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Tectonics and Structural Geology

Tectonics

Minds over Methods: Dating deformation with U-Pb carbonate geochronology

Minds over Methods: Dating deformation with U-Pb carbonate geochronology

Credit: Nick Roberts

For this edition of Minds over Methods, we have invited Nick Roberts, a research scientist at the British Geological Survey, working within the Geochronology and Tracers Facility (GTF) running a LA-ICP-MS laboratory. Nick has a background in ‘hard-rock’ geology, incorporating geochemistry, geochronology, and magmatic and metamorphic petrology across a wide range of tectonic settings, and is now involved in projects covering a breadth of earth system science, from Archean ore deposits to early hominid evolution. Nick is experienced in developing LA-ICP-MS U-Pb methods. He was involved in the first characterisation of a natural carbonate for use as a reference material, and in demonstrating the applicability of LA-ICP-MS U-Pb carbonate geochronology to a number of key applications, such as dating brittle deformation, ocean crust alteration, and paleohydrology.

 

Faults and fractures are ubiquitous in the Earth’s upper crust. As well as providing deformation histories of basins and orogens, they are critical for understanding the formation, migration and storage of natural resources. Determining the absolute timing of fault slip and fracture opening has lacked readily available techniques. Most existing methods require specific fault gouge mineralogy that is not always present, e.g. K-Ar illite dating. Other methods require a specific composition of fault-hosted mineralisation, e.g. U-Th/He dating of hematite, Th-Pb dating of hydrothermal monazite, U-Pb/U-Th dating of opal, and U-Pb/U-Th dating of carbonate. The latter is the most widely applicable, since carbonate minerals (e.g. calcite, dolomite) are common to many fault and fracture systems throughout a wide range of geological settings. The ability to date carbonate mineralisation with the popular method of U-Pb Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS), is opening up new doors in tectonics and structural geology.

 

The method

The key to the LA-ICP-MS method is its high spatial resolution (~20-200 microns); the method involves sampling of material with a laser and measuring the composition of that material with an ICP-MS. Uranium concentrations in carbonate are low when compared to most other U-Pb chronometers, typically 10 ppb to 10 ppm, which is one or two orders less than a typical zircon. Uranium concentration, particularly in vein-filling calcite, can also be highly variable within in a single sample, spanning orders of magnitude over length-scales of 10s of microns or less. A major benefit of LA-ICP-MS dating, is that this variation in composition can be targeted and sampled with the laser, which can lead to precise age estimates through measurement of wide-ranging U/Pb ratios (see example in Figure 1). High uranium zones can also be rather elusive and searching for a needle in a haystack is often an appropriate analogy. Another benefit of LA-ICP-MS is that many samples can be screened in a single session, to select those of the most favourable composition for dating, and to find within those samples the regions with highest U. The method can be utilised on polished thick sections or blocks, or chips or grains mounted in epoxy-resin blocks. This in situ technique makes it highly versatile and allows for complementary analysis that are crucial for providing context to the dates, such as cathodoluminescence, optical microscopy, trace element mapping, and in-situ Sr isotope analyses.

 

Figure 1. Example of vein-filling calcite with LA-ICP-MS elemental map of uranium, and corresponding U-Pb concordia (Tera-Wasserburg) plot from LA-ICP-MS U-Pb spot analyses across the same region. Credit: Nick Roberts

 

Dating deformation

Carbonate, primarily calcite, occurs as vein-filling mineralisation in fractures and along fault planes (see Figure 2 for examples). Such occurrences of carbonate have precipitated from a fluid, and thus, provide a record of past fluid-flow. Carbonate vein-fill can also be used to provide absolute timing constraints on brittle deformation. Carbonate that infills a fracture provides a minimum age for the timing of fracture opening. Many fractures and fault planes exhibit multiple opening and/or slip events that lead to multiple episodes of carbonate precipitation. From these, carbonate dates can be used to bracket the timing of fault slip, and potentially provide constraints on the longevity and periodicity of fault slip events.

Figure 2. Examples of calcite vein-fill in various faults and fracture types. Credit: Nick Roberts and Jonny Imber

 

Good dates need good petrography

The key to dating fault slip and fracture opening is detailed petrography. Vein mineralisation takes on a wide variety of growth textures, and these can used to infer the history of mineral precipitation as well as fault kinematics. For example, veins may exhibit syntaxial (growth from the margin towards the centre), antitaxial (growth from the centre towards the margin) or with no preferred growth direction. Calcite morphologies are wide-ranging, from flattened, to blocky, to elongate, to fibrous (see example in Figure 3). Calcite can also exhibit deformation twinning which can be used to infer the strain regime. These textures and morphologies, along with features such as inclusion trails, shear bands and healed fractures, allow us to interpret the combined history of fracture opening, displacement and fluid precipitation.

Figure 3. Example of a complex calcite fracture-fill from a mudstone-hosted normal fault, showing cathodoluminescence and reflected light imaging, schematic textural interpretation, LA-ICP-MS trace element mapping, and a simple model of vein evolution. Credit: Nick Roberts

 

Carbonate – a treasure trove of geochemical proxies

One of the key benefits to dating carbonate mineralisation, is that its elemental and isotopic composition provides a wide-ranging archive of fluid chemistry and temperature. Traditional methods include stable carbon and oxygen isotopes and radiogenic Sr isotopes; these provide information on the source of fluids and allow the characterisation of subsurface fluid pathways. Trace elements provide further information on fluid source and rock-fluid interaction and are particularly useful for characterising proxies such as the reducing potential of precipitating fluids. Clumped isotopes are a novel method that has been explored over the last decade and provide a proxy for the fluid temperature during carbonate precipitation. In combination with U-Pb dating, clumped isotopes can characterise ancient hydrothermal fluid-flow in the upper crust (MacDonald et al., 2019). A benefit of LA-ICP-MS geochronology, is that various in-situ methods can be combined, allowing fluid composition and dating to be analysed from the same domain. For example, carbon and oxygen isotopes can be measured in situ using an ion microprobe, and Sr isotopes and trace elements can be measured using LA-ICP-MS.

 

Application to tectonics

To date, LA-ICP-MS carbonate geochronology has been applied successfully to a range of tectonic settings to constrain the timing of brittle deformation. These include the far-field effects of the Pyrenean orogeny in southern England (Parrish et al., 2018) and Alpine orogeny in Sweden (Goodfellow et al., 2018), rift-related faulting during opening of the North Atlantic in the Faroe Islands (Roberts & Walker, 2016), graben formation in the Alpine orogen (Ring & Gerdes, 2016), the timing of the Dead Sea and North Anatolian transform fault zones (Nuriel et al., 2017 and 2019, respectively), nappe stacking in the Arabian Peninsula (Hansman et al., 2018), foreland deformation of Sevier-Laramide orogenesis (Beaudoin et al., 2018) and brittle deformation within an accretionary wedge (Smeraglia et al., 2019).

 

Edited by Derya Gürer

References

Beaudoin, N., Lacombe, O., Roberts, N.M. and Koehn, D., 2018. U-Pb dating of calcite veins reveals complex stress evolution and thrust sequence in the Bighorn Basin, Wyoming, USA. Geology46(11), pp.1015-1018.

Goodfellow, B.W., Viola, G., Bingen, B., Nuriel, P. and Kylander‐Clark, A.R., 2017. Palaeocene faulting in SE Sweden from U–Pb dating of slickenfibre calcite. Terra Nova29(5), pp.321-328.

Hansman, R.J., Albert, R., Gerdes, A. and Ring, U., 2018. Absolute ages of multiple generations of brittle structures by U-Pb dating of calcite. Geology46(3), pp.207-210.

MacDonald, J.M., Faithfull, J.W., Roberts, N.M.W., Davies, A.J., Holdsworth, C.M., Newton, M., Williamson, S., Boyce, A. and John, C.M., 2019. Clumped-isotope palaeothermometry and LA-ICP-MS U–Pb dating of lava-pile hydrothermal calcite veins. Contributions to Mineralogy and Petrology174(7), p.63.

Nuriel, P., Craddock, J., Kylander-Clark, A.R., Uysal, I.T., Karabacak, V., Dirik, R.K., Hacker, B.R. and Weinberger, R., 2019. Reactivation history of the North Anatolian fault zone based on calcite age-strain analyses. Geology47(5), pp.465-469.

Nuriel, P., Weinberger, R., Kylander-Clark, A.R.C., Hacker, B.R. and Craddock, J.P., 2017. The onset of the Dead Sea transform based on calcite age-strain analyses. Geology45(7), pp.587-590.

Parrish, R.R., Parrish, C.M. and Lasalle, S., 2018. Vein calcite dating reveals Pyrenean orogen as cause of Paleogene deformation in southern England. Journal of the Geological Society175(3), pp.425-442.

Ring, U. and Gerdes, A., 2016. Kinematics of the Alpenrhein‐Bodensee graben system in the Central Alps: Oligocene/Miocene transtension due to formation of the Western Alps arc. Tectonics35(6), pp.1367-1391.

Roberts, N.M. and Walker, R.J., 2016. U-Pb geochronology of calcite-mineralized faults: Absolute timing of rift-related fault events on the northeast Atlantic margin. Geology44(7), pp.531-534.

Smeraglia, L., Aldega, L., Billi, A., Carminati, E., Di Fiore, F., Gerdes, A., Albert, R., Rossetti, F. and Vignaroli, G., 2019. Development of an Intrawedge Tectonic Mélange by Out‐of‐Sequence Thrusting, Buttressing, and Intraformational Rheological Contrast, Mt. Massico Ridge, Apennines, Italy. Tectonics38(4), pp.1223-1249.

Minds over Methods: Virtual Microscopy for Geosciences

Minds over Methods: Virtual Microscopy for Geosciences
The next “Minds over Methods” blogpost is a group effort of Liene Spruženiece (left) – postdoctoral researcher at RWTH Aachen and her colleagues Joyce Schmatz, Simon Virgo and Janos L. Urai.

Credit: Liene Spruženiece

The Virtual Microscope is a collaborative project between RWTH Aachen University and Fraunhofer Institute for Applied Information Technology (Schmatz et al., 2010; Virgo et al., 2016).

In the multitude of tools to analyze rocks, the optical microscope is still one of the first go-to methods for rock characterization. A look on a petrographic thin section under an optical microscope gives a quick overview of the mineralogy and fabric of a rock and helps to determine the areas of interest for more detailed analysis with other methods. It is also a crucial part for teaching geology classes.

Most geoscience institutions already have a petrographic microscope that is equipped with a camera and maybe an automated stage for capturing image mosaics. However, these images make only limited use of the vast amount of information available, such as the overview of the sample fabric at different magnifications or the change of optical mineral properties (pleochroism and extinction behavior) with the rotation of polarizers. Although there have been recent developments in the field of virtual microscopy (NASA, 2007; The Open University; 2010; Tetley and Daczko, 2014), to our knowledge, none of the existing systems has been able to fully emulate a user experience in a virtual environment that is equal to using a traditional analogue microscope.

 

Figure 1. The setup of the PetroScan microscope. Credit: Fraunhofer FIT

How does it work?

The idea behind the virtual microscope is to capture the full information of entire rock thin-sections in a digital format, including the possibility to switch between plain light and crossed polarizers, rotate the polarizers and zoom everywhere in the sample at high magnifications.

The hardware part of our system consists of a fully automated petrographic microscope, connected to a computer (Figure 1). The software contains three modules; a PetroScan launcher for setting up the image acquisition, optimizer for combining and interpolating the scanned images and a viewer program (Figure 3).

 

Figure 2. Automated sample stage allows capturing high resolution images of entire thin sections by combining thousands of images taken sequentially along a predefined grid. Credit: Jochen Hürtgen

High-resolution mosaics (Figure 2) containing up to a million images can be acquired in a few hours. With crossed polarizers, the scans are performed for several rotation angles. The movement accuracy of the sample stage is about 10 nm. This ensures precise overlap of the scanned mosaics at different rotation angles, where each corresponding pixel has the same x-y coordinates. Thus, the images of each rotation angle are precisely overlapping and allow to interpolate the change in brightness values for every individual pixel in the mosaics, producing smooth curves (Figure 3) that reflect extinction behavior in minerals (Heilbronner and Pauli, 1993).  Each attribute of these interpolated curves (phase, amplitude) of the sample can be viewed at any rotation angle.

 

Quantification of optical images

Figure 3. Screenshot of the PetroScan TileViewer window showing a thin section that is scanned under crossed polarizers. On the right side the scanned image is overlain by the phase map, where each pixel is color-coded according to the mineral extinction angles. Credit: Liene Spruženiece, Joyce Schmatz, Simon Virgo and Janos L. Urai

Virtual microscopy offers powerful means for quantitative analysis of giga pixel images to extract multi-scale information. It has a resolution of up to few micrometers for areas in sizes of 10 square centimeters. In its current form the viewer software that we developed contains a built-in toolbox for displaying and tresholding the intensity, saturation and hue values of the scanned images. This can be used for a quick estimation of sample porosity or proportions of different minerals. In addition, the scans with interpolated crossed-polarizer rotation also contain information of the mineral extinction behavior that is used to produce a “phase map” (Figure 3). The “phase map” displays the variations in the mineral optical axis orientations. Although it cannot provide absolute values for the crystallographic orientations, it allows a clear distinction between differently oriented mineral grains and allows easy visualization of qualitative lattice misorientations inside individual mineral grains. Furthermore, import and export functions are incorporated in the image viewing software. Thus more advanced image analysis can be carried out in specifically designated external softwares (e.g. Fiji/ImageJ, GiS, Matlab, Python), then inserted back in the PetroScan viewer software.

 

Applications in teaching

Figure 4. Mineralogy and fabrics of the scanned samples can be segmented and quantified using built-in toolboxes. The recognition of features is greatly improved by the possibility to zoom in the samples, switch between the plain light and crossed polarizers and rotate the polarizers. Credit: Liene Spruženiece, Joyce Schmatz, Simon Virgo and Janos L. Urai

At RWTH Aachen University, virtual petrography has been used as a teaching tool for several years. For example, it is extensively used in the microtectonics course. Each class consists of a short introduction in some of the common rock microstructures, such as veins, cataclasites, mylonites, dissolution-precipitation features and others. This is followed by an hour-long presentation by a student group, where they describe and discuss the respective microstructure in a scanned thin-section, projected to a screen with a high-resolution beamer. The students use laser pointers for characterizing the features and origins of the microstructure, switch between views in plain-polarized or crossed-polarized light, zoom in and out across the sample, adjust illumination settings, rotate polarizers and do basic image analysis (Figure 4). This has proven to be a highly engaging teaching method. The students in the audience question the presenting group asking to provide a closer look or more details on any interesting microstructural feature. Often hypotheses are formed by the listeners and immediately tested by the presenters, allowing to perform an investigation and agree on a reasonable explanation at the end of the class. Virtual petrography can be especially important in teaching institutions that do not own well-equipped microscopy labs. It only requires a computer and a projector. The thin section scans and PetroScan viewing software will be available for download online. Thus, such a method will not incur additional costs to the institutions, at the same time will provide a large collection of a high variety of geological samples.

 

The vision

We imagine a world-wide community built around the platform of virtual microscopy, where the information from different analytical methods can be exchanged between users and stored in open databases, available for teaching or research purposes. Many advantages arise from such a transformation. Work can be carried out anywhere without a need to access microscopes, several users can simultaneously view the same samples, data can be easily quantified and integrated between different methods, and thin-section libraries can be shared and exchanged by the user community.

Further plans for developing this method include collaboration with the computer vision department at RWTH Aachen University in order to create deep learning algorithms that allows quick and precise segmentation of rock microfabrics, such as mineral content and distribution, grain boundaries, grain sizes, porosity, etc. This has been made possible by recently obtained funding from the RWTH Aachen University “Exploratory Research Space – ERS”. The final data set will be public and shared with communities.

 

Edited by Derya Gürer

 

References

Heilbronner, R.P., Pauli, C. (1993). Integrated spatial and orientation analysis of quartz c-axes by computer-aided microscopy. Journal of Structural Geology, 15, 369-382.

Tetley, M.G., Daczko, N.R. (2014). Virtual Petrographic Microscope: a multi-platform education and research software tool to analyse rock thin-sections. Australian Journal of Earth Sciences 61, 631-637.

NASA (2007). Virtual Microscope. Available at: http://virtual.itg.uiuc.edu/

Schmatz J., Urai J.L., Bublat, M., Berlage, T. (2010). PetroScan – Virtual microscopy. EGU General Assembly, EGU2010-10061.

The Open University (2010). The Virtual Microscope for Earth Sciences Project. Available at: http://www.virtualmicroscope.org/.

Virgo S., Heup, T., Urai J.L., Berlage, T. (2016). Virtual Petrography (ViP) – A virtual microscope for the geosciences. EGU General Assembly, EGU2016-14669.

Meeting Plate Tectonics – Jean-Philippe Avouac

Meeting Plate Tectonics – Jean-Philippe Avouac

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


Meeting Jean-Philippe Avouac


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

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

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

Where lies your main research interest?

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

 

How would you describe your approach and methodology?

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

 

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

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

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

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

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

 

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

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

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

 

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

I do not think relevance to society should drive academic research

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

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

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

What do you consider to be your biggest academic achievement?

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

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

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

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

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

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

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

I do not like the way the funding system works today

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

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

 

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

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

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

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

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

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

 

Jean-Phillipe Avouac. Credit: Trish Reda.

 

Interview conducted by David Fernández-Blanco

Meeting Plate Tectonics – Francis Albarède

Meeting Plate Tectonics – Francis Albarède

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


Meeting Francis Albarède


Francis Albarède started his career as an undergraduate student in Natural Sciences at the University of Montpellier in southern France. He moved to Paris to get a PhD in Geochemistry, supervised by Claude Allegre, at the Institut de Physique de Globe de Paris (IPGP). After his PhD he remained at IPGP as researcher and teacher. He then moved to Caltech, where he stayed for two years. The National School of Geology in Nancy, France, offered him a professorship, a position he happily fulfilled for 12 years. In 1991 he switched to the Ecole Normale Superieure in Lyon, France where he holds a director position of the department of earth and life sciences until today.

 

Hi Francis, could you briefly introduce your research interests and methods?

Francis Albarède. Credit: Francis Albarède.

Sure. I’m a geochemist, and I apply geochemistry to understand mantle dynamics and the evolution of the mantle. I also use geochemistry to investigate other planets and work on ocean dynamics. Besides geochemistry, I also use isotopes and trace elements to understand the mantle dynamics and I use models to predict the complexness of magmatic and oceanic processes. Besides earth scientific questions, my methods can be used in archaeological problems or medical issues. My interests are mainly within the field of earth sciences, but I sometimes venture to different fields of research.

Interdisciplinary research needs to be enhanced.

You have quite an extensive career. What do you consider your biggest accomplishment in your field?

The introduction of geochemical modelling I consider one my most significant achievements. And of course, the introduction of the MC-ICP Mass Spectrometry in the mid-’90s within geosciences had tremendous success. At the time it was a new technique and it has become one of the most dominant geochemical tools in many different laboratories around the world. In geodynamics, the idea that continents grow from the head of superplumes was also successful.

Besides these big accomplishment, do you have personal projects too?

Yes, I do have a couple fun projects of my own. They often have to do with introducing new data. For example, I am currently working on using a panel of isotopes (silver, lead, copper) to understand the origin of money.

Science is actually quite hard work.

You have seen many changes in your field. What do you consider one of the biggest challenges in your field nowadays?

Interdisciplinary research needs to be enhanced. The geochemists are good at their own job, and so are the geophysicists. But we need more people that are knowledgeable in both geochemistry and geophysics, a gap that is difficult to bridge. In addition, few scientists ask the right questions. Always ask yourself why other people should care about your own research.

A two-stage history of He in the marble-cake mantle made of fertile (e.g., U- and Th-rich “pyroxenite” in beige) and refractory (e.g., U- and Th-poor “dunite” in green) rocks. Francis Albarède, 2008. Rogue Mantle Helium and Neon, Science, Vol. 319, Issue 5865, pp. 943-945, DOI: 10.1126/science.1150060

 

When you were at the early stages of your career, what were your expectations?

I never expected to be successful at an international level, but it happened anyway. I was craving to make great discoveries, and, even though the road to it was very bumpy, it happened, at least to the best of my capacities.

The most important is to think out of the box.

What is the most valuable advice you have received in your career?

As an early career scientist, I was very arrogant, even more than today. I received the advice, mainly from foreigners, to be more rigorous or demanding to myself. I was told that science is not just a quick effort, or that you do not get important results with a snap of your fingers. It is actually quite hard work. Claude Allegre, my PhD supervisor predicted I would always be a student and sure enough, I still am a student. I am not sure if I became less arrogant, but I definitely took his advice to work harder and to become more rigorous.

So, as the last question, do you have any advice for Early Career Scientists that are aiming for a career in science?

An Early Career Scientist needs to be exposed to other groups and individuals, preferably those who think differently. Perk is the great strength of being young but the danger is to reinvent the wheel. Do not think that something understood 50 years ago is necessarily obsolete. The most important is to think out of the box. This is not an easy thing to do, but if you manage it will make you a different scientist and therefore much more valuable to the community. Being scholarly will multiply and enlarge your sources of information. Read a lot, cultivate your memory, and most of all, have faith in your own capacities.

 

Francis Albarède. Credit: Société Française d’Exobiologie.

Interview conducted by Anouk Beniest

Meeting Plate Tectonics – Dietmar Müller

Meeting Plate Tectonics – Dietmar Müller

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


Meeting Dietmar Müller


Dietmar Müller is Professor of Geophysics at the University in Sydney and leads the EarthByte research group. He started his academic career in Germany at the University of Kiel and obtained his PhD in Earth Science at the Scripps Institution of Oceanography, UC San Diego, in 1993. Throughout his career he has straddled the boundary between geology, geophysics and computing.

Figure out what you actually enjoy doing and just go and do that.

You were educated in Germany and in the USA. How did you end up in Australia?

After I finished my PhD in 1993, I saw an advert for a lectureship in geophysics at the University of Sydney in EOS. I had never been to Australia and had no idea what life in Sydney might be like, but I thought, I might as well send off an application. A couple of months later I got a postcard from Sydney University, informing me that I had been shortlisted for the position. I thought this was vaguely interesting, but as a fresh PhD graduate, I mainly had my eyes on a couple of postdoctoral fellowships in Europe. Then I got a phone call for an interview. They had clearly decided that flying me to Sydney, all the way across the vast Pacific Ocean, was far too expensive. But they seemed to be interested in my vision for the future, and at the end of the phone interview they asked me: “If we offered the position to you, would you take it?” The thing is, this was the first real job anyone offered to me, so I thought, it’s probably a good idea to say “sure, why not”. Soon they faxed me a contract (these were not yet the days of the internet). Then I thought, hmm, what is this place actually like? So I went to the public library in San Diego and borrowed a VHS tape on Sydney. It included footage of Bondi Beach, Sydney Harbour and the Blue Mountains, with a few kangaroos and koalas thrown in for good measure. I thought this looks ok, it could be a liveable place. After I finally got my visa, I booked a 1-way flight to Sydney, and in late October 1993 I showed up in the Department of Geology and Geophysics and ran into a guy who turned out to be the Head of Department. He looked at the Scripps T-shirt I was wearing, and said: You must be the guy we hired! Of course, he had never seen me, so my T-shirt was my main identifying feature. Remarkably, over 25 years later, I am still there.

Dietmar settling into life in Australia in the 90s, mapping Devonian carbonates in Yass. Credit: unknown.

What is your main research interest? How would you describe your approach and methods?

I lead an Australian research effort, with many international links, to develop and continue to refine something that could be called a virtual Earth Laboratory. I have been an advocate for open-source software and open-access data during my entire career to make science transparent and reproducible. Based on these principles we have spearheaded the development of custom software and global data sets to reconstruct the Earth through time. To understand the Earth’s evolution we need to change our geographic reference system as we go back in time, because of plate tectonics. The plate tectonic revolution in the late 60s and 70s established the principles of how plate tectonics works. Applying these principles to build an Earth model is essentially what I have focussed my career on. I have always had a fascination with Earth evolution over geological time because its comprehension lies so far outside the everyday experience of humans. Most people cannot grasp the relevance of processes on vastly longer timescales than our own lifetime. But understanding the rhythms of Earth’s deep past and thinking about time like a geologist can perhaps give us the perspective we need for a more sustainable future. To dive into the Earth’s past, plate tectonics is indispensable. We need to be able to reconstruct geological data to their original environments. Doing this effectively requires open-source software and open-access data sets that can be shared amongst the community, enabling collaboration.

How do you build an open-source software system from scratch?

Dietmar Müller and Mike Gurnis in Altadena, 2006, taking a break from planning GPlates development. Credit: Melanie Symonds.

When I arrived in Sydney (over 25 years ago) there was no open software to build plate tectonic models, let alone to link plate motions to mantle convection models so that we can investigate the evolution of the entire plate-mantle system. I assembled a small team, partnering with Michael Gurnis at Caltech, to build the community GPlates software. This effort was initially supported by the Australian Partnership for Advanced Computing (APAC) enabling the development of GPlates1.0 on Linux and PCs and its Geographic Markup Language-based information model. In 2005, we managed to get a small educational grant from Apple Computers to develop the GPlates for Macs. We are lucky that shortly afterwards the AuScope National Collaborative Research Infrastructure was established which has supported GPlates development since 2007. That allowed us to fully develop reconstructions of plate boundary networks through time, which is essential for coupling plate tectonics to mantle convection models, as well as the 3D interactive visualisation of mantle volumes and lastly the functionality to model plate deformation, a key step beyond the classical rigid plate tectonic theory. We also developed a python library, pyGPlates, that allows users to link our plate models to many other forms of spatiotemporal data analysis and to other types of models, including geodynamic and paleoclimate models.

The slow carbon cycle is like slow cooking… over millions of years

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

Understanding the Earth as a system. I am interested in integrating observations from Plate Tectonics and mantle convection with landscape evolution and surface environments through time. I would like to adopt a definition of Earth System Science that actually includes the entire solid Earth, as well as the atmosphere, oceans and biosphere.

What are the real world applications of your research?

There are many applications of plate tectonics. They include understanding solid Earth evolution, palaeogeography, paleoclimate, paleoceanography, paleobiology, and spatiotemporal data mining, for instance for resource exploration. Most mineral deposits are associated with plate boundaries, so being able to link ore deposit formation with plate motions and the kinematic and geodynamic history of plate boundaries allows us to start understanding why certain mineral deposits form at specific windows in space and time, something we have recently started doing using the Andes as a case study.

Students will be entering a transformed workplace unlike any their parents knew

What do you consider to be your biggest academic achievement?

I am most well-known for my work on the age and palaeophysiogeography of the ocean basins. I started working on this as a PhD student. My thesis supervisor, John Sclater, made a name for himself with the first isochron map of the ocean basins. But there was no digital map. Having a digital grid, linked to a global plate model, was going to be critical for studying a whole range of processes from subduction, plate-mantle interaction, the evolution of ocean gateways through time, dynamic surface topography, and many others. I decided to synthesize all the data that we had available at the time to create the first digital map of the ocean basins, followed by a set of reconstructed paleo-age maps. This has enabled a lot of research, both my own and that of the community. For example, it has allowed us to look at the volume of the ocean basins through time (via the connection between the age and the depth of the ocean floor). A more recent achievement, fresh off the press, represents an epic decadal effort on part of the EarthByte group to complete a global plate model for the Mesozoic/Cenozoic period that includes plate deformation. Classical plate tectonics requires plates to be rigid and separated by narrow boundaries. It’s astonishing that it’s taken about 30 years since diffuse deformation was first widely recognised in the 80s to get to the point of systematically building a global model incorporating diffuse deformation for the geological past (soon to appear in Tectonics). It reveals that about a third of the continental crust has been deformed since the breakup of Pangea, about 77 million km2, partitioned into 65% extension and 35% compression. That roughly corresponds to the total area of North and South America and Africa together. The model can be used to investigate the evolution of crustal strain, thickness, topography, temperature, and heat flux, globally.

Total distributed continental deformation accumulated over 240 million years of rifting and crustal shortening. In Dietmar et al. (to come in 50th anniversary plate tectonics volume in Tectonics). A global plate model including lithospheric deformation along major rifts and orogens since the Triassic.

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

Recently, I became involved in the Deep Carbon Observatory. There are a few quite exciting problems involved in understanding the Earth’s deep carbon cycle and, being an area I have not traditionally worked in, it’s a new adventure for me to try to understand how plate tectonic drives the geological carbon cycle. One of the problems that we tackled in the course of connecting plate tectonics to the “slow carbon cycle” is to investigate seafloor weathering. The slow carbon cycle takes place over tens of millions of years, driven by a series of chemical reactions and tectonic activity and is part of Earth’s life insurance, as it has maintained the planet’s habitability throughout a series of hothouse climates punctuated by ice ages. We were able to build on ocean drilling results and laboratory experiments from other groups to understand how of the storage of C02 and carbon in the ocean crust changes through time, as a function of the age of the ocean crust and of the bottom water temperature, which is quite important, because temperature strongly modulates this process. This is something we published in Science Advances in 2018.It is quite a cool paper!

We actually need geochemists and geophysicists to work together

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

The biggest change in my time in academia is the emergence of artificial intelligence (AI) and data science as a universal, rapidly growing research area and set of tools to analyse big or complex data, to assimilate data into models and to quantify uncertainties in process models and predictions. There is an urgent need for all Earth science students to become literate in these areas. By the time this year’s first-year students will graduate, they will be entering a transformed workplace unlike any their parents knew. However, the need for changing staff profiles and undergraduate curricula are often recognised and implemented much more slowly than the evolution of the world outside of our ivory towers. But this change needs to happen.

What you just exposed, goes to some extent in line with my next question: What are the biggest challenges right now in your field?

Most of the problems that we are left with are complicated problems that aim at understanding the complexity of the Earth system. That could be anything from structural geology to understanding physical and chemical problems. An example is the field of geodynamics. It is mostly dominated by looking at the physics of mantle convection. And then there is another bunch of people who look at the chemistry of the mantle. These fields have not been properly connected. We actually need geochemists, geophysicists and geologists to work together to try to understand how the Earth system works. Then we need to connect deep Earth evolution to surface environments, understand the exchange of fluids and volatiles between the solid Earth and the oceans and atmosphere.

You actually have to be in for the long game

Building a geological time machine at the University of Sydney, 2009. Credit: Rhiannon McKeon

What was your motivation, starting as an Early Career Researcher? Did you always see yourself staying in academia?

As a kid I was inspired to become a scientist by taking long walks along Germany’s Baltic Sea beaches, picking up unusual rocks and fossils along the way, none of which really belonged there. They had all originated in Scandinavia, where they had been scraped off by moving glaciers and dropped much further south after being transported in the ice over 1000 km. I still have a small collection of these rocks and fossils which include remains of sea urchins and squids from the Cretaceous period and over 400 My old pieces of ancient reefs that had once been buried deeply in the Scandinavian crust. I always wanted to be an academic, I wanted to understand how the Earth works, over geological time. I never had any second thought about that. I can see today that students are often quite confused about what they want to do. Because they are unsure about where the future might take them, they don’t end up focussing on any one subject and are not necessarily inclined to acquire skills that are deep and broad enough to excel. If you want to be successful at anything, you need to become really good at something, and persevere. Be good at something that you actually enjoy, and be in it for the long game.

Who inspires you?

I am inspired by the pioneers of open source software and open access data. Open science is the key to forming global research teams and advancing studies of the Earth system. I am inspired by Paul Wessel at the University of Hawaii, who, together with his colleagues, built one of the most extensive geo-software systems, the Generic Mapping Tools, over the past ~30 years; I started using an early version of it during my PhD and am still using it! In terms of open access data, one of my heroes in Earth Sciences is David Sandwell at the Scripps Institution of Oceanography, who revolutionised our knowledge of the deep structure of the ocean basins by making his global satellite gravity maps freely available to the community. On the geochemistry side, Kerstin Lehnert at the Lamont-Doherty Earth Observatory has accomplished an amazing feat by leading the EarthChem database effort, and now the Interdisciplinary Earth Data Alliance, a nice example for bringing geochemistry and geophysics together.

What is the best advice you ever received?

Not long after I arrived at the University in Sydney, the then professor of geophysics pulled me aside and said: “I have one piece of advice for you: Stay away from University politics and just do your own thing“. That’s exactly what I have done and that’s the best advice I have ever received. It is easy to get carried away with politics at many different levels…

Stay away from University politics

What advice would you give to students?

You have to figure out what you enjoy and what you would like to do. You should not choose a career because you think this career will pay more money than another one, or it may seem there are more jobs in one field than another. The advice I would give to students is to try to figure out what you actually enjoy doing and just go and do that. The future will be driven by big and complex data analysis and simulation and modelling, but there will still be a need for people who can identify a rock. If you can do both, you’ll have a job without any doubt!

 

Dietmar Müller, November 2018 in his office. Credit: Jo Condon, AuScope

 

Interview conducted by David Fernández-Blanco

Minds over Methods: Reconstructing oceans lost to subduction

Minds over Methods: Reconstructing oceans lost to subduction

Our next Minds over Methods article is written by Derya Gürer, who just finished a PhD at Utrecht University, the Netherlands. During her PhD, she used a combination of many methods to reconstruct the evolution of the Anadolu plate, which got almost entirely lost during closure of the Neotethys in Anatolia. Here, she explains how the use of these multiple methods helped her to obtain a 3D understanding of the Anatolian double subduction system and the demise of the Anadolu plate. 

Credit: Derya Gürer

Reconstructing oceans lost to subduction

Derya Gürer, Postdoctoral Researcher, Utrecht University, the Netherlands. 

Subduction represents the single biggest recycling process on Earth and takes place at convergent plate boundaries. One plate subducts underneath another into the mantle, generating volcanism, earthquakes, tsunamis and associated hazards. Subduction zones come and go, and nearly half of the subduction zones active today formed in the Cenozoic (after ~65 Ma) (Gurnis et al., 2004). The negative buoyancy of subducted lithosphere (‘slab pull’) is thought to be the major driver of plate tectonics (Turcotte and Schubert, 2014). Changes in the configuration of subduction zones thus change the driving forces of plate tectonics, making the reconstruction of the kinematic evolution of subduction key to understanding past plate motions. Such reconstructions make use of data preserved in the modern oceans (marine magnetic anomalies and fracture zone patterns). But because subduction is a destructive process, the surface record of subduction-dominated systems is naturally incomplete, and more so backwards in time. Sometimes, relicts of subducted lithosphere are preserved in active margin mountain belts, holding valuable information to restore past plate motions and the dynamic evolution of subduction zones.

But how does one recognize a plate that has been almost entirely lost to subduction? And how do we reconstruct the evolution of subduction zones through space and time?

 

Archives of plates that were (almost) lost due to subduction

Subduction occurs in a variety of geometries and leaves behind a distinct geological record that holds key elements for the analysis of the past kinematics of now-subducted plates. Where subduction occurred below oceanic lithosphere, fragments of the leading edge of this overriding lithosphere may be left behind as remnants of oceanic crust (ophiolites). Subduction of oceanic plates may also be associated with accretion of its volcano-sedimentary cover to the overriding plate as an accretionary complex (Matsuda and Isozaki, 1991). Forearc basins associated with intra-oceanic subduction zones form on top of ophiolites and accretionary complexes and may record permanent deformation (syn-kinematic) of the overriding plate in response to tectonic interaction with the down-going plate (e.g., accretion, subduction erosion, slab roll-back) (Fig. 1).

Fig. 1: The location of archives of the evolution of “lost” oceanic plates (ophiolites, accretionary complexes, forearc basins) in a subduction zone setting.Credit: Derya Gürer.

The sedimentary infill of forearc basins implicitly records the nature and stress state of the overriding plate. Forearc basins may therefore hold the most complete record of the motion of the oceanic plate relative to the trench. However, many accretionary complexes and forearcs are deeply submerged and buried below sediments, making them highly inaccessible, and therefore expensive to study. As a consequence, our understanding of such systems is primarily based on well-studied examples in the East Pacific (e.g. Franciscan Complex, California (Wakabayashi, 2015)). Other such systems exist in the Mediterranean realm – for example in the geological record of Anatolia. The unique and direct archive of past plate motion in the geological record of Central and Eastern Anatolia is independent from constraints provided by marine magnetic anomalies, and provides a key region to unravel the evolution of destructive plate boundaries.

 

How many oceans were lost in Anatolia?

Fig. 2: The multidisciplinary approach used in my PhD research consisted of structural field analysis and stratigraphy of Anatolian sedimentary basins with focus on syn-kinematic deformation (top) with time constraints provided by absolute age dating of accessory minerals and biostratigraphy (middle). Paleomagnetic analysis (bottom left) provided information about vertical axis rotations. The combined information from these methods were integrated in a kinematic reconstruction and tested against mantle tomography (bottom right). Credit: Derya Gürer

To answer this question, I studied the deformation of sedimentary basins overlying Anatolian ophiolites (remnants of oceanic crust), and the deformation record of rocks which were buried and exhumed below these ophiolites. The Cenozoic deformation of the Anatolian orogen allowed for identifying the timing of arrest of the subduction history and revealed the simultaneous activity of two subduction zones in Late Cretaceous time. These two subduction zones bound a separate oceanic plate within the Neotethys Ocean – the Anadolu Plate (Fig. 3, Gürer et al., 2016). The aim of my PhD research was to reconstruct the birth, evolution and destruction of this oceanic plate.

Tectonic problems require a multidisciplinary approach, in order to study the evolution of orogens and associated sedimentary basins. My research involved the integration of (1) structural analysis, (2) stratigraphy, (3) geochronology, (4) paleomagnetism, (5) plate reconstruction, and (6) mantle tomography (Fig. 2). The main goal was to obtain new data on the evolution of the Central and Eastern Anatolian regions through the analysis of spatial and temporal relationships of deformation archived in the geological record.

First, I collected kinematic data from sedimentary basins (Fig. 2) overlying ophiolitic relicts of the oceanic Anadolu Plate, as well as from the underlying accretionary complex (Gürer et al., 2018a). Here, it was especially useful to focus on syn-kinematic deformation recorded by sediments. To constrain the timing of this deformation, I used geochronological data coming from absolute age dating and biostratigraphy. The integrated reconstruction of the kinematic history of basins was used to develop concepts quantitatively constraining the tectonic history of the Anadolu Plate and its surrounding trenches in 2D (Gürer et al., 2016).

 

Fig. 3: The Ulukışla Basin (Central Anatolia) represents a forearc basin in Late Cretaceous to Eocene time which recorded the evolution of the Anadolu Plate. The basin has subsequently been strongly deformed during Eocene and younger collisional processes and is juxtaposed against the Aladağ range along the Ecemiş Fault. Credit: Derya Gürer.

 

There are, however, large vertical axis rotations constrained through paleomagnetic analysis within Anatolia, not taken into account in the workflow described in the previous paragraph. Therefore, paleomagnetic data from the Late Cretaceous to Miocene sedimentary basins were collected. These data identified coherently rotating domains and major tectonic structures that accommodated differential rotations between tectonic blocks (Gürer et al., 2018b).

Fig. 4: Simplified interpretation of the Late Cretaceous double subduction geometry in Anatolia and the Anadolu Plate.Credit: Derya Gürer.

Subsequently, a kinematic reconstruction of Anatolia back to the Late Cretaceous was built (Fig. 4) incorporating the timing of deformation obtained through structural analysis, stratigraphy, geochronology, and vertical axis rotations. This reconstruction provided first-order implications for the timing and geometry of subduction zones and revealed that the demise of the Anadolu Plate and collision in Anatolia was variable along the strike of the orogen, younging from the west to the east. The exact timing of collision in Eastern Anatolia will require future studies applying structural field geology, systematic analysis of the age and nature of magmatism, and thermochronology to constrain timing of regional exhumation, as well as detrital geochronology, providing information on the relative proximity of tectonic blocks through the provenance of sediments.

 

Finally, the resulting 2D kinematic reconstruction was tested against a mantle tomographic model (UU-07, Amaru, 2007; van der Meer et al., 2017) to gain insights into its 3D geometry. Mantle tomography images the present-day structure and positive seismic anomalies (blue colours in Fig. 5), which may be interpreted as subducted slabs. Comparing the convergence estimate obtained from the kinematic reconstruction with the imaged subducted lithosphere allowed to infer that the mantle structure in the Eastern Mediterranean holds record of not only the two strands of the Neotethys Ocean that existed in Anatolia, but also of the Paleotethys Ocean.

 

Fig. 5: Map view tomographic structure below the Eastern Mediterranean region at variable depths (increasing in depth from left to right). Blue colours generally represent positive, whereas red colours represent negative wave speed anomalies. Credit: Derya Gürer & Wim Spakman.

The combination of methods to unravel the geological record of Anatolia quantitatively constrained the evolution of subduction zones and of the Anadolu Plate. The reconstruction of the Anatolian double subduction system that existed in Late Cretaceous time has implications for the dynamics of multiple simultaneously active subduction zones.

 

References

Amaru, M.L., 2007. Global travel time tomography with 3-D reference models. PhD thesis, Utrecht University, The Netherlands.

Gürer, D., van Hinsbergen, D.J.J.D.J.J., Matenco, L., Corfu, F., Cascella, A., 2016. Kinematics of a former oceanic plate of the Neotethys revealed by deformation in the Ulukışla basin (Turkey). Tectonics 35, 2385–2416. https://doi.org/10.1002/2016TC004206

Gürer, D., Plunder, A., Kirst, F., Corfu, F., Schmid, S.M., van Hinsbergen, D.J.J., 2018a. A long-lived Late Cretaceous–early Eocene extensional province in Anatolia? Structural evidence from the Ivriz Detachment, southern central Turkey. Earth Planet. Sci. Lett. 481. https://doi.org/10.1016/j.epsl.2017.10.008

Gürer, D., Hinsbergen, D.J.J. van, Özkaptan, M., Creton, I., Koymans, M.R., Cascella, A., Langereis, C.G., 2018b. Paleomagnetic constraints on the timing and distribution of Cenozoic rotations in Central and Eastern Anatolia. Solid Earth 9, 1–27. https://doi.org/10.5194/se-9-1-2018

Gurnis, M., Hall, C., Lavier, L., 2004. Evolving force balance during incipient subduction. Geochemistry Geophys. Geosystems 5, Q07001. https://doi.org/10.1029/2003GC000681

Matsuda, T., Isozaki, Y., 1991. Well-documented travel history of Mesozoic pelagic chert in Japan: from remote ocean to subduction zone. Tectonics 10, 475–499.

van der Meer, D.G., van Hinsbergen, D.J.J., Spakman, W., 2018. Atlas of the Underworld: slab remnants in the mantle, their sinking history, and a new outlook on lower mantle viscosity. Tectonophysics 723, 309–448.

Wakabayashi, J., 2015. Anatomy of a subduction complex: architecture of the Franciscan Complex, California, at multiple length and time scales. Int. Geol. Rev. 37–41. https://doi.org/10.1080/00206814.2014.998728

 

How Rome and its geology are strongly connected

How Rome and its geology are strongly connected

Walking through an ancient and fascinating city like Rome, there are signs of history everywhere. The whole city forms an open-air museum, full of remnants of many different times the city has known, from the Imperial to the Medieval times, the Renaissance, the Fascist period, and finally the present day version of Rome. For historians and archaeologists, unravelling the exact history of the city proves to be a major challenge, since things are only partly preserved or have been renovated or moved to serve a different purpose. This might sound familiar to geologists, since they deal with the same type of problems, just on much larger scales, both spatially and temporally.

Although you might expect to find the keys to the geological history of Rome and its surroundings outside the city, there’s actually a great deal of hints within the city itself. Let’s start with the roads you would walk on, during a visit to Rome. If you’ve ever been to Rome, you might remember the black cobblestones, which form the pavement for many streets in the historical centre of Rome. The Italians call them ‘sanpietrini’, cubic-shaped blocks made from volcanic rocks coming from the surrounding volcanic regions.

 

Volcanic activity

Two of these volcanic regions are the Alban Hills, southeast of Rome, and the Sabatini volcanic complex, northwest of Rome. They are part of a line of volcanic fields along the edge of the Italian peninsula, stretching from Naples, all the way to Tuscany. Eruptions in these areas were mainly explosive and created large volcanic plateaus and craters. One of those plateaus was formed by an eruption of the Alban Hills volcanic field and consists of volcanic tuff stone. Over time, erosion has altered this plateau and created a topography of valleys and hills, including the seven hills that Rome was built on. These hills are still remarkable features in the city today, for example when you climb the stairs to the Capitoline Hill and have a gorgeous view of the Imperial Forum or when standing on the Aventine hill in the south, looking down on Circus Maximus in the valley below you, and seeing the ruins of the imperial palaces on the Palatine hill in front of you.

 

Left: Map showing the regional relief and the two volcanic complexes north and south of Rome. Credit: modified from Funiciello et al., 2003 by Francesca Cifelli. Right: The seven hills of Rome. Credit: theculturetrip.com.

 

The volcanic rocks in the Roman area did not only shape the landscape, they also served (and still do!) as an important water supply to the city. Springs in the areas, but also freshwater lakes formed in the volcanic craters are important sources for the city’s water budget. In fact, last summer Rome was in a state of panic, since severe drought and extremely hot temperatures had a big impact on the water level of volcanic lakes providing water to Rome and city officials were considering rationing drinking water for the Roman citizens.

 

The Apennines

Another important water supply to Rome are the springs in the Apennines, a NW-SE trending mountain chain, also called ‘the backbone of the Italian peninsula’. This mountain chain is the result of a collision between the African and Eurasian plates, which was part of a series of complex collisions and extensions of the Earth’s crust in the Mediterranean region, lasting from roughly 100 million years to 2 million years ago.  During the last 20 million years, the Italian Peninsula rotated counter-clockwise, resulting in the formation of what we now call the Tyrrhenian sea. This period of extension also formed the onset of volcanic activity in the region.

 

Map of the Mediterranean highlighting the main tectonic processes. Credit: Introduction to the Geology of Rome.

 

The rocks in the Apennine mountain range are limestone, deposited in ancient shallow seas as long as 300 million years ago. These rocks became very important to Rome, since they formed major rock reservoirs, which have been used for water supply for many centuries. Many remains of ancient aqueducts carrying water to Rome can still be found nowadays, and some of them are still being used, like the Vergine aqueduct, bringing water to the Trevi fountain. Also the ‘fontanelle,’ little fountains on the streets everywhere in Rome, are part of this water supply system and always provide clear, cool, and drinkable water. And if you’ve ever spend a day in Rome during summer, you know how valuable these fontanelle are!

 

Left: view on the Imperial Forum from the Capitoline Hill. Many of the buildings at the Forum have been built with travertine. Right: remants of the Aqua Claudia, one of Romes many acqueducts bringing water from the surrounding regions to the city. Credit: Elenora van Rijsingen

 

The limestone that ended up in the Apennines often were converted into marble due to the high pressures and temperatures during collision. This marble  can be found everywhere in Rome, since they have been used as building blocks for various structures like the Pantheon and Trajan’s column. Another rock which has been used a lot for Roman buildings is travertine, which forms by the evaporation of river and spring waters. Many temples, aqueducts, amphitheatres, and monuments have been built with travertine, but the most famous one is the Colosseum, which is the largest building in the world constructed mainly of travertine blocks.

Have you ever wondered why part of the outer ring of the Colosseum is missing? It is actually also linked to geology, since the southern part of the Colosseum collapsed during a historical earthquake. The tectonic processes which formed the Apennines still produce irregular movement along all kinds of faults on the Italian Peninsula, generating frequent earthquakes. The reason why only the southern half of the Colosseum collapsed (fortunately!) is because it had been partly built on unconsolidated alluvial deposits. When shaken by an earthquake, these loose sediments amplified the shaking and therefore caused severe damage to the southern part of the amphitheatre.

 

The site effect: amplification of seismic waves due to the properties of the subsurface. Credit: Ciaccio and Cultrera (2014) Terremoto e rischio sismico.


The Tiber
These type of alluvial deposits can also be found at the floodplains of the Tiber, the river which passes through Rome and played an important role in the city’s development. Romans in the imperial times did not build any houses on the floodplains of the Tiber, because they knew the river would flood every once in a while. Instead, they built theatres, temples, and army training facilities which could easily be restored and would not harm the societies too much.

Another reason not to build along these floodplains is the same reason which damaged the Colosseum: the increased risk of earthquake damage due to amplification of the shaking. Unfortunately, nowadays, many areas close to the river are covered with residential areas and even though the risk of flooding has decreased due to the 12 meter high walls surrounding the Tiber today, the risk of increased earthquake damage still exists.

And now I think of it, I am living in one of those areas myself, in Testaccio, a neighbourhood just south of the Aventine hill. I guess this amplification of the shaking due to the alluvial deposits below my feet is the reason why I feel a slight shaking (even when living on the fourth floor!) every time a large truck passes by. Roughly 2000 years ago, Testaccio was not a residential area, but was used as the location for an olive oil warehouse along the Tiber. We even have an ancient garbage dump in our neighbourhood, which is now part of the local landscape and is referred to as ‘Monte Testaccio,’ literally meaning ‘Testaccio mountain’. Romans would pile up discarded amphorae, which were used to store the olive oil, leaving a hill composed of fragments of roughly 53 million amphorae.

 

Left: the Tiber river bounded by its 12 meter high walls, which should prevent the city from future floods. Credit: Elenora van Rijsingen. Right: millions of amphorae fragments piled up in an organized way and together forming the Monte Testaccio. Credit: Flickr.

 

Clearly, in Rome not only geological processes shaped the landscape, but also deposits called human debris played a role. Digging an imaginary hole below your feet anywhere in Rome might reveal more ancient houses, businesses, or roads, all buried during the continuous evolution of the Eternal City. And that’s one of the reasons why, for example, the work on the new metro line here in Rome is taking so long! Every ten meters, they stumble upon a new archaeological site, all revealing new hints about what the city was like hundreds to thousands of years ago.