes using scaled materials that reproduce the rheological behavior (i.e ductile or brittle) of crust and mantle. In structural geology and tectonics analogue modelling is often used to understand the nature and mechanism of geological processes in a reasonable spatial and temporal scale.

d_grad_dike57_080Field evidence and theoretical models indicate that the direction of dike propagation is controlled by many factors including magma buoyancy and topographic loads. The relative weight of these factors in affecting vertical and lateral propagation of dikes is still unclear and poorly understood. My PhD project focuses on investigating the controlling factors on dike propagation by establishing a hierarchy among them and discriminating the conditions favoring vertical or lateral propagation of magma through dikes. I am applying my results to selected natural cases, like Bardarbunga (Iceland) and Etna (Italy). To achieve this goal, I performed analogue experiments on dike intrusion by injecting dyed water in a plexiglass box filled with pig-skin gelatin. The dyed water and the gelatin act as analogues for the magma and the crust, respectively. Pig-skin gelatin has been commonly used in the past to simulate the brittle crust, since at the high strain rates due to dike emplacement it shows brittle-elastic properties representative of the Earth’s crust. We record all the experiments with several cameras positioned at different angles, taking pictures every 10 seconds. This allows us to make a 3D reconstruction of the dike propagation during the experiment.

In order to have a complete understanding of the dike intrusion process it is essential to compare the laboratory results with natural examples. Hence, we went to the field and studied dikes outcropping in extinct and eroded volcanic areas, with the aim of reconstructing the magma flow direction (Fig. 3). This allows validating and interpreting correctly the observations made during the laboratory simulations of the natural process that we are investigating.

fig-1

Fig. 3 Outcrop of dikes intruding lava flows. Berufjordur eastern Iceland.

 

Minds over Methods: Numerical modelling

Minds over Methods: Numerical modelling

Minds over Methods is the second category of our T&S blog and is created to give you some more insights in the various research methods used in tectonics and structural geology. As a numerical modeller you might wonder sometimes how analogue modellers scale their models to nature, or maybe you would like to know more about how people use the Earth’s magnetic field to study tectonic processes. For each blog we invite an early career scientist to share the advantages and challenges of their method with us. In this way we are able to learn about methods we are not familiar with, which topics you can study using these various methods and maybe even get inspired to use a multi-disciplinary approach! This first edition of Minds over Methods deals with Numerical Modelling and is written by Anouk Beniest, PhD-student at IFP Energies Nouvelles (Paris).

 

Approaching the non-measurable

Anouk Beniest, PhD-student at IFP Energies Nouvelles, Paris

‘So, what is it that you’re investigating?’ It’s a question every scientist receives from time to time. In geosciences, the art of answering this question is to explain the rather abstract projects in normal words to the interested layman. Try this for example: “A long time ago, the South American and African Plate were stuck together, forming a massive continent, called Pangea, for many millions of years. Due to all sorts of forces, the two plates started to break apart and became separated. During this separation hot material from deep down in the earth rose to the surface increasing the temperature of the margins of the two continents. How exactly did this temperature change over time, since the separation until present-day? How did this change affect the basins along continental margins?”

These are legitimate questions and not easy to answer, since we cannot measure temperature at great depth or back in time. In this first post on numerical methods, we will be balancing between geology and geophysics, highlighting the possibilities and limits of numerical modelling.

The migration of ‘temperature’ through the lithosphere is a process that takes time and depends heavily on the scale you look at. Surface processes that affect the surface temperature can be measured and monitored, yielding interesting results on the present-day state and variations of the temperature. The influence of mantle convection cycles and radiogenic heat production are already more difficult to identify, take much more time to evolve and might not even affect the surface processes that much. Going back in time to identify a past thermal state of the earth seems almost impossible. This is where numerical models can be of use, to improve, for example, our understanding on the long-term behaviour of ‘temperature’.

Temperature is a parameter that affects and is affected by a variety of processes. When enough physical principles are combined in a numerical model, we can simulate how the temperature has evolved over time. All kinds of different parameters need to be identified and, most importantly, they need to make sense and apply to the observation or process you try to reproduce. Some of these parameters can be identified in the lab, like the density or conductivity of different rock types. Others need to be extracted from physical or geological observations or even estimated.

Once the parameters have been set, the model will calculate the thermal evolution. It is not an easy task to decide if a simulation approaches the ‘real’ history and if we can answer the questions posed above. We should always realise that thermal model results at best approach the real world. We can learn about the different ways temperature changes over time, but we should always be on the hunt to find measurements and observations that confirm what we have learned from the simulations.

temperature_quick