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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.

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.

 

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

Minds over Methods: Linking microfossils to tectonics

Minds over Methods: Linking microfossils to tectonics

This edition of Minds over Methods article is written by Sarah Kachovich and discusses how tiny fossils can be used to address large scale tectonic questions. During her PhD at the University of Brisbane, Australia, she used radiolarian biostratigraphy to provide temporal constraints on the tectonic evolution of the Himalayan region – onshore and offshore on board IODP Expedition 362. Sarah explains why microfossils are so useful and how their assemblages can be used to understand the history of the Himalayas. And how are new technologies improving our understanding of microfossils, thus advancing them as a dating method?

 

                                                                          Linking microfossils to tectonics

Credit: Sarah Kachovich

Sarah Kachovich, Postdoctoral Researcher at the School of Earth and Environmental Sciences, The University of Queensland, Australia.

Radiolarians are single-celled marine organisms that have the ability to fix intricate, siliceous skeletons. This group of organism have captured the attention of artist and geologist alike due to their skeletal diversity and complexity that can be observed in rocks from the Cambrian to the present. As a virtue of their silica skeletons, small size and abundance, radiolarian skeletons can potentially exist in most fine-grained marine deposits as long as their preservation is good. This includes mudstones, hard shales, limestones and cherts. To recover radiolarians from a rock, acid digestion is commonly required. For cherts, 12-24 hours in 5 % hydrofluoric acid is needed to liberate radiolarians. Specimens are collected on a 63 µm sieve and prepared for transmitted light or scanning electron microscope analysis.

Animation of radiolarian diversity. Credit: Sarah Kachovich

Scale and diversity of modern radiolarians. Credit: Sarah Kachovich (radiolarians from IODP Expedition 362) and Adrianna Rajkumar (hair).

 

 

 

 

 

 

 

 

 

 

Improving the biostratigraphical potential of radiolarians

The radiolarian form has changed drastically through time and by figuratively “standing on the shoulders of giants”, we correlate forms from well-studied sections to determine an age of an unknown sample. A large effort of my PhD was aimed to progress, previously stagnant, research in radiolarian evolution and systematics in an effort to improve the biostratigraphical potential of spherical radiolarians, especially from the Early Palaeozoic. The end goal of this work is to improve the biostratigraphy method and its utility, thus increasing our understanding of the mountain building processes.

The main problem with older deposits is the typical states of preservation, where radiolarians partly or totally lose their transparency, which makes traditional illustration with simple transmitted light optics difficult. Micro-computed tomography (µ-CT) has been adopted in fields as diverse as the mineralogical, biological, biophysical and anatomical sciences. Although the implementation in palaeontology has been steady, µ-CT has not displaced more traditional imaging methods, despite its often superior performance.

Animation of an Ordovician radiolarian skeleton in 3D imaged through µ-CT. Credit: Sarah Kachovich

To study small complex radiolarian skeletons, you need to mount a single specimen and scan it at the highest resolution of the µ-CT. The µ-CT method is much like a CAT scan in a hospital, where X-rays are imaged at different orientations, then digitally stitched together to reconstruct a 3D model. The vital function of the internal structures provides new insights to early radiolarian morphologies and is a step towards creating a more robust biostratigraphy for radiolarians in the Early Paleozoic.

Linking radiolarian fossils to tectonics

Radiolarian chert is important to Himalayan geologists as it provides a robust tool to better document and interpret the age and consumption of oceanic lithosphere that once intervened India and Asia before their collision.The chert that directly overlies pillow basalt in the ophiolite sequence (remnant oceanic lithosphere) represents the minimum age constraint of its formation. In the Himalayas, over 2000 km of ocean has been consumed as India rifted from Western Australia and migrated north to collide with Asia. Only small slivers of ophiolite and overlying radiolarian cherts are preserved in the suture zone and it is our job to determine how these few ophiolite puzzle pieces fit together.

Another way I have been able to link microfossils to Himalayan tectonics is by studying the history and source of erosion from the Himalayas on board IODP Expedition 362. Sedimentation rates obtained from deep sea drilling can provide ages of various tectonic events related to the India-Asia collision. For example, we were able to date various events such as the collision of the Ninety East Ridge with the Sumatra subduction zone, which chocked off the sediment supply to the Nicobar basin around 2 Ma as the ridge collided with the subduction zone.

Left: Results from the McNeill et al. (2017) of the sedimentation history of Bengal Fan (green dots) and Nicobar Fan (red dots). Middle/right: Reconstruction of India and Asia for the past 9 million years showing the sediment source from the Himalayas to both basins on either side of the Ninety East Ridge.

 

 

 

 

 

 

 

 

 

 

 

 

Lastly, on board Expedition 362 we were able to use microfossils to understand how and why big earthquakes happen. We targeted the incoming sediments to the Sumatra subduction zone that were partly responsible for the globally 3rd largest recorded earthquake (Mw≈9.2). This earthquake occurred in 2004 and produced a tsunami that killed more than 250,000 people.

From the seismic profiles (see example below), we found that the seismic horizon where the pre-decollement formed coincided with a thick layer of biogenetically rich sediment (e.g. radiolarians, sponge spicules, etc.) found whilst drilling. Under the weight of the overlying Nicobar Fan sediments, this critical layer of biogenic silica is undergoing diagenesis and fresh water is being chemically released into the sediments. The fresh water within these sediments is moving into the subduction zone where it has implications to the physical properties of the sediment and the morphology of the forearc region.

The Sumatra subduction zone. The dark orange zone represents the rupture area of the 2004 earthquake. Also shown are the drill sites of IODP Expedition 362 and the location of seismic lines across the plate boundary.

Seismic profile: The fault that develops between the two tectonic plates (the plate boundary fault) forms at the red dotted line. Note the location of the drill site.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

References

Hüpers, A., Torres, M. E., Owari, S., McNeill, L. C., Dugan, & Expedition 362 Scientists, 2017. Release of mineral-bound water prior to subduction tied to shallow seismogenic slip. Science, 356: 841–844. doi:10.1126/science.aal3429

McNeill, L. C., Dugan, B., Backman, J., Pickering, K. T., & Expedition 362 Scientists 2017. Understanding Himalayan Erosion and the Significance of the Nicobar Fan. Earth and Planetary Science Letters, 475: 134–142. doi:10.1016/j.epsl.2017.07.019

Minds over Methods: Experimental seismotectonics

Minds over Methods: Experimental seismotectonics

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

 

Credit: Michael Rudolf

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

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

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

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

 

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

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

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

 

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

 

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

 

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

 

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

 

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

 

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

EGU – Realm and Maze? An interview with Susanne Buiter, the current chair of the EGU Programme Committee

EGU – Realm and Maze? An interview with Susanne Buiter, the current chair of the EGU Programme Committee

source: ngu.no

Susanne Buiter is senior scientist and team leader at the Solid Earth Geology Team at the Geological Survey of Norway. She is also the chair of the EGU Programme Committee. This means that she leads the coordination of the scientific programme of the annual General Assembly. She assists the Division Presidents and Programme Group chairs when they build the session programme of their divisions, helps find a place for new initiatives and tries to solve issues that may arise. This also includes short courses, townhall and splinter meetings, great debates, events on arts and other events. The programme group also initiates discussions on how to include interdisciplinary or transdisciplinary science and how to accommodate the growth of the General Assembly. Questions: Micha Dietze, Annegret Larsen (both GM Early Career Representatives), and Anouk Beniest (EGU TS Early Career Representative)

 

Susanne, you are a perfect example of a scientist bridging scientific work with scientific management. What brought you to this and how do you manage keeping the balance?
I would not call it perfect! And I find it not so easy to keep a balance. I am very fortunate that my employer, the Geological Survey of Norway, recognises the importance of organisations like EGU for the geoscience community in Europe. That means that I can partly use working hours for EGU activities and that is a great help. For me, EGU fulfils an important task in bringing people together for networking, starting new projects, discuss new ideas and I would like to contribute to making that possible. I guess one thing led to the other, but what is important for me is that all activities are truly fun and rewarding.

 

It seems you have filled almost all the different possible jobs within the EGU: giving talks, discussing posters, judging presentations, convening sessions, coordinating ECS activities like short courses, acting as Programme Group member and leader, serving as TS Division President, and now working as Programme Committee Chair. Could you describe what the main goals of the EGU are for you, and what brought you to become such an active member of the EGU community?
I see the role of EGU as serving the geoscience community through enabling networking, discussions and information sharing. Our General Assembly is very important for this and also our journals. I love the outreach and education that EGU does, through the GIFT programme and attempts to interact with politics and funding agencies. By the way, the short courses are for and by all participants, including the ECS, but not only!

I would really like to encourage ECS to submit session proposals during our call-for-sessions in Summer. And please consider to submit your abstract with oral preference, so conveners can schedule ECS talks.

 

Could you shed some light on the structure of this big ship called EGU in a few sentences?
What characterises EGU is that the union is by the community and for the community. EGU has a small office in Munich that oversees the day-to-day operations and coordinates our media activities (www.egu.eu). They are also EGUs long-term memory. We have 22 divisions from Atmosphere Sciences AS to Tectonics and Structural Geology TS. The division presidents are usually also chair of their associated Programme Group, with the same abbreviations AS, BG, CL etc that you see in our programme at the General Assembly in Vienna. They schedule their parts of the conference programme. For this, programme group chairs rely on the work of conveners (you!) to propose and organise sessions. Division presidents are also member of EGU’s council, together with EGU’s executives. Here decisions are taken on budgets, committee work, new executive editors of journals etc. EGU has among others committees for awards, education, outreach, publications and topical events (https://www.egu.eu/structure/committees/). Copernicus is hired by EGU for organisation of the General Assembly and publication of the 17 journals (https://www.egu.eu/publications/open-access-journals/). All EGU journals are open access. Sorry, that was rather more than a few sentences…

 

How flexible – in your experience – is the EGU administration and organization on a scale of 1-10?
A 9! I would have like to say a 10, but improvements are always possible. The EGU office, executives, divisions and committees put a lot of effort in coordinating all activities. We actually rely on flexibility as EGU is bottom-up. This is also how new initiatives find a place. For example, EGU2018 will have a cartoonist-in-residence and a poet-in-residence, a new activity I am very excited about and that was proposed by participants.

 

Regarding the ECS, which role do you feel should they play at EGU level? What is running very well and what would you like to change? Where do you think are fields where you see opportunities to become more active?
About half of participants to our General Assembly identify as ECS according to the survey from 2017 and abstract submission statistics for 2018. So they should play an important role! Not only in the General Assembly, but also in our committees. The ECS representatives are important for their feedback to council, making the ECS opinions heard, and starting new activities, such as the networking reception, many short courses, and the ECS lounge. What I would like to change? More ECS session conveners please! I would really like to encourage ECS to submit session proposals during our call-for-sessions in Summer. And please consider to submit your abstract with oral preference, so conveners can schedule ECS talks.

 

What is most important for ECS to know about the EGU structure?
Know your ECS representative. At the General Assembly, come to the ECS forum on Thursday at lunch time and the ECS corner at the icebreaker. Connect with scientists in your division(s) by attending the division meeting.

 

From your perspective, what can we do to motivate more ECS to actively shape “their” EGU?
It is building on what you already do: share information on EGU, the divisions, that we are bottom-up and therefore rely on suggestions by community members. Encourage ECS to suggest sessions, volunteer as committee member when there are vacancies (these are advertised on www.egu.eu and through social media), and organise activities at, before and after the General Assembly. Encourage ECS to use the conference in Vienna to network with all participants, not only through ECS channels, and find new opportunities that way. My observation is that many experienced scientists love to discuss with ECS and perhaps even start new collaborations.

 

Which ways and approaches do you see to better connect ECS within and between Programme Groups?
I find especially connections *between* Programme Groups very interesting, not only for ECS. EGU is growing to a size that it has become more difficult to find time to look outside your own bubble. We have been investigating ways to make our programme more interdisciplinary and perhaps in the future also transdisciplinary, to try to create new approaches. That said, I am happy to see at the ice-breaker and networking reception that many ECS identify with more than one division! It is important to cross borders, that is where a lot of exciting research happens.

 

The mentoring programme is a rather new feature for many divisions. Could you give some feedback on how it went last year? Will it be a permanent item during the EGU General Assembly?
We organised the mentoring programme in 2017 as a pilot, which we on purpose kept somewhat low profile to generate feedback and develop our tools. We see the programme as a networking opportunity for both first-time and experienced attendees. Feedback was very positive, so we are rolling out in full this year. We offer matching, two meeting opportunities at the General Assembly and some guidance.

I encourage people to try a PICO presentation or convening a PICO session. I have run some poster-only sessions last years, which have been great fun as we had so much more time for discussions.

 

The EGU General Assembly can be overwhelming at first. What would you advice young (and not so young) researchers to do to have a successful meeting?
Attend short course SC2.1 on how to navigate the EGU (Monday at 08:30), read the first-timer’s guide to the General Assembly, and make sure you are on the mailing list for your division ECS representatives if they have one. Some divisions have an ECS evening event, do attend! Consider taking part in the mentoring programme of course. And prepare a personal programme before heading to Vienna. Not to follow it in detail, but at least to know where to go for talks, PICO, short courses, posters, and events. I would definitely use the General Assembly to talk to other participants, this is a great chance to expand your network.

 

Time and space are precious during the EGU General Assembly. There are over 10.000 contributions, many aiming at a talk, but ending up as posters, the session rooms are often overcrowded, the lunch break brings a rush and long queues. Is there any way the Union Council considers to improve certain bottle necks or are we already at the maximum of optimizing some of the conference logistics?
In 2017, we had ca. 17,400 presentations and 14,500 participants. We rented a new hall on the forecourt of the conference centre, which we will also have in 2018. This increased the conference space, taking pressure off the rooms and surely reduced queues. Copernicus and EGU work continuously on optimising the scheduling. We also started a broader discussion on future formats of the General Assembly. I would like to take this opportunity to encourage trying a PICO presentation or convening a PICO session. I have run some poster-only sessions the last years, which have been great fun as we had so much better time for discussions.

 

Many ECS approach their representatives because they are worried or disappointed to see their initiatives for scientific session proposals not succeeding. Instead they find year after year the same names behind established and crowded sessions. Do you have any advice how to deal with this or do you think this is not really an issue?
I am aware that this may unfortunately play for some sessions, but overall I think we cater well to new initiatives. My advice to Programme Group chairs is to encourage ECS conveners for new sessions and also to include ECS as part of long-running sessions that should rotate, and renew, conveners each year. Our General Assembly offers place for sessions on the basics and fields that require long-term developments, and at the same time also on new, emerging topics. Sometimes these sessions on upcoming topics may be small in number of submissions, but large in attendance. The best I can say to anyone is to discuss concerns or feedback regarding convening with the division president and the ECS division representative.

 

With the growing amount of members and participants (almost) every year, how do you see the EGU’s future both as a community and as one of the most important events?
EGU is an important voice of the Earth and space science community in Europe. I think the union should continue to do what it is good at: providing a platform for networking, discussions on new and old fun topics, and information sharing. I would like EGU to stay flexible and cater to new formats in its journals and at its General Assembly, the latter also in light of discussions on CO2 costs of meetings.

 

Thank you Susanne!
Could I emphasize again that EGU is bottom-up and depends on input from our communities? So please contact your ECS representative, the division president or me (programme.committee@egu.eu) with ideas and feedback!

 

Some more information online here:
www.egu.eu
https://meetings.copernicus.org/egu2018/information/programme_committee
https://www.egu.eu/gm/home/
https://www.egu.eu/gm/ecs/
http://www.geodynamics.no/buiter/

 

 

Paris: From quarry to catacombs

Paris: From quarry to catacombs


Paris, 2000 ya.
Claude is sweating all over. It’s mid-July and the sun is burning on his skin. With his hammer and shovel he is digging up grey and white stones. The faults and fractures in the rock help him to get the rocks out easily. But still, it’s hot and humid and his shift isn’t over yet. Luckily he can’t complain about the view. Lutetia, one of the new Roman settlements lies right in front of him, on the left bank of the Seine river.

Paris, 700 ya. Pierre sighs out deeply, his back hurts, but he has to continue. In the small corridors of the underground quarries at Montrouge south of Paris, he is digging for gypsum and cutting limestone for building material. The quarries are narrow and low, so he has to bend over all the time. Mining gypsum is easier, it’s a softer material. It’s the limestone that makes him suffer.

Paris, 300 ya. Jean covers his mouth. The stench is unbearable! Over 12 generations of Parisians are buried at the cemetery ‘Les Innocents’ in the heart of Paris. Last week, the cellar of a house located below the cemetery collapsed under the weight of the buried. The king decided to close all cemeteries within the city centre and so the cemeteries are being emptied. Jean found a job in this chaos, pulling a wagon at night, from the cemetery  to entrance of the old, abandoned quarries south of the city.

 

Figure 1. Geological map of France. The Paris basin is outlined in red and Paris (the red dot) is located at the centre of the basin. Credit: Written In Stone blog

Paris, 200 Ma – today
Paris is located at the heart of the Paris Basin, a NW-SE trending oval feature that measures over 140.000 kmand extends into the United Kingdom (Figure 1). The basin is positioned on top of a Palaeozoic crystalline basement. The lower lithologies are marine and continental sediments of Permian and Mesozoic age. A major subaerial, Palaeocene erosion event was followed by the deposition of Eocene limestones that is known now as ‘Parisian limestone’. The beige-white rocks are among the most famous building materials that is nowadays exported all over the world, but they used to be the main building material that characterizes the city of Paris. Of course, it is no coincidence that these rocks are of ‘Lutetian’ age, as Lutetia was the Latin name for Paris. The Lutetian is followed by the Bartonian stage, in which gypsum was deposited in the Paris basin, the second most important material that was mined in the Paris region. Oligocene, Miocene and Neogene successions consist of sands, marls and clays and cover all older sediments.

The familiar light-coloured, 6-7 stories high buildings, decorated with balconies and ornaments are Paris’ most famous selling point. Throughout the 20 districts all kinds of structures can be found that are made from these rocks; from regular houses to famous sights, such as the Notre Dame and the Louvre Museum. The rocks do not come from far. Scattered through the city, old quarries can be found above and below the ground (Figure 2). They were intensively mined from 2000 years ago until the 17th century to provide building material for the city of Paris.

 

Figure 2. Map of Paris with in black the old limestone quarries. Gypsum was mined around Mont Montmartre (18th arrondissement) and Belleville (19th and 20th arrondissement). Credit: Papyserge blog.

 

The very first mining activities in Paris were in an ‘open-pit’ kind of fashion, as our friend Claude from the introduction experienced. Paris was relatively small and these quarries were located just outside the old city walls. From some of them, we can still see signs nowadays, like the ‘Arènes de Lutèce’, situated in the 5th arrondissement. This was an open-pit mine that later turned into a small Roman amphitheatre (Figure 3).

Figure 3. ‘Les Arènes de Lutèce’. Before this place turned into an amphitheatre for the amusement of the Roman people it served as a limestone quarry to provide building material. Credit: Anouk Beniest.

As Paris expanded, more building material was needed. Even though the already excavated blocks were re-used, underground quarries for limestone and gypsum emerged below Montrouge (14th district), Parc Montsouris (13th district), Parc Butte-Chaument (19th district) and Montmartre (18th district). People like Pierre were subsurface miners. Initially they only excavated the Lutetian limestones. Later also Bartonian gypsum was extracted. Eventually the quarries moved south of Paris, outside the present-day city-limits, leaving a whole network of galleries and corridors below the city (Figure 4).

These quarries below Paris were almost forgotten, as they were abandoned for several decades. When the largest cemetery in the city-center of Paris, ‘Les Innocents’, collapsed and conditions became untenable, the king decided to clear all cemeteries within the citywalls. Our friend Jean was paid for his duties, moving carriages full of bones to the southern end of the quarries, close to the present-day entrance of the catacombs at ‘Denfert-Rochereau’. The major clean-up took a year-and a half during the end of the 18th century and resulted in corridors full of nicely stacked bones, decorated with skulls (Figure 5). During a second wave, more cemeteries were emptied in the early 19th century. Even after World War II, the old quarries at Pièrre-Lachaise were used as final resting place of the thousands of Parisians that found death during the war.

 

Figure 4 (left). One of the restored quarries below Paris. The pillars were used to support the overlying rocks and to avoid collapsing of the galleries. Credit: Secret de Paris. Figure 5 (right). The galleries of the old quarries were filled with the buried from the Parisian cemeteries. The remains were neatly stacked into blocks with the skulls used as decoration. Marble sign were placed to remind visitors of the origin of the bones. Credit: Anouk Beniest.

 

For over 150 years, parts of the catacombs have been accessible so people could pay tribute to their dead ancestors. During periods of civil unrest, people would use the old quarries as hide-outs. This was of course not without any risk; people could easily get lost and never see daylight again. In calmer periods, people would still search for ways to get inside, to party all night, go on adventure or just enjoy the silence without leaving the city. In 2005, part of the catacombs was renovated and 1.7 km of galleries is now open for public. The other 300 km of abandoned quarries remains closed, and only those who know the secret entrances dare to descend into these ancient galleries, where Pierre used to mine his limestones and Jean dumped his load of bones.