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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.

Minds over Methods: Making ultramylonites

Minds over Methods: Making ultramylonites

“Summer break is over, which means we will continue with our Minds over Methods blogs! For this edition we invited Andrew Cross to write about his experiments with a new rock deformation device – the Large Volume Torsion (LVT) apparatus. Andrew is currently working as a Postdoctoral Research Associate in the Department of Earth and Planetary Sciences, Washington University in St. Louis, USA. He did his PhD at the University of Otago, New Zealand, although he is originally from the UK. His main research interest lies in understanding how micro-scale deformation processes influence the evolution of Earth’s lithosphere and tectonic plate boundaries. Hopefully we will be seeing more of him in the very near future” – Subhajit Ghosh.

Credit: Andrew Cross

Investigating strain-localisation processes in high-strain laboratory deformation experiments

Andrew Cross, Postdoctoral Research Associate at the Department of Earth and Planetary Sciences, Washington University in St. Louis, USA.

Below the upper few kilometres of the Earth’s surface – where rocks break and fracture under stress – elevated temperatures and pressures enable solid rocks to flow and bend, like a chocolate bar left outside on a warm day. This ductile flow of rocks and minerals plays a crucial role in many large-scale geodynamic processes, including mantle convection, the motion of tectonic plates, the flow of glaciers and ice sheets, and post-seismic and post-glacial rebound.

Fig. 1: Creep deformation occurs over very long timescales in the Earth. To replicate these processes on observable timescales, we must increase the rate of deformation in the laboratory. Credit: Andrew Cross

Unlike seismogenic slip that periodically accommodates large displacements over very short timescales, ductile flow occurs continuously, and at an almost imperceptibly slow rate: for example, rocks in the Earth’s interior creep at a rate roughly 10 billion times slower than that of the long-running pitch drop experiment1. Since few researchers are willing to wait millions of years to observe creep deformation in nature, we need ways of replicating these processes on much shorter timescales. Fortunately, by increasing temperature and the rate of deformation in the laboratory, we can generate creep behaviour in small samples of rock over timescales of a few hours, days, or weeks (Fig. 1).

In the Experimental Studies of Planetary Materials (ESPM) group at Washington University in St. Louis, we have spent the last couple of years developing a new rock deformation device – the Large Volume Torsion (LVT) apparatus (Fig. 2) – for performing torsion (twisting) experiments on geologic materials. By twisting small, disk-shaped rock samples, we are able to apply much more deformation (“strain”) than by squashing cylindrical samples end-on: this enables us to replicate deformation processes that operate in high-strain regions of the Earth (along the boundaries between tectonic plates, for instance).

Fig. 2: The Large Volume Torsion (LVT) apparatus. A 100-ton hydraulic ram applies a confining pressure, while electrical current passes through a graphite tube around the sample, generating heat through its electrical resistance. A screw actuator (typically used to raise and lower drawbridges) is used to rotate the lower platen and twist the sample, held between two tungsten-carbide anvils. Credit: Andrew Cross

Using the LVT apparatus, we are starting to investigate the microstructural and mechanical processes that lead to the formation of mylonites and ultramylonites: intensely deformed rocks that comprise the high-strain interiors of ductile shear zones and tectonic plate boundaries. It is widely thought that dramatic grain size reduction during (ultra)mylonite formation causes strain localisation, since strain-weakening deformation mechanisms (i.e., diffusion creep and grain boundary sliding) dominate at small grain sizes. However, grain size reduction (and therefore strain-weakening) is counteracted by the tendency of grains to grow over time, in the same way that bubbles in soapy water merge and grow over time.

An effective way of limiting grain growth is through “Zener pinning”, whereby the intermixing of grains of different mineral phases prevents grain boundary migration (and therefore growth). However, despite its suspected importance for ultramylonite formation and the occurrence of localised deformation on Earth (and possibly other planetary bodies), the processes leading to interphase mixing remain somewhat poorly understood and quantified.

Fig. 3: A comparison between our experimentally deformed calcite-anhydrite samples2 (backscattered electron (BSE) images), and natural metagranodiorite mylonites from Gran Paradiso, Western Alps3 (quartz grains, in black, mapped using electron backscatter diffraction (EBSD). Credit: Andrew Cross and Kilian et al., 2011.

To investigate phase mixing processes, we recently performed torsion experiments on mixtures of calcite and anhydrite. By deforming these mixtures to different amounts of strain, and then analysing the deformed samples in a scanning electron microscope, we were able to observe and quantify the evolution of deformation microstructures and mechanisms leading to ultramylonite formation. Backscattered electron (BSE) images show that clusters of the different minerals stretch out to form very thin, fine-grained layers, similar to foliation in natural shear zones (Fig. 3). At relatively large shear strains (17 < γ < 57) those layers disaggregated to form a fine-grained and homogeneously mixed aggregate. Electron backscatter diffraction (EBSD) analysis showed that calcite crystals became progressively more randomly oriented during phase mixing, indicative of a transition to the strain-weakening diffusion creep and grain boundary sliding regime.

The fact that a large amount of strain is required for phase mixing – and therefore strain-weakening – suggests that 1) only mature (highly-strained) shear zones are likely to maintain their weakness over long periods of geologic time, and 2) these features are therefore more likely to be reactivated after periods of quiescence. Inherited, long-lived mechanical weakness may well explain why tectonic plate boundaries are often reactivated over multiple cycles of continent accretion and rifting.

 

http://smp.uq.edu.au/content/pitch-drop-experiment

 Cross, A. J., & Skemer, P. (2017). Ultramylonite generation via phase mixing in high‐strain experimentsJournal of Geophysical Research: Solid Earth122(3), 1744-1759.

 3 Kilian, R., Heilbronner, R., & Stünitz, H. (2011). Quartz grain size reduction in a granitoid rock and the transition from dislocation to diffusion creepJournal of Structural Geology33(8), 1265-1284.

Teaching in the 21st century – a PICO session

Teaching in the 21st century – a PICO session

With the progress in the digital world there are more and more e-tools available for research and teaching. What are smart ways to make use of new techniques in teaching? For inspiration and learning, Hans de Bresser, Janos Urai and Neil Mancktelow convened a PICO session at the EGU 2017 General Assembly to showcase present-day e-learning opportunities to improve the efficiency and quality of teaching structural geology and tectonics. Despite an 8h30 morning slot and a limited number of abstracts, all spots during the 2 min madness were taken and all authors were talking the full 90 minutes – and some even well into the coffee break – about what they are using and how. Inspirational and fun!

Hans, you’re somewhat of an expert on teaching in Utrecht as former head of teaching in geosciences, how would you say that teaching has changed from back when you were a student?

”In my time as a student, I spent hours learning to identify rocks and recognizing minerals in thin sections, browsing back and forward in text books packed with determination tables and graphs of which more than half was not relevant for me. I also invested a lot of time in making maps in the field, carefully adding measurements and colors, hoping that I did it right the first time (never) and that I wouldn’t spill coffee over my precious products (it happened). And I sat in classes in which professors talked for hours, repeating the content of books that I had in front of me, while my level of activity in class was so low that preventing to fall asleep was a serious challenge.  I really learned a lot, definitely had a lot of fun, but looking back I feel it could have been done more efficiently. New styles of teaching, such as blended learning and flipping the class room, and state-of-the-art e-tools for data collection, modelling and visualization now help us to be very efficient and improve the quality of teaching. And it can be fun”.

 

Broadly speaking the presented aids can be divided in the following 3 categories, for each category we give examples below.

1. How to bring the real and experimental world into the classroom?

2. How to make life easier for a teacher?

3. Can we add extra information using the virtual world?

 

Category 1:  How to bring the real and experimental world into the classroom?

Benjamin Craven explaining his PICO: Fieldwork Skills in Virtual Worlds. Credit: Anne Pluymakers 

Virtual landscape, presented by Benjamin Craven: a 3D model of a field area, to bring the real world in the classroom, and increasing the efficiency of real world field teaching. Different packages of open source software make it easy to design your own landscapes, though you can also make use of the already made world. One could also teach students photogrammetry, to enable them to make their own 3D models using photos made with a smart phone, of rocks or other items in the classroom. One idea is to ask students to create a geological map in the virtual “field”, but then to give each student only a limited amount of time to do so. This allows them to learn how to plan their time in mapping projects.

Drones and 3D models , presented by Thomas Blenkinsop. Using drones one can make 3D surface models, which can be combined with cross sections to create a 3D MOVE (TM Midland Valley) project. It starts with the regular manual work, but adding the digital models improves 3D thinking as well as it allows students to check their own cross sections.

Deforming ice with students, presented by Dave Prior. A low-cost ice deformation rig, designed to be used by student teams. It brings Dave’s own research into the class room. Through a questionnaire teams were designated by Dave to get the right mix of skills to eventually present a poster. Some results are of high enough quality to publish.

 

Thomans Blenkinsop explaning about using drones, Lidar measurements and 3D models for undergraduate teaching. Credit: Anne Pluymakers.

Category 2: How to make life easier for a teacher?

Jupyter notebooks, presented by Florian Wellmann: software for those who can’t program. It makes it easier to create exercises, and to allow students to play around with parameters. Automatic and manual exercise grading are both easy.

STEREOVIDEO, presented by Jose A. Alvarez-Gomez. This is a (currently Spanish only) channel with various Youtube video instructions on how to use stereonets. The channel will bring more subjects later. It is very popular in Latin America.

 

Category 3:  Can we add extra information using the virtual world?

Zappar, presented by Friedrich Hawemann. Using an icon on a poster and a free download app one can add extra layers of information to images. It also recognizes objects such as a polished rock, allowing the teacher to add arrows, circles etc. to highlight features

 

By Anne Pluymakers (just a visitor) and Hans de Bresser (session convener)

Folding and Fracturing of Rocks – 50 years of research since the seminal textbook of JG. Ramsay

Folding and Fracturing of Rocks – 50 years of research since the seminal textbook of JG. Ramsay

John G. Ramsay1 wrote his seminal textbook on the folding and fracturing of rocks in 1957, almost 20 years before I was born (and I don’t count myself as young!). So why did I co-convene a session at EGU in 2017 to celebrate the book? Because the book, in many ways, expresses the legacy that John has given to structural geology. He followed it with a series of books of the same ilk – Ramsay and Huber ‘The Techniques of Modern Structural Geology’, published in two volumes (1984 and 1987), was a main stay for me as an undergraduate and postgraduate and well actually now; although, it is often not to be found on my bookshelf, but on the desk of one of my graduate students. The Techniques of Modern Structural Geology, like the Folding and Fracturing of Rocks, is beautifully illustrated and describes geometrically and mathematically many of the now commonly used techniques in structural geology. So to me Ramsay, although I did not know him personally, was the name behind this interesting subject structural geology.

John Ramsay opening the session on his book “Folding and Fracturing of Rocks”. Credit: Susanne Buiter

It was as a postgraduate student that I met John at a Tectonic Studies Group (TSG) meeting and had the pleasure of dancing with him after the conference dinner! Many years on I am now the Chair of TSG, one of several specialist groups affiliated to the Geological Society of London. John was instrumental in founding TSG in 1970 @TSG_since1970 , when he was working at Imperial College London. As the Chair of TSG I was approached by several people enquiring about plans to celebrate 50 years since the publication of the Folding and Fracturing of Rocks. Given John’s European career and global impact EGU seemed the place to celebrate and I contacted some of John’s PhD students and collaborators to join me in convening a session to celebrate the book, John and structural geology.

Early on the morning of April 25th John gave the opening paper for the session to a large audience. He spoke not to a series of power point slides but to the audience about the book, his interests and his life as a structural geologist. I think one of the key messages for me is the one that he ended his paper with; that fieldwork is fundamental to structural geology and models need to be derived from and tested against field observations. John was followed by a series of speakers from Rod Graham, of Ramsay and Graham (1970)2 on shear zones from the field to seismic interpretation, to Richard Lisle (co-author with John on the third volume in the Techniques of Modern Structural Geology series)3. Richard gave an overview of the citations for the Folding and Fracturing of Rocks; it is the most cited structural geology textbook. Further papers were given in the afternoon poster session, celebrating and giving modern, and old, twists on aspects of the book.

Afterwards at the reception. Credit: Susanne Buiter

John joined many well-wishers at a reception and dinner after the session. John has given a legacy to structural geology, through his focus on the geometry and mathematics of structures backed up with detailed field observations, work that very much forms part of modern structural geology today.

The Folding and Fracturing of Rocks is currently published by Blackburn press.

 

Click here to go to the webpage of EGU Session TS1.3.

 

 

Blog by Clare Bond,

University of Aberdeen

Thanks to TecTask and TSG for sponsorship

 

1.John was born in 1931 in London, he was educated at Imperial College, London gaining a first class degree in 1952 and a PhD, working on superposed folds at Loch Monar Scotland. After military service he returned to the staff at Imperial, before taking up Professorial positions at the University of Leeds and the ETH in Zurich. He has been recognized for his work in advancing structural geology with many awards: Bigsby (1973) and Wollaston (1986) medals of the Geological Society of London, the Société Géologique de France Prestwich Medal in 1989, Sir Arthur Holmes Medal of the European Union of Geosciences (EGU) in 1984, C. T. Clough medal (1962) of the Geological Society of Scotland, the University of Liège medal in 1988. In 1992 he was named a Commander of the Most Excellent Order of the British Empire in the Queen’s Honours list. He is a Fellow of the Royal Society(elected 1973), and holds Honorary Fellowships of the Geological Society of America, the Société Géologique de France, the Indian National Science Academy, the American Geophysical Union, the US National Academy of Sciences and the Geological Society of London.

2.Ramsay, J.G. and Graham, R.H., 1970. Strain variation in shear belts. Canadian Journal of Earth Sciences7(3), pp.786-813.

3.Ramsay, J.G. and Lisle, R.J., 2000. The techniques of modern structural geology. Volume 3: Applications of continuum mechanics in structural geology. Academic, San Diego, Calif.

 

You’re an early career scientist and you want to go somewhere… but where?

You’re an early career scientist and you want to go somewhere… but where?

Only a few more days and the General Assembly of the EGU 2017 will start! Five exciting days with science and the opportunity to meet your colleagues and collaborators, both the old and the new. Earlier this week the outgoing TS President Susanne Buiter and the incoming TS President Claudio Rosenberg posted a blog with TS highlights, but what are the must-see for the Early Career Scientists? This is where we, outgoing TS ECS reps, come in! So we provide here tips for the hottest TS ECS events the coming week.

 

Meetings/socials:

Don’t forget to come for dinner with the other TS ECS at Brandauers Bierbogen (Heiligenstädter Str. 31) on Monday, at 20h.

We welcome everyone to the ECS corner at the icebreaker on Sunday. Pick up your badge, meet old and new friends at Foyer E, from 18h30 to 21h.

http://meetingorganizer.copernicus.org/EGU2017/session/25204

The TS Division Meeting is on Wednesday, 12h15-13h15. Lunch will be provided. What happened within the TS Division last year? What are the plans for next year?

http://meetingorganizer.copernicus.org/EGU2017/session/25276

Come give feedback! Your new TS ECS representative Anouk Beniest organizes an informal get-together over lunch on Thursday. We meet at 12h15 at the EGU flags just outside the building, we’ll get a sandwich and find a nice spot to sit and chat on how to improve things for TS ECS. All are welcome!

Meet your reps! The EGU Division President Claudio Rosenberg and the outgoing ECS Representatives Anne Pluymakers and João Duarte plus the incoming ECS Representative Anouk Beniest! They will all be at the EGU booth on Thursday from 14h15 to 15h.

 

Short courses:

For those of you who have not been to Vienna before, start the week with a crash course on how to navigate EGU as a first-timer, on Monday from 8h30 to 10h in Room -2.31. http://meetingorganizer.copernicus.org/EGU2017/session/25606#

If you like fully open access and transparent publishing, you should consider the course on how to publish in EGU journals: Solid Earth and Earth Surface Dynamics. Meet the Editors on Monday, 13h30 – 15h in room -2.91.

http://meetingorganizer.copernicus.org/EGU2017/session/25617

Of course we would love to see more TS-related research, also by the youngsters! So come to ‘How to write a successful ERC Grant proposal’ on Tuesday, 13h30 – 15h, in room -2.91.

http://meetingorganizer.copernicus.org/EGU2017/session/25127

Publish more pretty pictures to show of those beautiful structures with ‘Virtual Polarizing Microscopy in Petrology and Microtectonics’ on Wednesday 10h30 – 12h00, in room -2.16. http://meetingorganizer.copernicus.org/EGU2017/session/25156

 

Debates:

After open access publishing, we seem to transition now towards open science: all data available for everyone. Is this really the way to go? Come to the panel debate on Thursday, 15h30-17h in Room E1.

http://meetingorganizer.copernicus.org/EGU2017/session/25038

With the decreased amounts of funding and less and less permanent jobs the pressure of ‘publish or perish’ is mounting. What is the best way of judging early career scientists? Come to the group debates on Wednesday 19h-20h30 in room G1.

http://meetingorganizer.copernicus.org/EGU2017/session/25040

If you have a disability or chronic condition and want to find out more about the Chronically Academic Network and the type of support it provides, this is the meeting to attend. This meeting will also gather some information that will be provided to EGU to make the GA and EGU in general more accessible – so your input is much appreciated. Come come! Thursday 12h15 – 13h15 to room Room 2.61

http://meetingorganizer.copernicus.org/EGU2017/session/25644

 

Sessions:

Every TS session is an ECS session! So browse the program, and look for something that captures your interest, either directly inside your field or just outside. Due to its size EGU is the perfect conference to look across the boundary into other fields. Create your program online and download it into the EGU2017 app for Apple and Android.

http://meetingorganizer.copernicus.org/EGU2017/sessionprogramme

http://meetingorganizer.copernicus.org/egu2017/app

If you have no other plans, come to the Arne Richter Award lecture by outgoing ECS rep João Duarte “The Future of Earth’s Oceans: consequences of subduction invasion in the Atlantic” on Wednesday 9h30–10h in Room D3.

http://meetingorganizer.copernicus.org/EGU2017/session/25355

Equal opportunities for all! On Friday there are talks from women in geosciences from 10h30 to 12h15 in Room L4/5  and on equal opportunities in general from 13h30–17h in Room L4/5.

http://meetingorganizer.copernicus.org/EGU2017/session/23685

http://meetingorganizer.copernicus.org/EGU2017/session/23686

 

These and other sessions for ECS across all program groups can be found at http://meetingorganizer.copernicus.org/EGU2017/sessions-of-special-interest/ECS

 

written by: Anne Pluymakers and João Duarte, outgoing TS ECS reps

photo credits: TS Twitter and Facebook pages and EGU blog

Highlights at EGU 2017 from the division for Tectonics and Structural Geology

Susanne Buiter (outgoing TS president, Geological Survey of Norway) and Claudio Rosenberg (incoming TS president, UPMC France)

It is with great pleasure that we write this blog welcoming everyone to EGU’s upcoming General Assembly in Vienna, and especially to the many events organised by our division for Tectonics and Structural Geology! We are highlighting some of the week’s many events below, though it is in fact really difficult to choose highlights as of course the contribution of every individual to the success of the meeting is a highlight!

The TS programme can be found directly at these two links:

http://meetingorganizer.copernicus.org/EGU2017/meetingprogramme/TS

http://egu2017.eu/EGU2017_schedule_TS.pdf

We would encourage you to download the meeting app (http://app.egu2017.eu) and/or make a personal programme (http://meetingorganizer.copernicus.org/egu2017/personal_programme) in addition to taking the above pdf schedule along to Vienna.

In the list below we emphasized award lectures, PICO sessions, poster-only sessions, short courses that TS co-organises, and of course the opportunities where you can give us feedback, such as the division meeting and Meet EGU. But please don’t forget that we are running several scientific sessions in parallel throughout the day!

At the General Assembly and throughout the year, you can follow the division via our webpage (http://www.egu.eu/ts/home/), Facebook (http://www.facebook.com/TSdivision), twitter (https://twitter.com/EGU_TS) and the division mailing list (http://lists.egu.eu/cgi-bin/mailman/listinfo/ts).

Don’t forget to tweet about the General Assembly using #EGU17TS and #EGU17!

We wish you a great conference!

Susanne and Claudio

 

Monday 24 April

  • Start of the TS scientific programme at 08:30 with three sessions in parallel: TS1.4 “New geochronological approaches for quantification of geological processes”, 3 “Structures and patterns in fractured and porous media: witnesses for paleostress and fluid flow”, and TS7.4 “Probing the subduction plate interface”
  • Short course SC48 “Publishing in EGU journals: Solid Earth and Earth Surface Dynamics – Meet the Editors”, 13:30-15:00, room -2.91
  • PICO TS8.1 “Digital mapping and 3D visualization approaches in the Earth Sciences”, 15;30-17:00, PICO spot 5a
  • TS division Early Career Scientists event at Brandauers Bierbögen (Heiligenstädter Str 31), starting at 20:00

 

credit: Susanne Buiter

 

Tuesday 25 April

  • TS1.3 “Folding and Fracturing of Rocks – 50 years of research since the seminal text book of JG. Ramsay”, 08:30 – 10:00/D3 and 17:30-19:00/Hall X2
  • PICO TS8.3 “Analogue and numerical modelling of tectonic processes”, 10:30-12:00, PICO spot 5a
  • The Stephan Mueller medal lecture by Cees Passchier, “Panta Rhei – the changing face of rocks “, 16:00-17:00, room D3
  • Poster-only session TS1.1 “Open Session on Tectonics and Structural Geology”, 17:30 – 19:00, Hall X2

 

credit: Susanne Buiter

 

Wednesday 26 April

  • PICO TS1.2 “Teaching Structural Geology and Tectonics in the 21st century”, 08:30-10:00, PICO spot 1
  • The Arne Richter Award lecture by João Duarte “The Future of Earth’s Oceans: consequences of subduction invasion in the Atlantic”, 09:30-10:00, room D3
  • Short course SC9/TS10.1 “Virtual Polarizing Microscopy in Petrology and Microtectonics”, 10:30-12:00 room -2.16
  • Division meeting of Tectonics and Structural Geology: Your chance to provide us with feedback on the division and our programme. A light lunch will be provided. 12:15 – 13:15, room G1
  • PICO TS3.3 “Microstructure and texture analysis: New methods and interpretations”, 13:30-15:00, PICO spot 5b
  • Poster-only sessions:
    • TS8.4 “Learning from failed models and negative results”
    • TS9.2 “Oceanic and continental transform plate boundaries: nucleation, evolution and tectonic significance”

 

credit: Susanne Buiter

 

Thursday 27 April

  • PICO TS8.2 “Unravelling the Earth subsurface structure from seismic imaging and interpretation, geological observations, and numerical Experiments” 10:30-12:00, PICO spot A
  • Arthur Holmes medal lecture by Jean-Pierre Brun, “The extending lithosphere”, 12:15-13:15, room E1
  • Meet the EGU Division Presidents of Tectonics and Structural Geology (us!), 13:30-14:15, EGU Booth
  • Meet the incoming EGU Division President (Claudio) and the ECS Representatives of Tectonics and Structural Geology (Anne Pluymakers and João Duarte (both outgoing), and Anouk Beniest (incoming)), 14:15-15:00, EGU Booth

 

Friday 28 April

  • 17:30 – 19:00 Friday evening poster sessions! All are in Hall X2:
    • TS5.1 “Bridging Earthquakes and Tectonics: give-and-take”
    • TS5.4 “Advances in understanding earthquake processes and hazards in regions of slow lithospheric deformation”
    • TS6.1 “The evolution and architecture of rifts, rifted passive margins, and mid oceanic ridges: from mantle dynamics to surface processes”
    • TS7.5 “The Caledonian orogen of the North Atlantic region: understanding tectonic processes in collisional belts”
    • TS7.6 “Lithospheric and crustal dynamics of the Wilson Cycle: The Iberia case study”

 

credit: Susanne Buiter

 

Minds over Methods: Reconstruction of salt tectonic features

Minds over Methods: Reconstruction of salt tectonic features

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

 

Credit: Michael Warsitzka

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

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

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

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

(text continues below figure)

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

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

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

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