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Tectonics and Structural Geology

Tectonics and Structural Geology

Minds over Methods: Massively dilatant faults in Iceland – from surface to subsurface structures

Minds over Methods: Massively dilatant faults in Iceland – from surface to subsurface structures
In this Minds over Methods we don’t have one, but two scientists talking about their research! Michael Kettermann and Christopher Weismüller, both from Aachen University, explain us about the multidisciplinary approach they use to understand more about massively dilatant faults. How do they form and what do they look like at depth?

Massively dilatant faults in Iceland – from surface to subsurface structures

Michael Kettermann & Christopher Weismüller, RWTH Aachen University

Michael (left) and Christopher (right) in the field. Credit: Michael Kettermann and Marianne Sophie Hollinetz.

Iceland is a volcanic island in a unique setting on the Mid-Atlantic Ridge, separating the Eurasian and North American plates. A deep mantle plume lies beneath Iceland, and the combination of rift and plume leads to very active basaltic volcanism. Ubiquitous features along the rift zone are normal faults, often exquisitely exposed at the surface. Normal faults in basalts are also common in many volcanic provinces, like Hawaii, the East African Rift, and along mid ocean ridges. These faults often form as massively dilatant faults (MDF), which show apertures up to tens of meters at the surface and supposedly have large volumes of open voids in the subsurface.

Figure 1. View along-strike a massively dilatant fault in layered basalt. The geometry of the vertical fault faces is prescribed by the cooling joints. Several basalt columns have been loosened and dropped into the fault, now being stuck in the fault (top) or filling the cavity (bottom). Opening width < 3 m. Credit: Michael Kettermann.

These openings form pathways for fluids like magma or hydrothermal waters and consequently are of importance for volcanic plumbing systems, mineralization and geothermal energy supply.

Iceland provides a perfect natural laboratory to study MDF. Due to its position on the Mid-Atlantic Ridge, Iceland is cut by extensional fault systems roughly from southwest to north. A wide range of oblique extensional to pure extensional faults can be observed mostly in flood basalts, but also in sub-glacially formed hyaloclastites (weaker volcanic sediments), pillow lavas and occasionally sediment layers formed in warmer times. Outcropping rocks in Iceland are younger than 20 Ma distal from the rift (eastern and western Iceland), while tectonic and corresponding volcanic activity at the ridge (central Iceland) constantly causes the formation of new rocks. The rough climate hinders soil formation and vegetation to overgrow faults, providing unique outcrop conditions.

While it is relatively easy to access and study the faults at surface level, investigations into the subsurface are much more challenging. Direct observations are only possible down to depths of some tens of meters by climbing into the fractures. Cavities are often filled with rubble, sediments, water or snow (Fig. 1). Steep, open fractures with meter-scale aperture are hard to detect with geophysical methods (seismic reflection/refraction, ground penetrating radar, electrical resistivity tomography) at depths greater than some meters.

We therefore started the massively dilatant fault project, a multidisciplinary, integrated project bringing together remote sensing, fieldwork, analogue modelling and numerical simulations. In essence, we utilize a modelling approach to recreate the structure and evolution of MDF at depth, using real 3D surface data as input and comparison data set.

 

Drone mapping and photogrammetry

In a first step, we capture and analyse the surface expressions of MDF at a number of representative fault areas in Iceland. To this end, we flew 27 drone surveys during our five weeks long field season in summer 2017, covering a total length of more than 42 km of faults. Luckily, in the Icelandic summer the days are very long, so the National Park Service allowed us to fly the drones early in the morning and late in the evening outside of tourist hours. For each area, we took several hundred to thousands of overlapping photographs (e.g. Fig. 2). We processed these sets with photogrammetry software, applying the Structure from Motion (SfM) technique. SfM is an increasingly popular, fast and cheap technique to reconstruct high-resolution 3D information from 2D images. This allows us to recreate digital elevation models and ortho-rectified photo-mosaics of the faults in resolutions better than 15 cm per pixel (Fig. 3). The largest area at the famous Thingvellir fissure swarm covers a length of almost 7 km with an average resolution of 11 cm per pixel.

We use these digital elevation models and ortho-photos to retrieve a wide range of structural data. Mapping the fault traces in a GIS software allows for the measurement of fault opening width, throw, orientation and length. From throw and aperture, we can then estimate the fault dip at depth. Digital elevation models further provide surface dip data that we then compare with observations from analogue models.

Figure 2. A drone photograph facing South of the Almannagjá fault in Thingvellir, where the Thingvallavegur road crosses the fault. The Almannagjá fault resembles the western shoulder of the Thingvellir graben system with locally > 50 m opening width and 40 m vertical offset. Credit: Christopher Weismüller. .

Figure 3. Digital elevation model created from drone photographs using photogrammetry software. It contains the faults at Sandvik on the Reykjanes Peninsula (SW Iceland). The detail panes (red square) show the DEM (right) at a higher zoom level and the corresponding ortho-rectified photograph (left). The bridge crossing the fault depicted in the detail panes is a famous touristic spot, known as „The bridge between the continents“, since the fault symbolically divides the North American and Eurasion plates. Credit: Michael Kettermann.

 

 

 

 

 

 

 

Figure 4. Sideview of an analogue model showing three timesteps of the development of a massivley dilatant fault and associated fractures in hemihydrate (Bücken, 2017). Note the tilted block developing at the surface of the model and the dilatant jogs and voids in the subsurface. The opening at the surface is not directly linked to the fault at depth, but caused by the rotation of the tilted block. Credit: Daniel Bücken..

Modelling approach

For the analogue modelling approach on the hundreds to thousand meter scale, we use cohesive powders as modelling material (Bücken, 2017). Especially hemihydrate powder has been proven suited to model dilatant fractures (Holland et al., 2006; Kettermann et al., 2016; van Gent et al., 2010) as it has a well characterized true cohesion and tensile strength. Faults in Iceland transform from opening mode fractures to shear mode faults at depth when overburden stress is high enough. As we are interested in the upper dilatant parts of the faults, i.e. above the shear mode faulting, we chose a basement-fault controlled approach, where a rigid basement represents the shear mode fault. It moves down-dip along a predefined surface, deforming the powder sieved on top. The basement fault dip follows the data we derived from the field and is set to 60° – 65°. The scale of the models calculates from strength and weight of the natural prototype and the modelling material. 1 cm of powder equals about 50 m of basalt.

 

Comparison of models and nature

Results show a close similarity between field and experiment at the surface structures. Open fractures form with large apertures at the surface and often we observe the formation of tilted blocks (Fig. 4). The existence and scaled dimensional similarity of fractures and tilted blocks in the field and using a scaled material suggest a validity of other observations in the models. Glass sidewalls in the analogue models provide the opportunity to examine how the faults evolve at depth. We observe that large caves form underneath these blocks and we predict that these must exist in the field as well, albeit potentially filled with rubble. Our models corroborate earlier predictions that extensional faults are open down to 800 – 1000 m (Gudmundsson and Bäckström, 1991). We also learned that below that, a hybrid failure zone exists where dilational jogs, open extensional fractures between shear mode faults, provide lateral pathways for magma or water, even at depths where the overburden stress prevents the formation of purely extensional faults.

 

Outlook

The previously shown experiments investigated MDF at a larger scale in purely dip-slip kinematics. However, faults at rifts often have strike-slip components, forming normal faults with oblique kinematics. In further experiments, we therefore explored the effect of varying basement fault obliquities, i.e. the range between dip-slip normal faults and strike-slip faults (Bitsch, 2017). As expected, early phases of faulting are dominated by Riedel shears. Surprisingly, the surface structure of mature faults, however, does not change distinctly up to obliquities of 60°, but the subsurface connectivity decreases with increasing obliquity.

Figure 5. Analogue model resembling successive layers of lava flows with cooling joints created by carefully stacking several layers of dried corn starch slurry (Winhausen, 2018). The dip of the basement fault is prescribed by the apparatus. The fault geometry generated in the model is very similar to the ones observed in Iceland. Large cavities develop and are partially refilled by loosened columns, as shown in figure 1. Tensile fractures develop on the surface of the footwall, similar to the hemihydrate model and the field. Credit: Lisa Winhausen.

Zooming in on the faults, an inherent mechanical anisotropy (orthotropy) of basalts gains more influence on the macroscale structure of faults. Due to the shrinking during cooling of flood basalts, polygonal to blocky columns form and present regular weak zones in the rockmass. Introducing mechanical anisotropy into a stronger modelling material (dried corn-starch slurry) beautifully illustrates how the small-scale structure of the faults is affected by the layering of flood basalts and cooling fractures therein (Fig. 5; Winhausen, 2018). Close to the surface the strong basalt does not fracture, but propagating faults rather localize at the pre-existing cooling joints. This causes a jagged structure of the fault, formation of caves, and eroded basalt columns filling the opening fractures.

We are currently working on implementing all these learning points into discrete element simulations, where we can adjust material properties in a way that allows for modelling deeper parts of the faults with better mechanical control.

 

 

References

Bitsch, N.D., 2017. Massively dilatant faults in oblique rift settings – an analogue modeling study (MSc Thesis). RWTH Aachen University, Germany, Aachen.

Bücken, D.H., 2017. Effect of mechanical stratigraphy on normal fault evolution – Insights from analogue models and natural examples in Iceland (MSc Thesis). RWTH Aachen University.

Gudmundsson, A., Bäckström, K., 1991. Structure and development of the Sveinagja graben, Northeast Iceland. Tectonophysics 200, 111–125. https://doi.org/10.1016/0040-1951(91)90009-H

Holland, M., Urai, J.L., Martel, S., 2006. The internal structure of fault zones in basaltic sequences. Earth Planet. Sci. Lett. 248, 301–315. https://doi.org/10.1016/j.epsl.2006.05.035

Kettermann, M., von Hagke, C., van Gent, H.W., Grützner, C., Urai, J.L., 2016. Dilatant normal faulting in jointed cohesive rocks: a physical model study. Solid Earth 7, 843–856. https://doi.org/10.5194/se-7-843-2016

van Gent, H.W., Holland, M., Urai, J.L., Loosveld, R., 2010. Evolution of fault zones in carbonates with mechanical stratigraphy – Insights from scale models using layered cohesive powder. J. Struct. Geol. 32, 1375–1391. https://doi.org/10.1016/j.jsg.2009.05.006

Winhausen, L., 2018. Influence of columnar joints on normal fault geometry and evolution An analog modeling study Master Thesis (MSc thesis). RWTH Aachen University.

Meeting Plate Tectonics – Walter Roest

Meeting Plate Tectonics – Walter Roest

These bi-weekly blogs present interviews with outstanding scientists that bloomed and shape the theory that revolutionised Earth Sciences — Plate Tectonics. Stay tuned to learn from their experience, to discover the pieces of advice they share, to find out where the newest challenges lie, and much more!


Meeting Walter Roest 


Walter Roest was born in Dordrecht, The Netherlands. He has had an impressive international career that started with an MSc in Physics and then Geophysics at Utrecht University in the Netherlands. He was the last one to obtain a PhD in Marine Geophysics from the Vening Meinesz laboratory for Marine Geophysics at Utrecht University, which closed afterwards. His career continued in Halifax, Canada where he contributed to geophysical data processing and interpretation, and subsequently in Ottawa, where he spent 12 years of his career in aeromagnetics. Since 2002 he is based at IFREMER in Brest, France, where he is active as a Marine Geophysicist.

 

Walter Roest – Credit: IFREMER annuaire

Walter, what was your reason to go into Earth Sciences? 

As a young adolescent, I wanted to become a physics teacher. That was my main reason to start with my studies in Physics in 1976. In 1978 an advertisement for a scientific cruise appeared. I applied and was allowed to embark, but unfortunately, the cruise got cancelled. There was another opening in 1979, which was aborted after a fire in the engine room. As a result, as an undergraduate, I had no real plan for about 6 months, until they proposed me to participate in the construction of a seismic streamer for the laboratory. After that, I was convinced that I wanted to work at sea. I got some opportunities abroad, so I basically dropped my physics-teacher wishes and continued in Geosciences.

Throughout my career I have never really planned anything, I never had any clear expectations neither

When you were very early in your career as a scientist, what kind of expectations did you have?

Throughout my career I have never really planned anything, I never had any clear expectations neither. Opportunities arose, in my case not in the Netherlands but in Canada and so I moved continents. I left the data acquisition at sea for a while. When I didn’t find a job after my PostDoc position I got the opportunity to go into aeromagnetics. Many years later, when I saw an advertisement for a position at IFREMER, the French marine research institute, I just applied. I thought I didn’t have any chance, but I was lucky enough to get the position! My career has been mainly a concatenation of events that happened.

It is very important to have knowledge on how data is collected.

What research interests, approaches and methods did you develop during your career?

Müller, D., et al., 2008. Geochemistry, Geophysics, Geosystems, 9. Q04006.

My research interests lie within global tectonics, using empirical research tools that are closely connected to data. It is very important to have knowledge on how data is collected. I try therefore to go on a research cruise at least once a year, so I stay updated about the newest data acquisition and processing techniques. I’m not so much interested in very detailed processes, but I’d rather try to understand the large scale tectonic setting of an area.

 

You have been around, working in quite some different fields. What accomplishment in your career are you most proud of?

Interesting question! I think I’m most proud of the Müller et al., 2008 paper I co-authored. It was published in G-cubed. We started this project in 1987 with a first edition of the ‘digital global plate tectonics map of the world’ in 1997. It basically took 20 years of work and I think the publication is a fantastic result, used and cited by many researchers. It shows that hard work pays off!

As soon as you can, start international collaborations […] they give you a different view on the world.

After all these years in the field of plate tectonics, you have seen many questions solved, but also arise. What do you think are the biggest challenges today?

Many questions still remain about the initiation of subduction. We basically do not understand how this works. Recently, we had two cruises in the South-East Pacific where we acquireseismic data to figure out how subduction starts. Also in terms of plate boundaries, there still many questions. For example between North and South America, we don’t exactly know where the plate boundary is, nor the style of deformation that is associated with it.

[…] you should force yourself to go a bit further every time you do something and make yourself capable of reflecting on the things you have done.

One last question, Walter, what would be your advice to Early Career Scientists that aspire a career in geosciences?

I actually have multiple tips and tricks that might boost your (early) career. As soon as you can, start international collaborations. I have worked with Chinese, Russian, Brazilian and American research groups, amongst others. They give you a different view on the world. For example, when I first worked with the Russians, they did not think that seafloor spreading was happening, even though we together interpreted magnetic lineations as isochrons. Another advice is that you should force yourself to go a bit further every time you do something and make yourself capable of reflecting on the things you have done. A last advice: every now and then go to conferences by yourself, don’t stick with your group or the people you already know. You will have the best encounters. For example, I met Dietmar Müller with whom I eventually wrote many papers, at a poster session at the AGU in San Francisco in 1978. So even when you are shy, just go for it, get out there!

 

Interview conducted by Anouk Beniest

Meeting Plate Tectonics – Richard Gordon

Meeting Plate Tectonics – Richard Gordon

These bi-weekly blogs present interviews with outstanding scientists that bloomed and shape the theory that revolutionised Earth Sciences — Plate Tectonics. Stay tuned to learn from their experience, to discover the pieces of advice they share, to find out where the newest challenges lie, and much more!


Meeting Richard Gordon


Prof. Richard Gordon is currently Professor at Rice University (William Marsh Rice University in Houston, Texas). He researches on how several areas such as paleomagnetism, plate tectonics, lithospheric deformation and space geodesy are tied together. While a student, Professor Gordon used paleomagnetic data to calculate the minimum velocity of a plate or continent in the past. In 2002, he was awarded by the GSA with the Arthur L. Day Medal for contributions to the development of the plate tectonic model, especially for the recognition and quantification of diffuse oceanic plate boundaries.

There will be some heated debates in the AGU

After being active for several decades in this field, where lies currently your main research interest?

My interest is in processes in the lithosphere. This could mean Plate Tectonics, deformation of the lithosphere, absolute plate motions, how plates move relative to the hotspots, how much hotspots move between them and how they all move relative to the spin axis. Plate motions, how standard they are, how motions from a million years compare with plates motions we see over decades with space geodesy. My particular interest right now lies in working out the polar wander path of the Pacific plate, because it is a key missing part of the puzzle for understanding Cenozoic global tectonics, and Pacific tectonics. Those are some of the highlights.

How would you describe your approach, which methods do you while conducting your research?

A lot of it involves looking at data, using as many data and as diverse datasets as we can to test different hypotheses. A little tiny bit of it involves modelling. The main focus in our research group right now is on looking at marine magnetic anomalies in the Pacific and coming up with novel ways of process them in order to squeeze out information on where the paleomagnetic pole lies.

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

When you discover something new about the Earth and understand the Earth better, and you are the first one to get that realization: that is such a high, that makes all the hard work worthwhile.

Kreemer and Gordon (2014). Pacific plate deformation from horizontal thermal contraction. Geology, 42 (10), 847-850.

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

A lot of the work I’ve done has been about relative motion of the plates and motion across deep deforming zones, for example in the western US. Some of that work has been used and can be used more to help assess seismic hazards. The seismic moment releases energy over time and spaces related to how much the earthquakes move and how fast the plates move, I think this is a very important implication. I’m hoping, in the future, to relate true polar wander to global climate change. Maybe it will work, maybe not. But if it did, I think that would be really relevant.

What do you consider to be your biggest academic achievement?

(sighs)… That’s a tough one… Something I am very proud at was leading a group with a couple of my graduate students, to put together one new global set of plate velocities. We did a really careful job, went back to all of original data and analyzed the results. We were able to discover a lot of things. You can discover a lot of new things by going back and looking at the data. It was a big project and we were all really worn out at the end, but I think we are all very proud of that.

That work led to the discovery and quantification of motion across several diffuse oceanic plate boundaries. Such boundaries are globally significant and occupy 10% to 15% of the ocean floor. At the scale of the boundaries and boundary zones, the physics of deformation in them is very different from that for narrow oceanic plate boundaries.

C. DeMets, R. G. Gordon, D. F. Argus, S. Stein (1990). Current plate motions. Geophysical Journal International, 101 (2), 425–478

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

We have some of the papers out and still some of them are in the pipeline, I will be talking about them in the AGU: We solved a problem that people didn’t think was a problem: what’s the paleolatitude of the Hawaiian hot spot, when the Emperor Seamount Chain was formed. What we showed is different from what everybody believed. We showed that it stayed in the same place, it did not change its latitude. So it is going to be very controversial and there may be some heated debates in the AGU and EGU, I am sure. But I am sure we have got this right!

After being many years active in the academia, looking back, what would you change to improve how science in your field is done?

The easy answer would be: more funding! (laughs) Also, more opportunities for young scientists.

What are the biggest challenges right now in your field?

For the project I am doing right now on paleomagnetism of the Pacific, one challenge is that we need more data from the Pacific. We can do better with more data. A lot of the data that we have is collected by ships. But we would like to have vector data, from airplanes or drones that can move fast enough. Finding better quality data than we have is a challenge. And this goes back to more funding (laughs).

I thought I was going to be a writer

Richard Gordon in 1971. Credit – East Side Union High School.

What were your motivating grounds, starting as an Early Career Student? Did you always see yourself staying in academia?

When I went to graduate school, I thought I was going to be a writer. A science writer, maybe a science-fiction writer too. Isaac Asimov was my role model! I thought I had to have a PhD to know enough to be a good science writer. But to do a PhD I had to do research. So I started doing it and got really excited about it. And I thought “Hey, I could do this! I’m pretty good at this!

I did an internship in the oil industry for a summer and I really liked that too, but I liked academics a lot more, so I made the decision to stay in academia. Although I am still keeping my options open to still become a science writer. Isaac Asimov actually was an assistant professor for I think 6 years. When he reached the point where he was earning more money from his writing than as a professor, he decided to become a full-time writer. But I never did the writing, I just got so excited about academia that I have been totally focused in that way.

A disproportionate number of new discoveries are made by early career scientists

What advice would like to you give to Early Career Scientists?

The first thing is: don’t get discouraged. Because part of being an academic is receiving critical feedback. The advantage for us is that the people who are giving us feedback are people who also are getting feedback from somebody else. Whereas in art & music, the critics don’t actually make the music or make the art, they are just professional critics. It doesn’t give them the perspective of the person who also has received critical feedback. Everyone is going to get criticism, and papers get rejected and proposals get rejected…just don´t let yourself get discouraged and do read the criticisms carefully. It may be mostly wrong but there will be a kernel of truth, which can help you write a better paper, write a better proposal or be a better scientist.

The other thing to remember is that a disproportionate number of new discoveries are made by early career scientist. The early career scientists own the future, the near future. And that is part of “don’t be discouraged” because if you’ve got bright ideas, you could be just around the corner of a big advance.

Those two things together are, I think, important.

Richard Gordon. Credit – Jeff Fitlow, Rice University.

 

Interview conducted by David Fernández-Blanco

Minds over Methods: Tectonochemistry of Melting Mud in the Mantle, evidence from the Oman/UAE ophiolite

Minds over Methods: Tectonochemistry of Melting Mud in the Mantle, evidence from the Oman/UAE ophiolite

For this first Minds over Methods of 2019, we invited Christopher Spencer, Senior Research Fellow at Curtin University in Australia, to tell us something about tectonochemistry. By applying geochemistry to tectonic processes, it is possible to get more insight into the different stages of the rock cycle. By combining fieldwork and geochemical analyses of the Oman/UAE ophiolite, Chris and his co-workers believe they found the first direct and in-situ evidence of sediment melting in the mantle.

 

Credit: Christopher Spencer

Tectonochemistry of Melting Mud in the Mantle, evidence from the Oman/UAE ophiolite

Christopher Spencer, Senior Research Fellow, Curtin University, Australia

The rock cycle is the first thing we learn in Geology 101. Magma and lava cool to form igneous rocks. Igneous rocks then erode to form sediment, which forms sedimentary rocks as it is compacted. Increasing pressure and heat then create metamorphic rocks, which eventually will melt. In each of the transitions described in the rock cycle, tectonics is usually involved. Granite batholiths form in subduction zones and are uplifted and eroded in collision zones. The sediments derived therefrom are deposited along continental margins that are often then returned to subduction zones where they contribute to new magmatic systems. There is a wide array of tools that we can use to evaluate the role of tectonics in the rock cycle, of which geochemistry is able to provide insight into each stage of the process.

Applying geochemistry to tectonics is (unsurprisingly) referred to as tectonochemistry. Similar to tectonophysics, where geophysics is applied to address large-scale tectonic questions, tectonochemistry provides a unique view into geochemical proxies of tectonic processes. The melting of sediment along convergent margins is a classic tectonochemical problem, as the unique chemical signature of sediment found in a granite provides unequivocal evidence for the melting of a sedimentary rock. In collisional systems, like the Himalaya, tectonochemistry has been used to constrain the melting of meta-sedimentary rocks as crustal thickening and decompression drives dehydration of micas which leads to melting. Collisional systems provide clear and in situ evidence for sediment melting.

Figure 1: Clockwise from top left: tourmaline-bearing leucogranite from the Himalaya in NW India, leucogranite dykes intruding meta-sedimentary rocks exposed at 5000m altitude, in situ melting of meta-pelite and formation of leucogranite, incongruent melting of muscovite + plagioclase + quartz to form leucogranite but leaving the biotite behind. Credit: Christopher Spencer.

 

Sediments are also thought to melt in subduction systems, but given the difficulty of accessing the asthenosphere directly, it is more challenging to constrain the processes occurring deep in a subduction zone. The incorporation of sediment in subduction zones is often constrained using the geochemistry of the resulting magmatic rocks. The chemical signature of sediment provides a clear indication of its incorporation in the magma, but it is often unclear whether the contamination is occurring in the asthenospheric wedge or in the upper crust. For example, many granite batholiths contain zircon grains that are foreign to the host magma and whose age spectra match the detrital zircon age spectra of the adjacent sedimentary units. This relationship is a clear indication that sedimentary contamination occurred in the upper crust. Unfortunately, the geochemical proxies used to establish the sedimentary contamination only provide indirect evidence for the subduction of sedimentary material into the asthenospheric wedge. Such indirect evidence includes seismic stratigraphy showing sedimentary units being subducted beneath the forearc and whiffs of sedimentary geochemical signals in arc volcanics. Although these evidences point towards sediment being subducted deep into the asthenospheric wedge where it melts and contaminates the magmas coming off the subducting slab, they do not preserve direct evidence of sediment melting in the mantle.

To acquire direct evidence of processes happening deep in the mantle, I set my sights on the Oman/UAE ophiolite, where a thick succession of mantle peridotite is preserved beneath a complete stratigraphic section of oceanic crust. Previous work has shown that this ophiolite not only preserves an intact record of oceanic crustal stratigraphy, but also geochemical features of a subduction zone in the oceanic crust. This implies the ophiolite formed in a supra-subduction setting, where during the earliest phase of subduction, extension in the upper plate caused rifting and formation of oceanic crust above a subduction zone.

 

Figure 2: Oceanic crustal stratigraphy of the Oman/UAE ophiolite comprised of (clockwise from top left): pillow basalts, sheeted dykes, layered gabbros, and mantle peridotite. Credit: Christopher Spencer.

 

During fieldwork in the ophiolite, while traversing the 8-15 km thickness of the mantle peridotite, I encountered a number of granitoid dykes that cross cut the peridotite, but do not cross the petrologic Moho. Many of these dykes contained tourmaline, muscovite, biotite, and even andalusite, minerals that would be expected from the melting of sedimentary material. Finding these minerals in the mantle indicates these grantoid dykes formed from the melting of sedimentary material and here they were within the mantle! Subsequent analysis of zircon grains from these granitoid dykes revealed the age of these dykes was equivalent to the age of the overlying ophiolite providing bullet-proof evidence that they intruded while the ophiolite was forming above a subduction zone. To provide the nail in the coffin for a sedimentary origin, I performed oxygen isotope analysis of the zircon and quartz. Sedimentary material has a distinct oxygen isotopic composition and igneous rocks that are thought to have experienced sediment contamination have δ18O values that lie along mixing lines between a sediment end member and the mantle. The oxygen isotopic analyses of the sub-Moho granitoids of the Oman/UAE ophiolite revealed the highest δ18O values ever measured in igneous rocks, providing unequivocal evidence that these granitoids represent pure sediment melts. In a paper published in Geology (Spencer et al., 2017), my coauthors and I argue these igneous rocks represent the first direct and in situ evidence of sediment melting in the mantle. Lucky for us, we have just scratched the surface of the exciting things left to learn about these fascinating granitoids and I look forward to the opportunity to return to the Oman/UAE ophiolite.

Figure 3: Sub-Moho granitoids of the Oman/UAE ophiolite: A) Cathodoluminescence image of a zircon shown with location and result of δ18O analyses. B) Photograph of sub-Moho granitoids. C) Hand sample of granite with tourmaline and lepidolite (lithium-bearing mica). Credit: Christopher Spencer.

 

Meeting Plate Tectonics – David Bercovici

Meeting Plate Tectonics – David Bercovici

These bi-weekly blogs present interviews with outstanding scientists that bloomed and shape the theory that revolutionised Earth Sciences — Plate Tectonics. Stay tuned to learn from their experience, to discover the pieces of advice they share, to find out where the newest challenges lie, and much more!


Meeting David Bercovici


David Bercovici started his scientific career with a BSc in Physics, and eventually graduated with a PhD in geophysics and space physics [from UCLA]. He was a professor at the University of Hawaii from 1990 until 2000. In 1996 he received the James B. Macelwane Medal from the AGU for his contributions to geophysical sciences as a young scientist. Since 2001 he has been at Yale as a professor in Geology and Geophysics, and is currently Department Chair (for the 2nd time).  In the last few years he was elected to both the American Academy of Arts & Sciences and the US National Academy of Sciences.

 

For me, the biggest question I still would like to answer is: why do we have plate tectonics?

Could you briefly describe your research interests, David?

My research interests are in geophysical and geological fluid dynamics, especially to understand lithosphere and mantle dynamics. I’m mostly a theoretician, which means I do more pen and paper work developing theories and models of geophysical processes, not so much in the way of numerical simulations. My area of interest right now is understanding rock rheology at the grain scale in a physical way. For example, the softening feedback mechanisms we think are working on rocks to generate plate boundaries are quite complicated. However, if we are able to understand them for Earth, perhaps we can use them to understand the conditions for whether plate tectonics can occur on other planets, too. Mylonites are a good example of rocks that probably undergo a self-softening feedback, since it appears that deformation causes their mineral grains to shrink, which makes them softer, which then focusses or localizes their deformation, and so on.

 

You have been around for some time. What do you consider your biggest achievement within your field of expertise?

Overall I believe we’ve made a lot of progress in trying to understand why Earth has plate tectonics (and maybe why other planets in our solar system do not).  A lot of the physics necessary to advance this field relates to exotic rock rheologies, and this has involved a collaboration between experimental rock physicists and geodynamical theoreticians and modelers.  Rock physicists like my Yale colleague Shun Karato, David Kohlstedt (at the University of Minnesota) and Greg Hirth (at Brown University) have been a big influence on me.  I think my own contribution has been in developing ‘grain damage theory’ which describes how mineral grains evolve under deformation and cause weakening as we see in mylonites. I and my colleagues (most notably Yanick Ricard at the ENS-Lyon, but also former students William Landuyt now at Exxon and Brad Foley now at Penn State, and my two current collaborators Elvira Mulyukova at Yale and Phil Skemer at Washington University in St. Louis) have developed and continue to develop  theories for how grains damage. I consider the physics that we’ve developed for this a significant accomplishment.

 

Bercovici, D. & Ricard, Y., 2013. Earth and Planetary Science Letters, 275-288.

 

So besides your projects related to ‘damage physics’, do you have side-projects too?

David Bercovici – Credit: David Bercovici

Yes, I do! I currently have a project working on oscillations and magmatic waves in volcanic systems before eruptions with various colleagues (most notably Mark Jellinek at the University of British Columbia and Chloé Michaut at the ENS-Lyon).  And I have worked on problems that are related to the presence and circulation of water in the mantle. I and my colleague Shun Karato proposed the reasonably well-known (and controversial) transition zone water filter model. This theory argued that the upper and lower mantle are kept somewhat chemically distinct but without actual layering (which is usually required to explain the difference in basalts coming up at mid-ocean ridges versus ocean islands like Hawaii) by hydrous melting of material upwelling out of the transition zone, just at the 410 discontinuity. This melting then cleans the rising mantle of incompatible elements, much like a coffee filter (or maybe more like a hookah), allowing mid-ocean ridge basalts to look depleted.

One of the biggest challenges today is to predict and understand how other planets function.

You have quite some different interests! Overall, what do you consider the biggest scientific challenges in your field nowadays?

One of the biggest challenges today is to predict and understand how other planets function. Do they have plate tectonics or not? If not, could they have had plate tectonics at one time, and then why did it stop? We need to do tests and get data from other planetary and extra-solar bodies. Currently, we only have data from the Moon, Mercury, Mars and Venus (and also outer-solar system icy bodies). We are a long way from understanding our universe and the objects residing in it. I think that the model of plate tectonics as we know it nowadays is maybe just a recipe describing our own planet, but will not necessarily work for others.

Any model or code is only as good as the physics being used

 

So to get to there, what do you think could be improved in your field?

In geodynamics, constructing and using big numerical models is very popular nowadays. There is a danger here though, because users of these models do not always understand the physics behind the code they are using, and that some of this physics is incomplete. Any model or code is only as good as the physics being used, and we do not necessarily understand all this physics yet.   It is very important to understand how the numerical tool is constructed, or at least its limitations, and we really need to emphasize this. Ideally, everyone using a model would understand how the model works, but as codes become more complicated this becomes less practical or feasible. But at the very least, before you use a model, you should think about how to interpret it.  One can first develop a simpler theory or scaling-law to hypothesize or predict what the model might do, and then treat the numerical simulation like an experiment to test (and perhaps disprove or perhaps refine) this hypothesis.  This will make your study much more valuable and long-lasting.

 

You still have some time to continue your research. How do you see the remainder of your career?

I certainly hope to have some more discoveries coming up. You often have a broad idea or hypothesis that gives some direction where you should go, but you’re often surprised about what you discover along the way.  One of the things I work on now is the metal asteroid Psyche. One question is as such an asteroid freezes completely, can it sustain a magnetic field? This is a completely new direction for me and I find it very exciting! Whether my ideas will work or not, I can’t say yet. But sometimes my more successful ideas developed in a folder on my computer called ‘Cool or Stupid?’. One of my more well-known papers had that as a working title for a couple of years.

I was a terrible student.

What advice would you like to give today’s Early Career Scientists?

When I was an undergraduate and for some time in graduate school, I was a terrible student.  I didn’t even make it into graduate school at first! So my expectations were rather low.  Probably my one redeeming quality was that I was stubborn and persistent.  I figured I would continue to try to make it in graduate school until the university police were called to escort me off campus, which luckily never happened. My best advice is that if you feel you have found out what you want to do, be stubborn, but not so stubborn and rigid as not to learn new things and try ideas outside of your comfort zone.

My second advice is that you should ask yourself what big question do you want to answer in your life? What would you like written on your tombstone that you tried to accomplish?  Find yourself that question and make it your life goal. For me, the biggest question I still would like to answer is: why do we have plate tectonics?

Interview conducted by Anouk Beniest

 

 

Meeting Plate Tectonics – Mathilde Cannat

Meeting Plate Tectonics – Mathilde Cannat

These bi-weekly blogs present interviews with outstanding scientists that bloomed and shape the theory that revolutionised Earth Sciences — Plate Tectonics. Stay tuned to learn from their experience, to discover the pieces of advice they share, to find out where the newest challenges lie, and much more!


Meeting Mathilde Cannat


Mathilde Cannat started her career at the early age of 26 when she obtained her Doctorate in Geology at the University of Nantes, France. After a PostDoc at Durham University, England, she took a position at the National Center of Scientific Research (CNRS). She researched at the University Paris 6 since 1992 and obtained her present position at the Institut de Physique du Globe de Paris (IPGP), France, in 2001. She was awarded with the ‘Médaille d’Argent’ of the CNRS in 2009.

Scientist should be able to take time to produce publications, even if this means that there would be fewer publications

Mathilde, could you share with us your research interests and the methods you use to solve your research questions?

I work on the processes of oceanic accretion. I want to understand how new oceanic domains are created at mid-ocean ridges. My focus lies on the specific case of slow-spreading ridges, where tectonic processes are prevalent, and I unravel the interactions between tectonics, magmatism and hydrothermalism. I’m primarily a geologist, but in addition to submersible studies and rock sampling I also use several geophysical methods, that include gathering time series data on active processes such as seismicity and the temperature of hydrothermal vent fluids.

That’s quite a lot different topics you address. What is the favourite part of your research?

Mathilde Cannat – Credit: ODEMAR scientific cruise

Participate in sea-going cruises is the best part of the job. In particular, the use of manned or remotely operated submersibles to explore the seafloor is a very exciting business. I also very much enjoy good collaborations with colleagues, and the last stages of writing a paper, when it is almost finished. Lastly I am also fond of working with and advising PhD students.

Creating new concepts and knowledge is highly relevant no matter the topic

What do you think makes your research relevant and connected to real world applications?

In my opinion, creating new concepts and knowledge is highly relevant no matter the topic. I completely disagree with the notion that creation of knowledge belongs to some other less real world. I even go further and believe that research is a fundamental part of our culture. In my view whether it can be applied to some material objective at short or longer term does neither increases or decreases its relevance.

After being in the field for quite some years now, what do you consider your biggest academic achievement?

In the ’90s, I proposed a new concept for the formation of seafloor that is partially made of tectonically uplifted rocks from the earth’s mantle. I was the principal proponent of this idea and until today it is still an accepted and commonly used concept.

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

I don’t believe that science problems are ever truly solved. It is more like conceptual hypotheses that are made based on our current understanding. These hypotheses can then be tested which in most cases results in updating the concept and so on. So for this question, I can say that in my most recent project I have been able to gather observations that appear consistent with the hypothesis that I made with a colleague a few years back concerning the formation of new seafloor at mid-ocean ridges that have a very low melt budget.

Scientist should be able to take time to produce publications, even if this means that there would be fewer publications

Over the years you have seen the system in which scientists manoeuvre their work being changed and adapted. What would you like to change to improve how science in your field is done?

I would definitely change science funding and general organisation to put the emphasis back on teamwork. Also, the pressure that scientists have on publishing their work should go down. Scientist should be able to take time to produce publications, even if this means that there would be fewer publications but these would have been more thought about!

Sauter, Cannat, et al., 2013. Nature Geoscience, 6, 314-320.

 

For the near future, what do you think are the biggest challenges right now in your field?

We should definitely look at plate tectonics in relation to a more global picture. This means that it would include the interactions and impacts between the solid Earth and the biosphere, the oceans, the atmosphere. This global picture should be regarded both in the present, with a better understanding of time variable processes, and in the past through the Earth’s history.

[To ECS] Do not become bitter when it seems to be so hard to get a stable position

One last question for the Early Career Scientists (ECS) that read this blog, when you were in the early stages what is the best advice you ever received and what advice would you give to them?

When I was an ECS myself, I saw myself staying in academia. The best advice that I was given at the time, I guess, was not to become bitter because it seemed to be so hard and take such a long time to get a stable position during my postdoc years. And so to ECS, I would definitely suggest not to hesitate to contact people, even senior people, if you like their work. Don’t be afraid to ask them questions, explain your own ideas and get into a scientific discussion with them.

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.

 

Meeting Plate Tectonics – Peter Molnar

Meeting Plate Tectonics – Peter Molnar

These bi-weekly blogs present interviews with outstanding scientists that bloomed and shape the theory that revolutionised Earth Sciences — Plate Tectonics. Stay tuned to learn from their experience, to discover the pieces of advice they share, to find out where the newest challenges lie, and much more!


Meeting Peter Molnar


Active in different research areas of the Earth Sciences, Prof. Peter Molnar has been Professor of Geological Sciences at the University of Colorado at Boulder for more than a decade.

Set your own standards for excellence and don’t let other people decide them

You have come a long way in academia! How do you remember the beginnings of your career?

Peter Molnar (1984) – Credit: what-when-how, In Depth Tutorials and Information

I studied physics in the United States, at Oberlin College, where I took one semester in Physical Geology. I remember a friend of mine said “Molnar, you ought to take Geology. If you take Geology, you will look at the landscape completely differently from the way you do.” And I liked looking at the landscape, so I took that semester. Then, I worked one summer at the Harvard Cyclotron Laboratory, and I realized that I wasn’t cut out for that kind of physics… So, I thought of going to geophysics. I applied and I was a good enough student that I got in, in both Columbia and Caltech. I went to Columbia University. During my second year, I attended a talk by Lynn Sykes. He had studied earthquakes on fracture zones and demonstrated that transform faulting occurred. This was a moment that changed me. I remember thinking “Oh my God! Continental drift does occur!” I had been introduced to it back in college, but I didn’t believe any of it! I heard Sykes, and I suddenly realized there is something exciting going on. I got interested and turned my attention to it. While a student I took a “sabbatical,” went to East Africa with a bunch of seismographs to study earthquakes there.

 

 

I attended a talk by Lynn Sykes… This was a moment that changed me

I graduated in 1970. Then I was a PostDoc for two years at Scripps Institution of Oceanography. Afterwards, I went to the USSR for four months, because I thought earthquake prediction offered a bright future. Next, I took a job at MIT where I had the good fortune to get to know Paul Tapponnier. He really taught me more geology than I knew by a long shot. I stayed at MIT for 27 years, but I wasn’t a very good teacher. So I decided to quit, and I supported myself on grants from NSF and NASA. Late in the 90s, after supporting myself for more than 10 years, I wanted to change directions. So I looked into moving to a place where they would pay me a little a bit so that I did not have to depend on grants. And there was the choice between University of Washington and the University of Colorado. I had gotten interested in climate change, and then other things since then, and I have been here for 18 years.

 

After being active for several decades in this field, where does your main research interest currently lie? 

Right now my main interest is related to how geodynamics affects climate on geologic scales. There are two problems that attract me: how does the high topography of Asia affect Asian climate and how do islands in the ocean affect rainfall and large-scale atmospheric circulation. The ultimate goal of the latter is Ice Ages, since I think they are all tied together. I have been working on what you might call geodynamics now for most of the last 50 years, so I still do that. I no longer do much seismology.

It’s almost a religion that I don’t believe what I don’t understand

Peter Molnar (2014) – Credit: Oceans at MIT

 

How would you describe your approach?

My wife says that what I do is to look for problems where everybody believes something, but there is an inconsistency, and that I try to find that inconsistency and expose it, and then revel in the pleasure of that exposure. That’s her observation of watching me, I certainly do not do this consciously. 

A concern I have with a younger generation is that, for some reason, they have not been encouraged or they have not learned to ask important questions…

 

 What about your methods?

Molnar & England (1990). Late Cenozoic uplift of mountain ranges and global climate change: chicken or egg? Nature, 346, 29–34.

I seek simple physical explanations for things. I do not like big models because I don’t understand them, and it’s almost a religion that I don’t believe what I don’t understand. I use big numerical codes. I use them to carry out “simple numerical experiments” where you vary one parameter and see what you get. To me this is an experiment. It’s just not done in a laboratory but on a computer. The strategy is to understand the physical processes while bringing data to bear. Another central element, which I often seen missing today, is that I try to direct my research towards problems that are “important”. It seems to me that an important problem is one that when you solve it, it changes the way people think. Sometimes you have to make incremental steps forward. As an example, both Tapponnier and I, over the years, have tried to constrain the kinematics of Asian deformation by studying slips on faults, and determining slip rates. One could argue, those studies are incremental steps forward, but of course, the big goal is to put the whole picture together. I no longer do this.

There are many people who do this better than I do. So, it would be pointless for me to do that. But I compile their data continually. And the question that I am asking, in this case, is what are the underlying physical processes that determine how the deformation occurs?

A concern I have with a younger generation is that, for some reason, they have not been encouraged or they have not learned to ask important questions.  There’s too much of a tendency to work on incremental problems. 

While you are learning, you are alive

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

Bringing two pieces together that don’t look like they fit, until you put them together. For example, I think that rainfall over the islands in Indonesia and the growth of Indonesia has made the Ice Ages in Canada. Now, who would have thought that? I have fun with this! You have to realize that when you do this type of things, most of the time you are wrong. So, I might be wrong about this one, but I am having fun. So it doesn’t matter. I’m learning. That’s the second favourite thing: learning. While you are learning, you are alive. And the third thing is fieldwork. I love being in the field. My head gets clear, I see things that I have not seen before, I learn about other cultures and people. I just have a wonderful time. I don’t think my own fieldwork contributed much to our field  – but it’s important to me.

I’m just having fun!

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

Peter Molnar – Credit: University of Colorado Boulder

I think my research is about as relevant as Goya’s paintings – Goya is one of my favourite artists. So if you think that Goya’s paintings are relevant, then maybe my research is relevant. And if you think his paintings are not relevant, then my research is not relevant either. And I shouldn’t be so pretentious as to equate my work to Goya’s paintings.

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

I don’t know if I solved any problem… that’s not a question I ask myself. I’m just having fun!

I wanted to ask what do you consider to be your biggest academic achievement, but perhaps I should ask you what is the one achievement that gave you the most fun?

I don’t spend time thinking about my biggest achievement. I prefer to look forward to what’s coming. You know, most people my age are retired, I can still work 50 or 60 hours a week. I love what I do. I rather look forward to the exciting stuff in the future.

…it troubles me when I see people worrying […] about artificial metrics

Looking back, what would you change to improve how science in your field is done today?

I see two aspects of the direction science is going that trouble me. One, can do nothing about, is the level of funding. Most of us struggle to get funded. I feel that back 50 years ago, it was much easier than it is now. Of course, we were fewer people. But in any case, limitations on funding really slow us down.

The other thing that troubles me is the focus on metrics. People counting the number of papers they write, worrying about their citations and not worrying about the quality of their work. These very poor measures of quality. So much today is focussed on these metrics, these indexes, that are meant to be a measure of your work. People are not thinking about the quality, they are thinking about how many people are going to cite it, where they are going to publish it, does the journal have a high -whatever it is called- impact factor. This is just crap, people should not waste time on this. This is just ridiculous! The focus should be on the quality of the work. We all have different ways of deciding quality. It is not something you measure, however; it’s something we determine in some subjective way. And it troubles me when I see people not worrying about the right thing, quality, and worrying instead about these artificial metrics. I am just so glad these things don’t matter to me. I am old enough, but I really don’t envy young people that have to cope with these sorts of artificial targets.

I don’t see anything like Plate Tectonics in the verge from happening.

But I do see still see very exciting stuff, but probably in different parts the science

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

Some of the challenges are too hard for me even to pursue them. In the climate world, we don’t know about the role of clouds. And I don’t know how to pursue this, so I don’t pursue it. Do clouds have a cooling effect, and what is the response from clouds to warming? Will they slow or accelerate the warming? We don’t know. The role of clouds is certainly a big, big question. Although I do not work on this, I think about it, but I don’t see what to do.

One of the problems I do work on is what brought us Ice Ages. How did we go through 300 My years without much ice in the northern hemisphere and then suddenly, beginning 3My years ago or so, we had 5 big Ice Ages? Why? An easy answer is that now CO2 is higher. But it’s really hard to measure, determining CO2 in the past is a big question.

Another big question for me is how does the convection in the mantle connect with deformation in the lithosphere? How do these connect to one another?

Another one I work on is where is the strength within the lithosphere? We still argue about it. This is a 40 years old question, and the points of view haven’t changed. There are still those who put the strength in the crust, while others put it in the mantle. I don’t think we know. And of course it’s going to be different in different places, so it’s a more complicated issue.

Molnar (2015). Plate Tectonics: A Very Short Introduction – Credit: Amazon

I think the prediction of earthquakes is often dismissed as something that we ought not to spend time on. But the progress that has been made in understanding earthquakes in the past 20 years is huge. This came up in Paris and I agree completely with what Eric (Calais), Jean-Philippe (Avouac), and others said. The use of GPS to study co-seismic and post-seismic deformation, and the realization of slow earthquakes are big advances. That’s a big question that I think we might be close to solving.

Another question I got really excited about is understanding how the upper mantle and the lower mantle are connected. In fact, some of us have had a discussion about it in Paris. The evidence shows the lower mantle is really chemically different from the upper mantle; that’s obvious. But how are the two connected; that’s not obvious. I don’t see this the same way as a bunch of other people do. I see the connection between the two, and this takes us back to the question of the early history of the Earth. How is the chemical difference manifested? How has the slower convection of the lower mantle slowed the cooling of the Earth?

I think the answer to your question is: I don’t see anything like Plate Tectonics on the verge of happening. I do see still very exciting stuff, but probably in different parts the science.

…that way I was not going to get killed

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

I don’t remember what expectations I had, I don’t think I was even aware enough to know what I wanted to do. When I decided to go into geophysics, people said to me “Oh, what’s geophysics?”, and I didn’t know. And “What would you do?” and I said, “Well, oil companies need people like that”. At that time I knew so little, that it never dawned on me that if I work for an oil company, I might be stuck having to live in Texas. And I can’t imagine living in Texas. What I did know is that if I did not go to graduate school, I would be sent to Vietnam. I was kind of trapped with having to go to graduate school and choosing a field that seemed possible and open to me. So, I just decided to go for the easy road. I stayed in school because that way I was not going to get killed. I stayed, and I thought about music and girls. But once I got excited about research, it was clear that that was the only place for me.

 What is the best advice you ever received?

Now, that’s a good question. One of them came from my father. He did not articulate this, but I sensed it in a conversation with him. And one of my three main advisors, Jack Oliver, emphasized this to me again, and that is to continuously ask yourself: What is the most important scientific question? As soon as you did something, Jack Oliver would say, “Ok. Now you have done this, what’s the next most important question?” Just because you ask it, it doesn’t mean that you have solved an important problem. But if you continue to ask yourself that question, you have a better chance of doing good science, than if you don’t ask that question.

Jack gave another piece of advice, which is almost counter opposite to this, and that was that when you can’t think of what to do, the worst thing you could do is to do nothing. Just because you can’t come up with the most important problem doesn’t mean you should do nothing. You should just keep going.

Another piece of advice is, set your own standards. None of us is Einstein. None of us is Newton (maybe not none of us, but very, very few of us are). So, if we set those standards, we fail. And the problem is that, if we let universities with low standards but counting and using metrics to set the standards, we will not do as well as we would, if each of us would set our own standards for excellence. We should strive on meeting our standards, rather than what others expect from us. Don’t let other people decide your standards.

 

Peter Molnar – Credit: David Oonk

Interview conducted by David Fernández-Blanco

Meeting Plate Tectonics – Xavier Le Pichon

Meeting Plate Tectonics – Xavier Le Pichon

These bi-weekly blogs present interviews with outstanding scientists that bloomed and shape the theory that revolutionised Earth Sciences — Plate Tectonics. Stay tuned to learn from their experience, to discover the pieces of advice they share, to find out where the newest challenges lie, and much more!


Meeting Xavier Le Pichon


Prof. Xavier Le Pichon is one of the pioneers of the theory of plate tectonics. He developed the first global-scale predictable quantitative model of plate motion. The model, published in 1968, accounted for most of the seismicity at plate boundaries. Among many substantial contributions to the field, he also published, together with Jean Francheteau and Jean Bonnin, the first book on plate tectonics in 1973.

 

Your contributions have led to great advancements of our understanding of Plate Tectonics as we know it today. What‘s your main interest and what motivates your research?

My interest is the Earth and how it behaves. Discovering what type of animal the Earth is. I think of the Earth as a living organism, and we have to understand it. It’s very interesting to take the Earth as something that evolves, that changes, and that you have to understand how it evolves. The whole thing about research is getting very intimate with it and knowing really its behaviour.

I think of the Earth as a living organism

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

I do not have any favourite aspect, but I think that to explain the change in the Earth is captivating. For example, how did we pass from an Earth where there were a single continent and a single ocean, ~200 Ma, to something where the continents are as dispersed as they are now… This had a tremendous influence on many things, including evolution, biology, climate… We know, for example, that when all the continents were together the pace of the evolution was much smaller than when continents are dispersed. All this fascinates me. I believe that if there is something that is not understood, you have to understand it. The basic question that proves you are a human is, you always have the “why” in your mind as the main thing that is present.

Claude Riffaud and Xavier Le Pichon – Credit: Jean-Claude Deutsch/Paris Match

 

What do you consider is the main problem that you solved during research?

I have been interested in many different aspects… I’m best known by the fact that I’ve been one of those who promoted plate tectonics. I made the first global model of quantifying the motion of the plates, knowing everywhere what would be the motion absorbed in the plate boundary. Also, I made the first finite and precise reconstruction of the configuration of the Earth, for nowadays, 70 Ma, 200 Ma, and so on. I also think that I was the first that proved that the Earth’s expansion did not work. Because if you take the shortening that is absorbed in the trenches of the world, in the mountain belts, and you claim there is no shortening there, then you are left only with the expansion of the ridges. And the expansion is asymmetric, and it’s produced much more in the east-west sense than it is in the north-south sense. And if you have that going on for several tens of millions of years, then the Earth would have a shape which is completely non-hydrostatic. It would not respect what the Earth has to have to be a planetary body turning on itself. So the Earth’s expansion was clearly impossible.

I believe that science that is completely regulated

top-down is not efficient

Le Pichon, X. (1968). Sea-floor spreading and continental drift. Journal of Geophysical Research, 73(12), 3661–3697.

 

After being many years active in the academia, looking back, what would you change to improve how science in your field is done today?

I never worried about “what is done”, I worried about “what I do”.  I have always found a way to get money, to get a position and to get a lab. I changed labs quite a few times. I created a few labs… I think it is a question of adjusting. I believe that science that is completely regulated top-down is not efficient. I think there has to be a lot of freedom. At least for fundamental science. For applied science, I don’t know but I think it is probably about the same. The reason is very basic: what is the purpose of research? It’s to discover something that is totally unexpected. If it is expected, then it’s not a discovery. When the guy who does the planification says: “we will focus all our energy to find out about that”, how does he know “that” is the thing that is going to come out? The most important things in the evolution of research have been totally unexpected and came from people that had no planification whatsoever of what they should find.

The most important things in the evolution of research have been totally unexpected

Where do you see the biggest challenges in your field right now?

Le Pichon, Francheteau, Bonnin (1973). Plate Tectonics: Developments in Geotectonics, 6 – Credit: Amazon

The plate tectonic was really a revolution that changed completely the concept. And it took a few tens of years to adjust to this revolution. Actually, we are still in the phase of adjusting to that. For example, we are adjusting to the fact that to understand that plate tectonics is not only what happens at the surface, but that it implies things that happen in the interior of the Earth, in the mantle and below. This is not fully understood. And we do not understand one very important thing, which is that plate tectonics is a relatively new thing on the Earth. In the beginning, there was no plate tectonics as we know it nowadays. And I think that even the style of the plate tectonics has changed in the last was 200 Ma for example. It probably was not the same before Pangea… So we have still lots of things to understand, and to incorporate. And then, the main thing about discoveries, again, is that they are unexpected. So, I would not be surprised that major discoveries focus our energy in a completely new direction in the near future. I think we are approaching a time where it seems that we need to trigger something else to get into something new.

 I am very afraid of people who get specialized too early

When you were an Early Career Researcher, what was your motivation, what stimulated you most?

Riffaud, Le Pichon (1976). Expédition ‘Famous’ à 3000 m sous l’Atlantique. Paris: Albin Michel. – Credit: Amazon

The fact that strikes me the most when I think about Europe is that the student’s mobility has been greatly increased and I think that this is extremely important. The mobility I had was not too frequent in my time – I have moved a lot: I moved to the United States, where I was offered a professorship, and came back, then I was an invited professor in other places, Oxford, Tokyo… I have created three different laboratories, and I’ve been in many places in the world. I think this is very important because you change with time and you cannot get stuck in a given thing. I think this is very basic in research. I mean, you learn a lot by comparing. You have to move, and confront yourself to other laboratories, to other ways to teach… Otherwise, you get stuck in a certain frame and that can be very dangerous. Then you become more interested in promoting your position and the place where you are than in the discoveries. Or you end up trying to be what your professor was and trying to imitate the guy that taught you is certainly one of the worst things you can do. I think anything that promotes mobility and independence and possibilities to change is a very good thing.

I am very afraid of people who get specialized too early. Of course, it is easier to get a job if you have a narrow speciality, you are more immediately usable. But I think the result is quite bad, quite often. You first have to see the different possibilities and then progressively you find out that you best express yourself in a certain direction, in a certain field. And that requests time and several tries and so on.

 

When you were a young researcher, did you always see yourself staying in academia?

I always wanted to do research. I wanted the freedom to choose. And I always went to places where I was sure that I would decide myself what type of research I would do. If that was not anymore the case, I quitted and I changed. I was very firm about the fact that I wanted to choose myself my own research direction. This has been a problem with financing. I had to change my source of financing. Whenever I had a problem with the state and the administration, I would go to oil people and other types of European financing in order to be able to keep this freedom.

You have to go to a place where research is thriving

The last question for today’s Early Career Scientists: what advice would you like to give the ECS that would like to stay in science?

Xavier Le Pichon – Credit: Instituto De Estudios Andinos Don Pablo Groeberg (IDEAN)

Basically, I have been an autodidact. I have always learned, in contact with other people, but mostly by myself. I cannot give any advice about what is best… but it is clear that you have to go to a place where research is thriving. If you go to a place where nothing happens, you will not start by yourself something unless you are a real genius. But even then, you don’t have the resources and so on. So you first need to identify the place where things are moving, where things are happening.

And then you try to go to this place and then, if possible, you try another one. Don’t get stuck to one thing only. Try to see the world, try to see how it moves, try to contact people…

One of the most interesting things in research is the contact with other people. Academia is a place where you have a lot of cooperation and you learn to interact with others and having a wide network of people with whom you interact is one of the gifts of this type of life. One very interesting thing is wherever you go you will agree if you talk about good science. Because when proper science is made, everybody agrees. This is not true in any other field. In philosophy, for example, you will never find people with whom you totally agree, it’s impossible. In science it’s so restricted, the rules are so clear that you are sure to come to a common agreement. So you can work with anybody on Earth that has the proper mind to do research and you will cooperate very well.

Xavier Le Pichon – Credit: Xavier Le Pichon

Interview conducted by David Fernández-Blanco

Lisbon at the dawn of modern geosciences

Lisbon at the dawn of modern geosciences

Here, where the land ends and the sea begins...
Luís de Camões (Portuguese poet)

Lisbon. Spilled over the silver Tagus River, it is known by its beautiful low light, incredible food and friendly people. Here, cultures met, and poets dreamed, as navigators gathered to plan their journeys to old and new worlds. Fustigated by one of the greatest disasters the world has ever witnessed, Lisbon is intertwined with the course of Earth Sciences. For some, modern seismology was born here. For others, this might even have been the place where it all begun; what we now call geology.

On the morning of All Saints day of 1755, a giant earthquake struck the city of Lisbon. With a magnitude of ~8.7, the event was so powerful that it was felt simultaneously in Germany, as well as in the islands of Cape Verde. The main shock occurred around 9.40 am, when a significant portion of the population was attending the mass in churches. Lasting several minutes, many of the roofs collapsed and thousands of candles set fires that would last for days. While people were looking for safety at open areas near the river, three giant tsunami waves were on their way. Forty minutes after the main shock, the waves rose the Tagus River and flood the city’s downtown. The death toll in Lisbon reached up to 50,000 people, about one quarter of Lisbon’s population at the time. This event is known as the Great Lisbon Earthquake of 1755.

 

Painting depicting the day of the 1755 Great Lisbon Earthquake. Credit: Wikipedia.

 

The 1755 Lisbon Earthquake was a terrific natural disaster. A few years ago, the French magazine L´Histoire, considered this earthquake as one of the 10 crucial events that changed history. At the time, Lisbon was a maritime power in a maritime epoch. This was also the age of Enlightenment, when man started to realize that many events such as earthquakes, volcanoes and storms, had natural causes, and were not sent by gods.

Convento do Carmo, destroyed during the 1755 earthquake and kept as a ruin for memory. Credit: Flickr.

Lisbon was in the spotlight of the modern world and some of the most prominent philosophers like Kant, Voltaire and Rousseau focused on the destructive event of the 1st of November, 1755. In particular, Emmanuel Kant published in 1756 (yes, 1756!) three essays about a new theory of earthquakes (see Duarte et al., 2016 and the reference list below for two of the Kant’s essays). I recommend all geoscientists to read these documents. It is incredible how Kant understands and describes how earthquakes align along linear features that are parallel to mountain chains. Does this sound familiar? Moreover, he uses the then new physics of Newton to calculate the forces that were needed to set the seafloor off Lisbon in movement in order to generate the observed tsunami. He even refers to experiments with buckets full of water to explain how the tsunami formed (analogue modelling!?). And Kant was not alone…

The minister of the King of Portugal at the time, the Marquis of Pombal, sent an enquiry to all parishes in the country with several questions. While some of the questions were intended to evaluate the extent of the damage, it is now clear that the Marquis was also trying to gain (scientific) knowledge about the event (see Duarte et al., 2016 and references therein). For example, he asks if the ground movement was stronger in one direction than in other, or if the tide rose or fell just before the tsunami waves arrived. Today, we can reconstruct with rigor what happened that day because of the incredible vision of this man.

 

The center of Lisbon today. The statue of Marquis of Pombal facing the reconstructed downtown. Credit: Wikipedia.

 

Coming back to Lisbon. If you visit the old city by foot, you will realize that houses on the hills are closely packed, separated by narrow streets and passages, while in the flat downtown streets are wide and orthogonal. The hilly parts of Lisbon are an heritage of the Moorish and Medieval times. Mouraria and Alfama are the ideal neighborhoods to visit. The organized downtown was the area that was totally floored during the earthquake, due to ground liquefaction and the impact of the tsunami, and was rebuilt using a modern architecture (see Terreiro do Paço and the downtown area in the first figure in the top). The Grand Liberty Avenue is clearly inspired by the style of the Champs-Élysées. Going up the Liberty Avenue, from the downtown, you will find the statue of the Marquis of Pombal (see figure above). And if you are already planning to visit (or revisit) Lisbon, you should definitely stop by the Carmo Archeological Museum, a ruin left to remind us all of what happened on that day of 1755, and the Lisbon Story Centre.

The hills of Lisbon, with the Castle in the top left and the 25 de Abril bridge in the background. Credit: Flickr.

Rebuilding plan after the 1755 earthquake. Credit: Wikimedia Commons.

The 1755 Great Lisbon Earthquake was however not the only earthquake that hit the city. On the 28th of February 1969, another major quake, with a magnitude of 7.9, struck 200 km off the cost of Portugal, at 2 am in the morning. The earthquake generated a small tsunami but luckily, given the late hours, did not caused any casualties. This event also occurred in a particular point in history: The time of plate tectonics. The paper that inaugurated plate tectonics had been published only 4 years before, by Tuzo Wilson. And in 1969, geoscientists already realized that some continental margins were passive and did not generate major earthquakes, such as the margins of the Atlantic, while others were active and fustigated by major earthquakes, such as the margin of the Pacific (Dewey, 1969). It was somewhat strange that this Atlantic region was producing such big earthquakes, which therefore immediately resulted in scientists coming to study this area (see map below).

Fukao (1973), studied the focal mechanism of the 1969 earthquake and concluded that it was a thrust event. Purdy (1975), suggested that this could result from a transient consumption of the lithosphere, and Mckenzie (1977) proposed that a new subduction zone was initiating here, along the east-west Africa-Eurasia plate boundary (see the thinner segment of the dashed white line in the eastern termination of the Africa-Eurasia plate boundary, map below), SW of Iberia. Later on, in 1986, António Ribeiro, professor at the University of Lisbon, suggested that instead, a new north-south subduction zone was forming along the west margin of Portugal (yellow lines in the map), a passive margin transforming into an active margin. This could explain the high magnitude seismicity, such as the Great Lisbon Earthquake of 1755.

 

Map showing the main tectonic features in the SW Iberia margin. The Eurasia-Africa plate boundary spans from the Azores-Tripe Junction (on the left) until the Gibraltar Arc (on the right, with its accretionary wedge marked in grey). The yellow lines mark a new thrust front that is forming and migrating northwards away from the plate boundary and along the west Iberia margin. The smaller yellow line marks the approximate location of the 1969 earthquake. The 1755 Great Lisbon Earthquake might also have been generated in this region (see Duarte et al., 2013 for further reading on the tectonic setting of the region; the figure is adapted from this paper).

 

Today, we know that the SW Iberia margin is indeed being reactivated (Duarte et al., 2013). Whether this will lead to the nucleation of a new subduction zone is still a matter of debate, and we will probably never know for sure. Nevertheless, subduction initiation is one of the major unsolved problems in Earth Sciences, and the coasts off Lisbon might constitute a perfect natural laboratory to investigate this problem. It may be the only case where an Atlantic-type margin (actually located in the Atlantic) is just being reactivated, which is a fundamental step in the tectonic conceptual model that we know as the Wilson Cycle (see also Duarte et al., 2018 and this GeoTalk blog). In any case, we know that there are two other locations where subduction zones have developed in the Atlantic: in the Scotia Arc and in the Lesser Antilles Arc. How they originated is still being investigated; which is precisely what we are doing now in Lisbon. That is however a topic that deserves its own blog post.

 

Written by João Duarte

Researcher at Instituto Dom Luiz and Invited Professor at the Geology Department, Faculty of Sciences of the University of Lisbon. Adjunct Researcher at Monash University.

 

Edited by Elenora van Rijsingen

PhD candidate at the Laboratory of Experimental Tectonics, Roma Tre University and Geosciences Montpellier. Editor for the EGU Tectonics & Structural geology blog

 

For more information about the Great Lisbon Earthquake of 1755, check out these two video’s about the event: a reconstruction of the earthquake and a tsunami model animation

 

References:

Dewey, J.F., 1969. Continental margin: A model for conversion of Atlantic type to Andean type. Earth and Planetary Science Letters 6, 189-197.

Duarte, J.C., Schellart, W.P., Rosas, F.R., 2018. The future of Earth’s oceans: consequences of subduction initiation in the Atlantic and implications for supercontinent formation. Geological Magazine. https://doi.org/10.1017/S0016756816000716

Duarte, J.C., and Schellart, W.P., 2016. Introduction to Plate Boundaries and Natural Hazards. American Geophysical Union, Geophysical Monograph 219. (Duarte, J.C. and Schellart, W.P. eds., Plate Boudaries and Natural Hazards). DOI: 10.1002/9781119054146.ch1

Duarte, J.C., Rosas, F.M., Terrinha, P., Schellart, W.P., Boutelier, D., Gutscher, M.A., Ribeiro, A., 2013. Are subduction zones invading the Atlantic? Evidence from the SW Iberia margin. Geology 41, 839-842. https://doi.org/10.1130/G34100.1

Fukao, Y., 1973. Thrust faulting at a lithospheric plate boundary: The Portugal earthquake of 1969. Earth and Planetary Science Letters 18, 205–216. doi:10.1016/0012-821X(73)90058-7.

Kant, I., 1756a. On the causes of earthquakes on the occasion of the calamity that befell the western countries of Europe towards the end of last year. In, I. Kant, 2012. Natural Science (Cambridge Edition of the Works of Immanuel Kant Translated). Edited by David Eric Watkins. (Cambridge: Cambridge University Press, 2012).

Kant, I., 1756b. History and natural description of the most noteworthy occurrences of the earthquake that struck a large part of the Earth at the end of the year 1755. In, I. Kant, 2012. Natural Science (Cambridge Edition of the Works of Immanuel Kant Translated). Edited by David Eric Watkins. (Cambridge: Cambridge University Press, 2012).

McKenzie, D.P., 1977. The initiation of trenches: A finite amplitude instability, in Talwani, M., and Pitman W.C., III, eds., Island Arcs, Deep Sea Trenches and Back-Arc Basins. Maurice Ewing Series, American Geophysical Union 1, 57–61.

Purdy, G.M., 1975. The eastern end of the Azores–Gibraltar plate boundary. Geophysical Journal of the Royal Astronomical Society 43, 973–1000. doi:10.1111/j.1365-246X.1975.tb06206.x.

Ribeiro, A.R. and Cabral, J., 1986. The neotectonic regime of the west Iberia continental margin: transition from passive to active? Maleo 2, p38.

Wilson, J.T., 1965. A new class of faults and their bearing on continental drift. Nature 207, 343– 347